The fan affinity laws describe the mathematical relationships between fan speed, airflow, pressure, and power. These relationships are fundamental to fan selection, energy analysis, and VFD sizing.
The three affinity laws state: (1) Flow varies directly with speed ratio. (2) Pressure varies with the square of the speed ratio. (3) Power varies with the cube of the speed ratio. These relationships hold for geometrically similar fans operating at the same point on their performance curve.
The cubic relationship between speed and power is the basis for VFD energy savings. Reducing fan speed to 80% of rated reduces power to 51% (0.8³ = 0.512). This is why even modest speed reductions yield significant energy savings.
The affinity laws also apply when changing fan size. A geometrically similar fan scaled up by 20% in diameter will deliver 44% more flow at the same speed (1.2³ = 1.728 for power, 1.2 for flow).
Limitations apply: the affinity laws assume constant system resistance and are most accurate near the best efficiency point. They become less accurate at low speeds where Reynolds number effects and mechanical losses become significant.
Aerotech's engineering team uses affinity law calculations as the starting point for all fan selection and retrofit analyses, combined with actual performance curve data to account for real-world deviations.