Power Plant Engineering
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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. 206 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. Download 3.45 Mb. Do'stlaringiz bilan baham: |
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