Abstract:
A method for operating a once-through steam generator including an evaporator heating surface is provided. A target value for the supply water mass flow is fed to a device for setting the supply water mass flow, which is predefined using the ratio of the heat flow currently being transferred in the evaporator heating surface from the hot gas to the flow medium to a target enthalpy increase predefined with respect to the desired live steam condition of the flow medium in the evaporator heating surface. A forced-flow steam generator used for carrying out the method is also provided. The heat flow transferred from the hot gas to the flow medium is ascertained for this purpose allowing for a specific temperature value characteristic of the current temperature of the hot gas at the evaporator inlet and a specific mass flow value characteristic for the current mass flow of the hot gas.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2008/065522, filed Nov. 14, 2008 and claims the benefit thereof. The International Application claims the benefits of European Patent Office application No. 07023081.8 EP filed Nov. 28, 2007. All of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to a method for operating a once-through steam generator having an evaporator heating surface in which a device for setting the supply water mass flow {dot over (M)} is fed a target value {dot over (M)}s for the supply water mass flow {dot over (M)}. It further relates to a forced-flow steam generator for carrying out the method. 
       BACKGROUND OF INVENTION 
       [0003]    In a once-through steam generator the heating of a number of steam generator tubes which together form an evaporator heating surface leads to a complete evaporation of a flow medium in the steam generator tubes in one pass. The flow medium—usually water—is generally fed before its evaporation to a preheater, usually also referred to as an economizer, connected upstream on the flow medium side from the evaporator heating surface and preheated there. 
         [0004]    As a function of the operating state of the once-through steam generator and associated therewith as a function of the current steam generator output the feed water mass flow in the evaporator heating surface is regulated. With changes in load the evaporator flow should be changed as synchronously as possible with the input of heat into the evaporator heating surface, because otherwise a deviation of the specific enthalpy of the flow medium at the exit of the evaporator heating surface from the target value cannot be securely avoided. Such an unwanted deviation of the specific enthalpy makes the regulation of the temperature of the fresh steam exiting from the steam generator and also leads to high material stresses and thus to a reduced lifetime of the steam generator. 
         [0005]    To keep deviations of the specific enthalpy from the target value and the resulting unwanted large temperature fluctuations in all operating states of the steam generator, i.e. especially also in transient states or during changes in load, as low as possible, the supply water flow regulation can be embodied as a type of a so-called predictive design. In such cases the necessary supply water target values, especially also during a change of load, should be provided as a function of the current operating state or of the state to be expected in the near future. 
         [0006]    A once-through steam generator is known from EP 0639 253 in which the supply water flow is regulated using a predictive calculation of the necessary amount of supply water. The calculation method is based in this case on the heat flow balance of the evaporator heating surface in which the supply water mass flow, especially at the entry of the evaporator heating surface, should be included. The target value for the supply water mass flow is predetermined on the one hand from the ratio of the heat flow transferred in the evaporator heating surface to the flow medium and on the other hand from a target enthalpy increase of the flow medium in the evaporator heating surface predetermined in respect of the desired live steam state. 
         [0007]    In practice the measurement of the supply water mass flow directly at the entry of the evaporator heating surface however proves technically complex and is not able to be carried out reliably in each operating state. Despite this the supply water mass flow is measured instead at the entry to the economizer and included in the calculations of the supply water mass flow at the entry of the evaporator heating surface. 
         [0008]    To counter the imprecisions caused by this in the predetermination of an especially demand-related target value especially during changes in load for the supply water mass flow, in an alternate concept of a predictive mass flow regulation, as is known in WO 2006/005708 A1, there is provision to take into consideration the supply water density at the entry of the economizer as one of the input variables for the supply water flow regulation. 
         [0009]    Both said concepts for a predictive mass flow regulation are based as a major input variable on the target value for the steam generator power, from which on the basis of stored correlations and especially referring back to previously obtained calibration or reference measurements, the characteristic values included in the actual target flow value determination are calculated. This however requires system characteristics which are sufficiently stable and able to be referred back to a firing power, as are usually present with fired steam generators. In other systems, such as when the once-through steam generator is designed as a waste-heat boiler for heat recovery from the flue gas of an upstream gas turbine for example, these types of conditions are not available. In addition, with such systems connected as waste-heat boilers, a firing power is not usable to the same degree as a free parameter as with directly-fired boilers, since with a connection as waste-heat boilers the operation of the gas turbine is usually seen as the primary criterion for controlling the overall system, to the system state of which the other components are adapted. 
       SUMMARY OF INVENTION 
       [0010]    The underlying object of the invention is thus to specify a method for operating a steam generator of the type specified above, which, while keeping outlay comparatively low, even when the steam generator is operated as a waste-heat boiler, makes possible a setting of the supply water mass flow the evaporator heating surface adapted especially well to the current or to the expected heat input into the evaporator heating surface. Furthermore a forced-flow steam generator especially suitable for carrying out the method is to be specified. 
         [0011]    With regard to the method this object is inventively achieved by the heat flow transferred from the hot gas to the flow medium being determined taking into account a specific temperature characteristic for the actual temperature of hot gas at the evaporator input and a specific mass flow characteristic for the current mass flow of the hot gas. 
         [0012]    The invention is based on the idea here that a sufficiently reliable predictive mass flow regulation also able to be used for a steam generator connected as a waste-heat boiler should be largely adapted to the peculiarities of the waste-heat boiler. In this case it should particularly be taken in account that, unlike with fired boilers, in this case the firing power is not a suitable parameter which allows a sufficiently reliable deduction of the underlying heat flow balance. In particular account should be taken in this case that for an equivalent value for waste-heat boilers, namely the current gas turbine power or parameters correlating with this, further gas-turbine-internal parameters can also occur, so that on the basis of these values it is not possible to draw any acceptable conclusion about the enthalpy circumstances on entry of the hot gas into the flue gas duct of the steam generator. For the heat flow balance used as a basis for determining the needed supply water flow there should therefore be reference back to other, especially suitable parameters. In this case the hot gas temperature on entry into the evaporator as well as the mass flow of the hot gas are provided for this purpose. 
         [0013]    In this way a pre-controlled calculation of the required amount of supply water is made possible on the basis of the heat flow balancing of the evaporator, which can if necessary optionally also include subsequent superheater surfaces. The specific temperature characteristic for the current temperature of the hot gas at the evaporator entry in this case especially makes it possible to determine a characteristic value for the hot gas enthalpy which is especially reliable and thus appropriate to demand taking into account the hot gas enthalpy at the evaporator outlet, which for its part can be calculated on the basis of the specific mass flow characteristic for the current mass flow and thereby an especially reliable and appropriate determination of the current heat provision or surplus from hot gas to the supply water. Taking into account the predetermined target enthalpy increase, i.e. especially the difference between the target enthalpy of the flow medium at the evaporator outlet taking into account the desired live steam parameter and the actual enthalpy at the evaporator outlet determined from suitable measured values such as pressure and temperature for example, the desired target enthalpy increase of the flow medium into the evaporator heating surface can be determined from this, with a target value for the supply water mass flow suitable for this able to be calculated from the ratio of these values. 
         [0014]    A characteristic value especially representative for the current situation is preferably taken into account as a specific temperature characteristic and/or a specific mass flow characteristic suitable for quantitative description of the hot gas entering into the evaporator. Such characteristic values can be suitably determined on the basis of measurement data currently present and can especially be suitably provided by referring back to stored characteristic memory values. An especially reliable evaluation of the heat flow balance and thus the determination of an especially accurate pre-calculated supply water target value are made possible however by a currently detected measurement value advantageously being taken into account as a specific temperature characteristic and/or as a specific mass flow characteristic. 
         [0015]    The heat flow transferred from the hot gas to the flow medium is advantageously determined on the basis of the heat flow balance, for which the difference in enthalpy of the hot gas between evaporator entry and evaporator exit is used as an underlying significant input variable. For an especially reliable characteristic value calculation in such cases account is also taken in a further advantageous embodiment that the reduction of the energy content in the flue gas reflected by the enthalpy difference on its passage through the evaporator heating surface, although it can lead on the one hand to an enthalpy increase in the flow medium within the evaporator heating surface, on the other hand can also lead to energy input or output effects in the components of the evaporator, i.e. especially in the steam generator tubes and other metallic components. For an especially reliable determination of the enthalpy difference actually transferred to the flow medium within the evaporator heating surface, this aspect of the energy input and/or output of heat into the metal masses will be suitably regarded as a characteristic correction value by which the enthalpy difference of the hot gas will be suitably modified. 
         [0016]    The current enthalpy of the hot gas will advantageously be taken into account in the determination of the enthalpy difference of the hot gas by being determined on the basis of the pressure of the flow medium at the evaporator inlet, taking into account the specific mass flow characteristic for the current mass flow of the hot gas. The specific mass flow characteristic, which is preferably present in such cases in the form of a measured value, but alternately can be calculated using further parameters by referring back to stored correlation values or other characteristic values, is in such cases advantageously converted into the so-called “pinchpoint” of the steam generator, i.e. into the temperature difference between the outlet temperature of the flue gas and the boiling temperature of the flow medium and the evaporator inlet, with this temperature difference expediently being added to a boiling temperature of the flow medium determined on the basis of the pressure at the evaporator inlet and the enthalpy of the hot gas at the evaporator outlet being determined from this sum. 
         [0017]    The determination of the target enthalpy increase in the evaporator heating surface is advantageously based on the one hand, using suitable measured values such as the pressure and the temperature of the flow medium at the evaporator inlet for example, on the actual enthalpy determined. In addition, as a function of or taking into account the desired steam state, for example the specified steam parameters or also the steam content at the evaporator outlet, taking into account the current pressure of the flow medium at the outlet of the evaporator heating surface, a target value for its enthalpy at the evaporator outlet is predetermined. 
         [0018]    The once-through steam generator can be operated in this case in a so-called “Benson control mode”. In this case, in the event of control in the Benson control mode, there is overheating of the flow medium at the outlet of the evaporator heating surface. However in this mode the oversupply of a water reservoir connected downstream of the evaporator heating surface can be taken into account and the subsequent heating surfaces can partly be supplied with still unevaporated flow medium so that the full evaporation of the flow medium is only undertaken in the subsequent heating surfaces. In such a mode the setting of a target temperature above the saturation temperature of the flow medium by a predetermined temperature difference of for example 35° C. can especially be predetermined for the flow medium at the output of the evaporator. Precisely with such a mode of operation of the steam generator it can be desirable to take suitable account of the current operating state of the superheater heating surfaces connected downstream from the evaporator heating surface in that their cooling requirement is transferred to a suitable increased supply of the system with supply water. For this purpose the predetermination of the target value for the enthalpy of the flow medium at the outlet of the evaporator heating surface takes account of the current cooling requirements at injection coolers connected downstream from the evaporator heating surface. The target live steam temperature should thus especially as far as possible be achieved by a suitable setting of the supply of water flow so that the additional cooling requirement at the injection coolers can be kept especially low. Conversely, in the event of a live steam temperature which is too low being established, the enthalpy target value of the flow medium at the evaporator outlets can be suitably increased so that a supply water amount dimensioned correspondingly low can be supplied via the target value for the supply of water mass flow modified in such a way. 
         [0019]    Alternately the steam generator can also be operated in a so-called “level control mode” in which the water level in a reservoir connected downstream from the evaporator heating surface is varied and adjusted, with an oversupply of the reservoir being avoided where possible. In this case the water level within the reservoir is kept as far as possible within a predetermined target range with, in an advantageous embodiment of the target value for the supply water mass flow, a fill level correction value being taken into account which characterizes the deviation of the actual state of the fill level in the reservoir from an assigned target value. 
         [0020]    In relation to the once-through steam generator the desired object is achieved by a supply of water flow regulation assigned to a device for adjusting the supply water mass flow being designed to predetermine the target value for the supply water mass flow on the basis of the said method. The once-through steam generator is embodied in this case in an especially advantageous manner as a waste-heat steam generator to which the waste heat from an assigned gas turbine system is supplied on the hot gas side. 
         [0021]    The advantages achieved with the invention are particularly that explicitly taking into account a characteristic value for the current temperature of the flue gas on entry into the hot gas duct and/or for the current mass flow of the waste gas, a predictive or preventive determination of a supply water mass flow target value especially largely oriented to the expected demand is made possible, whereby even in the event of the steam generator being used as a waste-heat boiler and a consequential only insufficient correlation of the corresponding enthalpy characteristic values with the power or supply value of the system, an especially reliable and stable regulation behavior is able to be achieved. This means that an especially reliable predictive adaptation of the supply water flow through the evaporator heating surface to the current or expected heat input of the evaporator heating surface is made possible in an especially simple and reliable manner in all possible operating states of the once-through steam generator, with the deviation of the specific enthalpy of the flow medium at the outlet of the evaporator heating surface from the target value able to be kept especially low. 
         [0022]    An exemplary embodiment of the invention is explained in greater detail with reference to a drawing. The figures show: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIGS. 1 and 2  respectively a once-through steam generator with assigned supply water flow regulation. 
       
    
    
       [0024]    Both parts are provided with the same reference signs in the two figures. 
       DETAILED DESCRIPTION OF INVENTION 
       [0025]    The forced-flow steam generators  1 ,  1 ′ in accordance with  FIG. 1 ,  2  each feature a preheater referred to as an economizer  2  for supply water intended as a flow medium which is located in a gas pipe not shown a greater detail. The economizer  2  is connected on the flow medium side upstream from a supply water pump  3  and downstream from an evaporator heating surface  4 . On the output side the evaporator heating surface  4  is connected via a water reservoir  6  which can also especially be embodied as a water separator or separation vessel, to a number of downstream superheater heating surfaces  8 ,  10 ,  12 , which for their part can be provided, for adapting the steam temperatures and the like, with injection coolers  14 ,  16 . The forced-flow steam generators  1 ,  1 ′ are each embodied as a waste-heat boiler or waste-heat steam generator, with the heating surfaces, i.e. especially of the economizer  2 , the evaporator heating surface  4  as well as the superheater heating surfaces  8 ,  10 ,  12  being arranged in a hot gas duct to which the exhaust gas is applied from an assigned gas turbine system on the hot gas side. 
         [0026]    The forced-flow steam generator  1 ,  1 ′ is designed to have supply water applied to it in a regulated manner. To this end the supply water pump  3  is connected downstream from a throttle valve  22  activated by a control motor  20 , so that by suitable activation of the throttle valve  22  the amount of supply water demanded by the supply water pump  3  in the direction of the economizer  2  or the supply water mass flow can be adjusted. To determine a current characteristic value for the supply water mass flow provided, the throttle valve  22  has a measurement device  24  for determining the supply water mass flow {dot over (M)} through the supply water line connected downstream from it. The control motor  20  is activated by a regulator element  28 , to the input side of which a target value {dot over (M)}s supplied via a data line  30  for the supply water mass flow {dot over (M)} and the current target value of the supply water mass flow {dot over (M)} determined via a measurement device  24  are applied. By forming the difference between these two signals an adjustment requirement is transferred to the regulator  28  so that, for a deviation of the actual value from the target value, a corresponding adjustment of the throttle valve  22  is undertaken by the activation of the motor  20 . 
         [0027]    To determine a target value {dot over (M)}s especially suited to demand for the supply water mass flow {dot over (M)} as a type of setting which is in the nature of a prediction, forecast or value oriented to the future or current demand of the supply water mass flow, the data line  30  is connected on the input side to a supply water flow regulator  32 ,  32 ′ designed for predetermining the target value {dot over (M)}s for the supply water mass flow {dot over (M)}. This is designed for determining the target value {dot over (M)}s for the supply water mass flow {dot over (M)} on the basis of a heat flow balance in the evaporator heating surface  4 , with the target value {dot over (M)}s for the supply water mass flow {dot over (M)} being determined on the one hand on the basis of the ratio of the heat flow currently transferred into the evaporator heating surface  4  from the hot gas to the flow medium and a predetermined target enthalpy increase of the flow medium into the evaporator heating surface  4  in respect of the desired live steam state on the other hand. A use of this type of concept for providing a target value for the supply water mass flow based on a heating balance even for a forced-flow steam generator  1 ,  1 ′ constructed as a waste-heat boiler is especially achieved in the exemplary embodiments in accordance with  FIG. 1 ,  FIG. 2  by the heat flow transmitted from the hot gas to the flow medium being determined taking into consideration a specific temperature characteristic for the current temperature of the hot gas at the evaporator inlet and a specific mass flow characteristic for the current mass flow of the hot gas. 
         [0028]    To this end the supply water flow regulation  32  features a division element  34  which is supplied as a numerator with a suitable characteristic value for the actual heat flow transferred in the evaporator heating surface  4  from the hot gas to the flow medium and as a denominator a suitably predetermined characteristic value in respect of the desired live steam state for the desired target enthalpy increase of the flow medium in the evaporator heating surface  4 . On the numerator side the division element  34  is connected on its input side in this case with a function module  36  which, on the basis of a specific temperature characteristic supplied for the current temperature of the hot gas at the evaporator inlet, outputs a value for the enthalpy of the hot gas at the evaporator inlet. In the exemplary embodiment in this case the supply of a characteristic measured value for the current temperature of the hot gas at the evaporator inlet is provided as a specific temperature characteristic. The characteristic value for the enthalpy of the hot gas at the evaporator is output to a subtraction element, where a characteristic value for the enthalpy of a gas at the evaporator outlet delivered by a function module  40  is subtracted from this characteristic value. 
         [0029]    To determine the enthalpy of the hot gas at the evaporator outlet, the sum of two temperature values is formed by a summation element  42  on the input side for the function element  40 . In this case on the one hand the saturation temperature of the flow medium determined by a function element  44  which is connected on the input side to a pressure sensor  46  on the basis of the pressure of the flow medium at the evaporator inlet is taken into consideration. On the other hand the so-called pinch point, namely the temperature difference determined from the mass flow of the hot gas of the hot gas temperature at the evaporator outlet minus the boiling temperature of the flow medium at the evaporator inlet is taken into account via a function element  48 , which for its part is supplied on the input side via a further function element  50  with a specific mass flow characteristic for the current mass flow of the hot gas. From these two temperature contributions added via the summation element  42  an enthalpy of the hot gas at the evaporator outlet is thus provided by function element  40 , if necessary while referring back to suitable tables, diagrams or the like. On the output side the subtraction element  38  thus delivers the enthalpy difference or balance of the hot gas, i.e. the difference between hot gas enthalpy at the evaporator inlet and hot gas enthalpy at the evaporator outlet. 
         [0030]    This enthalpy difference is passed on to a multiplier element  52  which is likewise supplied with the specific mass flow characteristic which can additionally be present as the currently recorded measurement value. On the output side the multiplication element  52  thus delivers a characteristic value for the heat power output by the flue gas to the evaporator heating surface  4 . 
         [0031]    In order to be able to determine the heat flow actually transferred to the flow medium from this heat power output by the hot gas, a correction by heat injection and/or ejection effects into the components of the evaporator heating surface  4 , especially into the metal masses, is initially provided. For this purpose the said characteristic value for the heat power output by the hot gas is initially supplied to a subtraction element, where a characteristic correction value for the heat injected into or ejected from the evaporator components is subtracted. This is provided by a function element  56 ′ This in its turn has the output value of a further function element  58  applied to it on its input side by an average temperature value for the metal masses of the evaporator heating surface  4  being determined. For this purpose the further function element  58  is connected on its input side with a pressure sensor  60  arranged in the water reservoir  6 , so that the further function element  58  can determine the average temperature of the metal masses on the basis of a pressure of the flow medium, e.g. by equating it with the boiling temperature belonging to this pressure in the water reservoir  6 . 
         [0032]    On the output side the subtraction element  54  thus transfers a characteristic value for the heat power output by the hot gas reduced by the heat power stored in the metal of the evaporator heating surface  4  and thus for the heat power to be output to the flow medium. 
         [0033]    This characteristic value is used in the division element  34  as the numerator, which is divided there by a denominator which corresponds to a predetermined target enthalpy increase in respect of the desired live steam state of the flow medium in the evaporator heating surface  4 , so that from this division or this ratio the target value {dot over (M)}s for the supply water mass flow {dot over (M)} can be fanned. To provide the denominator, i.e. the characteristic value for the desired target enthalpy increase on the water, steam or flow medium side, a division element  34  is connected on its input side to a subtraction element  70 . This has a characteristic value provided by a function element  72  for the desired target value for the enthalpy of the flow medium at the evaporator outlet applied to it on its input side. Furthermore the subtraction elements  70  has a characteristic value actual value for the current enthalpy of the flow medium at the evaporator inlet provided by a function module  74  applied to it on its input side, which is subtracted in the subtraction element  70  from the said characteristic value for the target value of the enthalpy at the evaporator outlet. On the input side the function module  74 , for forming the said characteristic value for the actual enthalpy at the evaporator input, is connected to the pressure sensor  46  and to a temperature sensor  76 . Thus, by forming the difference in the subtraction elements  70 , an enthalpy increase to be included in the evaporator heating surface  4  as a function of the desired live steam state in the flow medium is determined, which can be used as a denominator in the division element  34 . 
         [0034]    The forced-flow steam generator  1  and the forced-flow steam generator  1 ′ in accordance with  FIG. 1  or  2  differ in respect of the design of their supply water flow regulation  32 ,  32 ′, especially as regards the formation of the target value for the enthalpy at the evaporator outlet and thus in respect of what is applied to the input side of the function module  72 . The forced-flow steam generator  1  in accordance with  FIG. 1  is in this case designed for operation in so-called “level control mode” in which the water level in the reservoir  6  is controlled, with exclusively steam being passed on to the superheater heating surfaces  8 ,  10 ,  12  connected downstream from the evaporator heating surface  4  and the water still carried on the evaporator outlet side being collected in the water reservoir  6 . In this operating mode the function module  72  on the one hand has a measured value delivered by the pressure sensor for the pressure in the water reservoir  6  applied to it on its input side. On the other hand a parameter characteristic for the desired live steam state, for example a desired steam content at the evaporator outlet, will be supplied to the function module  72  via an assigned input  78 . From this parameter together with the said pressure characteristic value, the target value for the enthalpy of the flow medium at the evaporator outlet is then formed in function module  72 . 
         [0035]    In the embodiment depicted in  FIG. 1  the division element  34  on the basis of the said division delivers on the output side a target value for the supply water mass flow which is aligned and determined on the basis of the said heat balance. This target value is subsequently further corrected however in a downstream addition element by a correction value which reflects a desired change of the level in the water reservoir  6  over the supply water inflow. For this purpose the level in the water reservoir  6  is detected using a fill level sensor  82 . The actual value for the fill level is subtracted in a subtraction element  84  from a stored target value or a target value able to be predetermined in some other way for the fill level in the water reservoir  6 . On the basis of the deviation of the actual value of the fill level in the water reservoir  6  established in this way from the assigned target value, in a subsequent control element  86  an effective supply water mass flow value is determined which is to be applied to the water reservoir  6  for correcting its fill level. This correction value is added in addition element  82  to the target value for the supply water mass flow determined on the basis of the heat flow balance, so that a value combined from the two components will be output as target value {dot over (M)}s for the supply water mass flow. 
         [0036]    By contrast the forced-flow steam generator  1 ′ depicted in  FIG. 2  is designed for operation in so-called “Benson Control Mode”, in which an oversupply of a water reservoir  6  also intended as a water separator and the complete evaporation of the flow medium is only possible in the subsequent superheater heating surfaces  8 ,  10 ,  12 . In this operating variant the function element  72  via which the target value for the enthalpy of the flow medium at the evaporator outlet is to be output also on the one hand has the actual value that the pressure in the water separator  6  determined with the pressure sensor  60  applied to it on its input side. Furthermore a further function module  90  is connected upstream from the function module  72  on the input side, which on the basis of the actual pressure in the water reservoir  6  determined by the pressure sensor  60 , determines a suitable target value the temperature of the flow medium in the water reservoir  6  on the basis of a stored functionality or of the desired live steam state. For example for an operation of the system in “Benson Control Mode”, a temperature value could be stored here as they target value of the temperature which corresponds to the saturation temperature of the flow medium at the determined pressure plus an intended minimum overheating of for example 35° C. The function module  72  determines from this target value from the temperature, taking into account the current pressure value, the said target value for the enthalpy of the flow medium at the evaporator outlet. 
         [0037]    In the exemplary embodiment depicted in  FIG. 2  this target value provided by function module  72 , which is substantially oriented to the properties of the flow medium as such, is subsequently modified again in a downstream addition element by a further correction value. This further correction value supplied by a function module  94  essentially takes account in the form of a trim function of the deviation of the currently established live steam temperature from the live steam temperature actually desired in respect of the desired live steam state. Such a deviation can especially become evident by a need for cooling arising if the live steam temperature in the injection coolers  14 ,  16  is too high and thus cooling medium needs to be applied to the injection coolers  14 ,  16 . If this type of mass flow is established for the injection coolers  14 ,  16  a design objective of the function module  94  is to transfer this cooling requirement away from the injection coolers  14 ,  16  and into an increased supply water feed. With an accordingly established cooling requirement in the injection coolers  14 ,  16  the desired enthalpy of the flow medium at the evaporator outlet will be lowered accordingly in function module  94  in order to minimize the cooling requirement. Otherwise, i.e. if a live steam temperature which is too low is established, the enthalpy target value is increased by the correction value provided by function module  94  and its addition in addition module  92 . 
         [0038]    To ensure this the supply water flow control  32 ′ of the forced flow steam generator  1 ′ according to  FIG. 2  also comprises a downstream direct control loop in which, in a function module  100  on the basis of the measured values in the water reservoir  6 , an actual value for the enthalpy of the flow medium at the evaporator outlet is determined and is compared in a differentiation module  102  with the desired enthalpy, i.e. with the target enthalpy value. In this case the target-actual deviation is established by forming the difference in the differentiation module  102 , which via a downstream control  104  in an addition module  106  is overlaid on the target value for the supply water mass flow provided by the division element  34 . This overlaying occurs suitably delayed in time and damped so that this control intervention only occurs if necessary, i.e. for a control deviation which is too coarse.