Forward curved blades
These impellers first became popular at the end of the 19th Century and almost superseded all other types. A diagrammatic representation of the impeller is shown in Figure 1.60. They are considerably smaller for a given duty than all other designs.
Flowrate can be as high as 2.5 times that of the same size of backward-bladed fan. This is now seen to be not necessarily an advantage since casing losses, which are a function of velocity, will therefore be about six times a great. Thus even with an impeller total efficiency approaching the theoretical optimum of about 92%, the overall fan total efficiency would still be down to about 75%.
Such fans are now only used where space is at a premium, as they will be the most compact. Due to their smaller size they are usually cheaper, although the differences are much reduced with the greater possibility for automated manufacture of backward bladed fans. Nevertheless the scope for improvement has been appreciated and current designs achieve static efficiencies of 63% and total efficiencies of 71 % at even lower speeds.
It will be noted that the performance curve has discontinuities due to stall and/or recirculation (see Figure 1.61). A large margin over the absorbed power is necessary where the system resistance cannot be accurately determined, or where it is subject to variation, to take account of the rising power characteristic.
Figure 1.61 Forward curved fan — typical characteristic curves
The impeller has a large number of shallow blades in widths from 0.25 to 0.5D and runs at lowertip speed for the duty. Structural considerations have in the past limited the pressure development to about 1 kPa, but the narrower widths are now suitable for pressures up to 14 kPa.
Apart from low-pressure ventilation requirements, these fans are widely used for mechanical draught on shell-type boilers, oil burners, furnace recirculation etc.
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