External Static Pressure
External Static Pressure can best be defined as the pressure difference (drop) between the Positive Pressure (discharge) and the Negative Pressure (intake) sides of the blower. External Static Pressure is developed by the blower as a result of resistance to airflow (Friction) in the air distribution system EXTERNAL to the
Resistance applied externally to the
External Static Pressure is affected by two (2) factors.
1.Resistance to Airflow as already explained.
2.Blower Speed. Changing to a higher or lower blower speed will raise or lower the External Static Pressure accordingly.
These affects must be understood and taken into consideration when checking External Static Pressure/Airflow to insure that the system is operating within design conditions.
Operating a system with insufficient or excessive airflow can cause a variety of different operating problems. Among these are reduced capacity, freezing evaporator coils, premature compressor and/or heating component failures. etc.
System airflow should always be verified upon completion of a new installation, or before a
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 specific
1. Set up to measure external static pressure at the |
supply and return air. |
2. Ensure the coil and filter are clean, and that all the |
registers are open. |
7.Recheck the external static pressure with the new speed. External static pressure (and actual airflow) will have changed to a higher or lower value depending upon speed selected. Recheck the actual airflow (at this "new" static pressure) to confirm speed selection.
8.Repeat steps 8 and 9 (if necessary) until proper airflow has been obtained.
EXAMPLE: Airflow requirements are calculated as follows: (Having a wet coil creates additional resistance to airflow. This addit ional resistance must be taken into consideration to obtain accurate airflow 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 airflow. 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 flow call be calculated by measuring the temperature rise, then using tile following criteria.
KILOWATTS x 3413
3. Determine the external static pressure with the |
Temp Rise x 1.08
= CFM
blower operating. |
4. Refer to the Air Flow Data for your |
system to find the actual airflow for |
fan speeds. |
5. If the actual airflow is either too high or too low, the |
blower speed will need to be changed to appropriate |
setting or the ductwork will need to be reassessed |
and corrections made as required. |
6. Select a speed, which most closely provides the |
required airflow for the system. |
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
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