A steam turbine is a prime mover which converts heat energy into mechanical energy. In a conventional steam turbines cycle, water is used as the working fluid. The water is heated in a boiler by burning fuel. It evaporates into steam which is expanded in a turbine where mechanical power is generated. The steam generated is of high temperature and high pressure. The temperature is often in the 450 to 540 degrees centigrade range. The pressure ranges between 60 and 120 bar.
The essential parts of all steam turbines are similar, consisting of nozzles through which steam flows and expands (dropping in temperature and gaining kinetic energy) and the blades against which the swiftly moving steam exerts pressure. The blades are mounted on rotor drum, and an outer casing confines the steam to the turbine.
Both temperature and pressure fall as the steam passes through the turbine. The greater the pressure drop, the more energy can be captured from the steam. The more efficient power plants condense the steam back to water at the end of the turbine.
The theoretical maximum efficiency of a steam turbine- based power plant is determined by the difference between the temperature at which steam enters the high pressure turbine and the temperature at which it exits the low pressure turbine. The greater the temperature difference, the more energy can be extracted.
Steam turbines are finding greater use in process industries (like steel and chemicals) producing large quantities of waste heat. The waste heat produced can be used to generate steam as well as power. The capital cost of such plants can be slightly higher but the generation of power represents a useful by-product when the waste must be burnt in any case.
Steam turbines can also be deployed advantageously in industries with greater requirements of both steam and power. They are used in cogeneration or combined heat and power applications where process steam is also used in the turbine to generate electricity. This also results in substantial improvements in overall process efficiency.
TOWARDS HIGHER EFFICIENCY :
More efforts are being made to improve the efficiency of steam turbines. The areas are
1. Super critical technology advances aiming for 50 percent efficiency. 2. Renovating and Upgrading for more value for money.
3. Combined Heat and Power for low cost, more flexibility.
4. Steam turbines in Combined Cycle, a new market
5. Clean coal technologies FBC, PFBC, IGCC etc to improve the overall efficiency and to reduce the pollution level.
COMPOUNDING OF STEAM TURBINES :
Steam jet does maximum work with good economy when the blade speed is just half the steam speed. Due to very large rate of expansion, the steam leaves the nozzle at a very high velocity ( Supersonic, since the pressure ratio exceeds the critical pressure ratio and the nozzle thus used is Converging – Diverging ) of about 1000 m/sec. Even though the rotor diameters are kept fairly small the rotational speed of 30000 rpm may be obtained. Such high speeds can be used to drive the machines only with a large reduction gearing arrangement. In actual De-Laval turbine the velocity of steam leaving the blade is also quite appreciable resulting in energy loss. This amount to as high as 10-12 percent of the steam.
One of the chief object in the development of steam turbines is to reduce the high rotational speed of the rotor to practical limits. Several methods are used to reduce this high rotor speed by absorbing the steam pressure or the steam velocity in stages as it flows over the rotor blades. This is known as Compounding.
TYPES OF COMPOUNDING :
1. Velocity Compounding ( Curtis principle )
2. Pressure Compounding
3. Mixed Compounding
IMPULSE STEAM TURBINE:-
In impulse steam turbine, the overall transformation of heat into mechanical work is accomplished in two distinct steps. The available...