Centrifugal Fan or Squirrel-cage Fan

 By: Vara Lakshmi
A Centrifugal Fan is a mechanical device for moving air or other gases. It has a fan wheel composed of a number of fan blades, or ribs, mounted around a hub. The gas enters from the side of the fan wheel, turns 90 degrees and accelerates due to centrifugal force as it flows over the fan blades and exits the fan housing.

Centrifugal fans can generate pressure increases in the gas stream. Accordingly, they are well suited for industrial processes and air pollution control systems. They are also common in central heating/cooling systems.

Ratings found in centrifugal fan performance tables and curves are based on standard air SCFM. Fan manufacturers define standard air as clean, dry air with a density of 0.075 pounds mass per cubic foot (1.2 kg/cubic m), with the barometric pressure at sea level of 29.92 inches of mercury (101.325 kPa) and a temperature of 70 degrees F (21 degrees C). Selecting a centrifugal fan to operate at conditions other than standard air requires adjustment to both static pressure and power. The volume of air will not be affected in a given system because a fan will move the same amount of air regardless of the air density.

If a centrifugal fan is to operate at a non-standard density, then corrections must be made to static pressure and power. At higher than standard elevation (sea level) and higher than standard temperature, air density is lower than standard density. Centrifugal fans that are specified for continuous operation at higher temperatures need to be selected taking into account air density corrections. Again, a centrifugal fan is a constant volume device that will move the same amount of air at two different temperatures.

The centrifugal fan performance tables provide the fan RPM and power requirements for the given CFM and static pressure at standard air density. When the centrifugal fan performance is not at standard conditions, the performance must be converted to standard conditions before entering the performance tables. Centrifugal fans rated by the Air Movement and Control Association (AMCA) are tested in laboratories with test set-ups that simulate installations that are typical for that type of fan. Usually they are tested and rated as one of four standard installation types as designated in AMCA Standard 210.
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