Abstract:
The aim of the invention is a device for coolant cooling in a gas turbine which, with a relatively simple construction and low plant complexity permits a particularly high degree of efficiency in using the heat produced on cooling the coolant from a gas turbine. Said aim is achieved, whereby a number of interconnected evaporator tubes for a flow medium, are arranged in a coolant channel, connected to a gas turbine, to form forced throughflow steam generator. Said device is preferably used in a gas and steam unit with a waste heat steam generator on the exhaust gas side of a gas turbine, the heating surfaces of which are connected into the water-steam circuit of a steam turbine. The evaporator tubes of the device are thus connected on the inlet side by means of supply line to the feed water train of the water-steam circuit of the steam turbine.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/EP02/05571, filed May 21, 2002 and claims the benefit thereof. The International Application claims the benefits of European application No. 01113305.5 EP, filed May 31, 2001, both applications are incorporated by reference herein in their entirety. 
     FIELD OF INVENTION 
     The present invention relates to a device for coolant cooling in a gas turbine. Furthermore, it relates to a gas and steam turbine with a waste heat steam generator on the exhaust gas side of a gas turbine, the heating surfaces of which are connected into the water-steam circuit of a steam turbine. 
     BACKGROUND OF INVENTION 
     A gas turbine, in particular in a gas and steam turbine, is usually used to generate electrical energy. To increase the performance of the gas turbine and thus to achieve the highest possible degree of efficiency, efforts are made to achieve a particularly high temperature of the working substance on the inlet side of the turbine of e.g. 1200° C. to 1500° C. However, such a high turbine inlet temperature may entail material problems, in particular in relation to the heat resistance of the turbine blades and vanes. 
     In order also to be able to operate reliably at such a raised turbine inlet temperature for a long service period, cooling of high-temperature turbine parts, such as, for example, rotating and/or guide blades, is usually provided in modern gas turbines. To this end, a coolant, for example, cooling air is applied to these turbine parts. In particular, a partial flow of the compressor air supplied by the compressor of the gas turbine can be enlisted as cooling air. In order to be able to enlist this partial flow of compressor air, the temperature of which may exceed 400° C. depending on the operating mode of the gas turbine, as coolant for the gas turbine this partial flow is usually cooled to temperatures of, for example, less than 200° C. 
     Such coolant cooling of a gas turbine usually takes place in a coolant cooler assigned to the gas turbine, in which cooling of the coolant takes place via heat exchange. The coolant cooler designed as a heat exchanger to this effect can be designed secondarily as a low-pressure steam generator in which a flow medium evaporates and the steam thus generated is fed into the water-steam circuit of a steam turbine or is also supplied to a district heating network to recover the energy. Generators known as water pipe steam generators or flue pipe steam generators which produce saturated steam are used for this purpose. 
     Precisely in the design of gas and steam turbines, a particularly standard design objective is the achievement of an especially high level of efficiency when converting the energy content of a fuel into electrical energy. With regard to this design objective, the results achieved to date for the transfer of heat produced during the cooling of the coolant of the gas turbine into the water-steam circuit of an assigned steam turbine have been only limited. To increase the level of efficiency attainable when using the heat produced during cooling of the coolant of the gas turbine, combined solutions with a two-stage coolant cooler were also taken into consideration in which both low-pressure and medium-pressure steam is generated during the cooling of the coolant. However, as has emerged, though slightly increased efficiency with regard to the use of heat produced during cooling of the coolant of the gas turbine is attainable in the case of such a solution, there is disproportionately high expenditure on plant technology. 
     SUMMARY OF INVENTION 
     The object of the invention is therefore to specify a device for coolant cooling of a gas turbine which makes possible particularly high efficiency with a relatively simple construction and low plant complexity when using heat produced during cooling of the coolant of the gas turbine. In addition, a gas and steam turbine of the aforementioned type should be specified in which on the one hand good gas turbine cooling is ensured, while on the other hand, high overall efficiency of the gas and steam turbine is attained by means of a particularly effective use of the heat gained in this way. 
     With regard to the device for coolant cooling of the gas turbine, this object is achieved in accordance with the invention by arranging a number of interconnected evaporator tubes for a flow medium in a coolant channel connected to the gas turbine to form a forced throughflow steam generator. 
     The invention is based on the consideration that high efficiency when using heat produced during cooling of the coolant of the gas turbine is attainable by means of the production of relatively high-quality steam during coolant cooling. In a departure from the concepts provided for use to date, in which relatively high-quality energy is converted from compressor air into relatively low-quality steam, the production of relatively high-quality steam, in other words, of steam at high pressure and a high temperature, is proposed. The steam supplied is particularly advantageous in thermodynamic terms and, to this effect, of particularly high-quality if it can be supplied not as saturated steam but as superheated steam. This is with relatively low plant complexity of the coolant cooler in a relatively wide parameter range of the operational parameters where the coolant cooler is designed in accordance with the so-called BENSON principle, in other words, as a forced throughflow steam generator. In a steam generator designed in this way, complete evaporation of the flow medium takes place in the evaporator tubes in a single operation. Precisely such a coolant cooler design permits a variable evaporation end point independent of operation in the series-connected evaporator tubes across a relatively wide range, where overheating of the steam occurs in the section downstream of the evaporation end point of the evaporator tubes. 
     For a relatively simple construction, advantageously compressor air from the gas turbine is provided as coolant for the gas turbine. Thus compressor air from the gas turbine can preferably be applied to the coolant channel, and in this case the device for coolant cooling of the gas turbine is a cooling air cooler. 
     Particularly stable operating performance with low susceptibility to failure by the device for coolant cooling or of the cooling air cooler is attainable if this is effected in an advantageous embodiment in what is known as a “horizontal construction”. For this the coolant channel is advantageously designed for throughflow of the coolant for the gas turbine in an essentially horizontal direction with the longitudinal axis of the evaporator tubes essentially aligned in a vertical direction. In such a construction, relatively few pressure drops in the flow medium can be achieved, with in particular no lower limit for minimum throughflow in the evaporator tubes. 
     Such a design is therefore particularly reliable to operate particularly in light-load or start-up mode. In addition, in such a “horizontal construction”, simple installation of the cooling air cooler is possible without an expensive support frame on relatively simple continuous footings, with good accessibility to the actual coolant channel ensured in addition. Maintenance and inspection work on a cooling air cooler designed in this way are kept particularly simple, while lateral extraction of the heating surfaces formed by the evaporator tubes is possible. 
     In order to ensure particularly good heat transmission from the coolant to be cooled to the flow medium secondarily flowing through the evaporator tubes and thus to ensure reliable cooling of the evaporator tubes in all operating states, these preferably each have internal finning. By means of such internal finning, a prewhirl is generated in the respective flow medium flowing through the evaporator tube. On account of this prewhirl, the flow medium is compressed against the internal wall of the respective evaporator tube particularly reliably as a result of the centrifugal force produced. The consequence is particularly good heat transmission from the internal wall of the evaporator tube to the flow medium conducted within the same. 
     Alternatively or in addition, the evaporator tubes have external finning as required. Such external finning entails, for example, a spiral metal band wound around the respective evaporator tube. This increases the surface of the evaporator tube exposed to the hot gas flow and thus makes an additional contribution to the heat yield of the same. 
     As complete evaporation of the flow medium with subsequent overheating takes place in the evaporator tubes, the cooling capacity of the coolant cooler is to a certain extent dependent on the throughflow rate of the flow medium. In order to enable adjustment of the cooling capacity of the device for coolant cooling to possibly varying operating states of the gas turbine, it is advantageously possible to adjust the flow medium to the evaporator tubes. It is preferably possible to apply the flow medium to the evaporator tubes via a supply line preceding them on the inlet side, with means to adjust the throughflow rate of the flow medium in the supply line. In particular, the means to adjust the throughflow rate of the flow medium preferably include a flow restrictor connected to the supply line. 
     In order to enable a particularly high degree of flexibility when cooling the coolant of the gas turbine, the cooling capacity is advantageously adjustable as a function of a temperature value of the coolant to be cooled and with regard to a desired temperature of the coolant. To this end, the means to adjust the throughflow rate of the flow medium are, in a particularly advantageous development, part of a control system in which the temperature of the coolant of the gas turbine serves as a reference variable and is compared with a desired value dependent on the operating point. The means to adjust the throughflow rate of the flow medium are advantageously assigned a control system which is connected on the inlet side to a temperature sensor assigned to the coolant channel. As a result of the flexibility achievable with such an arrangement when adjusting the cooling capacity to the actual operating status and the actual cooler requirements, such a coolant cooler can be used for a multitude of standard types of gas turbine. 
     With regard to the gas and steam turbine of the aforementioned type, the object is achieved by assigning a device of the type mentioned to the gas turbine where the evaporator tubes of the device are linked on the inlet side via a supply line to the feedwater train of the water-steam circuit of the steam turbine. 
     Precisely when used in a gas and steam turbine, the coolant cooler designed as a forced throughflow steam generator contributes the relatively high efficiency achievable when using the heat produced during cooling of the coolant of the gas turbine in a particularly advantageous manner to a destination usually specified in any case in the design of a gas and steam turbine. The feedwater train from which a partial flow for applying flow medium to the evaporator tubes of the coolant cooler is branched off, comprises in the usual version the partial area of the water-steam circuit of the steam turbine from the condenser to the evaporator heating surfaces. When said application to the evaporator tubes of the coolant cooler takes place from the feedwater train, condensate flowing from the condenser or also feedwater flowing to the heating surfaces can be provided. 
     The feedwater, which is usually under relatively high pressure as a result of a preceding feedwater pump, can be fed directly to the evaporator tubes of the coolant cooler. The pressure necessary for running through the evaporator tubes is supplied by the feedwater pump similarly to the heating surfaces connected to the actual water-steam circuit of the steam turbine. When using condensate for application to the evaporator tubes of the coolant cooler, the necessary pressure level for running through the evaporator tubes while avoiding the feedwater pump of the water-steam circuit of the steam turbine can also be generated via an independent compressor pump. 
     Depending on the type of construction of the gas and steam turbine, the water-steam circuit of the steam turbine may comprise several pressure levels, in particular two or three pressure levels. Precisely in such a multistage design of the water-steam circuit of the steam turbine, a particularly effective use of the heat produced during cooling of the coolant of the gas turbine is made possible by adjusting the coolant cooler for supplying relatively high-quality steam to the highest pressure level of the multistage water-steam circuit of the steam turbine. For this purpose, the evaporator tubes of the device for coolant cooling assigned to the gas turbine are advantageously connected on the outlet side to a high-pressure level of the water-steam circuit of the steam turbine. In the process, the steam generated in the coolant cooler can, for example, be fed into a high-pressure drum or in the event that the waste heat steam generator is also designed as a forced throughflow steam generator in the high-pressure range, into a high-pressure separator vessel. 
     As a rule, a preheater is connected to the feedwater train of the water-steam circuit of the steam turbine of a gas and steam turbine. This may be a condensate preheater also designated as an economizer and/or feedwater preheater. In a particularly advantageous further development, coolant cooling of the gas turbine that is variable and particularly flexibly adjustable to the respective operating situation can be achieved as flow medium may be applied to the evaporator tubes of the coolant cooler both with unpreheated and with preheated flow medium (that is to say, feedwater or condensate), in which the mixture ratio between preheated and unpreheated flow medium is adjustable as necessary. To this end, the supply line preceding the evaporating tubes of the coolant cooler on the inlet side is for its part advantageously connected on the inlet side via a first partial flow line to a first partial component of the feedwater train preceding the preheater and via second partial flow conduction to a second partial component of the feedwater train downstream of the preheater. In the process, feeding of the evaporator tubes of the coolant cooler with relatively cold flow medium can take place via the first partial flow line, whilst feeding of the evaporator tubes of the coolant cooler with preheated flow medium is made possible via the second partial flow line. 
     Advantageously, the mixture ratio of unpreheated and preheated flow medium is adjustable when feeding the evaporator tubes of the coolant cooler. To this end, the means to adjust the throughflow rate of the respective partial flow of flow medium are connected to the first and to the second partial flow line in a particularly advantageous embodiment. This is expediently assigned a control system via which the flow ratio of the partial flows into the partial flow lines is adjustable as a function of a characteristic value for a temperature value of the coolant to be cooled. 
     In particular, the main flow of the flow medium supplied to the evaporator tubes of the coolant cooler may consist of preheated feedwater supplied from the feedwater pump, whilst cold condensate is purposefully added to adjust the temperature of the incoming flow medium in the evaporator tubes of the coolant cooler. 
     The advantages achieved with the invention are in particular that through the embodiment of the coolant cooler as a steam generator for the flow medium in forced throughflow design, the provision of relatively high-quality steam in terms of thermodynamics is made possible during coolant cooling for the gas turbine. Precisely because of its construction as a forced throughflow steam generator, in addition in a relatively simple design, use of the coolant cooler in the supercritical or also in the subcritical pressure range is made possible so that reliable coolant cooling is also guaranteed in modern power plants with high design pressures or in the retrofitting of existing plants using relatively simple means. The coolant cooler is also particularly suited to what is known as solo operation in which no further use is made of the generated steam in an assigned steam turbine plant. 
     However, use of the high-quality steam generated in a corresponding pressure stage of the water-steam circuit of a steam turbine is particularly advantageous. Precisely the design as a forced throughflow steam generator makes possible reliable cover of a relatively wide range of operational parameters as a result of the variable evaporation end point. In particular, at temperatures of up to approximately 500° C. produced when using compressor air from the gas turbine as a coolant, when using heat-resisting materials such as, for example, high-temperature steels like 13 Cro Mo 44 or 15 Mo 3, a risk of overheating can be almost ruled out in all operating states. In particular, therefore, it is not necessary to feed the evaporator tubes with a minimum quantity of flow medium. Thereby, precisely in the case of the embodiment of the coolant cooler as a steam generator according to the forced throughflow principle, start-up or light-load operation with dry or partially filled evaporator heating surfaces is possible without the need for a relatively expensive separator vessel between individual pressure stages of the evaporator tubes. 
     The design of the coolant cooler as a forced throughflow steam generator therefore also makes it possible to influence the coolant temperature for the gas turbine immediately after start-up of the plant. As a result of the option of starting the coolant cooler with “dry” evaporator tubes, immediately after starting the gas turbine there are no substantial water masses to be heated in the coolant cooler, so that also when starting the gas turbine there is no risk of unintentionally intense cooling of the gas turbine cooling air which might in particular lead to cooling below the dewpoint of the water vapor incorporated in the cooling air with subsequent condensation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the invention is explained in more detail with reference to a drawing: 
         FIGS. 1–3  each show a diagrammatic view of a gas and steam turbine, the gas turbine of which is assigned a device for coolant cooling, 
         FIG. 4  shows a longitudinal section of the device for coolant cooling assigned to the gas turbine of the gas and steam turbine according to  FIGS. 1 to 3 , and 
         FIG. 5  a cross-section of the device according to  FIG. 4 . 
     
    
    
     The same parts have the same reference characters in all the figures. 
     DETAILED DESCRIPTION OF INVENTION 
     The gas and steam turbine  1 ,  1 ′,  1 ″ according to  FIG. 1 ,  FIG. 2  and  FIG. 3  each comprises a gas turbine unit  1   a  and a steam turbine unit  1   b . The gas turbine unit  1   a  comprises respectively a gas turbine  2  with connected air compressor  4  and a combustion chamber  6  preceding the gas turbine  2  which is connected to a fresh air pipe  8  belonging to the air compressor  4 . A fuel line  10  flows into the combustion chamber  6  of the gas turbine  2 . The gas turbine  2  and the air compressor  4  as well as a generator  12  are on a common shaft  14 . 
     The steam turbine unit  1   b  comprises a steam turbine  20  with connected generator  22  and in a water-steam circuit  24 , a condenser  26  downstream of the steam turbine  20  as well as a steam generator  30  intended as a waste heat steam generator for the gas and steam turbine  1 . The steam turbine  20  consists of a first pressure stage or a high-pressure component  20   a  and a second pressure stage or a medium-pressure component  20   b  as well as a third pressure stage or a low-pressure component  20   c , which drive the generator  22  via a common shaft  32 . 
     To supply working substance AM expanded in the gas turbine  2  or flue gas in the steam generator  30 , an exhaust gas line  34  is connected to an inlet  30   a  of the steam generator  30  designed as a waste heat steam generator. The expanded working substance AM from the gas turbine  2  leaves the steam generator  30  via its outlet  30   b  in the direction of an unspecified flue. 
     The condenser  26  downstream of the steam turbine  20  is connected via a condensate line  35 , to which a condenser pump  36  is connected, with an economizer or condensate preheater  38  arranged in the steam generator  30 . The condensate preheater  38  is connected on the outlet side via a supply line  40  to which is connected a feedwater pump  42  designed as a high-pressure pump, to a high-pressure preheater or feedwater preheater  44  arranged in the steam generator  30 . Together with the condensate preheater  38 , the supply line  40  and the feedwater preheater  44 , the condensate line  35  therefore forms the feedwater train of steam turbine unit  1   b.    
     The high-pressure component of the water-steam circuit  24  of the steam turbine unit  1   b  could be designed as a circulating evaporator with a high-pressure evaporator connected to a high-pressure drum on the inlet and outlet side to form a closed evaporator circuit. In the embodiment, however, the steam generator  30  is designed as a throughflow steam generator in its high-pressure range. To this end, the feedwater preheater  44  is connected on the outlet side to an evaporator  46  designed for throughflow operation. For its part, the evaporator  46  is connected on the outlet side via a steam line  48 , to which a moisture separator  50  also designated as a separator vessel is connected, to a superheater  52 . In other words, the water separator  50  is connected between the evaporator  46  and the superheater  52 . 
     The moisture separator  50  can be fed with fresh steam F from the evaporator  46  via the steam line  48 . In addition, a drain line  54  that can be shut off with a valve  53  for decanting of water W from the moisture separator  50  is connected to the moisture separator  50 . 
     The superheater  52  is connected to the steam inlet  55  of the high-pressure component  20   a  of the steam turbine  20  on the outlet side. The steam outlet  56  of the high-pressure component  20   a  of the steam turbine  20  is connected via an intermediate superheater  58  arranged in the steam generator  30  to the steam inlet  60  of the medium-pressure component  20   b  of the steam turbine  20 . Its steam outlet  62  is connected to the steam inlet  66  of the low-pressure component  20   c  of the steam turbine  20  via an overflow line  64 . The steam outlet  68  of the low-pressure component  20   c  of the steam turbine  20  is connected to the condenser  26  via a steam line  70 , resulting in the formation of a closed water-steam circuit  24 . 
     The feedwater train of the gas and steam turbine  1 ,  1 ′,  1 ″ could still have a feedwater tank for degassing of the condensate as required and for temporary storage of the condensate required as feedwater at the appropriate place, that is to say, in particular in the flow direction of the condensate or feedwater before the feedwater pump  42 . In the embodiment, however, the feedwater train is designed without a feedwater tank, while a circulation circuit  72  is provided for intermediate storage of condensate K as need be. To form this circulation circuit  72 , a branch line  76  that can be shut off with a valve  74  is connected in a medium-pressure range to the feedwater pump  42  which empties into the condensate line  35  again at an infeed point  78  located before the condensate preheater  38 . The circulation circuit  72  is therefore formed by the condensate preheater  38 , the supply line  40  and the branch line  76 . 
     In an additional pressure stage which is described in the embodiment as a medium-pressure stage, the steam generator  30  comprises a medium-pressure drum  80 . For feeding with preheated condensate K, the medium-pressure drum  80  is connected via a branch line  84  that can be shut off with a valve  82  to supply line  40 . Furthermore, the medium-pressure drum  80  is connected to a medium-pressure evaporator  86  arranged in the steam generator  30  to form a water-steam circuit  88 . To dissipate fresh steam F, the medium-pressure drum  80  is connected via a steam line  90  to the intermediate superheater  58 . 
     The water-steam circuit is arranged as a natural flow in the embodiment, in which the pressure gradients necessary for maintenance of the circuit are provided by the geodetic pressure difference. Alternatively, however, the water-steam circuit  88  can also be arranged as a forced flow, or the entire medium-pressure stage can also be designed as a forced-flow evaporator without the medium-pressure drum  80 . 
     In the embodiment, the water-steam circuit  24  therefore comprises two pressure stages. Alternatively, however, the water-steam circuit  24  can also have another appropriate number of pressure stages, in particular, it can be designed in three stages. The gas and steam turbine  1 ,  1 ′,  1 ″ is designed for particularly high efficiency. To this end, among other things, for thermodynamic reasons, operation of the gas turbine  2  is intended at relatively high temperatures of, for example, 1200° C. or more of the working medium AM flowing from combustion chamber  6 . In order to reliably avoid material problems at such a high turbine inlet temperature, in particular with regard to the heat resistance of the turbine blades and vanes of the gas turbine  2  in longer-term operation as well, gas turbine  2  is designed in such a way that at least its high-temperature components can be cooled. To this end, it is proposed that a partial flow of the compressor air L flowing from the air compressor  4  be fed into the gas turbine  2  as coolant while bypassing combustion chamber  6 . 
     In order to ensure a reliable and adequate cooling effect from the partial flow envisaged as coolant or cooling air, cooling of this partial flow before its entry into gas turbine  2  is envisaged. To this end, gas turbine  2  is assigned a device  100  for coolant cooling or cooling air cooling, which cools the partial flow flowing from the air compressor  4  from a temperature of, for example, more than 400° C. before its entry into gas turbine  2  to a temperature level of approximately 200° C. The device  100  for coolant cooling of gas turbine  2  has a coolant channel  102  which is connected to a cooling air line  104  branching off from the fresh air pipe  8  of the air compressor  4  and flowing into gas turbine  2 . The coolant channel  102  of the device  100  is therefore connected to the gas turbine  2  via the cooling air line  104 . 
     For particularly high overall efficiency of the gas and steam turbine  1 ,  1 ′,  1 ″, for coolant cooling of the gas turbine  2 , the device  100  is also designed for particularly effective use of the heat produced during cooling of the cooling air for gas turbine  2 . In order to incorporate this heat into the water-steam circuit  24  of the steam turbine unit  1   b  to particular advantage, the device  100  for coolant cooling of the gas turbine  2  is designed as a heat exchanger to which the cooling air to be cooled for the gas turbine  2  can be applied primarily, and which secondarily has a number of interconnected evaporator tubes for a flow medium for the formation of a forced throughflow steam generator. Feedwater or condensate K from the water-steam-circuit  24  of the steam turbine unit  1   b  is proposed as the flow medium. 
     To supply this flow medium, a supply line  112  which can be shut off with a throttle valve or flow restrictor  110  is connected on the inlet side to the feedwater train of the water-steam circuit  24  of the steam turbine unit  1   b . On the outlet side, the supply line  112  flows into a first evaporator heating surface  120  arranged in a flow channel  102  formed by an internal housing  114  which is for its part enclosed by a pressure vessel  116 . The first evaporator heating surface  120  on the flow medium side is series-connected to a second evaporator heating surface  122  and a third evaporator heating surface  124 , which for their part are likewise arranged in the coolant channel  102  of the device  100 . The evaporator heating surfaces  120 ,  122 ,  124  are designed as forced throughflow evaporator heating surfaces so that the flow medium evaporates completely on crossover through the series-connected evaporator heating surfaces  120 ,  122  and  124 . The evaporation end point is variable on account of the forced-flow evaporator principle, while in the heating surface area after the evaporation end point overheating of the steam generated occurs. For advantageous recirculation of the steam D thus acquired in the water-steam circuit  24  of the steam turbine unit  1   b , the third evaporator heating surface  124  is connected on the outlet side via a steam line  126  to the moisture separator  50 . 
     If necessary, as is indicated by the dotted line in  FIGS. 1 to 3 , the third evaporator heating surface  124  can also be series connected to another superheater heating surface  128 . From this superheated steam provided by the superheater heating surface  128 , the fresh steam flowing from the superheater  52  for the high-pressure component  20   a  of the steam turbine  20  can then be admixed via an overflow line  130 . 
     On account of the design of the evaporator heating surfaces  120 ,  122 ,  124  as forced throughflow steam generators, the provision of relatively high-quality steam D for recirculation in the water-steam circuit  24  of the steam turbine unit  1   b  is possible. This means that the heat produced during cooling of the cooling air for gas turbine  2  can be recirculated particularly advantageously. Supply of the superheated steam D generated in the device  100  during cooling of the coolant for gas turbine  2  therefore takes place in the embodiment in the high-pressure stage or highest pressure stage of the water-steam circuit  24 . Alternatively, however, supply to another pressure stage, in particular, to a medium-pressure stage, may be proposed in particular with regard to external, predetermined marginal conditions. 
     In order to facilitate reliable operation of the device  100  in the light-load range as well, the first evaporator heating surface  120  is designed in such a way that it can be bypassed if need be. To this end, a three-way valve  131 , via which a bypass line  132  branches off from the supply line  112 , is connected in series to the first evaporator heating surface  120  on the inlet side. On the outlet side, the bypass line  132  flows into an infeed point in the output area of the first evaporator heating surface  120 , in particular into its penultimate intermediate collector. This ensures that in the light-load range as well, in which only a relatively small amount of flow medium can be made available, there is no evaporation at all in the first evaporator heating surface  120  and the incoming flow medium is therefore exclusively liquid without incorporated steam parts from the series-connected evaporator heating surface. If need be, that is to say, in particular in the light-load range, by means of appropriate switching of the three-way valve  131 , the flow medium can therefore be largely directed past the majority of the heating surface pipes of the first evaporator heating surface  120  via the bypass line  132 . 
     The precise construction of the device  100  for coolant cooling of the gas turbine  2  can be seen in a longitudinal section in  FIG. 4  and in a cross-section in  FIG. 5 . As is evident there, the external housing of the device  100  comprises a relatively thick-walled pressure vessel  116 , in which the internal housing  114  for the formation of the coolant channel  102  is arranged. The device  100  is designed in a horizontal style for an essentially horizontal coolant channel  102 . In other words, the coolant channel  102  of the device  100  is designed for direct flow of the coolant for the gas turbine  2  in an essentially horizontal direction. 
     As is evident from  FIG. 4  in particular, the first evaporator heating surface  120 , the second evaporator heating surface  122 , the third evaporator heating surface  124  and the superheater heating surface  128  are arranged in the coolant channel  102 . The first evaporator heating surface  120  is designed as a preheater heating surface or economizer and is composed of a number of evaporator tubes  140  connected in parallel for the flow medium. The evaporator tubes  140 , to which a number of appropriately positioned inlet collectors  142 , also designated as inlet distributors, are connected upstream and a number of appropriately positioned outlet collectors  144  are connected downstream to form the first evaporator heating surface  120 , are connected to the incoming line  112  on the inlet side. The outlet collectors  144  connected downstream to the evaporator tubes  140  are for their part connected to an overflow line  146  to which a number of the inlet collectors  148  from the second evaporator heating surface  122  are assigned. In addition, the inlet collectors  148  are connected on the inlet side to the bypass line  132  which is connected to the three-way valve  131  in the manner not shown in greater detail in  FIG. 4 . 
     A number of evaporator tubes  150  forming the second evaporator heating surface are connected upstream to the inlet collectors  148 . The second evaporator heating surface  122  is designed as an actual evaporator heating surface in particular with regard to the dimensioning and positioning of the evaporator tubes  150  forming it. On the outlet side, the evaporator tubes  150  are connected to a number of outlet collectors  152  assigned to the second evaporator heating surface  122 . 
     For their part, these outlet collectors  152  are connected on the outlet side via an overflow system  154  to a number of inlet collectors  156  assigned to the third evaporator heating surface  124 . These are connected upstream to a number of evaporator tubes  160  forming the third evaporator heating surface  124 . On the outlet side, these evaporator tubes  160  flow into a number of outlet collectors  162  assigned to the third evaporator heating surface  124 . The third evaporator heating surface  124  is also designed as an actual evaporator heating surface. 
     The outlet collectors  162  assigned to the third evaporator heating surface  124  are connected on the outlet side to a number of evaporator tubes  170  forming the superheater heating surface  128 . 
     The device  100  is therefore designed in the manner of a horizontally constructed throughflow steam generator. This “horizontal construction” in particular enables simple and robust operational performance of the device  100  with a high level of operational stability and only slight pressure losses on the flow medium side. In addition, precisely the horizontal construction enables simple installation of the device  100  without an expensive support framework on relatively simple continuous footings. The evaporator tubes  140 ,  150 ,  160 ,  170  series-connected on the flow-medium side to form the throughflow steam generator are each vertically aligned, that is to say, with their longitudinal axis in an essentially vertical direction. To ensure high heat transmission from the cooling air flowing through the coolant channel  102  to the flow medium flowing through the evaporator tubes  140 ,  150 ,  160 ,  170 , in addition the evaporator tubes  140 ,  150 ,  160 ,  170  can each be provided with internal and/or external fins. 
     For particularly flexible coolant cooling of the gas turbine  2 , adjusted to the respective operating status of the gas and steam turbine  1 ,  1 ′,  1 ″, the cooling capacity of the device  100  for coolant cooling of the gas turbine  2  is adjustable and adaptable to the respective operating status. To this end, the device  100  is assigned a control system  180  for selection of the throughflow rate of the flow medium through the device  100 , as shown in  FIGS. 1 to 3 . The control system  180  is connected on the outlet side to the flow restrictor in the supply line  112  of the device  100  for transmission of a control command from the actuating signal S via a signal line  182 . Via the control system  180 , the valve positioning of the flow restrictor  110  and consequently the application of slow medium to the device  100  as a secondary coolant can be selected by inputting an appropriate control command or actuating signal S. On the inlet side, the control system  180  is connected via a first signal line  184  to a first temperature sensor  186  and via a second signal line  188  to a second temperature sensor  190 . The first temperature sensor  186  is arranged in an area before the device  100  on the cooling air line  104 . The second temperature sensor  190 , on the other hand, is arranged in an area after the device  100  on the cooling air line  104 . In this way, measured values for the temperature of the cooling air to be cooled for the gas turbine  2  can be supplied to the control system  180  before its entry into the device  100  and after its exit from the device  100 . Furthermore, additional parameters such as desired temperature values or manually selected settings may be supplied to the control system  180 , as indicated by the arrow  192 . 
     The control system  180  can therefore establish a characteristic value for the cooling requirement on recooling of this cooling air on the basis of a variance comparison for the temperature of the cooling air to be cooled for the gas turbine  2 . As a function of this characteristic value for the cooling requirement, an actuating signal S can then be output to the flow restrictor  110 , via which a throughflow rate of flow medium through the device  100  adjusted to the cooling requirement on recooling of the cooling air can take place. 
     For a particularly flexible and in addition, precise selection of the cooling capacity in the device  100  on recooling of the cooling air for the gas turbine  2 , a mixture of flow medium at various temperatures can additionally be applied to the device  100 . To this end, the supply line  112  of the device  100  is connected to the feedwater train of the water-steam circuit  24  of the steam turbine unit  1   b  on the inlet side both at a place before a preheater and at a place after a preheater. In the embodiment according to  FIG. 1 , the supply line  112  is connected on one side at a place  200  after the feedwater pump  42  to the feedwater train of the water-steam circuit  24 . The feedwater fed into the supply line  112  at the place  200  has therefore on the one hand run through the condensate preheater  38  and therefore displays a relatively high temperature. On the other hand, the feedwater at this place is also under relatively high pressure generated by the feedwater pump  42  with the result that the feedwater can be conveyed to the evaporator heating surfaces  120 ,  122  and  124  of the device  100  without additional means of increasing the pressure. 
     In addition, however, in the embodiment according to  FIG. 1  the supply line  112  is also connected via a partial flow line  202  to the condensate line  35 . Via the partial flow line  202 , to which a condenser admixture pump  204  and a flow restrictor  206  are connected, condensate K removed at a place  208  before the condensate preheater  38 , and therefore relatively cold, can be fed into the supply line  112 . The pressure necessary to feed this condensate K into the supply line  112  is generated by the condenser admixture pump  204 . 
     An actuating signal S can be applied to the flow restrictor  206  connected to the partial flow line  202  from the control system  180  via a signal line  210 . Admixture of the relatively cold condensate K to the supply line  112 , adjusted to the respective operational situation, in particular, depending on the cooling requirement, can therefore take place via the control system  180 . In other words, the flow restrictor  110 ,  206  in the embodiment according to  FIG. 1  shows means by which the throughflow rates of various partial flows of the flow medium intended to be applied to the device  100  can be selected. The control system  180  is assigned to these means in such a way that the incoming ratio of the partial flows is adjustable as a function of a characteristic value for a temperature value of the coolant of gas turbine  2  to be cooled, in particular, with regard to a comparison of this value with a desired value. 
     In the embodiment according to  FIG. 2 , that is to say in the gas and steam turbine  1 ′, it is likewise envisaged that an adjustable mixture of condensate K removed before flowing through the condensate preheater  38  and after flowing through the condensate preheater  38  will be applied to the device  100 . In the embodiment according to  FIG. 2 , however, the supply line  112  is connected on the inlet side of the supply line  40 , that is to say, before entry of the condensate K into the feedwater pump  42 . 
     In order to provide the necessary pressure for entry into the device  100  in the flow medium, a pressure boosting pump  212  is connected to the supply line  112  in the embodiment according to  FIG. 2 . For any necessary admixture of relatively cold condensate K to the partial flow of preheated condensate K branching off from the supply line  40  as flow medium for the device  100 , a partial flow line  214  is also envisaged for the gas and steam turbine  1 ′ according to  FIG. 2 . The partial flow line  214  is connected to the condensate line  35  on the inlet side and flows into the incoming line  112  on the outlet side at a place before the pressure booster pump  212 . To select an admixture rate of cold condensate K into the supply line  112  which meets requirements, a flow restrictor  216  is connected to the partial flow line  214  to which a control variable S from the control system  180  can be applied via a signal line  218 . 
     In the gas and steam turbine  1 ″ according to  FIG. 3 , feeding the device  100  with a selected mixture of unpreheated and preheated feedwater is envisaged. To this end, the supply line  112  branches off at a place  220  after the feedwater pump  42  from the feedwater train of the water-steam circuit  24 . Thus, feedwater under high pressure, not yet preheated in the feedwater preheater  44  reaches the supply line  112  through the feedwater pump  42 . For a relatively finely dosed temperature setting, a partial flow line  222  which branches off from the feedwater train of the water-steam circuit  24  at a place  224  after the feedwater preheater  44  is also envisaged in this embodiment. The partial flow line  222 , to which a flow restrictor  226  is connected, flows into the supply line  112  on the outlet side so that preheated feedwater under high pressure can continue to be added to the supply line  112  via the partial flow line  222  in the feedwater preheater  44 . 
     A control signal S can be applied to the flow restrictor  226  via a signal line  228  from the control system  180 , so that in this embodiment as well the mixture ratio of the partial flows of relatively warm and relatively cold flow medium can be selected as required and in particular as a function of the measured temperature parameters of the cooling air to be cooled for the gas turbine  2 . 
     The device  100 , to which a mixture of partial flows of a flow medium of various temperatures adjusted to the actual cooling requirement can be applied in each of the three embodiments mentioned, is designed in the embodiments as a forced throughflow steam generator without a separator vessel connected between the evaporator heating surfaces  120 ,  122 ,  124 ,  128 . 
     In this embodiment, starting the device  100  with unfilled (dry) or partially filled evaporator heating surfaces  120 ,  122 ,  124 ,  128  is envisaged. This is possible without a notable risk of overheating, in particular, with regard to the anticipated temperatures of the coolant to be cooled for the gas turbine  2  of up to approximately 500° C. precisely when using heat-resisting materials such as, for example, 13 Cro Mo 44 or 15 Mo 3 for the evaporator tubes  140 ,  150 ,  160 ,  170 . Thus, with a relatively simple construction, the device  100  is already highly usable on start-up, while in particular, the risk of excessive cooling of the coolant for the gas turbine  2  during start-up is avoided.