What a Fan Performance Curve Plots

A fan performance curve is a graph of pressure on the vertical axis against volumetric flow on the horizontal axis, drawn for one fan speed. Pressure may be shown as static pressure or total pressure depending on the manufacturer; always confirm which one before comparing curves. Total pressure equals static pressure plus velocity pressure, so the two curves differ, and mixing them leads to an undersized or oversized selection.

The curve slopes downward from left to right. At zero flow the fan produces its highest pressure — shut-off, a condition no fan should run in for long. At the far right the curve reaches free delivery, where flow is maximum and useful pressure is zero. Neither end is a working condition. The useful range lies between them, and the shape of the slope tells you how the fan behaves when system resistance changes. A flat curve delivers large flow changes for small pressure changes; a steep curve is more stable against varying resistance.

Finding the Fan Operating Point

The fan operating point is the intersection of the fan curve with the system curve. The system curve represents the resistance of the actual installation: duct friction plus any static head, rising roughly with the square of flow. The fan does not decide its own flow — the system does. Once the two curves cross, flow and pressure are fixed.

This is why a catalogue list of a single duty point is not enough. If a damper is closed, a filter loads up, or a branch duct is added, the system curve shifts and the operating point slides along the fan curve. Ask for the full curve, then check the expected range of system resistance, not only the design condition.

Reading a Multi-Curve Selection Chart

Most manufacturers publish a family of curves: one fan size at several speeds, or several impeller diameters at one speed. Selection procedure is straightforward:

  • Plot the required flow and pressure.
  • Find the curve that passes through or near that point.
  • Confirm the resulting speed matches the available motor, drive and permissible tip speed.
  • Check the power curve and the efficiency curve at that same point.

The Stall Region on the Left

Toward the left of the curve, at low flow and high pressure, the blades can stall. Air separates from the blade surface, pressure becomes unsteady, and the fan produces pulsation, extra noise and vibration, with a risk of mechanical damage. Some curves show this as a dip or saddle; on others it is not plotted at all, which does not mean the region is safe. Keep the fan operating point to the right of stall with a working margin, and never select a fan so that its normal duty sits near the peak of the curve.

Efficiency Islands and Best Duty

Efficiency is usually plotted as islands or contours on the same sheet, sharing the flow axis. The innermost island is the region of highest efficiency for that fan. A duty point inside it means lower shaft power, lower running cost and less heat and noise for the same airflow. A point just outside may still work, but the penalty shows up in power consumption over years of operation. Also check the power curve at both ends of the expected flow range — a fan selected at its design point can overload its motor if system resistance falls and flow increases.

What SINDFAN Publishes

SINDFAN publishes fan curves based on AMCA 210 / ISO 5801 testing. Rotors are balanced to ISO 1940-1 G4.0. The axial fan range covers Ø250–3150 mm and includes a contra-rotating mining fan developing approximately 3400 Pa, together with jet fans rated at thrust from 152 N to 1691 N. These curves let you verify the operating point against your own system calculation rather than a single quoted figure.

Why It Matters

Most fan problems reported in the field — shortfalls in airflow, high noise, vibration, motor overload — trace back to a selection made from one duty point without checking the curve. Reading the curve takes a few minutes and prevents a costly correction after installation.

Send your duty — required flow, pressure, gas temperature and installation details — to SINDFAN on WhatsApp +852 9524 8434 or by email at info@sind-tech.com. We will confirm a selection against the published curve and the expected range of system resistance.