Blower Curves, System Curves and Operating Point Drift

A blower curve shows the pressure a blower can develop at each airflow, at a fixed speed and gas density. A system curve shows the pressure your ducts, filters, burners, and equipment need at each airflow. The blower runs where the two curves cross, and that point sets the real airflow you get. Because system resistance changes over time, that point moves, and understanding how it moves is the difference between a system that holds its duty and one that slowly drifts out of specification.

The Two Curves and the Operating Point

Blower curve. Pressure plotted against flow. At zero flow, the blower produces its shut-off pressure, then pressure typically rises to a peak at moderate flow and falls as flow increases further.

System curve. For most ducted systems, resistance rises with the square of flow. Double the flow, and the resistance rises about fourfold. Fixed elements, such as a static head, lift the curve.

Operating point. Where the curves cross. It sets flow, pressure, power, and efficiency, and everything else follows from it.

An Illustrative Example

The figures below are from an illustrative backward-curved blower curve, not from a NextAir product. They show how the operating point moves.

Flow (m³/s)	0	5	10	15	20	25	30
Total pressure (Pa)	3,900	4,200	4,300	4,100	3,600	2,900	2,000

System condition	Operating flow	Operating pressure
Design system	22.1 m³/s	3,312 Pa (338 mm WC)
Filters loaded, resistance up 50%	19.0 m³/s	3,698 Pa (377 mm WC)
Duct opened up, resistance down 50%	27.2 m³/s	2,510 Pa (256 mm WC)

Two lessons appear. First, a 50% rise in resistance cuts flow by about 14%, not 50%, and raises pressure. The system still works, but airflow has drifted from the design value, and if the process needs steady flow, that matters. Second, a 50% fall in resistance raises flow by about 23%, which pushes power up on a forward-curved wheel and, on a backward-curved wheel, stays within the motor’s rating because power falls beyond its peak.

Filter Loading Is the Classic Drift

A dust collector’s filter resistance rises as it loads and falls after each cleaning cycle. Over hours or days, the operating point swings between the clean-filter and loaded-filter positions. A blower with a steep curve holds airflow steadier as resistance rises, while a blower with a flat curve loses more flow. Where flow must stay constant, hold it with speed control, and size the blower for the loaded condition. See dust collection and filtration.

Staying on the Right Side of the Peak

A blower’s curve rises to a peak and then falls. Operating to the right of the peak is stable, and operating far to the left, at high resistance and low flow, can be unstable in some designs. Select the operating point on the falling part of the curve, comfortably right of the peak, and check the worst-case resistance, including any partly closed damper. Single-stage backward-curved blowers are known for stable, no-surge airflow, which makes them forgiving, but the check is still worth making.

Power and Efficiency on the Curve

A full blower chart also shows shaft power and efficiency against flow. Efficiency peaks in a region near the design point, and running far from it wastes energy and adds noise. On a backward-curved blower, power peaks and then falls, so the motor is protected if resistance drops. On a forward-curved blower, power keeps rising with flow, so the motor must be sized for the highest flow the blower can reach. Read the power curve, not just the pressure curve, before choosing the motor.

Parallel and Series Operation

Two blowers in parallel add their flows at the same pressure. They should be matched and their combined curve checked, because unlike curves can interfere. Two blowers in series add their pressures at the same flow, which is how some multistage arrangements achieve higher pressure. Both arrangements need isolating dampers so an idle blower does not let air flow backward through it.

Curves Are Rated at Standard Air

Published curves assume standard air density. In hot gas or at altitude, the blower develops less pressure at the same speed, so the curve moves down. Correct the curve, or convert the duty to a standard-air rating, before reading the operating point. See gas temperature, altitude and density correction.

Reading a Curve from a Test

Ask whether the curve comes from a test to a recognised standard such as ISO 5801 or AMCA 210, whether it is for the installation arrangement you will use, and at what density and speed it was corrected. See blower performance testing.

Checklist for Reading Any Blower Curve

1. Confirm speed, gas density, and pressure basis.
2. Plot the system curve at design and worst-case resistance.
3. Read flow, pressure, power, and efficiency at each operating point.
4. Check the operating point is right of the peak, and near best efficiency.
5. Check the motor at the highest power the blower can draw.
6. Repeat for start-up and for any parallel or series arrangement.

Where Our Range Fits

NextAir Systems manufactures centrifugal blowers up to 800 HP (about 597 kW) and up to 500 mm WC (about 4.9 kPa) static pressure, with backward-curved and forward-curved wheels. Send your system resistance at clean and worst-case conditions, and we will plot the operating point against a blower curve.

Have Us Check Your Operating Point

Send your system resistance and gas conditions, and we’ll plot the operating point and confirm the margin. Contact us for a free quote.

Frequently Asked Questions