High efficiency fans

When the power absorbed is measured in hundreds of kilo­watts, every effort is made to achieve high efficiency. Among

High efficiency fans

Figure 1.90 Typical high pressure axial fan performance curves

Features distinguishing such designs from the general-purpose types are:

A) Hub diameters of 50% or more to improve the aerody­namic balance of the design from blade root to tip.

B) Blade form designed specifically for the required duty. When die forming is not justified, this entails increased la­bour to provide a good surface finish.

C) Aerofoil-section guide vanes, again designed specifically for the required duty.

D) Careful streamlining of the annulus passage, and fairing of bearing supports or other obstructions. This may entail moving the driving motor right out of the casing, introduc­ing the necessary transmission elements to the impeller.

E) Space for a long tail fairing following the impeller hub and guide vanes to maximize fan total pressure by conversion of annulus velocity pressure.

F) Space for a long gradually expanding diffuser to minimize outlet velocity pressure, and maximize fan static pressure.

These measures may raise the peak fan total efficiency to 90%, compared with 80% for a good general-purpose model at opti­mum duty. Figure 1.91 show the constructional arrangement and Figure 1.92 shows typical performance curves.

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Figure 1.91 High efficiency axial fan — construction

High efficiency fans

Figure 1.92 High efficiency fan — typical performance curves 30 FANS & VENTILATION

These are available in very large sizes for volumetric flowrates from 50 m3/s upwards at fan static pressure from 100 to 200 Pa. As an example they may be applied singly, discharging from the top of evaporative cooling towers, or in multiple, circulating air across extensive banks of heat exchange tubes.

Hubs are small and the blades long and few in number — three, four or six. Blades were at one time made of timber, but are now of hollow glass-reinforced polyester or similar. Mouldings or hollow aerofoil sections from steel or aluminium sheet are more usual. Guide vanes are unnecessary. (See Figure 1.93.)

High efficiency fans

Figure 1.93 Low-pressure axial fans

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