Let us define, Pressure Coefficient,
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(40.9) |
Volume Coefficient,
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(40.10) |
and Power Coefficient,
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(40.11) |
Substitution of (40.8) in the above yields
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(40.12) |
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(40.14) |
Equations (40.13) and (40.14) are plotted in Fig 40.1 for different values of volume coefficient  with  as a parameter.
Figure 40.1 Performance curves of a Fan |
The characteristics in Figure 40.1 depict the following
(i) Forward curved fans  develop the highest pressure for a given impeller diameter and speed.
(ii) Power requirement of a forward curved fan increases steeply for a small change in flow rate.
(iii) Pressure developed decreases fast with increasing flow rate in a backward curved fan
In conclusion, the forward curved fans have large volume discharge and pressure rise but they demand higher power. However, forward curved fans are unstable for off-design operating conditions.
Backward curved fans are very efficient and the drooping power characteristic makes them suitable for a better off-design performance
Radial curved fans are preferred for dust-laden fluids. Due to their shape, the solid particles are not stuck and deposited on the blade surface.
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