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All About the Squirrel Cage Fan

All About the Squirrel Cage Fan

The squirrel cage fan is also known as a centrifugal fan or blower, and is a common component in heating and air conditioning systems as well as having many other applications.  A squirrel cage fan features a number of blades that are mounted to a central hub and encased in a housing.  This hub is then attached to a driveshaft which turns it.  Air or gas enters through one end and passes over the fan blades to exit the fan through the opening in the housing, increasing the pressure in the gas stream.

The squirrel cage fan was invented by a Russian engineer in 1832 and eventually came into industrial use in many different areas.  It is a basic device that works so well that it is still in use coming up on two hundred years later, even with all of the advanced technology available today.

A squirrel cage fan can use one of three types of drives to turn the fan.  These are direct drive, belt drive and variable drive.  The first, direct drive, involves connecting the fan to an electric motor.  Although reliable, a direct drive system does not allow for changes in fan speed unless the motor speed itself can be varied.  As the name implies, a blet drive uses a belt and a set of sheaves to power the fan.  In this type of drive the fan speed is also fixed.  Belt slippage is one of the concerns with this type of squirrel cage fan drive.  Finally, a variable drive is the third option.  This uses a hydraulic or magnetic system to control the fan and allow th speed of the fan to be adjusted.  Automated systems can be used to allow control over the fan speed.

The fan ribs, also known as blades, can also be of three types in a squirrel cage fan.  These are forward curved, backward curved and straight radial blades.  Forward curved blades curve in the same direction that the fan rotates.  They are excellent for any use that requires a high flow with low pressure.  Backward curved blades curve in the opposite direction, against the direction of the fan’s rotation.  These are usually considered best for applications requiring a more moderate speed and high pressure.  Finally, straight radial blades have no curve and extend straight out from the hub of the squirrel cage fan.  These are often used for low volume, high speed and high pressure applications.

The Air Movement and Control Association (AMCA) has created standard performance tables to determine the expected performance of each of the squirrel cage fan types.  These are based on static pressure and standard air density.  These standards allow for testing of fans in lab conditions to measure accurately the the power, air flow rate and pressure as well as the efficiency of a squirrel cage fan at any given speed.  

Although a simple device, the squirrel cage fan can be used for very complex operations, and offers a wide variety of uses across many industries.

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