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Fan Affinity Laws

The affinity laws can be used to calculate resulting volume capacity, head or power consumption when speed or wheel diameters are changed.

The Affinity Laws for centrifugal pumps and fans are used to express the influence on volume capacity, head (pressure) and/or power consumption due to

  1. change in wheel speed - revolutions per minute ( rpm ), and/or
  2. geometrically similarity by change in impeller diameter

Note that the affinity laws for pumps are not identical with fans.

Fan Affinity Laws

Volume Flow Capacity

The volume flow capacity of a centrifugal fan can be expressed as

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

Head or Pressure

The head or pressure of a centrifugal fan can be expressed as

dp1 / dp2= (n1 / n2)2(d1 / d2)2(2)

where

dp = head or pressure  (m, ft, Pa, psi, ..)

Power

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, ..)

Changing the Wheel Velocity

If the wheel diameter is constant - the affinity laws for change in wheel velocity can be simplified to

Volume Flow Capacity

q1 / q2= (n1 / n2 )                                        (1a)

Head or Pressure

dp1 / dp2= (n1 / n2) 2 (2a)

Power

P1 / P2= (n1 / n2)3 (3a)

fan affinity laws velocity diagram

Note! - if the speed of a fan is increased with 10%

  • the volume flow is increased with 10%
  • the head is increased with 21%
  • the power is increased with 33 %

- if the volume flow capacity of an existing system is increased with 10% - the power supply increases with 33% and the electrical motor and the power supply may need an upgrade.

Fan Affinity Laws Calculator - Changing Wheel Velocity

Replace the default values with the actual values. The calculator is generic and can be used with all common units as long as the units use is consistent.

Changing the Impeller Diameter

If the wheel velocity is constant - the affinity laws for change in impeller diameter can be simplified to

Volume Capacity

q1 / q2= (d1 / d2)3 (1b)

Head or Pressure

dp1 / dp2= (d1 / d2)2(2b)

Power

P1 / P2= (d1 / d2)5 (3b)

fan affinity laws diameter diagram

Fan Affinity Laws Calculator - Changing Wheel Diameter

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.

Equivalent Static Pressure

Use of the fan laws can sometimes be simplified by using Equivalent Static Pressure - ESP - defined as the pressure that would be developed by a fan operating at standard air density instead of the actual air density.

ESP = dp act ρ std / ρ act (4)

where

ESP = Equivalent Static Pressure   (Pa)

dp act = actual pressure  (Pa)

ρ std = standard density of air (kg/m3 )

ρ act = actual density of air (kg/m3 )

ESP can be useful when selecting fans from published data based on standard conditions.

Related Topics

  • Ventilation Systems

    Design of systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more.

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