What is the difference in function between backward curved vanes and forward curved vanes in pumps?

The power of a pump is related to discharge as follows:

Power=K₁Q + [K₂Q²]/tan A

where k₁ and k₂ are constants, Q is discharge and A is the angle between the tangent of impeller at vane location and the tangent to vane.

For A less than 90^o (forward curved vanes) it is unstable owing to unrestricted power growth. Large losses result from high outflow velocity. The preferred configuration is achieved when A is more than 90^o (i.e. backward curved vanes) because it has controlled power consumption and presents good fluid dynamic shape.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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