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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)
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