Pump Laws are specific applications of the Affinity Laws that govern how centrifugal pump performance changes with impeller speed and diameter, enabling prediction of flow, head, and power relationships. These laws form the foundation for variable frequency drive (VFD) calculations in HVAC chilled water, condenser water, and heating systems.
Mathematical Relationships
Pump Laws establish that gallons per minute (GPM) varies directly with speed, pressure head varies with speed squared, and brake horsepower varies with speed cubed. A pump operating at 50 percent speed requires only 12.5 percent of full-speed power while delivering 50 percent flow at 25 percent pressure head.
HVAC Pump Applications
Secondary chilled water pumps, condenser water pumps, and circulating pumps in heating systems follow Pump Laws exactly. These relationships enable engineers to select appropriately sized VFDs and predict annual energy consumption for variable-speed systems serving systems with modulating loads.
Practical Implementation
Pump curves provided by manufacturers show actual performance across speed ranges, validating Pump Law calculations. System designers use these laws to estimate energy savings from VFD installation, typically achieving 15 to 30 percent reduction in pump energy consumption compared to constant-speed operation.