Power Plant Engineering

Fig. 6.10. Impulse Reaction Turbine.
In this type of turbine, the fixed blades which are set in a reversed manner compared to the
moving blades, corresponds to nozzles mentioned in connection with the impulse turbine. Due to the
row of fixed blades at the entrance, instead of the nozzles, steam is admitted for the whole circumfer-
ence and hence there is all-round or complete admission. In passing through the first row of fixed
blades, the steam undergoes a small drop in pressure and hence its velocity somewhat increases. After
this it then enters the first row of moving blades and just as in the impulse turbine, it suffers a change in
direction and therefore in momentum. This momentum gives rise to an impulse on the blades.
But in this type of turbine, the passage of the moving blades is so designed (converging) that
there is a small drop in pressure of steam in the moving blades which results in a increase in kinetic
energy of steam. This kinetic energy gives rise to reaction in the direction opposite to that of added
velocity. Thus, the gross propelling force or driving force is the vector sum of impulse and reaction
forces. Commonly, this type of turbine is called Reaction Turbine. It is obvious from the Fig. 6.10 that
there is a gradual drop in pressure in both moving blades and fixed blades.

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POWER PLANT ENGINEERING
As the pressure falls, the specific volume increases and hence in practice, the height of blades is
increased in steps i.e. say upto 4 stages it remains constant, then it increases and remains constant for the
next two stages.
In this type of turbine, the steam velocities are comparatively moderate and its maximum value is
about equal to blade velocity. In general practice, to reduce the number of stages, the steam velocity is
arranged greater than the blade velocity. In this case the leaving loss is about 1 So 2 per cent of the total
initial available energy. This type of turbine is used mostly in all power plants where it is great success.
An example of this type of turbine is the Parsons-Reaction Turbine. The power plants 30 MW and above
are all impulse-reaction type.