METHODS OF CONTROLLING FANS

There are various ways of varying fan performance, all of which have advantages and disadvantages. The main alternatives are set out below.

Variable Pitch Blades

 Axial fans only. Efficient and power conscious, but can be mechanically unreliable. There is nothing to prevent fan wind-milling in any induced draft which can cause serious problems on starting since the fan wind-mills 'back-wards'.

Two speed drive

Power conscious, but fixed speeds can mean that optimum matching of output cannot be achieved. Again, no protection against wind-milling.

Variable Speed Drive

Most energy efficient, but highest capital cost. Not as reliable as might be expected. Any slight fault tends to result in the fan stopping. No protection against wind-milling.

Inlet Vane Control

Controls by producing a forced vortex on the fan inlet in the same sense as the rotation of the fan and has as similar effect to reducing fan speed. Works best in conjunction with Centrifugal and Mixed Flow fans. As good as variable speed at near full volume. Low capital cost. Very reliable and robust. Excellent for modulating control.

Inlet Vane Control in conjunction with with 2 speed drive

Extends IVC efficiency to lower volumes. It is particularly useful for High Fire/Low Fire type applications.

Inlet Box dampers

Louvre type inlet dampers. These deflect the incoming gas to one side of the inlet box and produce a swirl by conservation of angular momentum as the gas is drawn into the fan inlet. The power savings are approximately half those achievable with an IVC and the characteristic is not as good.

Discharge Dampers

Slightly cheaper than IVCs but carry a high penalty in terms of power consumption at anything other than full volume. Poor control characteristic. With blades 10% open, flow can be as high as 40%. Little change in volume over the last 30% of actuator stroke.