Steam turbine? |
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A steam turbine extracts the energy of pressurized superheated steam as mechanical movement.
It has completely replaced the reciprocating piston steam engine (invented by Thomas Newcomen and greatly improved by James Watt) primarily because of its greater thermal efficiency and higher power to weight ratio. Also, because the turbine generates rotary motion, it is particularly suited to be used to drive an electrical generator--it doesn't require a linkage mechanism to convert reciprocating to rotary motion. The steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency to the use of multiple stages in the expansion of the steam (as opposed to the one stage in the Watt engine), which results in a closer approach to the ideal reversible process.
Principle of working
In a steam turbine nozzles apply pressurized supersonic steam to a set of curved blades mounted on a rotor. Each blade whips the steam back in the opposite direction, simultaneously allowing the steam to expand a little. A stationary blade then redirects the steam towards the next set of blades toward the exhaust end with the gap between acting like a nozzle. The process repeats in successive stages until the steam is exhausted at nearly atmospheric pressure . The moving blades are mounted radially on the rotor, while the stationary blades are mounted to the case of the turbine. Turbines always consist of a number of stages, with each stage being carefully optimised for the pressure and volume of steam that it contacts. Because high pressure (several hundred psi) steam exhausted through a nozzle into the air travels so fast, the turbine, in order for it to be efficient, must rotate very fast. This requires that the rotor and its blades be well balanced to protect it against vibrations, and creates difficulties with the seals around the rotor. The centrifugal force experienced by the blade is so strong that it must be carefully designed and made out of the strongest available materials to prevent it from failing catastrophically.
History and present
The first steam engine was little more than a toy, the classic Aeolipile made by Heron of Alexandria. The modern steam turbine was invented by an Irishman, Charles A. Parsons, in 1884 whose first model was connected to a dynamo that generated 7.5 kW of electricity, His patent was licensed and the turbine scaled up shortly after by an American, George Westinghouse. A number of other variations of turbines have been developed that work effectively with steam. The de Laval turbine (invented by Gustaf de Laval) accelerated the steam to full speed before running it against a turbine blade. This was good, because the turbine is simpler, less expensive and does not need to be pressure-proof. It can operate with any pressure of steam. It is also, however, considerably less efficient. The Parsons turbine also turned out to be relatively easy to scale up. Within Parson's lifetime the generating capacity of a unit was scaled up by about 10,000 times.
Problems with turbines are now rare and maintenance requirements are relatively small. Any unbalance of the rotor can lead to vibration, which in extreme cases can lead to a blade letting go and punching straight through the casing. If water gets into the steam and is blasted onto the blades rapid erosion of the blades can occur, possibly leading to unbalance and failure. The control of a turbine with a governor is essential, as turbines need to be run up slowly, to prevent damage. Uncontrolled acceleration of the turbine rotor can lead to an overspeed trip, closing the steam inlet valves and shutting off the fuel supply. If this fails then the turbine may continue accelerating until it breaks apart, often spectacularly. Turbines are expensive to make, requiring precision manufacture and special quality materials. This purchase cost is offset by much lower fuel and maintenance requirements and the small size of a turbine when compared to a reciprocating engine having an equivalent power.
Electrical power stations use large steam turbines driving turbo-generators to produce most of the world's electricity. These centralised stations are of two types: fossil fuel power plants and nuclear power plants. Another use of steam turbines are in ships, where their small size is an advantage. Steam turbine locomotives were also tested, but with limited success.