Fan Affinity Laws Calculator
Calculate new fan performance when changing speed or impeller diameter using ASHRAE affinity laws.
Fan Affinity Laws
Law 1 (Flow): CFM2 = CFM1 x (N2/N1) x (D2/D1)^3
Law 2 (Pressure): SP2 = SP1 x (N2/N1)^2 x (D2/D1)^2
Law 3 (Power): HP2 = HP1 x (N2/N1)^3 x (D2/D1)^5
Speed Change Effects
| Speed Change | Flow | Pressure | Power |
|---|---|---|---|
| +10% | +10% | +21% | +33% |
| +20% | +20% | +44% | +73% |
| -10% | -10% | -19% | -27% |
| -20% | -20% | -36% | -49% |
Important Notes
Affinity laws assume constant system efficiency and are most accurate for speed changes less than 30%.
Power increases with the CUBE of speed - small speed increases cause large power increases!