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# 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:

Fan Inlets - Pressure Correction Factor vs. Suction Pressure
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.)

## Related Topics

• ### Ventilation Systems

Design of systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more.

## Related Documents

• ### Fan Capacity Diagrams

Pressure, head, air flow volume and fan capacity diagrams.
• ### Fans - Capacity Control

How to modulate fans and their capacities.
• ### Pumps and Fans - Energy Equation and Head Rise

The energy equation can be used to calculate the head rise in pumps and fans.

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## Citation

• The Engineering ToolBox (2005). Fan Inlets - Air Density vs. Suction Pressure. [online] Available at: https://www.engineeringtoolbox.com/fan-inlet-pressure-d_914.html [Accessed Day Month Year].

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7.25.9

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