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Fan Inlet - Suction Pressure and Air Density

High suction pressure at a fan inlet decreases the density of air - and should be corrected for proper selection of the fan

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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 -
(mm)(in)
-135 -5 1.01
-200 -7.5 1.02
-270 -10 1.03
-380 -14 1.04
-490 -18 1.05
-600 -22 1.06
  • 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)

where

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.)

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