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Engineering ToolBox > Fan Affinity Laws
The Affinity Laws of centrifugal pumps or fans can be used to express the influence on volume capacity, head (pressure) or power consumption of a pump or fan due to
- change in speed of wheel - revolutions per minute (rpm)
- geometrically similarity - change in impeller diameter
Note that the affinity laws for pumps are not identical with fans.
The volume capacity of a centrifugal fan can be expressed like
q1 / q2 = (n1 / n2)(d1 / d2)3 (1)
where
q = volume flow capacity (m3/s, gpm, cfm, ..)
n = wheel velocity - revolution per minute - (rpm)
d = wheel diameter
The head or pressure of a centrifugal fan can be expressed like
dp1 / dp2 = (n1 / n2)2 (d1 / d2)2 (2)
where
dp = head or pressure (m, ft, Pa, psi, ..)
The power consumption of a centrifugal fan can be expressed as
P1 / P2 = (n1 / n2)3 (d1 / d2)5 (3)
where
P = power (W, bhp, ..)
If the wheel diameter is constant - change in fan wheel velocity can simplify the affinity laws to
q1 / q2 = (n1 / n2) (1a)
dp1 / dp2 = (n1 / n2)2 (2a)
P1 / P2 = (n1 / n2)3 (3a)
Note! If the speed of a fan is increased with 10%
If we want to increase the volume flow capacity of an existing system with 10% we have to increase the power supply with 33%. In this situation the electrical motor and power supply may need to be upgraded.
Replace the default values with the actual values. The calculator is generic and can be used with all common units as long as the use is consistent.
If wheel velocity is constant a change in impeller diameter can simplify the affinity laws to
q1 / q2 = (d1 / d2)3 (1b)
dp1 / dp2 = (d1 / d2)2 (2b)
P1 / P2 = (d1 / d2)5 (3b)
Replace the default values with the actual values. The calculator is generic and can be used with all common units as long as the use is consistent.