Fan Affinity Laws

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)

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. 

q₁ - volume capacity - (m³/s, gpm, cfm, ..)

dp₁ - head or pressure (m, ft, Pa, psi, ..)

P₁ - power (W, bhp)

n₁ - initial wheel velocity (rpm)

n₂ - final wheel velocity (rpm)

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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 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. 

q₁ - volume capacity - (m³/s, gpm, cfm, ..)

dp₁ - head or pressure (m, ft, Pa, psi, ..)

P₁ - power (W, bhp)

d₁ - initial wheel diameter (m, in, ft  ...)

d₂ - final wheel diameter (m, in, ft ..)

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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.