The Affinity Laws are mathematical relationships governing how pump and fan performance changes with speed and impeller diameter, fundamental to predicting system behavior in HVAC applications. These laws quantify changes in flow rate, pressure, and power consumption when motor speed or impeller geometry varies.
Core Relationships
The three primary laws state: flow rate is proportional to speed (Q2/Q1 = N2/N1); head or pressure is proportional to speed squared (H2/H1 = (N2/N1)²); and power is proportional to speed cubed (P2/P1 = (N2/N1)³). These relationships apply to all centrifugal pumps and fans at similar operating points on their curves.
HVAC System Applications
Affinity Laws enable engineers to predict energy consumption when implementing variable frequency drives (VFDs) on chilled water pumps, condenser water pumps, and cooling tower fans. A 50 percent speed reduction (0.5 ratio) reduces power consumption to 12.5 percent of full-speed power, delivering significant energy savings.
Design Importance
Accurate application of Affinity Laws prevents oversizing or undersizing equipment. These laws apply when operating within the normal range of pump or fan curves but become inaccurate near runout conditions or at very low speeds where cavitation or surge may occur.