Fan Affinity Laws describe how centrifugal fan performance changes with impeller speed and diameter, enabling accurate prediction of cubic feet per minute (CFM), static pressure, and power consumption in HVAC applications. These laws apply identically to the mathematical relationships governing pump performance.
Core Fan Relationships
Fan Affinity Laws state: CFM is proportional to speed (CFM2/CFM1 = N2/N1); static pressure is proportional to speed squared (SP2/SP1 = (N2/N1)²); and brake horsepower is proportional to speed cubed (BHP2/BHP1 = (N2/N1)³). These relationships remain accurate across normal operating ranges of fan curves.
HVAC System Applications
Cooling tower fans, air handler supply and return fans, and exhaust fans all follow Fan Affinity Laws. A variable frequency drive reducing fan speed to 50 percent reduces CFM by 50 percent, static pressure by 75 percent, and power consumption by 87.5 percent, delivering substantial energy savings on variable-load systems.
Design Value
Fan Affinity Laws enable engineers to model seasonal performance variations, demand-controlled ventilation (DCV), and terminal box control scenarios. Accurate application ensures proper VFD selection and realistic energy calculations for LEED or energy code compliance documentation.