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
- change in wheel speed - revolutions per minute ( rpm ), and/or
- 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
q 1 / q 2 = (n 1 / n 2 )(d 1 / d 2 ) 3 (1)
where
q = volume flow capacity (m 3 /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
dp 1 / dp 2 = (n 1 / n 2 ) 2 (d 1 / d 2 ) 2 (2)
where
dp = head or pressure (m, ft, Pa, psi, ..)
Power
The power consumption of a centrifugal fan can be expressed as
P 1 / P 2 = (n 1 / n 2 ) 3 (d 1 / d 2 ) 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
q 1 / q 2 = (n 1 / n 2 ) (1a)
Head or Pressure
dp 1 / dp 2 = (n 1 / n 2 ) 2 (2a)
Power
P 1 / P 2 = (n 1 / n 2 ) 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.
Changing the Impeller Diameter
If the wheel velocity is constant - the affinity laws for change in impeller diameter can be simplified to
Volume Capacity
q 1 / q 2 = (d 1 / d 2 ) 3 (1b)
Head or Pressure
dp 1 / dp 2 = (d 1 / d 2 ) 2 (2b)
Power
P 1 / P 2 = (d 1 / d 2 ) 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.
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/m 3 )
ρ act = actual density of air (kg/m 3 )
ESP can be useful when selecting fans from published data based on standard conditions.
Related Topics
• Ventilation
Systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more.
Related Documents
Belt Transmissions - Speed and Length of Belts
Calculate length and speed of belt and belt gearing.
Fan AMCA Classification
Fan classification established by AMCA.
Fan Capacity Diagrams
Pressure, head, air flow volume and fan capacity diagrams.
Fans - Calculate Air and Brake Horsepower
AHP - Air Horse Power and BHP - Brake Horse Power.
Fans - Troubleshooting
Guide to troubleshooting fan problems.
Pumps - Affinity Laws
Turbo machines affinity laws can be used to calculate volume capacity, head or power consumption in centrifugal pumps when changing speed or wheel diameters.
STP - Standard Temperature and Pressure and NTP - Normal Temperature and Pressure
The definition of STP - Standard Temperature and Pressure and NTP - Normal Temperature and Pressure.
Types of Fans - Capacity Ranges
Centrifugal, axial and propeller fans and their capacity ranges.