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Airflow Properties & Measurement
AIR PROPERTIES
Air
You can measure it. You can control it.
You can filter it. You can heat it.
You can cool it. You can circulate it.
But first
YOU MUST UNDERSTAND IT
AIR PROPERTIES
Density = 0.075 lbs. per cu. Ft. at sea level
Specific heat = 0.24 Btu per lb. Volume is measured in Cubic Feet Per Minute (CFM) Velocity is measured in Feet Per Min (FPM)
AIR PROPERTIES
Calculating Airflow
System CFM can be calculated
By using equipment blower performance charts. By multiplying: Air velocity in feet per minute x open area of duct in square feet. By sensible heat formulas.
Using air flow measuring tools.
Effects Of System Air Flow
23.4.3
Air flow will affect all of the below listed. The volume of air flow will change the sensible heat ratio of the air conditioning system in turn changing the amount of moisture the system can remove.
Effects of System Air Flow
Refrigerant Charging
System Efficiency
Air Filtering
Sound Levels
Human Comfort
Air Flow Measuring Tools
23.4.3Most air flow meters will measure the velocity of the air flow.
The actual CFM has to be calculated manually or with some of the instruments.
Measurements can be entered into the meter to output the CFM.
The Dwyer magnehelic gauge, inclined manometer, flow meter measure low pressures or velocity dependent on the meter attachments.
The TSI and Fieldpiece instruments are electronic vane and thermocouple types.
Air Flow Measuring Tools
Anemometer
Velometer
Flow Capture Hood
Courtesy of TSI Incorporated
Courtesy of TSI Incorporated
Courtesy of Fieldpiece
Courtesy of Dwyer Instruments
Courtesy of Dwyer Instruments
Courtesy of Dwyer Instruments
Capture Hood
Capture Hood is an electronic air balancing instrument used for reading air volume flow at diffusers and grilles.
Most can record the airflow CFM or FPM to be down loaded to a computer
for record keeping and printing.
Flow Capture Hood
23.4.4
Accurate
Easy to use
Preferred instrument for air balancing
23.4.4Most manufacturers provide performance charts that indicate the volume of air the blower can supply based on the motor horsepower, blower wheel Rpm, and system static pressure.
If the blower performance chart indicates that the blower can deliver the required CFM at 0.04” WC, the total pressure drop for the supply grilles, air filter, return grilles, supply duct, return duct, evaporator and any accessories on the air side must not exceed 0.04” WC.
Blower Performance Charts
Blower Performance Chart
23.4.4
Measuring Air Flow
23.4.4• Temperature rise method uses a version of one of the sensible heat equations.
• The 1.08 sensible heat factor is derived from the density and weight of standard air at sea level 0.24 sp. x 0.075 lbs x 60 minutes = 1.08 or rounded off “1.1”.
Measuring Air Flow
23.4.5
Temperature Rise Method:
Btu output ÷ (1.08 sensible heat factor x TD) = CFM
Net free area of grill or AK factor x FPM = CFM
23.4.51. Measure the average FPM of the air
passing through the grille.
2. Determine the net free area in square feet of the grille or refer to manufacturers’ literature for AK factor.
3. Multiply FPM x (AK factor or Net free area) to get CFM.
Measuring Grille Air Flow
Return Grille Air Flow
23.4.5
Most return air grilles installed for air conditioning systems are too small reducing the air flow.
The AK factor can be affected by conditions in the installation that are different from the manufacture’s test conditions. The percentage of free area can be as low as 70%.
Return Grille Air Flow
23.4.5
Example: 20” x 24” Return air grille with 80% free area
20” x 24” = 480 sq. in.480 x .80 = 384 sq. in. free area
384 sq. in. ÷ 144 sq. in. per sq. ft. = 2.66 sq. ft.or
2.7 AK factor
Grille Engineering Data
23.4.5
This data sheet gives the AK factor, CFM, Face Velocity, and pressure drop for various size grilles.
Supply Grille Air Flow
23.4.5
Rooms of equal size can have different heat loads, depending on the location in the house.
Example a corner room versus a room in the middle of a house.
Two exposed walls on the corner room versus one exposed wall on the middle room.
Return air versus supply air
Supply Grille Air Flow
23.4.5Measuring the airflow is not enough.
How much air should the grill supply?
Depends on the sensible heat loss/gain of the room
Air Flow Pressure Measurements
23.4.5Air flowing through a duct system creates three different pressures. Static pressure: the pressure pushing outward to the walls of the duct. Velocity pressure: the pressure from the force of the air moving.
Total Pressure: the combination of both static and velocity pressures.
Air Flow Pressure Measurements
23.4.5There are 3 pressures associated with duct systems.
STATIC PRESSURE VELOCITY PRESSURE
T0TAL PRESSURE
(Pt) Total Pressure (Ps)
Static Pressure
T0TAL PRESSURE – STATIC PRESSURE = VELOCITY PRESSURE
Measuring Air Flow using a Pitot Tube
23.4.5A pitot tube is designed to measure static
pressure, and total pressure when properly connected.
When both sides of the inclined manometer or a magnehelic pressure gauge are connected to the pitot tube, the measurement obtained is velocity pressure.
The double connection on the pitot tube cancels out the total pressure measurement.
23.4.5
Measuring Air Flow using a Pitot Tube
The velocity readings covering the whole cross section of the duct must be averaged.
Connected to measure velocity
(Pt) Total Pressure
(Ps) Static Pressure
Measuring Air Flow using a Traverse
23.4.5
Traverse is a method of establishing basic equalPoints for measurements.It is important to get an equal number of readings
covering the whole cross section of the duct.
The traverse must be made at a location at least five duct diameters downstream from elbows or constrictions in the ductwork.
Measure with the pitot tube facing into the airstream.
Measuring Air Flow using a Traverse
23.4.5The velocity of the air in the duct will vary from zero in the boundary layer at the duct wall to a maximum velocity near the duct centerline.
For this reason, a number of readings must be taken and averaged.
Remember to convert the square inches of the duct to square feet by dividing the square inches by 144.
The speed or velocity the air is moving in FPM times the square feet of the duct interior size equals the CFM.
Pitot Tube Traverse
23.4.5The velocity readings covering the whole cross section of the duct must be averaged.
Apply the formula: Velocity = 4004.4 x √ Velocity Pressure
FPM = 4004.4 x √ .04FPM = 4004.4 x .2FPM = 800.8
Note: This formula is based on standard air conditions.
Pitot Tube Traverse
23.4.5Velocity pressure must be converted into feet per minute. The square root of the velocity pressure is multiplied times 4004.4.
FPM= 4004.4 x √Velocity Pressure Example:
FPM = 4004.4 x √ .04FPM = 4004.4 x .2FPM = 800.8
Calculating Total Air Flow - Heating
23.4.5
Total CFM = Furnace output in Btu ÷ by the temperature rise X 1.08
If the total Btuh or CFM for the furnace is more than the total of the rooms, the excess must be equally distributed.
Example: 55,000 Btuh furnace 42,000 Btu total home heat loss55,000 ÷ 42,000 = 1.31 multiplierRoom Btu x 1.31 = New Btu
Calculating Total Air-Flow - Heating
23.4.5When the furnace Btu output rating used is greater than the total needed, the excess heating capacity of the furnace must be equally distributed to all of the rooms.
One of two methods must be used. Distribute the Btuh or the CFM.
Most heat load programs use the Btuh capacity if there is a built-in equipment selection feature. The math process is the same for both.
NOTE: The furnace output is sometimes referred to as the bonnet capacity.
Calculating Total Air Flow - Heating
23.4.555,000 Btuh furnace 42,000 Btu total heat loss55,000 ÷ 42,000 = 1.31 multiplier
Room 1 * 10,000 Btu x 1.31 = 13,100 BtuRoom 2 * 6,000 Btu x 1.31 = 7,860 Btu Room 3* 26,000 Btu x 1.31 = 34,060 BtuRoom 1 * 13,100 Btu ÷ (1.08 x 45 ΔT) = 270 CFMRoom 2 * 7,860 Btu ÷ (1.08 x 45 ΔT) = 162 CFMRoom 3 * 34,060 Btu ÷ (1.08 x 45 ΔT) = 700 CFM
Total = 1,132 CFM
Furnace 55,000 Btu ÷ (1.08 x 45 ΔT) = 1,132 CFM
Rm 110,000 Btu
Rm 26,000 Btu
Rm 326,000 Btu
23.4.5
The CFM total from the roomswill be very close to the CFMcalculated from the furnace Btuh.
Calculating Total Air Flow - Heating
Calculating Total Air Flow-Cooling
23.4.5CFM is based on the sensible capacity not the Total Capacity.
Sensible Capacity + Latent Capacity = Total Btu/h
Temperature difference is determined by the sensible heat ratio (SHR).
Sensible Capacity ÷ Total Capacity = Sensible Heat Ratio
Example: 29,520 Btu/h Sensible + 6,480 Btu/h Latent 36,000 Btu/h Total
29,520 Btu/h ÷ 36,000 Btu/h = 0.82 SHR
23.4.5
The sensible heat ratio is based on the humidity level that has to be controlled.
The more humidity that has to be removed, the lower the required air flow.
Calculating Total Air Flow-Cooling
23.4.5
Calculating Total Air Flow-Cooling
Sensible Heat Ratio Temperature Difference0.75 to 0.79 21 ΔT
0.80 to 0.84 19 ΔT
0.85 to 0.90 17 ΔT
Recommended Design Temperature Difference using SHR calculated from heat load.
23.4.5
Calculating Total Air Flow-Cooling
The outdoor environment with high humidity will have a higher temperature split because the air needs to be cooled below the dew point to release the moisture.
The “delta T”, “TD”, or temperature split in this case is the difference between the return air and supply air. (Dew point is the temperature at which moisture starts to condense out of the air.)
Calculating Total Air Flow - Cooling
23.4.5Rm 1
7,380 BtuRm 2
5,200 BtuRm 3
16,940 Btu
36,000 Btuh Total29,520 Btuh Sensible 6,480 Btuh Latent29,520 ÷ (1.08 x 19 ΔT) = 1,439 CFM
Room 1 * 7,380 Btu ÷ (1.08 x 19 ΔT) = 360 CFMRoom 2 * 5,200 Btu ÷ (1.08 x 19 ΔT) = 253 CFMRoom 3 * 16,940 Btu ÷ (1.08 x 19 ΔT) = 826 CFM
Total = 1,439 CFM
Calculating Total Air Flow - Cooling
23.4.5
400 CFM per ton is not always true for all systems. It is a rule of thumb use to play it safe.
29,520 Btu at 21 degree TD = 1301 CFM
29, 520 Btu at 17 degree TD = 1608 CFM
Return Grille
23.4.5
Grilles used for residential systems will have a percentage of free area equal to 90% to 92% of the grilles total area.
Velocity of air through the grille should be around 300 FPM with a maximum of 600 FPM.
Return Grille
23.4.5Recommended 2.7 CFM per square inch of net free area.
Maximum 2.7 CFM per square inch of gross area.Recommended Velocity 400 FPM
Recommended Free Area of return air grille
CFM of Return Air2.7 CFM/Sq. In.
Total Area of return air grill in sq. in.
Free Area of Return Air% Free Area of Grille
Return Filter Grille
23.4.5
The percentage of free area is usually 80% to 85% for a filter grille used in a residential application.
The velocity air passing through the filter grille should be around 300 FPM with a maximum of 450 FPM.
A filter will not clean the air if the velocity is too high or low.
Return Filter Grille
23.4.5
Recommended Free Area of return air grille
CFM of Return Air2 CFM/Sq. In.
Total Area of return air grill in sq. in.
Free Area of Return Air% Free Area of Grille
Recommended 2 CFM per square inch of net free area.
Maximum 2 CFM per square inch of gross area.
Recommended Velocity 300 FPM
Return Filter Grill
23.4.5Velocity should be checked and recorded for each 36 square inches of area.
The recorded velocity measurements are averaged and multiplied times the net free area or AK factor of the grille for CFM.
For a 20” x 30” Grille 20 measurements should be obtained and averaged.
Return Filter Grille Traverse
23.4.5The outcome of the CFM is only as accurate as the measurements obtained.Time and care in taking the measurements is very important. One measurement should be taken for each 16 to 36 square inches of total area. Depending on the type of instrument used, moving the instrument in a slow cross sectional pattern across the grille can be used instead of taking multiple measurements.Electronic meters today can average the velocity or calculate the CFM when the net free area is programmed into the instrument.
