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

2. UNIT OBJECTIVES Describe propeller and centrifugal fans and blowers
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. Mechanical means to introduce ventilation
Return air from the occupied space
Damper in fresh air duct
Supply duct
Air handler
Fresh air from outside the structure

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

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

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

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

13. Velocity 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. End View of Squirrel Cage Blower Wheel
Centrifugal Blower

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

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
Supply plenum
Branch ducts
Return duct

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

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

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

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
Drive cleat
Slip

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

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
Return
Damper
Supply
Flexible Duct

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. Balancing damper in the closed position
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. Central Return Supply plenum Return plenum
One central return grill in the common area

49. INDIVIDUAL RETURN AIR SYSTEM

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

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

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