Fan Inlet - Suction Pressure and Air Density

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

If inlet suction pressures exceeds 270 mm W.G (10 in W.G) the change in 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 P_s = -380 mm W.G. (-14 in W.G). Correction factor from table above 1.04. For proper fan selection the suction pressure should be corrected to

P_sc = -380 1.04 (mm W.G.) = -395 (mm W.G.)

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