How do Centrifugal Fans Work?

Figure 1: Centrifugal fan TRZ of Airtecnics.

The centrifugal fan is a device that uses the input mechanical energy to increase the pressure of the gas and discharge the gas. It is a driven fluid machine. It is widely used in mines, tunnels, cooling towers, vehicles, ships, etc. This article will describe its constitution and working principles.

The Constitution of Centrifugal Fans

Centrifugal fans are mainly composed of impeller, casing, coupling and shaft. The impeller is the main work component that generates wind pressure and transmits energy. The casing is mainly used to introduce and exhaust gas, and at the same time convert part of the kinetic energy of the gas into pressure energy. The coupling is used to connect motors and fans to transmit torque. The shaft mounts and fixes the impeller and passes through the coupling and the motor.

Figure 2: Constitution of a centrifugal fan.

The impeller of the small fan is directly mounted on the motor. The impeller of the large fan is connected with the motor through a coupling or a pulley. Centrifugal fans are generally single-inlet and use a single-stage impeller. Centrifugal fans with large flow can have two inlets, using two back-to-back impellers. They are also known as double suction centrifugal fans.

The Working Principles of Centrifugal Fans

When the centrifugal fan is working, the airflow enters the blade space from the axial direction of the fan. Driven by the impeller, the airflow rotates with the impeller on the one hand, and increases energy under the action of inertia on the other hand. It then leaves the impeller radially and does work by the centrifugal force generated. Two of the important links are air discharge and wind pressure reduction:

Air discharge: when the impeller rotates, the air between the blades of the centrifugal fan is subjected to centrifugal force to obtain kinetic energy (dynamic head) and is discharged from the periphery of the impeller. Guided by the volute-shaped casing, the air flows toward the ventilator outlet. Then, a negative pressure is formed in the center of the impeller, which makes the external airflow continuously flow in and supplement, so that the fan can discharge air.

Pressure reduction: the motor transmits power to the fan impeller through the shaft, and the rotation of the impeller transfers energy to the air. Under the action of rotation, the air generates centrifugal force, and the air spreads around the blades of the impeller of the fan. At this time, the larger the fan impeller, the greater the energy received by the air, that is, the greater the pressure head (wind pressure) of the fan. If the large impeller is cut small, it will not affect the air volume, but only reduce the air pressure.

In general, the centrifugal fan is essentially a variable flow and constant pressure device. When the rotational speed is constant, the pressure-flow theoretical curve of the centrifugal fan should be a straight line. But the actual curve is curved due to internal losses. The pressure developed in a centrifugal fan is greatly affected by changes in temperature and density of the inlet air.

Figure 3: How do centrifugal fans work.

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