Fan laws: what a change in speed does to flow, pressure, power and bearing load
Three simple laws and one efficiency formula tell you how a fan behaves when speed changes, and why bearings feel it before anything else.
The three laws
For one fan in one system, a change in speed n changes everything else in a fixed way. Flow goes up in proportion to speed. Pressure goes up with the square of speed. Power goes up with the cube of speed.
What it means in numbers
| Speed change | Flow | Pressure | Power | Unbalance force |
|---|---|---|---|---|
| +10% | +10% | +21% | +33% | +21% |
| +20% | +20% | +44% | +73% | +44% |
| -10% | -10% | -19% | -27% | -19% |
| -20% | -20% | -36% | -49% | -36% |
Why bearings care
- Thrust is roughly the pressure across the fan times the inlet area, so it follows the pressure: a 10% speed rise adds about 21% thrust.
- The force from a given unbalance grows with the square of speed too, so the same imbalance loads the bearings 21% harder after a 10% speed rise.
- Motor power rises with the cube of speed, so a small speed increase can overload the motor and the coupling or belt as well.
Estimating fan efficiency
Efficiency is the useful air power divided by the power into the motor: total pressure in pascals times flow in cubic metres per second, divided by 1,000 times the power in kilowatts. A fan that draws far more power than its pressure and flow explain may have an impeller problem or leakage, and unusually high power with the same flow loads the bearings through extra belt pull or coupling torque.
Fan noise follows speed
Sound power changes with speed by about 60 times the base-10 logarithm of the speed ratio. A 10% speed rise adds about 2.5 dB. Noise that rises faster than that, or changes tone, is worth a vibration check.
Using this
- Before raising fan speed, recalculate the bearing loads at the new speed. The Worked templates in the Load Calculator take the pressure and speed directly.
- When a fan is slowed down with a variable-frequency drive, check bearing grease and cooling, because less airflow over the motor and bearings means more heat at low speed.