Free Fan Affinity Laws Calculator
Calculate new fan performance (airflow, pressure, power) when speed changes using the fan laws.
New Airflow (Q2)
12,500 CFM
New Airflow (Q2) vs Original Airflow (Q1)
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.
Example Calculation
A fan at 1200 RPM moves 10,000 CFM at 2.0" w.g. using 5 HP. Speed increased to 1500 RPM.
- 01Speed ratio = 1500 / 1200 = 1.25
- 02Q2 = 10000 × 1.25 = 12,500 CFM
- 03SP2 = 2.0 × 1.25² = 2.0 × 1.5625 = 3.13 in. w.g.
- 04BHP2 = 5 × 1.25³ = 5 × 1.9531 = 9.77 HP
Frequently Asked Questions
Learn More
Beam Stress Calculation Guide: From Theory to Practice
Learn how to calculate beam stress step by step. Covers bending stress, shear stress, the flexure formula, stress distributions, and practical design checks for structural beams.