Fan Inlets - Air Density vs. Suction Pressure
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 - | |
|---|---|---|
| (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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