US3423933A

US3423933A - Cyclic process for steam power plants

Info

Publication number: US3423933A
Application number: US530653A
Authority: US
Inventors: Klaus Knizia
Original assignee: L&C Steinmueller GmbH
Current assignee: Hitachi Zosen Inova Steinmueller GmbH
Priority date: 1965-03-01
Filing date: 1966-02-28
Publication date: 1969-01-28
Anticipated expiration: 1986-01-28
Also published as: DE1551264A1; GB1152441A

Description

Jn. 28, 1969 KNlZlA 3,423,933

CYCLIC PROCESS FOR STEAM POWER PLANTS Filed Feb. 28, 1966 Sheet of '7 Jan. 28, 1969 K. KNlZlA CYCLI; PROCESS FOR STEAM POWER PLANTS Filed Feb. 28, 1966 Jan. 28, 1969 K. KNIZIA 3,423,933

CYCLIC PROCESS FOR STEAM POWER PLANTS Filed Feb. 28, 1966 Sheet 3 of 7 Jan. 28, 1969 K. KNIZIA 3,423,933

CYCLIC PROCESS FOR STEAM POWER PLANTS med Feb. 28. 1966 Sheet 4 of 7 55 1 I 56 58 l 5 v Jan. 28, 1969 K. KNIZIA 3,423,933 I CYCLIC PROCESS FOR STEAM POWER PLANTS Filed Feb. 28, 1966 7 Sheet 445 IN YENTOR z aar f/I/ Z/Q Jan; 28, 1969 K. KNIZIA I 3,423,933

CYCLIC PROCESS FOR STEAM POWER PLANTS- Filed Feb. 28, 1966 Sheet 6 of '7 402 g 1H V05 i //flo' Jan. 28, 1969 I K. KNlZIA 3,423,933

CYCLIC PROCESS FOR STEAM POWER PLANTS Filed Feb. 28, 1966 Sheet 7 fi 70. i

g 430 -f30 435 e United States Patent O" 6 Claims ABSTRACT OF THE DISCLOSURE Steam power installation and method of operating in which feedwater is passed through a feedwater line, having preheater means therein, to a primary steam circuit and wherein the primary circuit supplies superheated steam to a primary turbine. superheated steam is withdrawn from the turbine and is passed through steam producer means and then to the preheater means and then to the feedwater line. Water branched off from the feedwater line is supplied to the steam producer means where it is connected into steam and is then passed to a place of expansion and is expanded and then returned to the feedwater line.

The present invention relates to a cyclic process for steam power plants with process internal heat exchange.

With cyclic processes, the obtainable degree of efficiency depends on the ratio of the median lower temperature of heat elimination toward the outside T to the median upper temperature of heat elimination from the outside T in conformity with UB T With the steam power process employing water as working medium, low values for T are obtained in conformity with the surrounding temperature in view of the possible isothermic isobaric heat elimination. The efforts of improving the situation are therefore directed to an increase in T which is aimed at with the steam power process by high live steam variables of state (P T by introducing a simple or multiple intermediate overheating, and by process internal heat exchange when a regenerative preheating is involved. From the course of the isobars in the liquid, the wet steam, and the overheating phase of the water, it will be seen that by regenerative preheating of the feed water, it is possible considerably to increase T This regenerative preheating is effected by withdrawal from the turbine at different bleeder pressures, while the individual withdrawn quantities of steam give off their overheating heat and during the condensation in the preheaters at the saturation temperature pertaining to the respective pressure also give off their heat of vaporization for heating up the feed water flow. In view of the temperature differences occurring during the heat exchange, energy losses are encountered which heretofore have been reduced by a multi-step regenerative preheating operation and by deheater circuits.

By introducing single and multiple intermediate overheating and with increasing steam temperatures, the expansion lines of the steam in the i, s and T, s-diagram are transformed to values of higher entropy so that the steam which is being withdrawn at different bleeder pressures will be overheated to a higher degree. Inasmuch as thus also with the heater circuits to the feed water to be heated up, there exists still high differences in temperature, corresponding losses in energy are encountered. Expressed diiferently, with a cyclic process with regenerative preheating, the energy losses encountered with the 3,423,933 Patented Jan. 28, 1969 process internal heat exchange reflect the degree of efiiciency of the process. The lower these losses in energy, the higher will be the degree of efficiency of the process.

In addition to deheater circuits, also so-called preheating turbines have been suggested which could be employed with processes having at least one intermediate overheating step. With these processes, the steam leaving the topping turbine was to a great extent after effected intermediate heating, conveyed to the after-turbine, whereas the steam intended for the bleeder preheating at the individual pressure stages was not preheated but was expanded in the topping turbine to the individual bleeder pressures. The steam thus employed for the regenerative preheating was less overheated than would be the case if it had been withdrawn from the after-turbine. Consequently, also slight losses in energy were encountered with a process internal heat exchange.

It is, therefore, an object of the present invention to improve the cyclic process for steam power plants over the heretofore known processes set forth above.

It is another object of this invention so to improve the cyclic processes for steam power plants that the losses in energy during the process internal heat exchange will be reduced and the degree of efficiency of the process will be increased.

These and other objects and advantages of the invention will appear more cearly from the following specification in connection with the accompanying drawings, in which:

FIGS. 1 to 3 indicate successively how the values for T are increased first with regard to the simple process as shown in FIG. 1 by introducing intermediate heating (FIG. 2) and additionally by regenerative preheating (FIG. 3).

FIG. 4 shows a process in the T, s-diagram.

FIG. 5 illustrates the basic circuit of the improved method according to the invention.

FIG. 6 shows a modified circuit according to the invention in which the basic circuit of FIG. 5 is improved by the regenerative preheating by a secondary circuit.

FIG. 7 illustrates the diagram of a steam power process with single intermediate heating and prestage preheating while employing a plurality of secondary steam circuits.

FIG. 8 represents a diagrammatic circuit illustrating that the secondary steam produced in a secondary steam producer is conveyed to a primary circuit.

FIG. 9 diagrammatically illustrates a circuit according to which the secondary steam produced in a secondary steam producer is conveyed to a secondary turbine.

FIG. 10 diagrammatically illustrates a circuit according to which in a secondary steam producer a plurality of feed water flows are brought into heat exchange with a single quantity of bleeder steam while a feed water branch flow is conveyed to a further subsequent secondary steam producer for purposes of increasing its heat content by heat exchange with a further quantity of bleeder steam, while the other feed water branch flow is through heat exchange converted to variables of state which permit direct feeding of the produced secondary steam into a secondary turbine, into the primary circuit, or into the primary turbine.

In conformity with the present invention, the bleeder steam withdrawn from the turbine conveys its overheating heat to a branch flow of the feed water which, under a higher pressure, vaporizes said branch flow and heats the same up to a temperature which, in conformity with the employed heating surfaces, is still economic and which has a temperature lower than the bleeder steam temperature by AT. The thus produced steam can be usefully employed in various manners.

Expressed differently, by means of steam withdrawn from the turbine and prior to the entry of said steam in one or more preheaters while overheating heat is given off, a feed water branch flow under higher pressure or a plurality of feed water branch flows are vaporized, and the steam produced from the feed water branch flow or flows is additionally overheated to an economically still admissible temperature. With the production of steam in a so-called secondary circuit, due to the deheating of the steam withdrawn from the turbine, a reduction in the energy losses during the heat exchange in the regenerative preheating process will occur whereby the degree of efficiency of the process is increased.

According to a further development of the present invention, the produced steam is expanded in an additional turbine to the bleeder pressure and thus performs work. Depending on the starting pressure of this secondary steam power proces employed in the regenerative preheating, the expansion end point may be located in the secondary turbine in the neighborhood of the saturated steam line at the isobar pertaining to the bleeder pressure. The steam leaving the secondary turbine. and the deheated bleeder steam thus are in the form of saturated steam or only slightly deheated steam available for preheating the feed water in the corresponding stage. Particularly with power plant blocks of high output which are to be expected in the near future, the quantities of bleeder steam assume values which also with secondary circuits will result in quantities of steam which have to be economically employed.

According to a further development of the present invention, it is suggested to feed the secondary steam produced in the secondary circuit, directly into the primary circuit or directly into the primary turbine. There will thus be obtained two further groups of possible instances of application each of which will have its particular merit in conformity with the prevailing conditions. The production of secondary steam may in both groups of employment be effected not only in a single secondary steam producer but also in a plurality of secondary steam producers. Into the individual secondary steam producers there are introduced equal quantities of steam which have the same or different variables of state, or there may be introduced different quantities of steam which have the same or different variables of state. These quantities of steam are brought into a heat exchange with quantities of feed water of the same or different pressure. The arrangement of the secondary steam producer may in conformity with a preferred embodiment of the invention be so effected that in all steam producers, a secondary steam with the same or different variables of state will be produced while the feed-in into said secondary steam producers may be effected at the respective area in the primary circuit or in the primary turbine. It is obvious that this may also be effected when a secondary turbine is provided.

Heretofore in all groups of the possible application of the useful employment of the secondary steam namely feed-in into a secondary turbine, feed-in into the primary steam line, or feed-in into the primary turbine, the presumption was made that in the secondary steam producer a single definite quantity of feed water from a single quantity of bleeder steam was vaporized and overheated. However, according to a further preferred embodiment of the invention, it is also possible in a single secondary steam producer separately to vaporize and overheat feed water partial fiows of two or more different pressures which feed water flows originate from a single or several quantities of bleeder steam. These quantities of secondary steam, which have different variables of state, may be expanded for instance in a two or more pressure turbine. It is obvious that these quantities of secondary steam which have different variables of state can also be fed into the primary circuit at the areas corresponding to said variables of state or can be introduced into the primary turbine.

However, according to still another preferred embodiment of the invention, it is also possible in addition to a single secondary steam producer having a plurality of feed water branch flows of different pressure stages, to employ a plurality of secondary steam producers. In this instance, the feed water branch flows are preheated, partially vaporized or vaporized in the first secondary steam producer by a single or several quantities of bleeder steam, and the preheated or partially vaporized or vaporized feed water branch fiow or branch flows is or are in a secondary steam producer or producers following said first secondary steam producer transformed by a single or a plurality of quantities of bleeder steam to the respective variables of state in conformity with the feed-in point into the secondary turbine, in the primary circuit or in the primary turbine.

When employing secondary steam producers in which secondary steam of different variables of state or condition are produced at the same time, it is also possible according to a further embodiment of the invention to feed those quantities of secondary steam which have the same variables of condition to a common collecting line and from there either to a secondary turbine, the primary circuit or the primary turbine. Also in this instance, it will be obvious that the secondary steam is fed into the secondary turbine or primary steam line or primary turbine at a place where the same variables of condition prevail.

Referring now to the drawings, FIGS. 1, 2 and 3 indicate how the values for T can be raised first with regard to the simple process as illustrated in FIG. 1 and then by introducing intermediate heating as shown in FIG. 2 and further by empolying regenerative preheating as illustrated in FIG. 3.

The basic circuit of the improved process according to the invention is illustrated in FIGS. 4 and 5. FIG. 4 illustrates the process in a T, s-diagram. The feed water is by regenerative preheating, i.e., by process internal heat exchange and by the introduction of heat from the outside preheated in the boiler from point 1 to point 4, vaporized from point 4 to point 5 and overheated from point 5 to point 6. The steam expands subsequently from point 6 to point 8 in the topping turbine and after leaving the latter is from point 8 to point 9 again intermediateoverheated in the boiler by the feed-in of heat from the outside and is subsequently expanded in the after-turbine from point 9 to the pressure in the condenser at point 31. The withdrawn quantities of bleeder steam. for the regenerative preheating may in conformity with the selected example lie at the expansion points 7, 8, 10, 11, 12 and 13.

For the withdrawal at point 10, there will now be explained the turning on of the secondary steam circuit (FIG. 5). The bleeder steam withdrawn from the afterturbine at point 10 passes into the secondary steam producer 18 where it brings about the vaporization and overheating up to point 14 (FIG. 4) of the branch flow pressed by feeder pump 20 into the pipe system. This branch flow passes through the changes in condition from three or still lower values for the feed water to 14 and from there up to the overheating to 15. The steam will in secondary turbine 17 be expanded to point 16 while performing work. During the production of steam in the secondary steam producer 18, the bleeder steam will likewise be deheated to point 16. The thus deheated bleeder steam and the steam expanded in the secondary turbine will now from point 16 to point 2 convey their vaporization heat to the feed water in order to preheat the same from point 22 to point 2. The above described circuit of the present invention is over the deheater circuit advantageous for the reason that not only a deheating of the steam to the temperature of the feed water from the highest preheater is effected which with the temperature differences still existing in connection therewith at the same time leads to energy losses, but that a considerable portion of the overheating heat of the bleeder steam will be converted into work in the secondary circuit. Due to the fact that the quantity of steam available for perheating the feed water in within the range of the saturated steam condition, and due to the fact that the overheating heat to a great extent no longer is available for preheating, also the quantity of withdrawn steam at will rise. The degree of efficiency of the process thus increases additionally in view of the greater quantity of steam which circulates in the high temperature range of the process up to point 10 and performs work without the quantity of steam flowing into the condenser and thus the quantity of steam loss at the end of the process being increased.

The secondary circuit is advantageous over the preheating turbine inasmuch as due to the secondary circuit also bleeder steam which is withdrawn at high pressures of condition (for instance at point 7), can be heated at a lower loss in energy than was heretofore possible, and inasmuch as the live steam pressure of the secondary circuit can be freely selected for instance in such a way that during the steam expansion in the turbine of the secondary circuit, a predetermined expansion end point for instance within the range of the saturated steam curve will be obtained at the isobars pertaining to the bleeder pressure.

The above basic circuit of the improved regenerative preheating can be varied in a manifold Way by a secondary circuit. FIG. 6 shows such variation. With reference to FIG. 6, the steam flows from the steam withdrawal at points 7 and 10 (FIG. 4), gives off its overheating heat in the respective steam producers of the

secondary circuit

28, 18 and produces secondary steam at live steam pressure. The secondary circuit is fed from the main feeding line through a feed water branch flow branching off at point 30. In the turbine of the secondary circuit, the thus produced steam is expanded up to the isobar passing through point 7, and, more specifically, down to the range of the saturated steam curve, and subsequently the steam is fed into the preheater 29. The deheated steam withdrawn at point 7 and deheated in the secondary steam producer 28 will give off its vaporization heat in the preheater 29. In an analogous manner, the deheated bleeder steam withdrawn at 10 and deheated in the secondary circuit 18 is condensed in the preheater 19. In this connection, it is possible that the preheater 19 may be formed by the feed Water container. In such an instance, the feed water pump 23 would have to be arranged in the feed Water line between the preheater 26 and the feed water container 19. The circuit according to FIG. 6 does not contain a secondary steam circuit for the steam withdrawn at 8 because during the withdrawal at a separating pressure, the steam is generally not overheated to any material extent. It would, however, be possible to arrange this withdrawing stage and further withdrawing stages in the secondary circuit. It is also possible to operate a number of secondary steam circuits with different steam variables of condition which are operatively connected with a multi pressure turbine or different turbines. Similarly, the expansion end points in the secondary turbine may be located at different pressures.

FIG. 7 illustrates the circuit of a steam power process with simple intermediate overheating and with a six-stage preheating while including a plurality of secondary steam circuits. This circuit merely represents an example for some of the possibilities of interposing secondary circuits in the process. With this circuit, it is possible that all three secondary turbines be combined to a multipressure turbine with a number of inlets and outlets which may be employed for instance for aiding the drive of the feed pump.

The exhaust steam leaving the post turbine is condensed in the condenser 33 and through pump 23 is conveyed into the low pressure preheater 24 which is supplied with bleeder steam from the bleeder point 13. The condensate obtained at 24 is through pump 25 admixed at 36 again to the main condensate flow. The bleeder steam of this lowest stage is overheated to such a minor extent only that the insertion of a secondary circuit is uneconomical.

The condenser flow then enters the preheater 19. This preheater 19 is steam-wise connected to the secondary steam producer 18 and the exhaust outlets of the secondary turbine 17 of said first secondary steam circuit. The steam withdrawn from the after-turbine at point 12 is deheated in the steam producer 18 and in this way produces steam in the secondary circuit. Pump 40 increases the pressure of the feed water branch flow to such an extent that the exhaust steam leaving the secondary turbine 17 will after its expansion reach a condition near the saturated steam curve at the isobar pertaining to 12. The exhaust steam from the secondary turbine 17 and the deheated steam from the steam producer 18 give off their heat of vaporization in the preheater 19 for preheating the feed Water.

In the next higher secondary circuit, the overheating heat of the steam from the withdrawal at point 8 and from the withdrawal at point 11 is made use of in order in the respective

secondary steam producers

38, 28 pertaining thereto to produce steam of the same pressure as is expanded in the secondary turbine 37, while the exhaust steam is condensed in the condenser 61 between the preheaters 43 and 46 and is used for heating up the feed water. The condensate is pumped from this condenser 61 through pump 62 into the main condenser flow. It is also possible to permit this steam to expand to another end isobar than is effected in this circuit whereby the exhaust steam of the turbine could at another point of the circuit be employed for preheating the feed water. The steam leaving the

secondary steam producers

38 and 28 respectively will in the form of saturated steam at the isobars pertaining thereto heat up the feed water in the preheaters 53 and 43 by giving 011 its heat of vaporization. The feed water branch flow, which is Withdrawn from the feed water container 46 for producing steam in the secondary steam circuit, is through a pump 51 fed into the two secondary steam producers at point 49. It is advantageous to employ water of the respective highest possible temperature level as feed water for the secondary steam circuits in order thereby to maintain the quantity of secondary steam produced at the same given off overheating heat as large as possible.

Thus, for the first two above described secondary steam circuits, feed water could be withdrawn at approximately the point 59. However, inasmuch as this water has already been pressed by the feed pump to the entrance pressure in the boiler, considerable throttling losses Will be encountered if the pressure for the live steam of these secondary circuits is selected lower than the feed water pressure at 59.

Finally, a third secondary circuit may be provided in which again two withdrawal stages namely the withdrawal stages at

points

7 and 10 will in the

secondary steam producers

58 and 48 pertaining thereto produce secondary steam of the same pressure. The steam flows will in these two secondary steam producers give off their overheating heat and are then in the

preheaters

56, 46, pertaining thereto employed for preheating the feed water. The exhaust steam of the secondary turbine of this circuit is at point 57 admixed to the saturated steam leaving the preheater 58. The exhaust steam will at this point have approximately the same condition as the deheated bleeder steam. As feed water for this secondary circuit, feed water is at point 59 branched off from the main feed line.

Referring now to FIG. 8, it will be seen that from the high pressure part of turbine 101 through conduits 102, bleeder steam is withdrawn at the separating pressure through conduit 103. From the after-part 104 through

conduits

105, 106, 107, bleeder steam is withdrawn for preheating the feed water while an evaporization and a subsequent overheating of a portion of the feed water is effected. The bleeder steam from the high pres- I sure part of turbine 101 flows through conduit 102 to a secondary steam producer 108 and is heated therein while the feed water withdrawn from the main steam producer at point 109 is vaporized. This produced steam is at point 110 conveyed again to the main steam producer. The steam deheated in the secondary steam producer 108 is condensed in the preheater 111 pertaining thereto. The condensate flows through

preheater

112, 113 back to the feed water container 114. The bleeder steam withdrawn at the separating pressure (trenndruck) is through conduit 103 conveyed to the preheater 112 pertaining thereto and is there condensed. This system does not contain a secondary steam producer because the steam is generally at the separating pressure not highly overheated. The employment of a secondary steam producer, however, would be possible. The steam withdrawn through conduit 105 after the intermediate overheating in the first bleeder steam stage of the after-part 104 of the turbine again flows to a secondary steam producer 115 in which likewise secondary steam is produced from the feed water which has been withdrawn from the boiler at point 109. The steam obtained in this secondary steam producer 115 is at 116 fed into the conduit which leads into the primary circuit at 110. The steam deheated in the secondary steam producer 115 is condensed in the preheater 113 pertaining thereto. In

preheaters

111 and 113, the steam is condensed with variables of conditions which with the isobars pertaining thereto lie in the neighborhood of the saturated steam line. The overheating heat of these bleeder steam flows produces steam of a higher pressure level which is again added to the primary circuit. Since the overheating heat of the bleeder steam flows is, therefore, not available for the preheating of the feed water, it is necessary that at the same quantity of feed water to be preheated, the required bleeder steam flow must increase. The improvement of the degree of efficiency of the process is seen in the fact that while the heat, which in the condenser is conducted away toward the outside, remains the same, the flows at the high pressure and medium pressure side of the process rise and thus more electric work is produced which means that also the degree of efiiciency of the process is increased. This improvement of the degree of efficiency of the process can also be expressed by stating that with the process internal heat exchange with steam production in the secondary steam producers, the heat supply from the outside is effected at a higher median temperature level.

FIG. 9 illustrates a steam power process according to which in a secondary steam producer steam of difierent variables of condition is produced which in its turn is conveyed to a secondary turbine at points corresponding to said variables of condition. From the high pressure part of turbine 101 through

conduits

102 and 103 and from the low pressure part 104 through

conduits

105, 106 and 107 bleeder steam is withdrawn. The bleeder steam withdrawn from the high pressure part through conduit 102 is conveyed to a secondary steam producer 108 and is there heated. It produces secondary steam and subsequently after its deheating is condensed in the preheater 111 pertaining thereto. The bleeder steam withdrawn from the high pressure part of the turbine 101 through conduit 102 at the separating pressure of the process is through conduit 103 conveyed to the preheater 112 and is there condensed without the interposition of a secondary circuit. The bleeder steam withdrawn from the low pressure part 104 of the turbine through conduit 105 flows to a secondary steam producer 117 .in which two feed water branch flows of different pressure level are vaporized. The feed water withdrawn at 118 from the feed pump 119 flows through conduits 120 and 121 to the systems of the

secondary steam producers

108 and 117. The steam here produced passes through conduit 122 to the secondary turbine 123. Through conduit 121, feed water at low pressure is conveyed to the secondary steam producer 117 while the secondary steam produced in this part of the secondary steam producer is conveyed through conduit 124 to the secondary turbine 123. The steam expanded in secondary turbine 123 is at point 125 conveyed to the steam conduit leading to the preheater 112 and is there condensed. The expansion end point of the steam in the secondary turbine 123 could, if desired, also be so selected that the steam pertaining thereto is condensed in another preheater for instance in the preheater 113. The bleeder steam deheated in the secondary steam producer 117 is condensed in the preheater 113 and from here in the form of heating condensate flows to the feed water container 114.

As will be seen from FIG, 10,

secondary steam producers

126 and 127 may be provided in a steam power process while in the secondary steam producers 126 feed water branch flows of different or the same pressure stages could through

conduits

128 and 129 be fed into the secondary steam producer 126 and could be brought into heat exchange with bleeder steam introduced through conduit 130 into the secondary steam producer 126. In the secondary steam producer, the heat exchange may be carried out in such a way that the feed water branch flow introduced through conduit 128 does not yet have the condition required for the further employment, in other Words, this feed water branch flow merely preheats, partially evaporates or evaporates. In such instances, this feed water branch flow can through a conduit 131 be conveyed to the secondary steam producer 127 and here may be brought into heat exchange with bleeder steam introduced through conduits 132 into the secondary steam producer 127. In this stage, for instance, the condition of the secondary steam can be realized for the further employment of said secondary steam.

The feed water branch flow introduced into the secondary steam producer 126 through conduit 129 can in heat exchange with the quantity of bleeder steam introduced into the secondary steam producer 126 through conduit 130 be directly converted to the condition required for the further employment for instance in a secondary turbine,

in the primary circuit or in a primary turbine. In this instance, the feed water branch flow introduced into the secondary steam producer 126 through conduit 129 is after evaporation and corresponding overheating no longer passed through a subsequent further secondary steam producer but is directly conveyed to the corresponding consumer. In this instance, it is possible in the free circuit of the secondary steam producer 127 through conduit 138 to feed a further feed water branch flow into the secondary steam producer 127 and to preheat, partially vaporize, vaporize or overheat said branch flow accordingly in conformity with the prevailing heat of the bleeder steam introduced through conduit 132 into the secondary steam producer 127. The bleeder steam introduced into the

secondary steam producers

126 and 127 through conduits and 132 is after effected heat exchange conveyed through corresponding conduits 133 and 134 to the preheaters 135 and is there condensed. The condensate deposited in the vaporizer is through a corresponding collecting line 137 conveyed to a collecting container (not illustrated).

It is, of course, to be understood that the present invention is, by no means, limited to the particular circuits shown in the drawings but also comprises any modifications within the scope of the appended claims. Thus, the arrangement of secondary steam producers as shown in FIG. 10 is not limited to two secondary steam producers only but it is also possible to arrange a plurality of secondary steam producers accordingly while the arrangement has always so to be selected that in the end phase overheated steam of a condition is obtained which corresponds to the quantities of steam available at the corresponding feed-in points.

What I claim is:

1. The method of operating a steam powerplant having a feedwater line supplying a primary circuit with feedwater, preheater means in said feedwater line, and primary steam turbine means to which said primary circuit supplies superheated steam; withdrawing superheated steam at a predetermined pressure from said turbine from between the high and low pressure ends thereof, withdrawing Water at a pressure higher than said predetermined pressure from said feedwater line, passing the withdrawn feedwater and withdrawn superheated steam in heat exchange relation thereby to convert the said water to superheated steam while extracting at least the superheat from the withdrawn steam, delivering the said withdrawn steam to the feedwater line preheater means and then to the feedwater line, expanding the said superheated steam formed by the withdrawn feedwater to extract work therefrom and then returning it to said feedwater line.

2. The method according to claim 1 in which the expansion of the superheated steam formed by the withdrawn feedwater is effected in a secondary steam turbine.

3. The method according to claim 1 in which the superheated steam formed by the withdrawn feedwater is supplied to said primary circuit and the expansion thereof takes place in said primary turbine.

4. In a steam power plant; a feedwater line having feedwater preheater means therein, a primary circuit supplied by said feedwater line, a primary turbine supplied with superheated steam by said primary circuit, steam producer means, first conduit means leading from be tween the high and low pressure ends of said turbine to said steam producer, second conduit means branching otf from said feedwater line to supply water to said steam producer means to be vaporized therein by heat from the steam supplied by said first conduit means, third conduit means leading from said steam producer means to said preheater means to convey the steam supplied to said steam producer means by said first conduit means from the steam producer means to said preheater means and then to the feedwater line, and fourth conduit means leading from said steam producer means to convey the vaporized feedwater therefrom to a place of expansion of the vaporized feedwater and then from said place of expansion back to said feedwater line.

5. In a steam power plant according to claim 4 in which said place of expansion is a secondary turbine.

6. In a steam power plant according to claim 5 in which said place of expansion is said primary turbine.

References Cited UNITED STATES PATENTS 2,643,519 6/1953 Powell -67 2,991,620 7/ 1961 Nekolny 6067 3,016,712 1/1962 Taylor 60-107 XR 3,048,017 8/1962 Mary 6067 3,289,408 12/1966 Silvestri 60-67 FOREIGN PATENTS 1,021,451 12/1952 France. 1,245,852 10/1960 France.

441,317 3/1927 Germany.

952,347 11/1956 Germany.

957,948 2/1957 Germany.

345,022 4/ 1960 Switzerland MARTIN P. SCHWIADRON, Primary Examiner. ROBERT R. BUNEVICH, Assistant Examiner.

US. Cl. X.R.