High suction pressure at a fan inlet decreases the density of air - and should be corrected for proper selection of the fan.
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:
|Suction Pressure (Water Gauge)||Pressure Correction Factor|
- f -
- 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:
Psc = Ps f (1)
Psc= corrected suction pressure (mm W.G., in W.G)
f= correction factor
Ps= design (and measured) suction pressure (mm W.G., in W.G)
Example - Correcting Suction Pressure
Design or measured suction pressure for a fan is Ps = -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
Psc = (-380 mm W.G) 1.04
= -395 (mm W.G.)