Fan laws are mathematical relationships describing how fan performance changes with speed, diameter, and air density, expressed through four fundamental proportionality equations. These laws enable engineers to predict performance changes without detailed testing. Understanding fan laws is critical for system design, troubleshooting, and retrofit applications.
The Four Laws
Fan volume flow varies directly with speed (CFM1/CFM2 = N1/N2), pressure varies with speed squared (SP1/SP2 = (N1/N2)²), power varies with speed cubed (BHP1/BHP2 = (N1/N2)³), and diameter changes scale all parameters proportionally. A 10 percent speed increase boosts airflow by 10 percent, pressure by 21 percent, and power by 33 percent. These relationships hold across all fan types: centrifugal, axial, and mixed-flow designs.
Practical Applications
Fan laws enable quick capacity adjustments through variable frequency drives (VFD). Reducing speed by 20 percent decreases power consumption by 51 percent and reduces noise by 6 to 8 decibels. Engineers use fan laws for equipment selection, energy modeling, and equipment replacement planning to ensure proper system balance without excessive throttling or oversizing.