Hvac - Duct Design HP لبسكلفلا و تادعملاو ءاوهلا جرخم ديقافم = CFM * Static Pressure (Inwg) ÷(6356 * Efficiency)

1. SECTION 7
AIR CONDITIONING (COOLING)
UNIT 37
AIR DISTRIBUTION AND
BALANCE

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

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

9. 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

10. SYSTEM PRESSURES
• Duct system is pressurized by three
pressures
– Static pressure – air pressure in the duct
– Velocity pressure – pressure generated by the
velocity and weight of the air
– Total pressure
• Static pressure plus velocity pressure equals
total pressure

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

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

13. Velocity pressure
Velocity pressure = Total pressure – Static pressure
Total pressure
Static pressure

14. 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

15. 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

16. Propeller fan
Venturi

17. 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

18. Centrifugal Blower
End View of Squirrel Cage Blower
Wheel

19. CENTRIFUGAL BLOWER HOUSING

20. 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

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

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

23. 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

24. 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

25. Plenum system
Return duct
Supply plenum
Branch
ducts

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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

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

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

36. 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

37. 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

38. 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

39. Floor Register
Flexible Duct
Damper Supply
Return

40. 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

41. 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

42. Main supply duct
Takeoff fitting

43. 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

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

45. 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

46. 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

47. 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

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

49. INDIVIDUAL RETURN AIR SYSTEM
R
S

50. 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

51. 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

52. 1 foot
1 foot
Average 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

53. 18”
18”
Average air
velocity is
400 fpm
Air Volume (cfm) = 400 ft/min x 2.25ft2
= 900 cfm
Cross-sectional area =
18” x 18” = 324 in2
324 in2
/ 144 in2
= 2.25ft2

54. 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

55. RESIDENTIAL DUCT SYSTEM
• A 6-in. round duct delivers 100 cfm to the space
• An 8-in. round duct delivers 200 cfm to the space
• 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

56. 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

57. SUMMARY - 1
• 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

58. SUMMARY - 2
• 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

59. SUMMARY - 3
• 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

60. 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

61. 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