How to Read an Axial Fan Curve: Operating Point, Stall and System Resistance

A fan curve shows the pressure a fan can develop at each airflow, at a fixed speed and air density. A system curve shows the pressure your ducts, louvres, filters, and fittings need at each airflow. The fan runs where the two curves cross, and that single point decides the real airflow you get. Reading the curves correctly is how you avoid the most common axial-fan failures: too little flow, unstable running, and wasted energy.

The Two Curves and the Operating Point

Fan curve. It plots pressure against flow. At zero flow (a fully blocked system) the fan develops its shut-off pressure. As flow rises, pressure generally falls. Axial fan curves are distinctive because they often show a dip in the low-flow region, which we return to below.

System curve. For most ducted systems, resistance rises with the square of flow. Doubling the flow roughly quadruples the pressure needed. Fixed elements, such as a static head, shift the curve upward at zero flow.

Operating point. Where the curves intersect. Everything else follows from it: airflow, pressure, power, noise, and efficiency.

An Illustrative Example

The figures below come from an illustrative axial fan curve, not from any NextAir product. They show how the operating point moves when the system changes.

Flow (m³/s)	0	10	20	30	40	50	60
Fan total pressure (Pa)	780	700	820	780	650	470	240

Note the dip between 10 and 20 m³/s: pressure falls to 700 Pa at 10 m³/s and then rises again. That is the stall region.

System condition	Resistance factor	Operating flow	Operating pressure
Design system	1.0	44.8 m³/s	563 Pa
Filters loaded, resistance up 50%	1.5	39.5 m³/s	656 Pa
Damper closed further, resistance up 150%	2.5	32.6 m³/s	746 Pa

Two lessons appear. First, a 50% increase in resistance cut flow by about 12% here, not 50%. Fans are forgiving in flow terms but the pressure and power move too. Second, the third case is already approaching the region where the curve turns unstable.

What Stall Is and Why It Matters

Stall happens when the airflow across the blades separates from the blade surface, so the blades stop generating pressure efficiently. On an axial fan curve it shows as a dip, and operation on or near it brings pulsation, low-frequency noise, vibration, and reduced bearing and blade life.

In the example above, if the system resistance rose to roughly seven times its design value, the intersection would fall to about 20 m³/s, inside the unstable region. That sounds extreme, but partly closed dampers, blocked filters, and collapsed flexible ducts can produce large increases. Transient events, such as a door slamming or another fan starting, can push the operating point briefly into the dip. Mine engineers treat stall as a routine design constraint for this reason.

Good practice: – select the operating point to the right of the stall region, with margin, – check the worst-case resistance, not only the design value, – check start-up, when resistance and speed pass through transient values, – where pressure spikes are possible, consider anti-stall arrangements or a fan with a steeper, stall-free characteristic.

Pressure Basis: Static, Velocity, and Total

Fan total pressure is the sum of static pressure and velocity pressure. Manufacturers may publish curves on either basis. If a fan discharges freely into a large space, its velocity pressure is lost, so a comparison on total pressure can flatter the fan. Compare fans on the same basis, and state the basis in every specification.

Shaft Power and Efficiency on the Curve

A complete fan chart also shows shaft power and efficiency against flow. Efficiency peaks at a best-efficiency region. Running far to the left or right of it wastes energy and increases noise. Power on axial fans often rises toward lower flow, so a fan sized for a high-flow point may draw more power than expected if the system is throttled. Size the motor for the highest power the fan can draw at the worst-case point, not just the design point.

Parallel and Series Operation

Two fans in parallel add flows at the same pressure. Two identical fans in a system carry half the flow each at the operating pressure. Parallel operation needs matched fans and curves that do not interfere: if one fan’s curve has a stall dip and the other does not, the two can hunt against each other. Fans in series add pressures at the same flow, which is how multi-stage arrangements achieve higher pressure. Both arrangements should be checked on the combined curve.

How Density Changes the Curve

Fan curves are usually rated at standard air density, about 1.2 kg/m³. At altitude or high temperature, air is thinner, so at the same speed the fan develops proportionally less pressure and absorbs less power. At around 2,400 m, density is about three-quarters of standard. Read the curve using density-corrected pressure. See fan laws explained for the correction.

Reading a Test-Based Curve

Ask whether the curve comes from testing to a recognised standard such as ISO 5801 or AMCA 210, and whether it is for the actual installation arrangement, with free or ducted inlet and outlet. A curve from a different arrangement can differ noticeably. Our guide to fan performance testing explains the arrangements.

Checklist for Reading Any Fan Curve

1. Confirm speed, air density, and pressure basis.
2. Plot the system curve at design and at worst-case resistance.
3. Find the operating points and read flow, pressure, power, and efficiency.
4. Check the margin from the stall region.
5. Check the motor against the highest power point.
6. Repeat for start-up and for any parallel or series arrangement.

Have Us Check Your Operating Point

Send your airflow, system resistance, and site conditions, and we’ll plot the operating point against a fan curve and confirm the stall margin. Contact us for a free quote.

Frequently Asked Questions