Fan Affinity Calculator

Calculate new fan performance (airflow, pressure, power) when speed changes using the fan laws.

CFM
in. w.g.
HP
RPM
RPM

New Airflow (Q2)

12,500 CFM

New Static Pressure (SP2)3.13 in. w.g.
New Brake Horsepower (BHP2)9.77 HP

New Airflow (Q2) vs Original Airflow (Q1)

Vedi formula →

Formula

## Fan Laws (Affinity Laws for Fans) The fan laws are identical in form to the pump affinity laws. They predict how fan performance changes with rotational speed. ### Fan Laws for Speed Change **Q2/Q1 = N2/N1** (airflow scales linearly) **SP2/SP1 = (N2/N1)²** (pressure scales with speed squared) **BHP2/BHP1 = (N2/N1)³** (power scales with speed cubed) A small 10% speed reduction cuts power by 27%, making variable-speed fans highly efficient for part-load operation.

Esempio Risolto

A fan at 1200 RPM moves 10,000 CFM at 2.0" w.g. using 5 HP. Speed increased to 1500 RPM.

  1. 01Speed ratio = 1500 / 1200 = 1.25
  2. 02Q2 = 10000 × 1.25 = 12,500 CFM
  3. 03SP2 = 2.0 × 1.25² = 2.0 × 1.5625 = 3.13 in. w.g.
  4. 04BHP2 = 5 × 1.25³ = 5 × 1.9531 = 9.77 HP