Centrifugal Fans - Output Control
Howden supplies a number of different methods to control the output (flow and pressure) of a centrifugal fan. Each has its own merits and our experienced engineers work with customers to determine the optimum solution for a particular application.
Whilst it is possible to adjust flow by varying the system resistance eg with a damper somewhere in the ductwork, this is not recommended for fans absorbing more than a few kW, as it is inefficient and could stall the fan aerodynamically.
There are therefore two main options available to the plant designer: changing the speed of the fan, or changing the angle at which the air or gas enters the fan impeller.
Unless the system has an unusual characteristic speed control has the advantage that the fan will operate at design efficiency at lower loads. Inverter speed control systems are however expensive, bulky and require to operate in a controlled atmosphere, therefore constant speed drives combined with an aerodynamic control system provide an attractive option in many situations.
Inducing spin in the gas flow entering a centrifugal fan, in the same direction as the impeller rotation, will reduce the fan’s output and power consumption. Infinitely variable control of the fan can therefore be achieved by installing a set of movable vanes, either close to the impeller inlet (inlet vane control), or in the fan inlet casing (box vane control).
- Inlet vane control - The output from the fan is varied by adjusting radial vanes fitted immediately upstream of the fan inlet. The close proximity of the vans to the inlet gives the more effective control, and higher efficiencies at lower loads.
- Box vane control - The output from the fan is varied by adjusting a louvre-type damper fitted within the fan inlet box. Compared to inlet vanes the damper does not spin the gas as effectively therefore the power consumption at lower loads is higher. The lower velocities (and therefore aerodynamic losses) across the damper blades do however result in a slightly higher efficiency at peak load, and less wear if dust is present in the gas stream.