Greenheck Fan 240XP-CUb manual Fan Laws

Fan Laws

In a steady-state system, as the fan rpm changes, cfm, Ps and BHp (horsepower) also change. The equations below, known better as fan laws, show the relationship between these performance parameters.

cfmNew =	rpmNew	x cfmOld
rpmOld
	rpmNew	2
		) x PsOld
PsNew = (rpmOld

	rpm	3
BhpNew = (	rpmOldNew) x BhpOld

The first two equations have already been covered in the fan and system dynamics section. Refer to the examples in those sections on how to apply these equations.

The third equation relates horsepower to rpm. The change in horsepower can be determined when the rpm is increased by

25%. This is shown below:

BhpNew = (1.25)3 x BhpOld = 1.95 x BhpOld

NOTE: a 25% increase in rpm results in a 95% increase in horsepower. Considering this, initial fan selections should be sized with motor horsepowers greater than necessary if any increase in fan rpm is likely in the future.

P.O. Box 410 • Schofield, WI 54476-0410 • Phone (715) 359-6171 • greenheck.com	Fan Fundamentals Rev 2 June 2005
	Copyright © 2005 Greenheck Fan Corp.