Fan stall is an unstable aerodynamic condition that occurs when a fan operates at too little airflow for the pressure it is attempting to produce. Under these conditions, airflow separates from the blade surfaces, reducing the fan’s ability to generate pressure and move air efficiently. Stall can produce noise, vibration, pressure fluctuations, poor airflow, and accelerated mechanical wear.

The condition is most commonly associated with axial fans, backward-inclined centrifugal fans, and other fans that have an unstable portion of their performance characteristic. However, any fan can experience unstable operation if the fan and system are improperly matched.

What Causes Fan Stall?

A fan operates at the intersection of its fan performance curve and the system resistance curve. The operating point changes whenever fan speed, system resistance, airflow demand, or air density changes.

Stall generally develops when the operating point moves toward the low flow, high pressure region of the fan curve. Typical causes include:

• Excessive system resistance caused by closed dampers, blocked filters, undersized ductwork, fouled coils, or obstructions.
• Improper fan selection or an oversized fan operating far from its intended duty point.
• Inlet or outlet ductwork that creates turbulence, swirl, or an uneven velocity profile.
• Elbows, transitions, dampers, or other fittings installed too close to the fan.
• Incorrect blade pitch, damaged blades, excessive tip clearance, or fouling on the impeller.
• Operating a variable speed fan at an unsuitable speed.
• Changes to the system after commissioning, such as added filters, duct modifications, or increased back pressure.
• Operating conditions that vary beyond the stable range for which the fan was selected.

The U.S. Department of Energy and AMCA describe stall as occurring when insufficient air moves across the fan blades and flow separates from the blade surfaces. As separation develops, the aerodynamic force on the blades changes, further disturbing the airflow and potentially creating a cascading effect through adjacent blade passages.

Blade flow separation

Under normal conditions, air follows the blade surface as it passes through the fan. At low flow, the relative angle at which air approaches the blade becomes increasingly unfavorable. This increases the blade angle of attack, until the boundary layer can no longer remain attached.

Once separation occurs, part of the blade is covered by recirculating flow. The blade then produces less useful pressure, and the fan may experience a sudden reduction in airflow and efficiency.

In axial fans, stall may begin near the blade tip or shroud. Backflow can move upstream toward the fan inlet and form localized regions of separated flow that travel around the periphery of the fan. Research on axial fans has associated these flow structures with pressure fluctuations, noise, vibration, and system instability.

Where Is Stall Found on a Fan Curve?

The stall region is typically located toward the low volume, high pressure side of the fan performance curve. For many fans, this is to the left of the peak pressure point, although the exact location depends on the fan design and the way performance is measured.

A stable fan curve generally has a negative slope: as airflow increases, the pressure developed by the fan decreases. In an unstable region, the curve may flatten or develop a positive slope. Small changes in airflow or pressure can then cause large changes in the operating point.

The system and fan curves may intersect more than once, or their slopes may become nearly parallel. The fan can then “hunt” between operating points rather than settle at a steady condition. The DOE describes this cyclic behavior as a searching action that may sound like breathing and that promotes poor efficiency and increased component wear.

Stall Versus Surge

The terms stall and surge are often used together, but they describe related phenomena.

• Stall is primarily an aerodynamic flow separation condition occurring within the fan or blade passages.
• Surge is a larger scale oscillation involving substantial fluctuations or reversals in airflow and pressure.

A fan can experience stall without a complete system surge. However, stall can contribute to surge when the fan, ductwork, and system resistance interact unfavorably.

Problems Caused by Operating in the Stall Region

Operating continuously or repeatedly in the stall region can create several problems.

Reduced airflow and pressure

The fan may fail to deliver its required airflow even though it is running at the correct speed. Pressure readings can fluctuate, and the actual operating point may be difficult to determine from a single measurement.

For an HVAC system, the result may be inadequate ventilation, insufficient cooling or heating airflow, poor space pressurization, or inadequate exhaust. In an industrial process, low airflow can reduce production capacity or cause overheating of associated equipment.

Lower efficiency and higher operating cost

Separated flow does not transfer energy from the impeller to the airstream effectively. The fan therefore consumes power without producing the expected useful airflow and pressure. The fan may also need to operate at a higher speed to compensate for poor installed performance.

Excessive noise

Stall commonly produces a rough, pulsating, or rumbling sound rather than the relatively steady broadband noise expected from a properly operating fan. The sound may vary periodically as rotating stall cells pass through the fan.

Noise is an important warning sign, but it should not be used as the only diagnostic method. Duct resonance, bearing problems, imbalance, loose components, and system turbulence can produce similar symptoms.

Vibration and cyclic loading

Separated flow creates unsteady aerodynamic forces on the blades, shaft, bearings, housing, and support structure. Rotating stall can produce pressure pulsations and cyclic loads that are especially damaging when they coincide with a mechanical natural frequency.

Over time, this may lead to:

• Bearing fatigue.
• Shaft or coupling problems.
• Impeller cracking.
• Loosened fasteners and supports.
• Fatigue damage to fan housings and ductwork.
• Premature failure of vibration isolators.

Motor and drive problems

Stall does not always correspond to a predictable motor load. Depending on the fan type and operating point, the motor may experience fluctuating torque, current, or power. Repeated operation near the unstable region can cause motor heating, nuisance overload trips, or difficulty maintaining a stable control response.

The motor should not be assumed to be protected simply because the measured airflow is low. The fan’s actual brake horsepower curve must be checked at the operating condition.

Unreliable controls

A pressure or airflow control loop may repeatedly open and close dampers or change fan speed as it attempts to correct unstable measurements. This can create hunting, control oscillation, and unnecessary actuator movement.

In systems with multiple fans, one fan operating near stall can also disturb the pressure and flow distribution seen by the other fans.

Fan stall two-part series

 Part 2 Fan Stall: How to Avoid It (Publishing October 5)