Backward aerofoil blades
The impeller is shown in Figure 1.75. The blades produce lift forces, which counteract inter-blade circulation without requiring precise angles. Thus smooth flow conditions are maintained over a considerable portion of the characteristic.
Figure 1.75 Backward aerofoil bladed impeller
Pressure losses in the impeller are thus reduced, as are those in the casing volute. Fan static efficiencies up to 88% have been achieved and total efficiencies of 91 % are possible. An efficiency of at least 80% can be achieved over 40% of the volume flowrate at a given speed. It will be appreciated that at low flows the blades are stalled, resulting in a discontinuity in the pressure curve, which is not always acknowledged. (Figure 1.76).
Aerofoil should be used on low dust burdens, since particles penetrating the hollow welded blades can produce imbalance. Similar problems can arise with free moisture. Although precautions can be taken, such as solid nosing bars for dust or foam filling for moisture, the backward inclined is preferred for these applications, (see Section 1.5.8). Erosion of the blade noses will in any case reduce the efficiency. High temperatures may require “pressure relief for the air trapped within the blades.
Whenever operating costs are of paramount importance, as when large powers are involved and where there is continuous operation at high load factor, the aerofoil is to be preferred. In general the advantages are not significant for fans below size 1000 mm. Aerofoils may also be necessary when increased duty is required from existing power lines: in many cases the power saved may allow a smaller motor to be installed so that the overall cost is the same. In other cases the additional fan price may be recovered in energy cost differences long before expiry of the period allowed for amortizing plant costs.
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