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WHY STUDY PSYCHROMETRYIf HVAC problems and challenges are to be

properly diagnosed and designed, it is essential that the psychrometric chart and psychrometrics in general be clearly understood.

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OBJECTIVE

This course will review the introductory elements of psychrometrics (the behavior of mixtures of air and water vapor under varying conditions) and the chart that represents it.

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OBJECTIVESIf HVAC problems and challenges are to be

properly diagnosed and designed, it is essential that the psychrometric chart and psychrometrics in general be clearly understood.

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OBJECTIVES

This course will review the essential elements of psychrometrics (the behavior of mixtures of air and water vapor under varying conditions of heat) and the chart that represents it.

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PSME COURSES

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COURSE OUTLINE

FUNDAMENTALS – AIR AND WATER

THE PSYCRHOMETRIC CHART- PSYCHART PROCESSES

PRACTICAL APPLICATIONS DISCUSSIONS

* with WORKSHOPS-PROBLEM SOLVING

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IntroductionWHAT IS PSYCHROMETRY

Psychrometry is the study of MOIST AIR:

-its properties -changes in its conditions

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IntroductionWHAT IS PSYCHROMETRY

Psychrometrics deals with thermodynamic properties of moist air and uses these properties to analyze conditions and processes involving moist air'. (1997 ASHRAE Handbook, 6.1 p1)

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Definitions and ConceptsWhat is AIR

AIR is considered to be a mix of dry air and water vapor plus impurities.

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What is AIR

DRY AIR is mainly nitrogen (N2) and oxygen (O2) with traces of argon (Ar), carbon dioxide (CO2) and other gases

WATER VAPOR is H2O and constitutes about 0.40 per cent of the whole global atmosphere and typically makes up one to four per cent of the mass of the combined air mixture at the Earth’s surface.

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AMOUNT OF MOISTURE IN AIR

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PROPERTIES OF AIR

The Psychrometric Chart - an Overview

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BASIC PROPERTIES:SEVEN (7) PROPERTIES OF MOIST AIR

Dry-bulb temperatureWet-bulb temperatureRelative HumidityEnthalpySpecific volumeAbsolute HumidityDew PointPressure (usually Atmospheric)

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DRY BULB TEMPERATUREThe air dry bulb temperature, θ(°C), is the temperature measured using a standard thermometer that is shielded from any radiant heating or cooling effects.

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DRY BULB TEMPERATUREThe air dry bulb temperature, θ(°C), is the temperature measured using a standard thermometer that is shielded from any radiant heating or cooling effects.

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WET BULB TEMPERATURE

WET BULB - the temperature reading from a wetted bulb that gives a direct indication as to the total heat content of air

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ADIABATIC WET BULB

WET BULB - the temperature reading from a wetted bulb that gives a direct indication as to the total heat content of air

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PSYCHROMETER

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ADIABATIC WET BULB

WET BULB - the temperature reading from a wetted bulb that gives a direct indication as to the total heat content of air

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HUMIDITY RATIO WThe weight of the water vapor in each pound or kg of dry air

It is also same ratio as the partial pressures of water vapor to total pressure.

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HUMIDITY RATIO WThe mass of the water vapor in 1 kg of dry air is termed the moisture contentg (kg/kgda) – this is really a ‘vapor’ content and humidity ratio.

To avoid errors in writing down small numbers associated with moisture content, the units of g/kgda are frequently used as GRAINS.

7000 grains of water = 1 pound of water.

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MAKE A CHART

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MAKE A CHART

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MAKE A CHART

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RELATIVE HUMIDITYThe amount of water vapor in the air divided by the amount of water vapor the air can hold (at the same temperature and pressure.)

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RELATIVE HUMIDITY

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DEW POINTDEW POINT temperature, θ(°C), is the temperature at which water condenses out from moist air as it is cooled at constant pressure.

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DEW POINT

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SPECIFIC VOLUMESpecific volume indicates the space occupied by air. It is expressed as a volume per unit weight

(OPPOSITE of density -weight per unit volume).

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DENSITY OR SPECIFIC VOLUME

Values range from 12.5 to 15.0 cubic feet/ pound of dry air. Greater specific volume is associated with warmer temperatures (dry-bulb).

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ENTHALPYEnthalpy is the TOTAL heat energy content of moist air. It is expressed in Btu per pound of dry air.

Represents the heat energy due to:1 temperature 2 moisture in the air.

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ENTHALPYSENSIBLE heat is the heat (energy) in the air due to the temperature of the air.

LATENT heat is the heat (energy) in the air due to the moisture of the air.

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SUMMARY

Properties of moist air on a psychrometric chart. Wet-bulb temperature and enthalpy use the same chart line but values are read off seperate scales.

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EIGHT (8) PSYCHART LINES

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WORKSHOPPLOT THE POINT

DRY BULB – 95 DEG FWET BULB – 78 DEG F WHAT IS RH, HUMIDITY AND DEW

POINT?

And the…EnthalpySpecific volumeAbsolute HumidityDew PointPressure (usually Atmospheric)

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WORKSHOP- ANSWERS

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WORKSHOP

A sling psychrometer gives a dry-bulb temperature of 78°F and a wet-bulb temperature of 65°F.

Determine other moist air properties from this information.

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WORKSHOP-ANSWERS

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THE AIR CONDITIONING PROCESSCommon processes include: Sensible cooling / sensible heating Cooling and dehumidificationHeating and humidification Evaporative cooling / chemical

dehydration

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THE AIR CONDITIONING PROCESS

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SENSIBLE COOLING/HEATING

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SENSIBLE COOLING/HEATING

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SENSIBLE COOLING/HEATING

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HUMIDIFICATIONHumidification

Moisture and Water vapor is added into the air.

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DEHUMIDIFICATIONDehumidification

When water is removed or condensed from the air.

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COMBINATIONS ofTHE AIRCON PROCESS

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THE COOLING COIL PROCESS

Cooling and dehumidifying is the process of lowering both the dry-bulb temperature and the humidity ratio of the moist air.

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THE COOLING COIL PROCESS

If moist air comes in contact with a surface that is below the dew-point temperature of the air, moisture will condense on the surface and the air will be dehumidified

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THE COOLING COIL PROCESS

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Relationship of Dewpoint, RH, Wetbulb, and Drybulb

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WORKSHOPConsider Philippines, in a hot humid day 90°F and 90% RH. We need a room condition to 70°F at 50% RH.

We do this by chilling the air far enough to condense out enough moisture to dehumidify it: the goal is to have air with absolute humidity not exceeding 0.008 lbs of moisture per pound of air (~50 to 55 grains per pound of dry air).

Show the processes on the psychrometric chart.

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WORKSHOP-ANSWERS

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WORKSHOP-ANSWERS

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SENSIBLE HEAT FACTORor SHF RATIO

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DEHUMIDIFICATION AND HUMIDIFICATION

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SHF

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SHR

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ROOM SENSIBLE HEATFACTOR-RSHF

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ROOM SHF

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GRAND SHF

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GSHF

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TOTAL RELATIONSHIPSGSHF AND ROOM SHF

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ACTUAL CASES USINGGSHF AND ROOM SHF

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REAL CASES USINGGSHF AND ROOM SHF

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REAL CASES USINGGSHF AND ROOM SHF

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PRACTICE CASE STUDIES

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DEHUMIDIFICATION AND HUMIDIFICATION

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AHU PROCESS

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MIXING AIR

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MIXING AIR

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MIXING AIR

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MIXING AIR

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MIXING AIR

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AIR MIXING

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BLOW THRU AHU

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DRAW THRU AHU

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CASE STUDIESROOM COOLING

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ROOM COOLING

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CASE STUDIES-OVERALL

AA

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ALIGN THE SHR LINE @ 0.8 SCALE MARK

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ALIGN THE SHR LINE @ 0.8 SCALE MARK

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ANALOGY OF ROOM COOLING

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SHR LINE @ 0.6 SCALE MARK

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CASE STUDIES

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CASE STUDY-SAME ROOM

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CALCULATE SHR AND SUPPLY AIR TEMP AND RH

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CASE STUDIES

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CASE STUDIES-SOLUTION

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IF WE “GUESTIMATE” THE SUPPLY AIR CONDITIONS

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CASE STUDIES

THEREFORE, NO PSYCHROMETRIC CHART CALCULATIONS CREATES PROBLEMS!

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SUMMARY

PSYCHROMETRY IS A VERY IMPORTANT ASPECT OF HVAC ENGINEERING!