The steam is used to rotate the turbine because the steam is the vapourised water that can develop very high kinetic energy required to rotate the turbine. As well as the water is an easily available fluid and never harms the environment thus steam is preferred to rotate the turbine.
But before moving toward our question let’s discuss how steam helps to rotate the turbine.
What causes the steam turbine to rotate?
The above figure-A shows the single stage of a steam turbine which consist of the ring of the nozzles and the ring of moving blades.
The high-pressure steam from the boiler is passed through the nozzles which help to convert the pressure energy into kinetic energy. In other words, the nozzles drop the pressure of the steam to increase the velocity of the steam.
This high-velocity steam from the nozzles glides onto the inner surface of the moving blades. As shown in figure B, while passing over the moving blades the steam changes its direction and creates an impulsive force on the moving blades.
As the moving blades are mounted onto the rotor, the impulsive force acting on the moving blades rotates the rotor.
Why steam is used to rotate the turbine?
Following are the reasons that say why steam is suitable to rotate the turbine:-
1] High pressure and kinetic energy:-
The steam can attain high pressure and temperature by gaining maximum energy inside the steam generator.
By expanding in the nozzles, the high-pressure energy of the steam gets converted into high kinetic energy. This high kinetic energy of the steam is passed over the ring of moving blades to rotate the rotor shaft.
2] Easy availability:-
The steam is made by evaporation of the water into the steam generator and water is the most abundant and easily available chemical in comparison with other chemicals.
Thus by supplying heat to the water, we can easily obtain high-pressure steam.
The steam turbine extracts the energy from high-pressure steam and releases low-pressure steam to the condenser.
At the condenser, the low-pressure steam undergoes condensation and condensed water is pumped again to the steam generator.
Thus the maximum portion of the exhausts steam can be reused again and again.
4] No harm to the environment:-
The use of steam never make any harmful effects on the environment.
Thus because of all the above reasons steam is used to rotate the turbine.