Liquid heaters and vaporizers – Circulation – Once through
Reexamination Certificate
1999-12-30
2001-01-16
Ferensic, Denise L. (Department: 3749)
Liquid heaters and vaporizers
Circulation
Once through
C122S00100C, C122S00100C, C122S00700A, C122S45100R, C122S460000, C122S477000
Reexamination Certificate
active
06173679
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a waste-heat steam generator, in particular for a combined-cycle power plant, with a number of heating surfaces heatable by a flue gas and carrying a medium to be heated. More specifically, the generator contains a preheater following an evaporator on the flue gas side and a superheater precedes the evaporator on the flue gas side.
Such a waste-heat steam generator or waste-heat boiler is usually part of a combined-cycle power plant, in which the heat contained in the expanded working medium from the gas turbine is used to generate steam for the steam turbine. The heat is transmitted by a number of heating surfaces which are disposed in the form of tubes or tube bundles in the waste-heat steam generator. These, in turn, are connected into the water/steam circuit of the steam turbine, the circuit containing at least one pressure stage. In this case, each pressure stage usually has, as heating surfaces, a preheater or economizer and a evaporator as well as a superheater. A connection having a first or high-pressure stage and a second or low-pressure stage—a so-called two-pressure process—is known from European Patent EP 0 410 111 B1.
Due to the further development of the stationary gas turbine towards higher unit output and higher efficiency, in conjunction with an increased exhaust-gas temperature at the gas turbine outlet, it can be seen that it would also become necessary to adapt the fresh-steam parameters, namely the fresh-steam temperature and fresh-steam pressure, in order to achieve a further increase in plant efficiency. An increased exhaust-gas temperature of the gas turbine consequently results in an enhancement of the fresh-steam parameters, in order to achieve correspondingly high efficiency of the overall combined-cycle process.
In a waste-heat steam generator of this type, configured as a drum boiler and working on the circulation principle, evaporation is ended by separating the steam from the not yet evaporated water at deflecting plates in the drum. This separation requires circulation. The circulation and the steam separation make it necessary to have, between the water and steam, an appreciable density difference which, in turn, presupposes an appreciable pressure deviation (>50 to 60 bar) from the critical pressure (221 bar). Steam separation thus places an upper limit on the pressure, so that a drum boiler can work only below an operating pressure of 160 to 170 bar. Moreover, high steam pressures necessitate large wall thicknesses of the water/steam separating drum, thus sharply restricting the permissible rate of temperature change in the start-up and load-alternation modes undesirably.
In the forced once-through principle, such as a fossil-fuelled once-through steam generator, as an alternative to the circulation or natural circulation principle, the heating of the evaporator tubes forming a combustion-chamber wall leads to complete evaporation of the flow medium in the evaporator tubes in a single pass. In this forced once-through principle, the end of evaporation and consequently, at the same time, the commencement of steam superheating occur as a function of the load and are not locally fixed. In this case, with a once-through steam generator of this type, a freshsteam pressure above the critical water pressure may be implemented due to the absence of steam separation or water precipitation. A once-through steam generator of this type is known, for example, from European Patent EP 0 595 009 B1.
However, in conjunction with a gas turbine in a so-called combined-cycle power plant, a once-through steam generator of this type is usually not employed as a pure waste-heat steam generator. Instead, the oxygenous exhaust gas from the gas turbine serves merely as combustion air for the firing system of this fossil-fuelled steam generator. Disadvantages of a once-through steam generator of this kind are its high outlay in terms of connection and regulation and its high start-up losses due to so-called water ejection. This occurs when evaporation in the evaporator commences and the steam pushes out the downstream water quantity (water drops). Additional separating and monitoring components, provided in order to overcome the start-up losses caused thereby, additionally increase the technical outlay and consequently the investment costs which increase sharply with a desired implementation of high and very high steam pressures. Another essential disadvantage of the fired once-through steam generator, as compared with the pure waste-heat steam generator, is the comparatively uneven adaptation of the temperature profile of the heated medium (water/water-steam line) to the temperature profile of the heating medium (flue-gas line).
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a waste-heat steam generator which overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which a critical or supercritical steam pressure can also be implemented, while at the same time it operates in a fluidically stable manner in all load ranges, in particular even in the partload range.
With the foregoing and other objects in view there is provided, in accordance with the invention, a waste-heat steam generator, including:
a number of heating surfaces heatable by a flue gas and carrying a medium to be heated, the number of heating surfaces including:
an evaporator having an inlet side, an outlet side and evaporator tubes;
a preheater following the evaporator on a flue gas side; and
a superheater preceding the evaporator on the flue gas side, the preheater, the evaporator and the superheater disposed on a medium side in countercurrent to the flue gas;
an inlet distributor connecting the evaporator on the inlet side to the preheater;
a throttle configuration connecting each of the evaporator tubes to the inlet distributor; and
a pressure-equalizing header in a form of an equalizing tube connected to the evaporator.
Provided for this purpose is a waste-heat steam generator which works on the forced once-through principle and the drum-free evaporator of which is connected, on the medium side, in countercurrent to the direction of flow of the flue gas. The evaporator is connected on the inlet side to the preheater via an inlet distributor that is identical to an outlet header of the preheater. To achieve particularly stable operation of the evaporator, the latter is provided on the inlet side with a throttle configuration, a throttle expediently being connected into the inlet of each individual evaporator tube. Increased pressure loss over a particularly wide load range is thereby achieved in the evaporator. This ensures a uniform flow through the evaporator tubes.
Furthermore, instead of headers and distributors conventionally used, only a pressure-equalizing header is provided between the evaporator portions or evaporator parts. The header subdivides the entire evaporator virtually into two portions on the pressure side, since identical pressures prevail in each case at the ends of the evaporator portions.
This increases the through flow stability, while avoiding a further increase in the pressure losses. Expediently, the pressure-equalizing header has only a relatively thin tube for the connection of a tapping bore of each evaporator tube. The connecting tube of small dimensions influences the flow within the evaporator only insignificantly and therefore does not present any problems with regard to allocating to the evaporator tubes the two-phase mixture of the medium to be heated, the mixture being established in the evaporator.
Expediently, a preheater which follows the evaporator on the flue-gas side and a superheater which precedes the evaporator on the flue-gas side are also configured to be connected in countercurrent on the medium side.
A once-through or forced once-through waste-heat steam generator of this type makes it possible to implement high steam states up to supercritical values, since the generator is not subject to any pressur
Br{umlaut over (u)}ckner Hermann
Schwarzott Werner
Stierstorfer Helmut
Ferensic Denise L.
Greenberg Laurence A.
Lerner Herbert L.
Siemens Aktiengesellschaft
Stemer Werner H.
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