Match a fan to a duct system — the system resistance curve from friction and component losses, its crossover with the fan curve that sets the true operating point, the best-efficiency window, and the fan laws for speed and density changes.
≈ — CFM.
Static pressure drop (Pa) of each element at the design flow. They sum to the total, and the whole curve scales with Q².
Parabolic fan curve P = P₀(1 − (Q/Q_max)²). Use catalog shutoff & free-air points if known.
The system curve (blue parabola) passes through the origin — air ducts have no static lift, so resistance is pure Q². The fan curve (brass) falls from shutoff. Their crossing is the duty point; the shaded band is the ±15% best-efficiency window you want it to land in.
System curve. Unlike a liquid pump, a duct has no static lift — the resistance is pure friction and component loss, all of which scale with velocity² ∝ Q². So the system curve is a parabola through the origin: ΔP = K·Q², with K fixed by summing every element's loss at the design flow and back-solving K = ΔP_design / Q_design².
Operating point. The fan supplies pressure along its own curve; the duct demands it along the parabola. The intersection — solved numerically — is the only flow where they balance, so it's what you actually get. Good selection puts this point in the fan's high-efficiency region.
Best-efficiency window. Centrifugal fans peak around 50–60% of free-air flow. Operating within roughly ±15% of the BEP keeps efficiency high and avoids surge (too far left, unstable) or overload and noise (too far right). The shaded band marks it.
Fan laws. For the same system: flow Q ∝ N, pressure ΔP ∝ N², power ∝ N³. That cube on power is why trimming fan speed a little saves a lot of energy — the core case for a VFD. The tool reports the exact speed to land the duty flow on your design target.
Density. A fan develops the same pressure rise in pascals regardless of air density, but mass flow and power both scale with ρ. At altitude or high temperature the same fan moves the same volume at lower mass flow and draws less power — critical for hot-gas and high-altitude duty.
Volumetric, single-fan analysis. Fans in series/parallel, system-effect factors at inlet/outlet, sound power, and motor sizing margins are separate considerations.