Why is a valve used in front of the turbine

Steam turbine

Lexicon> Letter D> Steam turbine

Definition: a turbine that is driven by hot steam

More general terms: heat engine

More specific terms: saturated steam turbine, condensing turbine, extraction condensing turbine, back pressure turbine, low pressure turbine, high pressure turbine

English: steam turbine

Category: Engines and Power Plants

Author: Dr. Rüdiger Paschotta

How to quote; suggest additional literature

Original creation: 07.03.2010; last change: 03/15/2021

URL: https://www.energie-lexikon.info/dampfturbine.html

A Steam turbine is a turbine that is driven by hot steam (mostly water vapor) under high pressure. Its function is that of a heat engine, i. H. the (partial) conversion of heat into mechanical energy. The most common application is driving a generator to generate electrical energy. A so-called turbo generator is often used here, which can be driven directly (without a gearbox) by a turbine.

The steam turbine is basically a type of Steam engine, but it is mostly understood to mean reciprocating engines.

Working principle

The functional principle of a steam turbine and the other components required for a cycle process are initially described in a greatly simplified manner in the following, i. H. neglecting various technical details, which are then described in the following section.

A liquid medium (mostly fully demineralized water) is in a Steam generator evaporated with the addition of high-temperature heat at high pressure. This steam does work in the turbine by driving one or usually several rapidly rotating turbine wheels. In doing so, it is relaxed (i.e. its pressure is reduced) and cooled. After the turbine, the steam has to be cooled further in a condenser so that it condenses, whereby waste heat that can no longer be used is often produced. The condensed water is then fed back to the steam generator with a pump. So it will be a Circular process operated.

The drive of the feed water pump only requires a small part of the energy supplied by the turbine, since the volume of the condensate is much smaller than that of the steam.

The entire amount of water running through the turbine must come from the named pump (the Feed water pump or Boiler feed pump) are conveyed against the same pressure gradient. Nevertheless, the power required for the pump is only a small fraction of the power output by the turbine, since the condensed water has a much smaller volume than the steam. (The required pump capacity results from the product of pressure and volume flow.)

In a somewhat abstract form, the functional principle of the steam turbine can be represented by the so-called Clausius-Rankine cycle - named after the physicist Rudolf Julius Emanuel Clausius and the engineer William John Macquorn Rankine. Here it is assumed that an ideal adiabatic expansion of the water vapor takes place in the turbine, whereby the entropy does not increase. The heat supply in the steam boiler is isobaric. In the case of large steam turbine systems, the Rankine cycle can provide a reasonably precise description (more precisely than the Carnot process), and the efficiency achieved in this way is somewhat below the Carnot efficiency.

By far the most common working medium for steam turbines is water vapor. However, if only a heat reservoir with a relatively low temperature is available, water vapor is poorly or not at all suitable. In such cases it is possible to apply the principle of the Organic Rankine Cycle, in which a mostly organic working medium with a lower boiling point is used.

Further technical details

Saturated steam and live steam

Many steam turbines shouldn't work directly with it Saturated steam operated as it is produced in a steam generator. This saturated steam still contains a certain proportion of water droplets, which would additionally load the turbine blades from water hammer. For this reason, the saturated steam is usually first passed through a superheater, which raises the temperature slightly so that all remaining water droplets still evaporate. The resulting live steam (superheated steam) is then much better tolerated by the turbine, and the energy efficiency also increases.

There are, however Saturated steam turbineswho can tolerate saturated steam without any problems. They are used, for example, in nuclear power plants, where the high temperatures for a superheater would be difficult to achieve.

Blades and vanes

Each steam turbine contains a rotor (Wheel) on which the blades are located. As a rule, there are several sets (rings) of rotor blades, which the steam then traverses one after the other, the size of the rotor blades increasing in accordance with the increasing steam volume. (This increase in size is particularly pronounced in the case of low-pressure turbines.) Between these sets of rotor blades there are respectively Guide vanesthat are connected to the housing, so do not rotate. They optimally direct the steam onto the following rotor blades and increase the flow velocity at the expense of a decreasing pressure.

Multiple turbine stages

A steam turbine system often consists of several stages that work at different pressure levels.

Often combinations of several steam turbines with different pressure levels are used: high pressure, medium pressure and low pressure turbines. (Three turbine stages are often used, especially in high-performance thermal power plants.) The last turbine (one Low pressure turbine) usually works as a Condensing turbine, d. H. in it a large part of the water vapor is condensed into water droplets (Wet steam).

There is often another between two turbine stages Superheater, d. H. a heat exchanger with which the steam is heated up again. This increases the efficiency.

Regulation of performance

Short-term changes to the generated power are possible by actuating a control stage. This is a relatively small turbine stage directly after the steam generator, where the steam supply can be throttled with valves. For longer-term changes in output, the combustion output must be adjusted, but this takes considerably more time.

Capacitor; Back pressure turbines and extraction condensation turbines for waste heat recovery

After passing through the steam turbine (or the last turbine stage), the steam still contains a considerable part of its heat, which has to be dissipated for the purpose of condensation. This waste heat is often released into the environment via a cooling tower, possibly with additional flow cooling to further reduce the temperature and increase the efficiency. In thermal power stations, at least part of this heat is dissipated as useful heat (thermal heat). For this a significantly higher temperature level is necessary than that of the river cooling. There are different technical solutions for this:

There are different methods for utilizing waste heat. The electrical efficiency drops one way or another.
  • It can be a final stage Back pressure turbine can be used in which the exhaust steam is not expanded to the point of condensation, as in a condensation turbine, but remains as superheated steam. The remaining higher pressure causes a somewhat reduced mechanical power of the turbine. The steam obtained is only used in Heating condenser, a heat exchanger for extracting useful heat, condenses.
  • As an alternative there is Extraction condensing turbines, in which a (mostly variable) part of the steam can be withdrawn before the low-pressure part for heat generation, in order to be sent to the heating condenser. Again, the extraction reduces the mechanical performance of the low-pressure turbine. This process is more efficient than that of the back pressure turbine when the heat demand is low or fluctuating significantly.

Regenerative preheaters

In principle, the condensate (i.e. the cool water produced in the condenser), which is used again as feed water, can simply be heated up in the boiler of the power plant to generate steam. However, it is energetically more beneficial to do it first by several Preheater to send it z. B. already heat to over 250 ° C. The heat required for this is obtained by steam, which the turbines at different points (with different pressure and temperature levels) as Exhaust steam (or Intermediate steam) is removed. Even if this steam extraction reduces the output of the turbines, the achievable degree of efficiency is higher because the exergy of the steam is better used: The high exergy of the superheated steam is initially partially released in the high-pressure turbine, and the exergy content of the bleed steam is (per Joule Heat) correspondingly lower. This deviates somewhat from the basic Clausius-Rankine cycle and comes closer to the Carnot efficiency, which is why one is also from Carnotization speaks.

An additional feed water preheating can take place in an economiser, i. H. in a heat exchanger that uses residual heat in the exhaust gas.

Typical performance and efficiency

A good degree of efficiency of a steam turbine requires a high steam temperature.

Turbines in large power plants generate mechanical outputs of hundreds of megawatts, sometimes even well over one gigawatt. The steam temperatures are several hundred degrees Celsius - tending to be higher in fossil-fuel power plants than in nuclear power plants. At high steam temperatures, efficiencies a little above 45% can be achieved for the power plant as a whole, so a little more for the turbine alone.

The high temperature level (i.e. the live steam temperature) that can be used with steam turbines is limited by the load-bearing capacity of the materials used. The turbine blades are often operated at such a high temperature that their service life is only a few years. For many years the live steam temperature was limited to around 550 ° C. In recent years there have been increased efforts to increase the temperatures again considerably by using improved materials (e.g. nickel alloys); temperatures of well over 600 ° C are already being used. In the next few years, values ​​around 700 ° C could also become possible; the steam pressure would then rise to approx. 350 bar. The overall efficiency of a steam power plant could thus be a little over 50%.

Significantly higher temperatures can be used with gas turbines, but if used alone they would have a lower efficiency (e.g. 35%), since their exhaust gas temperature is quite high. The highest degrees of efficiency (around 60%) are achieved in gas power plants with combined gas and steam turbines, which are known as combined cycle power plants.

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See also: gas turbine, heat engine, steam engine, water vapor, steam generator, power plant, Carnot efficiency, condenser, Clausius-Rankine cycle, organic Rankine cycle
as well as other items in the engine and power plant category

Understand everything?


Question: Why do you condense the water after the turbine in a steam engine cycle, even though you have to evaporate it again afterwards?

Correct answers: (b) and (c)


Question: Why does it make sense to send the water in a steam turbine cycle after the condenser through a preheater that is operated with exhaust steam?

Correct answer: (a)


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