One chart holds everything about this fan: every airflow it can deliver, at what
pressure, at what speed, for how much power. Build it up one layer at a time.
Airflow
—CFM
Static pressure
—in. w.g.
Fan speed
—RPM
Shaft power
—BHP
Static efficiency
—%
Motor load (10 hp)
—%
⚠ in the surge zone⚠ motor overloaded
Layers
Controls
Resistance ↑ = dampers closing, filters loading, doors shutting.
Resistance ↓ = a more open system than calculated.
Scenarios
Try a selection
Schedule says: this CFM at this static. Can this fan do it, and at what speed?
The fan curve — hover, or focus and use ←/→
What each layer is telling you
Fan curve — every (airflow, pressure) pair this fan can produce at one speed. Not a prediction: a menu.
System curve — what your ductwork demands: pressure grows with the square of flow (P = C·Q²). Change the system, change the parabola.
Operating point — the only point on both lines. The fan must run here. You don’t pick it directly; you pick the two curves.
Speed family — the same fan at other RPMs (that’s the fan laws, demo 3). A belt-drive sheave change or a VFD moves you between these.
Surge zone — left of each curve’s peak the airflow separates from the blades: pulsation, noise, broken welds. Never select here.
Efficiency rails — parabolas through the origin along which efficiency stays constant. Speed changes slide the operating point along a rail — which is why VFDs are so effective.
Power curve — shaft BHP on the right-hand axis, exactly how manufacturer cut sheets print it. Same x-axis, different units: always check which axis a curve belongs to.