Checking External Static Pressure
The airflow through the unit can be determined by measuring the external static pressure of the system, and consulting the blower performance data for the specifi c
1.Set up to measure external static pressure at the supply and return air.
2.Drill holes in the supply duct for pressure taps, pilot tubes or other accurate pressure sensing devices.
3.Connect these taps to a level inclined manometer or Magnehelic gauges.
4.Ensure the coil and fi lter are clean, and that all the registers are open.
5.Determine the external static pressure with the blower operating.
6.Refer to the Air Flow Data for your
7.If the actual airfl ow is either too high or too low, the blower speed will need to be changed.
8.Select a speed, which most closely provides the required airfl ow for the system.
9.Recheck the external static pressure with the new speed. External static pressure (and actual airfl ow) will have changed to a higher or lower value depending upon speed selected. Recheck the actual airfl ow (at this "new" static pressure) to confi rm speed selection.
10.Repeat steps 8 and 9 (if necessary) until proper airfl ow has been obtained.
EXAMPLE: Airflow requirements are calculated as follows: (Having a wet coil creates additional resistance to airfl ow. This addit ional resistance must be taken into consideration to obtain accurate airfl ow information.
1 ½ TON SYSTEM ( 18,000 Btu)
Operating on high speed @ 230 volts with dry coil
measured external static pressure .20
Air Flow = 500 CFM
In the same SYSTEM used in the previous example but having a WET coil you must use a correction factor of
.94 (i.e. 500 x .94=470 CFM) to allow for the resistance (internal) of the condensate on the coil.
It is important to use the proper procedure to check external Static Pressure and determine actual airfl ow. Since in
the case of the
It is also important to remember that when dealing with
Checking Approximate Airflow
If an inclined manometer or Magnehelic gauge is not available to check the External Static Pressure, or the blower performance data is unavailable for your unit, approximate air fl ow call be calculated by measuring the temperature rise, then using tile following criteria.
KILOWATTS x 3413
= CFM
Temp Rise x 1.08
Electric Heat Strips
The approximate CFM actually being delivered can be calculated by using the following formula:
DO NOT simply use the Kilowatt Rating of the heater (i.e. 2.5, 3.4, 5.0) as this will result in a
current draw of all heaters (ONLY) in operation to obtain watts. Kilowatts are than obtained by dividing by 1000.
EXAMPLE: Measured voltage to unit (heaters) is 230 volts. Measured Current Draw of strip heaters is 11.0 amps.
230 x 11.0 = 2530 2530/1000 = 2.53 Kilowatts 2.53 x 3413 = 8635
Supply Air |
| 95°F | |
Return Air |
|
| 75°F |
|
| ||
Temperature Rise | 20° | ||
20 x 1.08 = 21.6
8635
= 400 CFM
21.6
IMPORTANT: FLEX DUCT CAN COLLAPSE AND CAUSE AIRFLOW RESTRICTIONS. DO NOT USE FLEX DUCT FOR: 90 DEGREE BENDS, OR UNSUPPORTED RUNS OF 5 FT. OR MORE.
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