Fan Power Equation

[ApertureScience]

What is the correct type of pressure (static or total) used in the compressibility factor, K_P, when calculating fan power? Howden's Fan Engineering book seems to indicate total pressures should be used, but I also have a PDF from Howden that indicates static pressures should be used. Online searching has also shown mixed answers. For my calculation, I did it both ways and the difference was small enough to be considered negligible by engineering judgement, however, I would still like to know the correct

 

[willard3]

Horsepower is based on kinetic plus potential energy, ie, velocity pressure plus static pressure.

 

[ApertureScience]

I agree, the pressure variable in the horsepower equation is based on total pressure, but the question is, what type of pressure is the compressibility factor based on?

 

[chicopee]

I don't ever remember compressibility factors unsed on fans or blowers, however, there will be a compressibility factor on compressed air emanating from compressors.

 

[ProcessHVAC]

Be careful with the word static in fan design and selection. Some folks (suppliers, contractors, TABS and some engineers) use the terms "total static pressure" and "external static pressure" and they are actually including the velocity pressure.

 

[ApertureScience]

chicopee
The power equation for a fan is:

H = (Q*P_T*K_P) / (6356*N_T)

Where,

H = fan shaft power [hp]
N_T = fan total efficiency [%]
Q = fan inlet volumetric flow [acfm]
P_T = fan total pressure [in. w.c.]
6356 is a conversion factor
K_P = compressibility factor

ProcessHVAC
Good heads up info. The pressures used in the compressibility factor equation are absolute fan inlet and outlet pressures. So far I haven't seen the words "external" or "total" paired with "static." Just "total" or "static" used seperately.