What Is Steam Turbine

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What is Steam Turbine?

A steam turbine is a mechanical device that extracts thermal energy from pressurized steam and converts it into rotary motion.
Steam turbine depends directly upon dynamic action of the steam.
As per the Newton’s second law of motion, the rate of change of momentum is caused in the steam by allowing high velocity jet of steam to pass over curved plate (blade) and the steam will impart a force to the blade.
If the blade is free then it will rotate in the direction of applied force.
The steam from the boiler is expanded in the nozzle where due to fall in pressure of steam, thermal energy of steam is converted into kinetic energy of steam.
This high velocity jet of steam is impinged on the blades mounted on a shaft.
The change in flow direction of steam causes a force to be exerted on the blades and due to rotation of these blades power is developed.

Working of Steam Turbine

Steam turbines are mainly divided into two groups:

In these turbines, the steam comes out at a very high velocity through the fixed nozzle and impinges on the blades fixed on the periphery of a rotor.
The blades change the direction of the steam flow without changing its pressure.
The resulting force due to the change in momentum causes the rotation of the turbine shaft.
Figure shows the blade arrangement for impulse turbine and the variation of pressure and velocity of steam passing through the turbine.
Examples of impulse turbine are De-Laval turbine, Rateau turbine, Curties turbine.
It is important to note that, in case of impulse turbine the shape of blades in profile type.

Impulse Turbine

In these turbines, the high pressure steam from the boiler is passed through the nozzles as shown in figure.
When the steam comes out through these nozzles, the velocity of the steam increases relative to the rotating disc.
This results in reacting force of the steam on nozzle which gives rotating motion to the disc and shaft.
In these turbines steam expands both in fixed and moving blades continuously when the steam passes over them.
Hence, pressure drop occurs gradually and continuously over both moving and fixed blades. For example, Parson’s reaction turbine.
Figure shows the blade arrangement for reaction turbine and the variation of pressure and velocity of steam passing through the turbine.
It is important to note that, in case of reaction turbine the shape of blades is aerofoil.

Reaction Turbine

Steam turbines offer following advantages

Steam turbine is highly simplified in operation and construction.
The thermal efficiency of steam turbines is much higher.
It is compact and it has low weight to power ratio.
It can operate at high speeds and greater range of speeds is possible.
Due to absence of reciprocating parts, the vibrations and noise are greatly minimized.
Steam turbine can take considerable over load.
It can be designed in sizes ranging from a few kW to 1000 MW in a single unit.
In steam turbines there is no condensation loss.
Life of steam turbine is high.
Initial cost, maintenance cost and installation cost are low.

In these turbines, pressure drops only in nozzle and not in moving blades channel.
The blades are of profile shape.
Blade channel area is constant.
By using these turbines much power cannot be developed.
These turbines occupy less space for same power.
Velocity of steam is slightly higher.
Blade manufacturing is simple and less costly.
Efficiency of these turbines is low.

In these turbines, pressure drops in nozzle as well as in moving blades channel.
The blades are of aerofoil shape.
Blade channel area is varying.
Much power can be developed by using these turbines.
These turbines occupy more space for same power.
Velocity of steam is lower.
Blade manufacturing is difficult hence costly.
Efficiency of these turbines is high.

Q.1 Differentiate between impulse and reaction turbine.

Q.2 Explain the working of reaction turbine.

Q.3 Explain the principle of impulse and reaction turbine.

Q.4 State applications of impulse turbine.