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Fan Inlet - Suction Pressure and Air Density
High suction pressure at the fan inlet decreases the density of air - should be corrected for proper fan selection
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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:
| 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 Ps = -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
Psc = (-380 mm W.G) 1.04
= -395 (mm W.G.)
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