8.1 Fan Types, Performance Curves, and Rotation

Fan Types, Curves, Rotation, and Multiple-Fan Systems

Centrifugal and axial fans

Centrifugal fans receive air near the wheel axis and discharge it radially into a housing or plenum. Common wheel families include forward-curved, backward-inclined or backward-curved, airfoil, and radial designs. Their stable range, efficiency, sound, pressure capability, and power curve differ, so identify the installed model and use its manufacturer curve.

Axial fans move air generally parallel to the shaft. Propeller, tubeaxial, and vaneaxial arrangements differ in casing and guide-vane construction. Do not assign universal pressure or efficiency ranges from the category name alone. Blade pitch, hub ratio, speed, inlet condition, and system effect materially change performance.

Record wheel type, diameter, speed, rotation arrow, drive arrangement, motor data, inlet and discharge configuration, and the exact curve revision. A curve for a similar wheel or nominal size is not evidence for the installed fan.

Fan and system curves

A fan curve shows pressure, flow, power, and often efficiency at a stated speed and density. The system curve represents the pressure required to move air through the connected path; for a fixed turbulent system it is commonly approximated as pressure proportional to flow squared. The intersection is the operating point.

Opening a path reduces resistance and moves the point toward higher flow; closing a damper raises resistance and moves toward lower flow. The actual fan power response depends on the wheel's published power curve. Avoid blanket statements that every forward-curved fan always overloads or every backward-inclined fan is safe at every point. Check the curve and motor limit.

Poor inlet geometry, swirl, abrupt transitions, missing straight connection, obstructed inlet area, and nonuniform discharge can cause system effect, so a correctly sized fan may not reproduce catalog performance in the installed arrangement.

Rotation verification

A centrifugal fan rotating backward can still discharge air in the normal duct direction while producing much less useful flow and pressure. The exact loss and current behavior are fan- and system-dependent. Low flow, low pressure, or low current can suggest reverse rotation, but they do not prove it.

Verify the shaft or wheel direction against the equipment arrow using an authorized safe method. Observe from a safe location without removing guards or approaching rotating parts. Any three-phase lead change is performed only by qualified authorized electrical personnel after de-energization, lockout/tagout, and absence-of-voltage verification. Repeat rotation, speed, pressure, airflow, and current after correction.

Series and parallel operation

Two fans in series primarily add pressure at a common flow; two fans in parallel primarily add flow at a common pressure. The combined curve—not simple arithmetic at arbitrary points—must be evaluated against the system curve. Parallel fans can share unequally because of different speeds, inlet conditions, dampers, or curves. A stopped fan can become a leakage or recirculation path unless isolation functions correctly.

For supply and return fans in the same air system, the fans are not necessarily a simple series pair because outdoor, relief, exhaust, and building leakage paths lie between their boundaries. Test the required control mode and reconcile each defined airstream.

Field test sequence

1. verify identity, readiness, guards, rotation, and speed;
2. establish the specified damper, filter, coil, terminal, and control mode;
3. measure fan inlet and outlet pressure at defined locations;
4. measure total airflow with a suitable boundary method;
5. record motor voltage and phase currents when authorized;
6. plot or compare the operating point with the correct curve; and
7. investigate system effect, leakage, restrictions, or control mismatch before proposing speed changes.

Diagnostic reasoning

Low flow with high pressure may indicate excess system resistance, but verify the pressure boundary and curve. Low flow with low pressure may indicate low speed, reverse rotation, an open bypass, an incorrect wheel, or a measurement problem. High flow with high power may reflect low system resistance or excess speed. One symptom does not establish one cause.

Report the observed values and conditions, not a categorical diagnosis unsupported by a direct check. Any speed, sheave, pitch, or control adjustment requires authorization and confirmation of fan, motor, drive, vibration, pressure, and power limits.

Curve identification checklist

Before plotting a field point, verify manufacturer, series, arrangement, wheel diameter, blade or pitch configuration, speed, air-density basis, and inlet condition. Check whether pressure on the curve is fan static or fan total pressure and whether power includes drive loss. Record any field feature—such as an inlet box, plenum, screen, elbow, or discharge transition—that differs from the catalog test setup.

A field point that appears off the curve should prompt a data check before a conclusion. Confirm simultaneous flow and pressure, tachometer setup, gauge references, duct area, density, and that multiple fans or bypass paths are in the expected state. The curve comparison is strongest when all quantities describe the same stable instant and system boundary.