Fan/Pump Affinity Laws Calculator

This calculator helps you analyze how changes in speed or diameter affect flow rate, head/pressure, and power consumption in fans and pumps based on the Affinity Laws.
Flow Rate

-

m³/h

Change: 0%

Head/Pressure

-

kPa

Change: 0%

Power

-

kW

Change: 0%

Calculation Details

Enter parameters and click Calculate to see results.

Parameter Original Value New Value Change
Flow Rate - - -
Head/Pressure - - -
Power - - -
Speed (RPM) - - -
Diameter - - -
Flow Rate vs Speed/Diameter
Head/Pressure vs Speed/Diameter
Power vs Speed/Diameter
System Curve
Affinity Laws Formulas
For Speed Changes (RPM):
Law 1

Flow Rate ∝ Speed

Q₂ = Q₁ × (N₂/N₁)

Where Q is flow rate, N is speed
Law 2

Head/Pressure ∝ Speed²

H₂ = H₁ × (N₂/N₁)²

Where H is head/pressure
Law 3

Power ∝ Speed³

P₂ = P₁ × (N₂/N₁)³

Where P is power
For Diameter Changes:
Law 1

Flow Rate ∝ Diameter³

Q₂ = Q₁ × (D₂/D₁)³

Where D is diameter
Law 2

Head/Pressure ∝ Diameter²

H₂ = H₁ × (D₂/D₁)²

Where D is diameter
Law 3

Power ∝ Diameter⁵

P₂ = P₁ × (D₂/D₁)⁵

Where D is diameter
Affinity Laws Guide
What are the Affinity Laws?

The Affinity Laws (also known as the Fan Laws or Pump Laws) are a set of equations that predict the performance of centrifugal pumps and fans when their speed or impeller diameter is changed.

When to Use Affinity Laws
  • Predicting performance changes when altering pump/fan speed
  • Estimating effects of impeller diameter changes
  • Energy savings calculations for variable speed drives
  • System troubleshooting and performance analysis
Limitations
  • Only valid for centrifugal pumps and fans (not positive displacement)
  • Assumes constant system characteristics
  • Assumes efficiency remains constant (though you can adjust for efficiency in this tool)
  • For large changes (>20-30%), actual performance may deviate
Practical Applications
HVAC Systems

Calculate energy savings from reducing fan speed to match reduced airflow requirements.

Water Systems

Estimate new pump performance when changing impeller diameter to match reduced flow needs.

Energy Savings

Demonstrate potential energy savings from variable speed drives instead of throttling valves/dampers.

Troubleshooting

Analyze whether performance issues might be caused by incorrect speed or impeller diameter.