Liquid heaters and vaporizers – Plants – Feed water heaters
Reexamination Certificate
2001-09-28
2004-04-06
Lu, Jiping (Department: 3749)
Liquid heaters and vaporizers
Plants
Feed water heaters
C122S00700A, C122S235120, C122S406400, C122SDIG004
Reexamination Certificate
active
06715450
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a continuous-flow steam generator having a combustion chamber for fossil fuel which is followed on the fuel-gas side, via a horizontal gas flue, by a vertical gas flue, the containment walls of the combustion chamber being formed from vertically arranged evaporator tubes welded to one another in a gastight manner.
BACKGROUND OF THE INVENTION
In a power plant with a steam generator, the energy content of a fuel is utilized for the evaporation of a flow medium in the steam generator. In this case, the flow medium is conventionally carried in an evaporator circuit. The steam supplied by the steam generator may, in turn, be provided, for example, for driving a steam turbine and/or for a connected external process. When the steam drives a steam turbine, a generator or a working machine is normally operated via the turbine shaft of the steam turbine. Where a generator is concerned, the current generated by the generator may be provided for feeding into an interconnected and/or island network.
The steam generator may in this case be designed as a continuous-flow steam generator. A continuous-flow steam generator is known from the paper “Verdampferkonzepte für Benson-Dampferzeuger” [“Evaporator concepts for Benson steam generators”] by J. Franke, W. Köhler and E. Wittchow, published in VGB Kraftwerkstechnik 73 (1993), number 4, p. 352-360. In a continuous-flow steam generator, the heating of steam generator tubes provided as evaporator tubes leads to an evaporation of the flow medium in the steam generator tubes in a single pass.
Continuous-flow steam generators are conventionally designed with a combustion chamber in a vertical form of construction. This means that the combustion chamber is designed for the heating medium or fuel gas to flow through it in approximately vertical direction. In this case, the combustion chamber may be followed on the fuel-gas side by a horizontal gas flue, a deflection of the fuel-gas stream into an approximately horizontal direction of flow taking place at the transition from the combustion chamber into the horizontal gas flue. However, because of the thermally induced changes in length of the combustion chamber, combustion chambers of this type generally require a framework on which the combustion chamber is suspended. This necessitates a considerable technical outlay in terms of the manufacture and assembly of the continuous-flow steam generator. The higher the outlay, the greater the overall height of the continuous-flow steam generator. This applies particularly in the case of continuous-flow steam generators which are designed for a steam power output of more than 80 kg/s under full load.
A continuous-flow steam generator is not subject to any pressure limitation, so that fresh-steam pressures well above the critical pressure of water (p
cri
=221 bar), where there is only a slight difference in density between a liquid-like and a vapor-like medium, are possible. A high fresh-steam pressure is conducive to high thermal efficiency and therefore to low CO
2
emissions for a fossil-fired power station which, for example, can be fired with hard coal or else with lignite in solid form as fuel.
A particular problem is presented by the design of the containment wall of the gas flue or combustion chamber of the continuous-flow steam generator in terms of the tube-wall or material temperatures which occur there. In the subcritical pressure range to about 200 bar, the temperature of the containment wall of the combustion chamber is determined essentially by the height of the saturation temperature of water when wetting of the inner surface of the evaporator tubes can be ensured. This is achieved, for example, using evaporator tubes which have a surface structure on their inside. In this respect, in particular, internally ribbed evaporator tubes come under consideration, of which the use in a continuous-flow steam generator is known, for example, from the abovementioned paper. These so-called ribbed tubes, that is to say tubes with a ribbed inner surface, have particularly good heat transmission from the tube inner wall to the flow medium.
Experience has shown that it is not possible to avoid the situation where, when the continuous-flow steam generator is in operation, thermal stresses occur between adjacent tube walls of different temperature when these are welded to one another. This is the case, in particular, with regard to the portion of the combustion chamber connecting the latter to the horizontal gas flue following it, that is to say between the evaporator tubes of the exit region of the combustion chamber and the steam generator tubes of the entry region of the horizontal gas flue. These thermal stresses may markedly reduce the useful life of the continuous-flow steam generator and, in an extreme case, even give rise to tube fractures.
SUMMARY OF THE INVENTION
An object on which the invention is based is, therefore, to specify a fossil-fired continuous-flow steam generator of the abovementioned type which requires a particularly low outlay in terms of manufacture and assembly. Moreover, preferably a generator during the operation of which temperature differences at the connection of the combustion chamber to the horizontal gas flue following the latter are kept low. This is to be the case, in particular, for the mutually directly or indirectly adjacent evaporator tubes of the combustion chamber and steam generator tubes of the horizontal gas flue following the combustion chamber.
This object and/or other objects are achieved, according to the invention, in that the continuous-flow steam generator has a combustion chamber with a number of burners arranged level with the horizontal gas flue. A plurality of the evaporator tubes are preferably capable of being acted upon in each case in parallel by flow medium. Further, in the exit region of the combustion chamber, a number of the evaporator tubes are preferably capable of being acted upon in parallel by flow medium being led through the combustion chamber before their entry into the respective containment wall of the combustion chamber.
The invention proceeds from the notion that a continuous-flow steam generator capable of being produced at a particularly low outlay in terms of manufacture and assembly, should have a suspension structure capable of being executed by simple means. A framework to be produced at comparatively low outlay in technical terms and intended for suspending the combustion chamber may, in this case, be accompanied by a particularly small overall height of the continuous-flow steam generator. A particularly small overall height of the continuous-flow steam generator can be achieved by the combustion chamber being designed in a horizontal form of construction. For this purpose, the burners are arranged level with the horizontal gas flue in the combustion chamber wall. Thus, when the continuous-flow steam generator is in operation, the fuel gas flows through the combustion chamber in an approximately horizontal main direction of flow.
Moreover, when the continuous-flow steam generator with the horizontal combustion chamber is in operation, temperature differences should be particularly low at the connection of the combustion chamber to the horizontal gas flue, in order reliably to avoid premature material fatigues as a result of thermal stresses. These temperature differences should be especially low, in particular, between mutually directly or indirectly adjacent evaporator tubes of the combustion chamber and steam generator tubes of the horizontal gas flue, so that material fatigues as a result of thermal stresses are prevented particularly reliably in the exit region of the combustion chamber and in the entry region of the horizontal gas flue.
However, when the continuous-flow steam generator is in operation, that entry portion of the evaporator tubes which is acted upon by flow medium has a comparatively lower temperature than the entry portion of the steam generator tubes of the horizontal gas flue following the
Harness & Dickey & Pierce P.L.C.
Lu Jiping
Siemens Aktiengesellschaft
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