US3349756A - Steam generating, superheating and reheating apparatus - Google Patents

Description

Oct. 31, 1967 w. SKUBATZ 3,349,756

STEAM GENERATING, SUPERHEATING AND REHEATING APPARATUS Oiiginal Filed Aug. 25, 1964 5 Sheets-Sheet 1 FIG! FLUID TEMP-F BOILER OUTPUT /0 FIG? ' 1000- WE STEAM JOUTLET .0 v i BEFORE lsT HEAT EXCHANGER AFTER 2o HEAT EXCHANGER 700 AFTER 1ST HEAT EXCHANGER FLUID TEMP-F P a TB K AF PRE-EVPPOR EFORE -4OO B BOILER OUTPUT OR Wolfgang Skubarz Oct. 31, 1967 w. SKUBATZ 3,349,755

STEAM GENERATING, SUPERHEATING AND REHEATING APPARATUS Original Filed Aug. 25, 1964 3 SheetS-Shet 2 Oct. 31, 1967 w. SKUBATZ 3,

STEAM GENERATING, SUPERHEATING AND REHEATING APPARATUS Original Filed Aug. 25, 1964 a Sheet-Sheet s FIG.2 FIG.4Y

United States Patent 1964. This application Sept. 16, Claims priority, application Germany, Sept. 5, 1963,

7 Claims. (Cf. 122-476) This application is a continuation of application Ser. No. 391,990 filed Aug. 25, 1964, now abandoned.

This invention relates to steam generating units equipped for the superheating and reheating of steam, and especially to generators characterized as being of the forced circulation, once-through type.

Conventional apparatus includes groups of fluid-conduct ing tubes or conduits subjected to the heat of high temperature flue gas. These make up the fluid heating, vapor generating and superheating circuits of the apparatus; and, if serially arranged, there may be a pre-evaporator or economizer and an evaporator or boiler, and also a primary superheater, an intermediate superheater, and one or more secondary superheaters.

In order to accomplish reheating of steam which has been partly expanded through a turbine, it previously has been proposed to provide units of the type described with a single stage heat exchanger in which heat is transferred from fluid being superheated to the fluid in the reheat circuit. Such an arrangement is not entirely satisfactory at low-load conditions, however, because the superheated steam temperature ahead of the single heat exchanger tends to reach undesirably high temperatures in intermediate portions of the superheating circuit at lowdoad, thus presenting a problem.

Rather than install flue gas regulating dampers or employ austenitic material inthe intermediate superheater, it is recognized by the present invention as more economical and technically more feasible to cope with the problem by modulating intermediate superheated steam temperature at part load conditions. Such modulation is eifected and maintained by providing in the vapor generating and superheating circuit a first heat exchanger ahead of one superheater, a second heat exchanger located after that one superheater and ahead of another superheater in the same series, and means for moving the relatively cooler fluid in the reheat circuit into heat transfer relationship with the serially arranged heat exchangers.

The various objects, features and advantages of the invention will appear more fully from the detailed description which follows, taken in connection with the accompanying drawing forming a part of the present application, wherein like reference numerals designate similar parts, and in which:

FIG. 1 is a graph showing the fluid temperature relationships in the vapor generating and superheating circuits with respect to boiler output or load for prior art apparatus employing one heat exchanger;

FIG. 1A is a schematic diagram of prior art apparatus to which the operating curves of FIG. 1 are related;

FIG. 2 is a schematic diagram of a vapor generating, superheating and reheating apparatus arranged according to the present invention;

FIG. 3 is a graph similar to FIG. 1, but with the fluid temperature relationships in the vapor generating and superheating circuits with respect to boiler output being applicable to apparatus arranged according to the invention as in FIG. 2; and

FIG. 4 is a view similar to FIG. 2, but showing a modified form of the invention.

3,349,756 Patented Oct. 31, 1967 FIG. 1 relates to prior art apparatus arranged as in FIG. 1A. The apparatus of FIG. 1A comprises a preevaporator 10, an evaporator 12, a primary superheater 14, an intermediate superheater 16, and a secondary superheater 18. The foregoing components are serially connected and arranged to be in heat transfer relationship with high temperature flue gas, while fluid supplied by a pump 22 flows serially through the components in the order named. A separator 24 may be installed between the evaporator 12 and the primary superheater 14. The apparatus further includes a reheat circuit comprising reheat- superheaters 32 and 34. Upstream of the reheat- superheaters 32 and 34 is a heat transfer unit 25 including an indirect type of heat exchanger 27 connected on the high pressure side between the primary superheater 14 and the intermediate superheater 16. Heat exchanger 27 is enclosed within a vessel 29 positioned in the reheat circuit. ahead of the re heat- superheaters 32 and 34. Thus the steam to be reheated first enters vessel 29 and absorbs heat by indirect transfer from the superheated steam flowing through heat exchanger 27 and from thence flows through reheatsuperheaters 32 and 34. An attemperator 56 may be installed between the superheaters 16 and. 18 and between the reheat- superheaters 32 and 34.

In FIG. 1, the disadvantageous rise of superheater steam temperature in the high pressure circuit ahead of the single heat exchanger 27 is plotted as a function of boiler load, together with a similar plot of fluid temperatures in other identified locations of the vapor generating and superheater apparatus circuit of FIG. 1A. From the curve designated Before Heat Exchanger in FIG. 1, it can be seen that with a boiler output of about 20% the intermediate steam temperature, immediately ahead of the single heat exchanger 27, would reach about 1020 F., as compared with about 800 F. at full load.

The high steam temperature associated with low-load operation is Well above the permissible limit for ferritic steels and would therefore necessitate using costly austenitic material for this particular section of the superheater. The crosshatched area in FIG. 1 lying between the curves labelled Before Heat Exchanger and After Heat Exchanger is indicative of the temperatures resulting from the transfer of heat to the reheat steam prior to its entering reheat-superheater 32.

The present invention seeks to modulate intermediate superheater steam temperatures at part load and to thereby avoid the expense and fabrication difficulties involved in the use of austenitic material at that. location in the circuit.

Referring now to FIG. 2, the high-pressure side of the apparatus includes an economizer or pre-evaporator 10, an evaporator 12, a presuperheater or primary superheater 14, an intermediate superheater 16, and two secondary superheaters 18 and 20. The foregoing components may be one or more fluid-conducting tubes or conduits serially connected and suitably arranged so as to be subjected to the heat of high temperature flue gas while fluid, supplied by a pump 22, flows serially through the components in the order named above. A separator 24 is preferably installed between the evaporator 12 and the primary superheater 14 to guard against excessive moisture carry-over from the evaporator 12 to the superheater 14.

A separate, but associated, reheat circuit is supplied with partly expanded, relatively low pressure steam, as from a turbine (not shown). That reheat circuit includes a proportioning by-pass valve 28, a vessel 30', a primary reheat-superheater 32, and a secondary reheat-superheater 34, all serially connected and arranged so that the reheatsuper heaters 32 and 34 are subject to the heat of high temperature flue gas. In addition, a parallel by-pass line 36 is connected between the bypass valve 28 and a line 38 connecting the vessel 30 and the primary reheatsuperheater 32. Manual or automatic adjustment of bypass valve 28 controls the quantity of fluid diverted from passage through the vessel 30 and thus by-passed through line 36 to supply line 38 of the primary reheat-superheater 32. Therefore, by-pass valve 28 is employed to control, for a given load, the temperature of the steam leaving the reheat-superheater 34 by apportioning the fluid flow through the by-pass and reheat superheaters.

According to the present invention, the preferred solution to the problem of modulating intermediate steam temperatures involves providing the high-pressure side of the apparatus with a two-stage heat transfer unit 41) enclosed within vessel 30. Heat transfer unit has a first heat exchanger 42 for the first heat transfer stage and a second heat exchanger 44 for the second heat transfer stage. The heat exchangers 42 and 44 are in the superheating portion of the fluid flow path, with the first heat exchanger 42 disposed between the primary superheater 14 and the intermediate superheater 16. The second heat exchanger 44 is disposed in the fluid flow path between the intermediate superheater 16 and the secondary superheater 18.

Still referring to FIG. 2, the working fluid flows through the pre-evaporator or economizer 1t) and on through evaporator 12 where vaporization occurs; it then passes through separator 24 from whence vapor, separated from the residual water in the fluid, is routed to the primary superheater 14 and then piped via inlet line 46 to the first heat exchanger 42 of the two-stage heat transfer unit 40. The working fluid leaves the first heat exchanger 42 via outlet line 48 and passes through the intermediate superheater 16, after which it is conducted by line 50 to the second heat exchanger 44 and delivered by outlet line 52 to the secondary superheater 18 of the serially arranged secondary superheaters. The second and last secondary superheater 249 in such series elevates the fluid to final steam temperature, and thereafter the fluid is piped via discharge line 54 to a point of use.

Spray attemperators, of known design and designated by reference numeral 56, may be located, as shown, at various points in the circuit for the purpose of modulating the steam temperature as necessary.

In FIG. 3 the plot of fluid temperature as a function of load or output, at various locations relative to the several components of the high-pressure steam generating and superheating circuit, graphically illustrates the modulating eflect on intermediate steam temperature of the two-stage heat transfer unit 40. It can be seen that at 20% of rated output of the apparatus of FIG. 2, the steam temperature is about 915 F. in line 46 and at about 815 F. in line 48 (leaving heat exchanger 42); and, as can be seen from the curve designated Before 2d Heat Exchanger in FIG. 3, steam temperature is about 930 F. in line 50 and at about 830 F. in line 52 (leaving heat exchanger 44). These steam temperature reductions result from a transfer of heat to the reheat steam entering primary reheater 32. The employment of a two-stage heat transfer unit 40 here ensures that the intermediate steam temperature does not exceed 930 F., as compared with 1020 F. in the FIG. 1 plot of the single-stage heat transfer unit. Thus, the FIG. 2 arrangement of the invention obviates the necessity for austenitic material in the intermediate superheater 16 and secondary superheater 18.

Referring now to FIGS. 1 and 3, a comparison may be made between the crosshatched areas on each of the graphs. In FIG. 1, the crosshatched area between the curves designated After Heat Exchanger and Before Heat Exchanger represents the wide range of intermediate steam temperatures experienced in the apparatus of FIG. 1A for boiler loadings in the range of between 20% and 100% of load. In FIG. 3 there is a first area, crosshatched in one direction, lying between the curves 4 designated After 1st Exchanger and Before 1st Heat Exchanger; and there is also another area, crosshatched in the opposite direction, lying between the curves designated After 2d Heat Exchanger and Before 2d Heat Exchanger.

The two crosshatched areas in FIG. 3 are indicative of the temperatures resulting from the transfer of heat to the reheat steam prior to its entering reheat-superheater 32. A comparison of these two crosshatched areas of FIG. 3 with the single crosshatched area of FIG. 1 reveals that for any given boiler output or load the sum of the two temperature reductions in the heat exchangers 42 and 44 of the FIG. 2 two-stage heat transfer unit 40 matches the temperature reduction in the heat exchanger 27 of the FIG. 1A single stage heat transfer unit 25, but with the added advantage of accomplishing reheat with significantly reduced magnitude and range of intermediate steam temperatures with the two-stage heat transfer unit 49.

It will be noted in FIGS. 1 and 3 that the temperature designated by the curve Before 3d Stage superheater shows a somewhat lower temperature than that of the curve designated After 2d Stage superheater. This is as a result of the action of the attemperators 56 located in the line interconnecting the second and third stage superheaters.

The modified form of the invention shown in FIG. 4 is closely related to the apparatus of FIG. 2, and like reference numerals are employed to designate similar parts. The apparatus of FIG. 4 has a high-pressure series flow circuit including a pump 22, an economizer 10, a primary evaporator 60, a secondary evaporator 62, a primary superheater 14, an intermediate superheater 16, and a secondary superheater 18. A steam-water separator 24 is disposed in the circuit between the secondary evaporator 62 and the primary superheater 14, and there are also suitably disposed and arranged attemperators 56. The low-pressure, or reheat circuit comprises a proportioning by-pass valve 28 for vessels 64 and 66, a primary reheat-superheater 32 and a secondary reheatsuperheater 34, arranged in series. In addition, there is a bypass line 36 arranged in parallel with the vessels 64 and 66 by connection between valve 28 and line 38 ahead of reheat-superheater 32. In this arrangement, however, there is a first heat exchanger 42 connected between the primary evaporator 60 and the secondary evaporator 62 by means of inlet line 46 and outlet line 48, as shown. Also, a second heat exchanger 44 is connected between primary superheater 14 and intermediate superheater 16 by means of inlet line 50 and outlet line 52, as shown. Heat exchangers 42 and 44 are respectively enclosed in vessels 64 and 66 to facilitate transfer of heat to the relatively low pressure low temperature steam characteristic of the reheat circuit. As in the former arrangement, the proportioning or by-pass valve 28 may divert some of the reheat steam flow into the by-pass line 36. The more fluid that is directed through the vessels 64 and 66, the greater will be the temperature reduction of the fluid flowing through the intermediate portions of the high-pressure circuit, and vice versa. Stated another way, the by-pass valve 28 is employed to control outlet temperature of reheated steam for a given load by controlling the quantity of steam which receives heat from the superheated steam in the high-pressure circuit.

It is a further advantage of the invention that it may be simply and economically incorporated in apparatus of known construction without extensive modification thereof.

Although the invention has been shown in but two forms, it will be obvious to those skilled in the art that it is not so limited, but that it is susceptible of various changes and modifications without departing from the spirit thereof as covered by the claims.

What is claimed is:

1. In a vapor generator, a vapor generating and superheating circuit subject to high temperature heating gases and comprising a first fluid heating unit, a first heat exchanger unit, a second fluid heating unit, and a second heat exchanger unit; means supplying vaporizable fluid to the inlet of said circuit and directing all of the fluid so supplied through said units in the order named; a single reheat circuit connected for flow of expanded fluid from the vapor generating and superheating circuit and arranged in indirect heat absorbing relation With said first and second heat exchanger unit; and means for directing expanded fluid through the reheat circuit to progressively increase the temperature of the expanded fluid.

2. In a vapor generator according to claim 1 further including an evaporating unit in said vapor generating and superheating circuit ahead of said first fluid heating unit, and wherein the fluid flowing through said first and second fluid heating units is superheated and is reduced in temperature in each of said heat exchanger units by the transfer of heat to the expanded fluid.

3. In a vapor generator according to claim 1 wherein said reheat circuit further includes a by-pass circuit arranged to direct expanded fluid for flow therethrough in parallel to said reheat circuit, a valve connected be tween said reheat circuit and said by-pass circuit ahead of the point of heat absorbing relation with said first heat exchanger unit for adjustably regulating the quantity of fluid diverted from said reheat circuit for flow through said by-pass circuit.

4. In a vapor generator according to claim 3, wherein said by-pass circuit is out of heat transfer relationship with said first and'second heat exchanger units, and

wherein the expanded fluid flowing through said by-pass circuit merges with the expanded fluid in said reheat circuit after said last named fluid has been in heat absorbing relation with said second heat exchanger unit.

5. In a vapor generator according to claim 4, at least one reheat superheater subject to high temperature heating gases and connected in said reheat circuit to receive the merged fluids from said by-pass circuit and reheat circuit.

6. In a vapor generator according to claim 5, an evaporator unit in said vapor generating and superheating circuit ahead of said first fluid heating unit and being also subject to the heat of high temperature gases; wherein said first fluid heating unit is a primary superheater unit and said second superheater unit is an intermediate superheater unit; and wherein the superheated fluid flowing through said heat exchanger units of said vapor generating and superheating circuit transfers heat to the expanded fluid flowing through said reheat circuit.

7. Apparatus according to claim 6 wherein said intermediate superheater unit is constructed of ferritic steel material.

References Cited UNITED STATES PATENTS 2,848,983 10/1958 Leeberherr 122-479 2,966,896 1/1961 Vogler 1221 3,035,556 5/1962 Brunner 122-476 KENNETH W. SPRAGUE, Primary Examiner.

Claims (1)

1. IN A VAPOR GENERATOR, A VAPOR GENERATING AND SUPERHEATING CIRCUIT SUBJECT TO HIGH TEMPERATURE HEATING GASES AND COMPRISING A FIRST FLUID HEATING UNIT, A FIRST HEAT EXCHANGER UNIT, A SECOND FLUID HEATING UNIT, AND A SECOND HEAT EXCHANGER UNIT; MEANS SUPPLYING VAPORIZABLE FLUID TO THE INLET OF SAID CIRCUIT AND DIRECTING ALL OF THE FLUID SO SUPPLIED THROUGH SAID UNITS IN THE ORDER NAMED; A SINGLE REHEAT CIRCUIT CONNECTED FOR FLOW OF EXPANDED FLUID FROM THE VAPOR GENERATING AND SUPERHEATING CIRCUIT AND ARRANGED IN INDIRECT HEAT ABSORBING RELATION WITH SAID FIRST AND SECOND HEAT EXCHANGER UNIT; AND MEANS FOR DIRECTING EXPANDED FLUID THROUGH THE REHEAT CIRCUIT TO PROGRESSIVELY INCREASE THE TEMPERATURE OF THE EXPANDED FLUID.

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