Axial flow fans

Axial flow fans have developed rapidly since the Second World War due to the creation of a range of high strength aluminium alloys. These permit running at the rotational speeds necessary to produce worthwhile pressure. Axial fans adhere closely to classical theory and require less “know-how” than centrifugal fans. They may be placed in three general classifications ac­cording to how the flow is constrained:

• Ducted fan where the air has to flow through a duct thus en­couraging it to enter and leave the impeller in an almost ax­ial direction.

• Diaphragm or ring mounted fan where the air is trans­ferred from one relatively large air space to another.

• Circulator fan where the impeller rotates freely in an unre­stricted space. Examples are pedestal or ceiling fans.

Performance at 8°, 16°, 24° and 32s pitch angle settings Figure 1.79 Tube axial fans — typical characteristic curves

Figure 1.80 Vane axial fan (DSGV — downstream guide vanes)

The various components possible in a ducted axial flow fan are shown in Figure 1.77. Not all the elements are present in a par­ticular fan and the terminology for the various types is as fol­lows:

Figure 1.77 Components of a ducted axial flow fan Tube axial fan The tube axial fan is a fan without guide vanes and comprising only the impeller and casing. Fairings up and downstream of the impeller may be fitted. Such fans are usually selected for pressures up to about 750 Pa. (See Figures 1.78 and 1.79). Blades may have adjustable pitch at rest to cater for varying flowrates.

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Short Cosing Figure 1.78 Examples of tube axial fans

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