Fan selection

This Chapter is concerned with the general choice of fan and ancillaries and is aimed primarily at the user. It must however, be emphasised that the choice should be finalised after detailed discussions with a reputable supplier. In the following Sections, the fan is chosen on the basis of the desired flowrate, fan pressure, air/gas temperature, humidity and density. Where solids are handled such as wood waste, textile remnants or dust particles, then these also can affect the fan selection.

Whilst centrifugal fans dominate the world market, they are not always the best choice. There are many applications where axial flow fans can be a valid alternative, especially when space is at a premium. Outside the pressure and noise limitations of the axial, the mixed flow fan should also be considered.

The correct fan selection may be complicated by the attitudes of some manufacturers. It is as well to remember that not all of them have a complete range in their armouries. Discussions with several companies may be necessary before becoming aware of what exactly is available to suit a particular application.

Fan choice is further complicated by the widely varying abilities of fan salesmen! Some companies are diligent in the training of their representatives — others are not. It is often necessary to by-pass the salesman to have sensible direct discussions with an applicational specialist.

Globalisation of fan companies has not necessarily improved the service to prospective customers. Competition between factories within a group is often just as intense as that between companies — especially where long term survival is at stake. Corporate finance arrangements may not reward the salesman or the factory for selling a fan made elsewhere in their “empire”. Background information, as given in the Classification guide to manufacturers and suppliers, Chapter 24, assists in this respect.

Of ever increasing importance is the total Life Cycle Cost of the fan and its drive. In an age where global warming is seen by some as a threat to the future of mankind, it is no longer ethical to purchase on first cost considerations alone. We must consider the energy costs over the fan lifetime together with routine and breakdown maintenance, spares costs and disposal costs.

Finally, to keep the fan manufacturers “on-side” a section is devoted to the mathematics of fan selection. With the advent of computers, curve methods may be unknown to the younger generation of engineers, but can still be of value in indicating exactly where a fan is operating on its characteristic, and can also show possible alternative selections.

Posted in Fans Ventilation A Practical Guide