Airflow Properties & Measurement

Air AIR PROPERTIES You can measure it. You can control it. You can filter it. You can heat it. You can cool it. You can circulate it.

AIR PROPERTIES 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)

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 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.  

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

Air Flow Measuring Tools Anemometer Flow Capture Hood Courtesy of Fieldpiece Courtesy of Dwyer Instruments Velometer Courtesy of Dwyer Instruments Courtesy of TSI Incorporated Courtesy of TSI Incorporated 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 Accurate Easy to use Preferred instrument 23.4.4 Accurate Easy to use Preferred instrument for air balancing

Blower Performance Charts 23.4.4 Most 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 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 Temperature Rise Method: 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

Measuring Grille Air Flow 23.4.5 Measure 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.

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%.

384 sq. in. ÷ 144 sq. in. per sq. ft. = 2.66 sq. ft. 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 Measuring the airflow is not enough. How much air should the grill supply? Depends on the sensible heat loss/gain of the room 23.4.5  

Air Flow Pressure Measurements 23.4.5 Air 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 There are 3 pressures associated with duct systems. STATIC PRESSURE VELOCITY PRESSURE T0TAL PRESSURE 23.4.5 (Pt) Total Pressure (Ps) Static Pressure T0TAL PRESSURE – STATIC PRESSURE = VELOCITY PRESSURE

Measuring Air Flow using a Pitot Tube 23.4.5 A 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.

Measuring Air Flow using a Pitot Tube The velocity readings covering the whole cross section of the duct must be averaged. 23.4.5 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 equal Points 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 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

Pitot Tube Traverse The velocity readings covering the whole cross section of the duct must be averaged. Apply the formula: Velocity = 4004.4 x √ Velocity Pressure 23.4.5 FPM = 4004.4 x √ .04 FPM = 4004.4 x .2 FPM = 800.8 Note: This formula is based on standard air conditions.

Pitot Tube Traverse Velocity 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 √ .04 FPM = 4004.4 x .2 FPM = 800.8 23.4.5

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 loss 55,000 ÷ 42,000 = 1.31 multiplier Room Btu x 1.31 = New Btu

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

Calculating Total Air Flow - Heating 55,000 Btuh furnace 42,000 Btu total heat loss 55,000 ÷ 42,000 = 1.31 multiplier Room 1 * 10,000 Btu x 1.31 = 13,100 Btu Room 2 * 6,000 Btu x 1.31 = 7,860 Btu Room 3* 26,000 Btu x 1.31 = 34,060 Btu 23.4.5 Rm 2 6,000 Btu Rm 1 10,000 Btu Rm 3 26,000 Btu Room 1 * 13,100 Btu ÷ (1.08 x 45 ΔT) = 270 CFM Room 2 * 7,860 Btu ÷ (1.08 x 45 ΔT) = 162 CFM Room 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

Calculating Total Air Flow - Heating 23.4.5 The CFM total from the rooms will be very close to the CFM calculated from the furnace Btuh.

Calculating Total Air Flow-Cooling CFM 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 23.4.5 29,520 Btu/h ÷ 36,000 Btu/h = 0.82 SHR

Calculating Total Air Flow-Cooling 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 Recommended Design Temperature Difference using SHR calculated from heat load. Sensible Heat Ratio Temperature Difference 0.75 to 0.79 21 ΔT 0.80 to 0.84 19 ΔT 0.85 to 0.90 17 ΔT

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

Calculating Total Air Flow - Cooling 23.4.5 36,000 Btuh Total 29,520 Btuh Sensible 6,480 Btuh Latent 29,520 ÷ (1.08 x 19 ΔT) = 1,439 CFM Rm 2 5,200 Btu Rm 1 7,380 Btu Rm 3 16,940 Btu Room 1 * 7,380 Btu ÷ (1.08 x 19 ΔT) = 360 CFM Room 2 * 5,200 Btu ÷ (1.08 x 19 ΔT) = 253 CFM Room 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 Recommended 2.7 CFM per square inch of net free area. Maximum 2.7 CFM per square inch of gross area. Recommended Velocity 400 FPM 23.4.5 Recommended Free Area of return air grille CFM of Return Air 2.7 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 Recommended 2 CFM per square inch of net free area. Maximum 2 CFM per square inch of gross area. Recommended Velocity 300 FPM 23.4.5 Recommended Free Area of return air grille CFM of Return Air 2 CFM/Sq. In. Total Area of return air grill in sq. in. Free Area of Return Air % Free Area of Grille

Return Filter Grill Velocity 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. 23.4.5 For a 20” x 30” Grille 20 measurements should be obtained and averaged.

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