hvac air distribution fundamentals

NASEEM AP

[email protected]

AIR DISTRIBUTION SYSTEM

AIR DITRIBUTION SYSTEM ..

AIR FLOW PATTERN

AIR DITRIBUTION SYSTEM ..

HEAT LOAD ESTIMATION CONTD ..

AIR DISTRIBUTION CONTD..

SOME FUNDAMENTALS

1. DUCT IS SIMPLY A PATH TO DISTRIBUTE & MANAGE

THE AIR FLOW TO THE REQUIRED /DESINED

ZONES/AREAS

2. DUCT SHAPE MAY BE CIRULAR,SQUARE,RECTANGULAR

OR SPIRAL

3. GALVANIZED IRON,ALUMINIUM,STAINLESS STEEL,

BLACK IRON ETC ARE SOME OF THE COMMONLY USED

DUCT MATERIALS


SOME FUNDAMENTALS

1. THE THICKNESS OF THE DUCT SHEET IS MENTIOINED BY ITS

GAUGE

2. ASPECT RATIO IS THE RATIO OF DUCT WIDTH TO HIGHT

3. ASPECT RATIO 1 TO 4 IS GENAERALLY ADVISED FOR DUCT

SIZING.

4. THE SIZE & SHAPE OF THE DUCT WILL DEPEND UP ON THE

CFM ,AVAILABLE SPACE FOR DUCTING, APPLICATION ETC


SOME FUNDAMENTALS

1. THE DUCT IS CONNECTED TO THE EQUIPMENT THROUGH

FLEXIBLE CONNECTOR

2. ALL THE BRANCEHS ARE FITTED WITH VCD

3. FIRE DAMPER IS PROVIDED AT THE EQUIPMENT OUTLET &

OTHER DUCT PENITRATING FIRE WALL AREAS

4. THERMAL INSULATION IS DONE FOR SUPPLY AIR DUCT

5. ACCAUSTIC INSULATION IS DONE FOR SUPPLY DUCT


SOME FUNDAMENTALS

1. AIR OUTLETS ARE SELECTED BASED ON THE MANUFACTURES

DESIGN DATA

2. TYPE & LOCATION OF THE AIR TERMINALS WILL DEPEND UP

ON THE ROOM DESIGN AND AESTHATICS

3. LOUVERS ARE FITTED AT THE FRESH AIR DUCT ENTRY

4. FITTINGS AND TRANSITIONS ARE DESIGNED ACCORDING TO

SMACNA & ASHRAE


SOME FUNDAMENTALS

1. FLEXIBLE CANVAS CONNECTION SHOULD BE USED TO

CONNECT THE EQUIPMENT & DUCTING SYSTEM

2. ACCESS SHUOLD BE PROVIDED FOR DAMPERS

3. AHU SHOULDE BE PROVIDED WITH DRAIN PAN & U TRAP

4. GASKETS TO BE USED BETWEEN DUCT JOINTS TO PREVENT

LEAKAGE

5. NEVER RUN THE AHU UNLESS THE AHU ROOM IS CLEANED

AND FILTERS ARE MOUNTED


TYPE OF DUCTS

1. SUPPLY AIR DUCT

2. RETURN AIR DUCT

3. FRESH AIR DUCT

4. EXHAUST AIR DUCT


DUCT DESIGNING METHODS

1. VELOCITY REDUCTION METHOD

2. EQUAL FRICTION METHOD

3. STATIC REGAIN METHOD


VELOCITY REDUCTION METHOD

1. VELOCITY(FPM) = VOLUME(CFM)/AREA (SQFT)

2. MAIN DUCT VELCOITY -1200 TO 1500 FPM

3. BRANCH DUCT VELOCITY - 750 TO 900 FPM

4. COLLAR/DROPPER VELOCITY - 300 TO 500 FPM

5. PLENUM BOXES – 500 FPM

6. RETURN AIR DUCT VELOCITY SHALL BE LEES THAN

SUPPLY AIR DUCT VELOCITY


EQUAL FRICTION METHOD

1. CONSIDERS THE TOTAL DUCTING WILL BEAR THE

SAME FRICTIONAL LOSS

2. 0.08”WATER/100 FT TO 1” WATER/100 FEET

3. THE RANGE WILL DIFFER DEPENDING UP ON THE

APPLICATION AND USE OF SPACE


DUCT CLASSIFICATION DW-

144DUCT

PRESSURE CLASS

STATIC

PRESSURE LIMIT

+ve -ve

pa pa

MAXIMUM AIR

VELOCITY

PERMISSIBLE AIR

LEAKAGE

M/secLitters per sec per

square meter

Low Pressure

Class-A500 500 10 0.027 X p^0.65

Medium Pressure Class-B

1000 750 20 0.009 X p^0.65

High Pressure Class-C

2000 750 40 0.003 X p^0.65

Where p is in Pascal


● Duct Classification as per SMACNA

1

7

DUCT PRESSURE CLASS

OPERATING PRESSURE

IN WG Pa

1/2 125 Not exceeding 1/2” wg

1 250 Exceeding 1/2” wg but not exceeding 1” wg

2 500 Exceeding 1” wg but not exceeding 2” wg





AIR FLOW MANAGEMENT● Duct seal class.

SEAL CLASS

APPLICABLE STATIC PRESSURE CLASS

SEALING REQUIREMENTS

AMore than 3” wg

(750 Pa)All transverse joints, longitudinal seams

and duct wall penetrations.

BMore than 2” & upto 3”

wg (750 Pa)All transverse joints & longitudinal seams

C Upto 2”wg (500Pa) Transverse joints only

PRASHANT

DESAI

10

AIR FLOW MANAGEMENT

PRASHANT DESAI 19


DUCT DESIGN & CONSTRUCTION CONSIDERATIONS

1. THE DUCT SHOULD CONSIDER THE AVILABLE SPACE FOR

DUCTING

2. DUCTS SHOULD BE TESTED FOR LEAKAGE

3. TRANSITIONS & FITTINGS PRESSURE LOSE SHOULD BE

CONSIDERED FOR BLOWER DESIGNING

4. 20 % MAX AREA REDUCTION IS ALLOWED IN DUCTING IN

CASE OF OBSTRUCTION

AIR TERMINAL DESIGN..

FACTORS TO BE CONSIDERED

1. AIR FLOW

2. THROW

3. NOISE LEVEL

4. APPERANCE

5. SPACE RESTRICTIONS

DISTRIBUTION SYSTEM PLANS & SYMBOLS

Positive Pressure (supply)

Negative Pressure (return or exhaust)

Arrow indicates

air flow direction

FLOW PATTERNS

1-way

4-way

2-way

3-way

Thermostat

Smoke/Fire

Damper

T


AHU FAN DESIGNING

1. THE CFM TO BE DELIVERED IS OBTAINED FROM HEAT LOAD

CALCULATION

2. THE FAN STATIC WILL DEPENED UP ON:

1. THE PRESSURE LOSS THROGH DUCT AND DUCT FITTINGS

2. THE PRESSURE LOSS AT DAMPERS

3. THE PRESSURE LOSS AT FLEXIBLE DUCT

4. THE PRESSURE LOSS AT TERMINAL

5. INTERNAL FAN LOSSES

FAN SELECTION..

CALCULATING THE PRESSURE DROP

1. IDENTIFY THE PATH HAVING THE MAXIMUM PRESSURE

2. CALCULATE THE PRESSURE DROP IN THE SELECTED PATH

DUCTING & DUCT FITTINGS

3. CALCULATE THE PRESSURE DROP IN THE PATH DAMPERS &

OTHER ACCESSORIES

4. CALCULATE THE PRESSURE DROP @ THE TERMINAL

5. ADD INTERNAL LOSS & SUM UP THE CALCULATION

hvac air distribution fundamentals

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