STEAM TURBINE

STEAM TURBINE

These are turbines or we can say them as prime movers which are capable of converting  steam of high temperature and high pressure as input into useful work i.e. mechanical work as output. The steam is directed on the blade of turbine through nozzle, where there is large pressure drop and it attains high velocity at the exit of nozzle. As the high temperature and pressurized steam strikes the blades of the turbine, it losses momentum. This lost momentum of steam is utilized in production of torque ( as the blades rotates), which has verity of mechanical requirements.

Impulse type Steam turbine

In such type of turbines, pressure drop occurs in nozzle instead in between the blades of turbine. In other words, as the steam passes the blades of turbine, the pressure remains constant. Suppose P0 be the pressure at inlet of nozzle and V0 is velocity at inlet undergoing expansion. After expansion the pressure is P1 and velocity is V1. This final pressure P1 remains same as it moves over the blade of turbine.
We can use law of momentum conservation. Considering the pressure drop occurring at the inlet and exit of blades instead of entry and exit of nozzle. We can write as 
Momentum absorbed by wheel = Momentum of steam at inlet - Momentum of steam at exit.
Also mean speed of turbine blade can be given as:-

                        Vb =3.14*Dm*N/60

Where Dm is wheel diameter, N is speed in RPM.

Diagram efficiency:-

Diagram efficiency, commonly known as blade efficiency is the ratio of amount of work done on the blade to amount of energy input to the blade.

                                      Work done on blade of turbine
Diagram efficiency = ------------------------------------------------
                                  Energy provided to blades of turbine  

Compounding of steam turbine

There are two types of compounding done in steam turbine. These are

1) Pressure compounding
2) Velocity compounding

CONCEPT BEHIND COMPOUNDING - Why it is done

In order to reduce the excessive high velocity at the exit of nozzle we do compounding. If the velocity of steam will be extremely high, there will be frictional losses and high centrifugal stresses which will reduce the efficiency of steam turbine. To avoid this, arrangement is made in such a way that one row of nozzle is followed by one row of blades. This is called staging of turbine.

Velocity of steam at exit of single stage turbine is given as

                      V1 = 44.72*sq root (h0-h1)

If the turbine has single stage then steam is allowed to expand from boiler to condenser in a single row. In this case the velocity of steam at exit is very large. De  laval turbine is example of single stage turbine having high rotational speed.

PRESSURE COMPOUNDING

It involves keeping number of impulse stages in series. No pressure drop occurs across the blades of turbine rather pressure drop occurs only across inlet and exit of nozzle. The pressure drop results in gain in kinetic energy. This kinetic energy is absorbed by blades of turbine to perform mechanical work. In such compounding, total enthalpy drop is divided equally among the number of stages.

VELOCITY COMPOUNDING

The kinetic energy of steam coming out of nozzle is used to move the blade  of rotor resulting in decreased velocity of steam. The guide blade or fixed blade directs this steam to another set of turbine blades which is further used to develop shaft output.
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