Backward curved blades
These impellers are shown in Figure 1.72 and are preferred for certain applications where there may be disadvantages in the use of the backward inclined type. Due to the curvature, the blade angle at inlet can be made steeper for a given outlet angle. This generally enables shock losses to be kept low, whilst the curvature itself develops a certain degree of lift. It is therefore possible to arrange such fans with a pressure curve continually rising to zero flow.
They can be extremely stable, with none of the “bumps" in their curves found with other types, and most suitable for operation
Figure 1.72 Backward curved bladed impeller
In parallel on multi-fan plants. With the special blade curvatures now used, efficiencies exceed 82% static, approaching those attained by aerofoil bladed fans.
The steeper inlet angle also results in a stronger blade, which can rotate at higherspeeds. This is offset to a large extent, however, by the need to run at higher speeds for a given duty as compared with the backward inclined type. They are also more expensive as, unless complex press tools are used to “stretch” the metal, the blades cannot be flanged for rivetting or spot welding and have to be arc welded in position.
The curvature of backward curved blades (concave on the underside of the blades) is inclined to encourage the build-up of dust. As the impeller in its rotation tends to develop a positive pressure on the working convex face of the blade and negative effect on the underside, dust can lodge within the camber. This becomes more pronounced on the narrowest fans where the
Camber is substantial and the chord is very much shorter than the developed blade length. The wider units have less curvature, although the effects are offset by the shallow outlet angles.
Inlet volume flow m3/s Figure 1.76 Backward aerofoil fan — typical characteristic curves
Generally backward curved impellers are not so suitable for high temperature operation, as differential expansion between blades and shrouds can be severe inducing additional stresses. Gas temperatures should therefore be limited to 350°C. Other advantages are the same as those of the backward include type, including a relatively steep pressure characteristic and non-overloading power curve. (See Figure 1.73).
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