CB503VENTILATION & AIR CONDITIONING 3TOPIC 2 : AIR FLOW DESIGN

NAZRIZAM BINTI AB. WAHABpnnazz@gmail.com

017-612 5556

PSA/ CB503/ PNNAZZ

AIR DISTRIBUTION AND BALANCE

UNIT OBJECTIVES After studying this unit, the reader should be able to

• Describe propeller and centrifugal fans and blowers• Explain how to take air pressure measurements• Explain how to measure air quantities• List various types of air-measuring devices• Describe common types of motors and drive assemblies• Identify and describe various duct systems• Explain how air from the conditioned space returns to the air handler

CONDITIONING EQUIPMENT• Air has to be conditioned in most cases for us

to be comfortable• Equipment includes cooling coil, heating

device, device to add humidity, and device to clean air

• Forced air systems use the same room air over and over again

• Fresh air enters the structure by infiltration or by mechanical means

Supply duct

Return air from the occupied space

Air handler

Fresh air from outside the structure

Damper in fresh air

duct

Mechanical means to introduce ventilation

CORRECT AIR QUANTITY• The forced air system delivers the

correct quantity of conditioned air to the occupied space

• Different spaces require different air quantities

• Same structure may have several different cooling requirements

Living Room 9,000 btu

(cooling) 18,000 btu (heating) 300

cfm

100 cfm

100 cfm

200 cfm

100 cfm

200 cfm

50 cfm

100 cfm

50 cfm

THE FORCED-AIR SYSTEM • Components that make up the

forced-air system – The blower– Air supply system– Return air system – Grilles and registers

• Occupants should not be aware if the system is on or off

Supply duct

Return air from the occupied space

Air handler (blower)

Supply registers

THE BLOWER • Provides the pressure difference to force

the air into the duct system, through the grilles and registers, and into the room

• Typically 400 cfm of air must be moved per minute per ton of air conditioning

• Pressure in the ductwork is measured in inches of water column (in. W.C)

• Air pressure in the ductwork is measured with a water manometer

SYSTEM AIR PRESSURES • Duct system is pressurized by two

pressures – Static pressure – air pressure in the duct– Velocity pressure – pressure generated by

the velocity and weight of the air– Combined, these pressures are called– “Total pressure “

• Static pressure plus velocity pressure equals total pressure

Static pressure

Duct

Airflow

Probe located on the surface of the duct

Total pressureProbe located in the duct,

facing into the direction of

airflow

Velocity pressure

Velocity pressure = Total pressure – Static pressure

Total pressure

Static pressure

AIR-MEASURING INSTRUMENTS FOR DUCT SYSTEMS

• Velometer – Measures actual air velocity (how fast the air is actually moving in the duct)

• Air volume in cfm can be calculated by multiplying the air velocity by the cross-sectional area of the duct in square feet

• Pitot tube – Used with special manometers for checking duct pressure

PROPELLER FAN • Used in exhaust fan and condenser fan

application • Will handle large volumes of air at low

pressure differentials • Set into a housing called a venturi• The venturi forces airflow in a straight

line from one side of the fan to the other • Makes noise and is used where noise is

not a factor

Propeller fan Venturi

SQUIRREL CAGE OR CENTRIFUGAL FAN

• Desirable for ductwork• Builds more pressure from the inlet to the

outlet• Has a forward curved blade and a cutoff to

shear the air spinning around the fan wheel • Very quiet when properly applied• Can be used in very large high-pressure

systems

Centrifugal Blower End View of Squirrel Cage Blower Wheel

CENTRIFUGAL BLOWER HOUSING

TYPES OF FAN DRIVES • Belt-drive blowers have two bearings on

the fan shaft and two bearing on the motor• Motor pulleys and fan motor pulleys can be

adjusted to change fan speeds• Direct-drive motors use no pulleys or belts• Direct-drive motors can be multi-speed

motors • Speeds can be changed by changing motor

wire leads

BLOWER

MOTOR

BOTH THE DRIVE AND DRIVEN

PULLEYS MUST BE PERFECTLY

ALIGNED

Belt-driven Assembly

DIRECT DRIVE MOTOR ASSEMBLY

THE MOTOR AND THE BLOWER TURN AT THE SAME SPEED

THE SUPPLY DUCT SYSTEM • Distributes air to the terminal

units, registers, or diffusers in the conditioned space

• Duct systems– Plenum system– Extended plenum system– Reducing plenum system– Perimeter loop

THE PLENUM SYSTEM • Suited for a job where the room

outlets are all close to the unit• Supply diffusers are normally located

on the inside walls• Work better on fossil-fuel systems • Fossil-fuel supply air temperatures

could easily reach 130°F

Plenum system

Return duct

Supply plenum Branch ducts

THE EXTENDED PLENUM SYSTEM

• Can be applied to a long structure • This system takes the plenum

closer to the farthest point • Called the trunk duct system • Ducts called branches complete

the connection to the terminal units

Living Room 9,000 btu

(cooling) 18,000 btu (heating) 300

cfm

100 cfm

100 cfm

200 cfm

100 cfm

200 cfm

50 cfm

100 cfm

50 cfm

THE EXTENDED PLENUM SYSTEM

THE REDUCING PLENUM SYSTEM

• Reduces the trunk duct size as branch ducts are added

• Has the advantage of saving material and keeping the same pressure from one end of the duct system to the other

Living Room 9,000 btu

(cooling) 18,000 btu (heating) 300

cfm

100 cfm

100 cfm

200 cfm

100 cfm

200 cfm

50 cfm

100 cfm

50 cfm

THE REDUCING EXTENDED PLENUM SYSTEM

THE PERIMETER LOOP SYSTEM

• Well suited for installation in a concrete floor in a colder climate

• Warm air is in the whole loop when the furnace fan is running

• Keeps the slab at a more even temperature

• Provides the same pressure to all outlets

Living Room 9,000 btu

(cooling) 18,000 btu (heating) 300

cfm

100 cfm

100 cfm

200 cfm

100 cfm

200 cfm

50 cfm

100 cfm

50 cfm

THE PERIMETER LOOP SYSTEM

DUCT MATERIALS

• Ductwork must meet local codes • For years, galvanized sheet metal was

used exclusively • Other ductwork materials

– Aluminum– Fiberglass ductboard– Spiral metal duct– Flexible duct

GALVANIZED STEEL DUCT

• Gauge is the measurement of the thickness of galvanized steel duct

• The gauge size means how many pieces of that material would need to be stacked together to make a one-inch stack

• Metal duct can be round, square, or rectangular

JOINING SECTIONS OF GALVANIZED DUCT WITH SLIPS AND DRIVES

SlipDrive cleat

JOINING SECTIONS OF GALVANIZED DUCT WITH SLIPS AND DRIVES

Ends of drives are bent over to

secure

Slip

FIBERGLASS DUCT• Styles: Flat sheet or round prefabricated

cut• Duct is normally 1 in. thick with

aluminum foil backing• Special knives are used to make special

cuts to turn duct board into ductwork• All duct seams should be stapled and

taped

SPIRAL METAL DUCT • Used more on large systems• Comes in rolls of flat narrow

metal• Runs can be made at the job site• Can be located within the

occupied space for a more contemporary look

FLEXIBLE DUCT • Comes in sized up to about 24 in. in diameter• Some have a reinforced aluminum foil backing• Some come with vinyl or foil backing and

insulation on it• Keep duct runs as short as possible• Has more friction loss inside it than metal duct• Flex duct should be stretched as tight as

possible

Floor Register

Flexible Duct

Damper

Supply

Return

COMBINATION DUCT SYSTEMS• Metal trunk lines with round branch ducts• Metal trunk lines with flexible branch ducts• Ductboard trunk lines with round metal branch

ducts• Ductboard trunk lines with flexible branch ducts• Round metal duct with round metal branch ducts• Round metal trunk lines with flexible branch

ducts

DUCT AIR MOVEMENT • Branch ducts are fastened to the main

trunk by a takeoff-fitting • The takeoff encourages the air moving the

duct to enter the takeoff to the branch duct

• Air moving in the duct has inertia, meaning it wants to move in a straight line

• Using turning vanes will improve the air-flow around corners

Main supply duct

Takeoff fitting

BALANCING DAMPERS • Used to balance the air in various parts

of the system • Dampers should be located as close as

practical to the trunk line• The trunk is the place to balance airflow• Handles allow the dampers to be turned

at an angle to the airstream to slow the air down

Branch duct Balancing damper in the closed

position

Damper in the open position

DUCT INSULATION • A 15°F temperature difference from

the inside of the duct to the outside of the duct is considered the maximum difference allowed before insulation is necessary

• Metal duct can be insulated on the outside and on the inside

• The insulation is joined by lapping it, stapling it, and taping it

BLENDING THE CONDITIONED AIR WITH ROOM AIR

• When possible, air should be directed on the walls

• The diffuser spreads the air to the desired air pattern

• Cool air distributes better from the ceiling• Place diffusers next to the outside walls• How far the air will be blown from the

diffuser into the room depends on the air pressure behind the diffuser and the style of the diffuser blades

THE RETURN AIR DUCT SYSTEM

• Individual return air system will give the most positive return air

• The return air duct is normally sized slightly larger than the supply duct

• Central return systems are usually satisfactory for a one-level residence

• A path must be provided for the air to return to the central return

• The return air grille should be around an elbow from the furnace

Supply plenum

Return plenum

One central return grill in the common area

Central Return

INDIVIDUAL RETURN AIR SYSTEM

R

S

SIZING DUCT FOR MOVING AIR• Friction loss in ductwork is due to the actual

rubbing action of the air against the side of the duct and the turbulence of the air rubbing against itself while moving down the duct

• The smoother the duct’s interior surface is, the less friction there is

• The slower the air is moving, the less friction there will be

• Each foot of duct offers a known resistance to airflow

MEASURING AIR MOVEMENT FOR BALANCING

• Air balancing is accomplished by measuring the air leaving each register

• Measuring velocity of the duct in a cross section of the duct

• Determine the cfm by using the formula: CFM = area in square feet x velocity in feet per minute

1 foot

1 footAverage

air velocity is 400 fpm

Air Volume (cfm) = 400 ft/min x 1ft2 = 400 cfm

Cross-sectional area = 1 ft x 1 ft = 12” x 12” = 144 square

inches = 144 in2 / 144 in2 =

1ft2

18”

18”Average

air velocity is 400 fpmAir Volume (cfm) = 400 ft/min x 2.25ft2 = 900 cfm

Cross-sectional area = 18” x 18” = 324

in2 324 in2 / 144 in2 =

2.25ft2

THE AIR FRICTION CHART• Used by system designers to size ductwork

and duct systems• Gives recommended duct sized and

velocities for optimum performance • Can be used to troubleshoot airflow problems • Pressure drops in duct fittings have

equivalent lengths• All lengths and equivalent lengths are added

together to achieve the total

RESIDENTIAL DUCT SYSTEM

Common duct problems – Excessively long flexible duct runs– Disconnected duct runs– Closed dampers– Collapsed flexible duct– Loose insulation in the duct– Blocked grills and/or registers

COMMERCIAL DUCT SYSTEMS• Each area has specifications regarding the

required amount of airflow• Certified testing and balancing company to

verify airflow• Flow hoods measure air volume at supply

registers • Total airflow can be measured at the main

duct • Common problems include dirty filters,

partially closed dampers, and incorrect fan rotation

SUMMARY • Forced air systems use the same air over and

over• Fresh air enters the structure by infiltration• Forced air systems deliver the correct quantity

of conditioned air to the occupied space • Different spaces require different air quantities• Forced air systems are made up of the blower,

supply duct system, return air system and supply registers or grilles

SUMMARY• Typically, 400 cfm of air must be moved

per minute per ton of air conditioning• Pressure in the ductwork is measured in

inches of water column (in. W.C)• Static pressure plus velocity pressure

equals total pressure• Air volume in cfm can be calculated by

multiplying the air velocity by the cross-sectional area of the duct in square feet

SUMMARY• Propeller fans are used in exhaust fan and

condenser fan applications and can handle large volumes of air at low pressure differentials

• Centrifugal blowers are used in duct systems• Motor drives can be direct or belt driven

assemblies• The supply duct system can be configured as

a plenum, extended plenum, reducing extended plenum or perimeter loop system

SUMMARY - 4• Duct systems can be made of galvanized

metal, aluminum, fiberglass duct board, spiral metal, flexible duct or a combination of different materials

• Branch ducts deliver the proper amount of air to remote locations in the structures

• Balancing dampers are used to help ensure proper airflow to the remote locations

• The return air system can be configured as a central or individual return air system

SUMMARY - 5 • Friction in the duct slows the air flowing

in it• Slower air experiences less friction• Air balancing ensures the proper amount

of air is delivered to each supply register• CFM = velocity x cross sectional area• The friction chart is used to properly size

duct systems

TASBIH KIFARAHسبحانك اللهم وبحمدك أنت أشهد أن ال إله إال أستغفرك وأتوب إليك(Maha Suci Engkau Ya Allah dan Segala Puji BagiMu, aku bersaksi bahawa tiada Tuhan

melainkan Engkau, aku memohon keampunan dan taubat daripada Engkau)