Back to basics: fan affinity laws

Because fans impart velocity and convert velocity to pressure, the flow and pressure created can be changed by changing the fan impeller diameter size or width and by varying the fan speed by using different size motors and a variable frequency drive, which is now common.

These overall relationships can be simplified with fan equations or fan affinity laws. These laws can be used by a designer to compare a different sized, geometrically similar fan model to an already tested fan. The fan laws can be used to quickly predict changes assuming the fan diameter and air density are constant.

The designer can also calculate another fan’s performance at other speeds and gas densities if an additional step to correct for density performance is take. However, it’s important to understand these fan laws are only approximations and have limited accuracy across changes of speed, size or pressure within the same fan model or family.

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