For calculating the required static pressure for AC units or fans, the pressure drop in air distribution system need to be calculated.

Every AC unit has two static pressure numbers, the total static pressure (TSP) and external static pressure (ESP).

TSP represents the AC unit’s fan static pressure which need to satisfy the pressure drop in coils, filters and other components which are in row with that fan such as humidifier and etc. in AC unit. The final static pressure off the AC unit is ESP which will be used for the air distribution pressure drop satisfaction. For selecting the AC units only the ESP need to be determined but TSP is manufacturer concern.

To estimate the ESP, the below items need to be calculated:

- Pressure loss in the longest run of the ductwork in system.
- Pressure loss in last zone damper (VAV box or VVT damper) in the longest duct run.
- Pressure loss in typical diffuser which is used in the last zone.

Example:

The longest duct run in a VAV air distribution system is 120 ft. The pressure loss in the VAV box in last zone at the end of the longest run is 0.28” w.c (from manufacturer spec) and the pressure loss in typical ceiling diffuser in last zone is 0.25” w.c. (from diffusers spec – it is also typical for calculation), determine the required ESP for selecting the AC unit for serving this system.

Solution:

The practical pressure loss in ductwork is 0.08” w.c/100 ft, (disregarding the duct dimension) therefore:

120 X1.25** x 0.08/100 =0.12”

0.12 + 0.28 + 0.25 = 0.65” w.c.

** 1.25 is the factor for considering the 25% of the longest duct run as an equal length for the fittings such as elbow, etc.

The required ESP is 0.7” w.c.

The same procedure is used for estimating a single supply or exhaust fan static pressure.