Stalling in Fans

What is stall?

Separation of air flow from one or more fan blades. stalling is a major setbacks in axial fans.

Why does it occur?

In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases.This occurs when the critical angle of attack of the foil is exceeded. The critical angle of attack is typically about 15 degrees, but it may vary significantly depending on the fluid, foil, and Reynolds number.

Air is a mysterious substance. It is difficult to pin point the exact reason for stalling in a specific case of a Fan, but will occur in well designed air flow system only due to following reasons;

1. Turbulent flow

       

    Imagine a fan's blade that on one rotation cuts a part of air and pushes it forward. Now the gap created between fan blade back and non-pushed air can cause flow separation. But this occurs in a very high speed fans like Airplane propellers. 

    Now imagine ID fans(Induced Draft) where inlet turbulence is high then there is a good chance of flow separation at blades as shown in the picture.

2. Unexpected system Resistance

      

    Now when in equilibrium state, suppose an unexpected resistance occurs in air flow path, e.g. sudden closure of inlet duct damper or mechanical blockage. This causes the same effect as explained in a turbulent flow.

3. Pressure spikes

    

    In case of FD fan (Forced Draft) and ID fan (Induced Draft fan) working together balancing the furnace pressure in boiler. Suppose due to some furnace pressure deviations high enough to cause a temporary resistance in the flow will cause the same effect on the fan as explained in turbulent flow.

When or where it occurs in Power plants?

In case of  fans stalling occurs when unequal loading in a set fan occurs or change over form one  fan to another is in progress.

What is the effect?

Stalling may affect fan's loading, causing nonuniform load sharing.
Banging noise.