• Steam entering from a small opening attains a very high velocity.
• The velocity attained during expansion depends on the initial and final content of the steam.
• The difference in initial and final heat content represent the heat energy to be converted to kinetic energy.
There are two types of steam turbines:
1) Impluse turbine and
2) Reaction Turbine
In this turbine there are alternate rows of moving and fixed blades. The moving blades are mounted on the shaft and fixed blades are fixed to the casing of the turbine.
A set of fixed nozzle is provided and steam is passed through these nozzles. The P.E in steam due to pressure and internal energy is converted to K.E. The steam comes out of the nozzles with very high velocity and impinges on the rotor blades.
The direction of steam flow changes without changing its pressure.
Thus due to the change in momentum the turbine rotor starts rotating.
Reaction turbine have no nozzles. These two have alternate rows of moving and fixed blades. The moving blades are mounted on shaft, while fixed blades are fixed in casing of turbine.
When high pressure steam passes through fixed blades, then steam pressure drops down and velocity of steam increases.
As steam passes over moving blades, the steam expands and imparts energy,resulting in reduction in pressure and velocity of steam.
Note: Turbines used in thermal power stations are Impuse, Reaction or combined. Generally multistage turbines are used. H.P steam after doing work in the H.P stage passes over l.P stage . more workis extracted thereby, with consequent increase in thermal efficiency.
Compounding of steam turbines:
Single stage turbines are of low efficiency.
In compounding, a number of rotors are connected or keyed to the same shaft
Two types of compounding are used: velocity compounding and pressure compounding
Governing of steam turbines:
Governing signifies the process of controlling the volume of steam to meet the load fluctuation.