Patent Publication Number: US-2012024241-A1

Title: Continuous evaporator

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. National Stage of International Application No. PCT/EP2010/051361, filed Feb. 4, 2010 and claims the benefit thereof. The International Application claims the benefits of German application. No. 10 2009 012 320.2 DE filed Mar. 9, 2009. All of the applications are incorporated by reference herein in their entirety. 
    
    
     FIELD OF INVENTION 
     The invention relates to a through-flow evaporator for a horizontally constructed waste heat steam generator with a first evaporator heating surface which incorporates a number of first steam generation tubes, the arrangement of which is essentially vertical and through which the flow is from the bottom to the top, and another second evaporator heating surface, which on the flow substance side is connected downstream from the first evaporator heating surface, which incorporates a further number of second steam generation tubes the arrangement of which is essentially vertical and through which the flow is from the bottom to the top. 
     BACKGROUND OF INVENTION 
     In the case of a combined cycle gas turbine plant, the heat contained in the expanded working substance or heating gas from the gas turbine is utilized for the generation of steam for the steam turbine. The heat transfer is effected in a waste heat steam generator connected downstream from the gas turbine, in which it is usual to arrange a number of heating surfaces for the purpose of preheating water, for steam generation and for superheating steam. The heating surfaces are connected into the water-steam circuit of the steam turbine. The water-steam circuit usually incorporates several, e.g. three, pressure stages, where each of the pressure stages can have an evaporator heating surface. 
     For the steam generator connected downstream on the heating gas side from the gas turbine as a waste heat steam generator, several alternative design concepts can be considered, namely a design as a through-flow steam generator, or a design as a recirculatory steam generator. In the case of a through-flow steam generator the heating up of steam generation tubes, which are provided as evaporation tubes, results in the flow substance being evaporated in a single pass through the steam generation tubes. In contrast to this, in the case of a natural or forced circulation steam generator, the water which is fed around the circulation is only partially evaporated during its passage through the evaporator tubes. After the steam which has been generated has been separated off, the water which has not yet been evaporated is then fed once more to the same evaporator tubes for further evaporation. 
     Unlike a natural or forced circulation steam generator, a through-flow steam generator is not subject to any pressure limitations. A high live steam pressure favors a high thermal efficiency, and hence low CO 2  emissions from a fossil-fuel fired power station. In addition, a through-flow steam generator has, by comparison with a recirculatory steam generator, a simple construction and can thus be manufactured at particularly low cost. The use of a steam generator, designed in accordance with the through-flow principle, as the waste heat steam generator for a combined cycle gas turbine plant is therefore particularly favorable for the achievement of a high overall efficiency for the combined cycle gas turbine plant together with simple construction. 
     A through-flow steam generator which is designed as a waste heat steam generator can basically be engineered in one of two alternative forms of construction, namely as a vertical construction or as a horizontal construction. A through-flow steam generator with a horizontal construction is then designed so that the heating substance or heating gas, for example the exhaust gas from the gas turbine, flows through it in an approximately horizontal direction, whereas a through-flow steam generator with a vertical construction is designed so that the heating substance flows through it in an approximately vertical direction. 
     Unlike a through-flow steam generator with a vertical construction, a through-flow steam generator with a horizontal construction can be manufactured with particularly simple facilities, and with particularly low manufacturing and assembly costs. In this case, an uneven distribution of the flow substance can arise across the steam generation tubes located downstream on the flow substance side, in particular within each individual row of tubes in the steam generation tubes of the second evaporator heating surface, said tubes being located downstream on the flow substance side, leading to temperature imbalances and, because of different thermal expansions, to mechanical stresses. For this reason expansion bends, for example, have hitherto been incorporated to compensate for these stresses, in order to avoid damage to the waste heat steam generator. However, this measure can be technically comparatively expensive in the case of a waste heat steam generator with a horizontal construction. 
     SUMMARY OF INVENTION 
     The object underlying the invention is thus to specify a through-flow evaporator, for a waste heat steam generator of the type identified above, which has a particularly long service life while permitting a particularly simple construction. 
     This object is achieved in accordance with the invention in that on the flow substance side an aperture system is connected downstream from the second steam generation tubes. 
     The invention then starts from the consideration that it would be possible to achieve a particularly simple construction for the waste heat steam generator or through-flow evaporator, as applicable, by eliminating the previously-usual expansion bends. In doing so however, the mechanical stresses caused by the temperature imbalances in the steam generation tubes, which are connected in parallel with one another in each individual row, must be reduced in some other way. These occur, in particular, in the second evaporator heating surface, to which is admitted a water-steam mixture. The temperature imbalances are here caused by the different proportions of water and steam at the flow side entry to the individual tubes in a row of tubes, and the resulting different through-flow through these tubes. It has been recognized that this different through-flow in the tubes is caused by a frictional pressure loss in the steam generation tubes which is small by comparison with the geodetic pressure loss. That is, a flow which has a high proportion of steam in the flow substance flows through individual steam generation tubes comparatively fast with a low frictional pressure loss, whereas a flow with a high proportion of water is disadvantaged by its greater geodetic pressure loss, caused by its mass, and can tend towards stagnation. In order to even out the through-flows, the frictional pressure loss should therefore be increased. This can be achieved by connecting into flow substance side downstream from the second steam generation tubes an aperture system which causes an additional frictional pressure loss of this type. 
     It is advantageous if the aperture system incorporates a plurality of apertures, arranged in the individual second steam generation tubes. Such a distributed arrangement of the apertures ensures that separately in each steam generation tube a sufficient additional frictional pressure loss arises to provide a static stabilization of the flow, and thereby an equalization of temperature imbalances. 
     This frictional pressure loss should be appropriately determined by reference to the other operating parameters, such as the pipe geometry, the dimensions of the heating gas duct and the temperature conditions. Advantageously, the aperture opening of each aperture should then be chosen in such a way that the prescribed frictional pressure loss for the flow substance is established via the system of apertures. This permits even better avoidance of temperature imbalances. 
     Advantageously, each aperture will have as the aperture opening a bore with a diameter between 10 and 20 mm. Namely, such a choice leads to a particularly good static stabilization of the flow in the second steam generation tubes, and thus to a particularly good equalization of the temperatures in steam generation tubes which are connected in parallel in the individual rows of tubes in the second heating surface. 
     In order to permit an even more flexible structuring of the aperture system, it should incorporate a plurality of apertures connected one after another on the flow substance side. By this means, an even more uniform temperature distribution can be achieved. 
     In an advantageous embodiment, a number of first steam generation tubes are connected one after another on the heating gas side as rows of tubes. This makes it possible to use as an evaporator heating surface a larger number of steam generation tubes connected in parallel, which means a better heat input from the enlarged surface. However, in this case the steam generation tubes which are arranged one after another in the direction of flow of the heating gas are differently heated. Particularly in the steam generation tubes on the heating gas entry side, the flow substance is comparatively strongly heated. However, by the downstream connection of an aperture system as described, a through-flow which is matched to the heating can also be achieved in these steam generation tubes. By this means, a particularly long service life is achieved for the waste heat steam generator together with a simple construction. 
     In an advantageous embodiment, the first evaporator heating surface is connected downstream from the second evaporator heating surface on the heating gas side. This offers the advantage that the second evaporator heating surface, which is connected downstream on the flow substance side and is thus designed to further heat up a flow substance which has already been evaporated, also lies in a comparatively more strongly heated region of the heating gas duct. 
     It is expedient to use a through-flow evaporator of this type in a waste heat steam generator, and the waste heat steam generator is used in a combined cycle gas turbine plant. In this case it is advantageous to connect the steam generator downstream on the heating gas side from a gas turbine. With this connection, a supplementary heat source can expediently be arranged behind the gas turbine, to raise the heating gas temperature. 
     The advantages achieved by the invention consist, in particular, in the fact that connecting an aperture system downstream achieves a static stabilization of the flow, and thus a reduction in the temperature differences between second steam generation tubes connected in parallel and in the resulting mechanical stresses. This makes the service life of the waste heat steam generator particularly long. An appropriate arrangement of an aperture system makes further expensive technical measures such as expansion bends unnecessary, and thus at the same time permits a particularly simple cost-saving construction for the waste heat steam generator or a combined cycle gas turbine power station, as applicable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the invention is explained in more detail by reference to a drawing. This shows: 
         FIG. 1  a simplified representation of a longitudinal section through a steam generator with a horizontal construction, 
         FIG. 2  a graphical representation of the tube temperature against its steam content at the entry to the heating tube, with no aperture system arrangement, and 
         FIG. 3  a graphical representation of the tube temperature against its steam content at the entry to the heating tube, with an aperture system arrangement. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The same parts have been given the same reference numbers in all the figures. 
     The through-flow evaporator  1  for the waste heat steam generator  2  shown in  FIG. 1  is connected downstream from a gas turbine, not shown here in more detail, on its exhaust gas side. The waste heat steam generator  2  has a surrounding wall  3  which faints a heating gas duct  5  through which the exhaust gas from the gas turbine can flow in an approximately horizontal direction as heating gas, as indicated by the arrows  4 . Arranged in the heating gas duct  5  is a number of evaporator heating surfaces  8 ,  10 , designed according to a through-flow principle. In the exemplary embodiment shown in  FIG. 1 , each of two evaporator heating surfaces  8 ,  10  is shown, but a larger number of evaporator heating surfaces could also be provided. 
     Each of the evaporator heating surfaces  8 ,  10  shown in  FIG. 1  incorporates a number of rows of tubes,  11  and  12  respectively, each in the nature of a nest of tubes, arranged one behind another in the direction of the heating gas. Each row of tubes  11 ,  12  incorporates in turn a number of steam generation tubes,  13  and  14  respectively, in each case arranged beside each other in the direction of the heating gas, of which in each case only one can be seen for each row of tubes  11 ,  12 . The first steam generation tubes  13  of the first evaporator heating surface  8 , which are arranged approximately vertically and connected in parallel so that a flow substance W can flow through them, are here connected on their output sides to an outlet collector  15  which is common to them. The second steam generation tubes  14  of the second evaporator heating surface  10 , which are also arranged approximately vertically and connected in parallel so that a flow substance W can flow through them, are also connected on their output sides to an outlet collector  16  which is common to them. Here, a comparatively expensive collection system could also be provided for both the evaporator heating surfaces  8 ,  10 . For flow purposes, the steam generation tubes  14  of the second evaporator heating surface  10  are connected downstream from the steam generation tubes  13  of the first evaporator heating surface  8 , via a downpipe  17 . 
     The evaporation system formed by the evaporator heating surfaces  8 ,  10  can have admitted to it the flow substance W which, in a single pass through the evaporation system, is evaporated and after it emerges from the second evaporator heating surface  10  is fed away as steam D. The evaporation system formed by the evaporator heating surfaces  8 ,  10  is connected into a steam turbine&#39;s water-steam circuit, which is not shown in more detail. In addition to the evaporation system which incorporates the evaporator heating surfaces  8 ,  10 , the water-steam circuit of the steam turbine has connected into it a number of other heating surfaces  20 , indicated schematically in  FIG. 1 . The heating surfaces  20  could be, for example, superheaters, medium-pressure evaporators, low-pressure evaporators and/or preheaters. 
     An aperture system  22 , which incorporates apertures  23  arranged in the individual steam generation tubes, is now connected downstream from the second steam generation tubes  14 . The bore of the apertures  23  is chosen such that the frictional pressure loss of the flow substance W in the steam generation tubes  14  is appropriately high to ensure a uniform through-flow within a row of tubes  11 . By this means, temperature imbalances are reduced. For this purpose, the apertures  23  incorporate bores between 10 and 20 mm in diameter. 
     The effect of the aperture system  22  on the temperature differences is shown in  FIGS. 2 and 3 . Each of these shows a graphical representation of the mean tube wall temperature  25  and the tube exit wall temperature  27 , plotted against the proportion of steam DA in the flow substance. Here,  FIG. 2  shows the situation without a downstream aperture system  22 . In this case, the mean tube wall temperature  25  varies between approx. 460° C. and 360° C., the temperature of the tube exit wall 27 between 480° C. and 370° C., depending on the steam content DA.  FIG. 3  shows that these variations are reduced to approx. 440° C. to 390° C. or 470° C. to 405° C. respectively. The temperature differences between tubes with a different steam content are also clearly reduced. 
     The reduction in the temperature differences, between tubes with differing steam content at the flow-side entry, reduces the mechanical stress loads on the waste heat steam generator  2 , and guarantees a particularly long service life and at the same time a simple construction due to the elimination of the previously usual expansion bends.