1. CB503
VENTILATION & AIR CONDITIONING 3
TOPIC 2 : AIR FLOW DESIGN
NAZRIZAM BINTI AB. WAHAB
pnnazz@gmail.com
017-612 5556

2. PSA/ CB503/ PNNAZZ

3. AIR DISTRIBUTION AND
BALANCE

4. 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

5. 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

6. Mechanical means to introduce
Return air from the ventilation
occupied space
Damper in
fresh air
duct
Supply duct
Air handler
Fresh air from outside the structure

7. 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

8. 50 100 cfm
100 200 cfm 100
cfm cfm cfm
50 cfm
Living Room 100
9,000 btu (cooling) cfm 200 cfm
18,000 btu
(heating) 300 cfm

9. 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

10. Return air from the
Supply
occupied space
registers
Supply duct
Air handler (blower)

11. 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

12. 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

13. Probe
located on Static pressure
the surface
Airflow of the duct
Duct

14. Probe located in
the duct, facing
into the Total pressure
direction of
airflow

15. Velocity pressure
Total
pressure
Static pressure
Velocity pressure = Total pressure – Static pressure

16. 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

17. 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

18. Propeller fan
Venturi

19. 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

20. End View of Squirrel Cage
Centrifugal Blower Blower Wheel

21. CENTRIFUGAL BLOWER HOUSING

22. 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

23. BLOWER Belt-driven
Assembly
BOTH THE DRIVE
AND DRIVEN
PULLEYS MUST BE
PERFECTLY
ALIGNED
MOTOR

24. DIRECT DRIVE MOTOR ASSEMBLY
THE MOTOR AND THE BLOWER TURN AT THE SAME
SPEED

25. 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

26. 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

27. Plenum system
Supply plenum
Branch
ducts
Return
duct

28. 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

29. THE EXTENDED PLENUM SYSTEM
50 100 cfm
100 200 cfm 100
cfm cfm cfm
50 cfm
Living Room 100
9,000 btu (cooling) cfm 200 cfm
18,000 btu
(heating) 300 cfm

30. 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

31. THE REDUCING EXTENDED PLENUM SYSTEM
50 100 cfm
100 200 cfm 100
cfm cfm cfm
50 cfm
Living Room 100
9,000 btu (cooling) cfm 200 cfm
18,000 btu
(heating) 300 cfm

32. 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

33. THE PERIMETER LOOP SYSTEM
50 100 cfm
100 200 cfm 100
cfm cfm cfm
50 cfm
Living Room 100
9,000 btu (cooling) cfm 200 cfm
18,000 btu
(heating) 300 cfm

34. 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

35. 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

36. JOINING SECTIONS OF GALVANIZED DUCT WITH
SLIPS AND DRIVES
Slip
Drive cleat

37. JOINING SECTIONS OF GALVANIZED DUCT WITH
SLIPS AND DRIVES
Slip
Ends of drives are
bent over to
secure

38. 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

39. 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

40. 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

41. Floor Register
Return
Damper Supply
Flexible Duct

42. 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

43. 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

44. Main supply duct
Takeoff fitting

45. 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

46. Branch duct
Balancing damper in
the closed position
Damper in the open position

47. 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

48. 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

49. 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

50. Central Return
Supply plenum
Return plenum
One central return grill in the common
area

51. INDIVIDUAL RETURN AIR SYSTEM
S
R

52. 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

53. 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

54. Cross-sectional area =
1 ft x 1 ft = 12” x
12” = 144 square
inches =
1 foot
144 in2 / 144 in2 = 1ft2
Average
air 1 foot
velocity is
400 fpm
Air Volume (cfm) = 400 ft/min x 1ft2 = 400 cfm

55. Cross-sectional area =
18” x 18” = 324 in2
324 in2 / 144 in2 =
18” 2.25ft2
Average
air 18”
velocity is
400 fpm
Air Volume (cfm) = 400 ft/min x 2.25ft2 = 900 cfm

56. 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

57. 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

58. 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

59. 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

60. 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

61. 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

62. 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

63. 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

65. TASBIH KIFARAH
َ ‫س ُ ب ْ ح َا ن َ ك َ ال ل ّ ه ُ م ّ و َ ب ِ ح َ م ْ د ِ ك‬
َ ‫أَ ش ْ ه َ د ُ أَ ن ْ ل َ إِ ل َ ه َ إِ ل ّ أَن ْ ت‬
َ ‫أ َ س ْ ت َ غ ْ ف ِ ر ُ ك َ و َأ َ ت ُ و ْ ب ُ إ ِ ل َ ي ْ ك‬
(Maha Suci Engkau Ya Allah dan Segala Puji
BagiMu, aku bersaksi bahawa tiada Tuhan
melainkan Engkau, aku memohon keampunan