As an aerodynamic phenomenon, stall will happen if a fan runs outside the scope of the performance range and flow separation occurs around the blade.  

Stall Condition

When the airflow impacts on the fan blades, an angle forms between the airflow and blades and it is called “the angle of attack”. For an axial fan, if the angle of attack exceeds a set value, the airflow that separates from the surface is thrown by the centrifugal force towards the rim of blades. It causes the pressure at the tip to accumulate and the pressure increases until it can be released at the gap between the tip and the housing. In this case, the fan operates in an unstable manner, the vibration starts, and the oscillation of airflow begins. If the fan size is too large or the system resistance is on the abrupt rise, the risk of stall will increase.

The fan characteristic has a point and stall happens beyond the point. When linking these points, you can have a stall curve. When integrated into the plant control system (DCS), the curve can act as a warning for the operator so that the stalling condition can be predicted and corrected to stay away from the danger region. The fan manufacturer shall offer stall line data with the machine.        

Stall of the axial fan can cause calamitous failure of the equipment and endanger the safety of the operator. Long-term operation of the fan in stall conditions can cause the blade to break and has to cost a lot of money to replace the fan components, and the equipment can experience considerable downtime.

Stall of the fan has another negative effect that will reduce the efficiency of the fan. Smaller fans have lower cost, lower operating costs, and may be more durable than larger ones.

Tips On How to Avoid Stall

1. Do not choose to have too large a fan. Some people think that choosing a larger fan will have more safety margins in the event of a calculation error. However, this causes the fan to not operate in the highest efficiency region of the performance curve, causing stall and surge. And the fan is too big to be cost-effective (there is a waste of horsepower). If the system calculates an error, a fan of higher class that can be sped up is also much better.

2. For the axial fans with adjustable blade angles, stall can be avoided by reducing the pitch and raising the speed.

3. Don’t run fans too long under a seriously throttled condition.