Fan Inlets - Air Density vs. Suction Pressure

If the inlet suction pressure of a fan exceeds 270 mm W.G (10 in W.G) the reduced air density should be compensated by increasing the suction pressures with a correction factor:

• 1 in water = 248.8 N/m²= 0.0361 lb/in² = 25.4 kg/m² = 0.0739 in mercury

For suction pressures below 270 mm W.G (10 in W.G) the correction is normally neglected.

Corrected suction pressure can be expressed as:

P_sc = P_s f                                             (1)

where

P_sc= corrected suction pressure (mm W.G., in W.G)

f= correction factor

P_s= design (and measured) suction pressure (mm W.G., in W.G)

Example - Correcting Suction Pressure

Design or measured suction pressure for a fan is P_s = -380 mm W.G. (-14 in W.G). The correction factor from table above 1.04. For a proper fan selection the suction pressure should be corrected to

P_sc = (-380 mm W.G) 1.04 

    = -395 (mm W.G.)

Temperature ^oC
K
^oF

Length m
km
in
ft
yards
miles
naut miles

Area m²
km²
in²
ft²
miles²
acres

Volume m³
liters
in³
ft³
us gal

Weight kg_f
N
lbf

Velocity m/s
km/h
ft/min
ft/s
mph
knots

Pressure Pa
bar
mm H₂O
kg/cm²
psi
inches H₂O

Flow m³/s
m³/h
US gpm
cfm