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HVAC FUNDAMENTALS

AND TESTING

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HEATING, VENTILATION, AND AIR CONDmONING TABLE OF CONTENTS

CHAYfER ONE: BASIC LA WS AND APPLICATIONS

INTRODUCTION . .......... . .... . . . ................ . ..•... 1·1

BASIC AIR LAWS ......• . . . . . . • ... ' ...... . .. • ... • . . . . .. . •... 1-1

PULLEY LAWS ........ . .............. . . .. . ............. . . 1-7

FINDING RPM INCREASE OR DECREASE BY AMPERAGE . .. . . . .• . .... 1-12

FORMULAS FOR ADJUSTING SHEAVES ....... . ...... . . . •.. , .... 1-14

PERFECT GAS LAWS ... . ............. , ., . . ....... . . . ...... 1-15 Pascal's Principle ......... . .. . ......................... 1-15 Charles's Law . .. . . . .. . ............... . ........ . ...... 1-15

Pressure Varies Directly with Absolute Temperature For a Constant Volume .... .. ..... . . . ..... . .............. . 1-15

Gay-Lussac's Law . .. . . ... .......... . .................. 1-17 Volume Varies Directly with Absolute TemperabJre For a Constant

Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . • .. . . . .. 1-17 , . Boyle's l..aw . ..................... . ................. . 1-18

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Pressure Varies Inversely with Volume if the Temperature Remains Constant .............................. . ... 1-18

Effects of Changing Temperature, Pressure, and Volume at Same Time . ... 1- 19

HEAT TRANSFER ............ . .. . . . .......... . ..... .. ... . . 1-20 Conservation Of Energy . . • .. • • . .• ... . . •... ,. . . . .. . . ..... 1-20 Heat Flow ........... , •. . ... . , . .... , ... , . • . . • . . , . . .. 1-20

Conduction ..... . , ... ........................... 1-21 Convection . . ... . •...•...• .. .. .. ...•. .. " .. ..... 1-24 Radiation ...... • , .. • ... • ..... , ...•.... ,.. . . . ... 1-27

Insulation .............. . . . ......... • . . ...... . .. , . . .. 1-28

PSYCHROMETRIC PROPERTIES OF AIR .. . . , • • ... " . . . • ...• . . , .. 1-29 Psychrometric Chart ................••.... , ............. 1-30

SUMMARY . . . . . . ....... . ... . ... , ... , ' • . .. • .. .•...•..... 1-39

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CHAPTER TWO: HVAC SYSTEMS

INTRODUCTION .. . . . ...... .. ........ • ... . . . • ........ . .... 2-1

PURPOSE OF HV AC ..• • ... . .. • .. . ... . .. . ........... • ...•.. . 2-1 Temperature .. .. ........... . ... . .•.........•..•.. .. ... 2-1 Humidity .. . . ................ .. ........... • ...... . ... 2-1 Suspended Particulates (Dust and Gases) . . . . .... . ..........•.... 2-2

AIR SYSTEMS . ........ .. ............. . . . .. . . . . ....... . . . . 2-2 Single Zone System ..........•.. . •....•..•..•......•.... 2-3 Variable Air Volume System .....•...•...... . • .. • .. . ...• . ... 2-4 Terminal Reheat System ...... • •. . .•...• . . • ......•........ 2-5 Induction System . . . . . . . . . . . . . . . . . . . . . • . . • . . • . . . . . . . • . . . 2-6 · Dual Duct System ....... . ........ . . ..... . . .•. .• ...• .... 2-7

Dual Duct System (Low Velocity) .... . • ..... ... .. .. • . ... . 2-7 Dual Duct System (High Velocity) ... . .. . . . .. • .. .... • . .. . . 2-8

Multizone System . . ... . .. ... .. . ........ • ... . .. • ... • .... 2-9

FILTRATION SYSTEMS .... ... ... ........... . .. . ...... .. ..... 2-10 Fibrous Media Filters .............•... . ... •. .• . ......... 2-10 Electronic Air Cleaners . . . . . . . . . . . . . . . . • . . • . . . . . . • . . . • . .. 2-12 High Efficiency Particulate Air Filter ....... •. .• . .• . ..•..•.... 2-13

HYDRONIC SYSTEMS . . . . . . . . . . . . . . . . . . . . • • . • . . . • . . • . . . • . . . 2-16 Low Water Temperature System (L TW) . . . . . . . . • . . . • . . • . . • . . . . . 2-16 Medium Temperature Water System (MTW) ... . . . . ..•..•........ 2-16 High Temperature Water System (HTW) .. .. . .. . .... .... .. .. . ... 2-17 Chilled Water System (CW) .............. . • ...• . . . . .. . .... 2-17 Dual-Temperature Water System (DTW) . . . .... .. ..... . ..•..... 2-17 Series Loop System ... ... .... . ..... . . . .• •. . .• . . •. . . .... 2-18 One-Pipe System (Diverting Fitting) .....•.......•......•..... 2-19 Two-Pipe Systems . . .... . . ..... . . . ..•. .. • .. .. . .. . .• . .. . 2-20 Combination Piping System . .. . . . .. .. ........ . ...•..•.. . .. 2-22 Three-Pipe System . . . . . . . . . • . . . . . . . . • . . . • . . . . . . • . . • . . . . 2-22 Four-Pipe System . . . . . . . . . . . . . . • . . . • . . . • . . . • . . • . . . . . . . . 2-24 Hydronic Piping ... ...... .. .. . .. . .•• . ... . .•..• • .. . . .. . 2-26

Air Control and Venting .. . .•...•.. . .. .... ..... • ..... 2-26 Drains and Shutoffs . . . . . . . • • . . . . • . . • . . • • . . . . . • . . . . . 2-27 Balance Fittings .........•... • . . . . •. . ... . • .. ...... 2-27 Pitch . ... ....... . .• • . .•. . .• •. ....... . •.. ... ... 2-27 Strainers . . . . . . . . . . • . . . • . . . • . . . . . . . • . . . . . . • . . . . . 2-27

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Thermometers . ..... . . •........... ... . ..... . . ... . 2-27 Flexible Connectors . . . . . . . . . • • . . . . • . . • . . . • . . . . . . . . . 2-28 Gauges . . . . . . . . . . . . . . . . . . . • . . . . . . . . . . . • . . . . . . .. 2-28 Pump Location . . . . . . . . . • . . . . . . . . • . . . • . . . . . . . . . . . . 2-28

SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CHAPTER THREE: HVAC EQUIPMENT

INTRODUCTION ........................... .•. . .•. .. . .. ..

CRITERIA FOR EQUIPMENT SELECTION ...... . . • •• .. •. . .• . ..... Demand of Comfort or Process .. . . . . . . . . . . . . . . . . . . . . . . . . . . . Energy Conservation . .............. . ... . ....... . ... . ... . First CostlLife Cost ... . . . . . . . . . . . . . . . . . . . . . . • . . . . . . . • . . Desires of Owner, Architect, or Design Office .... . .. . .. ... ..... . Space Limitations ...........................•...... •. .. Maintainability ............... . ... . .. . .•..... . ..... • . . Central Plant Versus Distributed Systems . ..•. .. •...•...• .. • .. .. Simplicity and Controllability .......... . ... . .... • ...•......

HEATING ....... .. ... . ... . .....•.....•...•...•......•..

BOILERS ............ . ..•...•...•.... . •...•...•......•.. Hot Water Boilers ... . . . . . . . .. .... . . ... . ... . . . .. . .. . .. . Steam Boilers ...........•...•....•• . ..•...•... . .. . . . .

ELECTRIC HEATERS . .. . .............•....•.. . • ...•... •.. .

2-30

3·1

3·1 3·2 3·2 3-3 3-3 3-3 3-4 3-4 3-4

3·5

3·7 3·7 3·7

3-7

TERMINAL HEATING EQUIPMENT ...... • . . ....... • .. . •.. • .... 3·9 Radiators and Convectors . . . . . . . . . . . • . . . . . . . . • • . . . . . . . . .. 3-10 Radiant Panels .......... . ... .. •• . . . . . . ...•... • . . •... 3-12

HEAT PUMPS ............. . . . • . . ..•.....•...•...•...... 3·13 Packaged Heat Pumps . . . . . . . . • . . . . • . . . . . . . . . • . . . • . . . . .. 3·13

COOLING ................ •.. . • ..... .. . .•.... •.. .. ..•.. 3·18 Refrigeration ......•.. • ...•...•.. ... •. .. • •.. • ...• . .. 3· 18

Steam Jet .......•.. . • .. . • .. . .. • .. . .... • . .. •... 3·19 Heat Sink ..............• . ....• . . .. •...... . .... 3-20 Absorption ... . .... ...... . . ... . .. .. • . . . • . . . . . .. 3-20

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HEATING, VENTILATION, AND AIR CONDmONING TABLE OF CONTENfS

Compressed Gas ..... ..... ... .... . . .. ... . ......• . ... . 3·23 Compressor . . . . . . . . . . . . • . . . • . . . . . . . . . . . . . • . . . .. 3·23 CondenserlReceiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 3·23 Metering Device . . . . . . . . . • . . . • . . . . . . . . . • . . . . . . . .. 3·24 Evaporators . . . . . . . . . . . . • . . . • . . . . . . . . . • . . . . . . . . . 3·25

Chillers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . . . 3·28 Flooded Chillers . . . . . . . . . . . . . . . . . . . . . . . • . . • . . . . .. 3·29 Direct Expansion (DX) Chillers . . . .. . .. . . . . • ... . .. . • . . 3·29 Package Chillers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 3·29

Cooling Towers. . . . . . . . . . . . . . . . . . . . . . • . • . . . . . . • . . . . .. 3·31 Open·Circuit Cooling Towers . . . . . . . . • . . . . . • . . . • . . . • .. 3·33 Closed·Circuit Towers .... .. . . . .... .. •.. • ...•. ..•.. 3·34

Cooling Coils .................. • .. ........ • ...•..... 3·36 Plpmg ....................... . . . . .. •..... .... 3·36 Pumps . . ...................... • ..... • .. . .... .. 3·36 Pump Configurations and Types ..... . ..... .. .. • ..... .. 3·37 Performance Curves. . . . . . . . . . . . . . • . . . . . • . . • . . . . . .. 3·38 Pump Selection . . . . . . . . . . . . ." . . . . • . . . . . . . . . • . . . • .. 3-40

AIR· HANDLING . . . . . . . . . . . . . . . . • . . . . . . . . . . • . . . . . . . . . . . .. 3·40

FANS ...... . .. ... . .. . . . . . . . . . . . . • . . . . . . . . . . . . . . . . . . .. 3-41 Classifications of Fans. . . • . . . • . . . . • . . . • . . . . . . • . . • . . . . . .. 3·42 Fan Control ......... . ....• • . . . . ...• . . • . . .. . .• ...... 3-48 Fan Drives . . . . . . . . . . . . . . • . . . . • . . . • . . . . . . • . . . . . . . • .. 3·50 Fan Laws . . . . . . . . . . . . . . . . • . . . . . . . . • . . . . . . . . . • . . . • .. 3·50 Fan Characteristic Curves ... • .... •...•... ... •...... • .... 3·51

DUCTWORK ................•...•....•......•.......... 3·59 Classification .... . ..... . .• . . . .•...• . . . . . .• . . . . .. ... . 3·59 Duct System Accessories .. . • • ..... . . • . . . . . . . . . . • . . . • . . .. 3·60

SUMMARY

Turning Vanes . . . . . . . . . . . . . . . • . . . • . . . • . • • • . • . . .. 3·60 Dampers . . . . . . . . . . . . . . . . . . • . . . . . . . • . . . . . . • • . .. 3·64 Louvers . .... . ..............•... •. .• .. . •. .. •... 3·66 Grilles. Registers and Diffusers . . . . . . . . • . . • . . • . . . . . . . .. 3·67 Silencers . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . . . . . . . .. 3·71

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CHAPTER FOUR: FIELD INSTRUMENT OVERVIEW

INTRODUCTION ........ . ............... . ................ 4-\

AIRFLOW MEASUREMENT DEVICES . . . . . . . • • . . • . . . • . . . • . . . • . .. 4-\ U-Tube Manometer ............. . . ..••...... . • .. . ...... 4-\ InclinedNertical Manometer. . . . . . . . . . . • • . . • . . . • • . . . . . . • . . . 4-2 Micro-Manometer .... . ...... . . . .....• . .• . .. . •. .• . ..•. . 4-3 Pitot Tube . . .. ... . .. .. ..•......... • ..•...•...•...•.. 4-3

Construction .. . .... . • . ..• . .. . ... . •. . .•. . .•.. . •. . 4-3 Pitot Tube Use . . . . . . . . . . • • . . . • . . . • . . . • . . . . . . . • . .. 4-5 Use of Readings . . . . . . . . . . • . . . . • . . . • . . . • . . . • . . . . . . 4·9 Pitot Tube Duct Traverses . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-11 Round Duct Traverses ...... ....... . •... • . . . • ...... 4- \3 SquarelRectangular Duct Traverses ......... . ....... . ... 4· 15 Correcting For Non-Standard Conditions. . . . . . • . . . • . . • . . .. 4·16

Pressure Gauge (Magnehelic) . . .. .......... • ...• . .. •...•.. 4-2\ Rotating Vane Anemometer . .....................•• . .•... 4-22 Bridled Vane Anemometer . . . . . . . . . . . . . . . . • . . . • . . . • . . . • .. 4-24 Deflecting Vane Anemometer ....•...••.......•........•.. . 4-25 Hot Wire Anometer . . .. ... . . ..•... . ....• . ..• . ......•.. 4-27 Smoke Devices ... . . . . . . . . . . . . . . • . . . . . . . . . • . . . • . . • . .. 4-3\

HYDRONIC MEASURING EQUIPMENT . . . • . . . . . • . . • . . . . . . . • . . .. 4-31 U-Tube Manometer .............. . • .. .. . ...• . . . . .. . ... 4-32 Pressure Gauge . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . • . . . . . .. 4-33 Differential Pressure Gauge . ......... .. . . . .. . . .. ......... 4-34 Venturi Tube and Orifice Plate (Flow Devices) ....•. ... •.. . . . ... 4-36 Annubar Flow Indicator. . . . . . . . . . . . . . . . . . . . . • . . . • . . . . . .. 4-38 Calibrated Balancing Valve . . . . . . . . . . . . . . . . . . • . . . • . . . • . . .. 4-39 Location of Flow Devices . . . ............ _ . . . . . . . . . . . . . .. 4-40

TEMPERATURE MEASURING INSTRUMENTS. . . . . .• . . . • . . . • . . . . . 4-42 Glass Tube Thermometers ......................•.. ... ... 4-42 Dial Thermometers . . . . . . . . . . . . . . . . . . . • . . . . . . . . . . . • . . .. 4-44 Pyrometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . • . . . • . . .. 4-45

HUMIDITY MEASURING DEVICES . . . . . . . . . . . . . . • . . . . . . . . . . . .. 4-46 Psychometric Measurement Devices ... . . . . ... . ... . ..... . .... 4-46

Dry Bulb Thermometer. . . . . . . . . . • . . . . • . . . • . . . • . . . .. 4-46 Wet Bulb Thermometer .. .......•........•... . ..... 4-46 Psychrometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . .. 4-47

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Dew Point .. ........... . .....•...•. . . . . . . . . • . . . • . .. 4·50 Wick·Type Dewcells .... . .. ••. .• . . ... . ... •. . . • . .. . 4·50 Capacitance Probe Dewcell ...... •... . .. •. .•.. .•..... 4·52 Chilled Mirror Dewcell ..... •.. . •.. .. . . •. ... . .•. . .. 4·53

ELECTRICAL MEASURING DEVICES .. .. . . . .. . • ..•• . . . . .... . .. 4·54 Volt-Anuneter . . . . . . . . . . . . . . . . . . . . . • . . . . . • . . . . . . . . . .. 4-54 Insulation Resistance Monitoring ..... .. ... . .......•... . .... 4-57 Two Fundamental Properties of Insulation . ....... • .. • ......... 4-57 Factors Affecting Insulation Resistance ... . ..... • ... •. .. ... ... 4-58 Measuring Insulation Resistance . . . . . . . . . . . . . . . • . . . . . . . . . . .. 4-58 Conditions for Measuring Insulation Resistance . .......... . ...... 4~59

Instruments ... .. .......................•........... 4·61 Testing Guidelines .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4·61

MINIMUM VALUES AND FREQUENCY OF INSULATION RESISTANCE TEST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . . . . .. 4·64

Minimum Insulation Resistance Value ... . . . . . . . . . . . . . . • . . . • .. 4-64 Frequency of Inspection . . . . . . . . . . . . . • . . . • . . • . . . . . . . • . . .. 4·65 Interpretation of Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . : . . . . 4-65

ROTATION MEASURING INSTRUMENTS ... ..... • •. . •.. . • ... • .. 4·67 Revolution Counter (Odometer) . . . . . . . • . . . . . . . . . . . . . . • . . . .. 4-68 Tachometers. Centrifugal . . . . . . . . . . . . . . . • . . . . . . . . . . • . . . .. 4-68 Tachometer, Chronometric . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-69 Tachometer, Electronic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-70 Tachometer, Photo. . . . . . . . . . • • . . . • . . . . . . • . . . . . . . • . . . .. 4-71

VIBRATION MEASUREMENT. . . . . • . . . . . . . . • . . . . . . . . . . • . . . . . . 4-72 Vibration Probe. . . . . . . . . . . . . . . . • . . . . • . . . . . . • . . . . . . . . . 4·72 Measuring Vibration . . . . . . . . . . . . . • . . . . • . . • . . . . . . . • . . . .. 4-75

Measuring Displacement . . . . . . . . . . . . . . . . . . . . . . . . . . .. 4-77 Measuring Velocity .......... . ..•...... • ...•... • .. 4-77

GAUGE MANIFOLD ......... .. ..... • . . . .. ....... ........ . 4-77 Using the Gauge Manifold ....•........•..•... . .. . ... . . .. 4-78 Special Attaching Devices ... . . . .. ... . • ... • . . . . ....... • .. 4·82

SUMMARY ....... . ......... .. ... . ... . ... •.... .. ....•.. 4·83


CHAPTER F1VE: SYSTEM TEST AND BALANCE PROCEDURES

INTRODUCTION 5-1

AIR FLOW MEASUREMENT IN DUCTS. . . . . . . . . . . . . . . . . . • . . . • . .. 5-1 Air Flow Measurement of Diffusers .... ............. ... . .. ... 5-2 Air Flow Measurements of Supply Grilles and Registers. . . . . . . . . . . . . . 5-2

Use of Hoods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 5-3 Air Flow Measurement of Return Grilles and Registers ... . ... . ...... 5-4 Testing of Motor Amperage .... . . . . . .. .. . .. . .. . . . • . ....... 5-5 Measuring Static Pressures .. . . ..... ....... ...... .. ...... . , 5-5 Testing of Hot and Cold Mixing Dampers .....• ... • . .. . . . .. . . .. 5-5 Testing and Setting Static Pressure Dampers . . . . . . . • . . . • • . . . . . . .. 5-5 Testing of Face Velocities Across Coils . . . . . . . . . . . . . . . . . . . • . . .. 5-6 Conditions of System During Tests . . . . . . . . . . . . . • . . . • • . . . • . . .. 5-6 Setting of Outside Air and Return Air Volumes . . . . . • . . . . . . . . . . . .. 5-7 Testing of Ceiling Plenum Systems . . . . . . . . . . . . . • . . . • . . . • . . . .. 5-8 Testing of Air Shafts. . . . . . . . . . . . .. . . . . . . . . . . . . . . • . . . • .. 5-11 Procedure for Testing Air Shafts ..... • •....•....... • .. ..•... 5-12 Procedure for Testing Shaft Wall. . . . . . . . . . . . . . . . . . . • . . . • . .. 5-13 Fume Hood Testing .. . .................... • ........... 5-14 Air Distribution Duct Leakage Test ...•. .. . •...•. . . •. . .• . ... 5-18

Methods and Standards . . . . . . . . . . . . . . . . . . . . . • . . • . . .. 5-18 Test Equipment ........... • . .... • ... •. ..•... • .. . 5-19 Field Test Procedure . .. . . ... • . . .. .... . ....• .. . • . .. 5-19 Test Verification . . . . . . . . . . . . • . . . . . . . • . . . . • . . . • . .. 5-22

HYDRONIC SYSTEM TESTING . ..... .. . ; . . . . . . . . • . . . • . . . . . . .. 5-22 Balance Procedure - General . . . . . . . . . . . . . • . . • • . . . • . . . . . . .. 5-23 Chilled and/or Hot Water Systems ..... .... ... . . ..• .. . . ..... 5-26 Condenser Water/Cooling Tower Systems ....•..••... •. . . •.. .. 5-27 Steam and Hot Water Boilers ... ... . .... • ...••... •... •.... 5-29 Heat Exchangers/Converters . . . . . . . . . . . . . . . . . . . . . . . . . • . . .. 5-30

Balancing Data Required. . . . . . . . . . . . . . . . . . . . . . . • . . .. 5-31 Water Balance with Coil, Control Valve and Measuring Station .... . • . . 5-33

GPM ESTABLISHED THRU COIL ..... . ....... . .. . .. . .. ... . ... 5-35 Cabinet Unit Heaters . . ........... ..•....... . • . .. . •. ... 5-36 Fan Coil Unit and Unit Ventilator .. • ............ . .... . .. . .. 5-37 Unit Heaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . • . . .. 5-38 Pumps ...... .. ............ • . .. . . ....•. . . ..... .... 5-39 Chiller ... . ........ ... . .... • ... •• ... . ... • ... . .. ... 5-40 Cooling Tower ...•......•. . . . .. . . •. . . . • . . . • ... . . .... 5-41

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HEPA FILTERS ... ... ............. . ...••. . ... .. .• •...•.. 5-42 Problems in HEPA Filter Use . . . . . . . . . . . . . . . . . . . . . . . . . • . . . 5-42 HEPA Filter Testing . . . . . . . . . . . . . . . . . . . . • . . . . . . • . . . . . .. 5-45 HEPA Filter Testing Problems ..... .. ..•. . ....•...•... • .. . 5-45 HEPA Filter Test Procedures . .. ...•... • . ......... . ...... . 5-46

Summary of Method . . . . . . . . • . . . . . . . . . . . . . • . . . • • . .. 5-46 Prerequisites for Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 5-47 Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . . . .. 5-47

CHARCOAL ADSORBER REQUIREMENTS AND TESTING ... ...•.... . 5-48 Adsorberl Adsorbent Requirements . .. ... . .............•..... 5-48 Charcoal Adsorber Test Procedures .. .... . ... . ... .... . . . .... 5-51

Purpose ... . ..............•...•..•. ..•.. .•.... 5-52 Summary of Method .. ....... . • ...•.. •. .. . . . .• . .. . 5-52 Prerequisites for Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 5-52 Apparatus . . . . . . . . . . . . . . . . . . • . . • . . • . . . • . . . . . . .. 5-53

SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

CHAPTER SIX: SOUND AND VIBRATION TESTING

INTRODUCTION Sound .. . ... . .......... . . . ... . ....... • ...... • •... ..

Sound Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sound Pressure Level .. ...... . ... . .... . .. ... ... .. . . . . . . . Loudness and Frequency .. . ... . .... . ... . .......... . ..... . NC Curves . . .... . .. .. .... . • ....... . .. . .. . • .. .•. ..... Architectural Acoustics ..... . .•... • ... • ... • .. . .... . ......

Reverberation Time . .. . . ... .. .. . ... . .... ... . ..... . . Sound Trap Selection . .. . ... • ...•... . ... .... • .... ..

Sound Testing ...... . ........ ......•• .. • ..... . . • . . . . . Sound Testing Specification ........ . . . . ..... ... . . .. . .

VIDRATION ........ . . . . . ........ .• .. . • • .. . . . . • . ..• .. ... Vibration Testing ... .. ......... . ....... . ...... . •.. . ...

Vibration Testing Procedure . ... . . . . . . . .. . ... . . .. . ... . Vibration and Noi~ Identification .... . . . . .... ... ...... .. .. .

Analysis Procedure ................... .. . .. . ... .. . Vibration and Noise Source Identification . .... ... . ... .. .. . Noise Analysis .. .. .. . . ........... ...... .. .... .. .

Relative Probability Ratings ...... . . . . . ...... . ... • . .. ... .. Application of the Chart ... • ..... ...• .... . .. . ... • . ..

SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5-53

6-1 6-3 6-3 6-3 6-5 6-6 6-7 6-8

6-10 6-10 6-11

6-12 6-13 6-15 6-16 6-24 6-26 6-28 6-28 6-29

6-29


CHAYfER SEVEN: MAINTENANCE AND TROUBLESHOOTING

INTRODUCTION ............... .. . . ... . .................. 7-1

SYSTEMATIC TROUBLESHOOTING TECHNIQUES ........ _ _ ... _ . . .. 7-2

TROUBLESHOOTING FANS . .......... . . . ... .. ... _ ... _ . .. _ • .. 7-3 Noise ........... ... ... .... ... . . . . .. .... . .. . ....... 7-4 Performance Reduction . ... . .. . .. . . .. . . ... . .. ... ..... . ... 7-4

Checking for Spin . . . . . . . . • • . . . . . . • . . . • . . . . . . . • • . .. 7-5 Rotation .......................................... 7-18

TROUBLESHOOTING ABNORMAL AIR CONDITIONING OPERATIONS ... 7-20 High Head Pressure ... .. .... . .... ... .. . ........ . .. . ... 7-22

Dirty or Partially Blocked Condenser . . . . . . . . . . . . . . . . . . . . 7-22 Air or Noncondensable Gases in System .... . • .. . • • .. • . . . . 7-22 Overcharge of Refrigerant . . . . . . . . . . . . . . • . . . • • . . . • . .. 7-24 Insufficient Condensing Medium . . . . . . . . . . • . . . • . . . . . . .. 7-25 High Temperature Condensing Medium .. .. • . . ... ... .. . .. 7-25 Restricted Discharge Line . . .... . . . . . • . . . . . . . • . . . • . .. 7-25

Low Suction Pressure .... . . . . . . . . . . . . . . . . . . • . . . • . . . • • .. 7-26 Insufficient Air on Evaporator Coil ... . . ...• ... • .. . .. .. . 7-26 Poor Distribution of Air on Evaporator Coil. . . • . . . . . . . • . . .. 7-27 Restricted Refrigerant Flow . . . . . . . . . . . . . • . . . • . . . • . . .. 7-27 Undercharge of Refrigerant .... ... . . ....• _ ..• _ .... _ .. 7-28 Faulty Metering Device . . . ... .•.. . ... . •....•. . .. • ... 7-28

Low Discharge Pressure ...........•... _ .... • .... . . _ • . .. 7-29 High Suction Pressure. . . . . . . . . . . . . • . . . . . . . • . . . • . . . . • . .. 7-29

Heavy Load Conditions ....... .... ... • • ... • ...... . . 7-29 Low Superheat Adjustment . . . . .... .. ..•...•.... _ . . .. 7-30 Improper Expansion Valve Adjustment ........ _ • . . . . . . . .. 7-30 PoorInstallation of Feeler Bulb ..... . ............ . .... 7-30 Inefficient Compressor .. . . . . . . . . . . . . . . . . . . . . . . . . . .. 7-31 High Discharge Pressure on Capillary Tube Systems . . .•... • .. 7-31

SUMMARY 7-32

," ..

(

CHAPTER ONE

BASIC LAWS AND APPLICATIONS

I .

CHAPTER ONE BASIC LAWS AND APPLICATIONS

OBJECTIVES

At the completion of this chapter, the student will be able to:

1. State the primary objective of HV AC.

2. Explain the fan laws as they relate to fan performance.

3. Define static and velocity pressure .

4. Calculate duct capacity.

5. Explain how the fan laws are used to determine the affect of various fan speeds.

6. Use the fan laws to determine fan speed.

7. Given a change in rpm, be able to determine the charge in amperage for a motor.

8. Given a change in cfm, be able to determine a new pulley setting.

9. Explain Pascal's Principle.

10. Explain Charles's Law.

11. Explain Gay-Lussac's Law.

12. Explain Boyle'S Law.

13. State the first law of thermodynamics.

14. State the second law of thermodynamics.

15. State the three ways heat can flow.

16. Explain the relationship of thermal resistance and thermal conductance.

17. Given two points of information on a psychrometric chart, be able to determine the other five.

CHAPTER ONE BASIC LAWS AND APPLICATIONS

INTRODUCTION

The primary objective of Heating, Vent ilation and Air Conditioning (HV AC) is to control the characteristics of the air in a contro lled environment. This chapter introduces the characteristics and properties of air that affect HVAC system des ign and construction. These bas ics must be comprehended in order to proceed effecti ve ly with the course .

The glossary in the back of the text provides an extensive listi ng of HV AC terms and should be referred to as needed throughout this course. Standard Heating, Ventilating and Air Conditioning symbols are located in Appendix A.

BASIC AIR LAWS

The performance of air handl ing, transmission and distribution systems will follow certa·in establ ished laws which make it possible to calculate the expected perfor mance of an air moving system after adjustment or changes within the system have been made.

Tllese laws are the most commonly used 10 system design and ba lanc in g and are listed in Figure 1-1.

1- 1

FAN PERFORMANCE:

a. CFM va ries in direct proportion to RPM.

or CFM =

b. SP varies as the square of the RPM.

SP, RPM2 RPM, , or SP, = SP j x =

SP j RPM; RPM j

SP or RPM, = RPM x -'

SP j

c. Hp varies as the cube of the RPM.

= RPMf3

RPM or RPM, = RPM x

,

d. BHP varies as the cube of the CFM.

BHp _-,-I = CFMf3 BHp;

Figure 1-1 Air Laws

1-2

CFMf3

CFM j

e. CFM & RPM varies as the square root of the pressure ratio.

or RPM - RPM x - /

f. HP varies as the square root of the pressure ratio cubed.

HP -/ -

g. CFM varies as the square of the fa n size ratio (at given SP & ra ting).

h. RPM va ries as the square of the fa n size ratio (at given SP & rating).

RPM = RPM x

Figure 1·1 Air Laws (cont ' d.)

l·3

I. HP varies inversely as the fan size ratio (at given SP & rating).

J. CFM varies as the size ratio cubed times the RPM ratio.

x

k. SP varies as the size ratio squared times the RPM ratio squared.

x RPMf 2

RPM I

1. Hp varies as the size ratio raised to the 5th power times the RPM ratio cubed.

x

Figure 1-1 Air Laws (cant'd.)

1-4

DVCfED AIR FLOW: ), ~1v\.J

'Y' " .-/

J~c. iii '1':1 "-

a. Total Pressure TP. \\\iJ ~~~,\

TP = SP (Static Pressure)

~\~ .,) ,,( ~

~\i fI' \ + VP (Velocity Pressure)

b. Duct Capaci ty CFM.

CFM = Duct Velocity FPM x Duct Area A

c. CFM varies in direct proportion to duct FPM.

= FPM _----'-f or CFM = FPM f

I

d. SP varies as the square of the duct CFM and FPM.

SPf = CFMf 2

= FPMf 2

SPi CFMi FPMi

CFMf 2 FPM2

SPf SPi x SPi X f = =

CFMi FPM I

Figure I-I Ai r Laws (cont'd.)

[-5

e. CFM va ries as the square root of the static pressure ratio.

CFM -f -

f. CFM varies in direct proportion to duct area A (at given velocity).

CFM -f -

g. Duct FPM varies as the square root of the static pressure ratio.

FPM -f -

Figure 1-1 Air Laws (coned.)

1-6

: .j "

. ,

1 I

I . . .

h. The veloc ity indicated is fo r dry a ir at 70"F, 29.9" Barometric Press ure and a resul ting density of .075#/cu. ft.

Air Ve loc ity =

where:

Air Dens ity

where:

Pv = D =

T =

11096.2 Pv

D

velocity pressure in inches of water Air dens ity in - leu. ft.

= 1.325 x PB T

Barometric Pressure in inches of

mercury

Absolute Temperature (indicated

temperature plus 460)

Figure 1-1 Air Laws (cont'd.)

[ ·7

The three main fan laws are identical to the three "pump laws".

PumQ Fan V a N CFM a RPM Hp a N' SP a RPM' P a N' BHP a RPM'

where: V = volumetric flow rate CFM = air flow rate Hp = pump head SP = static pressure P = power BHP - horsepower

The above relationships are extremely useful in determining the affect that varyi ng fan speed has on overall fan performance.

Static pressure is the pressure exerted by reason of weight or existence of fluid or gas confined within a space. Velocity pressure, commonly referred to as impact pressure is pressure exerted by air moving through a confined space and impinging on a stati onary object. The sum of static and velocity pressures is total pressure which is defi ned as total a ir "pressure" energy or energy of the air within the duct relating to atmospheric pressure. This pressure energy results in air flow within a duct, and if duct s ize is known (area), then total duct air flow may be determined. Static and velocity pressures are measured in inches of water gauge or inches of mercury.

Mathematica lly, these pressure relationships are:

TP = CFM =

SP + VP Duct Velocity (FPM) x Duct Area

1-8

Duct ve loc ity in fee t per minute is determ ined us ing ve loc ity pressure and a conversio n facto r to convert inches o f water gauge to feet per minute a nd will be disc ussed in deta il unde r a ir fl ow measurement.

PULLEY LAWS

Dri ve sets for fans and blowers cons ist o f a dri ver pulley on the motor shaft , a dri ven pu ll ey on the blower shaft, and a belt or set of matc hed be lts to tra nsmit the powe r. Pulley formu las are usually given in pulley di ameters; for accuracy, they should be considered in actua l pitch diameters.

Figure 1-2 shows an exa mple of a fully closed shows the sa me sheave in the full y open posi tion .

SHEAVE FULLY CLOSED

F igu re 1-2 Fu lly Closed Sheave

1-9

sheave. Figure 1-3

CORNERS

BREAK CORNERS

Figure 1-3 Fully Open Sheave

Figure 1-4 is an example of a multiple groove pulley with fully open sheaves.

F Open ~ SeN 9 - 1) + 2Se

1------ (Ng ~ No. of Grooves) -----

S e-"~I~ -

Figure 1-4 Multiple Groove Pulley with Fully Open Sheave

1-10

c,"" Section

14JOV 1930V 2S30V 32JOV 4430V

Table I-I gives dimensions for standard variable sheaves.

Table 1-1 Variable Sheave Groove Dimensions

b, b, h, 20 Closed 0"," Minimum ( Inches) (Inches) (Inches) (Inches)

0.875 !. 0.005 1.582 !. 0.005 1.758 0.20 LlSB ! 0,005 2.142! Q,()()5 2.]4 1 0.25 1.563 :!: 0.007 2.823 :!: 0.007 3.0:'18 0.30 2.000 ! 0.007 3.665 ! 0.007 3.855 0.35 2.750 ! 0.007 5.132 ! 0.007 5.258 0.40

The four basic pulley laws are :

rpm P,

rpm Pm

dia P,

dia Pm

where:

diaPm xrpm

dia Pr

dia P, X rpm

dia Pm

dia Pm X rpm

dia P, X rpm

P, fan pulley or driven sheave

2a.

( Inches)

2.64 3.56 4.74

6.21 8.89

Pm = motor pulley or driver sheave

1-11

S. Open M inimum ( Inches)

0 .882 1.163 1.501 [.954 2.687

S Minimum ( Inches)

1.765

2.325 l003 3.908 5.375

Pulley Speed-O-Graph for rapid calculations of the pulley laws is shown in Figure 1-5. Using this nomograph, the speed or size of either pulley can be determ ined when the other three factors are known.

1. Enter the chart from any given factor and follow the straight grid line to the point where it intersects, on the diagonal, the other given factor.

2. Follow the diagonal line to the point where it meets the third given factor.

3. From this point of intersection, move along the straight grid line to the fourth side of the margin for the solution.

EXAMPLES:

Example 1: Given: Diameter of Drive = 3 in. DIAMETER OF DRIVEN = 12 in. rpm of Driver = 5000 FIND: RPM OF DRIVEN

1-12

RI'~1 or: TIll; driull" ( dl

Q Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q QQ Q Q Q Q Q Q Q Q QQ Q Q Q Q Q Q QQ -. N ~ ~ ~ ~

, ~ 0-. N ~ ~ ~ ~ ,

~.

9000 8000 7000

'0 6000

SO SOOO

<0 4000

• • ~ JO JOOO u c .- 25

-. 20 2000 ·u -< • " , ... < -. ·u Q -~ , 0 1000 , ~ e- .. 0

~ , • ~ 8 800 ~

0 , 0

~ 7 700 ~

~

e- O 600 ~

~ e-

'" ~ ~ 5 SOD 0 c

'" , <00 ~ ~

J JOO

2 ZOO

100

/l IMtETER or TIlE DRIVeN (0) -inches

Figure 1-5 Speed-O-Graph: Pulley Laws

1-13

Example 2: Given: Diameter of Drive = 30 in. DIAMETER OF DRIVEN = 4 in. RPM OF DRIVER = 3750 FIND: rpm of Driver

Example 3: Given: rpm of Driver = 1000 RPM OF DRIVER = 5000 Diameter of Driver = 10 in. FIND: DIAMETER OF DRIVEN

1-14

I . I

Example 4: Given: rpm of Driver = 2500 RPM OF DRIVEN = 5000 FIND: Diameter of Driver

FINDING RPM INCREASE OR DECREASE BY AMPERAGE

To determine the percent of rpm increase or decrease by reading the ammeter, the following formula applies:

rpm23 ( )

Example:

A fan is turning 600 rpm and reading 20 amps. To deliver the proper cfm it is necessary to increase the fan speed to 700 rpm. Find the new amperage.

20 x ( 700)3 = 200 x 1.588 = 31.75 amps2 600

I-IS

Table 1-2 gives calculated data fo r this equat ion. Us ing the above example, the increase in speed is 100 rpm; th is is an increase of 100/600 or 16.66%. By interpolation the table shows that the original amps would have to be mult ip lied by 1.588, or 20 x 1588 = 31.75 amps.

Table 1-2 Rpm Increase/Decrease (To determine the required change in fang speed multiply the measured amps by the given faclor )

% RPM Mult iply Amps % RPM Multi ply Amps Increase By: Decrease By:

1 I 2 1.06 2 0.94 3 1.09

, 0.92 J

4 1.13 4 0.88 5 1.16 5 0.86 6 1.19 6 0.83 7 1.23 7 0.80 8 1.26 8 0.78 9 1.33 9 0.75

to 1.33 to 0.73 11 l.37 II 0.70 12 1.40 12 0.68 13 1.44 13 0.66 14 1.48 14 0.64 15 1.52 15 0.61 16 1.56 16 0.59 17 1.60 17 0.57 18 1.64 18 0.55 19 1.69 19 0.53 20 1.73 20 0.5 1 21 1.77 21 0.49 22 1.82 22 0.47

" _J 1.86 23 0.45 24 1.90 24 0.44 ,-- ) 1.95 25 0.42 30 2.20 30 0.34 35 2.46 35 0.28 40 2.75 40 0.22 45 3.05 45 O.t 7 50 3.38 50 0.12

1-16

FORMULAS FOR ADJUSTING SHEA YES

1. Given a change in cfm, find the new pulley setting.

=

Example:

( Cfm2) d X P 1

cfml

Determine the new pitch diameter for 4000 frm when a fan output is 3500 cfm at a 10 in . pitch d iameter.

10 = 11.43 In .

2. Given an increase in cfm, wi ll the new brake horsepower overload the ex ist ing motor?

l-17

Example:

Determine the new brake horsepower required to increase the cfm from 5000 to 5500 when the bhp is 0.8 and the motor is rated at 1 hp.

= 1.06

Therefore, the motor needs to be changed to 1-1/2 hp .

3. G iven a maximum brake horse power, find the new pitch diameter required to change from an ex isting pitch diameter.

3[j rnax bhP2 y( bh )

PI

Example:

Determine the new pitch diameter to bring a 1 hp motor up to maximum when the present pd is 10 in. and bhp is 0.8.

10 3jI.25 X 10 = 1.077 X 10 10.77pd

where: pd = pitch diameter bhp = brake horsepower cfm = a ir quantity at the fan

1-18

PERFECT GAS LAWS

Pascal's Principle.

Pasca l's Principle is defined as follows: "Fluid pressure is due to the weight of the fluid pushing on an area. [t is normally measured in pounds per square inch. If it is referenced to atmospheric pressure, it is psig. Pressures below atmospher ic are measured in inches of mercury (in Hg). If the reference point fo r pressures is below atmospheric it is a vacuum, then the un its are in psia or in Hg abs.

The expansion and contraction of solids and liquids with change in tem perature are enough that they must be considered in many situations. But they are comparatively small, for the molecules of the solids and liquids are held rather closely and not allowed to fly off by themselves. [n gases, the expansion and contraction with change of temperature are very large compared to those of solids and liquids. [n addition, the volume of the gas varies with the co nta iner. A gas automatically fill s any container that it is put into, regardless of whether the container is small or large .

Nei ther a liqu id nor a solid does this . A container fo r solids or liquids can be partly filled, bu t a gas contai ner is always full.

Charles's Law

Pressure Var ies Directly with Absolute Temperature if Volume Stays the Same

Because a gas adapts itself to its container, regardless of conta iner size or the amoun t of gas in the container, another factor is introduced -- pressure. If the conta iner is already filled, then a rise in temperature cannot cause an increase in volume, but it does resul t in an increase in the pressure of the gas aga inst the inner wa lls of the cyl inder.

1- 19

This change in pressure with a change in temperature can be eas il y calcu lated as long as the volume stays the same. The calcu lation is very s imple: Gas pressure goes up at the same rate as the temperature. If the temperature rises 25% or 1/4, the pressure goes up 1/4. [f the gas cools down to 2/3 its temperature, the pressure does down to 2/3 of what it was. Years ago, a sc ient ist named Charles discovered this princ ipl e, so it is called "Char les' Law rl

•

Cha rles' Law says that if the vo lume remains the same, pressure of a gas varies as the absolute temperature va ries. illustrates Cha rles' Law.

1 CU.FT. 70'F

700.0 PSIG 714.7PSIA

1 CU.FT. gOT

727.0PSIG 741.7PSIA

Figure 1-6 Ill ustration of Charles ' Law

1-20

the absol ute Figure 1-6

Gay-Lussac 's Law

Vo lu me Va ri es Directly with Absolute Temperature if Pressure Stays th e .' Same ,

Now suppose that instead of hav ing the gas in a steel cylinder that keeps the gas from expanding, the gas is in a cylinder that has a loose bottom that can slide up and down just like a piston in a compressor. If the gas in the cyl inder is warmed, it can expand and push the piston downward, but the pressure ins ide the cylinder remains the same, because the piston would mere ly s lide downward if the pressure inside the cylinder tended to become greater than that outside the cylinder and below the piston.

Now we have a condition of the volume changing with change of temperature, but the pressure re maining constant. This princ iple, known as Gay-Lussac's Law, states that the volume changes with the change in temperature. Figure 1-7 illus trates this principle.

700 PSIG 700 PSIG

707.dF gOT

1 CU.FT. 1. 0 4 CU .FT.

© ©

Figure 1-7 Illustrat ion of Gay-Lussac's Law

1-21

Boyle ' s Law

Pressure Varies Inverse ly with Volume if the Temperature Stays the Same

There is a thi rd cond ition: What happens to the pressure if the volume changes but the temperature stays the same?

In the two prev ious conditions, the proportion was direct; that is, the pressure and volume went up as the temperature went up and down as the te mperat ure went dow n.

In this third condi tion, the temperature remains constant, and the pressure goes down as the volume goes up, or vice versa, the pressure goes up as the vo lume goes down. This relationship is ca lled an inverse proport ion, known as Boyle 's Law.

For example, Figure 1-8 shows a cy linder with a loose piston. In th e piston shown in Figure 1-8(a), the volume of the cy linder above the piston is one cubic foot and the pressure is 20 psig or 34.7 ps ia. In Figure 1-8(b), the piston has been slowly lowered twice as far, so now the volume is two cub ic feet. What happens to the pressure? It goes down in the same proportion as the volume went up.

70T 1 cu. FT.

3 4.7 PSI A 70T

2 cU.n. 17.35 P$lA

Figure 1-8 Illustration of Boyle's Law

1-22

Effects of Changing Temperature, Pressure, and Volume at Same Time

[n Charles' Law, the volume remains constant and the pressure varies with a change of temperature .

[n Gay-Lussac's Law, the pressure remains constant and volume varies with change of temperature. [n Boyle's Law, the temperature remains constant and the pressure varies with change in volume. All three of these laws help us understand how pressures, volumes, and temperatures change in containers of gas. In each of these three laws, one of the variables remains constant: The pressure, the volume, or the temperature.

However, gases are not always so considerate: Pressure, volume and temperature may change at the same time with none of them remaining constant. Therefore, combination of these laws must be used, namely the general law of perfect gases. Figure 1-9 shows an actual case where pressure, temperature, and volume have changed.

70 ·F 1 CU.FT.

, .014.7 PSIA

@

40·F

2 CU.FT.

478.6 PSIA

@

Figure 1-9 lllustration of Temperature, Volume, and Pressure Variations

1-23

HEAT TRANSFER

Conservation Of Energy

Chemical energy in coal can be changed into heat energy in steam. Heat energy in s team is changed into mechanica l energy in a turb ine and then into electrical energy in a generator. Electrical energy can then be changed back into heat energy in a toaster, to mechanical energy in a motor, or to chemical energy in a storage battery. In this example, all of the e lectrica l energy in the motor did not go into mechanica l energy. Some of it was "lost" as heat. We say "lost" , beca use we did not get any use out of the heat of the motor. Actually it was not "lost", for the heat was energy , transformed from electr ica l energy.

Energy cannot be destroyed nor created . It can merely be transformed to or fro m some ot her kind of energy. Th is princ iple is known as the Law of Conservation of Energy, also known as the first Law of Thermodynamics. It is well to remember it, fo r it expla ins many things.

For example, the Law explai ns efficiency. In changing the electrical energy in the motor, so me went into mechanica l energy (or power) and some in to heat. T he efficiency is the percentage of the electric ity that becomes power. The mechanical energy (the output energy) determ ines the effi ciency. If three-fourths of the electrical energy became mechanical energy, then the effic iency is 75%.

In a good boiler, over one-third of the heat energy in the coal burned goes up the chimney or is rad iated from the boiler; about two- thirds goes into hea t energy in the steam, so the effi ciency is about 60% to 65%. Effic iency is the useful output energy fro m a machine, divided by the input energy to the machine, and expressed as a percentage.

1-24

The speed with which heat transfers by means of conduction varies with different substances or materials if the substances or materials are of the same dimensions. The rate of heat transfer varies according to the ability of the materials or substances to conduct heat. Solids, on the whole, are much better conductors than liquids; liquids conduct heat better than gases or vapors.

Most metals, such as silver, copper, steel, and iron, conduct heat fairly rapidly, whereas other solids such as glass, wood, or other building materials transfer heat at a much slower rate and, therefore, are used as insulators.

Copper is an excellent conductor of heat, as is aluminum. These substances are ordinarily used in the evaporators, condensers, and refrigerant pipes connecting the various components of a refrigerant system although iron is occasionally used with some refrigerants.

Heat transfer by conduction depends upon (1) the driving force, which is caused by a temperature difference Il.T, and (2) the resistance to heat transfer, which depends on the nature and dimensions of the heat transfer medium. There are several ways to relate these parameters. One of the most useful relates the rate of heat transfer Q to the cross-sectional area A, the temperature difference Il. T and a quantity called the heat transfer coefficient U.

Q =UMT

where: Q = rate of heat transfer (Btu/hr) U = heat transfer coefficient (Btu/hr-ft2 _OF) A = cross-sectional area for heat transfer (ft2) Il.T = temperature difference CF)

1-26

The rate of heat transfer Q divided by the cross-sectional area A is commonly referred to as the heat flux. The heat transfer coefficient U is equivalent to the reciprocal of resistance to heat transfer. The temperature difference l>T is the driving force.

where

Heat Flux =

Q = l>T A 1

U

Driving Force Resistance

Q = heat flux (Btu/hr-ft') A

l>T - temperature difference ("F)

U = heat transfer coefficient (Btu/hr-ft' -' F)

The heat transfer coefficient U is a measure of the resistance of the med ium to heat transfer. It depends on both the heat transfer characteristics and the dimensions of the heat transfer medium. The heat transfer characteristics of a material are measured by a property called the thermal conductivity k. The thermal conductivity of liquids and solids depends on temperature. For vapors, it depends also on pressure . Table 1-3 gives the thermal conductivity for zirconium, aluminum and water at several temperatures.

1-27

Table 1-3 Thermal Conductivity of Common Materials

Material Thermal Conductivity Temperature (BtulHr-Ft-OF) (OF)

Zirconium 12.1 120 11.8 200 11.5 300 11.0 500 11.6 750

Aluminum 132 68 131 390 131 750

Water 0.343 32 0.393 200 0.4 300 0.356 600

The heat transfer coefficient U depends also on the dimensions of the heat transfer medium. For the simplest case of steady-state heat transfer by conduction through a slab, the temperature profile is linear and the heat transfer coefficient U equals the thermal conductivity k divided by the thickness of the slab x. Thus, the basic relationship for heat transfer by conduction through a slab can be written as follows:

Q = kA t.T x

where: Q = rate of heat transfer (Btu/hr) k = thermal conductivity (Btu/hr-ft-OF) A = cross-sectional area for heat transfer (ft') x = thickness of s lab (ft) t.T = temperature difference COF)

1-28

Metals with a high conductiv ity are used in the refr igeration system itself because it is des irab le that rapid transfer of heat occur in both evaporator and condenser. The evaporator is where heat is removed fro m the conditioned space or substance or from air that has been in d irect contact with the substance; the condenser dissipates this heat to another medium or space.

Convection

In convec ti on, heat is transferred by motion of the heated material itse lf and is limited to liquid or gas. When a material is heated, convection currents are set up within it. The warmer portions rise, s ince heat brings about the decrease of a fluid 's density and an increase in its specific volume.

Figure 1-10 shows a generalized diagram of heat transfer by convection. It involves the transfer of heat between a surface at temperature T, and a fl ui d at temperature T" referred to as the bulk temperature of the fluid. The exact definit ion of the temperature of the fluid T, is the temperature fa r fro m the surface. For boiling or condensation, T, is the saturati on temperature.

'!-<;..-- SURFACE

t FLOW

Figure 1-10 Heal Transfer by Convection

1-29

The basic relationship for heat transfer by convection has the same form as that for heat transfer by conduction.

where Q h A t.T

= = = =

Q = hN.T

rate of heat transfer (Btu/hr) heat transfer coefficient (Btu/hr-ft2_0F) cross-sectional area for heat transfer (ft') temperature difference ("F)

The heat transfer coefficient h, more precisely referred to as the convective heat transfer coefficient, has been measured and tabulated for the commonly encountered situations for heat transfer by convection. Table 1-4 shows representative values of the convection heat transfer coefficient h.

Table 1-4 Representative Values of the convective Heat Transfer Coefficient

Operation Heat Transfer Coefficient (Btu/hr-ft2

-OF)

Drop-wise condensation of Steam 5000 - 20,000 Film condensation . 1000 - 3000 Boiling of water 300 - 9000 Heating of water 50 - 3000 Superheating of steam 5 - 20

The temperature difference t. T in heat transfer by convection is the difference between the temperature of the surface T, and the bulk temperature of the fluid Tb .

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Ai r in a refri ge rator and wa ter be in g hea ted in a pan are exa mples of the resu lts of co nvection currents (see F igu re 1- 11). The air in contact wi th the cool ing coi l of a refri gerator becomes coo l and more dense and beg ins to fa ll to the bottom of the refri gerator. [n doing so, it absorbs hea t from the food and the wa lls of the refri gerator, which through conduction, has picked up heat fro m the room .

............ ............ ... ........ .... ............ .... -.......... . .................. ...... .. -........... l~~===~;;;;;;;;;l ..... .................... , .. . .............. .... . ................ ... .. ... ........... ... ..... ................. . ..... . -.............. . ....................... ............... .....

:.":::: -::.":,"::,"::",";:,": , ....... " ..

....................... ....................... ............... ........ .......... ...... ................. · ... ................. . ...... .. .. ......... .... ..... .... .............. ....................... ....................... ........... .... ... :: :;:: .:;:::;: ::: ;;: . ....................... ';.' ............. ,':::. '::::: .. :: . . ':::: :::;:;;; .. : .. ;:;;".' . . :.;.:.;.;.;.:.;.;.;.:.:.:.:.:.:.:.:.;.:.;.:.: ....................... ....................... ...... .. ............... .... ........ .. ..... .... ............. ... ...... .. ...................... . ....................... ...... ..... ........... ................ ...... ... ': .... : ............... ':. ,.: .. :: .... . ............•...... .-.. ; ...... : .... ;;: .. . .......... ..... ................. ..................... ................. . .. .......... .. ........ ...... .. . ..... ....................... ......................

....................... ....................... ....................... ....................... ::;:::::::::: .. .. ...................... ..................... ....................... ....................... ....................... · ..... ... ........ .. .. . ....................... ........... ........ ... ... ........ ...... ..... . ....................... .... ............. .. .. . ............... ................... ... · ..................... Ll:::::::::::::::::::::::::::::::::::::::::::::::::::::J ,/:::::::::::::,:i ................. . .... ... .................. .............................. . ... ..... .......... ........................... . .......... , ... . ...... .............. . ............. .............................. - .............. , ........... ...... .. .... . . ......... ........... .. ... . ............... . .. ............... ................. . ................. ... ............................ ...... ...... . .... ........... ........ ..... ............. ............. . ............... ..... .".':: ..... :::::: .. .";; ........... . . ..... ... .......... . ...................... . .

. . . . . .. . ............................ ::. ':. ''-:. '. ''-: ..... '.'.-::: ...... :. ': .. : . . .......................................................... . ................................................. . :: .... :::. ' .. :::. ' .. :: .... ::. ':. ' .. :. '. '. ;'. '. '. '. , ': .. :: ...... ::. ; '. '.-. '. ''':.':. '. ':::.; '::.

Figure 1-11 Convection Cu rrents

1-3 1

After heat has been absorbed by the air, it expands, becoming lighter, and rises until it again reaches the cooling coil where heat is removed from it. The convection cycle repeats as long as there is a temperature difference between the air and the coil. [n commercial-type units, baffles may be constructed within the box so that the convection currents will be directed to take the desired patterns of air flow around the coil.

[n the case of the evaporator, the product or a ir is at a higher temperature than the refrigerant in the tubing and there is a transfer of heat downhill. In the condenser, the refrigerant vapor is at a higher temperature than the cooling medium traveling through or around the condenser, and here again there is a downhill transfer of heat.

Plain tubing, whether copper, aluminum, or another metal, transfers heat according to its conductivity or "k" factor, but this heat transfer can be increased through the addition of fins on the tubing. They increase the area of heat transfer surface, thereby increasing the overall efficiency of the system. [f the addition of fins doubles the surface area, it can be shown that the overall heat transfer should itself be doubled, when compared to that of plain tubing.

Water heated in a pan is affected by the convection currents set up in it through the application of heat. The water nearest the heat source becomes warmer and expands. As it becomes lighter, it rises and is replaced by the other water which is cooler and more dense. This process continues until all of the water is at the same temperature.

Convection currents are natural, and, as in the case of the refrigerator, a natural flow is a slow flow. [n some cases, convection must be increased through the use of fans or blowers. [n the case of liquids, pumps are used for forced circulation to transfer heat from one place to another.

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Radiation

A third means of heat transfer is through radiation by waves similar to light or sound waves. The sun 's rays heat the earth by means of radiant heat waves which travel in a straight path without heating the intervening matter of air. The heat from a light bulb or from a hot stove is radiant in nature and is felt by those near them, although the air between the source and the object, which the rays pass through, is not heated.

If you have been relaxing in the shade of a building or a tree on a hot sunny day and move into direct sunlight, the direct impact of the heat waves will hit like a sledge hammer even though the air temperature in the shade is approximately the same as the sunlight.

At low temperatures, there is only a small amount of radiation and only minor temperature differences are noticed, so radiation has very little effect in the actual process of refrigeration itself. But the results of radiation from direct solar rays can cause an increased refrigeration load in a building in the path of these rays.

Radiant heat is readily absorbed by dark or dull materials or substances, while light colored surfaces or materials reflect radiant heat waves, just as they do light rays. Wearing apparel designers and manufacturers make use of this by supplying light-colored materials for summer clothes.

This principle is also carried over into the summer air-conditioning field where, with light colored roofs and walls, less of the solar heat will penetrate into the conditioned space, reducing the size of the overall cooling equipment required. Radiant heat also readily penetrates clear glass in windows, but will be absorbed by translucent or opaque glass.

When radiant heat or energy is absorbed by a material or substance, it is converted into sensible heat - that which can be felt or measured. Every body or substance absorbs radiant energy to some extent, depending upon the temperature difference between the specific body or substance and other

1-33

substances. Every substance will radiate energy as long as its temperature is above absolute zero and another substance within its proximity is at a lower temperature.

If an automobile has been left out in the hot sun with the windows closed for a long period of time, the temperature inside the car will be much greater than the ambient air temperature surrounding it. This demonstrates that radiant energy absorbed by the materials of which the car is constructed is converted to measurable sensible heat.

Insulation

In the section on heat transfer by conduction, it was pointed out that certain substances are excellent conductors of heat, while others are poor conductors. The poor conductors are classified as insulators. Any mater ial that deters or helps to prevent the transfer of heat by any means is called and may be used as insulation. Of course, no material will completely stop the flow of heat. If there were such a substance, it would be very easy to cool a given space down to a desired temperature and keep it there.

Such substances as cork, glass fibers, wool, and polyurethane foams are good examples of insulating materials; but numerous other substances are used in insulating refrigerated spaces or buildings. The compressible materials, such as fibrous substances, offer better insulation if installed loosely packed or in blanket or batt form than if they are compressed .or tightly packed.

The thermal conductivity of materials, the temperature to be maintained in the refrigerated space, the ambient temperature surrounding the enclosed space, permiss ible wall thicknesses of insulating materials, and the cost of the various types of insulation are all points to consider in se lecting the proper materials for a given project. Most service personnel are not involved in the select ion or the installation of insulating material in a refrigeration application, but they may come in contact with different types of insulation, and under var ious conditions.

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Insulation should be fire and moisture resistant, and also vermin proof. Large refrigeration boxes or walk-in types of coolers are usually insulated with a rigid-type of insulation such as corkboard, fiber glass, foam blocks, and the like, while smaller boxes or receptacles might be filled or insulated with a foam that flows like a liquid and expands to fill up the available cavity with foam.

Low temperature boxes require an insulation that is vapor-resistant, such as unicellular foam, if the walls of the refrigerated enclosure are not made of metal on the outside. This foam ensures that water vapor will not readily penetrate through into the insulation and condense there, reducing the insulating efficiency. The most common unit for evaluating insulation materials is thermal resistance (R) or resistance to heat flow. Basically thermal resistance "R" is the inverse of thermal conductance "k". R = 11k. The units for "R" are (hr)x(ft") x ("F).

BTU x in

Psychrometric PROPERTIES OF AIR

Psychrometry is the science and practice of dealing with air mixtures and their control. The science deals mainly with dry air and water vapor mixtures.

Psychrometry deals with the specific heat of dry air and its volume. It also deals with the heat of water, heat of vaporization or condensation, al)d the specific heat of steam in reference to moisture mixed with dry air.

Tables and graphs have been developed to show the pressure, temperature, heat content (enthalpy), and volume of air and its steam content. The tables and charts are based on one pound of dry air, plus the water vapor to produce the air conditions being studied.

1-35

A standard pressure of 29.92 m. Hg. abs. IS used as the standard atmospheric pressure.

Psychrometric Chart

The psychrometric chart in Figure 1-12 is probably the best way of showing what happens to air and water vapor as these properties are changed. The chart is published by ASHRAE and is one most commonly used in the industry. Some manufacturers have developed their own charts which vary only in style and construction but the relationship of the air properties are all the same.

.. •

/ .I

" ' I "" ';' , "',' "I " . ~ " r '." ... , .. . , .... ,... .. [

." i d ,t "

~ .. " '" f.1. ~

U" : . " , .-. . ~

~:m;m : I !t .. ; " ." I , ,,~ . .. , I '·Yif1HmH+lw,l-< '~o8'i"llnl" . I.. ' .• , ] : IT ' ~ I f ~

J ~ ' ''' '" ~ ffi'''ffi' lfll l'' ' I I. i1!, ~ ~

II ! '" R • .. : .. ~ "' .

: '" 8'~

. ! ; Ii" . , . • ~ '" ; to

, '"

I" l T" , I, '"

j , 'I'

<:r:rTT ' ..... ,..,..., . , U ) .1 1="""",,, u:u:. ;U:IJ: 1 ::t ,..~. t:1 .L1 _iQWL 1=0.1. C.I::J .• C1.'XO"""""," C"':;:LL ... O ..... C • • _ . I~. U .J40 I. • i • . .. .L) .••. . . , " ,." .. .. ~ ... - , ~ ... ~ .... . __ . -.-. . _-_ ..... __ .•. "'--' -' -" '-

Figure 1-12 Psychrometric Chart

1-36

To make this chart, all we do is start with the ordinary temperature scale called the dry bulb temperature. Just extend the thermometer scale as shown in Figure 1-13. Note on the actual chart that these lines are not truly perpendicular. This is done so that other lines will come out straight instead of curved.

'" ~ ::J

'" >-a: 0 "-, 8 -

.

C:==~=c=i?==2c0=i40==Wc=WI==I00CI ='~20==~

Figure 1-13 Thermometer Scale

Next the horizontal scale is set up according to the amount of water vapor mixed with each pound of dry air. This scale (Figure 1-14), called the humidity ratio, is expressed in pounds of moisture per pound of dry air. More recent versions of the psychrometric chart express the humidity ratio in units of grains of moisture per pound of dry air. To convert pounds of moisture to grains of moisture simply multiply the reading in pounds by a conversion factor of 7000. We know that air can hold different amounts of moisture depending on its temperature; if it is holding all the moisture it can (100%), it is termed saturated.

1-37

(

-------------j 0.030

-------------40.015

- ____________ ...-....J 0.002

Figure 1-14 Humidity Ratio Scale

o z :::> o

-0.. ;:0:

W 00..0: - ->-W« «0:)CI:::J0: )->-0 >-'C' -o~

~:20 :2V1 :::>0 IZ

:::> 2

From the ASHRAE Guide and Data Book we can find out exactly how much moisture air can hold at saturated conditions. Following is a simple table taken from this reference book:

SAlURATEO HUMIDITY RA 110 HUMIDITY RA 110 TEMPERA lURE Lb/lb OF Gr/lb OF

t DB DRY AIR DRY AIR

70' 0.01582 110.74 7Z' 0.01697 118.79 75' 0.01882 131.74 78' 0.02086 146.02 80' 0.02233 156.31 82' 0.02389 167.23 85' 0.02642 184.94

Returning to the psychrometric chart construction. we can now plot saturation points (Figure 1-15) for each condition of dry bulb temperature, and when these are connected they form a curve or saturation line.

1-38

~ - -- - - - - --- 0.02642 0

>--- - --- -- - 0.01882 ~

>---------- 0.01582 ~

a ~ ::> r

70 75 85

DRY BULB of

Figure 1·15 Saturation Points

Assume an air sample (point A, Figure 1-16) with a dry bulb temperature of 80°F, holding 77.0 gr of moisture. If we were to heat the air without adding moisture, the point would move to the right on the horizontal line, showing an increasing dry bulb temperature but an unchanging moisture content.

HUMIQ IF Y , ' A

COO l --', '-- HE AT 17.0

DEHUMIDIFY

81) FD8

Figure 1·16 Air Sample

1-39

If we were to add moisture (humidity) without changing the dry bulb temperature, the point would move vertically up. If the moisture were reduced (dehumidifying), it would move vertically down. If temperature and moisture were added, the point would move up and to the right, and if the air were cooled (without changing its moisture content), the point would move horizontally to the left.

Continuing the example, if the air sample is cooled, it eventually reaches the saturation line (Point B, Figure 1-17) where it cannot hold any more water vapor, and on further cooling some water would start to condense. That temperature is just below 60"F, or about 59.7"F. This is known as the dew point temperature of the sample. It can be read from the vertical dry bulb index temperature. In summary, at point B, we have a 59.7"F dry bulb temperature, a 59.7"F dew point temperature, and a moisture content of 77.0 gr of moisture per lb of dry air.

DEW POINT TEMP.

, B COOLED A

59.7 so ~F DB

Figure 1-17 Saturation Line

Now if the sample is further cooled, for example to 50"F dry bulb, moisture will condense out and following along the saturation line to point C (Figure 1-18), where it will have a dew point of 50"F and a humidity ratio of only 53.2. Thus, the sample has lost 23.8 gr of moisture. It has been cooled and dehumidified.

1-40

50" 59.7· 80"

~F DB

77,0

53 ,2

Figure 1-18 Saturation Line

A practical example of this process is a cold supply air duct (Figure 1-19) running through a moist unconditioned area. Will the duct sweat and need to be insulated? Assume the air temperature inside the duct is 55°F and the unconditioned air surrounding the duct is at 95°F with 99.4 gr of moisture content. This condition means that the outside air would have a saturated (dew point) temperature of 67°F. Thus, as the 55°F duct temperature cools the air touching its surface to below the 6rF dew point, condensation will likely occur. Depending on conditions, it will be necessary to take some corrective action using appropriate insulation to prevent sweating.

DUCT TE. MP ~SF

• • ••• • • • CONOENSATION

WILL OCCUR

UNCONQITIONE O ,-- AIR AROUNQ

DUCT AT 9S·'F ANO 99,4 '.

z a

~-.... -.- 99.4 ........ <" , ~ 0

I z ~ I • • _ __ ..... - - - _ - _ 64.4 ~ a:

a u

9S'F

Figure 1-19 Cold Supply Air Duct

1-41

The next element in our chart is the construction of relative humidity lines for partly saturated conditions (Figure 1-20). We know the relative humidity is 100% at the saturation line. Lines for 80%, 60%, 40%, etc., can be plotted, since we know specific moisture contents in relation to temperatures. As an example, one pound of air at 75°F dry bulb will hold 131.74 gr of moisture (point A) at saturation (100% relative humidity). Point B (50% relative humidity) can be located at approximately 65.87 gr moisture (1/2 of 131.74 gr). The same method can be used for each dry-bulb temperature, and eventually a connecting line is drawn that represents a 50% relative humidity for any chosen condition of a dry-bulb temperature. Similar lines can be drawn for different relative humidity conditions. We already know how useful it is to be able to express relative humidity, since it affects human comfort.

7S' F DB

Figure 1,20 Relative Humidity Lines

Unfortunately, it's not practical or convenient to measure the amount of moisture content or dew point of the air except under laboratory conditions, so we need to plot another element that will give us an easier method. It has been noted that the wet bulb temperature also reflects the

1-42

amount of moisture in the air. The rate of evaporation on the sling psychrometer determined the wet bulb depression below the dry bulb temperature or the wet bulb temperature and from Table 1-5, we can determine the relative humidity.

For example, Table 1-5 showed that for an 800 dry bulb temperature and an 110 wet bulb depression (69"F actual measured WB), the relative humidity is 57%. Transferring this information to our psychrometric chart, we can plot point A (Figure 1-21).

Table 1-5 Psychrometric table: Percent Relative Humidity from Dry Bulb Temperature and Wet Bulb Depression

.... , D',,,,, .. ~ D~ " ... ,. , , , , , • , • , 10 II 11 !J " J} t'l7 II " 20 II ll~lHlJl6 " lS 19 M

" 90 19 ,9 60 ro " " l! 11 , " 91 II 1) 6J 16 (I " " " " • " " .. 76 U " " " " " " "

, , ~ " " 11 11 6-11111 " 11 11 " .. " , " " " aD 7l 61 60 H '. ., " " " " " • " ,. .. " " 69 61 sa " "" " " " " " " • 0

" " .. It n 11 66 6 1 " 10 H <0 " " " " " " I • " 94 19 84 11 n u 6) " " .. ~ <0 " " " " " " • , .. 91 90 II 79 7S 70 66 " " n 48 .,

" " 14 11 " " " " , .. 9J90111i 76 12 " " " lJ JI H 4J " l! II " " " " " " ~J91a6nJ11l 69 6J " " " .9 4' ., J1 H " " " " " " 96 91 "U7~7. 70 67 " " " !Z H ., ' Z II " " " " " !O 96 91 a7 I) 79 16 " .... " " " " n •.• 4' " " " " " " " .. 16 J) " • • .. ?~ 91 a& ' 4 ao 11 11 )0 66 " " " " \o.J,u. , " " " " " " " " " " " " .. 9691U"" " H 7' " " " " " ll49464141 " " " JO " " " " " " '4 I!

" 96 91 U IS II 11 H 12 69 " " " " " It .1 ., '1 <0 " " " JO " " " " " " " .. '6 9l " .. U7916l)70 U ,. " " " H ro 47 ., " <0 " " " " " " " " " " "" .. " 00 .. I) ao 71 " " .. " " " " " " .. " ~ " <0 " " " " " " IJ lJ 11

'" 91 9J 00 " It 10 J7 H " .. .. " " " " H 11 .. " ~ " " " " " " " " " " ". 91 91 00 " " " " " " "u " " " " " " ro " . , 4) 4' n J1 " lJ )1 " " "

1-43

75 69° WB

60 -( 50

-' / .

.' , , -, , ,

" '-{ 50 60 70 76 80

of DB

Figure 1-21 Constant Wet Bulb

If we were to cool the dry bulb temperature to 76° and the wet bulb temperature actually stayed at 69" on the sling psychrometer, we now have a WB (wet bulb) depression of only 7"F, and, from Table 1-5, a relative humidity of 70%. Point B can now be located. By connecting points A and B, we create a constant wet bulb line. This process could be repeated over and over until a complete grid of wet bulb lines fill the chart. Wet bulb temperature is read at saturation temperature line, because at that point it can hold no more moisture and becomes the same as the dry bulb and dew point temperatures.

This completes the construction of the simplified psychrometric chart (Figure 1-22). Although it is not 100% accurate, this description should help you understand the relationship of the lines on the real chart. Fortunately, precise and accurate information has gone into the construction of the ASHRAE chart, and it may be used with confidence. Remember, if any two of the five properties of air are known, the other three can be found on the psychrometric chart by locating the point of intersection of the lines representing the two known conditions.

1-44

SUMMARY

en >--' CI::;) Oen

HUMIDITY

Figure 1-22 Simplified Psychrometric Chart

In this chapter, we learned that the primary objective of HV AC is to control the characteristics of air in a controlled environment. In understanding those characteristics, we learned about the laws that govern those characteristics.

We looked at the Basic Air Laws which tells us about how fans perform under varying conditions and how that effects the flow of air through ductwork. We next discussed the Pulley Laws and their use in determining air flow and power consumption. After learning about the Pulley Laws, we went in to the discussion of the Gas Laws: Charles, Boyles, Gay-Lussac and how that are put together to come up with the perfect gas law which is used to understand how pressure, temperature and volume effect a gas . We briefly discussed Pascal's Principle, which explains how a liquid acts under pressure.

1-45

· .

We learned the first and second law of thermodynamics and how they relate to heat transfer. The three methods of heat transfer are conduction, convection and radiation. The last topic discussed was the psychrometric properties of air and how those properties are used to make the psychrometric chart, which we use for determining relative humidity and dewpoint. This is used to give you the necessary background to understand the following chapters.

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CHAPTER TWO HV AC SYSTEMS

CHAPTER TWO HV AC SYSTEMS

OBJECTIVES

Upon completion of this chapter, the student will be able to:

I. State the three main environmental characteristics that are controlled in an HV AC system.

2. List the ways a hydronic system may be classified.

3. State the purpose of filters in an HVAC system.

4. Describe the various filter types.

CHAPTER TWO : HVAC SYSTEMS

I'

i INTRODUCTION

r ,

There are many different types of HV AC systems. The purpose of this chapter is to introduce you, the student, to these systems. We will be studying the overall construction and use of these systems. In the following chapter, we will study how each component is sued in the system as a whole. We will be discussing the following topics:

Purpose of HV AC Air Systems Filtration Systems Hydronic Systems

PURPOSE OF HVAC

The purpose of HV AC is the control of an enclosed environment. The three main environmental characteristics that are controlled are:

• Temperature • Humidity • Suspended particulates (dust and gas)

{ . Temperature

Controlling the temperature of an environment involves the transfer of heat from one area to another. Increasing or decreasing the temperature of the environment can be accomplished by any of several methods. We will talk about some of these methods later in this chapter.

Humidity

The amount of water vapor contained in air is measured by relative humidity. Relative humidity is a ratio of how much water vapor is in the air to how much the air can hold at a specific temperature and it is expressed as a percentage. The lower the relative humidity, the more tendency the air has to

2-1

draw water from existing sources. When cold air is heated, its ability to hold additional water is increased. For this reason when providing a HV AC system for personnel, a humidifier is typically provided in conjunction with heaters to control the humidity in the designed range.

Suspended Particulates (Dust and Gases)

The final characteristic which is generally included in HV AC system design consideration is the cleaning of the air. This is accomplished through ventilation and filtra tion. Dust, gases and odors are unsatisfactory elements in environmental air, and as such they must be controlled. We will talk about some of the methods to control suspended particulates later in this chapter.

AIR SYSTEMS

There are many different types of air systems used to deliver the conditioned air to the areas or spaces requiring it. The single and dual duct systems are the two basic types of air duct systems that are used for distribution of conditioned air. The single duct system supplies air to each area at a constant ·temperature. Temperature control is obtained by adjusting the volume of the supply air furnished. The dual duct system provides warm and cool air in separate ducts . Individual room or area temperature control is obtained by adjusting the amount or ratio of warm and cool air being mixed and introduced to the environment.

The advantages of each system must be considered when selecting basic design. The single duct system is obviously less expensive to install. However, an evaluation of the operating costs should be made.

Efficiency of operation in the dual duct system often provides a cheaper overall cost. The systems we will discuss are:

• Single Zone Variable Air Volume Terminal Reheat Induction System Dual Duct

• Low • High • Multizone

2-2

, "

Single Zone System

The simplest form of a single zone system is a single conditioner serving a single temperature controlled zone. A single zone system responds to only one set of space conditions. Its use is limited to situations where variations occur almost uniformly throughout the zone served or where the load is stable. A single zone system would be applied to small department stores, small shops in a shopping center, individual classrooms of a small school, computer rooms, etc. Figure 2-1 shows a schematic of a single zone central unit.

POSSIBLE ADDIT ION QFVAVBOX ~ ( TYP ICAL EACH BRANCHI I

o A

UTOOOR IA INTAKE

• •

OUTDOOR AIR DAM PER j

-

POSSIBL E r SUPPLY FAN rPREoHEAT COIL

, , " . rz . , ."

~ .. ..

~ e.

\... FIL TERS

t "-RETURN AIR DAMPER

r- - '"" -1 , I

HEATING ,COIL

..h-

,./

COOLING COIL !

-

/ SUPPLY

~",,-~IA TE RMINAL

SPA CE LOAD

. , " , '

, -

u ....... \ , - -

EXHAUST LOUVERS \ texHAUST DAMPER 1- ~ RETURN AIR REG ISTER L POSSIBLE RETUAN AIR FAN

Figure 2-1 Single Zone System

2-3

Variable Air Volume System

Control of dry-bulb temperature within a space requires that a balance be established between the space load and the air supplied to offset the load. To maintain the balance you can choose between varying the supply air temperature or varying the volume as the space load changes. Variable air volume systems may be applied to interior or perimeter zones witb common or separate fans systems, common or separate air temperature control and with or without auxiliary heating devices . It is possible to vary zone air volume only, while keeping fan and system volume constant by dumping excess air into a return air ceiling plenum or directly into the return air duct system. Figure 2-2 shows a schematic of a variable volume system.

ouraOOA AlA !NlAKE 7

(

CooUNG SUPPl Y

SUPPLY

FAN I HEATING COIL

- i==!=ll=;;=~={=

OUTDOOR AIR DAMPEA

EXHAUST

DAMPER 1 EXHAUS T LOUVEA 7

\ \ .... FIL TEAS

"-- RE TURN AIR DAMPER

POSSIBLE RETURN l AIR FAN

rJ. , , ,

. - -

T

COOLING COIL

[PAI~AAY AIR DUC T

\. BYPASS SOXES

T

RETURN AIR REGISTER

Figure 2-2 Variable Volume System

2-4

Terminal Reheat System

The reheat system is a modification of the single zone system. Conditioned air is supplied from a central unit at a fixed cold air temperature designed to offset the maximum cooling load in the space. The control thermostat simply calls for heat as the cooling load in the space drops below maximum. This system is generally applied to hospitals, laboratories or spaces where wide load variations are expected. Figure 2-3 shows a schematic of a terminal reheat system.

OU TDOOR AIR INTAKEj

POSSIBLE [PREHEAT COIL r COOLING COil

SUPPLY fAN

;i;1 _ 1==+i#=nr===&I=={== B:: AEHEATCQll· 1

REHEAT COil 2-. OU TDOOR I AIR DAMPER ...J

EXHAusr\ LOUVERS

" ~FILTERS

~ AETURN AlA DAMPER

~EXHAUST DAMPER

TO ZONE ·1

T

- \ POSSIBLE RETURN/EXHAUST AIR FAN

\ SUPPLY AlA TERMINAL

TO ZONE 2

SPACE LOAD

~AETUANAIR REGISTER

Figure 2-3 Terminal Reheat System

2-5

Induction System

Primary air is discharged from nozzles arranged to induce room air into the induction unit approximately 4 times the volume of the primary air. The induced air is cooled or heated by a secondary water coil. Induction type units are generally located under the window to offset winter downdrafts. Figure 2-4 shows a schematic of an induction system.

OUTDOOR AlA IN TAKE 7

OUTDOOR J AIR DAMPER

EXHAUST t DAMPER

eXHAU ST LouvE AS

\ FILTERS

'- RETURN AIR OAMPER

CooUNG COIL

t SECONDARY 11 WATERCQIL 'l INDUCTION _ ~ . ~ : INOUCED I '==cf~ 1j UNIT : .... AIR -POSSIBLE AETURN

'\ AIR FAN

I ==li=o!h==~ \~~i==~"~~===~ j// F ,.... - - - SPACE LOAD

'-; I " ' ... -."..' '- RETURN AlA

AEGISTER

Figure 2-4 Induction System

2-6

(

Dual Duct System

The dual duct system comes in two types: low velocity and high velocity. We will take a look at each.

Dual Duct System CLaw Velocity)

The dual duct system conditions all the air in a central apparatus and distributes it to conditioned spaces through two parallel mains or ducts. One duct carries cold air and the other duct carries warm air, thus, providing air sources for both heating and cooling at all times. In each conditioned space or zone, a mixing valve controlled by a room thermostat mixes the warm and cold air in proper proportions to satisfy the prevailing heat load of the space. Figure 2-5 shows a schematic of a dual duct system (low velocity).

HEAliNG COIL

POSSIBI.E SUPPLY

ourooon ' . .• PRE .. HEAT COil \" FAN _ ~IR INTAKE 1 \ \ - u:·= ~====81===f=

ouraOOR AIR DAMPER

EXHAUS T

O'MPERI E)OI"U!)T

LOU'IERS 1 :1 r

~ COOLING COIL LFll TEAS

I t \~RETURN "R

DAMPE R

POSSIBLE AE l UAN

(AIR FAN

r--'-, ~-~' ~==f=~=====i= ,

' ., I

T

HQT DuC T -, ... ,

COLD Ducr 1-

-TO ZONE I fa ZONE l

~ S PACE LOAD ~

1E!5F I I

RETURN AlA REGISTERS

Figure 2-5 Dual Duct Low Velocity System

2-7

Dual Duct System !High Velocity)

Dual duct high velocity systems operate in the same manner as the low velocity systems except that the supply fan runs at a higher pressure and each zone requires a mixing box with sound attenuation. a dual duct system (high velocity).

r\ ,,"'v ,("\

::,.6 ' :-". ;:o. .. pF

",If '0

Figure 2-6 shows a schematic of

-{-' "

\ ::/,(cI' J / 1\0 "It'

DO !\

'0.

HEATING CQ1l- ,

OUTDOOR"

AIR INTAKEl POSSl8LE

[PRE. HEAT COtl

SUPPLY \ FAN

\

T

-OUTDOOR AIR DAMPER

;' ,.

"

EXHAUST t DAMPER

"-FILTERS

, T

COO"NGWL-------~l' i I~M II [I I '--Y / tr ~ ~ ~ (>

'-RETURN AIR DAMPER

POSSIBLE RETURN r AIR FAN

~ TO l~NE , TO l~NE 2 ~ "HAUS< 1 I LOUVERS f ~ SPACE LOAD / .

~Ik==k~==~r~-~-~~~.====~~~~====~~==~~ - ,

RE TURN AI R nEGlsrERS

Figure 2-6 Dual Duct High Velocity System

2-8

.' -. ,

f I

Multizone System

The multizone system is applicable for serving a relatively small number of zones from a single central air handling unit. The requirements of the different zones are met by mixing cold and warm air through zone dampers at the central air handler in response to zone thermostats. The mixed conditioned air IS

distributed throughout the building by a system of single-one ducts as shown In

Figure 2-7.

OUTDOOR AIR INTAKE 1

HEATING COIL IHor DECKI

,- - - - - - - - - - - - - _L - - - ~;;ii"'':: I I PI.\!'. I ~ \ . : ~~ i DAMPERS

I 1-\ POSSIBLE : rSUPPlY \ t:j::r~:;:~:~~::: ---", ~PAE HEAT COil i ~ FAN ~ 1 : _ ,

- !2 " / ..J--l==='''~~ TO INDIVIDUAL ..J

D. ~" : ~ ZONES

F===~0~==~~~'~=

OUTDOOA "...J

AlA DAMPEA

2 rs: ~LTERS: -.'

I 9. ,.. I DAMPERS

: ~ COOLINGI COil

T

EXHAUST t DAMPER 7

EXHAUST 7 LOUVERS

~ t ____ __ __ ___ ______ . jCOLDDECKI

~~~~~~ AlA '-MULTI ZONE UNIT

POSSIBLE RETURN f_ AlA FAN

COMMON RETUAN

r

/

\ I

..

Figure 2-7 Multizone System

2-9

FILTRATION SYSTEMS

When the environmental air is to be recirculated, filtration is used to remove undesirable elements. Filtration allows previously conditioned air to be cleaned while maintaining desirable characteristics (temperature and humidity) thus increasing the system's efficiency. '

The placement of filters in HV AC systems obviously produces a differential pressure that system fans must overcome. The magnitude of this differential pressure can drastically reduce air flow and, therefore, system energy efficiency. Several methods of filtration are available for use in environmentally controlled systems. The types of filters we will discuss are:

Fibrous Media Filters Automatic Replacement Air Fitter$ Electronic Air Cleaners '. High Efficiency Air Filters Activated Carbon filters

Fibrous Media Filters '

.! " . '--

, ~ '- . \/, -....::::.. .. "- .-. ~- "

Fibrous media filters (Figure 2-8) are"composed of a coarse fiber material such as fiberglass, metal mesh, or vegetable fibers. The ..• ir to be filtered is forced through the material where du~t and other similar sized particulates become trapped in the matrix. Depending-on the type of fibrous material used, it may be washed or replaced when dust impedes the air flow. As .dust,'s trapped in the filter material, the effectiveness of the filter increases (the passageways for the air get smaller), but the pres~ure drop acrosS'lh~ filtet~lso increases creating a higher demand on the system's blower unit. ",

2-10

Figure 2-8 Typical Fibrous Media Filters

In some circumstances, it is necessary to continuously provide a clean filter. The used filter may be rolled up and discarded or cleaned and reused. Some systems include a mechanism to clean the filter which is arranged ina continuous belt (Figure 2-9). This system maintains a constant pressure drop and cleaning efficiency.

Figure 2-9 Automatic Replacement Air Filter

2-11

Electronic Air Cleaners

By passing air through an electric field (typically 12,000 volts) particulates receive a charge. When the charged particulates are subsequently passed through a matrix of oppositely charged plates, the particles are attracted and collected on the plates. Figure 2-10 diagrams this process. The plates may be removed for cleaning or washed in place periodically. Figure 2-11 shows a typical electronic air cleaner.

~ .. , .

,1- r~'i .·~~-·~~.>~,;~t~·

.'" -."

, .. '

Figure 2-10 Electronic Air Cleaning Process

2-12

~\

\- . I I I I I I I I I IjIL., ~/~ . '. ;L---, ''-''-\-'' \ y---' \ \

I \ \ I \ •

Figure 2-11 Industrial Electronic Air Cleaner

High Efficiency Particulate Air Filter

The high efficiency particulate air (HEPA) filter is the most efficient air cleaning system commercially available. Although it was developed for the nuclear industry, it has been found to be extremely useful in the medical and electrical fields.

HEPA filters provide a minimum efficiency of 99 .97 percent on 0.3 micron particulates . (A micron is one-millionth of a meter.) The filter media is typically a fibrous material with a high surface area to volume ratio. Design velocities are held down to about 5 feet per minute. This increases the particulate holding characteristics of the filter. In the fab rication and installation process of HEPA filters care must be taken to insure that all air that passes through the unit goes through the filter material. No cracks or voids may exist which allow unwanted particulates to avoid filtration.

HEPA filters are actually a specialized fibrous material filter. Figure 2-12 shows a box HEPA filter (HEPA filter bank.)

2-13

Figure 2-12 Box HEPA Filter

2-14

Activated Carbon Filters

Activated carbon filters are commonly used to remove gases and vapors from recirculated air. The process involVed is adsorption where the carbon adsorbs the paniculate in a Sponge-like process. Figure 2-13 shows a typical activated carbon filter.

Figure 2-13 Activated Carbon Filter

2-15

HYDRONIC SYSTEMS

A hydronic or all-water system is one in which hot or chilled water is used to convey heat to or from a conditioned space or process through piping connecting a boiler, water heater or chiller with suitable terminal heat transfer units located at the space or process. All water systems may be classified by temperature, generation of flow, pressurization, piping arrangement and pumping arrangement.

In terms of flow generation, hot water heating systems are of two types (I) the gravity system, in which circulation of the water is due to the difference in weight between the supply and the return water columns of any circuit or system; and (2) the forced system in which a pump, usually driven by an electric motor, maintains the necessary flow. Water systems can be either once-through or recirculating systems .

Examples of how water systems are classified according to temperature are discussed below.

Low Water Temperature System (LTW)

A hot water heating system operating within the pressure and temperature limits of the ASME boiler construction code for low pressure heating boilers. The maximum allowable working pressure for low pressure heating boilers is 160 psi with a maximum temperature limitation of 250"F. The usual maximum working pressure for boilers for L TW systems is 30 psi, although boilers specifically designed, tested and stamped for higher pressures may frequently be used with working pressures to 160 psi. Steam-to-water or water-to-water heat exchangers are also often used.

Medium Temperature Water System (MTW)

A hot water heating system operating at temperatures of 350"F or less, with pressures not exceeding 150 psi. The usual design supply temperature is approximately 250' to 325' F, with a usual pressure rating for boilers and equipment of 150 psi.

2-16

High Temperature Water System (JITW)

A hot water heating system operating at temperatures over 350"F and usual pressures of about 300 psi. The maximum design supply water temperature is 400" to 450' F, with a pressure rating for boilers and equipment of about 300 psi. It is necessary that the pressure-temperature rating of each component be checked against the design characteristics of the particular system.

Chilled Water System (CW)

A chilled water-cooling system operating with a usual design supply water temperature of 40' to 55'F and normally operating within a pressure range of 125 psi. Antifreeze or brine solutions may be used for systems (usually process applications) which require temperatures below 4O"F. Well water systems may use supply temperatures of 60"F or higher.

Dual-Temperature Water System (DTW)

A combination hot water heating and chilled water cooling system which circulates hot andlor chilled water to provide heating or cooling using common piping and terminal heat transfer apparatus. They are operated within the pressure and temperature limits. of L TW systems, with usual winter design supply of water temperatures about 100' F to 150"F and summer supply water temperatures 40"F to 55' F.

Generally, the most economical distribution system layout has mains that are run by the shortest and most convenient route to the terminal equipment having the largest flow rate requirements and branch or secondary circuits are then connected to these mains.

Water distribution mains are most frequently located in corridor ceilings, above hung ceilings, wall-hung along a perimeter wall, or in pipe trenches, crawl spaces or basements. Water system piping need not be run at a defutite level or pitch, but may change up or down as required by architectural or structural needs. Water system piping may be divided into two arbitrary classifications:

• Pipe circuits suitable for complete small systems or for terminal or branch circuits on large systems.

2-17

Series Loop One-pipe Two-pipe reversed-return Two-pipe direct-return

• Main distribution piping used to convey water to and from the terminal units or circuits in a large system.

Two-pipe direct-return Two-pipe reversed-return Three-pipe Four-pipe

We will discuss each type of system in the remainder of this chapter. Also because of the unique nature of hydronic systems, we will also discuss hydronic plpmg.

Series Loop System

A series loop is a continuous run of pipe or tube from supply connection to return connection. Terminal units are a part of the loop.

Figure 2-14 shows a system of two series loops on a supply alld return main (split series loop). One or many series loops may be used in a complete system. Loops may connect to mains or all loops may run directly to and from the boilers.

Water temperature drops progressively as each radiator transfers heat to the air, the amount of drop depending on radiator output and water flow rate. The true system operating water temperature and flow rate must be known to calculate the average water temperature (Awl) for each unit on the loop. If all terminal units are in series on one loop in one zone of interconnecting air space, the entire set of units can be sized at the A WT of the loop. One floor of a small dwelling with open interior doorways is such an interconnecting space. If individual units on a loop are in separate enclosed spaces, each unit must be sized to actual A@ for that unit.

2-18

Pump

-Soiler

Adjusting Cock

~ 1

Figure 2-14 A Series Loop System

A decrease in loop water flow rate increases temperature drop in each unit and in the entire loop. Average water temperature shifts downward progressively from first to last radiator in series. Unit output gradually lowers from first to last on the loop. Consequently, comfort cannot be maintained in separate spaces heated with a single series loop if water flow rate is varied. Control of output from individual terminal units on a series loop is impractical except by control of heated air flow. Manual dampers can be used on natural convection units; automatic fan or face-and-bypass damper control can be used on forced air units.

One-Pipe System (Diverting Fitting)

One-pipe circuits use a single loop main (see Figure 2-15). For each terminal unit, a supply and a return tee are installed on the same main. One of the two tees is a special diverting tee which creates a pressure drop in main flow to divert a portion of main flow to the unit. One (return) diverting tee is usually sufficient for up feed (units above main) systems. Two special fittings (supply and return tees) are usually required for down feed units to overcome thermal head. Special tees are proprietary; consult manufacturer's literature for flow rates and pressure drop data.

2-19

( Ii ( 1

One Special Return Fitting I'- - (Upfeed)

l i Boiler

i'-~ ~ ---.I

,~ l;) Pump r-

: _J Downfeed (Two Special Fittings)

Figure 2-15 A One-Pipe System

One-pipe circuits allow manual or automatic control of flow to individual connected heating units. On-off rather than flow modulation control is advisable because of the relatively low pressure and flow diverted. Length and load imposed on a one-pipe circuit are usually small because of the limitations listed.

Two-Pipe Systems

Two-pipe circuits may be direct-return (return main flow direction is opposite supply main flow; return water from each unit takes the shortest path back to the boiler) as shown in Figure 2-16, or reverse-return (return main flow is in the same direction as supply flow; after the last unit is fed, the return main returns all water to the boiler) as shown in Figure 2-17. The direct-return system is popular because less main pipe length is required; however, circuit balancing valves usually are required on units or sub-circuits. Since water flow distance to and from the boiler is virtually the same through any unit on a reverse-return

2-20

, .

system, balancing valves are seldom adjusted. Operating (pumping) cost is likely to be higher with direct return because of the added balancing fitting pressure drops at the same flow rate.

.. z

... y

.. X

t Terminal/ Unrts

Pump Boiler , ! or I

'- / Chiller

Figure 2-16 Direct-Return Two-Pipe System

... T

... S

... R

-J Terminal ,/ Units

Pump Boile, or

'-_.I Chiller

Figure 2-17 Reverse-Return Two-Pipe System

2-21

Combination Piping System

The four basic arrangements exist only to describe function; one type can grade into another; a piping system can contain from one to all four types and, thus, cannot be described as a particular type. Figure 2-18 illustrates a primary circuit and two secondary pumping circuits. As pipe lengths and number of units vary and as circuit types are combined, basic names for piping circuits become meaningless; flow, temperature and head must be determined for each circuit and for the complete system.

Control

i rerminal/ t t

Unit J·Way control

\ Valve lor secondary crfC1Jlt

t Common

Secondary~ Flow Common Flow

~ Pump Secondary .. Pump ... B C Balance Cock 0 E

Boiler 0' F ... Chiller

A ~Primllry ::>ump

Figure 2-18 Example of Primary and Secondary Pumping Circuits

Three-Pipe System

The three-pipe system satisfies variations in load by providing independent sources of heating and cooling to the room unit in the form of constant temperature primary and secondary chilled and hot water.

2-22

The unit contains a single secondary water coil. A three-way valve at the inlet of the coil admits the water from either the hot or cold water supply, as required. ne water leaving the coil is carried in a common pipe to either the secondary cooling or heating equipment. The usual room control for three-pipe systems is a special three-way modulating valve which modulates either the hot or cold water in sequence, but does not mix the streams. The primary air is cold and at the same temperatures year-round.

During the period between seasons, if both hot and cold secondary water is available, any unit can be operated within a wide capacity range from maximum cooling to maximum heating within the limits set by the temperature of the secondary chilled or hot water. Any unit in the system can be operated through its full range of capacity without regard to the operation of any other unit in the system, recognizing the operating cost penalty that will result from simultaneous heating and cooling loads. All units are selected on the basis of their peak capacity requirements.

The return mix three-pipe room unit is provided with a singl,e coil which receives either hot or cold water. A modulating three-way valve at the inlet to the unit admits either hot water or cold water to the secondary coil (see Figure 2-19). The three-way valves are a special design in which the hot port gradually moves from open to fully closed and the cold port gradually moves from fully closed to open. The valves are constructed so that at mid-range there is an interval in which both ports are completely closed. Room control action is the same during all seasons.

2-23

.: ... (e.o< I "- Ly E

\

"'COMMON

[!} UNIT r",(>I""OST.1.1

"OT ""ArE~~ SU PPLY

C()t.lIolON SECONDARY "",TEfl C01L

VALvE

co..o '."ER ~ SuPPLy

Figure 2-19 Return Mix System Room Unit Controls .

Four-Pipe System

Four-pipe systems for induction, fan-coil or radiant panel systems derive their name from the four pipes to each terminal unit. As noted before, the piping includes a cold water supply, cold water return, warm water supply and warm water return. The four-pipe system satisfies variation in cooling and heating to the room unit in the form of constant temperature primary air, secondary chilled water and secondary hot water.

The fou r-pipe terminal unit is usually provided with two completely separated secondary water coils, one receiving hot water and the second receiving cold water. The coils are operated in sequence by the same thermostat. The coils are never operated simultaneously, and the unit receives either hot water or cold water in varying amounts, or else no flow is present. This is shown in Figure 2-20. During peak cooling and heating, the four-pipe system performs in a manner similar to the two-pipe system, with essentially the same operating characteristics. During the period between seasons, any unit can be operated at any capacity level from maximum heating, if both cold water and warm water are being circulated. Any unit can be operated at or between these extremes without regard to the operat ion of any other unit.

2-24

..!., UNI r

THERMOSTAT

L';)" _or "or wATER

WATER SUPPLY AET URN

I"IOT COIL " CONTROL VAl.vE

, COLO COIL

COLO WAT(Q COLI: RETURN wATER

0 SUPPl Y

A- SEPAR ATE COILS

- T UNI T T"ERMOSTA T

(R Hor _OT WATER wATER RETURN SUPPl Y

COMMON SECOND ARY ;;

\ 2 - POSI TlON

WATfR COIL

SE~UENCE) COLO COLO wATER

DIVERT I NG vAL V E WATER RETUR N VALVE SUP P LY

Figure 2-20 Four-Pipe System Room Unit Control

2-25

Since the primary air is supplied at a constant cool temperature at all times, it is sometimes feasible for fan-coil or radiant panel systems to extend the interior system supply to the perimeter spaces, eliminating the need for a separate primary air system.

Figure 2-20 shows another unit and control configuration which is sometimes used. A single secondary water coil is provided at the unit, and three-way valves located at the inlet and leaving side of the coil admits the water from either the hot or cold water supply, as required, and divert it to the appropriate return pipe. This arrangement requires a special three-way modulating valve, originally developed for one form of the three-pipe system, which controls the hot or cold water selectively and proportionally but does not mix the streams. The valve at the coil outlet is a two-position valve open to either the hot or cold water return, as required.

When all aspects are considered, the two-coil arrangement provides a superior four-pipe system. The operation of the induction unit controls is the same year-round. Units with secondary air bypass control are not applicable to four-pipe systems. .

Hydronic Piping

Because of the nature of water systems, we will discuss some of the components that are associated only with hydronic systems. A further discussion of some of these components will follow in Chapter Three.

Air Control and Venting

If air and other gases are not eliminated from the flow circuit, they may cause air binding in the terminal heat transfer elements and noise in the piping circuit. High points in piping systems and terminal units should be vented with manual or automatic air vents. As automatic air vents may malfunction, valves should be provided at each vent to permit service without draining the system. The discharge of each vent should be piped to a point where water can be wasted into a drain or container. If a plain expansion tank is used, free air contained in the circulating water should be removed from the piping circuit and trapped in the expansion tank by a boiler dip tube or other air separation devices . If a diaphragmtype tank is used, all air should be vented from the system.

2-26

Drains and Shutoffs

All low points should be equipped with drains. Provisions should be made for separate shutoff and drain of individual equipment and circuits so that the entire system does not have to be drained for service of a particular item.

Balance Fittings

Balance fittings should be applied as needed to permit balancing of individual terminal and major sub-circuits. Such fittings should be placed at the circuit return when possible.

Piping need not pitch but can be run level, providing flow velocities 10

excess of 1.5 feet per second are maintained.

Strainers

Strainers should be used where necessary to protect the elements of a system. Strainers placed in the pump suction need to be analyzed carefully to avoid cavitation. Large separating chambers are available which serves as main air venting points and direct strainers ahead of pumps. Automatic control valves or spray nozzles operating with small clearances require protection from pipe scale, gravel, welding slag, etc., which may readily pass through the pump and its protective separator. Individual fine mesh strainers may, therefore, be required ahead of each control valve. Condenser water systems without water regulating valves do not necessarily require a strainer. If a cooling tower is used, the strainer provided in the tower basin will usually be adequate.

Thermometers

Thermometers andlor thermometer wells should be installed to assist the system operator and to use for troubleshooting. Permanent thermometers with correct scale range and separable sockets should be used at all points where temperature readings are regularly needed. Thermometer wells should be installed where readings will be needed only during start-up and balancing.

2-27

Flexible Connectors

Flexible connectors are sometimes installed at pumps and machinery to reduce pipe vibration and to allow for expansion and contraction of system piping. Vibrations are transmitted through the water column across a flexible connection and reduce the effectiveness of the connector. Flexible connectors, however, prevent damage caused by misalignment of equipment piping flanges.

Gauge cocks should be installed at points where pressure readings will be required. It should be noted that gauges permanently installed in the system will deteriorate due to vibration and pulsation and will not be reliable when needed, unless periodic inspection and calibration is performed.

Pump Location

Pump location varies with the size and type of system. A pump in the boiler , return is acceptable for small systems when pump head is low (12 foot head or less), the compression tank is on the boiler (or a nearby main), and the highest piping and radiation is maintained at a static pressure greater than full pump head. @ese conditions apply to most residential systems.

When pump head is equal to or greater than the difference between boiler fill and relief valve discharge pressures, or when highest piping or radiation can be at a static pressure less than total pump head, the pump must be located on the supply side of the boiler, with the compression tank at the pump inlet, as shown in Figure 2-21. This assures that pump cycling will not cause a vacuum at the topmost system points to allow air to be introduced into the system. Pump cavitation is prevented by locating a properly sized compression tank near the pump inlet that supplies a positive pressure to the pump suction.

2-28

(

Oil"'"

* I I I

~

STM!lOb$

GU( .... L Y (

'LOW Oil .(IG.U[O C .. [C~ Y~\I(

40JI,IS""O Cotl<

GLOI( " 'LY[

lUTO ."., ... , v aLV!

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&LTlfuO t '''Gf

r.'[IIIoIDIII(HJI

... 5(P OA ·o.TO"

I

~ , ___________ L ____ _

I

Figure 2-21 Boiler Piping for a Multiple-Zone, Multiple-Purpose Heating System

2-29

SUMMARY

In this chapter, we learned the purpose of an HV AC system and how it controls an enclosed environment for a specific purpose. We do this by controlling temperature, humidity, and suspe!lded particulates. Then we learned about the different types of air systems, hydronic systems and the different types of filters that are used in HV AC systems.

2-30

CHAPTER THREE

HVAC EQUIPMENT

CHAPTER THREE HVAC EQUIPMENT

OBJECTIVES


1. List the eight requirements that are looked at for the selection of equipment and/or HVAC system.

2. List the heat sources that are used in a HV AC System.

3. Explain how terminal heating equipment heating is controlled.

4. Explain the basic principle of how a heat pump operates.

5. Given a diagram, be able to explain the basic cycle of the following cooling systems:

a . Steam jet b. Heat sink c. Absorption d. Compressed gas

6. Explain the purpose of the following equipment used in a cooling system:

• • •

Piping Pumps Fans

7. State the purpose of a cooling tower.

8. Identify the three types of fan control.

9. Given a diagram of an HV AC System, identify the major components used.

CHAPTER THREE HVAC EQUIPMENT

INTRODUCTION

The equipment used in heating, ventilation, and air conditioning comes under three major headings which cover all the equipment in an HV AC system. Those major headings are heating, cooling, and air-handling.

In this chapter, we will cover the criteria used for equipment selection along with the following topics:

• Heating

• Cooling

• Air Handling

CRITERIA FOIt EQUIPMENT SELECTION

The criteria for the selection of equipment and/or HV AC systems are basically the same. It requires that these eight requirements be looked at:

1. Demands of comfort or process.

2. Energy conservation, code requirements.

3. First cost versus life cycle cost.

4. Desires of owner, architect and/or design office.

5. Space limitations.

6. Maintainability.

3-1

7. Central plant versus distributed systems.

8. Simplicity and controllability.

The problem-solving process used in the evaluation is done so that the person getting the equipment/system get the best product for the cost.

Demand of Comfort or Process

These include temperature always, humidity, ventilation, and pressurization sometimes, and zoning for better control if needed. In theory, at least, this criterion should have a high priority. In practice, the "comfort" requirement is sometimes subordinated to first cost or the desires of someone in authority. Process requirements are more difficult and require a thorough inquiry by the HVAC designer into the process and its needs. Until he fully understands the process, the designer cannot provide an adequate HV AC system. Most often, it will be found that different parts of the process have different parts of the process have different temperature, humidity, pressure, and cleanliness requirements; the most extreme of these can penalize the entire HV AC system.

Energy Conservation

This is usually a code requirement and not an option. State the local building codes almost invariably include requirements limiting the use of new, nonrenewable energy. Nonrenewable refers primarily to fossil fuel sources. Renewable sources include solar, wind, water, waste processing, heat reclaim, and the like. The strictest codes prohibit any form of reheat (except from reclaimed or renewable sources) unless humidity control is essential. Most HVAC systems for process environments have opportunities for heat reclaim and other ingenious ways of conserving energy. Off-peak storage systems are becoming popular for energy cost savings through these systems may actually consume more energy than conventional systems.

3-2

First Cost/Life Cost

First cost considers only the initial price, installed and ready to operate. It ignores such factors as expected life, ease of maintenance and even, to some extent, efficiency, though most energy codes require some minimum efficiency rating. Life-cycle cost includes all cost factors including first cost, operation, maintenance, replacement, and estimated energy use, and evaluates the total cost of the system over a period of years. The usual method of comparing the life cycle costs of two or more systems to convert all costs to "present worth" values. Typically, first cost governs in buildings being built for speculation or short-term investment. Life-cycle costs are most often used by institutional builders - schools, hospitals, government - and owners who expect to occupy the building for an indefinite period.

Desires of Owner, Architect, or Design Office

Very often, someone in authority lays down guidelines which must be followed by the designer. This is particularly true for institutional owners and major retailers. Here the designer's job is to follow the criteria of his employer or the client unless it is obvious that some requirements are unsuitable in an unusual environment. Examples of such environmental conditions are: extremely high or low outside air humidity, high altitude (which affects AHU and air-cooled condenser capacity) and contaminated outside air (which may require special filtration and treatment).

Space Limitations

The architect can influence the HV AC system selection by the space he makes available in a new building. In retrofit situations, the designer must work with existing space. Sometimes in existing buildings it is necessary to take additional space in order to provide a suitable HV AC system. For example, in adding air conditioning to a school it is often necessary to convert a classroom to an equipment room. Roof-top systems are another alternative where space is limited, if the building structure will support such systems. In new buildings, if space is too restricted, it will be desirable to discuss with the architect the implications of the space limitations

3-3

in terms of equipment efficiency and maintainability. There are ways of providing a functioning HVAC system in very little space, such as individual room units and roof-top units, but these systems often have a high life-cycle cost.

Maintainability

This criterion includes equipment quality (mean time between failures is a commonly used term); ease of maintenance (are high maintenance items readily accessible in the unit?); and accessibility (is the unit readily accessible? Is there adequate space around it for removing and replacing items?). Roof-top units may be readily accessible if there is an inside stair and roof penthouse, but if an outside ladder must be climbed the adj ective "readily" must be deleted. Many equipment rooms are easy to get to but too small for adequate across or maintenance. This criterion is critical in the lifecycle cost analysis and in the long-term satisfaction of the building owner and occupants.

Central Plant Versus Distributed Systems

Central plants may include only a chilled water source, both heating and chilled water, an intermediate temperature water supply for individual room heat pumps, or even a large, central air-handling system. Many buildings have no central plant. This decision, in part, influenced by previously cited criteria and is itself a factor in the life-cycle cost analysis. In general, central plant equipment has a longer life than packaged equipment and can be operated more efficiently. The disadvantages include the cost of pumping and piping, or, for the central AHU, longer duct systems and more fan horsepower. There is no simple answer to this choice. Each building must be evaluated separately.

Simplicity and Controllability

Though listed last, this is the most important criterion in terms of how the system will really work. There is an accepted truism that "The operator will soon reduce the HV AC system and controls to his level of

3-4

understanding." This not to criticize the operator, who may have had little or no instruction about the system. It is simply a fact of life. The designer who wants or needs to use a complex system must provide for adequate training - and retraining - for the operators. The best rule is: never add an unnecessary complication to the systein or its controls.

HEATING

Heating is the first word in the HV AC acronym. It is the most important part because without heating there would be difficulty in surviving. Proper design of the heating system is even more critical than that of ventilation or cooling. The history of man began to develop with the discovery of fire which increased his ability to survive in a harsh environment. In modem heating system design, the two things of primary concern are proper sizing to achieve comfort and system reliability. Capital and operating costs and pollution control are of secondary consideration. Energy conservation and operating costs go together and have a considerable effect on life cycle costs.

In a modem heating system, heating can be provided by:

1. Fuel fired boilers that produce steam, hot water, or thermal liquids for direct or indirect use.

2. Furnaces, unit heaters, duct heaters, and outside air heaters which provide hot air for direct circulation to the conditioned space.

3. Waste heat furnaces and boilers which utilize the waste energy from some other source such as an incinerator or refrigeration equipment.

4. Solar energy collectors, both passive and active, which heat either water or air, and in some cases, solid materials.

3-5

5. Heat pumps, either liquid or air.

6. Direct-fired radiant heaters, either electric or natural gas.

End users are provided heat by:

1. Direct air - furnaces, duct heaters, outside air heaters, reheat un its, ducted heat pumps.

2. Indirect air - coils and air-handling units, fan-coil units, unit ve ntilators.

3. Liquid - radiators, convectors, liquid-filled radiant heaters.

4. Radiation - direct radiation from panels or other radiators.

The topics that will be covered in this section of the chapter are:

• Boilers

•

•

•

Hot water boiler Steam boiler

Electric Heaters

Terminal Heating Equipment

Radiators and convectors Radiant panels

Heat Pumps

Packaged heat pumps

3-6

BOILERS

Boilers can produce low, medium, or high-temperature water, lowpressure steam, high-pressure steam (including process steam), and thermal liquid.

Hot Water Boilers

Low-temperature water boilers (to 250"F) are the most widely used type for residential, apartment, and commercial construction. Medium temperature water boilers (250 to 310°F) are generally applied to industrial and campustype facilities. High-temperature water (310 to 400"F) is used for extended campus-type facilities and industrial process facilities. It is often used where there are significant end-user steam requirements at pressures of 100 psi or more. Thermal liquid heaters are primarily found in industrial applications where both space and process heating are significant loads.

Steam Boilers

Low-pressure boilers (15 psig) are generally found in commercial, apartment house, and single-unit industrial facilities. They are used for space heating and domestic hot water, through end-use heat exchangers. High-pressure steam applications (15 to 150 psig) are generally found in campus-type facilities, hospitals, and industrial plants where there are significant process requirements. Cogeneration high-pressure steam boilers are in the range of 600 to 900 psig with some degree of superheat in order to obtain good turbine efficiency. Waste heat from the turbine is used for space heating, domestic hot water, and process requirements.

ELECTRIC HEATERS

A unit heater is a package which includes a heating element and a circulating fan. It is designed for installation in or adjacent to the space to be heated. Units are made for horizontal discharge (Figure 3-1) or vertical discharge (Figure 3-2). Most unit heaters have propeller fans. Units with centrifugal fans may be used with duct work to extend the area of coverage.

3-7

Figure 3-1 Horizontal Unit Heater

Figure 3-2 Vertical Unit Heater

3-8

The heating element may be a steam or water coil, or may be directfired using fuel gas or electric resistance. Gas heaters require proper venting and safety controls. Unit heaters are normally controlled by means of a room thermostat which starts the fan and energizes the heating element simultaneously.

A duct heater (or duct furnace) is a unit heater with out a fan and is installed in a duct or plenum. The duct heater depends on an AHU fan for air circulation. It may be the primary heating element - in the main duct or AHU plenum - or may be used for zone reheat control in branch ducts. Many package air-handling systems use duct heaters.

An outside air heater is a unit heater or duct heater used for preheating outside air, as required for exhaust make-up or combustion. To prevent freeze-up gas or electric heating is used, with gas preferred on an energy cost basis. In some installations, codes allow the use of unvented heaters - all the heat and products of combustion are in the air stream, but so diluted as to pose no danger. This situation requires that all of the supply air be exhausted.

Radiant unit heaters have no fans and utilize radiant heating rather than convective heating. For this purpose they are installed overhead and equipped with special high-temperature surfaces which radiate primarily in the infra-red spectrum. They are used mostly for "spot-heating" at work stations in otherwise unheated or poorly heated buildings. Another use is for heating of outdoor areas where people need to wait or stand in line, such as under theater marquees or in amusement parks. Radiant heating is a very efficient and economical method of achieving a level of comfort in an area which would be difficult or impossible to heat satisfactorily in any other way.

TERMINAL HEATING EQUIPMENT

Terminal heating equipment is equipment installed in or contiguous with the area served. In general, the heating source is remote - water or steam is used - but electric resistance heating is common. Duct heaters and

3-9

some heat pumps can also be included in this category. In many cases, the terminal equipment is used for both heating and cooling.

Radiators and Convectors

A radiator is a heating device which is installed in the space to be heated and transfers heat primarily by radiation. The most common example is the sectional cast-iron column radiator. There are many thousands of these in use throughout the world, although in new installations they have been largely supplanted by convector radiators or baseboard radiation. The heat source is hot water or low-pressure steam (5 psig or less). Small "electric radiators" include water and an electric immersion heater.

Radiators are rated in square feet of radiation or EDR (equivalent direct radiation). One square foot EDR is equal to 240 btuh for steam at one psig or 180 btuh for water at 200"F. These ratings are no longer readily available but may be obtained from the Hydronics Institute, formerly the Institute of Boiler and Radiator Manufacturers. Some representative data are available in the ASHRAE Handbook.

Radiators are controlled in several ways:

1. Manually, by means of a globe valve.

2. Automatically, by means of a modulating or two-position valve. Self-contained valves are very popular for this application.

3. In zones, by means of a zone control valve, sometimes with solar compensation. With zone control, orifices are used at radiator supply connections to ensure uniform distribution of steam throughout the zone.

4. Some small systems are controlled by cycling the boiler or hot water pump.

3-10

5. Steam heat may be controlled by means of vacuum system. This requires a closed system in which the absolute pressure may be varied by means of a vacuum pump in the condensate return. The steam system may then be operated at sub-atmospheric pressures with a consequent reduction in steam temperature.

A convector is a heating device which depends primarily on gravity convective heat transfer. The heating element is a finned-tube coil or coils, mounted in an enclosure designed to increase the convective effect (Figure 3-3). The enclosure (cabinet) is made in many different configurations, including partially or fully recessed into the wall. The usual location is on an exterior wall at or near the floor. Capacity depends on geometry - length, depth, height - and heating element design, as well as hot water temperatures or steam pressure. Ratings are usually based on the test methods specified in "Commercial Standard CS 140-47, Testing and Rating Convectors." Refer to manufacturers' catalogues for specific data.

Flecess ____

.. Wall face

Front panel with -- integral grilles

_ Fin-tube heating L,---- element

~ / Floor line

Figure 3-3 Convector

Baseboard radiation is designed for wall mounting in place of the usual baseboard. It is either a fin-tube system, similar to a convector but much smaller, or a cast-iron section, designed with convective heat channels to augment the radiant effect. Baseboard radiation is usually continuous along exterior walls. Blank covers may be used for appearance if the capacity if not needed.

3-11

Finned-tube, or finned-pipe, radiation uses larger tubing or pipe - 1-1/4" to 2" size - with fins bonded to the pipe. The fins are typically 3-1/2" to 4-1/2" square. The system is used mostly for perimeter heating, particularly at glass areas. Heat transfer is by convection and a variety of enclosure types are available; some examples are shown in Figure 3-4. Special enclosures are often made to suit an architectural decor.

•

0

W<Jllline

- Cover ~Ih grille ml~rat

Fin,plpEt healing elemlnl

/Floor~ 0'

A. Flal 101' cover S. SeDping lOp coyer

Figure 3-4 Typical Fin-pipe Enclosures

E .. p .. n.ded r mela! cover

- - - -, ~Fin.p,pe

C. E"panded melal cover

All of these heating elements may use either low pressure steam or hot water as the heating source. Either one-pipe or two-pipe distribution systems are used, though two-pipe is more common in modem practice. Zoning by exposure, using solar compensated sensors, is a frequent practice. Electric baseboard radiation is also available. It is sometimes more economical, for example, in an all-electric situation or where steam or hot water is not available.

Radiant Panels

A radiant panel is a heating surface designed to transfer heat primarily by radiation. There may also be a convective component, and in the case of

3-12

floor panels, convective transfer may be predominant. Panels may be located in the floor, wall, or ceiling and may occupy part or all of the available area. Panel surface temperatures are limited by the physiological response of the building occupants. That is, too high a temperature may result in an uncomfortably warm feeling. Typical limitations are 80" to 85°F for floor panels, about 100"F for wall panels, and 120" to 13O"F for ceiling panels. The heating source is hot water or electrical resistance heating cable. Hotwater supply temperatures should be consistent with the panel temperature limitations; for floor panels, for example, supply water temperature should be no more than 100°F.

Factory-assembled sidewall and ceiling panels and panel systems are available. Most panels are field fabricated using electrical heating cable, copper tubing, or steel pipe imbedded in the construction. For concrete floor panels, steel pipe is used (3/4" or 1" size), because steel has an expansion coefficient similar to that of concrete. Corrosion at the concrete-pipe interface can be severe. Electric heating cable may be used. ·Ceiling and

.< wall panels use 1/2" to 3/4" copper tube or electric cable. Air venting is a serious problem, especially with floor panels.

Control systems are conventional, since radiant-heat-sensitive devices are not readily available. Floor panels are very difficult to control, since the relatively large mass provides a slow response.

HEAT PUMPS

A heat pump is a mechanical refrigeration system arranged and controlled to utilize the condenser heat for some useful purpose, typically space heating. Systems may be packaged or built-up, air-to-air, water-to-air, or water-to-water. Earth-coupled systems are also used.

Packaged Heat Pumps

A packaged heat pump is factor-assembled system designed to provide either heating or cooling, as needed. The standard refrigeration cycle is modified as shown in Figure 3-5. The key to the operation is the reversing

3-13

valve. In the cooling position, refrigerant flow is directed first to the outdoor coil, which becomes the condenser. The liquid refrigerant then bypasses metering device no. 1 and flows through metering device no. 2 to the indoor coil. The metering device is a thermal expansion valve, throttling tube, or some other method of reducing the pressure. The indoor coil then becomes the evaporator and cooling is provided. With the reversing valve in the heating position, refrigerant flow is reversed, the indoor coil becomes the condenser and provides heating; heat is extracted from the outdoor air. Changeover from heating to cooling may be automatic but is usually manual. Most packaged heat pumps are air-to-air. Heating capacity decreases as outdoor air temperature decreases.

While most air-to-air heat pumps will operate satisfactorily down to zero degrees F outdoors, auxiliary heating will be needed except in very mild climates. Figures 3-6 and 3-7 illustrate the procedure for determining the auxiliary heat requires. Figure 3-6 shows the method of calculating the net heating load as a function of temperature. For buildings with 24-hour occupancy, solar heat effects should be ignored. Note that the net heat loss is less than the calculated heat loss because of internal heat gains due to people, lights, and other sources. In Figure 3-7, this net heating load is plotted against the heat pump capacity from manufacturer's data as a function of temperature. The shaded area is the excess of load over capacity, requiring auxiliary heat. Almost any fuel can be used for auxiliary heat, but electric resistance is the most common.

In a water-to-air package heat pump, a water-to-air heat exchanger is substituted for the outdoor coil. A central source for heating or cooling the water can then, in effect, provide the auxiliary heat. Systems of this type are used in apartment houses and hotels to allow maximum control of the room environment by the occupant. The water temperature is controlled at a range of values - perhaps 70°F to 85°F - which is suitable both as a heat source and heat sink for the heat pumps. In mild weather when some units are in heating mode while others are cooling, the central boiler and cooling tower may be idle.

3-14

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r , "

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In(lOO' ,.,

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I

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~)---,-'"'-"-'-"'-'~-<-'-'--------",--~o<r" '---j

Hnl,ng mod.

Figure 3-5 Packaged Heat Pump Cycles

3-15

During the 1950s and 1960s, several large office buildings were constructed using water-to-water heat pumps, with capacities up to several hundred tons. These systems typically use well water. Two wells are used, one for supply and one for disposal. One possible arrangement is shown in Figure 3-8. The supply and disposal wells are manually selected. Well water and return water are mixed, for both evaporator and condenser, on a temperature basis. Under some conditions, this system can become an internal source heat pump - that is, when the exterior zone heating and interior zone cooling loads are in balance, or nearly so, little or no well water is needed. Internal source heat pumps without wells are used where there is sufficient internal cooling load to supply the net heating requirements under all conditions. Excess heat is disposed of through cooling towers.

HR CHO

C>lS HS

, .... - --- --, ,

, , - - -r;;;-t - - To C_n$Ol' ,... - ---t.:...J To s~,~

..... """'''

WINS .n<! """"'.

Figure 3-8 Large Building Heat Pump, with Water Well Source

3-17

, .

;,.. ..

COOLING

There are many types of cooling systems. In this section of the chapter, we will give you an introduction to the equipment that is used in cooling systems, The topics that we will cover are:

• Refrigeration

• Chiller

• Cooling Towers

• Cooling Coils

• Piping

• Pumps

Refrigeration

Several different methods are used to cool air directly or indirectly, In this section of the chapter, we are going to study the equipment that is used in the various cooling systems. The purpose of this is that the student will understand how this equipment fits into the whole picture of the operation an HV AC system. Equipment from the following systems will be discussed along with a brief description of how each system operates;

• Steam Jet

• Heat Sink

• Adsorption (Chiller)

• Compressed Gas (Chillers)

3-18

Steam Jet

The steam jet refrigeration system may be used if an abundant supply of high pressure steam is available. The equipment that is used in the system is a nozzling jet (which works like an air ejector), a tank with an inlet and outlet for water supply to it. (Figure 3-9) The process creates a partial vacuum in a tank by nozzling a jet of steam over the single opening (aspiration). This reduced pressure in the tank permits water to boil at a substantially reduced temperature (40-50"F). The heat required for evaporation is extracted from the cooling water which is pumped through the tank.

P ST

co

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COOL WATf.? TO

• , lI IP- CONn I TIO. II NG EQutPttEN T

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....... ~ 40o -S0of

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Figure 3-9 Steam Jet Refrigeration System

3-19

LOW P •

STEN' ~

RESSURE

TO CONI

i '

, .

Heat Sink

The heat sink method of providing air conditioning requires a large body of cool water, usually subterranean or drawn from dee I lakes. The equipment that is used in this system is a pump, piping and hydronic coils. The cool water is circulated through hydronic coils and subsequently provides cooling to the controlled environment.

Absorption

The absorption systems utilizes a liquid with a low boiling point such as ammonia on water under a low vacuum. The equipment that is used in this system is a chiller, which is divided into four sections: the evaporator, absorber, generator and condenser, heat exchanger and three pumps. We will discuss the function of each piece of equipment.

Chiller

As noted from the above paragraph, the chiller is divided into four sections. The function of each section is:

Evaporator -

Absorber -

Generator -

Condenser -

The purpose of the evaporator is to cool a liquid for use in an air-conditioning system,

The purpose of the absorber is to provide an area where the absorbent can absorb the refrigerant and also store the excess absorbent.

The function of the generator is to concentrate the absorbent by removing some of the refrigerant from the dilute absorbent solution.

The function of the condenser is to condense the refrigerant vapors from the generator back into a liquid and that condensed liquid is returned to the evaporator.

3-20

Heat Exchanger -

Pumps

The function of the heat exchanger is to make the absorption cycle more efficient. It does this by bringing the warm, concentrated absorbent solution coming from the generator in contact with the relative cool dilute absorbent from the absorber. This lowers the heat input needed for input to the generator and increases the efficiency of the system.

There are three pumps usually associated with an absorption system. These pumps are used to circulate the fluids between the following components and are named for the component in which they service.

Because all absorption systems work basically the same, we will describe the operation of the lithium brome cycle.

Refer to Figure 3-10 as you go through the cycle. It will help you understand what is happening in each area of the absorption chiller.

Let's start the cycle by creating a vacuum in the absorber and evaporator, and starting these pumps. Water will boil at 40"F. -45°F with a vacuum of 29.53 inches of mercury (Hg). As the refrigerant (water) is sprayed on the 55°F chilled water coil, the refrigerant boils and absorbs the heat from the chilled water. The refrigerant vapor is then absorbed by the lithium bromide, and becomes weaker. To have continuous operations, the lithium bromide must be made stronger and the refrigerant must return to the evaporator. To do this, the generator pump is started and a steam valve is opened. The generator pump forces the weak solution through the heat exchanger (where the weak solution is preheated and the strong solution from the generator is cooled), then into the generator. Steam is used to make the refrigerant (water) go into a vapor again where it condenses into pure water in the condenser. As the refrigerant level rises in the condenser, the float opens to return the refrigerant into the evaporator for continuous operation.

3-21

'" "

'"

'"

.~.

HlAT VCCHAHGIIt

Figure 3-10 Lithium Bromide Absorption System

3-22

C"'ll( O LIOUIO

Compressed Gas

The compressed gas type of refrigeration system is the most used in every day application and the one most people are familiar with and the one we will discuss the most about. Just as with the other type of refrigeration system, we will look at the major equipment that is used to make the system. The topics that we will cover are:

• Compressor

• CondenserslReceivers

• Metering Device

• Evaporators

Compressor

The compressor removes the vapor from the evaporator, compresses and heats the vapor. This raises the pressure and temperature of the vapor so that it can be condensed at ordinary climatic temperatures. The compressor then discharges the vapor to the condenser. There are four primary types of compressors: reciprocating, rotary, screw and centrifugal. Regardless of the type of compressors, they all do the same thing and they are the heart of the compressed gas cycle.

Condenser/Receiver

The condenser transfers heat from a place where it is not wanted to a place where it can be discarded. The condenser is a coil of metal tubing which is exposed to a cooling medium, such as water or fan-forced air. the cooling medium absorbs enough heat from the vapor to condense it. There are three types of condensers normally used and the are water-cooled, aircooled, and evaporative.

3-23

Receivers are installed to collect the liquid refrigerant as it leaves the condenser. In some models, the lower section of the condenser is used as the receiver. A receiver serves as a stowage for refrigerant, maintains a liquid seal on the liquid line, and vents any air or non-condensable gases back to the condenser.

Receivers are usually designed to be large enough to hold the complete charge of refrigerant required to operate the unit. They are equipped with stop valves on the inlet and outlet lines to permit the serviceman to pump the unit down when work it to be performed on another component in the system. The liquid refrigerant is then collected and directed to the metering device.

Metering Device

The metering device is a device that limits or controls the flow of refrigerant passing through it on its way back to the evaporator. By controlling the flow, the pressure is reduced so that the liquid will again boil at low temperature in the evaporator.

So that the refrigerating unit may operate automatically, an automatic metering device must be placed in the circuit between the liquid line and the evaporator. This control reduces the high pressure in the liquid line to the low pressure in the evaporator. The six main types of automatic metering devices are:

• • • • • •

Automatic Expansion Valve (AEV or AXV) Thermostatic Expansion Valve (fEV or TXV) Thermal-Electric Expansion Valve (11IEXV) Low Side Float (LSF) High Side Float (HSF) Capillary Tube (Cap. Tube)

3-24

Evaporators

The evaporator or cooling coil is the part of the refrigeration system where heat is removed from the product; air, water, or whatever is to be cooled. As the refrigerant enters the passages of the evaporator, it absorbs heat from the product being cooled and, as it absorbs heat from the load, it begins to boil and vaporizes. In this process, the evaporator accomplishes the overall purpose of the system - refrigeration.

Manufacturers develop and produce evaporators in several different designs and shapes to fill the needs of prospective users. The blower coil or forced convection type evaporator is the most common design; it is used both in refrigeration and air-conditioning installations. The six main types of evaporators are:

• Plate • Bare Tube • Finned Tube • Fixed Convection • Dry • Flooded

Figure 3-11 shows the basic compressed gas cycle and use with the following cycle description will help you see and understand how the cycle works.

Liquid refrigerant enters the metering device which separates the high pressure side of the system from the low pressure side. This valve regulates the amount of refrigerant which enters the cooling coils of the evaporator. Because of the pressure differential, as the refrigerant passes through the metering device, some of it flashes to a vapor.

3-25

IlEAT fnOM CONTROlU:O EIlV l OONKENT

LOH PRESS URE S I DE ~~ ft! GI! PRESSURE S I DE:

SATUR.I\TED GAS I SUPE RHEATED V/I.POR

~ t EVAPORATOP CONDEN SE [l

CO MP r..ESSOR

LIQU I D REfRIGERANT AT ....J E)( P ANS ION r;,()(}-_____ ----'Hc:I.::GH::....:.'.:.:RE:.:S.::S.::URE:=-_---'

VALVE '<..Y

I.J" ! ~~,> , \ \I;;i q ~o<'

Figure 3·11 Basic Vapor-Compression Refrigeration Cycle

IIEAT

From the metering device, the refrigerant passes into the evaporator, The boiling point of the refrigerant under the low pressure in the evaporator is lower than the temperature of the space in which the cooling coil is installed, This causes the liquid to boil and vaporize, picking up latent heat of vaporization from the space being cooled. The refrigerant continues to absorb latent heat of vaporization until all the liquid has been vaporized. By the time the refrigerant leaves the cooling coil, it has not only absorbed this latent heat of vaporization but has also picked up some additional heat - that is, the vapor has become superheated.

The refrigerant leaves the evaporator as low-pressure superheated vapor. The remainder of the cycle is used to dispose of this heat and convert the refrigerant back into a liquid state so that it can again vaporize in the evaporator and absorb the heat again.

The low-pressure superheated.vapor is drawn out of the evaporator by the compressor, which also keeps the refrigerant circulating through the

3-26

system. In the compressor, the refrigerant is compressed from a lowpressure, low-temperature vapor to a high-pressure, high temperature vapor.

The high-pressure vapor is discharged from the compressor into the condenser. Here the refrigerant condenses, giving up its superheat (sensible heat) and its latent heat of condensation. The refrigerant, still at high pressure, is now a liquid again. From the condenser, the refrigerant goes to the metering device and the cycle begins again.

Figure 3-12 shows a graphic illustration of the pressure-temperature relationship for the refrigerant R-22, during each phase of its cycle.

Referring to Figure 3-12, the evaporator state is the point at which the boiling liquid refrigerant enters the evaporator and absorbs sensible heat form the chill water return. As heat is absorbed, the liquid becomes completely vaporized and rises above its saturation temperature. At this point, the vapor is said to be "superheated".

The superheated vapor is then compressed. Compressing the vapor raises the pressure-temperature state of the vapor which preconditions it for the condensation process.

Condensation is the exact reverse of evaporation, and serves to expel the heat absorbed by the refrigerant, and to condense the refrigerant vapor back to liquid form for reuse by the evaporator.

Liquid metering is the final stage of the refrigeration cycle. Metering allows just the right amount of refrigerant to enter the evaporator so that the proper cooling and superheating takes place.

3-27

f

, ,

"0

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84 90

"-° ~ -a • -~ Co E ~ f-

70

eo ., '" '" " <0

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.JO

Chillers

Superheated state vapor ----•. ,

. 0" ..,' ~,

state -------------------~~ -------~ . ,,0 -~. 1'1 (84°F. 153.2 psig)

Liquid zone

Ev'ror,tor,tat. (50 F. 84 p,lg)

Subcool ed state liquid

o 10 10 30 ~ &0 60 10 ao 90 lao 110 110 1.30 '''0 1~ 1110

84 psig 153.2 psig

Pressure, psig

Figure 3·12 Pressure-Temperature Chart for R-22

The term chiller is normally used in connection with a complete chiller package - which includes compressor, condenser, evaporator, internal piping and controls; or for a liquid chiller (evaporator) only, where the water or brine is cooled,

Liquid chillers are of two general types: flooded and direct expansion. There are several different configurations including shell-and-tube, double tube, shell-and-coil, Baudelot (surface), and tank-with-raceway. For HVAC applications, the shell-and-tube configuration is most common.

3-28

Flooded Chillers

A typical flooded shell-and-tube liquid chiller is shown in Figure 3-l3. Refrigerant flow to the shell is controlled by a high- or low-side float valve or by a restrictor. Water flow rate through the tubes is defined by the manufacturer but is generally in the range of 6 to 12 fps. Tubes may be plain (bare) or have a finned surface. The two-pass arrangement shown in most common, although one to four passes are available. The chiller must be arranged with removable water boxes so that the tubes may be cleaned at regular intervals, because even a small amount of fouling can cause a significant decrease in heat-exchange capacity. Piping must be arranged to allow easy removal of the water boxes.

Direct Expansion (OX) Chillers

In the OX liquid chiller (Figure 3-14), the refrigerant is usually inside the tubes with the liquid in the shell. Baffles are provided tei control the liquid flow. The U-tube configuration shown is typical and less expensive than the straight-through tube arrangement but can lead to problems with oil accumulation in the tubes if refrigerant velocities are too low. Refrigeration flow is controlled by means of a thermal expansion valve.

Package Chillers

A complete package chiller will include compressor, condenser, evaporator (chiller), internal piping, and operating, and capacity controls. Controls should be in a panel and include all internal wiring with a terminal strip for external wiring connections. In small packages - up to 100 tons -motor starters may also be included. Some units with air-cooled condensers are designed for outdoor mounting; freeze prevention procedures must be followed. Units with water-cooled condensers require an external source of condensing water.

3-29

II

Tube sheel

Tubes

Aelflgeranl SUChon

Shell Aelngeranl liquid in

"

" .' '

Figure 3-13 Flooded Liquid Chiller

LiqUId oul

Tube sheet

Liquid In

_ LIQUId

'"' _ liqUId

'"

Rehigerant

Aelrogerant tiquld In

Figure 3-14 Direct-Expansion chiller (U-tube type)

3-30

Chillers with reciprocating compressors are found mostly in the 5-to-100 ton range. Though larger units are made, economics usually favor centrifugal compressor chillers in sizes of 100 tons or more. Screw compressor systems are made in a limited range of sizes, as contrasted with centrifugal compressors. Motor starters are usually separate from the centrifugal or screw packages may be turbine-driven but more often use electric motors. The typical system is direct-driven at 36500 rpm. WyeDelta motors are used for reduced voltage starting. In larger units or 1000 tons or more, it is not unusual to use high-voltage motors; the lower current requirements allow smaller wire sizes and across-the-line starting. An unusual drive system is that used on one of the 8500-ton chillers at the Dallas-Ft. Worth airport. The utility plant manager replaced the original steam turbine driver with 5000-hp, 4160-volt, variable-speed, variablefrequency electric drive. The chiller capacity was reduced to 5500 tons, more in line with the actual load.

Cooling Towers

A cooling tower is a device for cooling water by utilizing the evaporative cooling effect of the water. The cooled water may be used for many purposes but the principle concern in this book is for its use as a heat sink in a refrigerant condenser.

The two main types of cooling towers are open circuit and closed circuit, described below. There are also two basic configurations: cross-flow and counter-flow. In either arrangement, the water enters at the top of the tower and flows downward through it. In the counter-flow arrangement, the air enters at the bottom and flows upward. In the cross-flow arrangement, the air enters at one side, flows across the tower and out the other side.

Towers may be forced- or induced-draft, using fans (Figure 3-15), or natural draft, using convective chimney effects. Typical of this latter group are the large hyperbolic towers seen at many power plants, Figure 3-16. In a forced-draft tower, the air is blown into the tower by the fans; in the induced-draft tower, the air is drawn through the tower.

3-31

.... ir

'"

Wiuer sprays

,,, -I

Water ----J out

Figure 3·15 Forced-Draft Cooling Tower

Air

i out

Water sprays

Figure 3·16 Natural-Draft Cooling Tower

3-32

Towers are spray-filled, with the water distributed through spray nozzles, or splash-filled, where the water flows by gravity and splashes off the tower fill material. In either case, the idea is to maximize the evaporation efficiency. The most important factors in this effort are 1) the effectiveness of spray or splash in atomizing the water, 2) the internal tower volume in which air and water corne into contact, 3) the air flow rate through the tower, and 4) the water flow rate. Tower fill material used to be redwood. Now most fill material is made of PVC or some similar plastic.

The two terms relating to tower efficiency are range and approach. The range is the difference between entering and leaving cooling water temperatures. For HVAC practice, this is usually 1000F, although gOF to 15°F are used. Approach is the difference between the leaving cooling water temperature and the ambient wet-bulb temperature. This is usually between 6 and 1000F, with gOF being typical.

Open-Circuit Cooling Towers

In Figure 3-17, it can be seen that there is only one water circuit, with a portion of the cooling water being evaporated to cool the remainder. Because the water is exposed to air, with all of its contaminants, and absorbs oxygen, which is corrosive to most piping, the water must be carefully treated. To avoid increasing the concentration of solids as water is evaporated, blowdown must be provided: a portion of the water is wasted to the sewer either continuously or intermittently. A blowdown rate equal to the evaporation rate is considered normal. Ideally, treatment additives and blowdown rate should be controlled automatically by a system which measures water quality and solids concentration.

3-33

L r "

ij i

I

! ,

i

Water r Air out Water

'" '0

1 1

---I I--

Air A"

'0 '" --, Water

'"' I--

Figure 3-17 Cross-Flow Cooling Tower

Closed-Circuit Towers

The closed-circuit tower (Figure 3-18) is desigued to mlmmlze corrosion and fouling in the cooling water circuit by making this a closed circuit. The cooling water flows through a bare tube coil in the tower and coolant water in a separate circuit is sprayed over the coil and evaporated. This is essentially the same system as the evaporative condenser previously described. The coolant water circuit is open and needs treatment and blowdown. Because of the temperature differential through the tube wall, this system is slightly less efficient than the open circuit, but the lower fouling effect improves the performance of, and decreases maintenance on, the condenser. This tower usually has a higher first cost than the open circuit tower.

3-34

'I' ' I' /1\ /1\

(

C

Bare pipe coil ~ J'o Air in ~

<6 ~ Wale, sump wil make-up and bl

Pump

Figure 3-18 Closed-Circuit Cooling Tower

3-35

cs

CR

h automatic owdown

l.

Cooling Coils

A cooling coil is a finned-tube heat exchanger for use in an airhandling unit. Chilled water, brine, or refrigerant is inside the tubes and air is blown over the outside, across the fins and tubes. When used with refrigerant, this element is the "evaporator" in the refrigeration cycle and is called a direct expansion (DX) coil.

Piping

Piping systems are the means by which thermal energy fluids are transported from one place to another. The type of fluid and its temperature and pressure influence and limit the choice of piping materials. Most systems are closed - that is, the fluid is continually recirculated and no makeup is required except to replace that lost due to leaks. Steam systems are partly to completely open - as when the steam is used for a process or humidification - and require continuous makeup. Cooling-tower systems are open and need makeup to replace the water evaporated in the tower.

Closed systems require some means of compensating for the changes in volume of the fluid due to temperature changes. Expansion (compression) tanks are used.

Piping must be properly supported, with compensation for expansion due to temperature changes and anchors to prevent undesired movement.

Pumps

Centrifugal pumps are used in HV AC for circulation of chilled, hot and condensing water, and brine. They are also used for pumping steam condensate and for boiler feed .

The operating theory of centrifugal pumps is exactly analogous to that of centrifugal fans. The rotating action of the impeller (equivalent to the fan wheel) in a scroll housing generates a pressure which forces the fluid through the piping system. The pressure and volume developed are functions of

3-36

pump size and rotational speed. For higher pressures, multistage pumps are used.

Pump Configurations and Types

The majority of the centrifugal pumps used in HVAC work have a backward curved blade impeller (Figure 3-19). For pumping hot condensate, a turbine-type impeller is used to minimize flashing and cavitation.

Most pumps are direct driven at standard motor speeds such as 3500 rpm, 1750 rpm, and 1150 rpm. Typical arrangements include combinations of alternatives such as end-or double-suction, in-line or base mounted, horizontal or vertical, and close-coupled or base mounted. Vertical turbine pumps are used in sumps, Le., in cooling-tower installations.

Larger inlet dia. '" lower NPSH required

Rolation ----

Number 01 vanes may vary

Figure 3-19 Backward-Curved Pump Impeller

3-37

In general, in-line pumps are used in small systems or secondary systems, such as freeze-prevention loops. Base-mounted pumps are used for most applications. Double-suction pumps are preferred for larger water volumes over 300 to 400 gpm, because the purpose of the double-suction design is to minimize the end thrust due to water entering the impeller.

Performance Curves

A typical pump performance curve (Figure 3-20) is drawn with coordinates of gpm and feet of head. The curves show the capacity of a specific pump-casing size and design at a specific speed (rpm) and with varying impeller diameters. The same impeller is used throughout, but when it is "shaved" (machined) to reduce its outside diameter, the capacity is reduced. This allows the pump to be matched to the design conditions. The graph includes brake horsepower curves for standard size motors, based on water with specific gravity of 1.0. For brines, or liquids with other specific gravities, the horsepower must be corrected in direct proportion to the specific gravity change. Also shown are efficiency curves.

9

• 6 , , • < ,

60 .,'" 9'" . '"

" , ,

I I

./

.~ c.6 """.~y" , ~ , "

'----<:.. , ' .., , ~ ,\11

I I -~ ~' ... r 19"" "----.l ~ 0., ~..,

" " , ,

\ I -r...., '---- Impcn~ , " /'\~ ,,\ ;;OI-lP

. di"m~Ii" 'lit 01 m"",.,um K ;BHP

l BltP

1

"" JOO <00

Figure 3·20 Pump Performance Curve

3-38

The point at which a pump curve intersects the zero flow line is the shutoff head. At this or a higher head, the pump will not generate any flow. If the pump continues to run under no-flow conditions, the work energy input will heat the water. The resulting temperature/pressure rise has been known to break the pump casing.

If the speed of the pump is varied, the result will be a family of curves similar to Figure 3-21. These data are needed to evalua!e a variable-speed pumping design.

60 L- ",,"'" 1d""''''Y j." ~

50

I~flnl " " -; 0 • • <

• ,. " ~

---20

~pm '0 t--

850 rpm

o o '00 '00 ,00 500 600

C;>(I:tC,ly ,n GPM

Figure 3-21 Pump Speed versus Capacity and Head

3-39

Pump Selection

In order to select a pump, it is necessary to calculate the system pressure drop at the design flow rate. Losses include pipe, valves, fittings, control valves, and equipment such as heat exchangers, boilers, or chillers. The design operating point or a complete system curve can then be plotted on a pump performance curve. Usually several different pump curves will be inspected in order to find the best efficiency and lowest horsepower. In general, for large flows at low heads, lower speed pumps - 1150 rpm or even 850 rpm - will be most efficient. For higher heads and lower flow rates, 1750 rpm or 3500 rpm will be preferable. Multistage pumps may be needed at very high heads. Always select a motor HP that cannot be exceeded by the selected pump at any operating condition, e.g., the HP curve should be above the pump curve at all points.

When two or more identical pumps are installed in parallel. The performance curve for two pumps has twice the flow of one pump at any given head. When the system curve is superimposed, it can be seen that the curve for one pump will intersect the system curve at about 70 percent of the design flow rate and about half of the design head. Similar curves can be drawn for three or more pumps in parallel.

Two or more identical pumps in series provide twice the head at any given flow rate. The flow with one pump will be about 75 percent of design flow. However, unless a bypass is provided around the second pump, the system curve will change somewhat with only one pump running, due to the pressure loss through the second pump. A bypass should be provided around both pumps to allow one to operate while the other is being repaired or replaced.

AIR-HANDLING

I It stands to reason that an air-handling unit of some kind is an essential k. part of an air-conditioning system. So if the student understands the equipment that is used in air systems, it will help him understand the overall

3-40

view of HV AC. In this section of the chapter, we will be studying about the equipment used in air handling. The topics that will be covered are:

• Fans

• Ductwork

FANS

A fan is a device used to cause a current of air by movement of a broad surface or a number of such surfaces within a sealed plenum. From this definition, the function of a fan can be stated as a device which moves air or gas from one place to another. In doing so it overcomes the resistance to flow by supplying the fluid (gas or air) with the energy necessary for continued motion. The resistance to flow is caused by duct configuration, the fluid being at rest, etc.

Large central station boilers, regardless of fuel and method of firing use mechanical draft fans. Forced-draft fans supply large amounts of fresh air for combustion. Induced-draft fans remove combustion products. These are a few types and uses of mechanical draft fans in a power plant.

A fan moves a quantity of air or gas by adding sufficient energy to the air stream to start motion and overcome resistance to flow. The bladed rotor or impeller does the actual work. The power required depends on (1) the volume of gas moved per unit time, (2) the pressure difference across the fan, and (3) the efficiency of the fan and its drive. The topics we will cover under fans are:

• Classifications of Fans • Fan Control • Fan Drives • Fan Laws • Fan Characteristics Curves

3-41

Classifications of Fans

There are two basic types of fans; centrifugal and axial flow. The axial flow fan (Figure 3-22) moves the gas in a path parallel to the fan rotor. These fans operate most efficiently with a low resistance to flow and so provide a high volume of air at low head pressures. Axial fans are normally used as forced-draft fans in a balanced-draft system.

INLE T BOX

MOTOA ROTOR ASSEM SL '(

REM OV ABLE UPPER FAN HOUSI NG

DI FFU SER

DR IV E SHAF T

REMOVABLE V AR IABLE ·PI TCH ROTA TING BLA DES

MAIN BEARING ASSEMBL'T'

SUPPORT BLADES BLADE PITCH CONTROL MECHANISM

Figure 3·22 Axial Flow Fan

The centrifugal or radial fan (Figure 3-23) moves the gas perpendicular to the fan rotor and operates most efficiently in a high head situation. The centrifugal is suitable for a forced-draft or pressurized system where induced draft fa ns are not .

3-42

Figure 3-23 Centrifugal (radial) Fan

3-43

~ -I-:J a.. Z

a: IJJ

~ 0 a..

The centrifugal (radial) fan has several advantages over the axial fan. It is cheaper and lighter and, therefore, requires less power. This can be seen on Figure 3-24. Also, because of its size and weight it is more easily controlled.

J 4

100

90

80

70

60

50

40

30

20

/ ~7 /

,,~ V AXIAL FLOW FAN - / ~

oJ' .".. /

•• ~ ~ V

V' ..,. 7

./ '" --V

RADIAL FLOW FAN 10 I I I I

o 10 20 30 . 40 50 60 70 80 90 100

PERCENT UNIT LOAD

Figure 3-24 Typical Fan Power Curve

3-44

-"*-->-u z UJ -u u.. u.. UJ

The blades of an axial fan are generally smaller than those of a centrifugal fan and the construction is such that a variable pitch control system can be easily installed. This type of control allows for a rapid change of output and increased efficiency over the centrifugal fan as shown in Figure 3-25.

90

80

70

60

50

40

30

20

10

o

I -""-

~ ~ .; ~

V ." ~ AXIAL FLOW FAN /

V' l.. ......

,... RADIAL FLOW FAN -r-

a 10 20 30 40 50 60 70 80 90 100

PERCENT UNIT LOAD

Figure 3-25 Typical Fan Efficiency Curve

3-45

The main disadvantage of an axial fan is that it requires high rotational speeds to generate the required air flow. This can result in noise pollution. However, the primary problem with high speeds is that the fan must be a precision machine which means that it can fail quickly and catastrophically with little or no warning.

In contrast the centrifugal (radial) fan has two primary advantages. It is more durable because of its lower rotational speeds, and it can supply air at high head pressure more efficiently than the radial machine. The axial machine can change the pitch of its rotating or stationary blade. Whereas the blades of a centrifugal (radial) fan are normally fixed. Therefore, a centrifugal (radial) fans blades establishes the fan's use and a particular fan is chosen for a specific system.

There are three basic blade shapes used in a fan: a forward curve, a straight blade and a backward curve. These shapes and the effects on the velocity are shown in Figure 3-26.

The straight bladed fan is generally used for industrial, dust-laden gas flow. This type of fan operates at an efficiency of 50% or less. The forward blade curve is a general purpose fan best used for medium pressure applications. The fan is fairly quiet during operation because of the low tip speed.

The backward curve blade fan can produce higher discharge pressures than the straight blade fan or the forward curved blade fan. Horsepower requirements are maximum at 60% airflow. Above or below 60% airflow the horsepower requirements are less than maximum.

3-46

v vr r---= v

(al FORWARD·CURVED BLADES (bl STRAIGHT

BLADES

v,.,

(cl BACKWARD ·CURVED BLADES

v = Absolute velocity of air leaving blade (sh own equal for all three blade types)

vr = Velocity of air leaving blade relative to blade

Vb = Velocity of blade tip

Figure 3-26 Types of Centrifugal Fan Blades

3-47

Fan Control

Very few instances of operations permit fans to operate continuously at the same pressure and volume discharge rates. Therefore, to meet the requirements of the system, a convenient means of varying the fan output becomes necessary. Common methods of controlling fan output are damper control, variable speed control and inlet vane control. In some cases, a combination of controls are used. Damper control provides variable resistance in the system to alter the fan output. However, damper control is inefficient because of the excess pressure energy which must be dissipated by throttling. The advantages to damper control are:

1. It has the lowest first cost of all control types.

2. It is easily operated and adapted to automatic control.

3. It incorporates the least expensive type of fan drive, a constant speed, induction type AC motor.

4. It has continuous rather than a step type of control, which makes it effective throughout the entire range of fan operation.

Variable speed control is the most efficient method of controlling fan output since it also reduces power consumption. Speed control results in the same loss in efficiently throughout the entire, fan load range. The loss in effectiveness depends on the type of speed variation. Commonly used variable speed systems include magnetic couplings, hydraulic couplings, special mechanical drives, variable speed DC motors, variable speed AC motors and variable speed steam turbines.

Magnetic couplings consist of two windings in a housing with a variable field. A change in field strength varies the slip and consequently the

~ speed of the fan.

3-48

A hydraulic coupling varies slip by varying the hydraulic pressure as the speed of the driver changes. The variable pitch V-belt and the variable speed planetary transmission are examples of special mechanical drives.

Two-speed AC motors can be used to supplement damper control. Two-speed AC motors cost less than the variable speed AC drives and improve fan efficiency when coupled with a simple damper control.

Inlet vane control (see Figure 3-27) regulates air flow entering the fan and requires less horsepower at fractional loads than outlet damper control. The inlet vanes give the air a varying degree of spin in the direction of wheel rotation enabling the fan to produce the required head at proportionately lower power and, therefore, greater efficiency. Although vane control offers considerable savings in efficiency over damper control at any reduced load it is most effective for moderate load changes close to full-load operation. Inlet vane control is often used for full load operation, and efficiency adj ustments.

Inlet vane leakage often makes it difficult to reduce fan air flow at low loads when using a single speed fan drive. Therefore, a supplementary damper is used to increase the control range of the vanes. Th is is especially applicable to forced-draft fans where a wide load range is required.

IN LET VANES

AIR FLOW

Figure 3-27 Inlet Vanes

3-49

Fan Drives

Electric motors are normally used for fan drives because they are less expensive and more efficient than any other type of drive. For fans of more than a few horsepower, squirrel-cage induction motors are most common. This type of motor is relatively inexpensive, reliable and highly efficient over a wide load range. It is frequently used in large sizes with a magnetic or hydraulic coupling for variable speed installations. For some variable speed installations, particularly in the smaller sizes, wound rotor (slip rings) induction motors are used. If a DC motor is required the compound type is usually selected. The steam turbine drive costs more than a squirrel-cage motor but is less expensive than any of the variable speed electric motor arrangements in sizes over 50 horsepower.

Fan Laws

Fan laws were introduced at the beginning of this course and can be stated as follows:

1. Capacity is proportional to the fan speed, or: CFM a RPM.

2. Pressure or head is proportional to the square of fan speed, or SP a RPM'.

3 . Power is proportional to the cube of fall speed, or HP a RPM'.

To help you in understanding how the fan laws are applied, the following problem is provided.

A fan delivers 10,000 cfm at a static pressure of 2 inches of water when operating at a speed of 400 rpm. The required power input is 4 Bhp. Find the speed, pressure and power of the same fan system if 15,000 cfm are desired. Assume constant fan efficiency over varying flow requirements.

3-50

Using the first fan law, speed is proportional to capacity, the new speed can be found as follows: CFM a RPM

so: RPM, = RPM, x CFM., _ 400 rpm x 15,000 cfm

CFM, 10,000 cfm = 600 RPM

Using the second fan law, speed square is proportional to pressure, the new pressure can be found as follows: SP a RPM2

so: SP, = SP, RPM) --::-:c---'--' = 2 in. RPM,

600 rpm 2

400 rpm

= 4.5 in. of water

Using the third fan law, speed cubed if proportional to brake horsepower, the new power requirement can be found as follows: BHP a RPM)

so: BHP, = BHP,

Fan Characteristic Curves

RPM) -="..:..' = 4 BHP RPM, = 13.5 BHP

600 rpm)

400 rpm

Fans are tested by their manufacturers and the results of the fans operations are presented in characteristic curves. The curves may include the variation in head, capacity, power and efficiency for a constant speed or can be a family of curves for a series of constant speeds . By careful review of

3-51

the various types of fans and their characteristics curves the most correct fan for a given system can be selected.

Within a given class or type of fan there are certain general characteristics that are common to the many different designs. These characteristics are power, pressure, and efficiency. The curves in Figure 3-28 show the variation in power, pressure and efficiency for differing capacities at a constant speed for an axial-flow fan. The fairly constant power output over a wide range of capacities is common to most axial-flow fans . Thus, there will be little tendency to overload the driving motor regardless of the change in conditions under which the fan operates. This is called a nonoverloading characteristics. The capacity decreases more or less at a constant rate for an increase in resistance or pressure. The efficiency of such a fan is generally somewhat lower than that of centrifugal fans except at low pressure. By varying such things as the pitch diameter, and width of the blades, the point of maximum efficiency can be varied to cover a wide range of conditions.

w 0: ::J (j) (j)

:2 :::! ::J Cl. :2 0 xZ <t<t :20: u. W

O~ ..... 0 z Cl. WW u (j) 0:0: wO Cl.I

J II I :- SOUND LEVEL-RELA TlV

E Itvr r--, f::....S,1y

11 0 100 90 80 70 60 50 40 30 20

f'I>o. HORSEPOWER ...... ....-........ TOTAL PRESSURE

¥. ~ 2 """-

\>-" ~ -.......~ """ -<-\>-"-td/ S t.<J::--'" "-

c, c,''<: t ic"" "' I-~\,~ "\,c, c,-\ '9",0''''- i\ "' 1-'<: s"\\>- ,,,-":<

/' "c, 0'v~ '9", 10 V "-' I 00 10 20 30 40 50 60 70 80 90 100

PER CENT CAPACITY

Figure 3-28 Axial Flow Fan Characteristic Curve

3-52

The characteristics of a radial-tip centrifugal fan are shown in Figure 3-29. the power increases with a decrease in pressure and an increase in capacity, but the increase is not sharp enough to overload the motor if proper selection of the motor is made. Generally, the characteristics of the radial-tip centrifugal fan will be a compromise of the backward-curve-blade and forward-curved-blade fans.

The backward-curved-blade centrifugal fan will have characteristics as shown in Figure 3-30. Best efficiencies are obtained with rotors having backward-curved blades and the power curve for these rotors shows a nonoverloading characteristic over the complete range of pressures and capacities. The point of maximum efficiency occurs at the point of maximum power. Above 50% of the maximum capacity a increase in capacity will decrease the pressure sharply. This fan is excellent for forceddraft service, for, as the fuel bed of a furnace closes and restricts the flow of air from the fan, the fan pressure will rise sharply. This increase in fan pressure will tend to open the fuel bed to admit more air to the' furnace.

The forward-curved-blade fan has an overloading power characteristic as shown in Figure 3-31. If reasonable care is exercised in figuring the conditions under which the fan will operate, a motor can be selected to prevent its overloading. The point of maximum efficiency occurs near the point of maximum pressure.

The straight-blade fan's discharge pressure rises from full flow to a maximum at no flow, where it falls off, as shown in Figure 3-32. The maximum efficiency occurs near the fan's maximum pressure.

Figure 3-33 represents a modification of a backward-curved-blade fan. The pressure characteristic does not have a steel slope, nor does the horsepower curve have a distinct hump, resulting in maximum efficiency obtainable over a wider operating range.

3-53

140

W 130 ec 120 :J til 110 til

:!w 100 :Jg: 90 :Eo xZ 80 <1:~ 70 :Eec u. W 60 oS 50 ... 0 za. 40 WW u tll 30 ecec wo 20 a.J:

10 0

~ 1'0

Sr ~l.4 ~ ~C ~~ ,0 Ss.

V " ~ (C'.s- ~ -'9 ~ ~(I, "- .-

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~:'\. to I: 1:1 '\"

~~~/ s:::: S l': ,t"-.:.£I to I\' C Y ~I: "1lfc 1\ ~

/ q..,S'c.'?Y I:IC/~~ \.

""~ C)-I'-. ' / .,/ " I\. 'r'f' (lJZ)'\J ,.

10 20 30 40 50 60 70 80 90 100

PERCENT CAPACITY

Figure 3-29 Radial-Tip Blade Fan Characteristic Curve

3-54

PERCENT CAPACITY

Figure 3-30 Backward·Curved·Blade Fan Characteristic Curve

3-55

I

'"

i~~S\\'j \\,,~ \~ _r--

I-SOUNO LEVEL-\l.~\.A a: ::J 1 10 (/) (/) 100

~~ 90 ::Ja..

~o 80 xZ 70 ~~ ~a: 60

'" O~ 50 .... 0 40 Za.. wW 30 u(/) a: a: 20 ",0 a..l: 10

0

,A rO r 4t

" /" ......... ..!:..RfSSUR ./ "" ...... S,. f /'

MECHANICA&C /'

J' S,. ·" V ~ ...... ~/C L ~ C,," EFFICIENCY-

'l'l~~ ./ ....... ""Ie "' ~ s~~o ~ ~C"'~ .

/ 'rIO~';"""-- ......;:. f'ss

~ I , Ol~

It -

10 20 30 40 50 60 70 80 90 100

PERCENT CAPACITY

Figure 3-31 Forward-Curved-Blade Fan Characteristic Curve

3-56

UJ a: 110 ::J Vl 100 Vl

:2 UJ 90 ::Jg: 80 :20 xZ 70 «« 60 :2 a: u. UJ 50 0::: 40 >-0 zo.. 30 UJUJ UVl 20 a: a: wO 10 0.. I

0

rorA, L PR

l/'"" "'" S

-f~sURf ~ rX'" MECHANICAL lie

EFFICIENCY I ~ /'" .....

~ ~O~~~ -,.... V l'\.""-s> --)-...

V Sr ~.s f~~ ..q ric .sv

) ,<,,-o~~,.... 1Clfl\tc» ~ / .."....,.. """ ....

'rl ~

"-0: 1 ~ ~.

10 20 30 40 50 60 70 80 90 I no

PERCENT CAPACITY

Figure 3·32 Straight-Bladed Fan Characteristic Curve

3-57

UJ 1 10 a: ::::> 100 (f) (f) 90

~~ 80 ::::> a. . ~o 70 xz 60 <1:<1: ~a: 50 u.. UJ

O~ 40 1- 0 30 .."a.

- UJ 20 ~(f) u a:

10 ~O a.I 0

1 TOTAL p . ?O\f'J t ?

STATIC PR -;: RfSSURE: 'r\O?St E:SSURf ..... _ 11. . ~ ~ ~~C,y

:i!: .......... :.~I c. ~ 1'/. ~

SiAiICEFFICIENCY...... ~ C~( ~

~, C).-

-' ~ .... ~, .'\...

V If ~ I'\. '\ / R\'\

/ ~ ~

~~ 10 20 30 40 50 60 70 80 90 100

PERCENT CAPACITY

Figure 3·33 Backward-Bladed Fan Characteristic Curve

3-58

DUcrwORK

Air duct is an enclosed conduit through which air is moved from one place to another. In this section of the chapter, we are going to discuss the equipment that is used. The following topics will be covered:

• Classification

• Duct System Accessories

Classification

Air duct design is broken into high and low pressure classifications by the Sheet Metal and Air Conditioning Contractors National Association (SMACNA). Most of the low pressure standards also apply to high pressure work. High pressure standards are intended for heavier industrial systems that require additional structural consideration.

High pressure duct is classified as having a static pressure rating greater than or equal to 3 inches of water. Leakage is limited to 1 percent. Low pressure duct design covers pressures up to 2 inches of water.

Table 1-1 shows the specific breakdown of the SMACNA PressureVelocity classifications.

Table 1-1 SMACNA Pressure-Velocity Classifications

SMACNA Duct Static Velocity Standard Class Pressure Limits

High High Ion > 2000 FPM High Medium 6" > 2000 FPM High Medium 4" > 2000 FPM High Medium 3" < 4000 FPM

Low Low 2" < 2500 FPM Low Low 1" < 2500 FPM Low Low as' < 2000 FPM

3-59

The use of the velocity to classify duct construction and design is not common. Velocity classifications are used to describe the system or individual duct run. Traditionally 2000 feet per minute is used to separate high and low velocity classifications.

Duct System Accessories

Several accessories are used in the process of distributing the air to make the operation more efficient. These include dampers, louvers, vents, diffusers and silencers. Each of these components performs a specified function within HV AC air ducting.

Turning Vanes

Turbulent air flow increases the amount of friction encountered in the movement of air. By minimizing turbulence, thereby creating laminar flow, the overall efficiency of the HV AC system is increased.

Turbulence occurs when changes in flow direction are encountered. Figure 3-34 graphically indicates what happens to air flow through a typical elbow. The air entering the elbow is laminar. The portion of the flow which travels along the outside edge of the elbow follows the curve of the outside wall. Air entering on the inside edge continues straight until it runs into the air stream on the outer edge. This collision sets up eddies which increase the friction loss.

To overcome this situation vanes can be installed to direct the air flow around the elbow (Figure 3-35). The vanes create a series of smaller elbows which reduce the amount of turbulence. In some rare instances, splitters can be used to actually establish smaller elbows (Figure 3-36). Splitters and vanes are also used to maintain a laminar flow with uniform pressure distribution when a tee or other fitting is near the downstream side of an elbow. This process is shown in Figure 3-37.

3-60

Figure 3-34 Turbulent Air Flow in Elbow

Figure 3-35 Reducing Turbulence with Turning Vanes

3-61

I~--;- RADIUS

/'

WID/TH /' I DEPTH

tV

Figure 3-36 Reducing Turbulence with Splitters

3-62

I

~ I

~ ~~! • • ~! • ~~ . •

---.. •

Figure 3-37 Pressure Distribution with Vanes

3-63

Dampers

Dampers are used to limit the amount of air low through a duct ·or piece of equipment. Three basic types are found in HVAC distribution systems: parallel blade, opposed blade and pivot blade. Figure 3-38 shows a schematic representation of each basic type.

,

\/ \ \/'. - , - ; , ,

\/ \ i': , , - , /\: ~ -, ,

\/ \ \/\ I , - , / -,

,'~: \/\ PIVOT

, - , --, -,

\/\ \/\ , ,

""RALLE L OPPOSED 3D,DE Br..;"OE

/

/uv' /.1IIIf1'V r ~ O\v \

Figure 3·38 Damper Types

3-64

Parallel blade dampers normally are used when only full-open or fullshut conditions are required. When parallel blade dampers are in a partially open position they tend to direct the air flow to one side of the duct, thus causing uneven pressure distribution.

Opposed blade dampers are best suited for situation where the air flow (volume) is to be regulated. They do not create the uneven air flow that a partially open parallel blade damper does. Instead a mixing or turbulent condition exists.

Pivot (or splitter) dampers generally are used to direct desired air volume flows at a duct branch (Figure 3-39). Pivot dampers are not commonly used due to the force required to move the damper. Parallel and opposed blades have approximately equal forces acting on each side of the rotating axis (Figure 3-40).

------=:::,,""-£)~ - - 1

\ \

'--""-.. __ -1 ././ ______

Figure 3-39 Pivot Damper at Branch

3-65

'.

INDIVIDUAL PARALLEL OR OPPOSED BLADE

NOTE:

AIR FLOW = PIVOT BLADE

( NO EXTERNAL STABILIZING -----FORCE REQ UIRED )

STABILIZIlIG

FORCE

DASHED ARROW INDICATES VELOCITY PRESS URE FORCE VECTORS AT BLADE AXIS IS NOT SHOWN

Figure 3·40 Forces Acting on Damper Blade

Louvers

Louvers are similar to dampers in appearance. The blades, however, are fixed. Louvers are installed where intake or exhaust air is vented through the external walls. They may be ducted or simply used for ventilation. Their primary function is to keep rain and snow from entering the building. In addition, louvered openings are often equipped with screens or mesh to prevent insects, birds, animals and trash from entering the ventilation system. Figure 3-41 shows a typical louver configuration.

3-66

fRN1E

.~======I

~l=====l t 4"TYP

LOUVC:? BLADE

SCREEN OR MESH

Figure 3-41 Typical Louver Configuration

Grilles, Registers and Diffusers

At the tenninal ends of an air duct system (where the conditioned air is withdrawn from or introduced to the controlled environment) vents are installed to control the distribution and collection of conditioned air. The term "vent" is a general expression for any apparatus which permits transfer of air due to pressure gradient.

In HV AC distribution systems, the term grille applies to a flushmounted grid. A typical grille is shown in Figure 3-42 when dampers are added to the duct side of a grille, the assembly is known as a register. A typical register is shown in Figure 3-43. Grilles and registers are used on both supply and return duct systems.

3-67

I '

I I

l ,

; ,

i : [

Figure 3·42 Typical Grille

3-68

. ~

~ ~

IEEEEEEEEEEB

Figure 3-43 Typical Register

3-69

When the blades of a grille are arranged so the air flow is spread out and distributed into the controlled environment, it is known as a diffuser. Diffusers may be fixed or adjustable blade variety. Diffusers are only used on the supply side. Figure 3-44 shows a typical ceiling supply vent with a diffuser and damper.

, .\ l

(- I~

.... ~ ....... " "-/ '\ /

/ \ I \ I / \\

/ ~

Figure 3-44 Ceiling Supply Vent with Diffuser and Damper

3-70

Silencers

Fans and air flow can create unwanted noise which is carried by the HV AC duct system. To eliminate this noise, devices are installed to baffle and adsorb the sound. The most basic of the silencers is an expansion box with the entrance and exit ports skewed as shown in Figure 3-45. Other silencing methods include encasing noisy equipment in insulated boxes and lining the inside of the duct with insulation. Noise generators produce sound waves at a reduced pressure variance and, therefore, impede the movement of discernable noise.

AIR ~ fLOW~

SHEST METI\L EOX WI TH rN SULAT I O~

BOARD r~I S I DE

Figure 3-45 Basic Silencer Box

3-71

, ,

,-I "

, I ,,,

SUMMARY

This chapter has discussed the equipment used in heating, cooling and air-handling in HVAC systems.

Along with this discussion, we also looked at the criteria used to select this equipment. It is important that you understand how this equipment is used and how they basically operate. This information will be of value to you in understanding how a HV AC system operates when all this equipment is used together in a system.

3-72

, ,

' ..

.,

CHAPTER FOUR FIELD INSTRUMENTATION OVERVIEW


OBJECTIVES

Upon completion of this chapter, the student should be able to:

1. Discuss the operation and applications of the following air flow measuring devices:

a. Manometers b. Pitot tubes c. Pressure gauges (magnahelic) d. Anemometer e. Smoke devices f. Venturi tube and orifice plate

2. State the purpose of and methods used for performing duct traverses.

3. Perform traverse calculations including actual air flow, temperature, altitude, and barometric pressure corrections.

4. Identify various temperature ineasurement devices and describe their construction and operation.

5. Given two known variables, find all other air properties from the psychometric chart.

6. Define "dew point" and describe the operation of dew cell measurement instruments.

7. List the electrical parameters measured on HV AC components and the methods used for each measurement.

8. Identify the instruments used for measuring the speed of HV AC motors, compressors and fans.


INTRODUCTION

The instrumentation necessary for HV AC work varies with the extent to which you get involved with the maintenance and testing of HV AC systems. Instruments for the measurement of air flow, water flow, rotational speed, temperature and electricity are tools of the trade for those who install, test and balance HV AC equipment. Those tasked with long term maintenance may also employ vibration measurement equipment in predictive maintenance or noise testing equipment to ensure that the noise from fans, blowers, etc. does not exceed legal limits for noise in the work place.

Like any other tools or equipment, their usefulness depends on proper operation and handling. Many of the instruments we are about to discuss are delicate and require special care in storage, transportation and use. Always follow the manufacturer's recommended intervals for calibration, and perform a cal-check whenever the operation of an instrument is suspect. Some of the instrumentation covered is of extremely simple construction and has been in common use for decades. Modern electronic equivalents with direct digital readout are also available which have several advantages in terms of set-up ease, readout resolution, accuracy, etc., but are usually more expensive.

AIRFLOW MEASUREMENT DEVICES

U-Tube Manometer

The U-tube (Figure 4-1) manometer is a simple and useful means of measuring partial vacuum and pressure, both for air and hydronic systems. It so universally used that both the inch of water and the inch of mercury have become accepted units of pressure measurements. A manometer consists of a U-shaped glass tube partially filled with a liquid such as tinted water, oil or mercury. The difference in height of the two fluid columns

4-1

denotes the pressure differential. U-tube manometers are recommended for measuring pressure drops above 1 in. w.g. across filters, coils, fans, terminal devices, and sections of ductwork. They are not recommended for readings of less than 1.0 in. w.g. because of poor resolution in that range.

OVER-PRESSURE TRAPS. WITH

~ SHUT·OFF COCKS

Figure 4-1 U-Tube Manometer Equipped with Over-Pressure Traps

Inclined/Vertical Manometer

The inclined and/or vertical manometer (Figure 4-2) for airflow pressure readings is usually constructed from a solid transparent block of plastic. It has an inclined scale that expands or lengthens the scale for a given amount of fluid displacement, increasing resolution and allowing more accurate air pressure readings from 0 io 1.0 in. w.g. and' a vertical scale for reading greater pressures.

•

Figure 4-2 Inclined-Vertical Manometer

4-2

All air pressures are given in "inches of water", which means that the air pressure on one end of a U-shaped tube is enough to force the water higher in the other leg of the tube. Instead of water, this instrument uses colored oil which is lighter than water. This means that although the scale reads in inches of water, it is longei than a standard rule. Whenever a manometer is used, the oil must be at room temperature or the reading will not be correct. The manometer must be set level and mounted so it does not vibrate.

The manometer (or inclined draft gauge) is the standard in the industry. It can be read accurately down to approximately 0.002 in w.g. and contains no mechanical linkage. It is simple to adjust by setting the piston at the bottom until the meniscus of the oil is on the zero line. This instrument is used with a pitot tube or static probe to determine pressure or air velocity in a duct.

Micro-Manometer

The micro-manometer is used in air system work to read accurately very small differences in pressure. There are several types used, but the most common contains two glass vials about 2 to 3 inches in diameter. A skilled technician can locate one hook relative to the other within' +0.001" in .w.g." e •.. ~

The pointed needle or hook is adjusted until the point "dimples" the water surface but does not break the tension. This instrument often is difficult to use in the field because of its stability and leveling requirements, and where the pressure has pulsations. The electronic micro-manometer is somewhat easier to use.

Pitot Tube

Construction

The standard Pitot tube, shown in Figure 4-3, is used in conjunction with a suitable manometer, to provide a simple method of determining the air velocity in a duct. The Pitot tube is of double tube construction, consisting of an inner tube which is concentrically located inside of the outer tube. The

4-3

outer "static" tube has 8 equally spaced holes around the circumference of the outer tube.

I"~------ 5"· 16D-------.~ I ~.~---2''1 • 8D---~~ ....

~+ I- i I: OOA

I

I

/

A ~ : c-kL ___ ~' ---,' , ~[·, ·3-+~, §§, A-~~E.'-·~g~5·uT~;;a§j%\;dIiHZ::;jj~;fu:~~~',~ ,ei t ' ' 0

IS"

,.R @ "'-t ", a HOLES-O 04" CIA. J2 RAO

/

'~.' " EOUALlY SPACED NOSE SHALL BE FREE W FREE FROM BURRS FROM NICKS ANO BURAS

SECTION A-A

other-Sizes 01 Pltot lubeS when reqUIred. may be !xlIII uSing the same geometriC proporuons WI!h the e~cep"on thaI the SialiC onhces on sizes larger Ihan

INNER TUBING-APPROX. 1:8"00, 21 sa. SGA.

standard may not exceed .04 " '" diameter. The ITIInllnUm Pll01lube stem (llameler recogrllZeo unoer IhlS coae shall be . 10" , In no case snail me stem diameter exceed ' ;30 cllhe lest duCI diameter

I'r-----'~- STATIC PRESSURE __ OUTER TUBING

5< 16" 00 . APPROX. 18B& SGA.

'-TOTAL PRESSURE

Figure 4-3 Pitot Tube

Both tubes have a 90 degree radius bend in them located near the measuring end to allow the open ended inner" impact" tube to be positioned so that it faces directly into the airstream when the main shaft of the Pitot tube is perpendicular to the duct and the side outlet static pressure tube outlet connector is pointed in a parallel direction with airflow facing upstream,

4-4

The pitot tube is actually a head-type flow element which measures fluid flow by creating a differential pressure. Figure 4-4 is an exaggerated view of a pitot tube, showing how it functions inside of the duct. The inner tube is· sometimes called the impact tube, or the total pressure tube, while the outer tube is the static tube. As we began to explain, the pitot tube creates a difference of pressure to measure flow (or, more directly, fluid velocity). Per Bernoulli's equation, the square root of the difference in pressure is proportional to flow. From Figure 4-4 we can see that the difference between the total pressure (fp) and the static pressure (Sp) is the velocity pressure (V p). The relationship between the pressures is expressed by the equation Tp = Sp + Vp.

" s, I c= ____ , --AIRFLOW , Tp-, ____ "\ ... .

, t II

" s, "

I I

I· T, ·1 I I . I I

·1 ..

I I • s, v, I I s, I !

How Preu uf' is lIIIerled on a Pilot Tube .

Figure 4-4 Pressure Relationships and the Pitot Tube

Pitot Tube Use

The Pitot tube is used for the measurement of airstream "total pressure" by connecting the inner tube outlet connector to one side of a manometer; for measurement of airstream "static pressure" by connecting the outer tube side outlet connector to one side of a manometer; and from measurement of airstream "velocity pressure" by connectors to opposite sides of a manometer or draft gauge. This instrument is commonly used with a draft gauge, manometer or micro-manometer. The pitot tube is a most reliable and rugged

4-5

instrument and is preferred over any method for the field measurement of air velocity system total air, minimum outdoor air and maximum return air quantities, fan static pressure, fan total pressure, and fan outlet velocity pressures.

Several shapes and sizes of Pitot tubes are available for different applications. A reasonably large space is required adjacent to the duct penetration for maneuvering the instrument. Care must be taken to avoid pinching the instrument tubing.

If static pressure, velocity pressure, and total pressure are to be measured simultaneously, three draft gauges are connected depending on the specific application.

. .... " .... •

"'0'--'U., f'~'----_../

,. '-----Lf

A) Pitot Tube Connections for Supply Airstream

~.

y ", '0

", -.. ,.

• .. ·IOW •

~

(: / ~~ /). /.-"-. .. -............

B) Pitot Tube Connections if Airstream is Exhausted from Duct & TP is Positive

Figure 4-5 Pitot Tube Connections

4-6

'''''.

~ ,, -." ". ,

'"

•

"

MOl'lOW ) / " -._ .. _.- UI ('

~. .... ... ,

..... " ..... "" J '"

C) Pitot Tube connections if Airstream is Exhausted from Duct & TP is Negative

'.H' uN UIIU llSllIl(l1OII SIIIIC HI""

flOW __

.. , IU", I' "

D) Filter Drop Hook-up

Figure 4-5 Pitot Tube Connections (Con' t)

4-7

INTAKE

IMPACT CONNECTION

, , \

, , , + , ,

FAN

MANOMETER

-------

+

DISCIIARGE

mPACT CONNF.CTION

E) Fan Total Pressure Hookup

INTAKE

, - : . ,

FAN

. '. IMPACT I ..... -- .. ' i. 1.

CONNEC TI ON - I::::] = [ibx::J MANOMETER

. - -+

DISCHARGE

. , .- ~-. ' STATIC

CON NECTI ON

F) Fan Static Pressure Hookup

Figure 4-5 Pitot Tube Connections (Can't)

4-8

r , i In conducting tests, it frequently is sufficient to measure only two of

these three pressures, since the third one can be obtained by simple addition or subtraction. Care must be taken, however, so that the signs of the various pressures are correct; supply duct pressures are positive, return and exhaust duct pressures are negative.

If the airstream is exhausted from the duct, the static pressure is negative and the hose connections will depend on whether the velocity pressure is larger or smaller than the numerical value of the static pressure. If it is larger, the total pressure will be positive; if it is smaller, the total pressure will be negative (see Figure 4-5).

The various connections between the Pitot tube and the draft gauges are frequently made with rubber hose . Caution must be used to ensure that all passages and connections are dry, clean and free of leaks, sharp bends and other obstructions. The branch ing out of the rubber hose can be accomplished by the use of a T-tube or by the use of a 4-stem nipple adapter which can be purchased as an accessory to the draft gauge.

Use of Readings

Airflow velocity pressure (Vp) readings are obtained which can be converted to velocities within the duct by using the following equation:

Where:

Example No.1

v = 4005 .;v;,

Duct velocity (fpm) Velocity pressure (in. w.g.) at standard air conditions

What is the duct velocity when the measured velocity pressure is 0.25 . ? . In. w.g ..

4-9

Solution

v V

= =

4oo5.[V; = 4005 .t 0.5

4005 10.25 = 2003 [pm

To save time, a table like Table 4-1 which lists the "velocity vs. velocity pressure" equivalents is often used.

Table 4·1 Velocities vs. Velocity Pressures

v._ v._ Ptt'uur. Velocity

Veloci!v P'ftsure Velocity

v_. v._ VelOClly PrHsure V"loeity Pr.SSUl'e Ve-Ioc.ly P'enur l

""" on. wg. ",m ,no "'9. """ " . ., ''''' on, wg. 'Om m., 300 0.01 20 .. '" '"'' ' .90 !OSSO 1.512 "" 3.32 ,SO 0.01 "" 0.27 "" '92 58" 1.95 " .. 3.37 ." '" " .. '29 ,goo 0 ,95 58" .. " "" '" <5, 0.01 2'" '" "" 0.9] "" 202 H50 '" 500 0.02 22" 0.32 "" .. " "" 2." "" 3.51

'SO 002 2300 '03 " .. .. 02 58" 2.10 "" '55 600 002 " .. ,." "" U). 56" 2" "" '50 'SO ,OJ "" ,." .... 1.07 ,goo '" "" ' " ,,, '03 " .. 0.37 .'" 1.10 59" 2.21 ))00 '" ,,, ,0- 2500 0.]9 " .. 1.13 '000 ". ))" '" "" ,0- 25" '" '300 '" 6OSO 2.28 , .", '" ." '05 2600 0 .42 " .. '" "" 2.32 '"SO , .. 900 ' 05 28" , .. "00 1.21 "" ". '900 '89 ... ' 06 2>00 0.45 .. " '" 6200 '" 7950 ". '000 '06 2> .. 0 .. C7 '500 1.26 625' 2.43 .'" ] .99

" .. ,0> 2800 0.49 ., .. 1,29 5300 ", 8050 ". "" 0.08 28 .. 0.51 .'" 1.32 6350 2.SI "00 '" " .. 0 .08 2900 ' .52 .... 1.35 '''' 2.55

I "" ". '''' 0.09 29 .. ,." "" "JO .. " 2.S11 .'" 4. Ig

>2" 0.10 '''' ' .56 " .. 1.41 '500 '" 82" .,. "" '" JO .. 0.51 .. " ". " .. 2.67 .JOO '" " .. '" "" '50 .,,' , .,

'600 2» .'"' '" "00 0,12 " .. 0.62 .'" .." "" '" .. " .. , "" 0.13 '''' , .. 49S0 .. 53 "" 28' .... .. , ' 500 0. 1' J2" ' .66 sooo ' .56 "" 28' ."" ... )5" 0.15 "" ' .68 S050 1.59 ''''' 2,88 " .. '56

'''' 0.16 "" 0.10 5>" 1.62 68" 2.112 .600 ." >6 .. ')) ,." 0.72 "" '" '900 2.97 86" .. , ))" 0.18 '<5' D.H 5200 '" 69" '" ,>0, '" )) .. 0.19 35" 0.76 525' U2 ,'" 3.05 " .. '" )8" 02' '" .. 0.79 "''' '" '05" 3. 10 .. " <8, )8" 0.21 "" 0.81 "' .. 1.78 )),. '" .. .. ." "00 0.22 "" ,." ,.,. "2 )) .. 3. 19 89" .,. "" 0.24 "" 0.85 , ... 1.85 >200 '" 8950 ." 2000 '25 " .. , ... ''''' 1.89 >2 .. 3.28 9000 ,OS

. ~ ("""')' Veloetty • 4005 V v . (Of) V, - 4005

4-10

To calculate t e actual air flow (cfm) in the duct, a series of velocity pressure readings are made, changed into velocity readings, and averaged for an average duct velocity. The cfm is calculated by the following equation:

Airflow (cfm) AxV

Where: A V

Example No.2

Area of duct cross-section (sq. ft.) Average Velocity (fpm)

A 30 " X 24" duct has an average velocity of 1825 fpm. What is the airflow rate?

Solution

Airflow A x V = 30 x 24 x 1825 144

= 5 x 1825 = 9125 cfm

Pitot Tube Duct Traverses

If the velocity of the airstream under measurement were uniform, one reading at any point would be sufficient. However, the air moving along a duct wall loses speed because of friction, consequently the velocity in the center of the duct will always be greater (assuming no special turbulent motion). Since the velocity pressure is seldom uniform, a series of readings must be taken across the duct section - called a duct traverse.

Various industry groups have described methodology for uniform measurements in round ducts (tangential traverse) and square ducts, and in the paragraphs that follow, the consensus that has been reached on proper technique is described in the following paragraphs and figures that follow, but here are some general pointers to remember when performing a pitot tube traverse:

4-11

\

1. Ensure that a large enough space adjacent to the duct is available for maneuvering the pitot tube.

2. Drill the holes for the pitot tube in a clear, straight duct section providing at least 8 duct diameters upstream and 2 diameters down stream of the pitot tube free of elbows, transitions or reductions.

3. Drill probe holes 9/16 inch diameter (assuming standard 5/16 in. pitot tube) to accommodate tube movement without chafing.

4. Plug holes when finished with snap buttons and square of duct tape. (#5 bottle corks are often used in lined ducts.)

5. Check the impact and static holes regularly for plugging (particularly with insulated ducts).

6. Check tubing for crimping or kinking, and especially at connection ends for leaks.

7. Check gage or manometer for zero reading at level before each set of traverses. Keep the manometer level.

\ ' \""'Velocity pressure readings are taken at equal intervals over a cross

section of the duct. Good practice indicates not less than 16 readings in any duct and in larger ducts readings should be taken on not less than 6" centers. The velocity pressures are then changed to velocity values, added together and divided by the total number of readings to get the average velocity. Do not average the velocity pressure readings. It is not unusual to make a negative pressure reading in ducts with considerable turbulence. The negative readings are added in at zero value but are counted in the number of readings to obtain the average velocity. Assume a duct 16 positive pressure readings and 4 negative pressure readings. The 16 positive pressure readings would be added together and averaged by all 20 readings (16 positive plus 4 zero readings).

4-12

, .

Round Duct Traverses

For round ducts, the tangential method is the most common traverse. The duct is divided into N zones of equal area by concentric circles of radii, R

" R2> R" etc., as shown in Figure 4-6. A series of ten readings is then

taken along the horizontal axis, and ten readings are taken along the vertical axis. One practical aspect to be considered is how do you know where the pitot tube is inside the duct? As Figure 4-6 shows, reading positions are calculated from the center of the duct, as some position multiplier times the radius of the duct. This is fairly common practice in the industry. Table 4-2 shows the calculated distance from the inside wall to the pitot tube reading point for several duct sizes. Figure 4-7 shows a pitot tube marked for a 20" diameter duct traverse, as per Table 4-2. The pitot tube should be marked carefully with a China marking pencil or small strips of duct tape to facilitate accurate placement of the tube for traverse readings.

PIlOT lUBE STATIONS INOICAIEO BY 0

ROUND OUCI R[CTANGUlAR DUCT

Figure 4-6 Tangential Pitot Tube Traverse

4-13

Table 4-2 Calculated Ten Point Pitot Tube Traverses for Round Ducts 12 to 40 in.

Trnern Point Humber 1 2 3 4 5 6 7 8 , Multiplier: Distant. Irorn Intlde Wlilio Pilot Point .025 .083 .146 .225 .342 .647 .774 .855 .918 Pipe Diameter

12 " " 1 ", 2l\ 41> 7% '" lOY, 11 z 13 0 " 11> 2 3 411 81> 10 11' 11 V. • 14 • \I I I> 2 3\-\ 4;-. g'/, IOV, 12 12V.

• IS • \I 1114 2~ • 3" 511 9r, 11'/, 12* 13* 16 > \I' I ~ 2\1 3>1 5\\ lOY, 1211 1311 14* • • 18 " I> II> 211 4V. 6\\ , In ... 13r. 1S1i 161>

20 0

" I> III 21> 4V, 6Ya 13" IS\\ 17\\ 1811

" 22 " 1I III 3V. • 5 7\\ 141(: 17 18~ 20V.

24 • II 2 3V, 511 8~ 15* 18\\ 2011 22 •

26 0

~ % 2V. 311 51,1. av. 17'/. 20V, 22';' 23V.

28 z 1I 21f. 4" 611 ,Yo 1811 21'1. 23'1. 25*

30 • II 2\\ 411 611 IOV, 19* 23V. 2S1I 27V, 0

32 • II 211 4\1 7~ II 20V. 24* 27~ 29l. ~

34 • V. 2V, 5 7\1 11\1 22 26Y. 29 3 1'1. • 36

x V. 3 SYO 8V. 12v.. 23'1. 27'4 3m'. 33 u z

38 I 3\\ SV, 811 13 24 II 2911 32\', 34 V,

40 I 3;, 51,1, 9 131\ 25',', 31 34\. 36~'.

10

.975

II I>

12~'

13'1,

14'1.

15'1,

17 V1

19'h

2B',

23%

2S1I 27V,

29V.

31 v. 331,<,

3S

37

39

• IlIC~c c~kulat~t.1 in(h( .~ fm markin~ the I'ito l Tube' for 3 tcn ~tation IIner.<r arr wO lked 10 Ihe ncarC1l cil!hrh •. for .... ucts ollit! thi n li<lcd . u~c the muhil'l iu . All figure, show" are in in(hn di ~t ~'Ke 1(1 Ihe illsidc w~1I

I - - \

I I I 1"- /, , , I I

I I I PITOT TUBE

I I I I I MARKINGS I

.:- I I I I I I I I , i"'''.~ I I I I I I I I i , ... -1 I I I I I I , -!"-- I I I I C .~ I I " ...

,,~ , I I ,

I II .. · I I , " .. - , I

I. "",. ,

Figure 4-7 Marking the Pitot Tube

4-14

. ~ .,

l

SquarelRectangular Duct Traverses

For square or rectangular ducts, traverse holes are drilled in one wall of the duct in such a manner as to establish equal areas as shown in Figure 4-8 for a 48 in. by 36 in. rectangular duct. Note that no reading location is more than six inches from another. The 48" x 36" duct in Figure 4-8 would be measured at 48 equally spaced stations, and the pitot tube would be marked accordingly to facilitate those readings. As with circular ducts, tables are available for square/rectangular ducts which eliminate the need to calculate the reading position. All velocity pressure readings would be recorded and transferred into velocity values, added together and averaged.

I' 1-' +, +, +, +, +, +' -t' 1 ! • • • • • • •

,-"-,

• • • • • • -+-, • • • • • • • -+-

~ ,

! • • • • • • • • -+- 01° , • • • • • • • • + frvVV' " ~ , • • • • • ~ '7" , ,.;0

'NSlfll,l"'EtH PQflIS-

• .. • 0 '8 '"'e ASI,IJl'NG STATIONS IN A _8 J6 over

Figure 4·8 Rectangular Duct Traverse

Note that testing/balancing professionals normally use traverse report forms to record test point pressures/velocities and a map of traverse readings to correlate the data and to act as a worksheet for determining reading locations. Sample report forms for rectangular and round duct traverses are included at the end of this chapter.

Example No.3

The following velocity pressures were obtained by making a horizontal duct traverse of a 20 inch diameter duct. Find the following:

a: Average velocity __ _ b. Air flow (cfm) ___ _

4-15

(fpm)

Vp (in. W.G.) 0.46 0.50 0.52 0.50 0.49

Solution

Vp (in. W.G.) 0.45 0.51 0.51 0.55 0.60

The following velocities were obtained from Table 4-1 and Equation 4-1.

Vp (in. W.G.)

0.46 0.50 0.52 0.50 0.49

V ave.

Airflow

Air flow

Velocity (fpm) Vp (in. W.G.)

2716 0.45 2832 0.51 2900 0.51 2832 0.55 2800 0.60

28.550 = 2855 fpm (a.) 10

AxV= r xV

144

3.14 (1OZ) x 2855

144

2.18 x 2855 = 6229 cfm (b.)

Velocity

2700 2850 2850 2970 3100

Correcting For Non-Standard Conditions

Table 4-1 equates velocity pressure to air velocity at standard conditions, i.e., air at sea level, at 70"F, 29.92 in. Hg, with a density of .075 lbs/cubic foot. The temperature of the air under test obviously affects its density, the air becoming less dense as it increases in temperature. To correct for air at other than 70"F use the following equation:

4-16

" . v

where:

.075

d

Actual Velocity Measured Velocity Actual Air Density at Test Temperature

Table 4-3 lists the Temperature-Density Corrections for Dry Air (at 29.92 in. Hg) for various temperatures. To correct for non-standard air the correction factor in Column 6 is multiplied by the measured velocity.

Table 4-3 Temperature-Density Corrections for Dry Air Atmosphere Pressure

2 , 4 5 6 _ . DEGREOS ~CREC:S DENSITY VO,UI1 E ~HSITY CORREC1'IOH FAREHIIEIT CELSIUS LBS/FTJ F'T Ill) R"Tto FACTOR'

)2 0 .00°7 12.)8 1.08 0·96 40 4.4 .0794 12·.59 1.06 0 . .,

'" 21.1 .0149 l).~ 1.00 1.00

100 )7·8 .0109 14.10 ·95 1.0)

150 66.0 .0651 15·)6 .81 1.0, 200 9' .0602 16.62 .80 l.IZ

250 121 .0559 17·88 ." 1.15

)00 1'9 .052) 19.1) ." 1.20

'50 l?? .049<> 20·)9 ." 1.24

400 204 .0462 21.6,5 .62 1.27 450 2)2 .04)0 22.98 .,. 1.)1

500 260 .04 14 24 . 1 ? ." 1.)5

550 288 .0)92 25·48 ·52 1.)9

600 ,16 .0)1.5 26.69 ·50 1.41

"" )4' .0)58 21·95 .48 1.44

"'" J71 .O~2 29·21 .46 1.47

150 '99 .0)28 )0 ,It] .44 1. 51

800 421 .0)15 J1. 7J .42 1.54

850 460 .0)0) )2.99 .40 1.!8

"'" 482 .0292 J4 .24 ·'9 1.60

950 , , 6 .0282 )5·,51 .J? 1.&1

1000 5)8 .0212 )8.,. .)4 1.71

' Tn < .. "et. f .. r "0""'''.1 .. <,1 ' ''. m~l"rly 'ho CO" • • I;on (", M (,om «.Iu"." 6 ~1 ,he 1',11., ,wI>< n"' •• ~ •• d ~tl<>C"1.

4-17

Table 4-4 lists the Altitude-Density Corrections for Dry Air at 70"F for readings taken at other than sea level. The correction factor from Column 6 is multiplied by the measured velocity.

Table 4-4 Altitude-Density Corrections for Dry Air at 70°F

ALTITUDE, I NCKES or DENSITY VOLUME DENSITY COflRECTION

FEET MERCURY LBS/F1'J IT) /IJl RATI O FACTOR

-1000 )1 .02 .0775 12·90 LO) 0 ·913 Sea l evel 29·92 . O~9 1) .69 1. 00 1.00

500 29·)9 .07)5 1) .60 .98 1.01

1000 28.86 .0721 1),87 .,6 1. 02

1500 28. )) .0708 14.12 ·'5 1.0)

2000 27·8.2 .0695 14·)9 .,) 1.04

)COO 26 .81 .0670 14·92 .8, 1.06 4000 25·84 .0646 15.4a .86 1. Of

5000 24 .89 ,0622 16.08 .8) 1.10

6000 2).98 .0600 16.67 .80 1.12

;>000 2) ,09 .0577 I? .J) -77 1. 14

8000 22.22 .0550 18.18 ·7) 1.17

9000 21 .)8 ,°5)0 18.87 ·7' 1.19 le,COO 20 . 58 .0510 19.61 .68 1.21

"To co ,n:C1 for non·,la"d .. d I i, . mult iply 11M: corn:Cllon faa o. (rom column (, b~ I". Pilol lllbc ,....ull •• d YdOP1~.

4-18

, .. , Finally, changes in barometric pressure due to weather conditions, an

unknown height above sea level, etc. will effect the density of the air being tested, and must be corrected for. Table 4-5, Barometric Pressure-Density Correction for Dry Air lists the density of air at 25 different temperatures and four different barometric pressures. The applicable correction factor can be obtained from corresponding densities on Tables 4-3 and/or 4-4. If the barometric pressure at test conditions is not shown on Table 4-5, then actual air density can be calculated as follows:

Density 1.325 x barometric pressure (in. Hg) air temp. COF) + 460"

This value for density is then used in our correction equation:

v V .075

mX d

Table 4-S Barometric Pressure-Density Correction for Dry Air

""'ROMETIIIC 'IlESSUIlE. INCHES OF WEIlCUR'I'

DEC REES FAHRENHEIT !9 .9!" 29.jO 29 .00 lI. SO

COIlRESPONDINC ALR DENSITI ES

" 0.016 o.ou 0.0" o.on

'" 0.015 o.u.) O.U.l 0.010 !O F o.on o.os! 0.010 0.019

'" 0.0' 1 . 0.0.0 0.019 0.071

'" 0. 019 0.011 0.011 0.016

'" 0.011 0.011 0.015 0.01' .. , 0.01 6 0.015 0.0 14 O.OH

'" O.OH 0.014 o.on 0.011

." 0.01' O.OH 0.011 0.070

"" o.on 0.011 0.010 0.069

'''' , 0.01 1 1I.Ql0 0.069 0.061

1I0 F 0.01 0 0.069 0.06 ' 0.06 5

nO F 0.1l" n.on , .... 0.06 5

IlO F 11.061 0.066 O.II6S 11.\164

1'0 F 0.066 11.06 5 , .... O.OU

I SO F 0.065 O.1l64 ,,., 0, 061

160 F 0.064 h.ll6l 1I.06! 11.061

110 F 0.06} 0.061 0. 06 1 0.060

"" 0.062 O.Oil 0.06 0 0.059

"" 0 .061 11.\160 0.059 o.on 200 F 0.060 0.059 O.OSI O.OH !IO r 1).05 9 iI.on O.O SI 0.0S6 no r 0.0" o.osa O.OH 0.0S6

!lo r O.Ola Il.O" 0.OS6 U H l,or O.OH 11.0" o.oss 1"1.054

"29.' 1;' ... f>d"~ b. ffI .... '~ prn.~",

4-19

When correcting for non-standard temperature and barometric pressure conditions, the individual correction factofs are multiplied to obtain a total correction factor.

Example No. 4

.; - " .:r" ~r

An exhaust system's average velocity as read by traverse is 3000 fpm. The system is being tested at an altitude of 4000 ft and the air temperature measures 250°F.

Find: the actual corrected air velocity

Solution

a. From Table 4-3, column 6, the temperature correction factor for 250"F is 1.15.

b. From Table 4-4, column 6, the altitude correction factor for 4000 ft above sea level is 1.08.

c. Multiplying both factors: 1.15 x 1.08 = 1.242

d. V = Vm x correction factor v = 3000 x 1.242 = 3726 fpm, actual velocity

To summarize: to correct for non-standard air conditions, proceed with the pitot tube traverse as if it were for standard air:

1. Record the velocity pressures, Vp, for each point.

2. Convert each Vp to velocity in fpm.

3. Total all velocities.

4. Divide the total of the velocities by the number of Pitot tube readings to find the average velocity .

4-20

5. Multiply the average velocity by the necessary correction factors to find the actual corrected velocity for the variance in air density.

6. Multiply the corrected velocity by the duct area in square feet to find the actual airflow in cfm.

Some rules of thumb for rough calculations:

1. To find the barometric pressure where the elevation is known (but not on Table 4-4) use the approximated correction of 0.1 inch pressure reduction for each 100 ft. above sea level, eg: 1780 £t/100 ft x 0.1 inc. = 1.78 in. Therefore, barometric pressure can be approximate as 29.92 in. - 1.78 = 28.14 in. Hg.

2. Allow a 2% increase in average velocity for each 10 degrees above 70"F.

3. Another altitude approximation is to allow a 4% Increase In

velocity for each 1000 ft altitude above sea level.

Pressure Gauge (Magnehelic)

The magnehelic gauge (Figure 4-9) is an easy to use pressure gauge for air system work which has many different pressure ranges from 0 to 0.25 in. w.g. up to 0 to 150 in. w.g. Two different ranges (0 to 0.5 in. w.g., 0 to 1.0 in. w.g.) are the most commonly used. Readings should always be made in the mid-range of the scale and the instrument should be held in the same position as when "zeroed."

The "high" pressure connection is used (relative to the atmosphere) for reading positive pressures and the "low" pressure connection for negative pressures. By using both, it is possible to measure a pressure drop or rise across components in HV AC systems.

4-21

Figure 4-9 Magnehelic Gauge

Rotating Vane Anemometer

The propeller or rotating vane anemometer consists of a lightweight, wind-driven wheel connected through a gear train to a set of recording dials that read the linear feet of air passing through the wheel in a measured length of time. The instrument is made in various sizes: 3",4", and 6" sizes being the most common (See Figure 4-10).

At low velocities, the friction drag of the mechanism is considerable. In order to compensate for this, a gear train that overspeeds is commonly used. For this reason, a correction factor or calibration curve must be used and the correction is often additive at the lower range and subtractive at the upper range, with the least correction in the middle of the range. Most of these instruments are not sensitive enough for use below 200 fpm. Their useful range is from 200 to 2000 fpm.

The instrument reads in feet, and so a timing instrument must be used to determine velocity. A stop watch should be used to measure the timed interval, although a wristwatch with a sweep-second hand may give satisfactory results for rough field checks.

4-22

(

Figure 4-10 Rotating Vane Anemometer

It has been found that a two minute timed traverse gives better averaging accuracy across the coil face or return air grille than the one minute pass recommended by some industry groups. It is recommended that two or more traverses be made across the air stream and then averaged. The formula for air flow is:

Where:

cfm =

Ftm -A = F =

2

measured anemometer reading in feet free face area of grill in ft2 instrument correction factor (provided by manufacturer) two minute timed interval

In the case of coils or filters, an uneven airflow is frequently found because of entrance or exit conditions. The SMACNA recommends that this variation be taken into account by moving the instrument in a fixed pattern to cover the entire amount of time over all parts of the area being measured so that the varying velocities can be averaged.

4-23

In practice it is quite difficult to end the pattern at precisely the proper time. The SMACNA recommends that the area be traversed horizontally, then vertically, and then end with an "x-type" pattern, so that if time runs out and only one bar of the "x" has been completed, it will still be a satisfactory ending point.

The Associated Air Balance Council (AABC) recommends a different approach to obtain accurate anemometer re'ildings. They recommend that the anemometer be held steady in the air stream for a given period of lime. The average anemometer reading should be determined by marking the grille off in sections, taking a reading in front of each section and averaging the results. A true average reading cannot be obtained by moving the anemometer back and forth across the face, because if the instrument happens to pass over a dead spot or a section where the velocity is low after having passed over one where it is high, the blades are likely to coast over the low section.

Bridled Vane Anemometer

The Florite anemometer shown in Figure 4-11 is direct reading; that is, it does not depend on a time interval. It measures velocity pressure and displays velocity (fpm) on the gauge. The Florite anemometer may be used in the same manner as the rotating vane anemometer except that discreet velocity points are best, such as 8" x 9" grids over a particular area, reading the velocities at each point. This will be very much like a velocity profile reading obtained by a Pitot traverse. To take an approximate reading for a rough balance, a traverse is made in a given time period for the area by moving the anemometer around and visually averaging the velocities present. This is an inaccurate method, but it is fast and it will put you in the "ball park."

4-24

r

Figure 4-11 "Florite" Anemometer

Deflecting Vane Anemometer

Instead of depending on a swinging vane to deflect and indicate a reading, the AInor 6000P velometer shown in Figure 4-12 operates on the Pitot tube principle, pressure exerted on a vane which is free to travel in a circular tunnel moves the vane and causes a pointer to indicate the measured value on a scale. It is not dependent on air density because of the sensing of pressure differential to indicate velocities. Note that the instrument is provided and always used with a dual-hose connection between the meter and the probes, except as noted below.

The model 6000AP set is an all purpose set which adequately meets the needs of TAB work. Most major air distribution device manufacturers have set up area factors based on its use. The velometer consists basically of the meter, measuring probes, range selectors, and connecting hoses. The meter is scaled through the following velocity ranges: 0-300; 0-1250; 0-2500; 0-5000; 0-10,000 fpm.

4-25

Figure 4.12 Velometer Set

4-26

Three velocity probes are provided - the low flow probe, the diffuser probe, and the Pitot tube. The low flow probe is used in conjunction with the 0-300 fpm scale for measuring terminal air velocities in rooms or open spaces, and to measure face velocities at ventilating hoods, spray booths, fume hoods, and the like. The low flow probe is directly mounted to the meter without the use of hoses. The diffuser probe is designed to measure the velocity at diffusers, registers and grilles. The volume of air being supplied or exhausted can be determined using the following formula:

cfm = fpm x K factor

Ai distribll,tiQ.l!a.de:6f.~s ·s!!.eh.a.as diffusers, grilles et cannot be_ meas~d withQl:l~ a· Ks,factor~omfioW.tfactor, .becauseJ,the. manufacturer-mtEt test-eac4 outle~along~it_h .a.P..~Iticular£instrument and designate the--precise

. poiJljs on the diffuser where the instrument probe must be placed. The technician must select the K factor for each diffuser type and size from the manufacturer's specification sheet. The Pitot tube is used 'to measure

r velocities in ducts and at return air or exhaust air grilles. The low flow and diffuser probes are shown in Figure 4-13 .

I , • ,

I I .

; .

r L.

Hot Wire Anometer ...... 1 "~'--\JVJ f>'

c,t:'

The operation of this instrument depends on the fact that the resistance of a heated wire will change with its temperature. The probe of this instrument is provided with a special type of wire element which is supplied with current from batteries contained in the instrument case. As air flows over the element in the probe, the temperature of the element is changed from that which exists in still air, and the resistance change is indicated as a velocity on the indicating scale of the instrument (Figure 4-14).

4-27

(C) P, obe Collar

(A, AI, Flow Dlrecl lon Pointer

,B,

(A , Ve loci ty Sensing Po, 1

Snap 0 11 Fins

/

(0, Connec ting Log

A) DIFFUSER PROBE

The velocity directional sensing port (A) senses the velocity at the diffuse, register, or grille.

The snap-off fins (B) allow you to accurately position the probe vertically, horizontally or radially.

The probe collar (C) acts a s a stop when connecting the probe to the Range Selector, and the O-ring acts as a seal.

The connecting leg (D) is mounted into the Range Selector.

B) LO·FLOW PROBE

An arrow (A) on the probe serves as a reminder of the direction you must orient the probe and the Meter when taking measurements.

Figure 4·13 Velometer Attachments

4-28

Figure 4-14 Hot-Wire Anemometer

In addition to measuring air velocity, some instruments can measure temperature, and also static pressure when a special sensing element is used. The meters have several scales, and the instrument case usually ~as dials or buttons to select the function and range.

An important part of using the instrument is that, before taking a reading, it is necessary to adjust the meter to a zero setting.

The probe is quite directional when used for air velocity measurement. It is therefore necessary to hold the probe at right angles to the air flow and, when used with grilles and diffusers, to place the probe exactly as indicated

i by the manufacturer of the grille or diffuser. i.

The instrument gives instantaneous spot readings and, as with other instruments, a number of readings across the airstream are required in order to determine an average velocity.

A device that covers the terminal device to facilitate taking air velocity or airflow readings is called a "flow measuring hood".

The conical or pyramid shaped hood can be used to collect all of the aIr discharged from an air terminal and guide it over flow measuring

4-29

instrumentation. Hoods generally are constructed so that the outlet tapers down to an area of 1 square foot. An anemometer (velometer) tip is installed in the neck to read cfm directly, regardless of the airflow quantity measured.

The balancing cone should be tailored for the particular job. To keep weight to a minimum, aluminum is normally used. The large end of the cone should be sized to fit over the complete diffuser and should have a sponge rubber seal to eliminate leakage and to avoid ceiling marks. When balancing a large number of ceiling diffusers of common size, a hood may permit reading from the floor and eliminate the need for a ladder as does the commercially made hood shown in Figure 4-15.

Figure 4-15 Flow Measuring Hood

The disadvantages are:

1. They should not be used where the discharge velocities of the terminal devices are excessive.

2. The hood redirects the normal pattern of air discharge which creates a slight, artificially imposed, pressure drop in the ductwork branch of the terminal device being measured.

4-30

3. Some of the larger hoods "get heavy" which could lead to inaccurate readings because of leakage due to carelessness and fatigue.

Smoke Devices

WARNING: Before using any smoke devices, the TAB Technician must warn all people within the area so that they are aware of its use.

These are devices generally used for the study of air-flows and for the detection of leaks.

Smoke bombs come in various sizes with different lengths of burning time from which highly visible, non-toxic smoke readily mixes with air simplifying the observation of flow patterns.

When testing for leaks sufficient smoke should be used to fill a volume 15-to-20 times "larger than the duct or enclosure volume to be tested.

Smoke sticks and candles are convenient in that they corne in different sizes and they provide an indicating stream of smoke. Some are like the puff from a cigarette and others smoke continuously for a few minutes to a maximum of 10 minutes.

Smoke guns are valuable in tracing air currents, determining the direction and velocity of airflow and the general behavior of either warm or cold air in conditioned rooms.

HYDRONIC MEASURING EQUIPMENT

Table 4-6 shows different types of measuring devices used in hydronic applications. Each one will be described individually.

4-31

Table 4-6 Hydronic Measuring Instruments

Accuracy of Instrument Recommended Uses Calibration Required Field

Measurement

U·Tube Measuring fluid pressure drops None (Zero adjustment Manometer through coils, chillers, condensers required for each sel-up)

and other heat exchangers, also across orifices and vent uris.

Pressure The same use as the U-Tube By an approved tesl agency 1/2 of 1% or Gauge Manometer but for higher every 24 months depending 1/2 of scate

pressures. on usage. division.

Di fferential Same as pressure gauge. Same as pressure gauge. l/2o[ l%or Pressure 1/2 of scale Gauge division.

Row Used for accuracy of measurement As required by the Depends on Measuring in fluid system when installed manufacturer. instrument Devices properly . used.

U-Tube Manometer

Since the pressure to be measured in hydronic systems are usually considerably greater than those associated with airflow, manometers for hydronic use usually contain mercury rather than water or oil. Manometers of the type used in hydronic systems, usually have considerably greater scales than those associated with airflow. A manometer of the type used in hydronic systems is available in tube lengths up to 36 inches. When filled with mercury, such a manometer can measure pressures up to 36 in. Hg., or 36 x 0.491 pounds per cubic inch = 17.7 psi, or 17.7 psi x 2.31 = 41 ft. w.g. Manometers are, therefore, useful for measuring pressure drops through coils, chillers, condensers, and other heat exchangers; also across orifices and venturis. They should not be used for measurements under one inch of water. (See Figure 4-16)

4-32

., •• . 3

3·Valvt Bypn :! luptn valvu Q)ifldCDwllh

Valve(DOptn, etOte V~lvt0SIDW I Y )

Sahty Rnt rva", IV . L'o .... d Volume Whtn Ul1n9 Melcury)

U Tube (U sually Glau lor MtlCufy and Pintle tor Tinted Wiud

+2 MUlur~menl Sule tU Tubl Inei St ile

o -I

-, -3

-4

-,

Vert ically Movublt wil li RUpet! 10 Each Othtf tor Zero Adjuurn,nt)

Flu id IMer cu ry lor Wi llI D. P, Tinttd Witt r lor A" li P)

Figure 4-16 U-Tube Manometer

One objection to the use of manometers is the possibility of excess pressure, beyond the range or length of the manometer, which would blow the mercury out of the tube.

Aside from the delay and expense of replacing the mercury, it is very objectionable for mercury to enter the water system because it can cause rapid deterioration of any copper (including copper alloys) that it contacts in the system.

U-tube manometers are ideally suited for differential pressure measurement on a small scale.

Pressure Gauge

The calibrated "test gauge" normally has a bourdon tube assembly made of stainless steel, alloy steel, monel or bronze, and a non-reflecting white face

4-33

with black letter graduations. Test gauges are usually 3-1/2" to 6" diameter with bottom or back connections. Many dials are available with pressure, vacuum or compound ranges. The test instrument minimum accuracy must be within 1% of full scale.

Dial gauges are used primarily for checking pump pressures; coil, chiller, and condenser pressure drops; and pressure drops across orifice plates, venturis, and other flow calibrated devices.

Pressure ranges should be selected so the pressures to be measured fall in the middle two-thirds of the scale range.

The gauge should not ·be exposed to pressures greater than the maximum dial reading. Sfmilarly, a compound gauge should be used where exposed to vacuum.

Reduce or eliminate pressure pulsations by installing a needle valve between the gauge and the system equipment or piping; if large pulsating conditions occur. Also, if necessary, install a pulsation dampener or snubber (available from gauge manufacturers).

In using a gauge, apply pressure slowly by gradually opening the gauge cock or valve, to avoid severe strain and possible loss of accuracy that sudden opening of the gauge cock or valve could cause, and/or to avoid a sudden release of pressure.

A cutaway of bourdon tube pressure gauge is shown in Figure 4-17.

Differential Pressure Gauge

In practically all cases of flow measurement, it will be necessary to meaSure a pressure differential, that is, a pressure drop across a piece of equipment, a balancing device, or a flow measuring device.

4-34

(

Moveme nt Sect or

Co nnecting li nk.

2:- 00 Ca l ibrat ed

Scale

Figure 4-17 Bourdon Tube

A differential pressure gauge is a dual inlet, dual bourdon tube pressure gauge with a single indicating pointer on the dial face which indicates the pressure differential existing between the two measured pressures. It can be calibrated in psi, inches w.g. or inches mercury. The Differential Pressure Gauge will automatically read the difference between two pressures.

With a single gauge connected, the gauge is alternately valved to the high pressure side and the low pressure side to determine the pressure differential. Such an arrangement eliminates any problem concerning a gauge elevations, and virtually eliminates errors due to gauge calibration.

Figure 4-18 shows the application of a gauge modification that uses a single standard gauge and eliminates the need for subtraction to determine differential. The gauge glass is calibrated to ft. w.g. at its outer periphery. During operation, the gauge glass is left loose so it can be rotated. To measure a pressure differential, the high pressure is applied to the gauge by

4-35

opening the valve to the high pressure side, and the gauge glass is then rotated so that its "zero" is even with the gauge pointer. Next, the high pressure valve is closed and the valve to the low pressure side is opened. The gauge pointer will now indicate a pressure that is directly equal to the pressure differential in ft. w.g. If the gauge is of large diameter, such as 8 inches, differential pressures can be read accurately to the order of 0.25 ft. w.g.

,.,

Figure 4·18 Single Gauge for Measuring Differential Pressures

Venturi Tube and Orifice Plate (Flow Devices)

The venturi tube or orifice plate is a specific, fixed area reduction in the path of fluid flow, deliberately installed to produce a flow restriction and, therefore, a pressure drop (see Figure 4-19).

You would expect that it would take more pressure upstream to force the fluid through the restricted opening. The faster the fluid is flowing, the more upstream pressure is required. In this way, the pressure differential (that is, the upstream pressure minus the downstream pressure) is related to the velocity of the fluid. But the pressure differential must be equated to

4-36

gpm through the use of the orifice plate in order for the measurement device to be useful. However, pressure drop is not equal to velocity (differential pressure is not velocity pressure). By accurate measurement of the pressure drop with a manometer at flow rates from zero fluid velocity to a maximum fluid velocity established by a maximum practical pressure drop, a calibrated flow range may be established. The flow range may then be plotted on a graph which reads pressure drop versus flow rate (gpm, steam pounds per hour, etc.) or the manometer scale may be graduated directly in the flow rate values.

ORIFice DIAMETER

o

DRAIN HOLE. LOCATe AT BOTTOM Of PIPE IF ORIFICE IS USED IN STEAM PIPE

ORIFICE SIZE lDENTIFICA TION

PRESSURE TAPPINGS FOR INSTRUMENT CONNECTIONS

ORIFICE Pl..A TE

AlA VENT HOLE: LOCATE AT TOP OF HOR IZONTAL PIPE IF CARRYING WI\. TEA

(A) ORIFICE PlATE

~ FLOW /' OI'1IFICE

(8 ) ORIFICE PlATE INSTALLED L_I>-~ BETWEEN SPECIAL FLOWMETER FLANGES

FLOWMETER FLANGES

Figure 4-19 Orifice as a Measuring Device

The diagrams in Figure 4-20 illustrate the difference between the venturi tube and an orifice plate. The venturi tube, because of the streamlining effect of the entrance and the recovery cone, produces a lower pressure loss for the same flow rate.

4-37

'<ow

MOOIFIED TUBE

~Ull VEN TURIIU9E

, ..:...--~

F'1 '"~:==--=-I " I

ENTRANCE ",., RECOVERY ~,

VEHlv mlveE

ORIFM:E ",-"IE

'~ b ~ TVRBUlENCE

, . '<ow

VENACQH I RAC'A

ORIFICE PLATE

Figure 4.20 Flow Meter Types

The full venturi tube can be extremely accurate with no appreciable system pressure loss, but it must then be extremely long. Unless such accuracy is required, a modified version with shortened entrance and recovery cones may be employed. The modified tube generally provides adequate accuracy with acceptable system pressure losses (still less than the orifice plate for the same accuracy) for environmental systems.

Annubar Flow Indicator

The Annubar Flow Indicator is a flow sensing and indicating system that is an adaptation of the principle of the pitot tube. The upstream sensing tube has a number of holes which face the flow and so are subjected to impact pressure (velocity pressure plus static pressure). The holes are spaced so as to be representative of equal annular areas of the pipe, in the manner of selecting pitot tube traverse points. An equalizing tube arrangement within the upstream tube averages the pressures sensed at the various holes, and this pressure is transmitted to a pressure gauge. The downstream tube is similar

4·38

to a reversed impact tube, and senses a pressure equal to static pressure minus velocity pressure at this point; this pressure is also transmitted to a gauge. The difference between the two pressures, when referred to appropriate calibration data, will indicate flow in gpm. A differential pressure gauge is used to directly read the pressure differential.

Figure 4-21 Annubar Flow Indicator

Calibrated Balancing Valve

Another useful device is the calibrated balancing valve (Figure 4-22). These valves perform dual duty as flow measuring devices and as balancing valves. They are similar to ordinary balancing valves, but the manufacturer has provided pressure taps into the inlet and outlet; and has calibrated the device by setting up known flow quantities while measuring the resistance which results from the different valve positions. These positions usually are graduated on the valve body (as a dial) and the handle has a pointer to indicate the reading. The manufacturer then publishes a chart or graph which illustrates the percentage open of the valve (the dial settings), the pressure drop and the resulting flow.

4-39

,

Figure 4-22 Calibrated Balancing Valve

Specialty devices that cover the entire range of instruments usually prove to be useful, even though some are used only occasionally.

Location of Flow Devices

Flow measuring devices including the orifice, venturi, and other types described above, and give accurate and reliable readings only when fluid flow in the line is quite uniform and free from turbulence. Various texts provide charts for standard piping configurations such as that in Figure 4-23.

4-40

f .

I

T 10

01 '"

1

• ORIFICE OR ~OW NO~

~A -+-1 B.J...S. ruBE nJRHS OR LONG RADIUS BENDS

l-' ORlACl OR A.OW N::,.J

/' :n l ~ -=tB.j 2 OIAM~' ,/ STRAlGHT[NING V~E

2 Dl AM. LONG

ElBOWS OR TUBE lURNS

A LONe RADIUS BENDS

,

~

c Q2Q c . .,. 0.110

DIAMETER RAnD, !3 (C) FOR ORIFlCES AND FLOW NOZZLES

FITTINGS IN OIFFERENT PLANES

II

0.80

r

JQ

w a.

20 a: ~

:z:

" ;;: i" '"

10 ~ W

~ cs

c

~f . ~',' .... r,t ~

c.~, ..- 'I \rJ \ G\.

C el)

Figure 4·23 Flow Meter Location 0 ~ V I( ~. ","

4-41

Pipe fittings such as elbows, valves, etc., create turbulence and nonuniformity of flow. Therefore, an essential rule is that flow measuring elements must be installed far enough away from elbows, valves and other sources of flow disturbance to permit turbulence to subside and for flow to regain uniformity . This applies particularly to conditions upstream of the measuring element, and it also applies downstream except to a lesser extent. The manufacturers of flow measuring devices usually specify the lengths of straight pipe required upstream and downstream of the measuring element. Lengths are specified in numbers of pipe diameters, so that the actual required lengths will depend on the size of the pipe. Requirements will vary with the type of element and the types of fittings at the ends of the straight pipe runs, ranging from about 5 to 25 pipe diameters upstream and 2 to 5 pipe diameters downstream.

TEl\1PERATURE MEASURING INSTRUMENTS

Table 4-7 shows the most common types of temperature measurmg devices. Each will be described individually.

Table 4-7 Temperature Measuring Instruments

Instrument Recommended Uses Calibration Required ~uracy 01 Field Measurement

Glass Tube Used to measure temperature of None I/Zof 1% or I!Zof Thennometers air or flu ids. scale division.

Dial Used to measure temperature of Check: against mercury 1/2 of 1% or 1/2 of Thermometers air or flu ids. Ihennomeler. scale division.

Pyrometers Used to measure surface temperature Every 12 months. 1/2 of 1% or 1(2 of devices such as pipe or duct. scale of division.

Psychrometers Used to measure both weI bulb and Non. 1/2 of 1% or 1/2 of dry bulb temperatures to delennine scale division. wet bulb depression and relative humidity.

Glass Tube Thermometers

Mercury-filled glass tube thermometers have a useful temperature range of from minus 40"F to 950"F. They are available in a variety of standard temperature ranges, scale graduations and lengths.

4-42

I

l

The complete stem immersion calibrated thermometers, as the name implies, must be used with the stem completely immersed in the airstream in which the temperature is to be measured. If complete immersion of the thermometer stem is not possible or practical, then a correction must be made for the amount of emergent liquid column. Thermometers calibrated for partial stem immersion are more commonly used in conjunction with thermometer test wells designed to receive them or by inserting them through small holes drilled in the ducts. No emergent stem correction is required for the partial stem immersion type.

When the temperatures of the surrounding surfaces are substantially different from the measured airstream, there is considerable radiation effect upon the thermometer reading if left unshielded or otherwise unprotected from these radiation effects. Proper shielding or aspiration of the thermometer bulb and stem can minimize these radiation effects, as well as careful location of the reading as shown in Figure 4-24.

Be sure to allow enough time for the thermometer to reach the temperature of the fluid being measured. Always take several readings, usually a few minutes apart. Assume that a reading is the final and correct one only after obtaining the same reading at least twice in succession.

-----" --"--...

Figure 4-24 Temperature Reading Locations

4-43

Dial Thermometers

Dial thermometers are made in a wide variety of dial sizes, stem lengths, and temperature ranges. Their advantages are that they are more rugged and more easily read than glass-stem thermometers, and they are fairly inexpensive. Small dial thermometers of this type usually use a bimetallic temperature sensing element in the stem. Temperature changes cause a . the bend or twist of the element, and this movement is transmitted. by a mechanical linkage. (See Figure 4-25.)

Figure 4-25 Dial Thermometer

The flexible capillary type dial thermometer, one variety of which is shown in Figure 4-26, has a rather large temperature sensing bulb which is connected to the instrument with a capillary tube. The instrument contains a bourdon tube, the same as in pressure gauges. The temperatures sensor consisting of the bulb, capillary tube, and bourdon tube, is charged with either liquid or gas. Temperature dfunges at the bulb cause the contained liquid or gas to expand or contract, resulting in changes in the pressure exerted within the bourdon tube. This causes the pointer to move over a graduated scale as in a pressure gauge, except that the thermometer dial is graduated in degrees. The advantage of this type thermometer is that it can be used to read the temperature in a remote location.

4-44

!

; ' , f •

, .

,

Figure 4-26 Flexible Capillary Type Dial Thermometer

In using a dial thermometer, the stem or bulb must be immersed a sufficient distance to allow this part of the thermometer to reach the temperature being measured. Dial thermometers have a relatively long time lag, so enough time must be allowed for the thermometer to reach temperature and the pointer to come to rest.

Pyrometers

! . Pyrom~ters normally used in measurements of surface temperatures in heating and air conditioning applications, use a thermocouple as a sensing device and a milli-voltmeter (or potentiometer) with a scale calibrated for reading temperatures directly. A variety of types, shapes and scale ranges are available.

Electric type thermometers have an instrument case containing items such as batteries, various switches, knobs to adjust variable resistances, and a sensitive meter. Temperature sensing elements are remote from the instrument case, and connected to it by means of wire or cables. Electric

4-45

type thermometers have advantages of remote-reading, good precision, and flexibility as to temperature range. Additionally, some electric type thermometers have multiple connection points on the instrument case, and a selector switch, enabling the use of a number of temperature sensors which can be placed in different locations, and read one at a time by use of the selector switch.

It should be remembered that the surface temperature of a pipe or duct is not equal to its temperature and that a relative comparison is more reliable than an absolute reliance on readings at a single circuit or terminal unit.

HUMIDITY MEASURING DEVICES

Psychometric Measuremen t Devices

Dry Bulb Thermometer

Human comfort and health depend a great deal on the air temperature. In air-conditioning, the air temperature indicated usually is Dry Bulb (DB) temperature taken with the sensitive element of the thermometer in a dry condition. It is the temperature measured by thermometers in the home.

Wet Bulb Thermometer

If a moist wick is placed over a thermometer bulb, the evaporation of moisture from the wick will lower the thermometer reading. The temperature indicated is known as "Wet Bulb" (WB) temperature. If the air surrounding a wet bulb thermometer is dry, evaporation from the moist wick will be more rapid than if the air is quite moist. Figure 4-27 compares dry bulb temperature and wet bulb temperature taken at the same place and at the same time.

When the air is saturated with moisture, no water will evaporate from the cloth wick and the temperature on the wet bulb thermometer will be the same as the reading on a dry bulb thermometer near it.

4-46

ORV BULB

01''1' 8Ul8

WICI(

WET BULB

TEMPERATURE

Figure 4-27 Dry Bulb and Wet Bulb Thermometers

However, if the air is not saturated, water will evapbrate from the wick. In doing so, it will lower the wick temperature. Then, heat will flow from the mercury to the wet wick and the reading will be lower.

The accuracy of the wet bulb reading depends on how fast the air passes over the bulb. Speeds up to 5000 ft. /min.or 60 mph are best but dangerous if the thermometer is moved at this speed. Also, the wet bulb should be protected from heat radiation surfaces (radiator, sun, electric heater, etc.). Errors as high as 15 percent may be made if the air movement is too slow, or if too much radiant heat is present.

A hygrometer is an instrument used to measure the amount of moisture in the air. By using a psychometric chart the relative humidity can be found.

Psychrometer

To insure that the recorded wet bulb temperature is accurate, airflow over the wet bulb should be quite rapid. A device designed to whirl a pair of thermometers, dry bulb and wet bulb, is called a sling psychrometer (Figure 4-28). This instrument consists of two thermometers, a wet bulb and a dry bulb. The wick on a sling psychrometer must be clean fabric, preferably white.

4-47

weT BULB

j i ..

c;;;;a..

t DRyaULB

~ --

Figure 4-28 A Sling Psychrometer

Because evaporation is taking place from the surface of the wick, there is likely to be deposit of lime substances on the wick. Therefore, to get accurate measurements, a clean wick should be used. Also, use distilled water on the wick. Sling psychrometers come in a variety of sizes.

There are certain places in which it is difficult to spin the psychrometer (narrow passages, etc.). To obtain accurate results in these places, an aspirating psychrometer (Figure 4-29) is used. With this instrument the air sampled is blown over the wet and dry bulb thermometer by suction created by an air pump.

A battery operated aspirating psychrometer is shown in Figure 4-30. It has illuminated thermometer scales and a fan which draws air over the thermometer bulbs.

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Figure 4·29 Aspirating PSYChrometer

Battery Powered Aspirating PSYChrometer

4-49

Dew Point

Dew point is defined as the temperature at which a given sample of moist air is fully saturated and begins to deposit dew. You can easily observe this on a cool morning when you walk outside and see water on the lawn. This is a result of the air temperature outside falling below the dew point during the night. In many industrial processes, the dew point is a more significant measurement than relative humidity.

Dew point temperature measurement is performed for many reasons in industry. Many HV AC systems use dewcells to monitor humidity to measure their effectiveness or to provide an input ' 0 their controls. The dew point temperature is an indication of the moistur ; content of many gases; the dew point o· instrument air systems is often monitored for this reason. Moisture in the instrument air system can freeze in the ports of instruments and shut down an entire plant. In power plants: dewcells are often used to monitor remote, inaccessible areas for steam leaks and to monitor for cooling system leaks in generators with hydrogen/water cooling systems.

Dew point sensors are available"iii many different styles, based on several different operating principles. These are three of the most common types of dew point measuring devices in use:

Lithium Chloride wick-type (Foxboro) Capacitance Probe (panametrics) Chilled Mirror (General Eastern) .-

Wick-Type Dewcells

The principle of operation of the Lithium Chloride Wick-type dewcell is derived by moisture determination. The principle is based on the fact that for every water vapor pressure, in contact with a saturated salt solution, there is an equilibrium temperature. At the equilibrium temperature the salt solution neither absorbs, nor gives up moisture. Below this equilibrium temperature, the salt solution absorbs moisture. Above this equilibrium temperature, the saturated salt solution dries out until the only crystals are left.

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, .

I

! I..

I .

i ..

A wick-type dewcell is shown in Figure 4-31. The typical element is a thin-walled metal socket (to fit over an RID) covered with a woven glass tape, and impregnated with lithium chloride. A low voltage (25 VAC) alternating current is supplied to a pair of gold wires wrapped over the tape. An RID is mounted inside the dewcell over which the socket, tape and gold wires are mounted.

If the temperature of the dewcell element is below equilibrium, the salt absorbs moisture from the atmosphere. The conductivity of the solution on the tape between the gold wires increases, and as a result, current flows between the gold wire "heating elements".

GOlD HEA nNG • • BOBBIN ELEJ.AENT WIRES FlBERGlASS WICK

btfuiimrmmnl---; r - - -- - -------------------~ , , ,

RID BODY

t

TERI.4INA nON HEAD FOR RID LEADS ANO +2SV TO HEATER

Figure 4-31 Foxboro Dewcell

Next temperature of the dew cell element raises to the equilibrium temperature. The RID measures the equilibrium temperature which is the dew cell element temperature. The dewcell element temperature is converted to dew point temperature using a chart supplied by the manufacturer.

Normally, response time of the dewcell is approximately 10-15 minutes for a 25° step change. The dewcell itself cannot be calibrated; however, the RID contained in the dewcell can be calibrated using normal RID calibration procedures.

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Capacitance Probe Dewcell

The capacitance probe type dewcell operates similarly to the lithium chloride dewcell. The Panametrics hygrometers are typical of capacitance type dew point measurement systems. The capacitance probe dewcell is shown in Figure 4-32.

Figure 4-32 Capacitance Probe Construction

The sensor in a capacitance probe system consists of a specially anodized aluminum strip, which provides a porous aluminum oxide layer and a very thin layer of gold evaporated over the aluminum strip. The aluminum base and the gold layer form the two electrodes of what is essentially an aluminum oxide capacitor.

When the sensor is placed in a water containing environment, water vapor is rapidly transported through the gold layer. The water vapor reaches equilibrium on the porous walls of the anodized aluminum strip. The equilibrium reached on the aluminum strip is related to the vapor pressure of the water in the atmosphere.

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The number of water molecules absorbed on the oxide structure determines the conductivity of the porous wall . Each value of the porous wall resistance provides a distinct value of electrical impedance which is a direct measure of the water vapor pressure.

Solid state circuitry is used to measure the impedance and provides an output in dew point temperature.

Chilled Mirror Dewcell

The chilled mirror type dewcell manufactured by General Eastern is typical of the optical-thermoelectric humidity analyzers or "chilled mirror" instruments available for dew point determination. It is also known as an optical condensation hygrometer. The chilled mirror type dewcell is shown in Figure 4-33.

...-- OPTlCAl BALANCE AQ..uSTNENT

(OUlPUT BIASED TO CHili MIRROR

'llttEN a..£AR)

r-t--lJ..,--=~=--,'t-.r--j---j CONTROL AJ,IPUFlER

-~~~~rrr~~~~~~~~~~~~~~SAJ,l~GAS •••••• • ••••• ••••• • ••••• •••••• • ••••• ••••• • •••••

lfD REGULATlON

FRON OOPOINT 'JEr,W, TRANSOU(;[R

THERl.lOEl.£ClR1C HEAT PUMP ORI\I{R

Figure 4-33 Chilled Mirror

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In a chilled mirror type dewcell, a light emlttmg diode (LED) illuminates the mirror surface. A photo transistor is located to observe the reflection. A second LED/photo transistor optical circuit completes the bridge circuit, and tracks the first opto-electronic pair over sensor (ambient) temperature.

A control circuit amplifier amplifies the difference in "photo current" and drives the thermoelectric cooler. The amount of mirror cooling is therefore proportional to the difference in "photo current" (output of the optical circuits). The optical bridge circuit is intentionally imbalanced for maximum mirror cooling when the mirror is dry.

The mirror is cooled by the thermoelectric heat pump to the point where dew forms. the formation of dew reduces the light seen by the photo transistor observing the mirror reflection, reducing its current output. The power to the thermoelectric heat pump is proportionally reduced. Dew density increases to a point where an equilibrium condition exists and the optical bridge is balanced.

At equilibrium the mirror temperature is maintained at the point where the saturation vapor pressure equals the partial pressure of the water vapor in the air.

At equilibrium no additional evaporation or condensation occurs. The saturation pressure of pure water is a temperature dependent variable. Measurement of the mirror surface temperature at equilibrium establishes the dew point temperature.

ELECfRICAL MEASURING DEVICES

Volt-Ammeter

The testing, balancing and adjustment of a mechanical system requires the measurement of voltages and electrical currents as a routine matter. The units involved in such measurements are:

Voltage - volts Current - amperes

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The clamp-on type volt-ammeter is the type usually used for taking field measurements. The clamp-on type volt-ammeter shown has trigger operated, clamp-on transformer jaws which permit current readings without interrupting electrical service. Most normally have several scale ranges in amperes and volts. Two voltage test leads are furnished which may be quick connected into the bottom of the volt-ammeter. Some of the volt-ammeter models are also furnished with a built-in ohmmeter. The instrument should be calibrated by an approved test agency every 6 months, and it should be checked against a recently calibrated on each project.

Figure 4-34 ,-",ml?-'-Jn Volt-Ammeter

Figure 4-35 Measuring Amperage

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When using the volt-ammeter, the proper range must be selected. When in doubt, begin with the highest range for both voltage and amperage scales.

Before using, be aware of the following safety precautions:

First

Second

Be careful not to contact an open electrical circuit. Hands should never be put into the electrical boxes. Do not attempt to pry wires over into position. Do not force the instrument jaws into position. These precautions reduce the risk of causing a short circuit which could injure both equipment and personnel.

The inrush current required to start a motor is from three to five times higher than the load rated full nameplate current. Therefore, starting the motor with the instrument attached can damage the instrument, unless 'the ammeter has a sufficient range to withstand the high starting current.

Readings may be taken at the motor leads or from the load terminals of the starter. To determine the current of single phase motors, place the clamp about one wire. When involved with three phase current, take readings on each of three wires and average the results.

To measure voltage with portable test instruments, set the meter to the most suitable range, and connect the test lead probes firmly against the terminals or other surfaces of the line under test, and read the meter, making certain to read the correct scale if the meter has more than one scale. When reading single phase voltage the leads should be applied to the two load terminals. The resulting single reading is the voltage of the current being applied to the motor.

When reading three phase current it is necessary to apply the voltmeter terminals to Pole No. 1 and Pole No. 2; then to Pole No.2 and Pole No.3; and finally to Pole No. 1. and Pole No.3. This will result in three readings,

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each of which will likely be a little different, but which should be close to each other. For practical purposes they may be averaged.

If the average voltage delivered to the motor varies by more than a few volts from the nameplate rating of the motor, several things can occur. A rise in voltage may damage the motor and will cause a drop in the current reading. A drop in the voltage will cause a rise in the current and can cause the overload protectors on the starter to "kick out". In either case, it is advisable to promptly report high or low voltage situations.

Insulation Resistance Monitoring

Insulation resistance monitoring has been recommended and used for more than half a century to evaluate the condition of electrical insulation. Whereas individual insulation resistance monitoring is of limited use, a carefully maintained record of periodic measurements accumulated over months and years of service is a trendable history of the · insulation's condition.

Two Fundamental Properties of Insulation

Two properties of insulation are dielectric strength and insulation resistance. These are two different and distinct properties of insulation and no simple relation between them has been found. However, extremely low values of insulation resistance, especially when measured values have decreased sharply or steadily over a period of time, should be taken as a warning that the dielectric strength may be low or may be decreasing to the point where the insulation will rupture at the service voltage.

Dielectric strength is the ability to withstand potential difference and is usually expressed in terms of voltage at which the insulation fails due to electrostatic stress. Maximum dielectric strength values can be measured only by testing to destruction.

Insulation resistance is the resistance to current leakage through and over the surface of insulation. Insulation resistance can be measured without

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damaging the insulation and furnishes a highly useful guide for determining the general condition of insulation, but is, by itself, not entirely conclusive. Measurements have shown that insulation resistance measurements at moderate voltages may actually increase after the insulation has been broken down by a high potential. Clean, dry insulation having cracks or other faults may show a high value of insulation resistance but obviously is not suitable for use. These limitations of insulation resistance values must be fully realized when the condition of insulation is appraised by such values.

Factors Affecting Insulation Resistance

Insulation resistance measurements are affected by several factors:

Surface conditions Moisture Temperature Magnitude of test direct potential Duration of application of test direct potential Residual charge in the winding

Measuring Insulation Resistance

There are two types of insulation resistance tests 1) a short-time test or one minute test and 2) a comparative short-time test or a ten minute test.

The one minute test is used as a quick evaluation of the insulation condition. Usually three readings are taken, one from each motor winding phase to ground. If all of the readings are above acceptable minimum insulation resistance values, the motor is considered operable for a preselected period of time, usually six months to a year. The one minute test may be performed 1) when the insulation resistance is assumed low due to adverse operating condition, 2) before start up after being secured for an extended period of time, 3) on a scheduled annual basis along with the ten minute test to compute the polarization index and trend the insulation condition, and 4) during a drying out process to determine progress (see Figure 4-36).

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100

80 ~

" :I: 0 Cl w

" 60 ui «-<, ~ POLARIZA TlON u ,," INDEX z I 2.0 OR MORE

" ~.., ... ,

~ ~ <t" ~ 40 w " a: z 0 ;::

" -' 20 :J ~ Z

0 0 20 40 60 80 100

TIME. HOURS

Figure 4·36 Drying Process of a Class B Annature Winding

The ten minute test is used with the one minute test to compute the polarization index, which is the ratio of the ten minute to the one minute test. The ten minute test, taken annually, also provides a highly reliable evaluation of the motor insulation condition and can be charted, compared, and interpreted to determine maintenance action and frequency of inspection. The technique for the ten minute test is essentially the same as the one minute test except that the test is performed for a longer period of time.

Conditions for Measuring Insulation Resistance

The conditions for performing insulation resistance testing will vary. Whenever possible, the conditions below should be met:

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The insulation surface must be clean and dry if the measurement is to provide the information on the condition within the insulation as distinguished from surface condition.

The winding temperature sliould be at least a few degrees above the dewpoint to avoid condensation of moisture on the winding insulation. It is also important that, when comparing insulation resistance of machine windings, values be converted to a 40"C basis.

It is not necessary that the machine be at a stand-still when insulation resistance test are made. It is often desirable to make insulation resistance measurements when rotating equipment is subject to centrifugal forces similar to those occurring in service. The test can be performed immediately after the machine is taken out of service and while it is still rotating.

In certain cases, it is practical to make periodic insulation resistance measurements while machines are rotating on shortcircuit dry-out, that is, when a small amount of current is passed through the winding to allow it to warm up and dry out.

Whenever machines are rotating during measurement of insulation resistance, precautions should be taken to avoid damage to equipment or injury to personnel. These precautions should include a tagout of the circuit breakers using a mechanical means to prevent accidental energizing of the equipment.

Test records of a given machine should indicate any special test conditions.

When data is being trended, every effort should be made to conduct the test under the same conditions. This permits easier and more accurate comparison. However, it is not always possible to duplicate conditions, but if major deviations are recorded with the data, they can be factored into the review. Along with the test results, the following information should be recorded:

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[

L

Motor nameplate data Winding temperature Ambient temperature Relative humidity Condition in the area Whether the motor was in service prior to the inspection Maintenance actions taken

Instruments

Direct measurement of insulation resistance may be made with the following instruments:

Direct-indicating ohmmeter with self-contained hand or powerdriven generator.

Direct-indicating ohmmeter with self-contained battery.

Direct-indicating ohmmeter with self-contained rectifier using an external alternating-current supply.

Resistance bridge with self-contained galvanometer and batteries.

Testing Guidelines

Generally two electricians carry out the one minute or the ten minute tests. Before performing the test, the motor data and past history can be reviewed to give an idea of what to expect and warn of past problems.

Next the applicable motor and related electrical equipment should be identified and completely disconnected from all power sources and tagged. Where possible, both disconnects and circuit breakers should be opened and tagged (see Figure 4-37).

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460 V ~

r- - - -tI

-, i 600A CB I

L_

rI I I I I I I I I L_

-1---- _J FLOOR-MOUNTED ~ MOTOR __ _ ...L, CONTROLLER

-- - ---t~._-l-I_ I - DISCONNECT I SWITCH I (UN FUSED)

-- -- ----O+-110VC01L

200 HP MOTOR

I I

500 V MEGOHMMETER

Figure 4-37 Meggering Diagram

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At this point, a voltage tester should be used to test for live circuits and residual voltage. Any residual voltage should be discharged to ground before continuing the test. When using the tester, leads should be attached to the appropriate terminal or ground using an · alligator clip, so only one hand is required in the proximity of possible energized equipment.

.. , The insulation resistance reading can be ·taken at a point closest to the

motor itself. The testers' reading with the leads shorted together should read zero. This serves as a check on the tester's accuracy.

Next, the ground lead is connected to the appropriate terminal or ground (frame of controller). The other lead is placed in contact with each phase terminal. It is recommended that each phase be isolated and tested separately when feasible. Testing each phase individually gives a comparison between phases which is useful in evaluating the phase to phase insulation resistance.

When testing individual phases, the neutral end of each phase winding should be disconnected and the phases not under test should be grounded. Another method of testing each phase separately is to use guard circuits on the phases not under test.

Tests may be made on the entire winding at one time, under certain conditions, such as when time is limited. However, this procedure is not preferred. One objection to testing all phases at a time is that only ground insulation is tested and no test is made of the phase-to-phase insulation. The phase-to-phase insulation is tested when one phase is tested at a time with other phases grounded.

The connection leads, brush rigging, cables, switches, capacitors, lightning arrestors, and other external equipment influence the insulation resistance test reading on a machine winding to a marked degree. Thus, it is desirable to measure the insulation resistance of a winding exclusive of the external equipment on the machine.

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The insulation resistance readings obtained are adjusted for the test method and then for temperature, to 40°C, so they can be compared with the insulation resistance recommended minimum value of the complete winding.

Total winding tested - no reading adjustment Each phase tested/others ground - divide reading by two Each phase tested/others on guard circuits - divide reading by three

, , Readings taken on a scheduled basis can be plotted for trending the

winding's insulation resistance. Trending of readings is more important than single readings. A significant drop in readings indicates failing insulation and can predict maintenance actions.

MINIMUM VALUES AND FREQUENCY OF INSULATION RESISTANCE TEST

Minimum Insulation Resistance Value

Presently, the acceptable industry practice permits one megohm as the absolute minimum value of insulation resistance. This value is for a 460 volt motor. Motors with higher rated voltages will have a higher minimum insulation resistance value.

The IEEE Standard 43-1974 recommends calculating the minimum value from:

where:

kV =

kV + 1

recommended minimum insulation resistance in megohms at 40°C of the entire machine winding.

rate machine terminal to terminal potential, in rms kilovolts.

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r

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L

, .

..

L

Note that the IEEE recommended value is always a value larger than one and increases for machines with higher terminal to terminal potentials. The actual complete winding insulation resistance to be used when comparing to the calculated value must be corrected to 40"C.

In addition, special motors may have winding resistances lower than the recommended since special materials may be used in the insulation. The minimum insulation resistance value may be established by the manufacturer. This information should be requested at the time of purchase.

The minimum value of polarization index for alternating current and direct current rotating machines recommended by the Institute of Electrical and Mechanical Engineers is:

For Class A (105°C): 1.5 For Class B (130"C): 2.0 For Class F (155°C): 2.0

Classes A, Band F correspond to the insulation cl~ssification system that correlates the limiting temperature for that insulation. Insulation that is subjected to higher temperatures will have an increased rate of thermal aging.

Frequency of Inspection

The current industrial practice is to test motors annually using both the one minute and ten minute tests. The results are recorded for trending and the polarization index is computed. Also, motors are tested using the one minute test anytime the insulation resistances value is in question. Causes for question may be operating in adverse conditions or standing idle for long periods of time.

Interpretation of Results

Insulation resistance history of a given machine, made and kept under uniform conditions, is recognized as a useful way of monitoring the insulation condition (see Figure 4-38).

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•

When the insulation resistance history is not available, recommended minimum values of the polarization index or of the one minute insulation resistance may be used to estimate the suitability of the winding for an over potential test or for operation. The one minute insulation resistance (corrected to 40°C) should be at least · that · of . the recommended minimum insulation resistance value obtained from RM = kV + 1.

It is recognized that it may be possible to operate machines with values less than the recommended minimum value; however, it is not normally considered good practice.

In some cases, special insulation material or designs, not injurious to the dielectric strength,. will provide lower values. These special cases should be identified by the motor vendor (Semi-conductor, high voltage- cable).

When the end windings of a machine are treated with a semiconducting material for corona elimination purposes, the observed insulation resistance may be somewhat lower than that of a similar machine which is untreated.

/

"L-_'-_'-_-:---: " .

nUl IYIA"S)

Figure 4·38 Insulation Resistance History

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The insulation resistance of one phase of a three-phase armature winding with the other two phases grounded is approximately twice that of the entire winding. Therefore, when the three phases are tested separately, the observed resistance of each phase should be divided by two to obtain a value which, after correction for temperature, may be compared 'with the recommended minimum value of insulation resistance for the complete winding.

If each phase is tested separately and guard circuits are used on the other two phases not undeJ test, the observed resistance of each phase should be divigrd by three to obtain a value, which, after correction for temperature, may be compared with the recommended minimum value of insulation resistance for the complete winding.

For insulation in good condition, insulation resistance readings of 10 to 100 times the value of the recommended minimum value of insulation resistance (RM) are common.

In application where the machine is vital, it has been considered good practice to initiate reconditioning should the insulation resistance, having been well above the minimum value, drop appreciably to near that level.

ROTATION MEASURING INSTRUMENTS

A tachometer is an instrument used to measure the speed at which a shaft or wheel is turning. The speed is usually determined in revolutions per minute.

The several types of tachometers (fable 4-8) described below vary in cost, in dependability, and in accuracy of results obtainable. One basic difference between the different types of tachometers is that some read directly in revolutions per minute (rpm), while others are primarily revolution counters that must be used with a timing device such as an accurate stop watch. Each will be discussed independently.

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Table 4-8 Tachometer Types

Revolution Counter (Odometer)

The revolution counter is a small hand-held counting device that is pressed to the center of a rotating shaft for a time period of from 30 to 60 seconds. Reasonable accuracy can be obtained by using a good watch with a sweep-second hand where a stop watch is not available. This instrument cannot normally be rest to zero, so that the measured shaft speed is the difference between the initial and final instrument readings divided by the time interval.

Many revolution counters cannot be used on shafts with flat ends. (Slip and inaccurate readings are inev itable.) Some types feature a clutch engagement in which a certain amount of force is required to activate the recording mechanism. All must be used and coordinated with an accurate timepiece.

Tachometers, Centrifugal

This type of instrument contains a centrifugally operated mechanism that is similar to the fly-ball governor on a stationary steam engine, or the governor on a gasoline engine. The instrument is held in contact with the

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•

, -.

rotating shaft, which then rotates the tachometer mechanism and moves the pointer to give instantaneous indication on the dial, directly in rpm. This type of tachometer will indicate properly regardless of the rotation of the shaft, and a stop watch or other time device is not required.

Figure 4-39 Centrifugal Tachometer

Tachometer, Chronometric

The chronometric tachometer combines a revolution counter and a stop watch in one instrument. In using this type of tachometer, its tip is placed in contact with the rotating shaft. The tachometer spindle will then be turning with the shaft but the instrument will not be indicating. To take reading, the push button is pressed and then quickly released. This sets the meter hand to zero, winds the stop watch movement, and then simultaneously starts both the revolution counter and the stop watch. After a fixed time interval, usually six seconds, the counting mechanism is automatically uncoupled so that it no longer accumulates revolutions even though the instrument tip is still in contact with the contact with the rotating shaft. After the meter hands have stopped, the tachometer may be removed from the shaft and read. The meter face has two pointers and two dials, the smaller one indicating one graduation for each complete revolution of the larger pointer, and the reading will be directly in rpm. Some instrument spindles must be rotating in order to be reset without damage.

4-69

Figure 4·40 Chronometric Tachometer

Tachometer, Electronic

The Stroboscope is an electron ic tachometer that uses an electrically flashing light. The frequency of the flashing light is electronically controlled and adjustable. When the frequency of the flashing light is adjusted to equal the frequency of the rotating machine, the machine will appear to stand still.

The stroboscope does not need to make contact with the machine being checked, but need only be pointed toward the machine so that a moving part will be illuminated by the stroboscope light and can be viewed by the operator. The light flashes are of extremely short duration, and their frequency is adjustable by turning a knob on the stroboscope. When the frequency of the light flashes is exactly the same as the speed of the moving part being viewed, the part will be seen distinctly only once each cycle, and the moving part will appear to stand still. The corresponding frequency, or rpm, can be read from a scale on the instrument.

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Figure 4-41 Stroboscope

Tachometer, Photo

The photo tachometer is a relatively new concept in rpm measurement: The instrument uses a photocell, or eye, which counts the pulses as the object rotates. Then, by use of a transistorized computer circuit, it produces a direct rpm reading on the dial indicator on the instrument's face. The instrument in Figure 4-42 has a dual range of 0-2,400 rpm and 0-12,000 rpm. Several features make it adaptable for use in measuring fan speeds. It is completely portable and is equipped with long-life mercury batteries for its light and power source. It weighs only about two-and-a-half pounds, with case and batteries. It has good accuracy and any error can be reduced by using more than one marker on the rotating device. Its calibration can be continually checked on most jobs by directing its beam to a fluorescent light and comparing the indicating reading against 7,200 on the 0-12,000 rpm scale.

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Figure 4-42 Photo Tachometer

The photo tachometer does not have to be in contact with the rotating device. It indicates instantaneous speeds, not average speed - whether constant or changing - thereby reading the speed as it is. It is easy to use, and easy to read. One need only place a contrasting mark on the rotating device by using chalk or colored tape. It is good instrument to use on in-line fans and other such equipment where shaft ends are not accessible. It also has good application for use on equipment rotating at a high rate of speed.

VIBRATION MEASUREMENT

Vibration Probe

A vibration probe is used to indicate the displacement of rotating equipment in an HV AC system. Larger facilities use vibration measurements as the basis for predictive maintenance programs.

Causes of displacement or vibration in HVAC equipment include the following:

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,.

r

i I ,

I -

Unbalance of rotating parts Misalignment of couplings and bearings Bent shafts Worn, eccentric or damaged gears Bad drive belts and drive chains Bad bearings - anti-friction type Torque variations Electromagnetic forces Hydraulic forces Looseness Rubbing Resonance Dirty Blades

The vibration probe and meter cannot determine the problem but does provide the data which indicates changes since the last reading. Once a problem is detected, analysis is performed using more sophisticated equipment. Typical meters for vibration probe analysis are shown in Figure 4-43.

r.::;:;71 ~

• Figure 4-43 Vibration Meters

4-73

The key item of a vibration pickup probe is the crystal. The crystal, when subjected to pressure caused by vibration, deforms which causes an electrical potential proportional to the applied force or pressure. This action of generating a voltage from the force or pressure is the principle used in the piezoelectric crystal. Piezoelectricity is a property of nonconducting solids which have a crystal lattice structure that does not have a center of symmetry to produce voltage. The crystal is a dynamic responding sensor and is not suitable for steady-state conditions, therefore, it only responds on a vibration pulse or change. A magnet may be used to attach the probe to the rotating equipment. Other probes are permanently attached and provide constant readings that are monitored.

VELOCITY PICKUP

CASE

-------- /" -------~

COIL "RINO

CONNECTOR PINS

oOLCH' ... ".IFIL.LEDWITH SILICONE Dill

CONNECTOR'INI -- , __ .. --- "..,>NVoN

....... CRYSTAL

(COY CURRENT P"OXIMITY PROIIE

1 .

ACCiLIEAOMITlR

MAGNET..,"l

IRON CORE THAIADI.IOOV CAS!

} OUll'UT LEAOS

Figure 4-44 Vibration Probe Schematic Representation

4-74

Typically the output from the crystal goes to an amplifier which then transmits a usable signal for indication or control. A vibration probe using this principle may be found on large forced-draft fan bearings.

Sometimes each bearing will have a permanently affixed vibration probe which will feed information to a vibration monitoring panel on the status of each bearing. The vibration probe may also be attached to a tripping device should equipment vibrations become excessive. The permanent probe is typically held to the equipment by a threaded connection while the hand held probe is held to the equipment by a magnet.

Preventive maintenance requires periodic vibration and noise analysis. The vibration test is important to the maintenance program in that vibration is a good "machine-health indicator".

Measuring Vibration

The most significant vibration measurement points using the vibration meter, will usually be at the bearings which support the major rotating component(s), or on solid machine structure as near the bearings as possible. These points are the best "transmitters" of the machine's vibration.

At a given measurement point (or bearing) on the machine, three "standard" measurement positions of the pickup may be used. These points are the best "transmitters" of the machine's vibration. These positions are "Axial", "Vertical", and "Horizontal". The three measurements, when compared with each other and/or with their previous values, can provide important clues as to type or magnitude of possible problems. When combined with analysis of the frequencies at which these vibrations occur, the source of the vibration can be pin-pointed to the source of the problem. Chapter 6 covers frequency analysis in more detail.

Methods for applying the pickup are listed below. Factors which may affect the accuracy (or desired ability) of these methods are given immediately after the list, and should be studied carefully.

4-75

1. Hand-held, with pickup directly against vibrating part.

2. Attached to machine with vise grip holder or magnetic holder (optional pickup accessories).

3. Installed on 1/4-28 stud welded on or threaded to machine part. (Mounting surface must be flat and stud square with surface to ensure good readings.)

When hand-holding the pickup (with or without probe installed), use just enough force to prevent any chattering between the part and the pickup (or probe) - don't "lean" against the pickup. Keep the pickup as steady as possible while taking the meter reading.

Repeat measurements of a given. point are taken to show an change in machine condition by comparing the measurement values. When taking repeat measurements, it is important to use the same relative pickup position each time, since a change in relative position can cause a change in the comparative measurements. To ensure "repeatability" when using methods 1. or 2., make a practice of always placing the pickup in a reasonably true horizontal or vertical plane when conditions permit.

If machine structure forces you to use an "unusual" position, be sure to repeat that position for later measurements at the same point.

Methods of pickup will vary with machine RPM and pickup device. The following is an example of speed limits (due to "resonance" limits of the pickup accessories).

APPLICATION METHOD SPEED LIMITS

Attached with vise holder 720 to 8,700 rpm

Hand-held, with 9-in. probe 720 to 16,000 rpm

Attached with magnetic 720 to 37,200 rpm

4-76

Measuring Displacement

Vibration can be measured in terms of how far the part moves back and forth. This is called the peak-to-peak displacement or simply the displacement. This measurement of displacement is normally expressed in mils.

Measuring Velocity

The vibrometer can also measure the vibration in terms of how fast the part moves. This is called the peak velocity and is measured in inches per second. Because the velocity is a function of both displacement and frequency, it provides an added sensitivity to high frequency vibrations.

For instance, a vibration displacement of 0.1 mil results. in the same meter reading when the frequency is 1800 cpm as it does at 18,000 cpm. The velocity for 0.1 mil at 1800 cpm is 0.0094 in/sec but at 18,000 cpm the velocity is ten times as large or 0.094 in/sec. This means that small vibrations occurring at high frequency are easier to detect if velocity measurements are used. Since troubles such as bad bearings and gears cause vibration at high frequency, velocity measurements are extremely valuable .

GAUGE MANIFOLD

In troubleshooting and repairing a refrigerant system, the steps below should be followed:

1. Attach a gauge manifold. 2. Evaluate performance of system. If repairs requiring opening the

system are needed, continue to next step. 3. Discharge systems refrigerant charge. 4. Open system and make repairs. 5. Pressure test system. 6. Evacuate and dehydrate system. 7. Charge system with oil and refrigerant. 8. Evaluate repairs made and trim the refrigerant charge.

4-77

All maintenance performed will include some of these steps. The primary tool used in diagnosing the problems in a refrigerant system is the gauge manifold.

Using the Gauge Manifold

The gauge manifold is a device that is used to evaluate a refrigerant system. It checks the different pressures in the system so that a technician can pinpoint problems. A typical gauge manifold is shown in Figure 4-45 with a schematic shown in Figure 4-46.

When connecting refrigerant lines or gauge manifolds to any refrigerating system, one must keep the system clean. The lines, gauges and manifold must be free of dirt, moisture and air. The manifold should be purged with the same refrigerant as is used in the system. The manifold and connecting lines must be purged before the system service valve is opened or before the piercing valve stem makes an opening in the tubing.

To check the pressure in a system, gauges must be connected to the system without allowing air, moisture or dirt to enter. The procedure for connecting gauge to a system depends on the system design. It is different for each system. See Figure 4-47.

Some systems have both a suction service valve and a discharge serv ice valve. See Figure 4-47 A.

Some have a suction service valve adaptor mounted on the compressor. See Figure 4-47B.

Some do not have any service valves but do have a process tube. See 4-47C.

Some have a process tube too short or not reachable. In such systems it is necessary to attach a piercing valve to either the liquid line, the suction line or both. See Figure 4-470.

4-78

,,'''CUo.' e ' l"Of' ll.f . ... ,

Figure 4-45 Gauge Manifold

MII:M 'IInsulU I:&UI:I

(OM"ICTIOH '0 IITIIIII,

1U'"Ic:lIUHT (YUHO" OIl. COMUINII 'UIIIOII.IIII 'I.U5111111: '1.1.110

\

Figure 4-46 Gauge Manifold Schematic

4-79

---f, I. ct.::; ~ ~

<=c?

I .~

'-

• .. OCfU lUll

.,,,. ~ ~-CO_ ~'BSO' rTJ-•

o'e'c ,"" ..... t

•• octa ~.· U .0"'0'

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c

j

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•

1

co, 0 Oil ~, .. ,

Figure 4·47 Four Different Methods for Connecting a Gauge Manifold

System A with two service valves is the easiest for attaching gauges. It also permits one to check both the low side pressure and the high side pressure.

Referring to Figure 4-48 the most popular way to purge the service lines is to loosen the line fitting on the system service valve at C, open valve B and then open cylinder line valve E just a little. Repeat the same procedure for valve D. The cylinder refrigerant will free or purge all the lines and the manifold of air and moisture.

4·80

l

r- - - - - -- -. - - ------ -- - - -- --- - --- -. --,

(( ))

((

))

,

Figure 4·48 Valves on Gauge Manifold Opened to Purge Service Lines

When only one connection is to be made to the system, it is to the low or suction side, at valve C. The flexible line between Band C is connected to the system valve, C. However, the use of the gauge manifold allows one to check both low side pressures when valve C is open and valve B is closed and high side pressures when valve 0 is open and valve A is closed.

It also allows one to charge a system. Valves C and B are open; cylinder valve E is opened slowly. It can also be used to evacuate the system. Vacuum pump line is connected to the middle connection of the gauge manifold; valve C is open and valve B is opened.

4-81

After installing the gauge manifold (if the unit will run), operate the system through at least three operating cycles. Carefully record the suction pressures, condensing pressures, evaporator temperature and the condenser temperature. Many times this information is provided on packaged units .

Special Attaching Devices

Some systems do not provide for gauge openings. Therefore, special attaching devices must be used to make it possible to use the gauge manifold. Figure 4-49 shows a gauge manifold with a vacuum pump valve and refrigerant cylinder valve added to the manifold.

~ 1

I

U"'~I"N' C'l'''O;>U l'''' "l"'

Figure 4-49 Gauge Manifold with Vacuum Pump Valve and a Refrigerant Cylinder Valve Added

4-82

SUMMARY

4-83

, .

! L

CHAPTER FIVE

SYSTEM TEST AND BALANCE PROCEDURES

;

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..

l.

CHAPTER FIVE SYSTEM TEST AND BALANCE PROCEDURES

OBJECTIVES


1. List the instruments used to measure flowrate in the following air systems components:

• • •

Ducts Diffusers Supply grilles and registers

2. Describe the conditions of a system during tests.

3. List the four critical factors that are checked in balancing plenum systems.

4. Describe the procedure for testing air shafts.

5. Describe the procedure for testing duct leakage.

6. List the checks that must be made before balancing a water system.

7. Describe the general procedure for balancing hydronic systems.

8. Describe the basic procedure used to balance flow (air/water) across the following components:

• Cabinet unit heaters • Fan coil and unit ventilator • Unit heaters • Pumps • Chillers • Cooling tower

9. Describe the two tests used to test HEPA filters .

10. Describe the charcoal adsorber test procedure.

CHAPTER FIVE SYSTEM TEST AND BALANCE PROCEDURES

INTRODUcrION

In order for an HV AC system to operate properly, it must be tested and balanced in accordance with proven procedures. This chapter discusses some of the methods used to properly test and balance HV AC systems and components. The areas that will be covered are:

• • • •

Air Flow Measurement in Ducts Hydronic System Testing HEPA Filters Testing Charcoal Adsorber Requirements and Testing

AIR FLOW MEASUREMENT IN DUCTS

Air flow measurement in duct work should be measured In the following manner and with the following approved instruments:

• • • •

Pitot Tube and Inclined gauge Manometer Pitot Tube and Magnehelic gauge Pitot Tube and Velometer Thermo-Anemometer

To establish air flow in ducts a complete traverse should be made using the above approved instruments. The traverse should be made in a duct having a minimum of four diameters in length from nearest transition duct or other obstruction.

The cross section of the duct should be marked off in square areas of equal proportions and the pitot tube inserted so as to be in the center of each square progressively and the gauge reading noted for each square. Readings of 700 FPM or below should not be made with this type of instrument as these readings will not be accurate . For readings of 700 FPM or below, the

5-1

micromanometer should be used in lieu of the inclined or magnehelic gauge. A maximum of six inch squares should be used.

Air Flow Measurement of Diffusers

Air flows from diffusers should be measured with one of the following type instruments:

• • •

Alnor Velometer Bacharach FloRite Scoop Hoods when applicable as noted herein

The Alnor Velometer consists of a special blade vane in a case containing calibrated scales over which a needle point moves correspondingly to the pressure upon the blade vane. A variety of special tips are provided and should be used as per the manufacturer's recommendations. The instrument is found to be commercially accurate and durable for field use and is not affected by minor temperature variations.

Each diffuser tested should be marked at locations of readings on face or vane. The velocity meter inlet jet should be placed in the vena contract of the face vanes of the diffuser. A minimum of six readings should be made to determine average velocity in feet per minute. All future readings and check readings should be made at the marked locations of each diffuser.

Air Flow Measurements of Supply Grilles and Registers

Air flow measurements from supply grilles and registers should be made using one of the following approved instruments and methods:

• • • •

4" Vane Type anemometer 4" Vane Type Bacharach FloRite Meter Alnor Velometer Hoods

5-2

The average anemometer reading should be made by marking off the grille in sections taking a reading in front of each section and averaging the results. Readings taken by moving the instrument back and forth across the face should not be used.

The manufacturer's published anemometer K factor of effective area should be used in determining the total CFM being discharged.

The vane type Bacharach Velocity meter can also be used in the same manner with the corrected factor.

The Alnor Velometer should be used with the correct tip and read at the vena contract of the blades using the manufacturer's published velometer factor of effective area.

Use of Hoods

Hoods should be used on perforated type supply diffusers. Hoods may be used on standard supply or return diffusers for proportioning only. Final settings and readings must be accomplished with the Alnor or Bacharach. Hoods used on perforated diffusers should not exceed the particular size of the diffuser face. Hoods should be constructed as per outlet manufacturer's recommendations and used with the proper correction factors for each size hood. The factor should be applied to the discharge free area of the hood.

When a hood is applied to an outlet a certain amount of backpressure is introduced against the flow of air. Since the exact shape of these hoods follows a carefully calculated design, you can determine just exactly how much backpressure can be expected from each of the various sizes so that correction factors can be applied to the results. For example, a 24" x 24" hood funnels down to a 12" x 12" discharge opening at the bottom. Obviously, this creates a 1 square foot free area where the velocity measurements are read. However, the lab tells us that a 1/4" S.P.W.G. is created by this particular size hood. Therefore, using 1.00 sq. ft. times the velocity will not give an accurate reading of what the diffuser would be handling without the hood's imposed S.P. Consequently the correction factor

5-3

of 1.25 is applied to the actual free area of the discharge end of the hood increasing it from 1.00 sq. ft. to 1.25 sq. ft.

It has been determined that when a hood is applied to any given outlet, the backpressure created by the hood causes the air to back up slightly forcing more air to be discharged from some other outlet either in the same zone or at the point of least resistance. This can be proven by testing two outlets individually on the same branch duct and comparing the added total against a traverse reading of the branch duct upstream from both outlets. Without applying the correction factor as outlined above, the reading of the traverse will be greater than the total of the two outlet readings.

NOTE:

Hood Size at Top

24 x 24 20 x 20 16 x 16 12 x 12 24 x 12

Correction factors stated below are for average velocities. When necessary to use a hood, a correction factor for that hood, at the velocity being used, should be reestablished in the field .

Hood Size Actual Corrected at Bottom Free Area Free Area

12 x 12 1.00 1.25 lOx 10 .69 .83 8x8 .44 .50 6x6 .25 .28

12 x 12 1.00 1.25

When these factors are applied to the free area of the bottom end of the hoods, the most accurate readings may be taken.

Air Flow Measurement of Return Grilles and Registers

The anemometer or Bacharach flow meter or velometer should be used to determine the flow through a return intake by marking off the face of the intake into sections as was done with supply grilles. The procedure for testing returns and intakes is similar to the testing of supply outlets with the exception of the effective area factor.

5-4

,

l.

Testing of Motor Amperage

To test motor amperage the test technician should use the clamp-on ammeter. Whenever possible readings should be made at the motor terminals. If the motor terminals are not accessible, readings should be made at the motor starter box or controller. Before the final reading is taken, fan drive or vanes should be set in position of final operation. No readings should be taken until the motor has come up to maximum speed after start up. Readings should be taken on all three legs of three phase motors.

Measuring Static Pressures

To determine static pressure in ductwork, plenum chambers, across filters or across coils, the inclined gauge or calibrated magnehelic gauge should be used in conjunction with a static pressure tip. Insertion of the tube end or the use of suction cups is not acceptable. Probes should be made in areas considered to have a stabilized pressure. Preferably two or more readings should be taken.

Testing of Hot and Cold Mixing Dampers

Each system that uses hot and cold mixing should be subject to the test so that a leakage factor can be determined. A temperature sampling should be made in the cold supply duct and in the main hot supply duct. With the room thermostat calling for full cooling, air temperature should be read at the outlet and compared to the temperature of the cold duct air. The same procedure should be followed for testing of the hot supply duct but with the

'~ use of full heating. !ii9rmal leakagetfactor, should,,oot. exceed 5%.

Testing and Setting Static Pressure Dampers

The setting and testing of static pressure dampers should be accomplished in the following manner:

During the balancing of the system, the cold static dampers should be held at a maximum open position of 90% with the system calling for full

5-5

cooling and with the hot dampers in the closed position. For the balancing of the hot side the same positioning should be made with the hot damper on call for full heating. The final settings of dampers should be made by reading the static pressure required at the sensing tip when the air column at all terminals is as specified on a call for full cooling. This procedure should be reversed for the heating side. Arbitrary settings at gauges should be avoided. This may involve a great deal of checking as the point of least static is generally, but most certainly not always, at the end of the system.

Testing of Face Velocities Across Coils

The measurement of coil face velocities should be made using a 4" vane anemometer. The test engineer must attach a long handle to the instrument and avoid blocking any air flow motion. Continuous movement across the face of the coil should be avoided. Individual spot readings at set intervals should be made to establish averages. Coil face velocities at best are not very reliable and should not be used as a method of establishing total air except when it is the only available method.

Conditions of System During Tests

A. Determine the total amount of air required to flow across the cooling coil and heating coil during maximum load conditions. If the cooling coil is designed to handle the total CFM during maximum load conditions, the system should be balanced with full air flow across the cooling coil. If the design calls for a percentage of cooled air and warm air across respective coils at maximum load conditions, the systems should be balanced under these conditions. The above applies to double duct systems.

B. Single duct or reheat type air supply systems should be balanced on a full call for cooling.

C. All tests should be run with supply, return and exhaust systems operating and all doors, windows, etc., closed or under projected operating conditions.

5-6

, .

D. Whenever possible, final readings and settings should be made with cooling coils operating (wet) in order that static pressure conditions should be a maximum.

E. Allowances should be made for air filter resistance at the time of the tests. The main air system should be at design air quantities and at an air resistance across the filter bank midway between the design specifications for clean and dirty filter conditions.

F. The room air supply and return or exhaust should'tM'±'10%~of design'" quantities for rooms with an air supply, return or exhaust under 1,000 CFM, and within ° and +5% in rooms where the total is 1,000 CFM or more. In all cases, the total air quantity supply to any floor or major zone should be at a maximum condition of + 10%.

G. Total system air quantities should be obtained by adjustment of the fan operating speeds.

H. The deflection pattern of all supply outlets should be adjusted to insure proper and uniform air distribution through the areas served by such outlets.

l. All damper positions should be permanently marked after air balancing is complete.

Setting of Outside Air and Return Air Volumes

Final balanced conditions must include the setting of outside air quantities and return air quantities. Setting of outside air quantities must be made by adj ustment of dampers using direct air flow readings or by temperature methods. Where possible a duct traverse should be taken to establish total O.S.A., or the use of 4" vane anemometer across outside air intake may be used. The temperature percentage method of calculation may be used whenever conditions of duct work or installation indicate improper readings or erratic readings at louvre face.

5-7

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Testing of Ceiling Plenum Systems (See Figure 5-1)

When air is delivered into the conditioned area through an acoustical ceiling, balancing the air distribution system cannot end at the duct outlet into the ceiling plenum. It must continue through the testing and adjusting of the plenum and the ceiling tiles themselves.

The ceiling plenum supply system is essentially a sealed area or box into which air is discharged at the required CFM with a predetermine pressure established to force the air from this sealed area through the ceiling, either by means of perforations in the tile itself or slotted runners that hold the tile in place.

O'"llrol ion

Ple num l uOOly 11)<1( '

i dileharQtd io ta illlr>um IhrOIlQh ducl

Comlorl ,II't I'. · ~6o rpmv .T.

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Pattern of air distribution

through plenum ceilinq

Figure 5-1 Pattern of Air Distribution through Plenum Ceiling

5-8

There are various types of ceilings on the market, but the principle of air supply is essentially the same in all of them. Typical design of such a system consists of a standard air handling unit or supply fan with a supply air duct running to the various sealed plenum areas or boxes over the space to be conditioned. Supply ducts terminate at the plenums, discharging the air into these sealed areas above the ceilings.

There are four critical factors that must be carefully checked In

balancing this type of system:

1. Penetration of air supply through ceiling to comfort level 6 ft. above the floor .

2. Even distribution of air over entire ceiling area inside plenum.

3. Required pressure to provide penetration through ceiling.

4. Final adjustment to air supply slots for correct velocities.

Here is a 6-step balancing procedure which, if followed, will enable the test and balance engineer to assure the satisfaction of these four major factors and to provide comfort conditions in the areas served:

1. Visual check, using high-powered light to check for leakage between barriers and seals.

2. Instrument check to establish leakage through tile joints and around perimeter area.

3. Instrument check to establish pressure in ceiling plenum at all locations.

4. Instrument and velocity check at ceiling level 2 ft. below ceiling level and at 6 ft. level above floor to establish correct velocity of air movement in FPM.

5-9

.IU ' f .. O(.ItOll'l",u. ,Uf e: ... t ... . • ..... . ~~Cfl'(O

Figure 5-2 Test Set-up

5-14

" I

Fume Hood Testing

A balanced hood and enclosure requires that the air flow through the opening and enclosure itself be such that full protection is maintained without interfering with the experiment or the personnel carrying on the experiment. Minimum air flow conditions which will furnish this protection, yet not waste conditioned or heated air, are also a requirement of good balanced conditions.

Two types of hoods are now generally in use. Type one introduces a source of make-up air and uses a very minimum amount, if any, of the surrounding conditioned air. The second type uses all surrounding air. In either case the air that flows through the enclosure must utilize an exhaust fa n system to move this contaminated air to the out-of-doors.

To achieve good balance in either type of system, the exhaust fan and supply air must be adjusted to accurate and correct amounts.

In general the following recommended face velocities should be applied under balanced condition across the hood face :

1. Low Toxicity Levels ... .. .. . .... ... . .. . 50 FPM

2. Average Toxicity Levels ........... . ... . 75 FPM

3. Low level radioactive materials or high toxicity levels .. . ...... .. .... .... . 100 FPM

4. Medium or high chemical toxicity levels ..... 150 FPM

Four separate tests must be made to properly balance fume hood systems:

A. Determine and adjust face velocity across the hood face or openmg.

B. Determine and adjust the exhaust air flow to out-of-doors .

5-15

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0)

A. B. C.

A. B.

® gj Roof Line .

Pitot traverse pasHia n. Static pressure readi ng. Exhaust air reading a t hood.

Roo! line

Position of face velocity readings. Position of measuring spillage or backdraft. Using smoke gun application.

Figure 5-3 Fume Hood Schematic

5-18

, l

..

When hood enclosures are banked together in a given area, particular attention must be given to the placement of air conditioning outlets. Placing these outlets close to the hoods will cause excessive draft conditions and in some cases, short circuiting of conditioned air. Optimum application is to place air outlets on oppcsite ends of room areas allowing hoods to draw this air across the room area. When hoods have a self-contained air supply, attention to the above details may not be needed. Because of the possible health hazards possible due to improper hood operations, hood operating tests should be performed at least once a year. Inspection and test results certificate should be placed in a conspicuous location by the testing agency.

Air Distribution Duct Leakage Test

Methods and Standards

To prevent the occurrence of leakage problems, the Consulting Engineer should include a duct leakage test section in the specification which would include a verification procedure and certification of tightness.

In most cases it is not practicable for the test and balance agency to perform these tests as it would require having a test technician continuously on the job as the ductwork is installed. From the standpoint of economy, it is more practicable to have the contractor conduct tests in accordance with AABC Test Standards, and have the agency verify the results obtained and issue a certificate. "The degree of air tightness in high velocity ductwork should not be compared with a water distribution system or gas system. Some degree of leakage will exist regardless of all the precautions taken during fabrication and installation, however, this leakage should be minimized to a degree that will not cause excessive problems.

Duct tightness can be determined by the application of proper pressure testing. When not otherwise specified, 1 % of the system air volume at 1.5 times the duct operating pressure is considered reasonably adequate.

5-19

Test Equipment

[n order to test sections of ductwork as it is installed, a portable means of testing is required. The most practical type of test apparatus which will facilitate field testing should consist of the following:

a. Source of High Pressure Air:

1. Rotary Type Blower Fan 2. High Power Tank Type Vacuum Cleaner

b. A Device to Measure Total Air Flow Accurately:

1. Calibrated Orifice Plate 2. Air Straightening Vanes 3. Pressure Tap and Receptacle Tube and Dampering Section

c. Instruments - Two Each

1. Magnehelic Gauge 2. U-Tube Manometer 3. Inclined Gauge

These items should be assembled into a portable device as shown in Figure 5-4.

Field Test Procedure

1. Seal all openings in duct section to be tested.

2. Connect test apparatus to the test section of duct using a flexible duct connection of hose.

3. Close damper on blower suction side to prevent excessive buildup of pressure.

5-20

A"

~Tr';'Ic."lrnlflt.

Figure 5-4 Portable Test Apparatus

4. Start blower and gradually open damper on suction side of blower.

5. Buildup pressure in the duct system being tested to the specified test pressure. (Use 1.5 times duct operating pressure if not specified.)

6. Read indicated pressure on the instrument that is connected to the section of duct under test. (See Figure 5-4)

7. Maintain this pressure for ten minutes which will indicate audible leaks.

8. Repair all visual and audible leaks. Shut down the blower and release the pressure when making repairs.

5-21

9. Upon completion of repairs, build up pressure to test pressure and read leakage pressure on the instrument connected across the test apparatus orifice plate. (See Figure 5-4)

10. Leakage CFM is read by consulting the calibrated chart as shown in Figure 5-5. If no leakage exists, zero pressure differential will be indicated.

11. Leakage factor allowable (1 %) should be based upon the total operating CFM of the section of duct under test.

•• .. o • • • • , • • • • . ,. .. ~

~

" ~ c-. " ,

., • • , • ,

, ,

o. • • .•• -0,"' ....... . ,.

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.,

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•• , • .. •

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l' ..... ~ . ... ... " ,. ... . . . ......... I •

Figure 5-5 Variation of Air Flow Rate with Orifice Differential Pressure

5-22

Test Verification

The air conditioning installation or sheet metal contractor should engage the services of a Certified AABC Test Agency to verify results and submit a certification certificate attesting to the results obtained.

Tested sections of the ductwork should be visually marked by the agency with a certification sticker and initials of the field test inspector. Tests should be made before duct sections are concealed.

HYDRONIC SYSTEM TESTING

A schematic piping diagram should be made by the field technician showing all locations of major components and flows required should be marked at all locations.

The entire system must be cleaned by the installing contractor prior to the start of balancing.

The following items must be checked before the start of balancing:

1. Check automatic fill valve setting and strainer.

2. Check expansion tank level.

3. Check all air vents at the coils and at the high points of the system.

4. Position all automatic valves, hand valves and balancing cocks for full flow through all coils, connectors and all items in the system requiring circulation of chilled or hot water.

5. Set all controls to maintain coil water inlet design temperatures with coil valves positioned for full flow through the coils during adjustments.

5-23

d. When systems have multiple coil sections, where possible, balance the water flow by establishing the design water pressure drop across each col. An alternate method of balancing multiple sections involves reading the water temperatures at each coil section with insertion thermometers or contact pyrometer probes, and adjusting the balancing cocks until uniform temperatures are obtained.

Condenser Water/Cooling Tower Systems

a. With the system off, confirm that the water level in the tower basin is at the correct level. On towers with variable pitch fan blades, verify that the setting of the blades is correct.

b. With pump(s) off, observe and record the system static pressure at the pump(s).

c. Place the system into operation and allow the flow conditions to stabilize.

d. Record the operating voltage and amperage of all fan and pump motors and compare these with nameplate ratings and thermal overload heater ratings.

e. Record the speed of each pump and/or fan as required.

f. With the pump(s) running, slowly close the balancing cock in each pump discharge line and record shutoff discharge and suction pressures at the pump gauge connections. Using the shutoff head, determine (and verify) the actual pump operating curve and the size of each impeller. Compare this data with the submittal data curves. If the test point falls on the design curve, proceed to the next step; if not, plot a new curve parallel with other curves on the chart, from zero flow to maximum flow. Make sure the test readings were taken correctly before plotting a new curve. Preferably one gauge should be used to read

5-28

"

differential pressure. It is important that gauge readings be corrected to center line elevation of the pump.

g. Establish uniform water distribution over the tower and check for clogged outlets or spray nozzles.

h. Record inlet and outlet pressures of the condenser( s) and check against the manufacturer's design pressure difference.

1. If there is a three-way valve used in the condenser water piping at the tower, check the pressure difference through valve with the water going both through the tower and/or through the bypass line. Set the bypass lien balancing cock to maintain a constant pressure at the pump discharge with the control valve set in the full bypass position.

J. Start the tower fan and check rotation, gear box belts, sheaves and water makeup valve. Check for vortex conditions at the tower suction connection. Check and record fan motor amperes, voltage, phase and speed.

k. Have the refrigeration system started. Verify the head and suction pressures and compare with design. After operation stabilizes under a normal cooling load, check and record the condenser water inlet and outlet temperatures. Observe and record the percent of load on compressor where possible.

l. After setting the three-way control valve (to control head pressure) in the condenser water line (paragraph i above), verify and record that it operates to maintain the correct head pressure by varying the flow at the tower. On units that have a fan cycling control verify that the fan cycles to maintain design condenser water temperature. If fan inlet or outlet damper control are used, verify that the dampers modulate to maintain the design condenser water temperature leaving the tower.

5-29

m. When electric or steam coils are used in the tower basic with low water cutoff controls to prevent freeze-up, verify that they will function properly.

n. Take another complete set of pressure, voltage and ampere readings on the pump system. If the pump capacity has fallen below design flow, open the balancing cock at the pump discharge to bring flow within 105-110% of the design reading, if possible.

o. Make a final check of all pump and equipment data, and record.

p. After all balancing work has been completed and the system is operating within plus or minus 10% of design flow, mark or score all balancing cocks, gauges, and thermometers at final set points and/or range of operation.

q. Verify the action of all water flow safety and shutdown controls.

Steam and Hot Water Boilers

a. Verify that the boiler(s) has been cleaned, flushed, and started; that all safety and operating controls have been tested, adjusted and set; and that the bumer(s) is operating properly.

b. With the boiler(s) operating under normal conditions, check the following:

•

•

•

Boiler feed pump( s) or makeup water system( s) operation.

Boiler, burner and pump nameplate data.

Boiler control settings (operating pressures and temperatures).

• Water flow rates and inlet and outlet temperatures (hot water boilers).

5-30

l.

• Steam boiler water level proper and steady.

c. On initial runs, hot water systems normally require additional air venting. Confirm that automatic air vents are operating and vent air manually as required . .

d. Steam traps can be checked for proper operation with a pyrometer.

e. Confirm that all automatic temperature control valves and steam pressure reducing valves in the system are in the proper position or mode of operation.

f. Confirm that all pipe strainers are clean.

g. The distribution of steam systems is set by the piping design and layout; therefore, no field balancing is required.

h. Follow the basic procedures for hot water or steam system work for items not mentioned above.

Heat Exchangers/Converters

a.

b.

c.

Determine the water flow pressure drop through the heat exchanger for all circuits. With the measured differential pressure, the water flow can be obtained from the manufacturer's submittal data curves or tables. Adjust the water flow to design conditions and record flow data.

Take inlet and outlet water temperature readings; check against design data and record.

Check and record the steam pressure; check the setting and/or operation of automatic temperature control valves, self-contained control valves, or pressure reducing valves where used. Record data.

5-31

d. Verify safety valve settings and operation.

e. Confirm that all pipe strainers are clean.

f. Check the operation of steam traps.

g. Check all automatic air vents; manually vent air as required.

h. Follow the basic procedures for hot water or steam system work for items not mentioned above.

Balancing Data Required

1. Obtain the following Pump Data:

a. Pump Manufacturer J. b. Pump No., Model No., etc. k. c. Impeller Size I. d. Pump Curve e. Motor Hp m. f. Service Factor g. Amperage n. h. Voltage 1. Cycles o.

2. Obtain the following Coil Data:

a. Location and Designation b. Design and Installed Data for:

1) CFM and BTU 2) GPM and Pressure Drop

Phase RPM Specified Pump GPM, RPM and Pump Head Running Amperes and Voltage Pump Suction and Discharge Pressure Final Pump Head and GPM

3) Water Temperature In and Out 4) Air Wet and Dry Bulb In and Out

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c. From Tests

1) Air Volumes across Coil 2) Water Temperature In and Out 3) GPM from coil pressure drop, orifice, flow station or

control valve (Cv)

It is considered good practice to use this cross check on all main coils regardless of whether or not flow station or pressure taps are provided.

3. Water Balance Final Report Data

Note:

a. 1) Pumps's flow and head 2) Flow Station Heads 3) Control Valve Cv heads 4) Terminal element pressure and temperature data

b. Field Data from Test and Balance

1) Pump heads, full flow 2) Pump running current, voltage and BHP. 3) Compare operating heads and BHP with pump curves

for verification of flow. 4) Flow station actual pressure drop and resulting flow

at final setting. 5) Compare with required pressure drop. 6) Check Cv of control valves through the coil and

through the bypass. 1) Compare Cv drops actual with required. 8) Test pressure drops, water temperature difference and

air temperature difference across terminal units . 9) Compare test figures with the required.

A heat balance should be run and reported on all main coils. It is well to remember that pump heads, orifice drops, etc. are a means of checking and balancing in an attempt to obtain a given result. The heat balance is the result.

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Water Balance with Coil, Control Valve and Measuring Station

A. Install gauge cocks at "C", "0", and "E". They should be as close to the valve as possible. (See Figure 5-6.)

B. Place the valve in the position for full flow through the coil. Check to be sure that the valve has seated against the bypass port. Connect the differential pressure gauge or manometer across gauge cocks "C" and "0". (See Figure 5-6.)

C. Adjust the balancing valve "A" until the differential matches the drop required with design flow and the control valve Cv. (See Figure 5-6.)

O. Place the differential gauge between "C" and "E". Read and record the coil pressure drop. (See Figure 5-6.)

E. Place the differential gauge between "0" and "E" and read this differential with full flow through the coil. (See Figure 5-6.)

F. Change the control valve to full flow through the bypass and adjust balancing valve "B" until the differential across "0" and "E" is the same as it was with full flow through the coil. (See Figure 5-6.)

G. Place the differential gauge across "0" and "F", read the differential and calculate flow using Cv of the valve on bypass. (See Figure 5-6.)

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A " B - Balancing ":tlves C, D, E, F, G & [I - ", auge Cock! I " J - The rmomete r Wells

Figure 5-6 Composite Coil Diagram

This example illustrates the various possible ways a coil may be tested for flow. It is a good practice to check by as many methods as are available .

Design

CFM EDB EWB LDB LWB GPM Drop Ft. Water In Water Out

. Installed COil

22,410 84 69 52.0 51.5 180 10.0 Max. 450

Control Valve CV

Flow Station Rinco 4" B

5-35

22,410 84 69 52.0 51.5 178 3.2 ft 450

58.40

80 Thm 100 Bypass

25" @ 178 GPM

FIELD TEST & SET

C to E D to E D to E C to D F to D G to H G to H CFM EDB EWB LDB LWB EW LW

40.0 In. Set Valve A - Flow Thru Coil 14.5 ft. Test Thru Coil 14.5 ft. Set Valve B - Flow Thru Bypass l1.2ft. Test Thru Coil 7.3 ft. Test Thru Bypass

28.0 In. Flow Thru Coil 28.0 In. Flow Thru Bypass 22,000 74.0 68.5 55.0 53.5 47.5 59.0

GPM ESTABLISHED THRU COIL

1. By Coil Drop

40" or 3.33' Test 185 GPM

2. By Valve Cv Thru Coil (C to D)

GPM

=

GPM head in psi

80 x

80 x 2.21

176 GPM

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(Use Spec. Gravity of 1.0)

11.0 2.3 Test 11 ft.

; ,

, ; L

(

r

f

3. By Flow Station - Rinco 4" B

28" Test Drop 190 GPM from Rinco Curve

4. By Total Heat Transfer

GPM - .BTU/CFM x CFl1 6TW x 500

Test EWE _ LWE _

CFM = LW = EW = T =

Cabinet Unit Heaters

Enthalpy 68.5 _ 53.5 _

22,000 59.0 47.5 11.5

5-37

147.74 100.44 _

47.30 BTU/CFM

GPM=

=

47.30 x 22.000 11.5 x 500

181 GPM

1. Check rotation of the fan.

2. Read and record the pressure drop across the coil and adjust if necessary.

3. Read and record the entering and leaving water temperature.

4. Read and record the entering and leaving air temperature.

Fan Coil Unit and Unit Ventilator

1. Set unit on high fan speed. Set outside air and thermostat on full cooling for fan coil units and full recirculated air for unit ventilators.

2. Measure the entering and leaving water temperature.

3. Measure the entering and leaving air temperature .

4. Measure the temperature or pressure difference.

5. Plot the temperature difference and balance flow or set design pressure difference.

6. Re-read the temperature or pressure difference.

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I

I L

Note: A flow mounted unit is defined as an "under-the-window" type. A ceiling mounted unit is defined as a "horizontal ceiling-hung uni t" to accept a duct system or free blow.

Unit Heaters --

....

- .-- .

1. Check rotation of the fan.

2. Read and record the pressure drop across the coil and adjust if necessary.

3. Read and record the entering and leaving water temperature.

4. Read and record the entering and leaving air temperature.

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Pumps

1. Read and record the nameplate data.

2. Check for the proper rotation.

3. Read and record the current and voltage.

4. Read and record the operating pressures.

5. Adjust to the desired quantity.

5-40

Chiller

1. All adjustments affecting chiller performance should be made in the presence of the chiller manufacturer's serviceman and to his specifications.

2. Read and record the condenser water temperatures and pressure entering and leaving the machine and adjust the cocks accordingly.

3. Read and record the chilled water temperature and pressures entering and leaving the machine and adjust the cocks accordingly.

5-41

Cooling Tower

1. Read and record all nameplate data on the tower and pumps.

2. Read and record the current and voltage on the pump motors and tower fan motors.

3. Read and record the pump pressures and flows and adjust accordingly.

4. Read and record fan CFM and adjust if necessary.

5. Read and record the wet and dry bulb temperatures of the inlet and outlet flows.

6. Read and record the water temperatures of the entering and leaving and make-up water.

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. .

HEPA FILTERS

A HEPA filter is an extremely high efficiency filter for sub micron particles and aerosols in the air. The minimum efficiency is 99.97% on an 0.3 micron particle. In the nuclear power industry, HEPA filters are standardized to a common size and general construction. 24 in x 24 in x 11-1/2 in (610 mm x 610 mm x 292 mm) with a flow from 1000 to about 1500 cfm (1700 m'lh to 2550 m'lh) is the universal standard.

Problems in HEPA Filter Use

The most common problems seen in design, operation, maintenance, and testing can be grouped into a relatively compact list.

Designing HEPA banks with a complete absence of prefiltration or a lack of sufficient prefiltration to protect them for a reasonable engineering and economic life is a very common problem. HEPA filters are very high efficiency "polishing" filters for a gas stream. They have relatively little capacity on a mass basis when compared on an equal size basis, and even lower on a cost basis, to a standard ASHRAE or NBS dust filter.

There is considerable confusion as to whether large (10 microns and larger) or small (under 10 microns to submicron) particles will raise the pressure drop faster. It is true that small particles, in the absence of large particles, will quickly fill the very small spaces between the filter fibers in the actual media and plug it to flow. The confusion arises in that this is a rare situation in actual practice. The tightly folded media in filter construction combined with the fact that only small particles are almost never found without large particles, the large particles blind the face of the folded media raising the pressure drop long before the small particles in the actual media become the significant factor. This has lead, in part, to the lack of adequate prefiltration .

Actual examples of poorly designed prefiltration systems in current plants are: (see Figure 5-7)

1. A HEPA bank for an emergency supply system with no prefiltration of any kind, drawing air from Midwest farmland.

5-43

During plowing in dry weather, it could plug up from large particles and fibers in hours, if not minutes, depending on wind conditions .

2. The nearly universal use of HEPA filters after a carbon adsorber bank (theoretically to catch carbon fines) with no prefiltration. Depending on the specific carbon used at any given time, the fines, if any, could be from quite large to very small. In either case, a 95% NBS filter would be more technically justified, either alone or as a first stage. In fact, this need for the HEPA bank is being formally questioned at the present time.

NO GREATF.R r mAN <5 C~·) /

V P ,! R H E

~ F E AIR FLOW I

L p

~ T E

L- A /

R

INLET 5' -0'

PLENUM

/ --! ,!

CHARCOAL

L- -

5'-0' 5'-0 '

20 ")( 50" ACCESS

/ ~OOR (TYP.)

H ~ E .. P

AI/( \

OUTL T E PLENU M

Figure 5·7 Ventillation Filtration Diagram

3. Large ventilation systems for major areas of plants, where any type of particles could be generated, with only a single stage of medium efficiency prefilters before the HEPA bank. These are often found to need (or would be greatly improved by) an additional stage of lower efficiency roll or bag filters to trap the very large fibers and particles actually existing. Conversely, some of these systems have only low efficiency prefilters that are not of sufficient efficiency to protect the HEPA filters.

5-44

In the area of operation and maintenance, the common problems are lack of understanding of the relative costs and importance of the prefilters vs. the HEPA filters. First, the prefilters have no regulatory significance as far as removal credit or requirements for testing. By Plant License requirements, most HEPA banks must be subjected to rigorous, aerosol leak tests whenever changed. It is, therefore, not simply the cost of the filters themselves to compare on a cost/benefit analysis of when to replace filters, but the regulatory (i.e., Plant License) significance of the HEPA bank and the cost to test it . At times this is not fully grasped by the maintenance staff. Pressure drop readings are the most common basis for prefilter changeout. The frequency of the readings and the basis for the schedules vary greatly. There has not been a formal program to check prefilter pressure drops after an event that would reasonably be expected to cause excess prefilter load and, therefore, reduced life. There have been cases where the pre filters were so loaded with dirt and debris that they had failed, and the HEP A filters were grossly loaded. Where the prefilters have not failed but are loaded in excess of system design, the system flow is often below design standards.

There are many events or situations that can change the normal particulate concentration in the airstream. In addition to the obvious ones of grinding, sanding, cutting, etc., such things as working on pipe insulation (fiberglass or "wool") or changing environmental conditions around the plant will have a direct effect on the prefilter life. The example of pipe insulation is quite real. A power plant had initially installed a HEPA filter system without prefiltration to exhaust air from a "clean" area. During construction, a great deal of pipe insulation was added in the area ventilated. This was bare insulation with the fibers exposed, as there was to be no normal traffic or access to this area. Fortunately, this potential problem was discovered by a consultant and prefiltration stages were added. Operating experience has shown most prefilters require at least annual changeout, which is very cost effective compared to changing the HEPA filters annually.

Lack of adequate access to changeout HEPA filters, deficient clamping devices that are not able to provide a good gasket compression and are difficult to manipulate while fully dressed in anti-contamination clothing, and lack of adequate space around the system housing are also common problem areas.

5-45

HEPA Filter Testing

Tests of an individual HEPA (high efficiency particulate air) filter are of two types. The test by the factory or quality assurance station is an efficiency determination using a monodisperse challenge aerosol of 0.3 + 0.03 m diameter droplets . The minimum specification value for HEPA filter efficiency is 99.97% efficiency (where efficiency equals 100 minus percent penetration). Most filters today run about 99.97% efficiency. In the efficiency test, the total filter is challenged at one time and a single reading of penetration is obtained.

In-place field tests of installed HEPA filters are made with a polydisperse DOP aerosol, and do not show the efficiency of the filters but only reveal the presence of leaks in the system, scanning may be used if necessary to locate the leaks. If the penetration observed in the test is equivalent to the penetration established during factory testing, however, it can be inferred that the particle-removal efficiency of the system is equivalent to that of the individual filters. This is the basis for many persons identifying the in-place test as an efficiency test. The in-place test is not an efficient test and should not be so considered.

I-IEPA Filter Testing Problems

Testing is another major area where lack of understanding of HEP A filters and basic aerosol physics contributes to system problems.

Even when a HEPA bank is to be tested, adequate provision seldom is designed and built in to perform an acceptable integrated aerosol leakage test. Where there is good prefiltration, the DOP aerosol usually must be injected far enough upstream of the housing to obtain good mixing in the duct, pass partly through the prefiltration stages, and then challenge the HEPA bank. For large systems with high efficiency pre filters, this is a problem for DOP aerosol generation, since a significant percent will be filtered out by the prefilters .

There is then the problem of a downstream sample when an adsorbent bank follows the HEPA bank (a common requirement). How is a downstream sample obtained when there is only 3 to 5 feet between a HEPA

5-46

bank and an adsorbent bank? The nearly universal provision provided by filter system vendors and accepted by both NE and utility engineers is an access hole in the door or housing. The N509 standards and NRC regulations 1.52 and 1.140 have clearly called for permanent test manifolds or other provisions to allow the mandated leak tests, but such provision has never been provided by the original vendor to date. Where they exist, they have been added by the plant after startup has proven their need.

HEPA Filter Test Procedures

The in-place test is a leak test of the installed system and should not be confused with the efficiency test of individual filters. This test is used during acceptance testing of the air cleaning system, after any filter replacement, or after any maintenance activity in the filter housing to verify (1) that the filters have not been damaged, (2) that they have been installed properly, (3) that there are no leaks in the mounting frame or ·between the mounting frame and the housing, and (4) that the system contains no bypassing (e.g., through defective or inefficient bypass dampers, through adjacent plenums, or through penetrations, such as electrical conduits, which penetrate the mounting frame) which would compromise the function of the filters. The test is also made periodically in both operating and standby systems to check on possible degradation of the filters or the filter installation (e.g., development of cracks in the mounting frame or mounting-frame-tohousing seal). This covers only the gross test but can be used as a basis for the development of procedures for a shrouded test. The shrouded test is sometimes used when extensive scanning of the bank (and therefore extended release of challenge aerosol or gas) is expected. The shrouded test is valid ONLY if a satisfactory pressure-leak test of the mounting frame has previously been completed.

Summary of Method

With the system fan or an auxiliary blower operating, DOP aerosol is injected upstream of the filters. Concentration measurements are made upstream and downstream of the filters and percent penetration is calculated from the ratio of DOP concentrations in the filtered air (downstream reading) and the unfiltered air ' (upstream reading). If penetration is greater than the

5-47

value specified in the test procedure, the test is stopped and the system reinspected for leaks or bypasses. If leaks or bypasses cannot be located visually, the fan and DOP generator are turned on again and the downstream face of the mounting-frame-to-housing seal, the peripheries of the individual filters, and finally the faces of the filters, in that recommended order, are scanned. After location and correction of leaks and bypasses or, if necessary, replacement of defective filters, the in-place test is repeated for record. The test should be performed at the airflow required for each individual system.

Prerequisites for Test

The downstream sample point should be located, if possible, at a point where a single-point sample, representative of the downstream concentration, can be taken; this may be a point downstream of the fan or auxiliary blower, or a point downstream of a flow disturbance which will provide adequate mixing of the DOP-air mixture emitting from the filters in the bank. Where it is impossible to obtain an adequate single-point downstream sample, a multiple sampling technique is required. There must be adequate room and safe working conditions for test personnel and equipment. Verify that DOP shall be injected at a point far enough upstream to disclose any possible system bypasses.

Apparatus

Dop Generator - An air-operated generator or gas-thermal generator certified by the manufacturer to be capable of producing the droplet-size distribution required. The generator output and/or penetrometer adjustment as specified in the test procedure shall ensure penetrometer sensitivity high enough to permit detection of leaks at least two times smaller than the maximum leak allowed by project specifications. The DOP concentration shall not exceed the linear response capability of the detector.

Penetrometer - An instrument with a linear read-out, near-forward lightscattering aerosol photometer having the following characteristics is recommended:

5-48

I , .

l

1. Threshold sensitivity to permit detection of test aerosol in concentrations of at least as low as 10-3 g per liter of air and having a minimum reading at this concentration of 1.0% when set on the most sensitive scale.

2. Capability of measuring concentrations of DOP in air of at least 10' times the threshold sensitivity of the instrument used for the test.

3. For testing of systems larger than 1000 cfm installed capacity, a sampling rate of at least 1 cfm.

4. Linear response from minimum detectable aerosol concentration to maximum upstream concentration.

System fan or auxiliary blower capable of producing the airflow and pressure specified in the test procedure.

CHARCOAL ADSORBER REQillREMENTS AND TESTING

Adsorber/Adsorbent Requirements

Pleated bed and tray-type adsorbers cells shall meet the requirements for Type I or Type II cells, respectively, of AACC CS-8; and for ESF systems they shall be filled with an adsorbent, each batch of which meets the requirements of Table 5-1.

1. Joints which are gasketed, caulked, or sealed with elastomeric materials shall not be employed between the upstream and downstream sides of the adsorbent bed, frames or any part of the installation except for removable test canisters.

2. The adsorbent bed shall be so arranged that no air can bypass the adsorbent and the minimum residence time of air in the adsorbent is 0.25 seconds per 2 inches bed depth. There shall be no internal structures within the adsorbent bed, such as throughbolts, where air bypass can occur.

5-49

3. Screens shall be supported by stiffeners which are external to the adsorbent bed to assure uniformity and integrity of the bed.

4. Means shall be provided for filling the unit with adsorbent and compacting it to uniform ' packing density throughout all cross sections of the bed. In a vertical direction, this density shall vary only to the extent that the lower portion of the bed supports the weight of the adsorbent placed above it.

5. All material in contact with the adsorbent shall be Type 300 Series stainless steel, or equivalent.

The capacity of an adsorber shall be determined by the equation:

c

Where: C -t =

A -

b -

T -

28.8 =

t(A - b)

28.8T

nominal capacity (cfm) thickness of adsorbent bed (in.), normally > 2 m. gross inlet or outlet screen area. The lesser of the two shall be used (in.') total area of baffles, blanks, and margins of all screens (in.2) residence time, seconds, in the stage required to achieve the specified iodine DF, using the adsorbent specified in the technical specification normally 0.25 seconds per 2 in. thickness conversion factor

For ESF units, the adsorbent shall meet the requirements of Table 5-1.

5-50

Table 5-1 Performance Requirements and Physical Properties of (unused) Activated Carbon

Test

Performance Requirements

Molecular Iodine, 30°C, 95% RH(I)

Molecular Iodine, 180°C Methyl Iodine, 30"C, 95% RH

Methyl Iodide, 80"C, 95% RH(I)

Methyl Iodide, 13O"C, 95% RH(2)

Physical Properties

Particle Size Distribution

Ball Pan Hardness CO, Activity (on base) Apparent Density Ash Content (on base) Ignition Temperature Moisture Content pH of Water Extract

NOTES:

Test Method

ASTM 03803

ASTM D2862

ASTM 03802 ASTM 03467 ASTM D2854 ASTM D2866 ASTM D3466 ASTM D2867 ASTM 03838

'Tests shall be performed only for qualification purposes.

Acceptance Value

0.1 % penetration, maximum 99.5% retentivity, minimum 3% penetration, maximum 1 % penetration, maximum 2% penetration, maximum

Retained on #6 Sieve: 0.1 % using 8 x 16 maximum U.S. Mesh Retained on #8 Sieve:-5.0% maximum Through #8, on #12 Sieve: 60% maximum Through #12, on #16 Sieve: 40% minimum Through #16 Sieve: 5.0% maximum Through #18 Sieve: 1.0% maximum

92 minimum 60 minimum 0.38 glcm 3 minimum state value 330°C minimum state value state value

2Test shall be performed only for qualification purposes on activated carbon to be installed in primary containment cleanup· system.

5-51

Charcoal Adsorber Test Procedures

Efficiency, in the usual sense, cannot be measured for adsorption systems. Adsorption is time dependent and therefore instantaneous contaminant-removal efficiency is meaningless. True efficiency tests are run on small, representative samples of the adsorbent using a radioactively tagged tracer having similar properties and composition of those of the contaminant of interest (e .g., radioactive elemental iodine or methyl iodide). The tagged challenge gas is mixed with air and flowed through a sample bed of the same thickness as the beds in the system, at the same airflow rate as the airflow through the beds in the system. The amount of challenge gas retained over a specified period of time (usually 2 hours), compared to the quantity in the unfiltered air establishes the efficiency of the adsorbent for that particular contaminant gas (adsorbate). Because of the difficulty of handling radioactive materials, this type of test is generally not made in the field.

Factory tests of full size cells and in-place field tests · of installed systems, using a refrigerant-gas, are leak tests only. The tests are designed to determine only the amount of leakage through or around the adsorbent in the cell (factory test) or through or around the installed bank of cells (field tests). Poor-performance adsorbent is not detected by these tests.

Penetration values shown on individual cells by the manufacturer, unlike the penetration values shown on HEPA filters, do not indicate contaminant-removal efficiency, but only leak-tightness. Therefore, the contaminant removal efficiency of an installed adsorber system cannot be inferred from the penetration values shown on the individual cells, as can be done with HEPA filters.

The efficiency of the individual cells and of the installed system can be assumed to have a given value only on the basis of the tests made on representative samples of the adsorbent used in those cells and systems. An installed system can be assumed to have an efficiency equivalent to that of the sample only if:

5-52

j ,

t.

1. The sample is actually representative of all of the adsorbent in all of the cells in the system.

2. All of the cells are filled properly in accordance with a qualified filling procedure which will ensure a "tight pack".

3. There are no leaks or bypasses in either the individual cells (factory tests) or the installed system (field tests).

Purpose

This test is used for both acceptance and surveillance leak-testing of the installed adsorber stage. If samples of adsorbent are to be taken for laboratory testing, remove such samples prior to this test, and restore stage to operating condition.

Summary of Method

A refrigerant tracer gas is injected into the air stream upstream of the adsorber bank, tracer concentrations are determined downstream and upstream of the bank, and penetration (percent leakage) is determined from the ratio of downstream to upstream concentration at time zero.

Prerequisites for Test

The downstream sample point should be located at a point where a single point sample, representative of the downstream concentration, can be taken; this may be a point downstream of the fan or auxiliary blower, or a point downstream of a flow disturbance which will provide adequate mixing of the tracer-air mixture emitting from the adsorber stage. There must be adequate room and safe working conditions for test personnel and equipment. Verify that tracer gas will be injected at a point far enough upstream to disclose any possible system bypasses.

5-53

Apparatus

Tracer Gas. R-ll is preferred; R-1l2 (or RlO1l2A) is an acceptable alternate.

Tracer Gas Detector. The tracer-gas detector shall have demonstrable capability to distinguish the tracer gas from background.

Tracer Gas Generator. The tracer gas output shall be at least 4 times the Minimum Workable Threshold Sensitivity (MWTS) of the tracer gas detector divided by the maximum acceptable leak rate, expressed as a fraction of total system airflow. The MWTS is the concentration of tracer gas which will produce response on the readout of the tracer gas detector. The generator output shall be held within +20% of the pre-set value.

System Fan or Auxiliary Blower. Capable of supplying required flow rate.

SUMMARY

[n this chapter, we have discussed some of the tests that are done to ensure that an HV AC is operating properly. It is important to remember that the results from a test is only as good as the data collected and the experience of the tester.

The first major topic was on Air Flow Measurement in which we discuss the different instruments used on supply and exhaust grilles and registers to the testing of air flow in a duct using the Pitot tube and the procedures used to do the test.

We next covered testing of hydronic systems. It is very important to remember to balance the hydronic system prior to balancing the air fl ow in an HV AC system and that a small change in one area can cause a big change in a ll HV AC systems. Prior to doing any changes on the air flow or water in an HV AC system, we should anticipate those changes and check that we get the desired results.

5-54

I

! t _

L

Along with balancing of air systems, we looked at the test we do to make sure that the quality of air is also right. The checks/tests we covered were the tests on HEP A filters and charcoal filters.

5-55

, . ,

CHAPTER SIX SOUND AND VIBRATION TESTING

, ,

CHAPTER SIX SOUND AND VIBRATION MEASUREMENT

OBJECTIVES

At the completion of this chapter, the student should be able to:

1. Discuss the purpose of performing and usefulness of vibration measurement and signature analysis.

2. Discuss ' the importance of sound measurement, methods of controlling sound levels and the characteristics affecting sound level strength.

3. Describe the procedure for the performance of sound measurement.

4. Discuss vibration and noise identification and methods used for source analysis.

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CHAPTER SIX SOUND AND VIBRATION TESTING

INTRODUCTION

Sound Testing, in accordance with the Noise Criteria Curve using the Sound Pressure reference of .0002 microbars, is the preferred testing method because we can truly measure "Sound Pressure."

AMCA Standard 300-67 Test Code for Sound Rating states" A person hears and judges sound on the basis of a Sound Pressure Level at the point of observation, and this is also what the Sound Meter detects, For a given source, however, the Sound Pressure Level varies with the environment. For example, a unit heater will sound louder in a hard walled room than in a room which is carpeted, has drapes and upholstered furniture. This is despite the fact that its Sound Power output is the same. That is the reason why this code is based on Sound Power Level rather than Sound Pressure Level."

AMCA also states under "Field Testing"; "It is a relatively simple matter to determine the Sound Pressure Level in the conditioned space resulting from the operation of the air system. However, it will rarely be possible to use this information to establish the Sound Power Level of the air moving device, This would be possible only where circumstances permitted simulation of a laboratory setup, etc."

Since it is a relatively simple matter to determine the Sound Pressure Level in the conditioned space, it is also a simple matter to isolate components until the noise source is found.

Engineers need realistic data on sound levels in order to design a system that will meet Noise Criteria (NC) levels acceptable to his client. The industry should push forward toward the development of a usable data system. With usable information, engineers could design the system such that it will not exceed a certain NC level. It is unfortunate that laboratory rating data is acceptable in designing a system, when field applications will

6-1

prove that the rating is reduced, often by as much as 50%. As Testing Engineers and Technicians, we must be alert and aware of these facts, and ensure the Design Engineer is also aware of them so he can design a predictable system.

The Sound Level in the space to be tested can be measured with a good Sound Meter and the Sound Level established. The problem is identifying the equipment that creates the noise. The fan manufacturer says, "You cannot be sure it's his equipment as there are other components that could cause the noise, such as mixing boxes, diffusers, and ducts. Also, the room acoustics are a factor." The diffuser manufacturer is more realistic and relates his equipment to velocity and pressure drop which can be easily measured. If properly selected, these can be removed as a noise source. Also, a reading at one diffuser compared with other diffusers will indicate it's relationship to the noise source. The same applies for most of the other system components. Thus, by the process of elimination, noise can be traced to its source.

In order for a person to get the whole picture of HV AC, they cannot neglect the consideration of the sound and noise generated by HV AC equipment. In this chapter, we will cover the following topics:

• Sound Architectural acoustics Sound testing

• Vibration Vibration testing

• Vibration and Noise Identification Analysis procedure Identification Relative probability ratings

6-2

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Sound is a form of energy, detected as a variation in pressure and stress in an elastic/viscous medium. Sound is an integral part of any system, but only when it reaches a level that interferes with speech, a person's well being, or a preconceived condition, does it become "objectionable", and becomes noise. There are several ways of describing the characteristics of sound. The ways we will be discussing are:

• Sound power

• Sound pressure level

• Loudness and frequency

• Noise curves (NC curves)

Sound Power

An acoustical source radiates energy in the form of sound. This power is expressed in watts. A "watts exponential" scale of sound power has been developed. A sound power level of 10-12 watts represent the threshold of hearing. Table 6-1 list the 'decibel valve corresponding to a given watts exponential, with an example of this power level, a scale from 0 to 200 dB.

Sound Pressure Level

Sound power cannot be measured directly but must be calculated from pressure measurements. If an imaginary sphere is placed around a sound source (with the source at the center of the sphere), all the energy from the source must pass through the sphere. Power flow through a unit area of the sphere is intensity, expressed in watts power unit area. Intensity varies

'decibel - A unit used to express relative difference in power, equal 10 lOx the common log of the ratio of the two levels.

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inversely as the square of the distance from the source. The intensity and the sound pressure level are nearly identical numerically if proper units are used.

Table 6-1 Sound Power Output

A".. I .r_o.p n~ nO> DldlMlm ...... U- EQal.llIal I,-Il .. a.

Sawm roct~ ]00,000,000 ," ... TUrbo;et~~ 100,000 ," ,,. J"aircnlll' takeoUt. 10,000 ," , .. 1\Irbopn)p I' takeoff '.010 ," ,,. Prop airaaft I: takcorr' '''' ," ,., Larp pipe Oq&n 10 10' ". SmaU aira'afl maine , ,e' '''' Blariq radio ., 10-1 II. Aulomobile.tllilh .... Y lpeed 0.01 10- 2 '''' Voice, .$houlinl 0.001 10- ) go OarbllJe disposailUlit 0.0001 ,.~ ., Voioee, o:onva'S.Itioa lrood 0.00001 10-' " El«lronic equip.

_til,Lionf.n 0.000001 , ... .. om« air dirrusu 0.0000001 10-7 ,. Slnall decuic ck.ct 0.00000001 10- ' ., Voioee, $Of' whisper O.OOXlOOOOI :0'" 10 RU$lUn,leava O.OOJOOOOOOI 10- 10 '" Human breath 0 .0000000000. 10- 11 ,. Tbrallold of hellrin. 0.00000000000. 10- 11 • 'With .rterbumcr. bFour jei en,ines. <Few: prOpdkr!"lina.

The conversion from Sound Pressure Level (SPL) to Sound Power Level is PWL = SPL + 20 log 0 + .5db (D is in feet and is the distance from the Sound Source). A Noise Level will decay over a distance; therefore, it is inversely proportional. The close to the sound or noise source the louder it is.

6-4

Loudness and Frequency

Sound may be visualized as traveling in a wave pattern similar to that of alternating electrical current (Figure 6-1). Variation above and below the reference is called amplitude and determines the loudness. The distance from one wave peak to the next is the wavelength, the reciprocal of which is the frequency or pitch of the sound. Frequency is measured in cycles per second (cps). The term Hertz (Hz) is used instead of cycles per second. Figure 6-1 represents a pure tone - one single frequency. Most sound is made up of several frequencies (tones), with each frequency having a different loudness. For example, air noise in a duct is made up of several high-frequency tones generated by turbulence of the air due to fittings and obstructions, as well as the due to straight line flow and friction against cut walls. This air noise will usually be accompanied by sound transmitted from the fan, with a predominant frequency which is a function of fan speed and number of blades.

-Wavelength I •

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Figure 6-1 Wavelength and Amplitude of Sound

A good human ear can hear, and distinguish, a wide range of frequencies - from a low of about 20 Hertz to a high of about 20,000 Hertz. For the purposes of analysis this range is divided into several octaves. Two tones are said to be an octave apart when the frequency of one is twice that of the other. A common example of this is the musical octave on the piano or other instrument. On the musical scale the A below middle C has a frequency of 440 cps.

6-5

NC Curves

Noise criteria curves were the standard for many years and define acceptable limits for sound pressure level in each octave band. Figure 6-2 shows the standard NC curves. The actual environment must not exceed the specified curve at any point, but can be at any level below the curve. The resulting sound may be too quiet in some frequencies. A higher sound power level is acceptable at lower frequencies. These curves emphasize the fact that high frequencies sound "louder" than low frequencies when sound power levels are equal.

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Figure 6-2 NC Curve

6-6

Architectural Acoustics

Architectural acoustics deal with the sound attenuation in a room or a building. The degree of acoustical control required depends on the use of the area.

Sound waves emitted from a sound source, vocal cords, etc., cause a deviation in atmospheric pressure above or below the static value. This is called Sound Pressure. Sound waves cause pressure and have frequency. Frequency is the number of times per second that the sound pressure alternates above and below the ambient atmospheric pressure. The higher the frequency, the higher the pitch. Sound waves travel at the speed of 1,125 feet per second at 77" F at sea level. Sound travels in all directions from the source. When it strikes a surface its direction of travel is changed by reflection. The reflection follows the law that the angle of incidence equals the angle of reflection. A person hears not only the direct and reflected sounds, but background sounds also, resulting in a total sound which is always louder than the direct sound alone. How much louder depends on the size of the room, the distance from the source and the sound absorbing properties of the room surface.

Sound absorption coefficients for various materials range from .05 to .999, meaning a .05 absorption has a 95% reflection and is considered hard. Such materials are concrete, glass, plaster, etc. An absorption coefficient of .90 reflects only 10% of the sound waves that reach its surface. Porous materials which permit penetration of sound waves or soft materials have large absorption coefficients. Such materials are carpets, drapes and specific acoustical material.

The amount of sound waves absorbed in a room is given in uni ts called Sabins. A Sabin is the equivalent of one square foot having an absorption coefficient of 1.00. An example is 100 square feet of room surface with an absorption coefficient of .80 equals 80 Sabins.

In large rooms, air itself absorbs sound particularly at high frequency and should be added to the room total Sabins.

6-7

The following example shows a typical sound absorption calculation. Note in the table of sound absorption coefficients that the coefficients vary with frequency.

Assume a 500 CPS room 100 X 40 X 10 feet Volume = 40,000 cubic feet Floor - Concrete - 4,000 sq. ft. @ .015 Walls - Plaster - 2,800 sq. ft. @ .03 Ceiling - Plaster - 4,000 sq. ft. @ .03

20 persons - - - - - - - - - - - - - - - @ 2.40 TOTAL ROOM

Reverberation Time

= 60 Sabins = 84 Sabins = 120 Sabins = 48 Sabins = 312 Sabins

The amount of reverberation in a room is measured by its reverberation time, which is the number of seconds required for the energy of the reflected sound in a room to die out to one millionth of the value it had at the moment the source was cut off.

This may vary from a fraction of a second in a very dead room to 5 to 15 seconds in a very live room.

Reverberation time depends only on the cubic volume of the room and on its total absorption in Sabins.

Formula T = .05V/a V = volume of room in cubic feet a = room absorption in Sabins

Reverberation Time

below 1 second - good for speech, probably too dead for music 1 to 1.5 seconds - good for speech, fair for music over 2 seconds - poor for speech, fair to poor for music

6-8

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Using the room in the previous example:

T

.05 x 40,000 312

2.000 = 6.4 seconds 312

6.4 seconds is a live room and any audible noise source would probably be objectionable.

To give this room a more acceptable reverberation time, the calculation is as follows :

a (Sabins) = .05V T

a = 2,000 = 1.5

a = 1,333 Sabins

A rug could be put on the floor. This now becomes 4,000 x .39 = 1,560 Sabins. This alone is more than enough to reduce the reverberation time.

A new calculation:

Floor with rug Wall Ceiling 20 persons

= 4,000 x .39 = 2,800 x .03 = 4,000 x .03 =20x2.4

= 1,560 = 84 = 120 = 48

1,812

2,000 = 1.1 seconds. This may be too much of a decrease if 1,812 it were a music room.

6-9

Sound Trap Selection

Catalog data will give what attenuation can be expected from a specific Sound Trap at a specific air velocity. Therefore, your sound test will give you the level before the sound trap. Calculate what you need to bring the sound level to the specified NC level and select a sound trap that will do the job.

Sound lined ductwork will give some sound attenuation. Chapter 33 of the ASHRAE Guide data book provides useful charts and data on the subject.

The two main sources of noise are the fan and turbulence in the duct system. Mixing boxes, diffusers and dampers also create noise, but to a lesser extent. High frequency noise levels are easier to attenuate than low frequency noise.

Dynamic insertion loss is the sound trap or attenuation between sound source and the space where the test is being performed. Insertion loss without air flow is used in rating of sound traps.

Self-Noise Power Levels (db 10,12 watts)

A sound trap will produce noise; therefore, self-noise power levels are given and determine the trap's acoustical characteristics. Note - this rating is given in Sound Power rather than Sound Pressure, so a conversion calculation has to be made.

Manufacturers of sound traps test and rate their sound traps In

accordance with ASHRAE 36-B-63 and SIW 42 of ASA

Sound Testing

Assume a specification calls for an Octave Band Analysis of a sound source of NC 35.

6-10

,

1. Calibrate the sound meter.

2. Thm off all equipment and take a background reading. If the background noise level is close to the actual noise level, make a correction to your db readings per the manufacturer's correction charts before plotting on the NC Chart.

It is important to clear the area, if possible, of persons other than yourself so their disturbances will not be recorded in background noise or final readings.

3. Take a series of readings per specification with all equipment turned on, as in normal operation.

4. If the specified NC Curve is exceeded, then turn off one piece of equipment at a time and take a series of readings.

Then take another series of readings until you have a set of readings of all possible noise sources. From this procedure you can tell which unit or units are causing the higher noise level.

5. Take readings at each diffuser or within 6 feet of each diffuser which will tell you if a damper or diffuser, grille, etc., is the cause of the higher noise level. Sometimes a simple adjustment of the damper will be sufficient. Splitter dampers are noisy when closed or open and should be avoided. A good ridged volume damper in the branch diffuser and main trunk is better for a quiet operation. Slowing a fan down will reduce the noise level if you have an excess of air or if you must compromise.

Sound Testing Specification

Instrument

Sound testing meters should be an Octave Band Analyzer which essentially complies to ASA Standards S1.4 - 1961 with a range of 24 db to 150 db Sound Pressure Level.

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As vibration is a source of sound waves or noise, the source of noise or noise level can be reduced by solving the vibration problem. Components causing vibration in HVAC equipment. are as follows:

1. Misalignment (drive, belts, etc.) 2. Defective bearings 3. Coupling misalignment 4. Fan wheel or pump impeller out of balance 5. Bent shafts 6. Drive Pulleys out of balance 7. Fan casing or support structure not rigid enough for

centrifugal load 8. Electrical - synchronous frequencies 9. Aerodynamic - ductwork pulsation 10. Hydraulic - water carried pulsations

If the vibration test exceeds recommended limits, isolate the motor from the equipment fan by disconnecting or removing belts of the coupling. Then test the motor separately; if vibration is normal, then trouble is in the HV AC equipment. Measurement of vibration in mils of deflection is sufficient to determine the severity of vibration. Other measurements, for instance velocity, frequency test, and stroboscopic light, may be used to locate the exact point of vibration.

The balancing agency should make vibration tests on all rotating units of equipment and other items. Tests shall be taken in general, on top and on the side of each bearing, on two points on the equipment housing and base 90° apart, and on duct or pipe after the flexible connection. Each point shall be read in mils of deflection, then compared with the allowable tolerance for the respective unit of equipment and recorded on the proper form. Where vibration readings deviate from normal, a separate report should be forwarded to the architect and engineer with a recommended action to be taken.

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Vibration Testing Procedure

Balance the unit for proper air or water flow in accordance with design requirements. Make certain that the equipment is working at or within the rated capacity. If not, then partial loading of curve conditions could cause excessive vibration.

After balancing proceed as follows:

1. Set up instrumentation for testing on a base other than where the equipment is being tested.

2. Clean the test area so as to be free from grease or dirt that could cause slippage and false readings.

3. Zero the instrument. If battery powered, be certain batteries are up to the required power level.

4. Secure the measuring device (accelerometer, reed, etc.) on the test area keeping clear of the rotating parts.

5. Read out vibration in mils of deflection or velocity as required by the specification.

6. Record and tabulate all vibration readings on the proper forms.

7. Test the following:

a. BHP b. operating pressure across the unit c. operating flow across the unit d. check belts for cracks if applicable e. visually check and note any condition that could be a

contributing factor such as vibration isolators not set up correctly, debris under unit grounding it to the structure.

f. give manufacturer 's vibration tolerance if there is any and where measured if poss ible

6-15

8. If vibration exceeds recommended limits per manufacturer's requirements or severity chart, disconnect the motor and test separately.

9. Shut off the unit and take a reading on the casing to determine if there are external forces of vibration.

10. Evaluate readings and data. Write the report on what you think the problems are and make your recommendations.

Vibration and Noise Identification

The primary objectives of control of vibration and noise in machinery are:

a. to achieve acceptable machinery operating conditions b. extend machinery life c. lower maintenance costs d. reduce machine failures e. reduce operator discomfort

Increased machinery speeds, greater complexity, increased power and higher equipment costs have created new problems that must be analyzed to achieve the above objectives. Vibration and noise analysis is used to diagnose specific machinery problems without major disassembly and physical inspection. New and rebuilt machinery installations can be checked for mechanical condition.

Vibration and noise analysis is the procedure of measuring the vibration and noise present in a machine and analyzing the characteristics of that vibration and noise to determine the cause. By comparing this information with known machine characteristics; machine speeds, vibration and nOIse, machine troubles can be pinpointed and corrective action prescribed.

6-16

POINT 1&2 3&4 5&6 7&8 9 & 10 11 & 12

Remarks

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VIBRATION TEST CENTRIFUGAL FAN __

10

---· 12

8

Fan bearing drive end top __ Fan bearing opposite end top __ Motor bearing drive end top __ Motor bearing opposite end top __ Casing top __ Duct or casing after flexible connection discharge __

6·17

side side--

side side--side __

suction __

VIBRATION TEST IN·LINE FAN __

--- ...... 9 / /" 1' 2 \ -( Gl -, - -1+

POINT 1&2 3&4 5&6 7&8 9 & 10 11 & 12

Remarks

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Fan bearing drive end Fan bearing opposite end Motor drive end Motor opposite end Casing Duct after flexible connection

6-20

top side --top side __ top side __ top side_~

bottom & top __ side __ discharge side --

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VIBRATION TEST HORIZONTAL SPLIT CASE PUMP __

6-21

POINT 1&2 3&4 5&6 7&8 9 & 10 11 & 12

Remarks

VIBRATION TEST END SUCfION PUMP __

Pump bearing drive end Motor bearing drive end Motor bearing opposite end Coupling or shaft support Structure Pipe after flexible connection

6-22

II

12

top side __ top side __ top side __ top side __ top side __

discharge __ suction __

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VIBRATION TEST AHU-UNIT __

14.

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POINT 1&2 Fan bearing drive end top side 3&4 Fan bearing opposite end top side 5&6 Fan bearing center (if applicable) top side 7&8 Motor bearing drive end top side 9 & 10 Motor bearing opposite end top side 11 & 12 Casing bottom & top side 13 & 14 Duct after flexible connection discharge suction

Remarks

6-23

Vibration and noise are the signals that reveal the presence of mechanical faults. The vibration and noise are measured and then compared to standards which enable the analyst to judge whether or not a fault is present, and if so, to classify its severity. If a fault reveals its presence, a careful analysis of the frequency components presented in the complex signal will allow you to positively identify the problem. Identification is a simple procedure involving relating observed frequencies to the frequencies generated by known mechanical sources within the machine. For example: vibration and noise caused by worn or damaged gears will have a definite frequency component related to the RPM times the number of gear teeth. Frequency of noise and vibration is the key. Phase measurements are also useful in many cases.

Various machine faults, identified in terms of predominant frequency, amplitude and noise characteristics are given in the vibration and noise identification chart shown in Table 1. The chart is used by following a definite analysis and troubleshooting procedure. Table 1 is located at the back of this chapter.

Analysis Procedure

Determining the frequencies present in a complex machine vibration or noise signal and relating these frequencies to predetermined possible sources of vibration and noise in that machine is the most powerful technique available for machinery analysis.

In general the analysis technique used is:

1. Calculate all the expected vibration frequencies of a machine based on the rotational speed of the main rotating components, gear reduction speeds, number of gear teeth, number of impeller vanes or blades, belt speeds, bearing frequencies, etc. This provides a rough guide as to the frequencies at which vibration energy might be expected.

2. Make a preliminary survey of the vibration data at various measuring points. Using a vibration analyzer with a tunable filter

6-24

carefully tune through the appropriate ranges, recording the amplitude and frequency at which significant frequency components are detected. This survey determines: what frequencies are present in the complex signal, what amplitude ranges are most desirable, and whether "displacement", "velocity," or "acceleration" is the most effective parameter to measure. In most cases involving purely analysis, "velocity" will be selected because both high and low frequencies receive equal weight when measuring velocity .

3. Once the measurement parameters and measurement locations are selected, each point is measured and the overall readings recorded. A narrow-band analysis is made on each range and the significant vibration components are recorded on the data sheet.

The vibration measured at any point on a machine can be resolved into its component frequencies by use of the tuneable electronic filter in the vibration analyzer. The tuneable filter has been designed to provide frequency analysis by manually tuning the filter dial to the known vibration frequencies that exist in the machine being analyzed. The vibration frequencies are noted and compared to the identification chart to determine the source.

The use of a vibration analyzer with an XY recorder will provide you with hard copy vibration signatures which can be used as a baseline against future signatures of the same machine. Vibratio·n trends and mechanical defects are then visually indicated. Good operating machinery history is also confirmed thus eliminating guesswork.

The tuneable filter characteristics are selected to provide ease of tuning operation and adequate narrow-band analysis capability to analyze a major number of problems found in all types of rotating machinery. Detailed narrow-band analysis is used to identify specific troubles by relating the observed frequency components to the known rotational elements in the machine.

6-25

Vibration and Noise Source Identification

After the characteristics of a machine's vibration have been measured and recorded, the next step is to compare the readings with the characteristics of vibration typical of various types of trouble. The Vibration and Noise Identification Chart in Table 1 lists the causes of vibration and the characteristics of each.

In the table, vibration frequency is listed in multiples of the main machine component rotational speed; i.e. 1, 2, 3 or more times RPM. The table identifies mechanical and electrical vibrations as well as aerodynamic or hydraulic. The key to vibration identification is frequency and phase. The remarks tell the operator the probable cause of the predominant vibrations by comparing the observed frequency and phase measurements to the machine's rotating speeds.

Unbalance

Vibration caused by mechanical faults will be related to the rotating speed or an exact multiple. For example, if frequency of vibration is exactly at rotational speed and the amplitude and phase angle remain constant, the vibration is probably simple unbalance. Simple unbalance may be caused by non-symmetrical rotating parts, a slightly bowed shaft or improperly assembled parts. Unbalance is corrected by following standard balancing procedures.

If the vibration frequency is exactly rotational speed but amplitude and/or phase angle drift or change after start-up, the unbalance may be caused by thermal effects where heat in the rotor affects shaft straightness or alignment. With unbalance caused by thermal condition the amplitude and phase readings will stabilize with time.

Unstable Machine Conditions

If the vibration frequency is at rotational speed and the amplitude and/or phase angle continues to change over long period of time, the problem may be related to a rotating part rubbing a stationary part, such as a shaft

6-26

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seal, or a changing mechanical condition such as a bearing race slipping or moving on a shaft or in its housing. When the amplitude and/or phase are unstable the fault should be located and corrected.

Looseness of movement of machine parts may be thought of as a part making contact twice per revolution. Looseness may cause non-repeatable vibration readings from run to run and the readings will be somewhat erratic. Machine components may become loose during operation or after repairs have been made.

Misalignment

Vibrations occurring at twice rotational speed may indicate looseness, or misalignment. Misalignment may manifest itself in a variety of ways depending on the type. Often the vibration occurring at 1 X RPM and 2 X RPM will be large in the axial direction.

Electrical

Electrical causes of vibration will disappear quickly when power is turned off. Electrical causes of vibration are related directly to the impressed 60-CPS voltage and will show up at 60-CPS and 120-CPS. Eccentric rotors, unbalanced voltages, rotor misalignment, a bent shaft, unequal air gaps, and defective rotor bars may produce vibration of this type. Another common type of electrically caused vibration occurs at rotational speed with a slow periodic variation in amplitude occurring at exactly slip speed times the number of poles. For example, a four-pole, 1800 synchronous speed induction motor running at 1750 RPM will have a "slip-beat" vibration of 300 CPM.

This electrically caused vibration is not to be confused with the "beatfrequency" vibration that occurs when two or more machines, operating at essentially the same speed "beat" one against the other. This "beat frequency" is often heard in areas where two or more machines are operating, each producing some vibration. The slow variation in amplitude of "beat" occurs as the vibration produced by each machine alternately adds or reinforces then subtracts or cancels each other.

6-27

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CHAPTER SEVEN MAINTENANCE AND TROUBLESHOOTING

OBJECTIVES

State the likely consequence on HV AC systems of radial fans being installed backward.

State a likely cause for the following HV AC problems:

The unit runs continuously with insufficient cooling. The unit short-cycles with insufficient cooling.

State two likely causes for each of the following HV AC system conditions:

High head pressure. High suction pressure.

State one easily-checked indication of refrigerant undercharge.

State the likely result of a restriction in the refrigerant discharge line.

Describe the result of a metering' device with a low setting; of a metering device with a high setting.

• Systematic Troubleshooting Techniques

• Troubleshooting fans

• Troubleshooting Abnormal Operations (Air Conditioning)

Troubleshooting techniques was chosen to give a basic method of attacking a problem. The other two topics were chosen because if there is a problem involved with an HV AC system, they are usually involved.

SYSTEMATIC TROUBLESHOOTING TECHNIQUES

Troubleshooting is nothing more than problem solving, and as with any problem, there are five major steps in reaching a conclusion of the problem. These steps are:

• Verify a problem exists.

• Identify the problem.

• Locate the cause of the problem.

• Solve the problem.

• Verify that the problem has been corrected.

The complexity of the problem determines how long it will take to solve the problem and in some cases there are more than one right answer. But usually when dealing with machinery, there is only one way to actually solve the problem or that will correct the cause of the problem. When dealing with machinery or systems, there is some type of malfunction we want to fix.

Approaching troubleshooting in a logical step-by-step manner can save a considerable amount of time. The first step which should be accomplished

7-2

is to verify that a malfunction actually exists. In other words, ensure that the r system/components are set up for and operating normally.

Identifying and locating the cause of the malfunction can be the hardest step in troubleshooting. This task ·is made much easier based on the technician's level of knowledge of the overall system and its operation.

The next step is to solve the malfunction. It is very important to correct the cause of the fault, not just replace or repair a component. Determine if a component failure or another component is in a degraded condition causing the failure of other components.

The last step in troubleshooting is to verify that the malfunction has actually been corrected. Operate the system under normal conditions and, if possible, monitor the area of the malfunction. At this time, all operational characteristics should be monitored to ensure they are correct for normal conditions.

TROUBLESHOOTING FANS

From Chapter Three, we know that fans are used to move air in an HV AC system. How well a fan performs its job depends on a few factors such a size, speed and design of the system. Because the fan plays such a big part of the system, it is very necessary for an HV AC Technician to understand what common problems a fan have and how to troubleshoot for them. The topics we will discuss are;

• Noise • Performance Reduction • Rotation

Also provided is a chart with sources and probable cause of problems 1 with fans. '~. ,

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7-3

Figure 7.1 Probing For Spin

7·6

A. NOISE

Source Probable Cause

l. IMPELLER HITIING a. Impeller not centered in INLET RING inlet ring.

b. Inlet ring damaged. c . Crooked or damaged

impeller. d. Shaft loose in bearing. e. Impeller loose on shaft.

! f. Bearing loose in bearing , r

support.

I 2. IMPElLER HITIING Cutoff not secure in a. CUTOFF housing.

b . Cutoff damaged . .

r c. Cutoff improperly positioned.

3. DRIVE a. Sheave not tight on shaft (motor and/or fan).

b. Belts hitting belt tube. c. Belts too loose. Adjust for

! belt stretching after 48 hours

l operating. d. Belts too tight. e. Belts wrong cross section. f. Belts not "matched" in

length on multibelt drive. g. Variable pitch sheaves not

adjusted so each groove has I same pitch diameter (multi-\.. . .

belt drives). h. Misaligned sheaves. I. Belts worn.

7-7

l.

A. NOISE


J. Motor, motor base or fan not securely anchored.

k. Belts oily or dirty. 1. Improper drive selection.

4. COUPUNG a. Coupling unbalanced, misaligned, loose, or may need lubricant.

5. BEARING a. Defective bearing. b. Needs lubrication. c. Loose on bearing support. d. Loose on shaft. e. Seals misaligned. f. Foreign material inside

bearing. g. Worn bearing. h. Fretting corrosion between

inner face and shaft.

6. SHAFf SEAL SQUEAL a. Needs lubrication. b. Misaligned.

7. IMPELLER a. Loose on shaft. b. Defective impeller. Do not

run fan. Contact manufacturer.

c. Unbalance. d. Coating loose.

7-8

A. NOISE


e. Worn as result of abrasive or corrosive material moving through flow passages.

8. HOUSING a. Foreign material in housing. b. Cutoff or other part loose

(rattling during operation). , ,

9. ELECTRICAL a. Lead-in cable not secure. b. AC hum in motor or relay. c. Starting relay chatter d. Noisy motor bearings. e. Single phasing a 3 phase

• motor.

10. SHAFf a. Bent. b. Undersized. May cause

noise at impeller, bearings or sheave.

c. If more than two bearings are on shaft, they must be properly aligned.

11. mGH AIR VELOCITY a. Duct work too small for application.

b. Fan selection too small for application.

c. Registers or grilles too small I for application. t

d. Heating or cooling coil with insufficient face area for application.

7-9

j

B. LOW CFM, INSUFFICIENT AIR FLOW


6. OBSTRUCI'ED FAN a. Elbows, cabinet walls or INLETS other obstructions restrict air

flow. Inlet obstructions cause more restrictive systems but do not cause increased negative pressure readings near the fan inlet(s). Fan speed may be increased to counteract the effect of restricted fan inlet(s).

7. NO STRAIGHT DUer AT a. Fans which are normally FAN OUTLET used in duct system are

tested with a length of straight duct at the fan outlet. If there is no straight duct at the fan outlet, decreased performance will result. If it is not practical to install a straight section .. of duct at the fan outlet, the' fan speed may be increased to overcome this pressure loss.

8. OBSTRUCTIONS IN HIGH a. Obstruction near fan outlet. VELOCITY AIR STREAM b. Sharp elbows near fan

outlet. c. Improperly designed turning

vanes.

7-12

1. , . .

t

2. !

I. .

r I

, , "

B. LOW CFM, INSUFFlCIENT AIR FLOW


d. Projections, dampers or other obstruction in part of system where air velocity is high.

C. IDGH CFM, TOO MUCH AIR FLOW


SYSTEM a. Oversized duct work. b. Access door open. c. Registers or grilles not

installed. d. Dampers set to by-pass

coils. e. Filter(s) not in place.

FAN a. Backward inclined impeller installed backwards (HP will be high).

b. Fan speed too fast.

7-13

1.

D. WRONG STATIC PRESSURE

Source

SYSTEM, FAN OR INTERPRETATION OR MEASUREMENTS

7-14

Probable Cause

GENERAL DISCUSSION The velocity pressure at any point of measurement is a function of the velocity of the air or gas and its density.

The static pressure at a point of measurement in the system is a function of system design (resistance to flow), air density and the amount of air flowing through the system.

The static pressure measured in a "loose" or oversized system will be less than the static pressure in a "tight" or undersized system for the same air flow rate.

In most systems, pressure measurements are indicators of how the installation is operating. These measurements are the result of air flow and as such are useful indicators in defining system characteristics.

Field static pressure measurements rarely correspond with laboratory static pressure measurements unless the fan inlet and fan outlet conditions of the installation are

, . •

• l

• , l

l l •

•

i , j

, i l

2.

3.

4.

1.

D. WRONG STATIC PRESSURE

Source

SYSTEM

GAS DENSITY

FAN

Probable Cause

exactly the same as the inlet and outlet conditions in the laboratory.

Also see D-2 through D-6, E-2, F-1, and G-1, for specific cases.

System has less resistance to flow than expected. This is a common occurrence. Fan speed may be reduced to obtain desired flow rate. This will reduce HP (operating cost).

Pressures will be less with high temperature gas or at high altitudes.

a.

b.

Backward inclined impeller installed backwards. HP will be high. Fan speed too high.

E. STATIC PRESSURE LOW, CFM LOW

SYSTEM

7-15

a. Fan inlet and/or outlet conditions not same as tested. See general discussion (D-1).

Also see B.1-S.

F. STATIC PRESSURE IDGH, CFM LOW


2. SYSTEM a. Obstruction in system. b. Dirty filters. c. Dirty coil. d. System too restricted.

Also see B.1-8.

G. HORSEPOWER IDGH

1. FAN a. Backward inclined impeller installed backwards.

b. Fan speed too high.

2. SYSTEM a. Oversized duct work. b. Face and by-pass dampers

oriented so coil dampers are open at same time by-pass dampers are open.

c. Filter( s) left out. d. Access door open.

3. GAS DENSITY a. Calculated horsepower requirements based on light gas (e.g., high temperature) but actual gas is heavy (e.g., cold start up).

4. FAN SELECTION a. Fan not operating at efficient point of rating. Fan size or type may not be best for application.

7-16

1.

1.

H. FAN DOES NOT OPERATE

Source

ELECTRICAL OR MECHANICAL

Probable Cause

Mechanical and electrical problems are usually straightforward and are normally analyzed in a routine manner by service personnel. In this category are such items as: a. Blown fuses. b. Broken belts. c. Loose pulleys. d. Electricity turned off. e. Impeller touching scroll. f. Wrong voltage. g. Motor too small and

overload protector has broken circuit.

I. PREMATURE FAILURE

BELTS, BEARINGS, SHEA YES IMPELLER, HUBS, ETC.

• 7-17

GENERAL DISCUSSION Each fan component is designed to operate satisfactorily for a reasonable life time. Fans intended for heavy duty service are made especially for that type of service. For example, Class I fans are intended for operation below certain limits of pressure and outlet velocity. Class II fans are designed for higher operating limits. Not all components are

Source

2. COUPLINGS

3. SHAFf

Rotation

I. PREMATURE FAILURE

Probable Cause

limited by the same factors, e.g., limiting factors may be HP, RPM, temperature, impeller tip speed, torque, corrosive atmospheres, expected life, etc.

Also see A3, AS, A6.

See A4.

Also see AID.

No system should be tested without first checking the fan rotation. Fan rotation, as defined by the fan manufacturer, is either the "clockwise" or "counterclockwise" spin of the fan impeller. But the rotation would depend upon the position of the viewer relative to the fan. When checking rotation for centrifugal fan the fan must be viewed from the drive side while it is coasting to a stop. For tubular centrifugals, the fan must be viewed from the outlet side. For axial fans the fans must be viewed from the inlet side.

Figures 7-2 and 7-3 show the correct rotation for centrifugal and axial fan impellers.

7-18

,

, ,

,

~OTATION

RADIAL BLADE

BACKWARD INCLINED AIRFOIL

~(@)J~ RADIAL TIP BACKWARD FORWARD

CURVED CURVED

Figure 7-2 Centrifugal Fan Impellers

~

ROTATION

d ~ 2) C(~;b t t t -t-

AIR FLOW t AIR FLOW t

'" ROTATION

~~)] ~ 0 tr t t t t

AIR FLOW t AIR FLOW -t-

Figure 7-3 Axial Fan Impellers

7-19

TROUBLESHOOTING ABNORMAL AIR CONDITIONING OPERATIONS

Regardless of the type of system, there will be some common operating problems encountered, and the service technician must, like a doctor, be able to recognize the symptoms, diagnose the cause, and take corrective action. In most cases, the medical doctor is able to prescribe medicine or treatment immediately to relieve the patient. The refrigeration serviceman may have to arrive at a satisfactory diagnosis through the process of elimination of several possible causes, each of which may be the source of the complaint or the problem in the refrigeration system.

Mechanics must be conscientious in their attempts to put the system back in proper operating condition. There have been complaints from the field that some servicemen do not always measure up to the higher standards. For example, one may have added refrigerant to a system when there was indications of a shortage and, when this action did not correct the trouble, the serviceman was negligent and did not remove the excess refrigerant. (This, in itself, might be the cause of a future service complaint.) Some manufacturers of components such as expansion valves have had parts returned that were not defective. Rather, the strainers in some of the valves were merely dirty or clogged, but the serviceman would replace the valve, blaming the trouble on its operation.

In small refrigeration units, the major problems that occur are:

• The unit runs continuously with insufficient cooling .

• The unit short cycles with insufficient cooling .

Of course, many other troubles may occur in the electrical circuitry but they will have to be determined on case by case basis.

7-20

Three mam conditions In units that are operating but not cooling satisfactorily are:

• High head pressure

• Low suction pressure

• High suction pressure

It is recommended that, if possible, the technician diagnose this problem without entering the sealed system. Some of the variables that could cause these problems and can be diagnosed by using gauges are:

1. High head pressure

Dirty or partially blocked condenser Air or other noncondensable gases in system Overcharge of refrigerant . Insufficient condensing medium (air, water, etc.) High temperature condensing medium Restricted discharge line

2. Low suction pressure

Insufficient air or heat load on evaporator coil Poor distribution of air over evaporator coil Restricted refrigerant flow Undercharge of refrigerant Faulty expansion valve or capillary tube

3. High suction pressure

Heavy load conditions Low superheat adjustment Improper expansion valve adjustment Poor installation of feeler bulb Inefficient compressor High head pressure on capillary tube systems

7-21

High Head Pressure

Dirty or Partially Blocked Condenser

An automobile engine will probably overheat if the radiator becomes clogged with leaves or insects. So will the operation of an air cooled condensing unit be seriously affected if its condenser becomes partially blocked with paper, leaves, or other debris. Particularly if the condenser is located outdoors. If the unit is located indoors, such as in the back room of a grocery store or restaurant, the condenser may not be subjected to leaves and other debris, but grease in the air collects on the fins, permitting dust and dirt to accumulate and prohibiting proper heat transfer.

This condition may be diagnosed during the visual check of the system by the service technician. External cleaning of the condenser fins and coil may be done with a stiff brush or, if a portable air tank is available, by the pressure of an air supply in the opposite direction to the normal air flow through the condenser. Accumulations of dirt and dust may have to be removed by the application of a soap and water solution, followed by flushing the condenser with water from a hose - again from the direction opposite to the normal air flow.

If grease has accumulated on a condensing unit in a restaurant or store, the condensing unit itself may have to be cleaned with a degreasing solvent applied with a brush or spray. This should be followed with a soap and water solution and external flush ing with water.

Care must be taken that electrical connections are protected when the unit is being cleaned.

Air or Noncondensable Gases in System

If there is only relatively dry air in a refrigeration system, it is less harmful than moist air, but in either case, oxygen may react with oil or metals to produce sludge, metal oxides, etc. The same applies if dry nitrogen or dry carbon dioxide has been used to pressure test a system and has not

7-22

been completely removed. However, moisture-laden air in the system indicates that it was opened for repair or component replacement and was not evacuated properly. Proper evacuation is absolutely necessary to eliminate both air and moisture.

Space in the condenser occupied by air or other noncondensables is not available for the proper function of that component, and can affect heat removal from the superheated vapor and condensation of the saturated vapor. A reduction of the heat transfer area in the condenser will make a greater temperature difference between the cooling medium and the condensing refrigerant necessary to permit removal of the required amount of heat from the refrigerant. At higher condensing temperatures, there will be a corresponding increase in head pressure.

The question now is how to determine if there is a noncondensable gas such as air in the condenser. To make a test, the temperature of the refrigerant in the condenser should be the same as the air surrounding it; so the compressor must be shut off (if it is in operation) and the refrigerant allowed to give up its heat to the surrounding air. This process can be speeded up if it is possible to bypass the controls and operate the condenser fan alone.

The difference in pressure within the condenser should not be more than 5 psig from the pressure corresponding to the temperature of the refrigerant being used . Assuming that R-12 is the refrigerant and that the ambient temperature (and that of the refrigerant in the condenser) is 95°P. Therefore, the pressure within the condenser - as indicated on the gauge -should be 108 psig but not exceed 113 psig. If it does, the air or noncondensable must be purged from the unit.

Most small condensers do not have purge valves at the top, so the purging must be done through the gauge manifold. The purging should be done in small amounts, with a few moments of time elapsing between brief periods of purging. This will permit the air or noncondensable gas to collect at the high point, which would be the gauge manifold, and allow it to be purged without losing too much refrigerant. It is impossible to purge without

7-23

the loss of some refrigerant, since complete separation cannot be obtained. Purging should continue until the head pressure drops down to the proper point corresponding to the temperature of the refrigerant. Purging of the capillary tubes or other critical charge systems is not recommended - only proper and complete evacuation procedures should be followed.

Overcharge of Refrigerant

If, as mentioned earlier in this section, unnecessary refrigerant had been put into the system during a previous service call, or if the system was improperly charged during start-up (and then not removed) high head pressure may result, although in a commercial system that has a receiver, the additional refrigerant will only raise the liquid level and should not affect the system unless it was greatly overcharged. An overcharge of refrigerant, like air or other noncondensable gases in the condenser, will occupy space in the condenser that is needed for proper heat transfer from the refrigerant vapor to the air used as a cooling system, unless the system has a receiver. With a smaller area available, the increased temperature difference will cause an increase in the head pressure. When other possible causes of the high head pressure are eliminated, and a surplus refrigerant charge is suspected, some of the refrigerant must be removed from the system.

If there is a sight glass installed in the liquid line just ahead of the liquid control, a full glass will indicate that there is either enough refrigerant in the system or a restriction ahead of the sight glass. If the possibility of a restriction has already been eliminated, and a surplus refrigerant charge is suspected, some of the refrigerant must be removed from the system.

Refrigerant should be removed from the system until the sight glass indicates a shortage, that is, when bubbles of gas entering the liquid line are visible in the sight glass. The refrigerant removed from the system should be placed in an empty, dry drum or carefully purged to the atmosphere and new refrigerant added. Only enough refrigerant to clear up the gas bubbles should be charged back into the system.

7-24

Insufficient Condensing Medium

As explained earlier, a partially blocked condenser will result in inadequate heat transfer between the refrigerant and the cooling medium. Even if the condenser itself is not obstructed, there may be other reasons why insufficient air reaches or is available to the condenser. If the condensing unit is located too close to a wall, partition, or other obstacle, it is possible that not enough air can be drawn across the condenser.

Insufficient air moving across the condenser can also be the result of a loose or slipping belt between the motor and fan, a loose fan wheel on direct drive equipment, or binding of the shaft of either the motor or fan because of bad shaft bearings or lack of lubrication.

High Temperature Condensing Medium

If the temperature of ambient air surrounding the condensing unit increases, it follows that the operating head pressure will increase correspondingly. If the unit is located outdoors, there is of course no control of the outdoor dry bulb temperature. The condensing unit may be protected from direct sun rays, although this factor is not too important. The unit should not be located indoors where it will be seriously affected by high ambient temperature.

If the condensing unit with a blow through fan is located indoors and too close to a wall or other obstacle, it is possible that the hot air exhausts from the condenser may be short cycled back into the inlet of the fan. This would increase the temperature of the available air for removal of heat form the refrigerant. For optimum operation, a condensing unit located indoors must have some provision for the removal of condenser discharge air. A poorly located outdoor condensing unit also can short cycle air into the fan.

Restricted Discharge Line

A kink that develops in the discharge line of a self-contained condensing unit, or in the hot gas line between a compressor and a remote

7-25

air cooled condenser would cause a restriction in the flow of the "hot refrigerant vapor. An increase in pressure measured at the compressor, along with a corresponding increase in temperature, would result.

Similar results might occur in a recently installed system where excess solder could cause a restriction to the flow of the discharge vapor. Usually a stoppage or major restriction of this type is diagnosed by the pulsation of pressure and the whistling sound of vapor trying to force its way past the restriction.

Low Suction Pressure

Insufficient Air on Evaporator Coil

This is the most common cause of low suction pressure ID a refrigeration or air-conditioning system. If the flow of air across the evaporator coil is reduced, the load on the coil is decreased. Since there is customarily a transfer of heat from the volume of warm air moving across the coil to the cool refrigerant within the coil, any decrease in the amount of air that passes through the coil will result in a loss of normal heat transfer. If the refrigerant is picking up less heat, its temperature will be lowered along with the suction pressure.

Insufficient air on the forced air evaporator coil may be caused by dirty air filters, too small return ducts, improper speed of the blower, a clogged cooling coil, a combination of these possible causes, or improperly adjusted duct dampers or registers. The service technician should check to see if there are filters in the air distribution system. If they are dirty, they should be cleaned or replaced. The cooling coil should be checked to make sure it is clean and free of dirt and lint, whether or not there is a filter in the system.

If the blower motor and/or blower shaft bearings have not been lubricated for some time and are not running freely, the flow of air through the cooling coil may be less than normal. An improperly adjusted blower belt could cause a slowdown in the blower speed and a reduction of the air flow across the coil. (It should be noted that a squirrel cage blower running

7-26

I

J6dbackward will deliver air in the proper direction, but at a greatly reduced volume.)

Poor Distribution of Air on Evaporator Coil

It is important that each circuit or portion of the coil receive fairly ., equal amounts of air so that the entire cooling load can be handled

proportionately by the entire cooling coil. Without a proper distribution of air over the coil, the capacity and the efficiency of the cooling coil will be reduced. When there is ductwork connected to the inlet side of the coil, it is highly possible that an unbalanced condition of air distributed across the coil will occur if the air must make a turn in direction prior to entering the cooling coil. When air changes direction, such as in turning a corner prior to entering the cooling coil, most of the air will probably flow to the outside radius of the curve. In so doing, only a small portion of the air will flow across the coil section located at the inner part of the curve and so cause an unequal distribution of the load on the cooling coil.

, . . "c' It is possible that, because of the design and circuiting of the cooling

coil, the liquid refrigerant in the circuit or sections of the coil nearest the inner radius of the curve will not be completely vaporized. In that case, the refrigerant will pass from the coil in a liquid form and will, in turn, cool the gas that is leaving the other sections of the coil to a temperature that is lower than normal. This lowering of temperature may cause the metering device to restrict the flow of liquid refrigerant to the other sections of the coil, thereby robbing the remainder of the coil of the refrigerant needed to handle the load properly. It may be necessary to install turning vanes in the sheet metal ductwork on the inner side of the cooling coil.

[ .'-Restricted Refrigerant Flow

For a cooling coil to vaporize enough refrigerant to satisfy the capacity ", of the compressor and to remove the proper amount of heat from the load,

"2 it must receive an adequate amount of liquid refrigerant. Any restriction to ". tbis flow of liquid refrigerant will mean a reduction in the capacity of the

. !" 'looling coil for heat removal. There must be no restriction to this liquid

7·27

(,0,,-, \ ~ , .'::

flpw between the outlet of the condensing unit and the inlet to the cooling COIL This inciudes the' receiver, drier, sight glass, and refrigerant conirols.

1" .If il , receiveLi~ psed, tpe.liquid line valve on the receiver may be partially ,: ... closed. The;'e~ai also be a restriction in the liquid flow due to a crimp in J JIJ- , , . , ' ,. , " I(;d~~ ~\ne, p~r~!a!ly ,~mashed tubl?g, a qUIck-connect couphng that i.s, only .. : partIajly .open, a cj[1~r full of mOIsture, or an obstruction of some sort il) the . I, meterigg device. rn:any case, a restriction to the flow of liquid refriger~tion

could affect the operation of the system. It may cause enough pre~ure, drop to reduce the boiling point of liquid available in the cooling coil. There will be a definite temperature drop across the point of restriction, which, depending on its location, mayor may not be easily located.

,54 ., ' .1

"c y~?~rcharge of ~~!rigera~~ ~u )

'.i!IJO:)~ ""'1 ,'"' ,~. "I

p ' ,." A shortage of refdgerant in the system is usually indicated by a warm ~;,·. suction line along with a low suction pressure. If there is an underchar~e in . the system, the refrigerant vapor may not condensate properly (iii the

condenser) before it is ready to reenter the cooling coil and remove additional heat from the load. A refrigerant that does not condense fully and enters the

" liquid line in a gaseous state may be indicated by a hissing noise at the metering device. In addition, as previously discussed, the refrigerant wi'n not pick up as much heat when it is in a vapor state as it would if it entered the cooling coil in its proper state - as a liquid. The cooling coil and the suction line will probably be warm to the touch, because there is little or no liquid refrigerant being supplied to the cooling coil. A liquid indicator or sight glass installed in the liquid line will show a shortage of refrigerant by

< '{". bJ~tq}~s J,n the sight glass . . , . . ,i I' .,' ' ..

.. ' Fa'ulty"Metering Device ':.J .,' .. _'

" • . ,j , j . '.f( ,

A meterIng deVIce such as an expansIon valve may have mechllnlcal problems. This valve may stick in a nearly closed position, a fully closed position, or a fully open position. Sometimes dirt or frozen moisture will restrict the flow of refrigerant liquid through the valve or stop the flow of any liquid at all into the evaporator. In such a case, the compressor will

7-28

• . short-cycle (that is, start and stop at frequent intervals) when the expansion ~~Ji valve is only partially closed and insufficient liquid is ' entering the coil. '. r v •

'il With the expansion valve completely closed, ~he compressor ~ill16wer the pressure in the evaporator down to the cutout point of the low pressure

· ' control, which will stop the compressor. If there is no low pressure control switch in the system, the compressor will continue to tun with no work being

':n done until the motor windings heat up and are cut off by the ' ,electrical .. . " · overload control operation. '''':; 1 I

' .. ,: '.,. I r;. , Low Discharge Pressure

Low discharge pressure on a system may be cau~ed by an Ull?ercharge of refrigerant. It could also be the result of low ambient air temperature being circulated across the condenser coil. This low head pre~ure could

, result in a higher compression' ratio and therefore a decrease in compressor efficiency, accompanied by a reduction in the expansion valve capacity. (Low refrigerant charge also can result in lower suction pressure, which may

IL. increase the compression ratio.) This reduction of capacity would be caused orO) by a lowering of the pressure drop across the expansion valve wi.th a ". decrease in the head pressure.

High Suction Pressure

Heavy Load Conditions , ..

A system might have a high suction pressure and yet no part of the system is faulty. Possibly load conditions increased considerably, accompanied by an increase in the temperature of the ambient air entering the condenser. In such a case, there would be a high discharge pressure along with the high suction pressure, with no fault in the mechanical operation of

, '[the system. These conditions may be remedied by the removal of the cause -: .. of the excessive load on the evaporator.

j" , ,

L "-

, 7-29

rises higher and higher as does the motor load. At the same time because of the reduction of suction gas to the hermetic compressor, the latter cannot properly cool the motor, and prolonged cycling and overheating may eventually result in total motor burnout.

SUMMARY

Troubleshooting a system malfunction does not have to be a time consuming and troublesome event. If a systematic and logical approach is taken, time can be held to a minimum. This process will only work if the person understands the entire operation of an HV AC system.

There are common problems associated with fans and air conditioners. Being able to recognize what those problems are allows you to be able to minimize downtime and the cost of maintenance.

7-32

hvac fundamentals & testing

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