Backward curved vanes vs. forward curved vanes in pumps 

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 tangent of impeller at vane location and tangent to vane. For A less than 90^o (forward curved vanes) it's unstable because of unrestricted power growth. Large losses result from high outflow velocity. The preferred configuration is achieved when A is more than 90^o (which is backward curved vanes) since it has controlled power consumption and represents good fluid dynamic shape.