Turbine.
Publié le 11/05/2013
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steam turbine has supplanted the reciprocating engine as a prime mover in large electricity-generating plants and is also used as a means of jet propulsion.
Steam turbines are used in the generation of nuclear power and in nuclear ship propulsion.
They operate with fuel-fired boilers for power generation.
In cogenerationapplications requiring both process heat (heat used in an industrial process) and electricity, steam is raised at high pressure in the boiler and extracted from the turbineat the pressure and temperature required by the process.
Steam turbines may be used in combined cycles with a steam generator which recovers heat that wouldotherwise be lost.
Industrial units are used to drive machines, pumps, compressors, and electrical generators.
Ratings range from a few horsepower to more than 1300Mw.
The steam turbine was not invented by any one individual but was the result of work by a number of inventors in the latter part of the 19th century.
Notablecontributors to the development of the turbine were the British inventor Charles Algernon Parsons and the Swedish inventor Carl Gustaf Patrik de Laval.
Parsons wasresponsible for the so-called principle of staging, whereby steam was permitted to expand in a number of stages, performing useful work at each stage.
De Laval wasthe first to design suitable jets and blades for the efficient use of the expanding steam.
The action of the steam turbine is based on the thermodynamic principle that when a vapor is allowed to expand, its temperature drops, and its internal energy isthereby decreased.
This reduction in internal energy is transformed into mechanical energy in the form of an acceleration of the particles of the vapor ( see Thermodynamics).
This transformation makes a large amount of work energy directly available.
In the case of expanding steam, a reduction of 100 Btu in internalenergy through expansion can result in increasing the speed of the steam particles to a rate of almost 2900 km/h (almost 1800 mph).
At such speeds the energyavailable is great, even though the particles are extremely light.
Although they are built according to two different principles, the essential parts of all steam turbines are similar.
They consist of nozzles or jets through which steamflows and expands, dropping in temperature, and gaining kinetic energy, and blades against which the swiftly moving steam exerts pressure.
The arrangement of jetsand blades, whether fixed or stationary, depends upon the type of turbine.
In addition to these two basic components, turbines are equipped with wheels or drumsupon which the blades are mounted, a shaft for these wheels or drums, an outer casing that confines the steam to the area of the turbine proper, and various pieces ofauxiliary equipment, including lubrication devices and governors.
The simplest form of steam turbine is the so-called impulse turbine, in which the turbine jets are fixed in place on the inside of the turbine casing, and the blades are seton the rims of revolving wheels mounted on a central shaft.
Steam passing through a fixed nozzle passes over the curved blades; these absorb some of the kineticenergy of the expanded steam, turning the wheel and shaft on which they are mounted.
The turbine is designed so that steam entering at one end of the turbineexpands through a succession of nozzles until it has lost most of its internal energy.
In the reaction turbine, mechanical energy is obtained to some degree by the impact of steam upon the blades, but primarily it is obtained by the acceleration of thesteam as it expands.
A turbine of this type consists of a set of fixed and a set of movable blades.
The blades are arranged so that each pair acts as a nozzle throughwhich the steam expands as its passes.
The blades of a reaction turbine are usually mounted on a drum and not on a wheel.
This drum acts as the shaft of the turbine.
In order to use the energy available in steam efficiently in a turbine of either type, it is necessary to employ a number of stages, in each of which a small amount ofthermal energy is converted to kinetic energy.
If the entire conversion of energy took place instead in a single expansion stage, the rotative speed of the turbine wheelwould be excessive.
In general, reaction turbines require more stages than impulse turbines.
It can be shown that for the same diameter and energy range, a reactionturbine requires twice the number of stages for peak stage efficiency.
Large turbines that are nominally of the impulse variety employ some reaction at the root of thesteam path to assure efficient flow through the buckets.
Many turbines that are nominally reactive have an impulse control stage first, which allows for a saving in thetotal number of stages.
Because of the increase in volume as the steam expands through the various stages of a turbine, the size of the openings through which the steam passes mustincrease from stage to stage.
In the practical engineering design of turbines, this increase is accomplished by lengthening the blades from stage to stage and byincreasing the diameter of the drum or wheel upon which the blades are mounted and by adding two or more turbine sections in parallel.
As a result, a small industrialturbine may be more or less conical in shape, with its smallest diameter at the high-pressure, or inlet, end, and its largest at the low-pressure, or exhaust, end.
A largeunit for a nuclear power station may have four rotors consisting of one double-flow high-pressure section followed by three double-flow low-pressure sections.
Impulse turbines usually employ pressure or Rateau staging, named after the French engineer Auguste Rateau, in which the pressure ratio across each stage is nearlyuniform.
Impulse turbines built in the past have made use of velocity-compounded, or Curtis, staging named after its American inventor, Charles Gordon Curtis, whichhas two sets of moving buckets with an intermediate set of fixed blades following the nozzles.
The staging of a reaction turbine may be called Parsons' staging, after itsBritish inventor, Charles Parsons.
Steam turbines are comparatively simple machines, having only one major moving part, the rotor; however, auxiliary equipment is necessary for their operation.Journal bearings support the shaft.
A thrust bearing positions the shaft axially.
An oil system provides lubrication to the bearings.
Seals minimize steam leakage withinthe steam path.
A sealing system prevents steam leaking from the machine and air leaking from the outside into the machine.
The speed of rotation is controlled byvalves at the inlet(s) of the machine.
In addition, reaction turbines develop considerable axial thrust owing to the pressure drop across the moving blades.
This isusually compensated for by the use of a dummy piston, which creates a thrust in the opposite direction to that of the steam path.
The expansion efficiency of a modern multistage steam turbine is inherently high because of the state of development of the steam-path components and the ability torecover losses of one stage in those downstream through reheating.
The efficiency with which a section of a turbine converts the theoretically available thermodynamicenergy to mechanical work is commonly in excess of 90 percent.
The thermodynamic efficiency of a steam-power installation is much less, owing to the energy lost inthe exhaust steam from the turbine.
See also Gas Turbine; Jet Propulsion.
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