Compressibility effects can be a point of confusion for fan users. Even experienced fan engineers have difficulty in accepting the need to include a compressibility coefficient when calculating fan efficiency.

The equation for fan total efficiency based on incompressible flow is provided. For a complete understanding, it is usually necessary to follow a step by step derivation. Such a derivation will be discussed in this paper.

The evaluation of compressibility coefficient has generally involved an iteration procedure, because the coefficient was expressed as a function of fan efficiency. It is possible to evaluate Kp directly from test data exclusive of efficiency. Two new coefficients, x and z, which make this possible, will also be derived.

Heretofore, the fan laws for compressible flow have been considered to be the same as those for incompressible flow except with respect to pressure. There are strong reasons to believe that this is not so. Compressible flow fan laws are derived in this paper.