Explain about 1)backward curved blades 2)forward curved blades 3)comparison of above 4)which is more advantageous and why?

Answer:  

             The shape of the blades depends on the details of the hydraulic design. A centrifugal pump operates on a principle of imparting an angular momentum to a fluid, i.e. literally the fluid must change direction as it passes through the impeller blade cascade. In other words, there is an exchange of energy, as a mechanical torque is transmitted from the motor shaft to, ultimately, the hydraulic energy, which manifests itself in building a pump pressure. Hydraulic designers refer to this as “velocity triangles”: one at the impeller blade inlet, and another at the exit. A velocity triangle has peripheral velocity (U), absolute velocity (V), and relative velocity (W), as shown below, with impeller rotating clockwise:

Figure Q8-1 “Backward-curved” blades, - most pump impeller designs

The ability of building up pressure depends directly on product U x V_theta , which means that for higher pressure the impeller OD must be larger, or the pump should rotate faster – these make velocity “U” vector longer. The relative velocity vector (W), including its magnitude and direction, must be such that the velocity triangle closes-up to produce the desired pressure and flow. For most centrifugal pumps, this relative velocity vector ends up (anti-intuitively) “backward”, against the direction of the velocity “U”. The angle between the vectors U and W is called a relative flow angle “beta”, and the blade angle is set approximately equal to that. This angle “beta” ranges between 10^O to 35^o , for most single-stage centrifugal pumps, but, at higher values of Specific Speed (NS), such as turbine pumps, it can be as high as 40^o to 50^o

Some machinery has significant space limitations, such as car hydraulic transmissions. There, it is not possible to “beef-up” velocity U by large impeller OD, and the only option is to curve the blades “forward”, to still create a large velocity V_theta , and thus build up the same pressure as was in Fig. Q8-1 above:

Figure Q8-2 Special “forward-leaning” blades, such as in a pump of a hydraulic transmission

While this allows significant size reduction, the downside is low efficiency, because the absolute flow velocity becomes too large, and would result in increased hydraulic losses for a “normal” pump. In hydraulic transmission, however, there is a pressure recovery turbine, which sits immediately after the pump. The blades of the turbine wheel are also curved in a “funny fashion”, to accommodate and match the exit velocity triangle of a pump. Thus, the turbine “picks up and recovers” the velocities produced by the pump.

As most pump impellers discharge directly into a volute, or a diffuser, without having a special recovery turbine wheel following the pump impeller, the majority of the designs have backward-leaning blades.