Elementary centrifugal fan theory

To fully understand therefore, Sections 1.5 and 1.6 in Chapter

1, dealing with fan characteristics, Chapter 5, Section 5.6 on system effect factors and Chapter 6 on flow regulation, some knowledge of the elementary theory is essential.

For the sake of simplicity the analysis which follows is not math­ematically exact and further assumes that the air or gas is in­compressible.

A centrifugal fan receives air or gas at the impeller eye and de­livers it to the casing volute at high velocity by imparting rota­tional energy. The kinetic energy produced by the impeller is converted into pressure energy within the volute. Fan efficiency therefore depends on how much kinetic energy is produced, how low the impeller losses can be kept, and how well this ki­netic energy is converted into potential energy (or static pres­sure) within the casing.

Considering the velocity triangles in Figure 3.1, the work done on the gas by the impeller will be the energy difference between exit and entry in the direction of rotation.

In most cases where equation 3.13 is used, the error due to the incorrect method of calculating pv (mean) is allowed for by an ex­perimentally determined loss factor or coefficient for the form of velocity distribution it is hoped will be encountered. It will be as­sumed here that Pv(mean) is based on the simple calculation in conjunction with this factor.

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