Theory: The steam turbine is the most versatile prime mover, which is capable of almost endless applications. A machine, which originates mechanical motion by using some natural force, is called prime mover. Steam turbine is a practical power source and is built in as small size as 5 KW to as large as 1300 MW. The steam turbine offers many advantages over other prime movers.The major advantage of the steam turbine over, a reciprocating steam engine is that in case of turbine the steam can be expanded and after the expansion the steam pressure is reduced to a very lower value as compared to the inlet pressure of the turbine, thereby it creates a larger enthalpy drop.
As the internal efficiency of the steam turbine is very high, so it has an ability to convert a high proportion of this relatively large enthalpy drop into mechanical work. Another advantage of the turbine is that the exhaust steam is not contaminated with oil vapors so it can be condensed in the condenser and fed back to the boiler without passing through filters.
Energy Conversion in Steam Turbine
A simple steam turbine basically consists of two major components or we can say sets of components and Those are :
- Nozzle
- Curved Blades/Rotating Blades
Nozzle:
The nozzle is attached to the casing of the turbine. The steam enters the nozzle at a high pressure and a relatively low velocity. Due to nozzle action the steam velocity increases at the cost of pressure and temperature. At the nozzle, there is a drop in pressure as well as temperature and the steam expands in the nozzle.
Curved Blades/ Rotating Blades:
The blades are attached to the turbine rotor. The rapidly moving particles of steam issuing from the nozzle enters the blades. As the blades are curved, the direction of motion of these particles of steam is changed. This causes the change of momentum of passing steam due to which resultant force in tangential to the rotor periphery is set-up. The summation of this force acting on all the blades constitutes the driving force of the turbine. Here the steam glides on and off the curved blades instead of striking them perpendicularly.Finally, when the steam comes out of the blades, the pressure and temperature of the steam are reduced. There is a drop of the enthalpy at the exhaust of the turbine due to expansion of steam inside the turbine.
The processes of expansion and direction changing may occur once (single stage) as in the simple impulse turbine, or a number of times (multi-stage) as in the pressure compounded impulse or the impulse-reaction turbine.
