Fan Affinity Calculator — 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.
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.
Exemplo Resolvido
A fan at 1200 RPM moves 10,000 CFM at 2.0" w.g. using 5 HP. Speed increased to 1500 RPM.
- Speed ratio = 1500 / 1200 = 1.25
- Q2 = 10000 × 1.25 = 12,500 CFM
- SP2 = 2.0 × 1.25² = 2.0 × 1.5625 = 3.13 in. w.g.
- BHP2 = 5 × 1.25³ = 5 × 1.9531 = 9.77 HP