A system curve is a graph representing the static pressure required by ductwork and components at varying airflow rates, created by plotting resistance against volume flow. System curves combine all friction losses, fittings, equipment, and terminal resistance into a single pressure-flow relationship. This curve is essential for matching fans to actual installation requirements.
Curve Development
System static pressure increases with the square of airflow velocity according to the equation SP = K × (CFM)². Friction losses in ductwork, filters, coils, and dampers accumulate to create the total curve shape. A clean filter might add 0.2 inches of water column pressure drop, while a dirty filter can add 0.8 inches. Engineers must account for all components including terminal dampers, VAV boxes, and future fouling factors.
Operating Point Selection
The intersection of fan curve and system curve identifies the operating point where airflow and pressure balance naturally. Improper system design places operating points in low-efficiency regions, wasting 15 to 30 percent of fan energy. VFD control enables dynamic curve matching by adjusting fan speed as system resistance changes, maintaining efficient operation across varying loads and conditions.