About Fan Affinity Laws

The fan affinity laws express the relationship between elements of fan performance and power use. They help to predict how a certain type of fan will behave when the impeller diameter or rotational speed are changed. Since it is impractical to test all sizes of a type of fan at all speeds and inlet densities, the fan laws are a great asset to industry.

The laws assume a constant efficiency and are limited to particular point on the fan characteristic curve. Two fans must be geometrically proportional (i.e. have similar dynamics) to be comparable.

It is also important to note that design and manufacturing tolerances affect the applicability of the fan laws. This means that some loss of accuracy is to be expected in normal ventilation, with better accuracy where the affinity laws are used to extrapolate from smaller impeller sizes to larger ones rather than vice versa.

The most common use of the affinity laws is to calculate how pressure, flow volume and power use changes with different impeller diameters and velocities. The laws can also be used to estimate variables where gas densities vary, but fan performance data are often based on a standard air density (1.20 kg/m³) at zero elevation.

Above fan pressures of 2000 Pa compressibility effects become noticeable and should be included in the equations to improve the accuracy of performance predictions. The fan laws are represented below by the following equations and are in their simplified form.

Fan law equations