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Aperture Labs
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What is the correct type of pressure (static or total) used in the compressibility factor, KP, 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 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 calculation.
The equations I am using are:
H = (Q*PT*KP) / (6356*NT)
Where,
H = fan shaft power [hp]
NT = fan total efficiency [%]
Q = fan inlet volumetric flow [acfm]
PT = fan total pressure [in. w.c.]
6356 is a conversion factor
KP = compressibility factor [dimensionless]
KP = [itex]\gamma[/itex]/([itex]\gamma[/itex]-1) * [(p2/p1)^(([itex]\gamma[/itex]-1)/[itex]\gamma)[/itex]-1] / ((p2/p1)-1)
Where,
p2 = total or static pressure at the fan outlet [in. w.c.]
p1 = total or static pressure at the fan inlet [in. w.c.]
The equations I am using are:
H = (Q*PT*KP) / (6356*NT)
Where,
H = fan shaft power [hp]
NT = fan total efficiency [%]
Q = fan inlet volumetric flow [acfm]
PT = fan total pressure [in. w.c.]
6356 is a conversion factor
KP = compressibility factor [dimensionless]
KP = [itex]\gamma[/itex]/([itex]\gamma[/itex]-1) * [(p2/p1)^(([itex]\gamma[/itex]-1)/[itex]\gamma)[/itex]-1] / ((p2/p1)-1)
Where,
p2 = total or static pressure at the fan outlet [in. w.c.]
p1 = total or static pressure at the fan inlet [in. w.c.]