4 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)
5 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.
6 Effects Of System Air Flow 23.4.3Air 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.
7 Effects of System Air Flow Refrigerant ChargingSystem EfficiencyAir FilteringSound LevelsHuman ComfortAir flow will affect all of the above 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.Note: Sensible Heat Ratio (SHR) will be explained in more detail in side 19.
8 Air Flow Measuring Tools Most 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.23.4.3
9 Air Flow Measuring Tools AnemometerFlow Capture HoodCourtesy of FieldpieceCourtesy of Dwyer InstrumentsVelometerCourtesy of Dwyer InstrumentsCourtesy of TSI IncorporatedCourtesy of TSI IncorporatedCourtesy of Dwyer Instruments
10 Capture HoodCapture 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 computerfor record keeping and printing.
11 Flow Capture Hood Accurate Easy to use Preferred instrument 23.4.4AccurateEasy to usePreferred instrumentfor air balancing
12 Blower Performance Charts 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.
14 Measuring Air Flow23.4.4Temperature 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 lbs x 60 minutes = 1.08 or rounded off “1.1”.
15 Measuring Air Flow Temperature Rise Method: 23.4.5Temperature Rise Method:Btu output ÷ (1.08 sensible heat factor x TD) = CFMNet free area of grill or AK factor x FPM = CFM
16 Measuring Grille Air Flow 23.4.5Measure the average FPM of the air passing through the grille.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.
17 Return Grille Air Flow23.4.5Most 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%.
18 384 sq. in. ÷ 144 sq. in. per sq. ft. = 2.66 sq. ft. Return Grille Air Flow23.4.5Example: 20” x 24” Return air grille with 80% free area20” x 24” = 480 sq. in.480 x .80 = 384 sq. in. free area384 sq. in. ÷ 144 sq. in. per sq. ft. = sq. ft.or2.7 AK factor
19 Grille Engineering Data 23.4.5This data sheet gives the AK factor, CFM, Face Velocity, and pressure drop for various size grilles.
20 Supply Grille Air Flow23.4.5Rooms 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
21 Supply Grille Air Flow Measuring the airflow is not enough. How much air should the grill supply?Depends on the sensible heat loss/gain of the room23.4.5
22 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.
23 Air Flow Pressure Measurements There are 3 pressures associated with duct systems.STATIC PRESSUREVELOCITY PRESSURET0TAL PRESSURE23.4.5(Pt)Total Pressure(Ps)Static PressureT0TAL PRESSURE – STATIC PRESSURE = VELOCITY PRESSURE
24 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.
25 Measuring Air Flow using a Pitot Tube The velocity readings covering the whole cross section of the duct must be averaged.23.4.5Connected tomeasure velocity(Pt)Total Pressure(Ps)Static Pressure
26 Measuring Air Flow using a Traverse 23.4.5Traverse 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.
27 Measuring Air Flow using a Traverse The 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.23.4.5
28 Pitot Tube TraverseThe velocity readings covering the whole cross section of the duct must be averaged.Apply the formula:Velocity = x √ Velocity Pressure23.4.5FPM = x √ .04FPM = x .2FPM = 800.8Note: This formula is based on standard air conditions.
29 Pitot Tube TraverseVelocity pressure must be converted into feet per minute.The square root of the velocity pressure is multiplied timesFPM= x √Velocity Pressure Example:FPM = x √ .04FPM = x .2FPM = 800.823.4.5
30 Calculating Total Air Flow - Heating 23.4.5Total CFM =Furnace output in Btu ÷ by the temperature rise X 1.08If the total Btuh or CFM for the furnace is more than the totalof the rooms, the excess must be equally distributed.Example: 55,000 Btuh furnace42,000 Btu total home heat loss55,000 ÷ 42,000 = 1.31 multiplierRoom Btu x 1.31 = New Btu
31 Calculating Total Air-Flow - Heating When 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.23.4.5
32 Calculating Total Air Flow - Heating 55,000 Btuh furnace42,000 Btu total heat loss55,000 ÷ 42,000 = 1.31 multiplierRoom 1 * 10,000 Btu x 1.31 = 13,100 BtuRoom 2 * 6,000 Btu x 1.31 = 7,860 BtuRoom 3* 26,000 Btu x 1.31 = 34,060 Btu23.4.5Rm 26,000 BtuRm 110,000 BtuRm 326,000 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 CFMTotal = 1,132 CFMFurnace 55,000 Btu ÷ (1.08 x 45 ΔT) = 1,132 CFM
33 Calculating Total Air Flow - Heating 23.4.5The CFM total from the roomswill be very close to the CFMcalculated from the furnace Btuh.
34 Calculating Total Air Flow-Cooling CFM is based on the sensible capacity not the Total Capacity.Sensible Capacity + Latent Capacity = Total Btu/hTemperature difference is determined by the sensible heat ratio (SHR).Sensible Capacity ÷ Total Capacity = Sensible Heat RatioExample:29,520 Btu/h Sensible + 6,480 Btu/h Latent36,000 Btu/h Total23.4.529,520 Btu/h ÷ 36,000 Btu/h = 0.82 SHR
35 Calculating Total Air Flow-Cooling 23.4.5The 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.
36 Calculating Total Air Flow-Cooling 23.4.5Recommended Design Temperature Difference using SHR calculated from heat load.Sensible Heat Ratio Temperature Difference0.75 to ΔT0.80 to ΔT0.85 to ΔT
37 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.)23.4.5
39 Calculating Total Air Flow - Cooling 23.4.5400 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 CFM29, 520 Btu at 17 degree TD = 1608 CFM
40 Return Grille23.4.5Grilles 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.
41 Return Grille Recommended 2.7 CFM per square inch of net free area. Maximum 2.7 CFM per square inch of gross area.Recommended Velocity 400 FPM23.4.5Recommended Free Area of return air grilleCFM 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
42 Return Filter Grille23.4.5The 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.
43 Return Filter GrilleRecommended 2 CFM per square inch of net free area.Maximum 2 CFM per square inch of gross area.Recommended Velocity 300 FPM23.4.5Recommended Free Area of return air grilleCFM of Return Air2 CFM/Sq. In.Total Area of return air grill in sq. in.Free Area of Return Air% Free Area of Grille
44 Return Filter GrillVelocity 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.23.4.5For a 20” x 30” Grille 20 measurements should be obtained and averaged.
45 Return Filter Grille Traverse The 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.23.4.5