Title of Invention	FOSSIL-FUEL FIRED CONTINUOUS STEAM GENERATOR
Abstract	A continuous-flow steam generator (2) has a combustion chamber (4) with evaporator tubes (10) for fossil fuel (B) , which is followed on the fuel-gas side, via a horizontal gas flue (6) , by a vertical gas flue (8) . When the continuous-flow steam generator is in operation, temperature differences in a connecting portion (2), which comprises the outlet region (34) of the combustion chamber (4) and the inlet region (32) of the horizontal gas flue (6) , are to be kept particularly low. For this purpose, of a plurality of evaporator tubes (10) capable of being acted upon in parallel by flow medium (S) , a number of evaporator tubes (10) are guided in the form of a loop in the connecting portion (Z).

Title of Invention

FOSSIL-FUEL FIRED CONTINUOUS STEAM GENERATOR

Abstract

A continuous-flow steam generator (2) has a combustion chamber (4) with evaporator tubes (10) for fossil fuel (B) , which is followed on the fuel-gas side, via a horizontal gas flue (6) , by a vertical gas flue (8) . When the continuous-flow steam generator is in operation, temperature differences in a connecting portion (2), which comprises the outlet region (34) of the combustion chamber (4) and the inlet region (32) of the horizontal gas flue (6) , are to be kept particularly low. For this purpose, of a plurality of evaporator tubes (10) capable of being acted upon in parallel by flow medium (S) , a number of evaporator tubes (10) are guided in the form of a loop in the connecting portion (Z).

Full Text	Description Fossil-fired continuous-flow steam generator 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. In a power plant with a steam generator, the energy content of a fuel is utilized for evaporating a flow medium in the steam generator. In this case, the flow medium is normally 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. If the steam drives a steam turbine, a generator or a working machine is usually 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 context, be designed as a continuous-flow steam generator. A continuous-flow steam generator is known from the paper "Verdampferkonzepte fur Benson-Dampferzeuger" ["Evaporator concepts for Benson steam generators"] by J. Franke, W. Kohler and E. Wittchow, published in VGB Kraftwerkstechnik 73 (1993), No. 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. - la- Continuous-flow steam generators are conventionally designed with a combustion chamber in a vertical form of construction. This means that - 2 - the combustion chamber is designed for the heating medium or fuel gas to flow through in an 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, in general, because of the thermally induced changes in length of the combustion chamber, combustion chambers of this type require a framework on which the combustion chamber is suspended. This necessitates a considerable technical outlay in terms of the production and assembly of the continuous-flow steam generator, this outlay being the greater, the greater the overall height of the continuous-flow steam generator is. This is true particularly with regard to 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 (pcri = 221 bar) , where there is still only a slight density difference between the liquid-like and steam-like media, are possible. A high fresh-steam pressure is conducive to high thermal efficiency and therefore to low CO2 emissions of a fossil-fired power station which can be fired, for example, 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 down to about - 2a - 200 bar, the temperature of the containment wall of the combustion chamber is determined essentially by the height of the saturation temperature of the water, when wetting of the inner surface of the evaporator tubes can be ensured. This is achieved, for example, by using evaporator tubes which have a surface structure on their inside. In this respect, consideration is given, in particular. - 3 - to internally ribbed evaporator tubes, of which the use in a continuous-flow steam generator is known, for example, from the paper quoted above. 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 evaporator tubes of the outlet region of the combustion chamber and steam generator tubes of the inlet region of the horizontal gas flue. These thermal stresses can markedly reduce the useful life of the continuous-flow steam. generator and in an extreme case may even give rise to tube cracks. The object on which the invention is based is to specify a fossil-fired continuous-flow steam generator of the abovementioned type, which requires a particularly low outlay in terms of production and assembly and, moreover, during the operation of which temperature differences at the connection of the combustion chamber to the horizontal gas flue following it are kept low. This is to apply particularly to 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 is achieved, according to the invention, in that the continuous-flow steam generator has a -3a - combustion chamber v;ith a number of burners arranged level with the horizontal gas flue, a plurality of the evaporator tubes being capable of being acted upon in each case in parallel by flow medium, and a number of the evaporator tubes capable of being acted upon in parallel by flow medium being led in the form of a loop - 4 - in a connecting portion which comprises the outlet region of the combustion chamber and the inlet region of the horizontal gas flue. The invention proceeds from the notion that a continuous-flow steam generator capable of being set up at a particularly low outlay in terms of production and assembly should have a suspension structure capable of being executed by simple means. At the same time, a framework capable of being set up at a comparatively low technical outlay for the suspension of the combustion chamber can be accompanied by a particularly low overall height of the continuous-flow steam generator. A particularly low 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 outlet region of the combustion chamber and in the inlet region of the horizontal gas flue. - 4a - However, when the continuous-flow steam generator is in operation, the inlet portion of the evaporator tubes which is acted upon by flow medium has a comparatively lower temperature than the inlet portion of the steam generator tubes of the horizontal gas flue following the combustion chamber. To be precise, comparatively cold flow medium enters the evaporator tubes, - 5 - in contrast to the hot flow medium which enters the steam generator tubes of the horizontal gas flue. Hence, when the continuous-flow steam generator is in operation, the evaporator tubes are colder in the inlet portion than the steam generator tubes in the inlet portion of the horizontal gas flue. Consequently, material fatigues as a result of thermal stresses are to be expected at the connection between the combustion chamber and the horizontal gas flue. However, if, then, preheated flow medium enters the inlet portion of the evaporator tubes of the combustion chamber, instead of cold flow medium, the temperature difference between the inlet portion of the evaporator tubes and the inlet portion of the steam generator tubes will no longer be as great as would be the case if cold flow medium were to enter the evaporator tubes. If, therefore, the flow medium is led first in a first evaporator tube, which is arranged further away.from the connection of the combustion chamber to. the horizontal gas flue than a second eyaporator tube,and is then introduced into this second eyaporator tube, flow medium preheated by firing enters the second evaporator tube when the continuous flow steam generator is in operation. The complicated connection between a first and a second evaporator , tube may be dispensed with if one evaporator tube has an inlet for flow medium in the middle of the containment wall of the combustion chamber. For, then, this evaporator tube can be led first from the top downward and then from the bottom upward in the combustion chamber. Consequently, when the continuous-flow steam generator is in operation, firing causes a preheating of the flow medium to take place in that portion of the evaporator tube which is led from the top downward, before the flow medium enters the so-called inlet portion of the evaporator tubes in the lower region of the combustion - 5a - chamber. It proves to be particularly beneficial, at the same time, if a number of the evaporator tubes capable of being acted upon in parallel by flow medium are led in the form of a loop in the respective containment wall of the combustion chamber. - 6 - The side walls of the horizontal gas flue and/or of the vertical gas flue are advantageously formed from vertically arranged steam generator tubes welded to one another in a gastight manner and capable of being acted upon in each case in parallel by flow medium. Advantageously, in each case, a number of parallel- connected evaporator tubes of the combustion chamber are preceded by a common inlet header system and followed by a common outlet header system for flow medium.To be precise , a continous -flow steam generator designed in this configuration allows reliable pressure compensation between a number of evaporator tubes capable of being acted upon in parallel by flow medium, so that, in each case, all parallel-connected evaporator tubes between the inlet header system and the outlet header system have the same overall pressure loss. This means that, in the case of an evaporator tube heated to a greater extent, the throughput must rise, as compared with an evaporator tube heated to a lesser extent. This also applies to the steam generator tubes of the horizontal gas flue or of the vertical gas flue which are capable of being acted upon in parallel by flow medium and which are advantageously preceded by a common inlet header system for flow medium and followed by a common outlet header system for flow medium. The evaporator tubes of the end wall of the combustion chamber are advantageously capable of being acted upon in parallel by flow medium and precede the evaporator tubes of the containment walls, which form the side walls of the combustion chamber, on the flow-medium side. This ensures particularly favorable cooling of the highly heated end wall of the combustion chamber. - 6a - In a further advantageous refinement of the invention, the tube inside diameter of a number of the evaporator tubes of the combustion chamber is selected as a function of the respective position of the evaporator tubes in the combustion chamber. The evaporator tubes in the combustion chamber can thereby be adapted to a heating profile - 7 - predeteminable on the fuel-gas .side. By means of the influence brought about thereby on the flow through the evaporator tubes, temperature differences of the flow medium at the outlet from the evaporator tubes of the combustion chamber are kept particularly low in a particularly reliable way. For particularly good heat transmission from the heat of the combustion chamber to the flow medium carried in the evaporator tubes, a number of evaporator tubes advantageously have in each case, on their inside, ribs which form a multiflight thread. In this case, advantageously, a pitch angle a between a plane perpendicular to the tube axis and the flanks of the ribs arranged on the tube inside is smaller than 60% unforable smaller than 55°. To be precise, in a heated evaporator tube designed as an evaporator tube without internal ribbing, a so-called smooth tube, the wetting of the tube wall, necessary for particularly good heat transmission, can no longer be maintained from a specific steam content onward. With a lack of wetting, there may be a tube wall which is dry in places. The transition to a dry tube wall of this kind leads to a so-called heat transmission crisis with an impaired heat transmission behavior, so that, in general, the tube wall temperatures rise particularly sharply at this point. In an internally ribbed evaporator tube, however, as compared with a smooth tube, this heat transmission crisis arises only in the case of a steam mass content > 0.9, that is to say just before the end of evaporation. This is attributable to the swirl which the flow experiences due to the spiral ribs. On account of the different centrifugal force, the water fraction is separated from the steam fraction and is transported to the tube wall. The wetting of the tube wall is thereby maintained up to high steam contents, so that - 7a - there are already high flow velocities at the location of the heat transmission crisis. This gives rise, despite the heat transmission crisis, to relatively good heat transmission and, consequently, to low tube wall temperatures. - 8 - A number of evaporator tubes of the combustion chamber advantageously have means for reducing the throughflow of the flow medium. In this case, it proves particularly beneficial if the means are designed as throttle devices. Throttle devices may, for example, be fittings in the evaporator tubes, which reduce the tube inside diameter at a point within the respective evaporator tube. At the same time, means for reducing the throughflow in a line system which comprises a plurality of parallel lines and through which flow medium can be fed to the evaporator tubes of the combustion chamber also prove to be advantageous. In this case, the line system may also precede an inlet header system of evaporator tubes capable of being acted upon in parallel by flow medium. In such cases, for example, throttle assemblies may be provided in one line or in a plurality of lines in the line system. Such means for reducing the throughflow of the flow medium through the evaporator tubes make it possible to adapt the throughput of the flow medium through individual evaporator tubes to the respective heating of these in the combustion chamber. As a result, in addition, temperature differences of the flow medium at the outlet of the evaporator tubes are kept particularly low in a particularly reliable way. Adjacent evaporator or steam generator tubes are welded to one another in a gastight manner on their longitudinal sides advantageously via metal bands, so-called fins. These fins can be connected fixedly to the tubes even during the tube production process and can form a unit with these. This unit formed from a tube and fins is also designated as finned tube. The fin width influences the introduction of heat into the evaporator or steam generator tubes. The fin width is therefore adapted to a heating profile predeterminable on the flow-gas side, preferably as a function of the - 8a - position of the respective evaporator or steam generator tubes in the continuous-flow steam generator. The heating profile predetermined in this case may be a typical heating profile determined from experimental values - 9 - or else a rough estimation, such as, for example, a stepped heating profile. By means of the suitably selected fin widths, even when different evaporator or steam generator tubes are heated to a widely differing extent, an introduction of heat into all the evaporator or steam generator tubes can be achieved, in such a way that temperature differences of the flow medium at the outlet from the evaporator or steam generator tubes are kept particularly low. Premature material fatigues as a result of thermal stresses are reliably prevented in this way. As a result, the continuous-flow steam generator has a particularly long useful life. The horizontal gas flue advantageously has arranged in it a number of superheater heating surfaces which are arranged approximately perpendicularly to the main direction of flow of the fuel gas and the tubes of which are connected in parallel for the throughflow of the flow medium. These superheater heating surfaces, arranged in a suspended form of construction and also designated as bulkhead heating surfaces, are heated predominantly by convection and follow the evaporator tubes of the combustion chamber on the flow-medium side. A particularly favorable utilization of the fuel-gas heat is thereby ensured. Advantageously, the vertical gas flue has a number of convection heating surfaces which are formed from tubes arranged approximately perpendicularly to the main direction of flow of the fuel gas. These tubes of a convection heating surface are connected in parallel for a throughflow of the flow medium. These convection heating surfaces, too, are heated predominantly by convection. In order, furthermore, to ensure the particularly full utilization of the heat of the fuel gas, the vertical gas flue advantageously has an economizer. - 9a - Advantageously, the burners are arranged on the end wall of the combustion chamber, that is to say on that side wall of the - 10 - combustion chamber which is located opposite the outflow orifice to the horizontal gas flue. A continuous-flow steam generator designed in this way can be adapted particularly simply to the burnup length of the fossil fuel. The burnup length of the fossil fuel refers, in this context, to the fuel-gas velocity in the horizontal direction at a specific average fuel-gas temperature, multiplied by the burnup time tA of the flame of the fossil fuel. The maximum burnup length for the respective continuous-flow steam generator is obtained, in this case, from the steam power output M under the full load of the continuous-flow steam generator, the so-called full-load mode. The burnup time tA of the flame of the fossil fuel is, in turn, the time which, for example, a coaldust grain of average size requires in order to burn up completely at a specific average fuel-gas temperature. Advantageously, the lower region of the combustion chamber is designed as a funnel. In this way, when the continuous-flow steam generator is in operation, ash occurring during the combustion of the fossil fuel can be discharged particularly simply, for example into an ash removal device arranged under the funnel. The fossil fuel may in this case be coal in solid form. In order to keep material damage and undesirable contamination of the horizontal gas flue, for example due to the introduction of high-temperature molten ash, particularly low, the length of the combustion chamber, defined by the distance from the end wall to the inlet region of the horizontal gas flue, is advantageously at least equal to the burnup length of the fossil fuel in the full-load mode of the continuous-flow steam generator. This horizontal length of the combustion chamber will generally amount to at least 80°of the height of the combustion chamber, measured from the - 10a - funnel top edge, when the lower region of the combustion chamber has a funnel-shaped design, to the combustion chamber ceiling. - 11 - For a particularly beneficial utilization of the combustion heat of the fossil fuel, the length L (given in m) of the combustion chamber is selected as a function of the steam power output M (given in kg/s) of the continuous-flow steam generator under full load, of the burnup time tA (given in s) of the flame of the fossil fuel and of the outlet temperature tBRK (given in 0C) of the fuel gas from the combustion chamber. In this case, with the given steam power output M of the continuous-flow steam generator' under full load, approximately the higher value of the two functions (I) and (II) applies to the length L of the combustion chamber: C1 = 8 m/s and C2 = 0.0057 m/kg and C3 = -1.905 -10-4 (m • s)/(kg0C) and C4 = 0.286 (s . m)/kg and C5 - 3 . 10-' m/(°C)2 and C6 = -0.842 m/°C and C7 = 603.41 m. By "approximately" is to be meant, here, a permissible deviation in the length L of the combustion chamber of +20%/-10% from the value defined by the respective function. The advantages achieved by means of the invention are, in particular, that, by some evaporator tubes being guided in the form of a loop in the containment wall of - 11a - the combustion chamber, temperature differences in the immediate vicinity of the connection of the combustion chamber to the horizontal gas flue when the continuous-flow steam generator is in operation are particularly low. Consequently, when the continuous-flow steam generator is in operation, the thermal stresses at the connection of the combustion chamber to the horizontal gas flue - 12 - which are caused by temperature differences between directly adjacent evaporator tubes of the combustion chamber and steam generator tubes of the horizontal gas flue remain well below the values at which, for example, there is a risk of pipe cracks. It is therefore possible to use a horizontal combustion chamber in a continuous-flow steam generator, even at the same time with a comparatively long useful life. Moreover, designing the combustion chamber for an approximately horizontal main direction of flow of the fuel gas affords a particularly compact form of construction of the continuous-flow steam generator. This makes it possible, when the continuous-flow steam generator is incorporated into a power station with a steam turbine, also to have particularly short connecting pipes from the continuous-flow steam generator to the steam turbine. . An exemplary embodiment of the invention is explained in more detail by means of a drawing in which: figure 1 shows diagrammatically a side view of a fossil-fired continuous-flow steam generator of the dual-flue type, and figure 2 shows diagrammatically a longitudinal section through an individual evaporator tube, figure 3 shows a coordinate system with the curves K1 to K6, figure 4 shows diagrammatically the portion connecting the combustion chamber to the horizontal gas flue. - 12a - figure 5 shows diagrammatically the portion connecting the combustion chamber to the horizontal gas flue, and figure 6 shows a coordinate system with the curves U1 to U4. Parts corresponding to one another are given the same reference symbols in all the figures. The fossil-firable continuous-flow steam generator 2 according to figure 1 is assigned to a power plant, not illustrated in any more detail. - 13 - which also comprises a steam turbine plant. In this case, the continuous-flow steam generator 2 is designed for a steam power output under full load of at least 80 kg/s. The steam generated in the continuous-flow steam generator 2 is in this case utilized for driving the steam turbine which itself, in turn, drives a generator for current generation. The current generated by the generator is in this case intended for feeding into an interconnected or island network. The fossil-fired continuous-flow steam generator 2 comprises a combustion chamber 4 which is designed in a horizontal form of construction and which is followed on the fuel-gas side, via a horizontal gas flue 6, by a vertical gas flue B. The lower region of the combustion chamber 4 is formed by a funnel 5 with a top edge corresponding to the subsidiary line having the end points X and Y. When the continuous-flow steam generator 2 is in operation, ash from the fossil fuel B can be discharged through the funnel 5 into an ash removal device 7 arranged under the latter. The containment walls 9 of the combustion chamber 4 are formed from vertically arranged evaporator tubes 10 which are welded to one another in a gastight manner and the number N of which can be acted upon in parallel by flow medium S. In this case, one containment wall 9 of the combustion chamber 4 is the end wall 11. In addition, the side walls 12 of the horizontal gas flue 6 and 14 of the vertical gas flue 8 are also formed from vertically arranged steam generator tubes 16 and 17 welded to one another in a gastight manner. In this case, a number of the steam generator tubes 16 and 17 can be acted upon in each case in parallel by flow medium S. A number of evaporator tubes 10 of the combustion chamber 4 are, on the flow-medium side, preceded by an inlet header system 18 for flow medium S and followed - 13a - by an outlet header system 20. The inlet header system 18 comprises, in this case, a number of parallel inlet headers. At the same time, a line system 19 is provided for feeding flow medium S into the inlet header system 18 of the evaporator tubes 10. The line system 19 comprises a plurality of parallel- - 14 - connected lines which are in each case connected to one of the inlet headers of the inlet header system 18. In the same way, the steam generator tubes 16, capable of being acted upon in parallel by flow medium S, of the side walls 12 of the horizontal gas flue 6 are preceded by a common inlet header system 21 and followed by a common outlet header system 22. In this case, a line system 19 is likewise provided for feeding flow medium S into the inlet header system 21 of the steam generator tubes 16. Here, too, the line system comprises a plurality of parallel-connected lines which are connected in each case to one of the inlet headers of the inlet header system 21. By virtue of this configuration of the continuous-flow steam generator 2 with inlet header systems 18, 21 and outlet header systems 20, 22, particularly reliable pressure compensation between the parallel-connected evaporator tubes 10 of the combustion chamber 4 and the parallel-connected steam generator tubes 16 of the horizontal gas flue 6 is possible in that in each case all the parallel-connected evaporator or steam generator tubes 10 and 16 have the same overall pressure loss. This means that the throughput must rise in an evaporator tube 10 or steam generator tube 16 heated to a greater extent, as compared with an evaporator tube 10 or steam generator tube 16 heated to a lesser extent. As illustrated in figure 2, the evaporator tubes 10 have a tube inside diameter D and, on their inside, ribs 40 which form a type of multiflight thread and have a rib height C. In this case, the pitch angle a between a plane 42 perpendicular to the tube axis and the flanks 44 of the ribs 40 arranged on the tube - 14a - inside is smaller than 55°. As a result, a particularly high transmission of heat from the inner walls of the evaporator tubes 10 to the flow medium S carried in the evaporator tubes 10 and, at the same time, particularly low temperatures of the tube wall are achieved. - 15 The tube inside diameter D of the evaporator tubes 10 of the combustion chamber 4 is selected as a function of the respective position of the evaporator tubes 10 in the combustion chamber 4. The continuous-flow steam generator 2 is thereby adapted to the different heating of the evaporator tubes 10. This design of the evaporator tubes 10 of the combustion chamber 4 ensures particularly reliably that temperature differences of the flow medium S at the outlet from the evaporator tubes 10 are kept particularly low. Some of the evaporator tubes 10 are equipped, as means for reducing the throughflow of the flow medium S, with throttle devices which are not illustrated in any more detail in the drawing. The throttle devices are designed as perforated diaphragms reducing the tube inside diameter D at one point and, when the continuous-flow steam generator 2 is in operation, have the effect of reducing the throughput of the flow medium S in evaporator tubes 10 heated to a lesser extent, with the result that the throughput of the flow medium S is adapted to the heating. Furthermore, one or more lines of the line system 19, which are not illustrated in any more detail, are equipped with throttle devices, in particular throttle assemblies, as means for reducing the throughput of the flow medium S in the evaporator tubes 10. Adjacent evaporator or steam generator tubes 10, 16, 17 are welded to one another in a gastight manner on their longitudinal sides via fins in a way not illustrated in any more detail in the drawing. To be precise, the heating of the evaporator or steam generator tubes 10, 16, 17 can be influenced by a suitable choice of the fin width. The respective fin width is therefore adapted to a heating profile which is predeterminable on the fuel-gas side and which depends on the position - 16 - a rough estimation. Consequently, even when the evaporator or steam generator tubes 10, 16, 17 are heated to a greatly differing extent, temperature differences at the outlet of the evaporator or steam generator tubes 10, 16, 17 are kept particularly low. Material fatigues as a result of thermal stresses are thereby reliably prevented, thus ensuring that the continuous-flow steam generator 2 has a long useful life. When the horizontal combustion chamber 4 is being fitted with tubes, it must be borne in mind that the heating of the individual evaporator tubes 10 connected to one another in a gastight manner varies greatly when the continuous-flow steam generator 2 is in operation-The design of the evaporator tubes 10 in terms of their internal ribbing, their fin connection to adjacent evaporator tubes 10 and their tube inside diameter D is therefore selected such that, in spite of different heating, all the evaporator tubes 10 have approximately the same outlet temperatures of the flow medium S and sufficient cooling of all the evaporator tubes 10 is ensured for all the operating states of the continuous-flow steam generator 2. A heating of some evaporator tubes 10 to a lesser extent when the continuous-flow steam generator 2 is in operation is in this case taken into account additionally by the fitting of throttle devices. The tube inside diameters D of the evaporator tubes 10 in the combustion chamber 4 are selected as a function of their respective position in the combustion chamber 4 . In this case, the evaporator tubes 10 which are exposed to greater heating when the continuous-flow steam generator 2 is in operation have a larger tube inside diameter D than evaporator tubes 10 which are heated to a lesser extent when the continuous-flow - 16a - steam generator 2 is in operation. What is ensured thereby, as compared with the situation where the tube inside diameters are the same, is that the throughput of the flow medium S is increased in the evaporator tubes 10 with a larger tube inside diameter D and temperature differences at the outlet of the evaporator tubes 10 are thereby reduced as a result of - 17 - different heating, A further measure for adapting the flow of flow medium S through the evaporator tubes 10 to the heating is to fit throttle devices into some of the evaporator tubes 10 and/or into the line system 19 provided for feeding flow medium S. In order, by contrast, to adapt the heating to the throughput of the flow medium S through the evaporator tubes 10, the fin width may be selected as a function of the position of the evaporator tubes 10 in the combustion chamber 4. All the measures mentioned give rise, despite a widely varying heating of the individual evaporator tubes 10, to an approximately identical specific heat absorption of the flow medium S carried in the evaporator tubes 10, when the continuous-flow steam generator 2 is in operation, and therefore to only slight temperature differences of the flow medium S at its outlet. The internal ribbing of the evaporator tubes 10 is in this case designed in such a way that, in spite of different heating and a different throughflow of flow medium S, particularly reliable cooling of the evaporator tubes 10 is ensured in all the load states of the continuous-flow steam generator 2. The horizontal gas flue 6 has a number of superheater heating surfaces 23 which are designed as bulkhead heating surfaces and are arranged in a suspended form of construction approximately perpendicularly to the main direction of flow 24 of the fuel gas G and the tubes of which are in each case connected in parallel for a throughflow of the flow medium S. The superheater heating surfaces 23 are heated predominantly by convection and follow the evaporator tubes 10 of the combustion chamber 4 on the flow-medium side. The vertical gas flue 8 has a number of convection heating surfaces 2 6 which are capable of being heated - 17a - predominantly by convection and are formed from tubes arranged approximately perpendicularly to the main direction of flow 24 of the fuel gas G. These tubes are in each case connected in parallel for a throughflow of the flow medium S. Moreover, an economizer 28 is arranged in the vertical gas flue 8. On the outlet side, the vertical gas flue 8 issues into a further heat exchanger, for example into an 18 air preheater and from there, via a dust filter, to a chimney. The components following the vertical gas flue 8 are not illustrated in any more detail in the drawing. The continuous-flow steam generator 2 is designed with a horizontal combustion chamber 4 of particularly low overall height and can therefore be set up at a particularly low outlay in terms of production and assembly. For this purpose, the combustion chamber 4 of the continuous-flow steam generator 2 has a number of burners 30 for fossil fuel B, which are arranged, level with the horizontal gas flue 6, on the end wall 11 of the combustion chamber 4. The fossil fuel B may in this case be a solid fuel, in particular coal. So that the fossil fuel B, for example coal in solid form, burns up particularly completely in order to achieve particularly high efficiency and material damage to the first superheater heating surface 23 of the horizontal gas flue 6, as seen on the fuel-gas side, and contamination of this surface, for example by the introduction of high-temperature molten ash, are prevented particularly reliably, the length L of the combustion chamber 4 is selected such that it exceeds the burnup length of the fossil fuel B in the full-load mode of the continuous-flow steam generator 2. The length L is in this case the distance from the end wall 11 of the combustion chamber 4 to the inlet region 32 of the horizontal gas flue 6. The burnup length of the fossil fuel B is in this case defined as the fuel-gas velocity in the horizontal direction at a specific average fuel-gas temperature, multiplied by the burnup time tA of the flame F of the fossil fuel B. The maximum burnup length for the respective continuous-flow steam generator 2 is obtained in the full-load mode of the respective continuous-flow steam generator 2. The burnup time tA of the flame F of the fuel B is. - 18a - in turn, the time which, for example, a coaldust grain of average size requires in order to burn up completely at a specific average fuel-gas temperature. - 19 - In order to ensure a particularly beneficial utilization of the combustion heat of the fossil fuel B, the length L {given in m) of the combustion chamber 4 is suitably selected as a function of the outlet temperature TBRK (given in °C) of the fuel gas G from the combustion chamber 4, of the burnup time tA {given in s) of the flame F of the fossil fuel B and of the steam power output M (given in kg/s) of the continuous-flow steam generator 2 under full load. This horizontal length L of the combustion chamber 4 amounts in this case to at least 80% of the height H of the combustion chamber 4. The height H is in this case measured from the top edge of the funnel 5 of the combustion chamber 4, marked in figure 1 by the subsidiary line having the end points X and Y, to the combustion chamber ceiling. The length L of the combustion chamber 4 is determined approximately by the functions (1) and (II): What is to be understood here by approximately is a permissible deviation of the length L of the combustion chamber 4 of +20%/-10% from the value defined by the respective function. In this case, the higher value from the functions (I) and {II) for the length L of the - 19a - combustion chamber 4 applies to the design of the continuous-flow steam generator 2 for a predetermined steam power output M of the continuous-flow steam generator 2 under full load. As an example of a possible design of the continuous-flow steam generator 2, six curves K1 to K6 are plotted - 20 - in the coordinate system according to figure 3 for some lengths L of the combustion chamber 4 as a function of the steam power output M of the continuous-flow steam generator 2 under full load. Here, the curves are in each case assigned the following parameters: K1: tA = 3s according to (I), K2: tA = 2.5s according to (I), K3: tA = 2s according to (I), K4: TBRK = 1200°C according to (II), K5: TBRK ="1300'C according to (II), K6: TBRK = 1400°C according to (II) . Thus, for example for the burnup time tA = 3s of the flame F of the fossil fuel B and the outlet temperature TBRK = 1200°C of the fuel gas G from the combustion chamber 4, curves K1 and K4 are to be used for determining the length L of the combustion chamber 4. This results, in the case of a predetermined steam power output M of the continuous-flow steam generator 2 under full load of M = 80 kg/s, in a length of L = 29 m according to K4, of M = 160 kg/s, in a length of L = 34 m according to K4, of M = 560 kg/s, in a length of L = 57 m according to K4, The curve K4 drawn as an unbroken line is therefore always applicable. For the burnup time tA = 2.5s of the flame F of the fossil fuel B and the outlet temperature of the fuel gas G from the combustion chamber TBRK - 1300°C, it is necessary, for example, to use curves K2 and K5. This - 20a - results, in the case of a predetermined steam power output M of the continuous-flow steam generator 2 under full load of M = 80 kg/s, in a length of L = 21 m according to K2, of M = 180 kg/s, in a length of L = 23 m according to K2 and K5, of M = 560 kg/s, in a length of L = 37 m according to K5. - 21 - Hence, up to M . 180 kg/s, that part of the curve K2 which is drawn as an unbroken line is applicable, not the curve K, drawn as a broken line in this value range of M. For values of M which are higher than 180 kq/s, that part of the curve K5 which is drawn as an unbroken line is applicable, not the curve K2 drawn as a broken line in this value range of M. The burnup time tA = 2s of the flame F of the fossil fuel B and the outlet temperature TBRK = 1400°C of the fuel gas G from the combustion chamber 4 are assigned, for example, the curves K3 and K6. This results, in the case of a predetermined steam power output M of the continuous-flow steam generator 2 under full load of M = 80 kg/s, in a length of L = 18 m according to K3, of M = 465 kg/s, in a length of L = 21 m according to K3 and Kg, of M = 560 kg/s, in a length of L = 23 m according to K5. Hence, the values of M up to 465 kg/s, the curve K3 drawn as an unbroken line in this range is applicable, not the curve K6 drawn as a broken line in this range. For values of M which are higher than 465 kg/s, that part of the curve K6 drawn as an unbroken line is applicable, not the part of the curve K3 drawn as a broken line. So that comparatively small temperature differences occur between the outlet region 34 of the combustion chamber 4 and the inlet region 32 of the horizontal gas flue 5 when the continuous-flow steam generator 2 is in operation, the evaporator tubes 50 and 52 are guided in a particular way in the connecting portion Z marked in figure 1. This connecting portion Z is illustrated in - 21a - detail in an alternative version in figures 4 and 5 and comprises the outlet region 34 of the combustion chamber 4 and the inlet region 32 of the horizontal gas flue 6. In this case, the evaporator tube 50 is an evaporator tube 10, welded directly to the side wall 12 of the horizontal gas flue 6, of the - 22 - containment wall 9 of the combustion chamber 4 and the evaporator tube 52 is an evaporator tube 10, directly adjacent to the evaporator tube 50, of the containment wall 9 of the combustion chamber 4. The steam generator tube 54 is a steam generator tube 16, welded directly to the containment wall 9 of the combustion chamber 4, of the horizontal gas flue 6, and the steam generator tube 56 is a steam generator tube 10, directly adjacent to the steam generator tube 16, of the side wall 12 of the horizontal gas flue 6. According to figure 4, the evaporator tube 50 enters the containment wall 9 of the combustion chamber 4 only above the inlet portion E of this containment wall 9. In this case, the evaporator tube 50 is connected on the inlet side to the economizer 26 via the line system 19. As a result, venting of the evaporator tube 50 before the start-up of the continuous-flow steam generator 2 and therefore a particularly reliable flow through the latter are achieved. The evaporator tube 50 is provided initially for carrying the flow medium S from the top downward. The routing of the evaporator tube 50 then changes through 180° in the immediate vicinity of the inlet header system 18, so that a flow of the flow medium S can then take place in the evaporator tube 50 from the bottom upward. Above the point at which the evaporator tube 50 has entered the containment wall 9 of the combustion chamber 4, the evaporator tube 50 is guided upward in the containment wall 9 so as to be laterally offset by one tube division in the direction of the burners 30. In the last portion, therefore, the evaporator tube 50 is guided in vertical alignment with the first portion of the evaporator tube 50. The steam generator tube 54 of the side wall 12 of the horizontal gas flue 6, after emerging from the inlet - 22a - header system 21, is guided firstly outside the side wall 12 of the horizontal gas flue 6. The steam generator tube 54 enters the side wall 12 of the horizontal gas flue 6 only above the point at which the evaporator tube 50 is guided further along in a laterally offset manner. At the connection 36 between the containment wall 9 of the - 23 - combustion chamber 4 and the side wall 12 of the horizontal gas flue 6, therefore, the lower part belongs to the containment wall 9 of the combustion chamber 4 and the upper part to the side wall 12 of the horizontal gas flue 6. In the same way as the other evaporator tubes 10 and steam generator tubes 16, the evaporator tube 52 and the steam generator tube 56 are guided vertically in the containment wall 9 of the combustion chamber 4 and in the side wall 12 of the horizontal gas flue 6 respectively and are connected on the inlet side to the inlet header system 18 and 21 and on the outlet side to the outlet header system 20 and 22. Another possible embodiment of the portion Z connecting the containment wall 9 of the combustion chamber 4 to the side wall 12 of the horizontal gas flue 6 is illustrated in figure 5. Here, the evaporator tube 50, connected to the economizer 26 on the inlet side via the line system 19, enters the containment wall 9 of the combustion chamber 4, so as to be laterally offset by one tube division, above the inlet portion E. What is meant here by laterally offset by one tube division is that the entry of the evaporator tube 50 into the containment wall 9 of the combustion chamber 4 takes place at a distance of one tube layer from the connection 36 of the combustion chamber 4 to the horizontal gas flue 6, The routing of the evaporator tube 50 changes through 90° in the immediate vicinity of the inlet header system 18, and the evaporator tube 50 is routed outside the containment wall 9 of the combustion chamber 4 in the direction of the side wall 12 of the horizontal gas flue 6. Before entry into the side wall 12 of the horizontal gas flue 6, the routing of the evaporator tube 50 changes again through 90° in the direction of the outlet header system 22. The - 23a - evaporator tube 50 is in this case guided vertically in the side wall 12 of the horizontal gas flue 6 at a distance of one tube layer from the connection 36 of the combustion chamber 4 to the horizontal gas flue 6. In the side wall 12 of the horizontal gas flue 6, a change of direction of the evaporator tube 50 in the vertical direction takes place again, laterally offset by one tube layer, below the entry of the evaporator tube 50 into the containment wall 9 of the combustion chamber 4, so that the evaporator IS directly adjacent to the connection- 36 of the combustion chamber 4 to the horizontal gas flue 6, Above the level of entry of the evaporator tube 50 into the containment wall 9 of the combustion chamber 4, a change in the routing of the evaporator tube 50 takes place once again, specifically from the side wall 12 of the horizontal gas flue 6 into the containment wall 9 of the combustion chamber 4. In the containment wall 9 of the combustion chamber 4, the evaporator tube 50 is then guided, in its last portion, vertically along the connection 36 of the combustion chamber 4 to the horizontal gas flue 6 towards the outlet header system 20. The routing of the evaporator tube 52 in this case matches the routing of the evaporator tube 50. The evaporator tube 52 enters the containment wall 9 of the combustion chamber 4 below the entry of the evaporator tube 50 and is connected to the economizer 28 on the inlet side by the line system 19. The entry of the evaporator tube 52 takes place, in this case, in the tube layer which is adjacent to the connection 36 of the combustion chamber 4 to the horizontal gas flue 6. After the evaporator tube 52 enters the containment wall 9 of the combustion chamber 4, the evaporator tube 52 is guided vertically from the top downward. A change in the routing of the evaporator tube 52 through 900 in the direction of the side wall 12 of the horizontal gas flue 6 takes place in the immediate vicinity of the inlet header system 18. It changes its directxon once again through 900 , level wrth the first tube layer which is adjacent to the connection 36 of the combustion chamber 4 to th. horizontal gas flue 6, and enters the side wall 12 of the horizontal gas flue 6 From this level, the evaporator tube 52 is guided vertically in the side wall U of the horizontal gas flue 6 It therefore forms the connecting tube of the - 24a - side wall 12 of the horizontal gas flue 6 to the containment wall 9 of the combustion chamber 4. The evaporator tube 52 leaves the side wall 12 of the horizontal gas flue 6 above the level of entry of the evaporator tube 52 into the containment wall 9 of the combustion chamber 4, in order to be guided in the vertical direction above the entry of the evaporator tube 52 in the containment wall 9 of the combustion chamber 4, specifically in vertical - 25 - alignment with the entry of the evaporator tube 52, Above the entry of the evaporator tube 50 into the containment wall 9 of the combustion chamber 4, the routing of the evaporator tube 52 changes once again, in order then to be guided vertically in the containment wall 9 of the combustion chamber 4 in vertical alignment with the first portion of the evaporator tube 50. The last portion of the evaporator tube 52 is therefore guided in vertical alignment with the first portion of the evaporator tube 50. Both the evaporator tube 50 and the evaporator tube 52 are connected on the inlet side to the line system 19 between the economizer 28 and the inlet header system 18 and on the outlet side to the outlet header system 20. The steam generator tube 52 is connected on the inlet side to the inlet header system 21. After the steam generator tube 54 emerges from the inlet header system 21, the steam generator tube 54 is guided outside the horizontal gas flue 6. Above the change of the evaporator tube 50 from the side wall 12 of the horizontal gas flue 6 into the containment wail 9 of the combustion chamber 4, the steam generator tube 54 enters the side wall 12 of the horizontal gas flue 6. The last portion of the steam generator tube 54, said portion being guided in the side wall 12 of the horizontal gas flue 6, is in this case guided along the connection 36 of the combustion chamber 4 to the horizontal gas flue 6, The side wall 12 of the horizontal gas flue 6 is therefore formed at the connection 36 by the evaporator tube 50 in the lower part and by the steam generator tube 54 in the upper part. The steam generator tube 56 is also connected to the inlet header system 21 on the inlet side in figure 5. - 25a - The steam generator tube 56 is first guided outside the horizontal gas flue 6. The steam generator tube 56 enters the side wall 12 of the horizontal gas flue 6 only above the point at which the evaporator tube 50 has changed its routing from being offset by one tube layer to the connection 36 to a routing which is directly adjacent to the connection 36. The steam generator tubes 54 - 26- and 56 are in each case connected to the outlet header system 22 on the outlet side. By virtue of the special tube routing of the evaporator tubes 50 and 52 and of the steam generator tubes 54 and 56, when the continuous-flow steam generator 3 is in operation temperature differences at the connection 36 between the combustion chamber 4 and the horizontal gas flue 6 are kept particularly low in a particularly reliable way. The flow medium S, and therefore also the evaporator tube 50 or 52, enters the containment wall 9 of the combustion chamber 4 above the entry portion E. The further tube routing of the evaporator tubes 50 and 52 and of the steam generator tubes 54 and 56 then takes place in such a way that, when the continuous-flow steam generator 2 is in operation, the evaporator tubes 50 and 52 and therefore also the flow medium S carried in them are preheated by heating, before a direct connection to the steam generator tubes 54, 56 and to a further steam generator tube 16 of the side wall 12 of the horizontal gas flue 6 takes place. As a result, when the continuous-flow steam generator' 2 is in operation, the evaporator tubes 50 and 52 have at the connection 3 6 a comparatively higher temperature than the evaporator tubes 10 of the containment wall 9 of the combustion chamber 4 which are directly adjacent to them. As an example of possible temperatures Ts of the flow medium S in the evaporator tubes 10 of the combustion chamber 4, and in the steam generator tubes 16 of the horizontal gas flue 6, the curves U1 to U4 are plotted, for the exemplary embodiment according to figure 5, in the coordinate system according to figure 6 for some temperatures TS (given in °C) as a function of the relative tube length R of that part of an evaporator - 26a - tube 10, 50, 52 or of the steam generator tubes 54, 5 6 through which the flow passes from the bottom upward {given in %). In this case, the horizontally routed region, that is to say the steps, is not taken into account in the curves shown. U1 describes, here, the temperature profile of a steam generator tube 16 of the horizontal gas flue 6. By contrast, U2 describes a temperature profile of an evaporator tube 10 - 27 - along its relative tube length R. U3 describes the temperature profile of that part of the specially routed evaporator tube 50 through which the flow passes from the bottom upward, and U4 describes the temperature profile of that part of the evaporator tube 52 of the containment wall 9 of the combustion chamber 4 through which the flow passes from the bottom upward. It becomes clear from the curves depicted that, owing to the special tube routing of the evaporator tubes 50 and 52 in the entry portion E of the evaporator tubes 10 in the containment wall 9 of the combustion chamber 4, the temperature difference from the steam generator tubes 16 of the containment wall 9 of the horizontal gas flue can be markedly reduced. In the example, the temperature of the evaporator tubes 50 and 52 in the entry portion E of the evaporator tubes 50 and 52 can be increased by 45 Kelvin. As a result, when the continuous-flow steam generator 2 is in operation, particularly low temperature differences in the entry portion E of the evaporator tubes 50 and 52 and in the steam generator tubes 16 of the horizontal gas flue 6 at the connection 3 6 between the combustion chamber 4 and the horizontal gas flue 6 are ensured. When the continuous-flow steam generator 2 is in operation, fossil fuel B, preferably coal in solid form, is fed to the burners 30. The flames F of the burners 30 are in this case oriented horizontally. Due to the form of construction of the combustion chamber 4, a flow of the fuel gas G occurring during combustion is generated in the approximately horizontal main direction of flow 24. This passes via the horizontal gas flue 6 into the vertical gas flue 8 oriented approximately toward the ground and leaves the latter in the direction of the chimney which is no illustrated in any more detail. - 27a - Flow medium S entering the economizer 28 passes into the inlet header system 18 of the evaporator tubes 10 of the combustion chamber 4 of the continuous-flow steam generator 2. In the vertically arranged evaporator tubes 10 of the combustion chamber 4 of the continuous-flow steam generator 2 which are welded to one another in a gastight manner, evaporation and, if appropriate, partial - 28 - superheating of the flow medium S take place. The steam or a water/steam mixture occurring at the same time is collected in the outlet header system 2 0 for flow medium S. The steam or the water/steam mixture passes from there, via the walls of the horizontal gas flue 6 and of the vertical gas flue 8, into the superheater heating surfaces 23 of the horizontal gas flue 6. In the superheater heating surfaces 23, further superheating of the steam takes place, the latter subsequently being fed for utilization, for example to the drive of a steam turbine. By means of the special routing of the evaporator tubes 50 and 52, the temperature differences between the outlet region 34 of the combustion chamber 4 and the inlet region 32 of the horizontal gas flue 6 are particularly low when the continuous-flow steam generator is in operation. At the same time, a choice of the length L of the combustion chamber 4 as a function of the steam power output M of the continuous-flow steam generator 2 under full load ensures that the combustion heat of the fossil fuel B is utilized particularly reliably. Moreover, by virtue of its particularly low overall height and compact form of construction, the continuous-flow steam generator 2 can be set up at a particularly low outlay in terms of production and assembly. In this case, a framework capable of being erected at a comparatively low technical outlay can be provided. In a power plant with a steam turbine and with a continuous-flow steam generator 2 having such a small overall height, moreover, the connecting pipes from the continuous-flow steam generator to the steam turbine can be made particularly short. - 29 - Patent Claims A continuous-flow steam generator (2) with a combustion chamber (4) for fossil fuel (B) , which is followed on the fuel-gas side, via a horizontal gas flue (6) , by a vertical gas flue (8) , the combustion chamber (4) having a number of burners (58) arranged level with the horizontal gas flue (6), and the' containment walls (9) of the combustion chamber {4) being formed from vertically arranged evaporator tubes (10) welded to one another in a gastight manner, a plurality of the "evaporator tubes (10) being capable of being acted upon in each case in parallel by flow medium (S) , characterited in that a number of the evaporator tubes (10, 50, 52) capable of being acted upon in parallel by flow medium (S) being guided in the form of a loop in a connecting portion (Z) which comprises the outlet region (34) of the combustion chamber (4) and the inlet region (32) of the horizontal gas flue (6). The continuous-flow steam generator (2) as claimed in claim 1, in which the side walls (12) of the horizontal gas flue (6) are formed from vertically arranged steam generator tubes (16) welded to one another in a gastight manner and capable of being acted upon in parallel by flow medium (S). The continuous-flow steam generator (2) as claimed in claim 1 or 2, in which the side walls (14) of the vertical gas flue (8) are formed from vertically arranged steam generator tubes (17) welded to one another in a gastight manner and capable of being acted upon in parallel by flow medium (S). - 29a - The continuous-flow steam generator (2) as claimed in one of claims 1 to 3, in which in each case a plurality of evaporator tubes (10) capable of being acted upon in parallel by flow medium (S) , on the flow-medium side, are preceded by a common inlet header system (18) and are followed by a common outlet header system (20). - 30 - The continuous-flow steam generator (2) as claimed in one of claims 1 to 4, in which in each case a number of steam generator tubes (16, 17) of the horizontal gas flue (6) or of the vertical gas flue (8) which are capable of being acted upon in parallel by flow medium (S) , on the flow-medium side, are preceded by a common inlet header system (21) and are followed by a common outlet header system (22). The continuous-flow steam generator (2) as claimed in one of claims 1 to 5, in which one containment wall (9) of the combustion chamber (4} is the end wall (11), the evaporator tubes (10) of the end wall (9) being capable of being acted upon in parallel by flow medium (S). The continuous-flow steam generator (2) as claimed in one of claims 1 to 6, in which the evaporator tubes (10) of the end wall (11) of the combustion chamber {4) precede the other containment walls (9) of the combustion chamber (4) on the flow- medium side. The continuous-flow steam generator (2) as claimed in one of claims 1 to 7, in which the tube inside diameter (D) of a number of the evaporator tubes (10) of the combustion chamber (4) is selected as a function of the respective position of the evaporator tubes (10) in the combustion chamber (4) . The continuous-flow steam generator (2) as claimed in one of claims 1 to 8, in which a number of the evaporator tubes (10) carry in each case, on their inside, ribs (40) which form a multiflight thread. - 31 - The continuous-flow steam generator (2) as claimed in claim 9, in which a pitch angle (a) between a plane (42) perpendicular to the tube axis and the flanks (44) of the ribs (40) arranged on the tube inside is smaller than 60°, preferably smaller than 55°. The continuous-flow steam generator (2) as claimed in one of claims 1 to 10, in which a number of the evaporator tubes (10) have in each case a throttle device. The continuous-flow steam generator (2) as claimed in one of claims 1 to 11, in which a line system (19) for feeding flow medium (S) into the evaporator tubes (10)- of the combustion chamber (4) is provided, the line system (19) ' having a number of throttle devices, in particular throttle assemblies, in order to reduce the throughflow of the flow medium (S). The continuous-flow steam generator (2) as claimed in one of claims 1 to 12, in which adjacent evaporator or steam generator tubes (10, 16, 17) are welded to one another in a gastight manner via fins, the fin width being selected as a function of the respective position of the evaporator or steam generator tubes (10, 16, 17) in the combustion chamber (4), the horizontal gas flue (6) and/or the vertical gas flue (8). The continuous-flow steam generator (2) as claimed in one of claims 1 to 13, in which a number of superheater heating surfaces (23) are arranged in a suspended form of construction in the horizontal gas flue (6). - 31a - The continuous-flow steam generator (2) as claimed in one of claims 1 to 14, - 32 - in which a number of convection heating surfaces (26) are arranged in the vertical gas flue (8). The continuous-flow steam generator (2) as claimed in one of claims 1 to 15, in which the burners (58) are arranged on the end wall (11) of the combustion chamber (4). The continuous-flow steam generator (2) as claimed in one of claims 1 to 16, in which the length L of the combustion chamber (4) , defined by the distance from the end wall (11) of the combustion chamber (4) to the inlet region (32) of the horizontal gas flue (6), is at least equal to the burnup length of the fuel (B) in the full-load mode. The continuous-flow steam generator {2} as claimed in one of claims 1 to 17, in which the length (L) of the combustion chamber (4) is selected as a function of the steam power output (M) under full load, of the burnup time (tA) , of the flame (F) of the fuel (B) and/or of the outlet temperature (TBRK) of the fuel gas (G) from the combustion chamber (4) approximately according to the two functions (I) and (II) - 32a - C5 = 3 - 10-4 m/(°C)2 and C6 = -0.842 m/°C and C7 = 603.41 m. - 33 the in each case higher value of the length (L) of the combustion chamber (4) being applicable for a predetermined steam power output (M) under full load. The continuous-flow steam generator (2) as claimed in one of claims 1 to 18, in which the lower region of the combustion chamber {4) is designed as a funnel (5) . A continuous-flow steam generator (2) has a combustion chamber (4) with evaporator tubes (10) for fossil fuel (B) , which is followed on the fuel-gas side, via a horizontal gas flue (6) , by a vertical gas flue (8) . When the continuous-flow steam generator is in operation, temperature differences in a connecting portion (2), which comprises the outlet region (34) of the combustion chamber (4) and the inlet region (32) of the horizontal gas flue (6) , are to be kept particularly low. For this purpose, of a plurality of evaporator tubes (10) capable of being acted upon in parallel by flow medium (S) , a number of evaporator tubes (10) are guided in the form of a loop in the connecting portion (Z).

Full Text

Description
Fossil-fired continuous-flow steam generator
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.
In a power plant with a steam generator, the energy content of a fuel is utilized for evaporating a flow medium in the steam generator. In this case, the flow medium is normally 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. If the steam drives a steam turbine, a generator or a working machine is usually 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 context, be designed as a continuous-flow steam generator. A continuous-flow steam generator is known from the paper "Verdampferkonzepte fur Benson-Dampferzeuger" ["Evaporator concepts for Benson steam generators"] by J. Franke, W. Kohler and E. Wittchow, published in VGB Kraftwerkstechnik 73 (1993), No. 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.
- la-
Continuous-flow steam generators are conventionally designed with a combustion chamber in a vertical form of construction. This means that
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the combustion chamber is designed for the heating medium or fuel gas to flow through in an 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, in general, because of the thermally induced changes in length of the combustion chamber, combustion chambers of this type require a framework on which the combustion chamber is suspended. This necessitates a considerable technical outlay in terms of the production and assembly of the continuous-flow steam generator, this outlay being the greater, the greater the overall height of the continuous-flow steam generator is. This is true particularly with regard to 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 (pcri = 221 bar) , where there is still only a slight density difference between the liquid-like and steam-like media, are possible. A high fresh-steam pressure is conducive to high thermal efficiency and therefore to low CO2 emissions of a fossil-fired power station which can be fired, for example, 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 down to about
- 2a -
200 bar, the temperature of the containment wall of the combustion chamber is determined essentially by the height of the saturation temperature of the water, when wetting of the inner surface of the evaporator tubes can be ensured. This is achieved, for example, by using evaporator tubes which have a surface structure on their inside. In this respect, consideration is given, in particular.
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to internally ribbed evaporator tubes, of which the use in a continuous-flow steam generator is known, for example, from the paper quoted above. 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 evaporator tubes of the outlet region of the combustion chamber and steam generator tubes of the inlet region of the horizontal gas flue. These thermal stresses can markedly reduce the useful life of the continuous-flow steam. generator and in an extreme case may even give rise to tube cracks.
The object on which the invention is based is to specify a fossil-fired continuous-flow steam generator of the abovementioned type, which requires a particularly low outlay in terms of production and assembly and, moreover, during the operation of which temperature differences at the connection of the combustion chamber to the horizontal gas flue following it are kept low. This is to apply particularly to 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 is achieved, according to the invention, in that the continuous-flow steam generator has a
-3a -
combustion chamber v;ith a number of burners arranged level with the horizontal gas flue, a plurality of the evaporator tubes being capable of being acted upon in each case in parallel by flow medium, and a number of the evaporator tubes capable of being acted upon in parallel by flow medium being led in the form of a loop
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in a connecting portion which comprises the outlet region of the combustion chamber and the inlet region of the horizontal gas flue.
The invention proceeds from the notion that a continuous-flow steam generator capable of being set up at a particularly low outlay in terms of production and assembly should have a suspension structure capable of being executed by simple means. At the same time, a framework capable of being set up at a comparatively low technical outlay for the suspension of the combustion chamber can be accompanied by a particularly low overall height of the continuous-flow steam generator. A particularly low 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 outlet region of the combustion chamber and in the inlet region of the horizontal gas flue.
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However, when the continuous-flow steam generator is in operation, the inlet portion of the evaporator tubes which is acted upon by flow medium has a comparatively lower temperature than the inlet portion of the steam generator tubes of the horizontal gas flue following the combustion chamber. To be precise, comparatively cold flow medium enters the evaporator tubes,
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in contrast to the hot flow medium which enters the steam generator tubes of the horizontal gas flue. Hence, when the continuous-flow steam generator is in operation, the evaporator tubes are colder in the inlet portion than the steam generator tubes in the inlet portion of the horizontal gas flue. Consequently, material fatigues as a result of thermal stresses are to be expected at the connection between the combustion chamber and the horizontal gas flue.
However, if, then, preheated flow medium enters the inlet portion of the evaporator tubes of the combustion chamber, instead of cold flow medium, the temperature difference between the inlet portion of the evaporator tubes and the inlet portion of the steam generator tubes will no longer be as great as would be the case if cold flow medium were to enter the evaporator tubes. If, therefore, the flow medium is led first in a first
evaporator tube, which is arranged further away.from
the connection of the combustion chamber to. the
horizontal gas flue than a second eyaporator tube,and
is then introduced into this second eyaporator tube,
flow medium preheated by firing enters the second
evaporator tube when the continuous flow steam generator is in operation. The complicated connection between a first and a second evaporator , tube may be dispensed with if one evaporator tube has an inlet for flow medium in the middle of the containment wall of the combustion chamber. For, then, this evaporator tube can be led first from the top downward and then from the bottom upward in the combustion chamber. Consequently, when the continuous-flow steam generator is in operation, firing causes a preheating of the flow medium to take place in that portion of the evaporator tube which is led from the top downward, before the flow medium enters the so-called inlet portion of the evaporator tubes in the lower region of the combustion
- 5a -
chamber. It proves to be particularly beneficial, at the same time, if a number of the evaporator tubes capable of being acted upon in parallel by flow medium are led in the form of a loop in the respective containment wall of the combustion chamber.
- 6 -
The side walls of the horizontal gas flue and/or of the vertical gas flue are advantageously formed from vertically arranged steam generator tubes welded to one another in a gastight manner and capable of being acted upon in each case in parallel by flow medium.
Advantageously, in each case, a number of parallel-
connected evaporator tubes of the combustion chamber
are preceded by a common inlet header system and
followed by a common outlet header system for flow
medium.To be precise , a continous -flow steam
generator designed in this configuration allows reliable pressure compensation between a number of evaporator tubes capable of being acted upon in parallel by flow medium, so that, in each case, all parallel-connected evaporator tubes between the inlet header system and the outlet header system have the same overall pressure loss. This means that, in the case of an evaporator tube heated to a greater extent, the throughput must rise, as compared with an evaporator tube heated to a lesser extent. This also applies to the steam generator tubes of the horizontal gas flue or of the vertical gas flue which are capable of being acted upon in parallel by flow medium and which are advantageously preceded by a common inlet header system for flow medium and followed by a common outlet header system for flow medium.
The evaporator tubes of the end wall of the combustion chamber are advantageously capable of being acted upon in parallel by flow medium and precede the evaporator tubes of the containment walls, which form the side walls of the combustion chamber, on the flow-medium side. This ensures particularly favorable cooling of the highly heated end wall of the combustion chamber.
- 6a -
In a further advantageous refinement of the invention, the tube inside diameter of a number of the evaporator tubes of the combustion chamber is selected as a function of the respective position of the evaporator tubes in the combustion chamber. The evaporator tubes in the combustion chamber can thereby be adapted to a heating profile
- 7 -
predeteminable on the fuel-gas .side. By means of the influence brought about thereby on the flow through the evaporator tubes, temperature differences of the flow medium at the outlet from the evaporator tubes of the combustion chamber are kept particularly low in a particularly reliable way.
For particularly good heat transmission from the heat of the combustion chamber to the flow medium carried in the evaporator tubes, a number of evaporator tubes advantageously have in each case, on their inside, ribs which form a multiflight thread. In this case, advantageously, a pitch angle a between a plane perpendicular to the tube axis and the flanks of the ribs arranged on the tube inside is smaller than 60%
unforable smaller than 55°.
To be precise, in a heated evaporator tube designed as an evaporator tube without internal ribbing, a so-called smooth tube, the wetting of the tube wall, necessary for particularly good heat transmission, can no longer be maintained from a specific steam content onward. With a lack of wetting, there may be a tube wall which is dry in places. The transition to a dry tube wall of this kind leads to a so-called heat transmission crisis with an impaired heat transmission behavior, so that, in general, the tube wall temperatures rise particularly sharply at this point. In an internally ribbed evaporator tube, however, as compared with a smooth tube, this heat transmission crisis arises only in the case of a steam mass content > 0.9, that is to say just before the end of evaporation. This is attributable to the swirl which the flow experiences due to the spiral ribs. On account of the different centrifugal force, the water fraction is separated from the steam fraction and is transported to the tube wall. The wetting of the tube wall is thereby maintained up to high steam contents, so that
- 7a -
there are already high flow velocities at the location of the heat transmission crisis. This gives rise, despite the heat transmission crisis, to relatively good heat transmission and, consequently, to low tube wall temperatures.
- 8 -
A number of evaporator tubes of the combustion chamber advantageously have means for reducing the throughflow of the flow medium. In this case, it proves particularly beneficial if the means are designed as throttle devices. Throttle devices may, for example, be fittings in the evaporator tubes, which reduce the tube inside diameter at a point within the respective evaporator tube. At the same time, means for reducing the throughflow in a line system which comprises a plurality of parallel lines and through which flow medium can be fed to the evaporator tubes of the combustion chamber also prove to be advantageous. In this case, the line system may also precede an inlet header system of evaporator tubes capable of being acted upon in parallel by flow medium. In such cases, for example, throttle assemblies may be provided in one line or in a plurality of lines in the line system. Such means for reducing the throughflow of the flow medium through the evaporator tubes make it possible to adapt the throughput of the flow medium through individual evaporator tubes to the respective heating of these in the combustion chamber. As a result, in addition, temperature differences of the flow medium at the outlet of the evaporator tubes are kept particularly low in a particularly reliable way.
Adjacent evaporator or steam generator tubes are welded to one another in a gastight manner on their longitudinal sides advantageously via metal bands, so-called fins. These fins can be connected fixedly to the tubes even during the tube production process and can form a unit with these. This unit formed from a tube and fins is also designated as finned tube. The fin width influences the introduction of heat into the evaporator or steam generator tubes. The fin width is therefore adapted to a heating profile predeterminable on the flow-gas side, preferably as a function of the
- 8a -
position of the respective evaporator or steam generator tubes in the continuous-flow steam generator. The heating profile predetermined in this case may be a typical heating profile determined from experimental values
- 9 -
or else a rough estimation, such as, for example, a stepped heating profile. By means of the suitably selected fin widths, even when different evaporator or steam generator tubes are heated to a widely differing extent, an introduction of heat into all the evaporator or steam generator tubes can be achieved, in such a way that temperature differences of the flow medium at the outlet from the evaporator or steam generator tubes are kept particularly low. Premature material fatigues as a result of thermal stresses are reliably prevented in this way. As a result, the continuous-flow steam generator has a particularly long useful life.
The horizontal gas flue advantageously has arranged in it a number of superheater heating surfaces which are arranged approximately perpendicularly to the main direction of flow of the fuel gas and the tubes of which are connected in parallel for the throughflow of the flow medium. These superheater heating surfaces, arranged in a suspended form of construction and also designated as bulkhead heating surfaces, are heated predominantly by convection and follow the evaporator tubes of the combustion chamber on the flow-medium side. A particularly favorable utilization of the fuel-gas heat is thereby ensured.
Advantageously, the vertical gas flue has a number of convection heating surfaces which are formed from tubes arranged approximately perpendicularly to the main direction of flow of the fuel gas. These tubes of a convection heating surface are connected in parallel for a throughflow of the flow medium. These convection heating surfaces, too, are heated predominantly by convection.
In order, furthermore, to ensure the particularly full utilization of the heat of the fuel gas, the vertical gas flue advantageously has an economizer.
- 9a -
Advantageously, the burners are arranged on the end wall of the combustion chamber, that is to say on that side wall of the
- 10 -
combustion chamber which is located opposite the outflow orifice to the horizontal gas flue. A continuous-flow steam generator designed in this way can be adapted particularly simply to the burnup length of the fossil fuel. The burnup length of the fossil fuel refers, in this context, to the fuel-gas velocity in the horizontal direction at a specific average fuel-gas temperature, multiplied by the burnup time tA of the flame of the fossil fuel. The maximum burnup length for the respective continuous-flow steam generator is obtained, in this case, from the steam power output M under the full load of the continuous-flow steam generator, the so-called full-load mode. The burnup time tA of the flame of the fossil fuel is, in turn, the time which, for example, a coaldust grain of average size requires in order to burn up completely at a specific average fuel-gas temperature.
Advantageously, the lower region of the combustion chamber is designed as a funnel. In this way, when the continuous-flow steam generator is in operation, ash occurring during the combustion of the fossil fuel can be discharged particularly simply, for example into an ash removal device arranged under the funnel. The fossil fuel may in this case be coal in solid form.
In order to keep material damage and undesirable contamination of the horizontal gas flue, for example due to the introduction of high-temperature molten ash, particularly low, the length of the combustion chamber, defined by the distance from the end wall to the inlet region of the horizontal gas flue, is advantageously at least equal to the burnup length of the fossil fuel in the full-load mode of the continuous-flow steam generator. This horizontal length of the combustion chamber will generally amount to at least 80°of the height of the combustion chamber, measured from the
- 10a -
funnel top edge, when the lower region of the combustion chamber has a funnel-shaped design, to the combustion chamber ceiling.
- 11 -
For a particularly beneficial utilization of the combustion heat of the fossil fuel, the length L (given in m) of the combustion chamber is selected as a function of the steam power output M (given in kg/s) of the continuous-flow steam generator under full load, of the burnup time tA (given in s) of the flame of the fossil fuel and of the outlet temperature tBRK (given in 0C) of the fuel gas from the combustion chamber. In this case, with the given steam power output M of the continuous-flow steam generator' under full load, approximately the higher value of the two functions (I) and (II) applies to the length L of the combustion chamber:
C1 = 8 m/s and
C2 = 0.0057 m/kg and
C3 = -1.905 -10-4 (m • s)/(kg0C) and
C4 = 0.286 (s . m)/kg and
C5 - 3 . 10-' m/(°C)2 and
C6 = -0.842 m/°C and
C7 = 603.41 m.
By "approximately" is to be meant, here, a permissible deviation in the length L of the combustion chamber of +20%/-10% from the value defined by the respective function.
The advantages achieved by means of the invention are, in particular, that, by some evaporator tubes being guided in the form of a loop in the containment wall of
- 11a -
the combustion chamber, temperature differences in the immediate vicinity of the connection of the combustion chamber to the horizontal gas flue when the continuous-flow steam generator is in operation are particularly low. Consequently, when the continuous-flow steam generator is in operation, the thermal stresses at the connection of the combustion chamber to the horizontal gas flue
- 12 -
which are caused by temperature differences between directly adjacent evaporator tubes of the combustion chamber and steam generator tubes of the horizontal gas flue remain well below the values at which, for example, there is a risk of pipe cracks. It is therefore possible to use a horizontal combustion chamber in a continuous-flow steam generator, even at the same time with a comparatively long useful life. Moreover, designing the combustion chamber for an approximately horizontal main direction of flow of the fuel gas affords a particularly compact form of construction of the continuous-flow steam generator. This makes it possible, when the continuous-flow steam generator is incorporated into a power station with a steam turbine, also to have particularly short connecting pipes from the continuous-flow steam generator to the steam turbine. .
An exemplary embodiment of the invention is explained in more detail by means of a drawing in which:
figure 1 shows diagrammatically a side view of a fossil-fired continuous-flow steam generator of the dual-flue type, and
figure 2 shows diagrammatically a longitudinal section through an individual evaporator tube,
figure 3 shows a coordinate system with the curves K1 to K6,
figure 4 shows diagrammatically the portion connecting the combustion chamber to the horizontal gas flue.
- 12a -
figure 5 shows diagrammatically the portion connecting the combustion chamber to the horizontal gas flue, and
figure 6 shows a coordinate system with the curves U1 to U4.
Parts corresponding to one another are given the same reference symbols in all the figures.
The fossil-firable continuous-flow steam generator 2 according to figure 1 is assigned to a power plant, not illustrated in any more detail.
- 13 -
which also comprises a steam turbine plant. In this case, the continuous-flow steam generator 2 is designed for a steam power output under full load of at least 80 kg/s. The steam generated in the continuous-flow steam generator 2 is in this case utilized for driving the steam turbine which itself, in turn, drives a generator for current generation. The current generated by the generator is in this case intended for feeding into an interconnected or island network.
The fossil-fired continuous-flow steam generator 2 comprises a combustion chamber 4 which is designed in a horizontal form of construction and which is followed on the fuel-gas side, via a horizontal gas flue 6, by a vertical gas flue B. The lower region of the combustion chamber 4 is formed by a funnel 5 with a top edge corresponding to the subsidiary line having the end points X and Y. When the continuous-flow steam generator 2 is in operation, ash from the fossil fuel B can be discharged through the funnel 5 into an ash removal device 7 arranged under the latter. The containment walls 9 of the combustion chamber 4 are formed from vertically arranged evaporator tubes 10 which are welded to one another in a gastight manner and the number N of which can be acted upon in parallel by flow medium S. In this case, one containment wall 9 of the combustion chamber 4 is the end wall 11. In addition, the side walls 12 of the horizontal gas flue 6 and 14 of the vertical gas flue 8 are also formed from vertically arranged steam generator tubes 16 and 17 welded to one another in a gastight manner. In this case, a number of the steam generator tubes 16 and 17 can be acted upon in each case in parallel by flow medium S.
A number of evaporator tubes 10 of the combustion chamber 4 are, on the flow-medium side, preceded by an inlet header system 18 for flow medium S and followed
- 13a -
by an outlet header system 20. The inlet header system 18 comprises, in this case, a number of parallel inlet headers. At the same time, a line system 19 is provided for feeding flow medium S into the inlet header system 18 of the evaporator tubes 10. The line system 19 comprises a plurality of parallel-
- 14 -
connected lines which are in each case connected to one of the inlet headers of the inlet header system 18.
In the same way, the steam generator tubes 16, capable of being acted upon in parallel by flow medium S, of the side walls 12 of the horizontal gas flue 6 are preceded by a common inlet header system 21 and followed by a common outlet header system 22. In this case, a line system 19 is likewise provided for feeding flow medium S into the inlet header system 21 of the steam generator tubes 16. Here, too, the line system comprises a plurality of parallel-connected lines which are connected in each case to one of the inlet headers of the inlet header system 21.
By virtue of this configuration of the continuous-flow steam generator 2 with inlet header systems 18, 21 and outlet header systems 20, 22, particularly reliable pressure compensation between the parallel-connected evaporator tubes 10 of the combustion chamber 4 and the parallel-connected steam generator tubes 16 of the horizontal gas flue 6 is possible in that in each case all the parallel-connected evaporator or steam generator tubes 10 and 16 have the same overall pressure loss. This means that the throughput must rise in an evaporator tube 10 or steam generator tube 16 heated to a greater extent, as compared with an evaporator tube 10 or steam generator tube 16 heated to a lesser extent.
As illustrated in figure 2, the evaporator tubes 10 have a tube inside diameter D and, on their inside, ribs 40 which form a type of multiflight thread and have a rib height C. In this case, the pitch angle a between a plane 42 perpendicular to the tube axis and the flanks 44 of the ribs 40 arranged on the tube
- 14a -
inside is smaller than 55°. As a result, a particularly high transmission of heat from the inner walls of the evaporator tubes 10 to the flow medium S carried in the evaporator tubes 10 and, at the same time, particularly low temperatures of the tube wall are achieved.
- 15
The tube inside diameter D of the evaporator tubes 10 of the combustion chamber 4 is selected as a function of the respective position of the evaporator tubes 10 in the combustion chamber 4. The continuous-flow steam generator 2 is thereby adapted to the different heating of the evaporator tubes 10. This design of the evaporator tubes 10 of the combustion chamber 4 ensures particularly reliably that temperature differences of the flow medium S at the outlet from the evaporator tubes 10 are kept particularly low.
Some of the evaporator tubes 10 are equipped, as means for reducing the throughflow of the flow medium S, with throttle devices which are not illustrated in any more detail in the drawing. The throttle devices are designed as perforated diaphragms reducing the tube inside diameter D at one point and, when the continuous-flow steam generator 2 is in operation, have the effect of reducing the throughput of the flow medium S in evaporator tubes 10 heated to a lesser extent, with the result that the throughput of the flow medium S is adapted to the heating.
Furthermore, one or more lines of the line system 19, which are not illustrated in any more detail, are equipped with throttle devices, in particular throttle assemblies, as means for reducing the throughput of the flow medium S in the evaporator tubes 10.
Adjacent evaporator or steam generator tubes 10, 16, 17 are welded to one another in a gastight manner on their longitudinal sides via fins in a way not illustrated in any more detail in the drawing. To be precise, the heating of the evaporator or steam generator tubes 10, 16, 17 can be influenced by a suitable choice of the fin width. The respective fin width is therefore adapted to a heating profile which is predeterminable on the fuel-gas side and which depends on the position
- 16 -
a rough estimation. Consequently, even when the evaporator or steam generator tubes 10, 16, 17 are heated to a greatly differing extent, temperature differences at the outlet of the evaporator or steam generator tubes 10, 16, 17 are kept particularly low. Material fatigues as a result of thermal stresses are thereby reliably prevented, thus ensuring that the continuous-flow steam generator 2 has a long useful life.
When the horizontal combustion chamber 4 is being fitted with tubes, it must be borne in mind that the heating of the individual evaporator tubes 10 connected to one another in a gastight manner varies greatly when the continuous-flow steam generator 2 is in operation-The design of the evaporator tubes 10 in terms of their internal ribbing, their fin connection to adjacent evaporator tubes 10 and their tube inside diameter D is therefore selected such that, in spite of different heating, all the evaporator tubes 10 have approximately the same outlet temperatures of the flow medium S and sufficient cooling of all the evaporator tubes 10 is ensured for all the operating states of the continuous-flow steam generator 2. A heating of some evaporator tubes 10 to a lesser extent when the continuous-flow steam generator 2 is in operation is in this case taken into account additionally by the fitting of throttle devices.
The tube inside diameters D of the evaporator tubes 10 in the combustion chamber 4 are selected as a function of their respective position in the combustion chamber 4 . In this case, the evaporator tubes 10 which are exposed to greater heating when the continuous-flow steam generator 2 is in operation have a larger tube inside diameter D than evaporator tubes 10 which are heated to a lesser extent when the continuous-flow
- 16a -
steam generator 2 is in operation. What is ensured thereby, as compared with the situation where the tube inside diameters are the same, is that the throughput of the flow medium S is increased in the evaporator tubes 10 with a larger tube inside diameter D and temperature differences at the outlet of the evaporator tubes 10 are thereby reduced as a result of
- 17 -
different heating, A further measure for adapting the flow of flow medium S through the evaporator tubes 10 to the heating is to fit throttle devices into some of the evaporator tubes 10 and/or into the line system 19 provided for feeding flow medium S. In order, by contrast, to adapt the heating to the throughput of the flow medium S through the evaporator tubes 10, the fin width may be selected as a function of the position of the evaporator tubes 10 in the combustion chamber 4. All the measures mentioned give rise, despite a widely varying heating of the individual evaporator tubes 10, to an approximately identical specific heat absorption of the flow medium S carried in the evaporator tubes 10, when the continuous-flow steam generator 2 is in operation, and therefore to only slight temperature differences of the flow medium S at its outlet. The internal ribbing of the evaporator tubes 10 is in this case designed in such a way that, in spite of different heating and a different throughflow of flow medium S, particularly reliable cooling of the evaporator tubes 10 is ensured in all the load states of the continuous-flow steam generator 2.
The horizontal gas flue 6 has a number of superheater heating surfaces 23 which are designed as bulkhead heating surfaces and are arranged in a suspended form of construction approximately perpendicularly to the main direction of flow 24 of the fuel gas G and the tubes of which are in each case connected in parallel for a throughflow of the flow medium S. The superheater heating surfaces 23 are heated predominantly by convection and follow the evaporator tubes 10 of the combustion chamber 4 on the flow-medium side.
The vertical gas flue 8 has a number of convection heating surfaces 2 6 which are capable of being heated
- 17a -
predominantly by convection and are formed from tubes arranged approximately perpendicularly to the main direction of flow 24 of the fuel gas G. These tubes are in each case connected in parallel for a throughflow of the flow medium S. Moreover, an economizer 28 is arranged in the vertical gas flue 8. On the outlet side, the vertical gas flue 8 issues into a further heat exchanger, for example into an
18
air preheater and from there, via a dust filter, to a chimney. The components following the vertical gas flue 8 are not illustrated in any more detail in the drawing.
The continuous-flow steam generator 2 is designed with a horizontal combustion chamber 4 of particularly low overall height and can therefore be set up at a particularly low outlay in terms of production and assembly. For this purpose, the combustion chamber 4 of the continuous-flow steam generator 2 has a number of burners 30 for fossil fuel B, which are arranged, level with the horizontal gas flue 6, on the end wall 11 of the combustion chamber 4. The fossil fuel B may in this case be a solid fuel, in particular coal.
So that the fossil fuel B, for example coal in solid form, burns up particularly completely in order to achieve particularly high efficiency and material damage to the first superheater heating surface 23 of the horizontal gas flue 6, as seen on the fuel-gas side, and contamination of this surface, for example by the introduction of high-temperature molten ash, are prevented particularly reliably, the length L of the combustion chamber 4 is selected such that it exceeds the burnup length of the fossil fuel B in the full-load mode of the continuous-flow steam generator 2. The length L is in this case the distance from the end wall 11 of the combustion chamber 4 to the inlet region 32 of the horizontal gas flue 6. The burnup length of the fossil fuel B is in this case defined as the fuel-gas velocity in the horizontal direction at a specific average fuel-gas temperature, multiplied by the burnup time tA of the flame F of the fossil fuel B. The maximum burnup length for the respective continuous-flow steam generator 2 is obtained in the full-load mode of the respective continuous-flow steam generator 2. The burnup time tA of the flame F of the fuel B is.
- 18a -
in turn, the time which, for example, a coaldust grain of average size requires in order to burn up completely at a specific average fuel-gas temperature.
- 19 -
In order to ensure a particularly beneficial utilization of the combustion heat of the fossil fuel B, the length L {given in m) of the combustion chamber 4 is suitably selected as a function of the outlet temperature TBRK (given in °C) of the fuel gas G from the combustion chamber 4, of the burnup time tA {given in s) of the flame F of the fossil fuel B and of the steam power output M (given in kg/s) of the continuous-flow steam generator 2 under full load. This horizontal length L of the combustion chamber 4 amounts in this case to at least 80% of the height H of the combustion chamber 4. The height H is in this case measured from the top edge of the funnel 5 of the combustion chamber 4, marked in figure 1 by the subsidiary line having the end points X and Y, to the combustion chamber ceiling. The length L of the combustion chamber 4 is determined approximately by the functions (1) and (II):
What is to be understood here by approximately is a permissible deviation of the length L of the combustion chamber 4 of +20%/-10% from the value defined by the respective function. In this case, the higher value from the functions (I) and {II) for the length L of the
- 19a -
combustion chamber 4 applies to the design of the continuous-flow steam generator 2 for a predetermined steam power output M of the continuous-flow steam generator 2 under full load.
As an example of a possible design of the continuous-flow steam generator 2, six curves K1 to K6 are plotted
- 20 -
in the coordinate system according to figure 3 for some lengths L of the combustion chamber 4 as a function of the steam power output M of the continuous-flow steam generator 2 under full load. Here, the curves are in each case assigned the following parameters:
K1: tA = 3s according to (I),
K2: tA = 2.5s according to (I),
K3: tA = 2s according to (I),
K4: TBRK = 1200°C according to (II),
K5: TBRK ="1300'C according to (II),
K6: TBRK = 1400°C according to (II) .
Thus, for example for the burnup time tA = 3s of the flame F of the fossil fuel B and the outlet temperature TBRK = 1200°C of the fuel gas G from the combustion chamber 4, curves K1 and K4 are to be used for determining the length L of the combustion chamber 4. This results, in the case of a predetermined steam power output M of the continuous-flow steam generator 2 under full load
of M = 80 kg/s, in a length of L = 29 m according
to K4,
of M = 160 kg/s, in a length of L = 34 m according
to K4,
of M = 560 kg/s, in a length of L = 57 m according
to K4,
The curve K4 drawn as an unbroken line is therefore always applicable.
For the burnup time tA = 2.5s of the flame F of the fossil fuel B and the outlet temperature of the fuel gas G from the combustion chamber TBRK - 1300°C, it is necessary, for example, to use curves K2 and K5. This
- 20a -
results, in the case of a predetermined steam power output M of the continuous-flow steam generator 2 under full load
of M = 80 kg/s, in a length of L = 21 m according
to K2,
of M = 180 kg/s, in a length of L = 23 m according
to K2 and K5,
of M = 560 kg/s, in a length of L = 37 m according
to K5.
- 21 -
Hence, up to M . 180 kg/s, that part of the curve K2 which is drawn as an unbroken line is applicable, not the curve K, drawn as a broken line in this value range of M. For values of M which are higher than 180 kq/s, that part of the curve K5 which is drawn as an unbroken line is applicable, not the curve K2 drawn as a broken line in this value range of M.
The burnup time tA = 2s of the flame F of the fossil fuel B and the outlet temperature TBRK = 1400°C of the fuel gas G from the combustion chamber 4 are assigned, for example, the curves K3 and K6. This results, in the case of a predetermined steam power output M of the continuous-flow steam generator 2 under full load
of M = 80 kg/s, in a length of L = 18 m according to K3,
of M = 465 kg/s, in a length of L = 21 m according to K3 and Kg,
of M = 560 kg/s, in a length of L = 23 m according to K5.
Hence, the values of M up to 465 kg/s, the curve K3 drawn as an unbroken line in this range is applicable, not the curve K6 drawn as a broken line in this range. For values of M which are higher than 465 kg/s, that part of the curve K6 drawn as an unbroken line is applicable, not the part of the curve K3 drawn as a broken line.
So that comparatively small temperature differences occur between the outlet region 34 of the combustion chamber 4 and the inlet region 32 of the horizontal gas flue 5 when the continuous-flow steam generator 2 is in operation, the evaporator tubes 50 and 52 are guided in a particular way in the connecting portion Z marked in figure 1. This connecting portion Z is illustrated in

- 21a -
detail in an alternative version in figures 4 and 5 and comprises the outlet region 34 of the combustion chamber 4 and the inlet region 32 of the horizontal gas flue 6. In this case, the evaporator tube 50 is an evaporator tube 10, welded directly to the side wall 12 of the horizontal gas flue 6, of the

- 22 -
containment wall 9 of the combustion chamber 4 and the evaporator tube 52 is an evaporator tube 10, directly adjacent to the evaporator tube 50, of the containment wall 9 of the combustion chamber 4. The steam generator tube 54 is a steam generator tube 16, welded directly to the containment wall 9 of the combustion chamber 4, of the horizontal gas flue 6, and the steam generator tube 56 is a steam generator tube 10, directly adjacent to the steam generator tube 16, of the side wall 12 of the horizontal gas flue 6.
According to figure 4, the evaporator tube 50 enters the containment wall 9 of the combustion chamber 4 only above the inlet portion E of this containment wall 9. In this case, the evaporator tube 50 is connected on the inlet side to the economizer 26 via the line system 19. As a result, venting of the evaporator tube 50 before the start-up of the continuous-flow steam generator 2 and therefore a particularly reliable flow through the latter are achieved. The evaporator tube 50 is provided initially for carrying the flow medium S from the top downward. The routing of the evaporator tube 50 then changes through 180° in the immediate vicinity of the inlet header system 18, so that a flow of the flow medium S can then take place in the evaporator tube 50 from the bottom upward. Above the point at which the evaporator tube 50 has entered the containment wall 9 of the combustion chamber 4, the evaporator tube 50 is guided upward in the containment wall 9 so as to be laterally offset by one tube division in the direction of the burners 30. In the last portion, therefore, the evaporator tube 50 is guided in vertical alignment with the first portion of the evaporator tube 50.
The steam generator tube 54 of the side wall 12 of the horizontal gas flue 6, after emerging from the inlet

- 22a -
header system 21, is guided firstly outside the side wall 12 of the horizontal gas flue 6. The steam generator tube 54 enters the side wall 12 of the horizontal gas flue 6 only above the point at which the evaporator tube 50 is guided further along in a laterally offset manner. At the connection 36 between the containment wall 9 of the

- 23 -
combustion chamber 4 and the side wall 12 of the horizontal gas flue 6, therefore, the lower part belongs to the containment wall 9 of the combustion chamber 4 and the upper part to the side wall 12 of the horizontal gas flue 6. In the same way as the other evaporator tubes 10 and steam generator tubes 16, the evaporator tube 52 and the steam generator tube 56 are guided vertically in the containment wall 9 of the combustion chamber 4 and in the side wall 12 of the horizontal gas flue 6 respectively and are connected on the inlet side to the inlet header system 18 and 21 and on the outlet side to the outlet header system 20 and 22.
Another possible embodiment of the portion Z connecting the containment wall 9 of the combustion chamber 4 to the side wall 12 of the horizontal gas flue 6 is illustrated in figure 5. Here, the evaporator tube 50, connected to the economizer 26 on the inlet side via the line system 19, enters the containment wall 9 of the combustion chamber 4, so as to be laterally offset by one tube division, above the inlet portion E. What is meant here by laterally offset by one tube division is that the entry of the evaporator tube 50 into the containment wall 9 of the combustion chamber 4 takes place at a distance of one tube layer from the connection 36 of the combustion chamber 4 to the horizontal gas flue 6, The routing of the evaporator tube 50 changes through 90° in the immediate vicinity of the inlet header system 18, and the evaporator tube 50 is routed outside the containment wall 9 of the combustion chamber 4 in the direction of the side wall 12 of the horizontal gas flue 6. Before entry into the side wall 12 of the horizontal gas flue 6, the routing of the evaporator tube 50 changes again through 90° in the direction of the outlet header system 22. The

- 23a -
evaporator tube 50 is in this case guided vertically in the side wall 12 of the horizontal gas flue 6 at a distance of one tube layer from the connection 36 of the combustion chamber 4 to the horizontal gas flue 6. In the side wall 12 of the horizontal gas flue 6, a change of direction of the evaporator tube 50 in the vertical direction takes place again, laterally offset by one tube layer, below the entry of the evaporator tube 50 into the containment wall 9 of the combustion chamber 4, so that the evaporator

IS directly adjacent to the connection- 36 of the combustion chamber 4 to the horizontal gas flue 6, Above the level of entry of the evaporator tube 50 into the containment wall 9 of the combustion chamber 4, a change in the routing of the evaporator tube 50 takes place once again, specifically from the side wall 12 of the horizontal gas flue 6 into the containment wall 9 of the combustion chamber 4. In the containment wall 9 of the combustion chamber 4, the evaporator tube 50 is then guided, in its last portion, vertically along the connection 36 of the combustion chamber 4 to the horizontal gas flue 6 towards the outlet header system 20.
The routing of the evaporator tube 52 in this case matches the routing of the evaporator tube 50. The evaporator tube 52 enters the containment wall 9 of the combustion chamber 4 below the entry of the evaporator tube 50 and is connected to the economizer 28 on the inlet side by the line system 19. The entry of the evaporator tube 52 takes place, in this case, in the tube layer which is adjacent to the connection 36 of the combustion chamber 4 to the horizontal gas flue 6. After the evaporator tube 52 enters the containment wall 9 of the combustion chamber 4, the evaporator tube 52 is guided vertically from the top downward. A change in the routing of the evaporator tube 52 through 900 in the direction of the side wall 12 of the horizontal gas flue 6 takes place in the immediate vicinity of the inlet header system 18. It changes its directxon once again through 900 , level wrth the first tube layer which is adjacent to the connection 36 of the combustion chamber 4 to th. horizontal gas flue 6, and enters the side wall 12 of the horizontal gas flue 6 From this level, the evaporator tube 52 is guided vertically in the side wall U of the horizontal gas flue 6 It therefore forms the connecting tube of the

- 24a -
side wall 12 of the horizontal gas flue 6 to the containment wall 9 of the combustion chamber 4. The evaporator tube 52 leaves the side wall 12 of the horizontal gas flue 6 above the level of entry of the evaporator tube 52 into the containment wall 9 of the combustion chamber 4, in order to be guided in the vertical direction above the entry of the evaporator tube 52 in the containment wall 9 of the combustion chamber 4, specifically in vertical

- 25 -
alignment with the entry of the evaporator tube 52, Above the entry of the evaporator tube 50 into the containment wall 9 of the combustion chamber 4, the routing of the evaporator tube 52 changes once again, in order then to be guided vertically in the containment wall 9 of the combustion chamber 4 in vertical alignment with the first portion of the evaporator tube 50. The last portion of the evaporator tube 52 is therefore guided in vertical alignment with the first portion of the evaporator tube 50. Both the evaporator tube 50 and the evaporator tube 52 are connected on the inlet side to the line system 19 between the economizer 28 and the inlet header system 18 and on the outlet side to the outlet header system 20.
The steam generator tube 52 is connected on the inlet side to the inlet header system 21. After the steam generator tube 54 emerges from the inlet header system 21, the steam generator tube 54 is guided outside the horizontal gas flue 6. Above the change of the evaporator tube 50 from the side wall 12 of the horizontal gas flue 6 into the containment wail 9 of the combustion chamber 4, the steam generator tube 54 enters the side wall 12 of the horizontal gas flue 6. The last portion of the steam generator tube 54, said portion being guided in the side wall 12 of the horizontal gas flue 6, is in this case guided along the connection 36 of the combustion chamber 4 to the horizontal gas flue 6, The side wall 12 of the horizontal gas flue 6 is therefore formed at the connection 36 by the evaporator tube 50 in the lower part and by the steam generator tube 54 in the upper part.
The steam generator tube 56 is also connected to the inlet header system 21 on the inlet side in figure 5.

- 25a -
The steam generator tube 56 is first guided outside the horizontal gas flue 6. The steam generator tube 56 enters the side wall 12 of the horizontal gas flue 6 only above the point at which the evaporator tube 50 has changed its routing from being offset by one tube layer to the connection 36 to a routing which is directly adjacent to the connection 36. The steam generator tubes 54

- 26-
and 56 are in each case connected to the outlet header system 22 on the outlet side.
By virtue of the special tube routing of the evaporator tubes 50 and 52 and of the steam generator tubes 54 and 56, when the continuous-flow steam generator 3 is in operation temperature differences at the connection 36 between the combustion chamber 4 and the horizontal gas flue 6 are kept particularly low in a particularly reliable way. The flow medium S, and therefore also the evaporator tube 50 or 52, enters the containment wall 9 of the combustion chamber 4 above the entry portion E. The further tube routing of the evaporator tubes 50 and 52 and of the steam generator tubes 54 and 56 then takes place in such a way that, when the continuous-flow steam generator 2 is in operation, the evaporator tubes 50 and 52 and therefore also the flow medium S carried in them are preheated by heating, before a direct connection to the steam generator tubes 54, 56 and to a further steam generator tube 16 of the side wall 12 of the horizontal gas flue 6 takes place. As a result, when the continuous-flow steam generator' 2 is in operation, the evaporator tubes 50 and 52 have at the connection 3 6 a comparatively higher temperature than the evaporator tubes 10 of the containment wall 9 of the combustion chamber 4 which are directly adjacent to them.
As an example of possible temperatures Ts of the flow medium S in the evaporator tubes 10 of the combustion chamber 4, and in the steam generator tubes 16 of the horizontal gas flue 6, the curves U1 to U4 are plotted, for the exemplary embodiment according to figure 5, in the coordinate system according to figure 6 for some temperatures TS (given in °C) as a function of the relative tube length R of that part of an evaporator

- 26a -
tube 10, 50, 52 or of the steam generator tubes 54, 5 6 through which the flow passes from the bottom upward {given in %). In this case, the horizontally routed region, that is to say the steps, is not taken into account in the curves shown. U1 describes, here, the temperature profile of a steam generator tube 16 of the horizontal gas flue 6. By contrast, U2 describes a temperature profile of an evaporator tube 10

- 27 -
along its relative tube length R. U3 describes the temperature profile of that part of the specially routed evaporator tube 50 through which the flow passes from the bottom upward, and U4 describes the temperature profile of that part of the evaporator tube 52 of the containment wall 9 of the combustion chamber 4 through which the flow passes from the bottom upward. It becomes clear from the curves depicted that, owing to the special tube routing of the evaporator tubes 50 and 52 in the entry portion E of the evaporator tubes 10 in the containment wall 9 of the combustion chamber 4, the temperature difference from the steam generator tubes 16 of the containment wall 9 of the horizontal gas flue can be markedly reduced. In the example, the temperature of the evaporator tubes 50 and 52 in the entry portion E of the evaporator tubes 50 and 52 can be increased by 45 Kelvin. As a result, when the continuous-flow steam generator 2 is in operation, particularly low temperature differences in the entry portion E of the evaporator tubes 50 and 52 and in the steam generator tubes 16 of the horizontal gas flue 6 at the connection 3 6 between the combustion chamber 4 and the horizontal gas flue 6 are ensured.
When the continuous-flow steam generator 2 is in operation, fossil fuel B, preferably coal in solid form, is fed to the burners 30. The flames F of the burners 30 are in this case oriented horizontally. Due to the form of construction of the combustion chamber 4, a flow of the fuel gas G occurring during combustion is generated in the approximately horizontal main direction of flow 24. This passes via the horizontal gas flue 6 into the vertical gas flue 8 oriented approximately toward the ground and leaves the latter in the direction of the chimney which is no illustrated in any more detail.

- 27a -
Flow medium S entering the economizer 28 passes into the inlet header system 18 of the evaporator tubes 10 of the combustion chamber 4 of the continuous-flow steam generator 2. In the vertically arranged evaporator tubes 10 of the combustion chamber 4 of the continuous-flow steam generator 2 which are welded to one another in a gastight manner, evaporation and, if appropriate, partial

- 28 -
superheating of the flow medium S take place. The steam
or a water/steam mixture occurring at the same time is
collected in the outlet header system 2 0 for flow
medium S. The steam or the water/steam mixture passes
from there, via the walls of the horizontal gas flue 6
and of the vertical gas flue 8, into the superheater
heating surfaces 23 of the horizontal gas flue 6. In
the superheater heating surfaces 23, further
superheating of the steam takes place, the latter subsequently being fed for utilization, for example to the drive of a steam turbine.
By means of the special routing of the evaporator tubes 50 and 52, the temperature differences between the outlet region 34 of the combustion chamber 4 and the inlet region 32 of the horizontal gas flue 6 are particularly low when the continuous-flow steam generator is in operation. At the same time, a choice of the length L of the combustion chamber 4 as a function of the steam power output M of the continuous-flow steam generator 2 under full load ensures that the combustion heat of the fossil fuel B is utilized particularly reliably. Moreover, by virtue of its particularly low overall height and compact form of construction, the continuous-flow steam generator 2 can be set up at a particularly low outlay in terms of production and assembly. In this case, a framework capable of being erected at a comparatively low technical outlay can be provided. In a power plant with a steam turbine and with a continuous-flow steam generator 2 having such a small overall height, moreover, the connecting pipes from the continuous-flow steam generator to the steam turbine can be made particularly short.

- 29 -
Patent Claims
A continuous-flow steam generator (2) with a
combustion chamber (4) for fossil fuel (B) , which
is followed on the fuel-gas side, via a horizontal
gas flue (6) , by a vertical gas flue (8) , the
combustion chamber (4) having a number of burners
(58) arranged level with the horizontal gas flue
(6), and the' containment walls (9) of the
combustion chamber {4) being formed from
vertically arranged evaporator tubes (10) welded
to one another in a gastight manner, a plurality
of the "evaporator tubes (10) being capable of
being acted upon in each case in parallel by flow
medium (S) , characterited in that a number of the evaporator tubes
(10, 50, 52) capable of being acted upon in
parallel by flow medium (S) being guided in the
form of a loop in a connecting portion (Z) which
comprises the outlet region (34) of the combustion
chamber (4) and the inlet region (32) of the
horizontal gas flue (6).
The continuous-flow steam generator (2) as claimed in claim 1, in which the side walls (12) of the horizontal gas flue (6) are formed from vertically arranged steam generator tubes (16) welded to one another in a gastight manner and capable of being acted upon in parallel by flow medium (S).
The continuous-flow steam generator (2) as claimed in claim 1 or 2, in which the side walls (14) of the vertical gas flue (8) are formed from vertically arranged steam generator tubes (17) welded to one another in a gastight manner and capable of being acted upon in parallel by flow medium (S).

- 29a -
The continuous-flow steam generator (2) as claimed in one of claims 1 to 3, in which in each case a plurality of evaporator tubes (10) capable of being acted upon in parallel by flow medium (S) , on the flow-medium side, are preceded by a common inlet header system (18) and are followed by a common outlet header system (20).

- 30 -
The continuous-flow steam generator (2) as claimed in one of claims 1 to 4, in which in each case a number of steam generator tubes (16, 17) of the horizontal gas flue (6) or of the vertical gas flue (8) which are capable of being acted upon in parallel by flow medium (S) , on the flow-medium side, are preceded by a common inlet header system (21) and are followed by a common outlet header system (22).
The continuous-flow steam generator (2) as claimed in one of claims 1 to 5, in which one containment wall (9) of the combustion chamber (4} is the end wall (11), the evaporator tubes (10) of the end wall (9) being capable of being acted upon in parallel by flow medium (S).
The continuous-flow steam generator (2) as claimed in one of claims 1 to 6, in which the evaporator tubes (10) of the end wall (11) of the combustion chamber {4) precede the other containment walls
(9) of the combustion chamber (4) on the flow-
medium side.
The continuous-flow steam generator (2) as claimed in one of claims 1 to 7, in which the tube inside diameter (D) of a number of the evaporator tubes
(10) of the combustion chamber (4) is selected as
a function of the respective position of the
evaporator tubes (10) in the combustion chamber
(4) .
The continuous-flow steam generator (2) as claimed in one of claims 1 to 8, in which a number of the evaporator tubes (10) carry in each case, on their inside, ribs (40) which form a multiflight thread.

- 31 -
The continuous-flow steam generator (2) as claimed in claim 9, in which a pitch angle (a) between a plane (42) perpendicular to the tube axis and the flanks (44) of the ribs (40) arranged on the tube inside is smaller than 60°, preferably smaller than 55°.
The continuous-flow steam generator (2) as claimed in one of claims 1 to 10, in which a number of the evaporator tubes (10) have in each case a throttle device.
The continuous-flow steam generator (2) as claimed in one of claims 1 to 11, in which a line system
(19) for feeding flow medium (S) into the evaporator tubes (10)- of the combustion chamber
(4) is provided, the line system (19) ' having a number of throttle devices, in particular throttle assemblies, in order to reduce the throughflow of the flow medium (S).
The continuous-flow steam generator (2) as claimed in one of claims 1 to 12, in which adjacent evaporator or steam generator tubes (10, 16, 17) are welded to one another in a gastight manner via fins, the fin width being selected as a function of the respective position of the evaporator or steam generator tubes (10, 16, 17) in the combustion chamber (4), the horizontal gas flue (6) and/or the vertical gas flue (8).
The continuous-flow steam generator (2) as claimed in one of claims 1 to 13, in which a number of superheater heating surfaces (23) are arranged in a suspended form of construction in the horizontal gas flue (6).

- 31a -
The continuous-flow steam generator (2) as claimed in one of claims 1 to 14,

- 32 -
in which a number of convection heating surfaces (26) are arranged in the vertical gas flue (8).
The continuous-flow steam generator (2) as claimed in one of claims 1 to 15, in which the burners (58) are arranged on the end wall (11) of the combustion chamber (4).
The continuous-flow steam generator (2) as claimed in one of claims 1 to 16, in which the length L of the combustion chamber (4) , defined by the distance from the end wall (11) of the combustion chamber (4) to the inlet region (32) of the horizontal gas flue (6), is at least equal to the burnup length of the fuel (B) in the full-load mode.
The continuous-flow steam generator {2} as claimed in one of claims 1 to 17, in which the length (L) of the combustion chamber (4) is selected as a function of the steam power output (M) under full load, of the burnup time (tA) , of the flame (F) of the fuel (B) and/or of the outlet temperature (TBRK) of the fuel gas (G) from the combustion chamber (4) approximately according to the two functions (I) and (II)

- 32a -
C5 = 3 - 10-4 m/(°C)2 and
C6 = -0.842 m/°C and
C7 = 603.41 m.

- 33
the in each case higher value of the length (L) of the combustion chamber (4) being applicable for a predetermined steam power output (M) under full load.
The continuous-flow steam generator (2) as claimed
in one of claims 1 to 18, in which the lower
region of the combustion chamber {4) is designed
as a funnel (5) .
A continuous-flow steam generator (2) has a combustion chamber (4) with evaporator tubes (10) for fossil fuel (B) , which is followed on the fuel-gas side, via a horizontal gas flue (6) , by a vertical gas flue (8) . When the continuous-flow steam generator is in operation, temperature differences in a connecting portion (2), which comprises the outlet region (34) of the combustion chamber (4) and the inlet region (32) of the horizontal gas flue (6) , are to be kept particularly low. For this purpose, of a plurality of evaporator tubes (10) capable of being acted upon in parallel by flow medium (S) , a number of evaporator tubes (10) are guided in the form of a loop in the connecting portion (Z).

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Patent Number

205852

Indian Patent Application Number

IN/PCT/2001/00829/KOL

PG Journal Number

15/2007

Publication Date

13-Apr-2007

Grant Date

13-Apr-2007

Date of Filing

14-Aug-2001

Name of Patentee

SIEMENS AKTIENGESELLSCHAFT

Applicant Address

WITTELSBACHERPLATZ 2, D-80333 MUNCHEN, GERMANY

Inventors:

#	Inventor's Name	Inventor's Address
1	WITTCHOW EBERHARD	SCHRONFELD 96, D-91054 ERLANGEN GERMANY

PCT International Classification Number

F 22 B 21/34

PCT International Application Number

PCT/DE00/00864

PCT International Filing date

2000-03-20

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	19914761.2	1999-03-31	Germany