Predict the new airflow, static pressure and power when fan speed, impeller diameter or air density changes — or find the speed needed for a target airflow — with the fan affinity laws.
The fan laws hold for the same fan on the same system curve (no damper or filter change). Check the new point against the manufacturer's fan curve, maximum safe speed (class limit) and motor rating.
| Quantity | Speed change | Diameter change | Density change |
|---|---|---|---|
| Airflow Q | ∝ N | ∝ D³ | no change |
| Pressure P | ∝ N² | ∝ D² | ∝ ρ |
| Power W | ∝ N³ | ∝ D⁵ | ∝ ρ |
The laws describe one fan moving along one system curve: change the speed and the operating point slides along that curve, with airflow, pressure and power each changing by a fixed power of the speed ratio.
Power rises with the cube of speed. Increasing speed by 10% to gain 10% more air needs 1.1³ = 1.33 times the power — a third more. Many balancing jobs fail here: the airflow target is reached but the motor runs overloaded. The tool flags the new power against the motor rating.
The cube law works in reverse too. Slowing a fan to 80% speed cuts power to 0.8³ = 51% — roughly half the energy for 80% of the air. This is why variable-frequency drives on fans pay back so quickly, and why VAV systems are efficient at part load.
A fan delivers 10,000 CFM at 2.0 in. w.g. and 5.0 HP at 1,000 RPM. The pulley is changed to run at 1,100 RPM.
For the same fan and system: airflow is proportional to speed, pressure to speed squared, and power to speed cubed. Airflow also scales with diameter cubed, and pressure and power scale with air density.
1.2³ = 1.73 times the original power — 73% more — and 1.44 times the static pressure.
Divide the target airflow by the measured airflow and multiply by the current speed: N2 = N1 × Q2 ÷ Q1. Then check the new power against the motor rating.
No — a fan moves the same volume of air at a given speed. Hot or thin air is less dense, so the fan develops less pressure and draws less power in proportion to the density.
Yes. The same affinity laws apply to centrifugal pumps: flow ∝ speed, head ∝ speed², power ∝ speed³.
Found a number that looks wrong, or a case the tool does not cover? Say so here — corrections are welcome and get answered.
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