2.2.4 Blade efficiency
The efficiency with which the transfer of energy to the moving blades occurs is essentially dependent upon the ratio of moving blade velocity and the steam absolute velocity (U/C, = £). As one might expect, the effect of the velocity ratio on efficiency in an impulse stage is somewhat different to that in a reaction stage.
This describes a parabola and is shown in Fig 1.11. The maximum value of efficiency may be derived by differentiating the above expression with respect to U and equating to zero.
Ideal reaction-stage blade efficiency
For an ideal 50% reaction stage the velocity diagram will appear as Fig 1.11. Since the fixed and moving blades have the same shape, /3 = a and 0 = 0, AV = 2C,(cos a U). Therefore, WD = mUAV = mU(2C,cos a - U).
In a reaction stage, there is an enthalpy drop across the moving blade as well as the fixed; the available energy is therefore:
Figure 1.11 shows the diagram efficiency of a reaction stage in comparison to the impulse stage.
In reality there are a number of factors that affect blade efficiency and these will be discussed in section 2.3 of this chapter. To achieve the best efficiency for both designs, reaction turbines require twice the number of stages of an impulse turbine. This is because the high velocity ratio (U/C|) for the reaction turbine means only a small heat drop can be accommodated at each stage.