Fan Laws - Greenheck 240XP-CUB Manual

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.
rpm
New
cfm = x cfm
New Old
rpm
Old
( ) 2
rpm
New
Ps = x Ps
New Old
rpm
Old
( rpm ) 3
New
Bhp = x Bhp
New
rpm
Old
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:
3
Bhp = (1.25) x Bhp = 1.95 x Bhp
New Old
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.
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Old
Fan Fundamentals Rev 2 June 2005
Copyright © 2005 Greenheck Fan Corp.