EP1614962A1 - Method for operating of an once-through steam generator - Google Patents

Abstract

The operating method is used for continuous flow steam generators with an evaporator heating surface (4), a preheater (2) on the flow medium side of the surface, a device for setting the mass of feed water flowing through the generator and a control (1) for ensuring this value. The mass of water is adjusted according to the steam production desired and is controlled by measuring the density of water at the preheater inlet.

Description

The invention relates to a method for operating a continuous steam generator with a Verdampferheizfläche and a Verdampferheizfläche flow medium side upstream preheater and a device for adjusting the feedwater mass flow Ṁ in the evaporator.

In a continuous steam generator, the heating of a number of steam generator tubes, which together form the gas-tight surrounding wall of the combustion chamber, leads to complete evaporation of a flow medium in the steam generator tubes in one pass. Before it evaporates, the flow medium-usually water-is fed to a preheater upstream of the evaporator heating surface upstream of the evaporating medium, usually also referred to as an economizer, and preheated there.

Depending on the operating state of the continuous steam generator and, consequently, on the current steam generator capacity, the feedwater mass flow is regulated in the evaporator heating surface. In the case of load changes, the evaporator flow and the heat input into the evaporator heating surface should be changed as synchronously as possible, otherwise an overshoot of the specific enthalpy of the flow medium at the outlet of the evaporator heating surface can not be reliably avoided. Such an undesirable overshoot of the specific enthalpy makes it difficult to regulate the temperature of the live steam emerging from the steam generator and, moreover, leads to high material loads and thus to a reduced service life of the steam generator.

In order to prevent an overshoot of the specific enthalpy and large temperature fluctuations in each operating state of the steam generator, is a feedwater flow control is provided, which provides the necessary feed water setpoints depending on the operating state even during load changes.

From EP 0639 253 a continuous steam generator is known, in which the feedwater flow is controlled by a preliminary calculation of the feedwater quantity. The basis for the calculation method is the heat flow balance of the evaporator heating surface into which the feedwater mass flow should enter, in particular at the inlet of the evaporator heating surface.

In practice, however, the measurement of the feedwater mass flow directly at the entrance of the evaporator heating surface proves to be technically complex and not reliably feasible in any operating condition. Instead, the feedwater mass flow at the inlet of the preheater is alternatively measured and included in the calculations of the feedwater quantity, which however is not always equal to the feedwater mass flow at the inlet of the evaporator heating surface.

Namely, if the temperature of the medium flowing to the preheater or the density of the flow medium within the preheater changes due to a change in heating, mass injection or Ausausicherungseffekten in the preheater, and the feedwater mass flow at the inlet of the preheater is not identical to that at the entrance the evaporator heating surface. If these injection and withdrawal effects are not or only insufficiently taken into account in the control of the feedwater flow, the above-mentioned overshoot of the specific enthalpy and thus large temperature fluctuations of the flow medium can occur at the outlet of the evaporator heating surface.

The size of the temperature fluctuations depends on the speed of the load change and a fast load change is particularly large. That's why it has been necessary make a limitation of the load cycle speed and thus take a lower efficiency of the steam generator in purchasing. In addition, the fast and uncontrollable load changes that occur in the event of malfunctions reduce the life of the steam generator.

The invention is therefore based on the object of specifying a method for operating a steam generator of the type mentioned above, which allows a largely synchronous change of the feedwater mass flow through the evaporator and the heat input into the evaporator in any operating condition without great technical effort.

This object is achieved in that the device for adjusting the feedwater mass flow Ṁ is associated with a control device whose control variable is the feedwater mass flow Ṁ and their setpoint Ṁ _s for the feedwater mass flow depending on one of the steam generator power associated setpoint L is performed, the control device as a the input values of the actual value ρ _{E of} the feedwater density at the inlet of the preheater is supplied.

The invention is based on the consideration that for the synchronous change of feedwater mass flow through and heat input into the evaporator heating a Wärmestrombilanzierung the evaporator heating should occur. Optimally, a measurement of the feedwater mass flow should indeed be provided at the inlet of the evaporator heating surface. However, because the direct measurement of the feedwater mass flow at the entrance of the evaporator heating surface has proven not to be reliably feasible, it is now provided at a location which is suitable upstream of the media, namely at the inlet of the preheater. However, since the possible mass injection and recovery effects in the preheater could distort the measured value, these effects should be appropriately compensated. For this purpose, a calculation of the feedwater mass flow at the entrance of the evaporator heating surface due to further easily obtainable measured variables. Particularly suitable measured variables for correcting the measured value for the feedwater mass flow obtained at the inlet of the preheater are the average density of the flow medium in the preheater heating surface and its temporal change.

verwendet, wobei Ṁ der Istwert des Speisewassermassenstroms am Eintritt des Vorwärmers, $\Delta \bar{\rho }$

die zeitliche Änderung der mittleren Dichte des Strömungsmediums im Vorwärmer und V das Volumen des Vorwärmers sind. Durch den Beitrag $\Delta \bar{\rho }\cdot V$

werden somit die genannten Ein- und Ausspeicherungseffekte berücksichtigt.For a particularly accurate calculation of the heat flow through the evaporator heating surface and also a particularly accurate correction of the measured value for the feedwater mass flow, the additional detection of the density of the flow medium at the outlet of the preheater heating surface is advantageously provided. This allows a particularly accurate detection and consequently also consideration of the aforementioned injection and Ausspeichernseffekte. In additional or alternative advantageous development is as the setpoint Ṁ _s for the feedwater mass flow of the expression $\dot{M}+\Delta \tilde{\rho }\cdot V$

where Ṁ is the actual value of the feedwater mass flow at the inlet of the preheater, $\Delta \tilde{\rho }$

the time change of the mean density of the flow medium in the preheater and V are the volume of the preheater. By the contribution $\Delta \tilde{\rho }\cdot V$

Thus, the aforementioned injection and withdrawal effects are taken into account.

näherungsweise die Dichte ρ_E des Strömungsmediums am Eintritt des Vorwärmers verwendet werden. In diesem Fall kann nämlich die zeitliche Änderung der Dichte ρ_E gleich der zeitlichen Änderung der mittleren Dichte $\bar{\rho }$

gesetzt werden, so dass eine zusätzliche Erfassung der Dichte ρ_A des Strömungsmediums am Austritt der Verdampferheizfläche nicht erforderlich ist.If the heat input into the flow medium within the preheater is stationary, ie does not change over time, the setpoint value Ṁ _s instead of the average density can be used for the calculation $\tilde{\rho }$

approximately the density ρ _{E of} the flow medium can be used at the inlet of the preheater. Namely, in this case, the change with time of the density ρ _{E can be} equal to the time change of the average density $\tilde{\rho }$

are set, so that an additional detection of the density ρ _{A of} the flow medium at the outlet of the evaporator heating surface is not required.

näherungsweise die Dichte ρ_E des Strömungsmediums am Eintritt des Vorwärmers verwendet wird. Daher wird der Istwert ρ_E der Eintrittsdichte vorteilhafterweise durch ein in der Regelungstechnik übliches Differenzierglied mit PT1-Verhalten in eine mit der Durchlaufzeit des Vorwärmers als Zeitkonstante verzögerte Eintrittsdichtenänderung umgewandelt.When calculating the setpoint Ṁ _s for the feedwater mass flow, it should be taken into account that the signal of the entry density change must be delayed according to the flow time of the system, if instead of the mean density $\tilde{\rho }$

approximately the density ρ _{E of} the flow medium is used at the inlet of the preheater. Therefore, the actual value ρ _{E of} the entry density is advantageously converted by a differentiating element with PT1 behavior customary in control technology into an entry density change delayed with the throughput time of the preheater as the time constant.

und ihrer zeitlichen Änderung $\mathrm{\Delta }\bar{\rho }$

allein durch die genäherte Verwendung der Eintrittsdichte nicht möglich. Da im arithmetischen Mittel ρ_E und ρ_A in die Berechnung von $\bar{\rho }$

jeweils zur Hälfte eingehen, kann im Fall eines instationären Wärmeeintrags, aber einer konstanten Eintrittsdichte ρ_E die halbe Änderung der Austrittsdichte ρ_A als Maß für die Dichteänderung im Vorwärmer verwendet werden.However, in particular in the case of a heating change in the preheater, that is to say a transient heat input into the flow medium within the preheater, for example during a load change, the calculation of the mean density is $\tilde{\rho }$

and their temporal change $\mathrm{\Delta }\tilde{\rho }$

not possible only by the approximate use of entry density. Since in the arithmetic mean ρ _E and ρ _A in the calculation of $\tilde{\rho }$

In each case, in the case of a transient heat input, but a constant entry density ρ _E, half the change in the exit density ρ _{A can be used} as a measure of the density change in the preheater.

Also in this case, the formation of the time derivative of the density signal is performed by a differentiator. However, since a change in the exit density is temporally downstream of the mass storage effect in the preheater, the density signal is advantageously PT1-delayed with a relatively small time constant of about one second.

With a separate detection of the densities of the flow medium at the inlet and at the outlet of the preheater can be considered in this way feed water injection and -ausspeichernseffekte in the preheater and the desired value of the feedwater flow can be easily adapted to the operating condition of the steam generator.

des Strömungsmediums im Vorwärmer ist aber bereits durch die Änderung der Dichte am Eintritt des Vorwärmers vollständig erfasst, so dass sich die Änderung der Dichte am Austritt der Verdampferheizfläche nicht mehr auf die berechnete Korrektur am Sollwert Ṁ _s des Speisewassermassenstroms auswirken darf. Daher ist vorzugsweise eine Korrekturschaltung vorgesehen, die die Reaktion des DT1-Gliedes, das das Dichtesignal am Austritt des Vorwärmers differenziert und verzögert, in diesem Fall kompensiert. Vorteilhafterweise wird dazu das Eintrittsdichten-Signal einem Totzeitglied mit einer Zeitkonstanten der Durchlaufzeit des Vorwärmers aufgeschaltet, entsprechend einer thermischen Zeitkonstanten des Vorwärmers PT1-verzögert und das so erzeugte Signal dem Austrittsdichtesignal negativ aufgeschaltet wird.For a particularly accurate control of the steam generator is also possible in cases where the temperature of the feed water changes abruptly before entering the preheater. This could be caused, for example, by the sudden failure of one done the preheater upstream external preheating. In such a failure, the jump in the density of the flow medium at the inlet of the preheater is largely unchanged until the exit through. The change of the mean density $\tilde{\rho }$

However, the flow medium in the preheater is already completely detected by the change in density at the inlet of the preheater, so that the change in density at the outlet of the evaporator heating surface must no longer affect the calculated correction at setpoint Ṁ _{s of} the feedwater mass flow. Therefore, a correction circuit is preferably provided which compensates for the reaction of the DT1 element, which differentiates and delays the density signal at the outlet of the preheater, in this case. Advantageously, the inlet density signal to a deadtime element with a time constant of the cycle time of the preheater is switched on, according to a thermal time constant of the preheater PT1-delayed and the signal thus generated the outlet density signal is switched negative.

In any case, this correction circuit ensures correct consideration of the density changes: In the event of an abrupt change in the temperature of the inflowing medium, the change in the outlet density ρ _{A is} not taken into account as described. However, if the entry density ρ _E remains constant, but the heat input in the preheater changes and thus the outlet density ρ _A , then no correction takes place at the outlet of the preheater and the effect of the change in the heat supply is completed when calculating the setpoint value Ṁ _s for the feedwater mass flow considered.

If, for example, the admission density ρ _E changes simultaneously with the heat input, for example during a load change, then both mass injection and recovery effects due to the density jump at the inlet and storage effects due to the changed heat input are considered separately. For the correction at the outlet of the preheater, only changes resulting from the changed heat input are considered, because the changes that occur due to the density jump at the inlet also at the exit time-delayed, are taken into account only at the entrance and compensated at the exit.

Advantageously, both the dead time and the thermal time constant of the preheater is adjusted reciprocally to the load of the steam generator.

Advantageously, the feedwater flow control is switched on and off depending on the operating state of the steam generator.

The advantages achieved by the invention are in particular that by the calculation of the feedwater mass flow taking into account the average density of the feedwater in the preheater as Korrekturterm the synchronous control of Speisewasserdurchflusses and the heat input into the Verdampferheizfläche in a particularly simple and reliable manner in all possible operating conditions Continuous steam generator safely prevents overshoot of the specific enthalpy of the flow medium at the outlet of the evaporator and large temperature fluctuations of the steam generated fresh, thus reducing material stress and increases the life of the steam generator.

FIG 1

eine Speisewasserdurchflussregelung für einen Durchlaufdampferzeuger,

FIG 2

eine alternative Ausführung der Speisewasserdurchflussregelung,

FIG 3a

ein Diagramm mit dem zeitlichen Verlauf der spezifischen Enthalpie des Strömungsmediums am Austritt der Verdampferheizfläche des Durchlaufdampferzeu- gers im Falle einer abrupten Temperaturänderung des zuströmenden Speisewassers im Volllastbetrieb des Durchlaufdampferzeugers,

FIG 3b

ein Diagramm mit dem zeitlichen Verlauf der spezifischen Enthalpie im Falle einer abrupten Temperaturänderung des zuströmenden Mediums im Teillastbetrieb des Durchlaufdampferzeugers, und

FIG 3c

ein Diagramm mit dem zeitlichen Verlauf der spezifischen Enthalpie im Falle eines Lastwechsels.

Embodiments of the invention will be explained in more detail with reference to a drawing. Show:

FIG. 1

a feedwater flow control for a continuous flow steam generator,

FIG. 2

an alternative embodiment of the feedwater flow control,

FIG. 3a

a diagram with the time course of the specific enthalpy of the flow medium at the outlet of the evaporator heating surface of Durchlaufdampferzeu- gers in the event of an abrupt change in the temperature of the incoming feedwater at full load operation of the continuous steam generator,

FIG. 3b

a diagram with the time course of the specific enthalpy in the event of an abrupt temperature change of the inflowing medium in part-load operation of the continuous steam generator, and

3c

a diagram with the time course of the specific enthalpy in case of a load change.

Identical parts are provided with the same reference numerals in all figures.

1 shows schematically a device 1 for forming the desired value Ms. for the feedwater mass flow of a continuous steam generator. The continuous steam generator has a designated as economizer preheater 2 for feed water, which is located in a throttle cable, not shown. The preheater 2 is on the flow medium side, a feedwater pump 3 upstream and a Verdampferheizfläche 4 downstream. In the guided from the feedwater pump 3 to the preheater 2 feedwater line a measuring device 5 for measuring the feedwater mass flow Ṁ is arranged through the feedwater line.

A drive motor on the feedwater pump 3 is assigned a controller 6, at the input of which the control deviation ΔṀ of the feedwater mass flow Ṁ measured by the measuring device 5 is located as a controlled variable. The controller 6 is associated with the device 1 for forming the set value Ṁ _s for the feedwater mass flow .

This device is designed for a particularly needs-based determination of the setpoint Ṁ _s . It is considered that the detection of the actual value of the feedwater mass flow M does not take place immediately before the evaporator 4, but already before the preheater 2. This could be due to unit of mass or -ausspeicherungseffekten in the preheater 2 inaccuracies in the measured value determination for the feed-water mass flow Ṁ result. To compensate for this, a correction of this measured value is provided taking into account the density ρ _{E of} the feedwater at the inlet of the preheater 2. The device 1 has among other things as input variables, on the one hand, a desired value L output by the setpoint generator for the output of the continuous steam generator and, on the other hand, the actual value ρ _{E of} the density of the feedwater at the inlet of the preheater 2 determined from the pressure and temperature measurement of a measuring device 9.

The setpoint value L for the output of the continuous steam generator, which changes over time in operation and which is given in the (not shown) Brennungsregelkreis directly to the fuel control, is also supplied to the input of a first delay element 13 of the device 1. This delay element 13 outputs a first signal or a delayed first power value L1. This first power value L1 is supplied to the inputs of function generator units 10 and 11 of the function generator of the feedwater flow control 1. At the output of the function generator unit 10 appears a value Ṁ (L1) for the feedwater mass flow, and at the output of the function generator unit 11 appears a value Δh (L1) for the difference of the specific enthalpy h _IA at the outlet of the evaporator 4 and the specific heat h _IE on The entry of this evaporator heating surface 4. The values Ṁ and Δh as functions of L1 are determined from values for Ṁ and Δh, which were measured during steady-state operation of the continuous steam generator, and stored in the function generator units 10 and 11 respectively.

The output quantities Ṁ (L1) and Δh (L1) are multiplied together in a multiplication element 14 of the function generator of the device 1. The recovered product value Q̇ (L1) corresponds the heat flow in the evaporator 4 at the power value L1 and, if necessary, after correction by a in a differentiator ?? From the entrance enthalpy determined, for injection or Ausspeichernseffekte in the evaporator characteristic power factor, entered as a counter in a divider 15. As a denominator, the difference between a desired value h _SA (L2) of the specific enthalpy at the outlet of the evaporator heating surface 4 and the actual value h _{IE of} the specific enthalpy at the inlet of the evaporator heating surface 4 formed by a summing element 19 is determined by means of the measuring device 9 is measured, entered.

The setpoint h _SA (L2) is taken from a third function generator unit 12 of the function generator of the device 1. The input value of the function generator unit 12 is produced at the output of a second delay element 16 whose input quantity is the first power value L1 at the output of the first delay element 13. Accordingly, the input value of the third function encoder unit 12 is a second power value L2 delayed from the first power value L1. The values h _SA (L2) as a function of L2 are determined from values for h _SA , which were measured during stationary operation of the continuous steam generator, and stored in the third function generator unit 12.

The output of the divider 15, the set value Ṁ _s for the feedwater mass flow for taking place in a summing 23 formation of the controller 6 supplied control deviation of the measured with the device 5 actual value for the feedwater mass flow into the preheater 2 are removed.

At the output of the second delay element 16 is the input of a differentiating element 17, the output of which is negatively connected to a summing element 18. This summing element 18 corrects the value for the heat flow Q̇ (L1) into the evaporator heating surface 4 to the output signal of the differentiating member 17th

The measured by the measuring device 9 actual values of temperature and pressure of the feedwater at the inlet of the preheater 2 are converted in a differentiator 22 in an actual value ρ _{E of} the feedwater density at the inlet of the preheater 2. This is given to the input of a differentiating element 22 and multiplied by the volume of the preheater. The thus calculated approximate value Δ Ṁ for the change in the feedwater mass flow due to injection and Ausspeichernseffekten within the preheater 2 is fed via an integrated into the differentiating element 22 delay element with the flow time of the feedwater through the preheater 2 as a time constant a summing 24, which sets the target value for corrected the mass flow Ṁ _s from the differentiator 15 by Δ Ṁ and thus allows the consideration of Massenein- and -Estspeicherseffekten due to a change in temperature and thus the density of the feedwater at the inlet of the preheater 2 in the control of the feedwater mass flow.

FIG. 2 shows an alternative embodiment of the feedwater flow rate control, which, even in the case of a temporal change in the heat input within the preheater 2, enables the reliable consideration of mass injection and recovery effects in the regulation of the feedwater mass flow.

The feedwater flow control according to FIG. 1 is supplemented in the exemplary embodiment according to FIG. 2 by the consideration of the density ρ _{A of} the flow medium at the outlet of the preheater 2. To determine the density of the flow medium at the outlet of the preheater 2, a measuring device 21 for measuring the pressure and the temperature of the flow medium is provided at the outlet of the preheater 2. The computing element 26 determines as an input signal for a downstream summing element 30 from the measurement of temperature and pressure, the actual value for the density ρ _{A of} the flow medium at the outlet of the preheater 2. The output of the summing 30 is fed to a differentiator 36, the time derivative multiplied by the volume of the preheater 2 as an output signal. This output signal, which represents the change over time of the feedwater mass flow Δ Ṁ _A at the outlet of the preheater 2, is applied to a summing element 36, which has as a second input variable the change Δ Ṁ _{E of} the feedwater mass flow at the inlet of the preheater 2.

The summing element 36 has as an output signal calculated from Δ M _A and Δ M _E mean change of the feed-water mass flow Δ Ṁ basis of unit of mass and -ausspeicherungseffekten in the preheater 2. The output of the Dividiergliedes 36 is at the summing element 24 to the output signal of the Dividiergliedes 15 for correcting the Setpoint of the feedwater mass flow switched.

In the case of a malfunction, which leads to an abrupt change in temperature of the incoming feedwater to the preheater 2, for example in the event of sudden failure of an upstream preheating section, the output signal of the computing element 26 must still be corrected by the effect of the changed input density. If this is not done, the effect of the density jump at the inlet of the preheater 2 is taken into account twice, namely in the detection of the density of the feedwater at the inlet and at the outlet of the preheater 2. To correct this, the output signal of the differentiator 20 is a deadtime member 28 with the flow time of the feedwater through the preheater 2 as a time constant switched. The signal thus generated is negatively connected to the summing 30 via a delay element 32 with a thermal storage constant of the preheater 2. Thus, the effect of the density jump is eliminated at the entrance of the preheater 2 in the exit density signal and thus only once and not considered twice in the calculation of the correction mass flow.

The feedwater flow control using the device 1 allows in any operating condition of the steam generator, a particularly simple determination of the setpoint Ṁ _s for the feedwater mass flow through the evaporator 4. By a precise vote of this feedwater mass flow on the heat input in the evaporator heating can large fluctuations in the outlet temperature of the live steam and a Overshoot of the specific enthalpy at the outlet of the evaporator 4 are reliably prevented. High material loads due to temperature fluctuations, which lead to a reduced service life of the continuous steam generator, can thus be avoided.

The curve shown in FIG. 3a (curves I to III) of the three specific enthalpies in kJ / kg at the outlet of the evaporator heating surface 4 as a function of time t was determined for a continuous steam generator in full load operation in the event of failure of a preheating line preceding the preheater 2. The curve I in FIG 3 is for the case that caused by the simulated malfunction density change of the feed water is not considered in the feed water flow control at the inlet of the preheater 2, so that _S m the uncorrected output signal for the feed-water mass flow as a target value of the divider element 15 according to FIG 1 or 2 is used.

The curve II applies to the case that only as shown in FIG 1, the temporal change in the density ρ _E at the inlet of the preheater 2 and thus only the Massenein- and -Estspeicherseffekte due to the temperature jump at the inlet of the preheater 2 is taken into account in the feedwater flow control. Massenein- and -Estspeicherseffekte due to a change in heating in the preheater 2 and thus a change in the heat input into the feed water stay unconsidered. This case corresponds to the feedwater flow control of FIG. 1.

The curve III finally shows the time course of the specific enthalpy with additional consideration of the mass injection and -ausspeichernseffekte due to a change in heating in the preheater 2, which corresponds to the feedwater flow control of Figure 2. In this case, the summing element 24 from FIG. 2 has, in addition to the output variable of the differentiating element 15, the mean change in the feedwater mass flow Δ berechn calculated from Δ Ṁ _A and Δ Ṁ _E as a second input variable. In this case, the feedwater flow control therefore not only takes into account the density ρ _E at the inlet of the preheater 2, but additionally the density ρ _A at its outlet. By separately recording both densities ρ _E and ρ _A , mass injection and recovery effects due to a change in heating in the preheater 2 as well as due to a changed temperature of the feed water at the inlet of the preheater 2 can be taken into account.

3 b shows the course (curves I to III) of the three specific enthalpies in kJ / kg at the outlet of the evaporator heating surface 4 as a function of the time t for a continuous steam generator in partial load operation (50% of the maximum power) in the event of a failure of the preheater 2 upstream preheating.

The curve I in FIG. 3b applies, as in FIG. 3 a, for the case where the density change of the feedwater caused by the failure of the preheater 2 preceding the preheater 2 is not taken into account at the inlet of the preheater 2 in the feedwater flow control, ie as the setpoint value Ṁ _s for the Feedwater mass flow the uncorrected output signal of the divider 15 is used according to FIG 1 or 2.

The curve II in FIG. 3b applies, as in FIG. 3 a, for the case in which only the temporal change of the density ρ _E at the inlet of the preheater 2 in the feedwater flow control is taken into account, as shown in FIG. Mass injection and storage effects due to a change in heating in preheater 2 are not taken into account. This case corresponds to the feedwater flow control of FIG. 1.

As shown in FIG. 3 a, the curve III in FIG. 3 b shows the time characteristic of the specific enthalpy with additional consideration of the mass injection and removal effects due to a changed heating in the preheater 2, which corresponds to the feedwater flow control of FIG.

FIG. 3c shows the course (curves I to III) of the three specific enthalpies in kJ / kg at the outlet of the evaporator heating surface 4 as a function of the time t for a continuous steam generator during a load change from full load to partial load operation (100% to 50% load).

The curve I in FIG. 3 c applies, as in FIG. 3 a, for the case in which the density change of the feedwater caused by the failure of the preheater 2 is not taken into account at the inlet of the preheater 2 in the feedwater flow control , ie the uncorrected setpoint value Ṁ _s for the feedwater mass flow Output signal of the divider 15 is used according to FIG 1 or 2.

The curve II in FIG. 3 c applies, as in FIG. 3 a, for the case in which only the temporal change of the density ρ _E at the inlet of the preheater 2 in the feedwater flow control is taken into account, as shown in FIG. Mass injection and storage effects due to a change in heating in preheater 2 are not taken into account. This case corresponds to the feedwater flow control of FIG. 1.

The curve III in FIG. 3c, as in FIG. 3 a, shows the time curve of the specific enthalpy with additional consideration of the mass injection and recovery effects due to a changed heating in the preheater 2, which corresponds to the feedwater flow control from FIG.

The diagrams according to FIGS. 3 a, 3 b and 3 c show that the feedwater flow control 1 from FIG. 1 or 2 is particularly suitable for preventing an overshoot of the specific enthalpy at the outlet of the evaporator heating surface 4.

Claims (8)

Method for operating a continuous steam generator with an evaporator heating surface (4), a Vorwärmer (2) upstream of the evaporator heating surface (4), a device for adjusting the feedwater mass flow Ṁ and a feed water flow control (1) associated with this device whose controlled variable is the feedwater mass flow Ṁ and whose Setpoint Ṁ _s for the feedwater mass flow Ṁ depending on a steam generator power associated setpoint L is performed, the feedwater flow control (1) as one of the input variables of the actual value ρ _{E of} the feed water density at the inlet of the preheater (2) is supplied. The method of claim 1, wherein the feedwater flow control (1) as a further input variable, the actual value ρ _{A of} the feed water density at the outlet of the preheater (2) is supplied. The method of claim 1 or 2, wherein the size $$\dot{M}+\Delta \tilde{\rho }\cdot V$$

as setpoint value Ṁ _s for the feed-water mass flow is used, wherein
Ṁ the actual value of the feedwater mass flow at the inlet of the preheater (2), $\Delta \tilde{\rho }$

the temporal change of the mean density of the feedwater within the preheater (2) and V is the volume of the preheater (2). Method according to one of claims 1 to 3, wherein the approximate value for the average density $\tilde{\rho }$

the density ρ _{E of} the feed water is used at the inlet of the preheater (2). Method according to claim 3 or 4, wherein the temporal change of the mean density $\Delta \tilde{\rho }$

of the feed water in the preheater (2) is formed by a functional element with differentiating behavior. Method according to one of claims 2 to 5, wherein the entry density signal to a dead time member with a time constant of the flow time of the preheater (2), corresponding to a thermal time constant of the preheater (2) PT1-delayed and the signal thus generated the outlet density signal negatively becomes. A method according to claim 6, wherein both the dead time and the thermal time constant of the preheater (2) are adjusted reciprocally to the load of the steam generator. Method according to one of claims 1 to 7, wherein the feedwater flow control (1) as needed switched on and off.

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