Compared to compressors, it is more common for fans or blowers to operate with variable speeds (adjustable in the Variable Speed tab in the Compressor/Fan Properties window). Affinity laws (also called homologous laws) allow an estimation of the compressor/fan performance at speeds other than the design speed.

Before discussing the affinity laws, it should be noted that the affinity laws are an approximation. In many cases, compressor/fan manufacturers will have performance data for a number of speeds. These data should be used if available. When not available, the affinity laws can be used with an understanding that an approximation is being made. The affinity laws are known to have much lower accuracy for compressors and blowers than for fans.

Compressor/fan curves can generally be curve fit to a polynomial:

The speed ratio is defined in terms of the design impeller speed, N1 and the desired operating speed N2 as:

Affinity Laws

For the following ratios, subscript 1 represents the parameter from the defined curve (assumed to be at 100% speed). Subscript 2 represents the given parameter at the desired operating speed N2 .

• Flow rate -

• Compressor/fan head -

• Power consumption -

• Efficiency -

Application to Transients

During a transient event, the speed of the compressor/fan may change. In such cases, the affinity laws are used to adjust the head/pressure curves appropriately for the new speed. This is a process that often requires additional iteration.

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