Fan Laws

A recent comment prompts me to list the basic fan laws here.

For a given change in RPM,

CFM2/CFM1 = RPM2/RPM1

Static pressure 2/ Static Pressure 1 = (Rpm2/ RPM1)^2

Horsepower 2/ Horsepower 1 = (RPM2/RPM1)^3

Reference: Trane Air Conditioning Manual, Page 270, The Trane Company, Fifty-Seventh Printing, December 1981

My experience during 40 years of air conditioning measurement and service confirms this to the 3rd digit when using adequate measuring instruments on a stable system operating within the design limits of normal fans. Of course, these equations fall apart under certain conditions, such as energy loss when the fan blade tips exceed the speed of sound or cyclic unloading of the fan due to flow restriction in either the intake side or the exhaust side. If the air flow is restricted beyond the static pressure limits of the fan blade design, the fan blade, "loses its grip" on the air and fails in a pulsing manner. Static Pressure means the difference in pressure from the intake to the output of the fan, usually measured in inches of water column.

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