A fan curve is a manufacturer-provided graph plotting the relationship between airflow (CFM) and static pressure (inches of water column) at a specific fan speed and installation. Fan curves are fundamental to system design, showing performance across the full operating range. Each curve represents a single speed and installation type (ducted inlet, free inlet, etc.).
Curve Characteristics
Most fan curves show pressure rising from zero CFM (shutoff head) to maximum flow at zero pressure (free delivery). Efficiency typically peaks at 60 to 80 percent of maximum flow. Fan curves include multiple speed lines for variable-speed fans, with pressure scaling by speed squared and flow scaling linearly. Standard ratings follow AMCA (Air Movement and Control Association) or ASHRAE standards ensuring consistency across manufacturers.
System Integration
Engineers overlay system curves on fan curves to identify operating points where airflow and pressure balance. Mismatched curves cause excessive noise, vibration, and energy waste. Modern software calculates performance across entire curves, enabling VFD selection and troubleshooting. Reading curves correctly prevents oversizing and undersizing, ensuring equipment operates within designed efficiency ranges of 70 to 85 percent.