If fluids other than Hg or water are used for measuring pressure, the conversion is based on the relationship:

hiPi = h2p2

where the subscripts 1 and 2 denote the fluids under consideration.

Fan total pressure (Pt) is the difference between the total pressure at the fan outlet and the total pressure at the fan inlet. Total pressure is expressed as:

t t2 ti

If the fan draws directly from atmosphere:

If the fan discharges directly to atmosphere:

t2 v2

Fan velocity pressure (Pv) is the pressure that corresponds to the average velocity at the fan outlet. This is expressed as:


Pv =

Fan velocity pressure (in wg)

qv =

Volumetric flow rate at fan outlet (in cfm)

A =

Flow area at fan outlet (in ft2)

Pa =

Fan air density at fan outlet (in lbm/ft3)

v =

Velocity (in ft/min)

In SI units, when Pv is in kPa, pa is kg/m3 and v is in m/s, Equation 32-5 becomes:

py 2

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