Other constructional features and an — cillaries
For more detailed information refer to Chapter 16, and Figure
8.8 may be helpful.
These permit examination of the fan impeller for material build-up or erosion. They are usually positioned on the scroll so that the impeller blades may be readily seen and cleaned. If positioned at a low level any dust may be easily removed.
Doors may occasionally, and additionally, be positioned on the volute sides to permit the shroud and/or backplate of the impeller also to be viewed and cleaned.
Figure 8.8 Constructional features and ancillaries for centrifugal fans
The inspection door usually consists of a steel plate positioned over a rectangular or circular hole in the casing. If positioned on the scroll, it must of course be rolled to match. Quick release fitting are not recommended — rather the door should be held by bolts and nuts, requiring a spanner to be used. Too easy a removal could be dangerous when the fan is running. The rotating impeller will be in close proximity and will be highly dangerous. It may even be advisable to have an electric interlock with the power supply, such that when the door is removed, the fan cannot run.
Where a fan is handling air contaminated with liquids or vapours, it is recommended that a drain point is positioned at the lowest point of the scroll. This may be screwed to accept piping or fitted with a closing plug.
A non-ferrous rubbing ring is attached to the inlet cone or Venturi, where the cone is adjacent to the eye of the impeller, and contact could take place, see Figure 8.9. A non-ferrous shaft washer is also necessary. These will minimise the possibility of incendiary sparks being produced. Such features are essential where explosive or inflammable gases or vapours are bing handled. The material pairings are especially important and are detailed in prEN14986.
The ATEX Directive 94/9/EC of the European Union came into force at the end of June 2003. This placed obligations on both users and manufacturers of equipment, such as fans, which could be the cause of explosions. As a result CEN (Commitйe Europйen Normalisation) was mandated to produce prEN 14986. Not only does this give detailed recommendations on the spark minimising features, it also details other requirements concerning bearing selection, vee belt drives, clearances, material stresses, etc.
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