US3286696A

US3286696A - Vertical steam generator with central downcomber

Info

Publication number: US3286696A
Application number: US505665A
Authority: US
Inventors: Charles R Green; Ronald B Creek
Original assignee: Combustion Engineering Inc
Current assignee: Combustion Engineering Inc
Priority date: 1965-10-29
Filing date: 1965-10-29
Publication date: 1966-11-22
Anticipated expiration: 1983-11-22
Also published as: GB1136295A

Description

Nov. 22 1966 c. R; GREEN ETAL VERTICAL STEAM GENERATOR WITH CENTRAL DOWNGOMER Filed Oct. 29, 1965 2 Sheets-Sheet 1 INVENTORS RONALD B. CRK 04421.55 2.6REEN ATTORNEY Nov. 22, 1966 c. R. GREEN ETAL 3,286,696

VERTICAL STEAM GENERATOR WITH CENTRAL DOWNCOMER "Filed Oct. 29, 1965 2 Sheets-$heet 2 -INVENTORS RONALD 5. CREEK BY CHARLES R. GREEN ATTORNEY United States Patent 3,286,696 VERTICAL STEAM GENERATOR WITH CENTRAL DOWNCOMER Charles R. Green and Ronald B. Creek, Chattanooga, Tenn, assignors to Combustion Engineering, Inc.,

Windsor, Conn., a corporation of Delaware Filed Oct. 29, 1965, Ser. No. 505,665

13 Claims. (Cl. 122--34) The present invention relates generally to vapor generators of the shell and tube type wherein heat is supplied by a heating fluid flowing through tubes that are immersed in a vaporizable liquid contained within the shell. More particularly, the invention relates to a novel vapor generator design of the aforementioned type having means for more effectively utilizing the heating surface available within the vapor generator in order to achieve greater operational efliciency and a more compact form.

It has been heretofore known in the art to orient a shell and tube type vapor generator such that its axis extends vertically in order to reduce the amount of floor space required for mounting such units. Vapor generators having such an attitude have, of necessity, been constructed with tube sheets of an extreme thickness in order to overcome stresses imposed upon the tube sheet by its own weight, by the weight of the tubes that it serves to mount and by the difference in fluid pressure that exists'on either side of the tube sheet during unit operation. It has been proposed to reduce the amount of tube sheet thickness required in such units by providing a structural member intermediate the ends of the tube sheet which extends between it and the Wall of the pressure vessel in order to thereby transfer some of the loading on the tube sheet to the vessel wall. In order to accomplish this it is necessary to maintain that portion of the tube sheet to which the support is attached void of tube attachments thereby eliminating a wide swath of heating surface from service. I

It has also generally been known that for most effective operation of vapor generators of the shell and tube type it is necessary to define distinct fluidvflow paths through which the vaporiza'ble liquid flows while being circulated through the unit. These paths are conventionally formed by means of fluid impervious baffles or the like. One of such flow paths is normally referred to as the riser path, since it is in this portion of the fluid circuit that the heat exchange tubes are located for heating the vaporizable liquid and transforming a portion thereof into steam that rises toward the top of the vessel. The other such path, referred to as the downcomer path, is that which is void of heat exchange tubes and therefore relatively unheated. It is through this portion of the circuit that liquid is passed downwardly toward the bottom of the vessel due to the greater density of the fluid that exists in this portion of the circuit. The necessity of eliminating heat exchange tubes from the downcomer path still further reduce the amount of heating surface capable of being provided Within the vapor generator shell.

By means of the present invention there is provided a vapor generator wherein means are employed for sup porting the tube sheet in order to reduce its requisite thickness and fluidly distinct flow paths are provided for directing the recirculating vaporiza'ble fluid through the unit. The tube sheet support and the plate means employed to define the fluid flow paths are so disposed as to reduce to a minimum the amount of vessel volume that is not available for containing heating surface, there- 'by increasing the over-all efficiency of the unit and reducing the amount of space necessary for containing the unit. Moreover, the present invention provides means for collecting the liquid separated from the vapor-liquid mixture created in the vapor generating portion of the unit and for returning it to the downcomer portion of the circuit in a minimum amount of vapor generation space and in a manner whereby its flow velocity is not unduly increased.

Various other objects and advantages will appear from the following description of one embodiment of the invention, and the novel features will be particularly pointed out hereinafter in connection with the appended claims.

The invention is described with reference to the accompanying drawings wherein:

FIGURE 1 is a vertical section of a vapor generator constructed in accordance with the invention;

FIGURE 2 is a vertical section taken along line 22 of FIGURE 1;

FIGURE 3 is a horizontal section taken along line 3-3 of FIGURE 1; and

FIGURE 4 is a vertical section of a modified form of vapor generator.

Referring now to the drawings the numeral 10 represents a vapor generator constructed in accordance with the invention. The vapor generator 10 comprises a vertically elongated pressure vessel including a first cylindrical wall 12 and a second cylindrical wall 14 of greater diameter than the first connected thereto by a generally conical transition portion 16. The upper and lower ends of the vessel are closed by hemispherical end portion 18 and 20, respectively. A generally annular tube sheet 22 extends normal to the axis of the vessel adjacent the lower end thereof dividing the vessel into a vapor generating section 24 and a heating fluid section 26. The tube sheet 22 contains a plurality of tube seats for mounting the ends of the tubes 28 that form a tube bundle 30. The tube bundle 30 comprises parallel rows of U-shaped tubes 28 having their bends positioned uppermost in the vessel and their ends attached to the tube sheet 22 in communication with the heating fluid section 26. Within the vapor generating section 24 is contained a body of liquid in which the tubes 28 of the tube bundle 30 are immersed. The liquid has a level 32, here shown in the position which it occupies during normal operating conditions, that divides the vapor generating section 24 into a lower liquid space 34 and an upper vapor space 36. Liquid is admitted to the vessel by means of a feedwater nozzle 33 attached to the vessel wall 12 and a duct 35 that conducts feedwater to the downcomer portion of the vessel. A vapor nozzle 38 atached to the end wall portion 18 is provided for removal of vapor created in the generator to it point of intended use.

As shown in the drawings, the tube sheet 22 is supported at its center by an axially disposed cylindrical support member 40 that is attached as by means of Welding at its .top to the lower surface of the tube sheet and at its bottom to the inside surface of the lower end wall portion 20. .By supporting the tube sheet 22 in this manner its unsupported length is considerably reduced, thereby reducing the amount of thickness that would otherwise be necessary to Withstand the stresses imposed by the weight of the tube sheet, the Weight of the tubes 28 and the differential fluid pressure that exist between the fluid contained in the vapor generating section 24 and that contained in the heating fluid section 26. As shown, tube sheet 22 and the end wall portion 20 contain

openings

42 and 44, respectively, that are aligned with the central opening through the support cylinder thereby providing a manway 46 for convenient access by a workman to the vapor generating section 24 for maintenance or replacement of the components contained therein. A cover plate 48 that may be attached to the outer surface of the lower end wall portion 20 as by means of threaded fasteners is provided for covering the manway.

Within the heating fluid section 26 are provided a pair of flat, diametrically opposed plates 50 for dividing the section into an inlet chamber 52 and an outlet chamber 54. The plates 50 conform generally to the shape of the heating fluid section 26 and are attached, as by means of a continuous, fluid impervious weld, to the outside of the tube sheet 22, the outer surface of the tube sheet support 40 and to the inner wall of the lower end wall portion 20 to form fluidly distinct chambers. Inlet and outlet nozzles 56 and 58, respectively, are attached to the end wall portion 20 to establish fluid communication between the respective chambers, 52 and 54, and a source of heating fluid (not shown). ,By means of this arrangement the vapor generator is provided with a continuous source of heating fluid.

The generation of vapor occurs within the vapor generating section 24 where the U-tubes 28 that conduct the heating fluid through the unit are immersed as shown in a body of vaporizable liquid, such as water. Heat contained in the heating fluid is given up to the cooler vaporizable liquid thereby transforming a portion of the liquid into vapor which rises toward the vapor space 36 due to thermal siphonic action thereby establishing fluid circulation through the vapor generator. It is generally known that circulation efficiency of a vapor generator of this type will be increased when the flow paths through the vapor generating section 24 are distinctly defined as by means of bafiies that separate the riser portion of the fluid circuit from the downcomer portion thereof. According to the present invention therefore, there is provided a cylindrical conduit 60 axially disposed within the vessel in spaced relation from the wall 12 thereof, thereby defining an inner downcomer chamber 62 for conducting liquid to the bottom of the vapor generating section 24 and an outer, annular chamber 64 within which the tubes 28 are located and which comprises the riser portion of the fluid circuit. As shown the conduit 60 is conveniently located within the tube-free void 66 formed at the axial center of the tube bundle 30. This void 66 is formed as a result of the fact that the tube bundle is formed of U-shaped tubes which must inherently provide such a void in the space enclosed by the tubes having bends of minimum dimension. The conduit 60 is closed at its upper end by a plate 61 and extends substantially to the bottom of the vapor generating section 24 but, as shown, is somewhat spaced from the upper surface of the tube sheet 22 in order to establish fluid communication between the downcomer chamber 62 and the outer annular chamber 64. The conduit 60 is further constructed of a diameter that substantially coincides with that of the tube sheet support 40 and is positioned in axial alignment therewith in order that the effective tube mounting surface of the tube sheet 22 is not reduced below that amount that is necessary for attaching the tube sheet support 40-.

Other baflie means further provided in the vapor generator 10 include an enlarged annular downcomer trough 68 spaced axially above the tube bundle 30 which, together with baflie plates 70 and 72, form a reversing hood type of vapor separator for removing entrained liquid from the mixture that emerges from the vapor generating space 24. As shown the downcomer trough 68 comprises an enlarged, open, annular receptacle formed by a substantially cylindrical plate 74 concentrically spaced from the wall 14 of the vessel as by means of circumferentially spaced brackets 76 and a substantially conically shaped bottom plate 78 that is adapted to discharge liquid collected therein to the downcomer chamber 62 for recirculation through the unit. Discharge from the downcomer trough 68 is accommodated by a laterally elongated opening 80 in the bottom plate 78. Communication between the trough 68 and the downcomer chamber 62 is effected by means of a diametrically elongated,

' rectangular duct 82 that attaches at its upper end to the opening 80 in the bottom plate 78 and at its lower end to closure plate 61 to thereby permit passage of liquid from the trough 68 to the downcomer chamber 62. The duct 82 is positioned in a lateral void 84 provided in the tube bundle 30 by the removal of one or more tube rows.

In keeping with the invention the duct is constructed with as narrow a width dimension as possible in order to minimize the number of tube rows that must be removed for accommodation of the duct. The dimensions of the duct 82 are dictated by the desired velocity of the liquid flowing through it from the downcomer trough 68. The velocity of the liquid must not be so high as to result in the liquid having an excessive pressure drop while flowing through the duct since such excessive pressure drop would tend to disrupt the fluid circulation stability of the unit by reducing the effective pressure head available for fluid circulation and thereby disrupt fluid flow through the unit. In order to reduce, therefore, the velocity of the liquid flowing through the duct 82 the cross-sectional area required to provide the desired velocity is determined and the duct dimensioned so as to contain such cross-sectional area With a minimum width dimension. In other words, the length of the duct is made as long as practicable in order that its width may be at a minimum.

In the arrangement disclosed in FIGURES 13 the vapor separating apparatus indicated generally as 86, is in the form of a reversing hood formed by the arrangement of plates 70 and 72. Plate 70 is an annular member that is disposed horizontally across the vapor axis with its outer peripheral edge attached as by means of a continuous weld, to the inner surface of wall 14. The plate 70 contains a central opening 73 from which depends a vertically arranged cylindrical plate 72 concentrically spaced from the trough wall 74 and depending below the top edge thereof in order to create a flow passage having an abrupt change of direction such that any liquid contained in the mixture will be flung therefrom by the centrifugal force developed in the mixture flowing through the passage.

The operation of the herein disclosed vapor generator is as follows. Heating fluid from a source is admitted by the heating fluid inlet nozzle 56' into the compartment 52 from whence it flows through the tubes 28 of the tube bundle 30 through the vapor generating section 24 to the heating fluid outlet compartment 54 and thence it is passed by means of the outlet nozzle 58 to the source for reheating. At the same time vaporizable liquid in the form of water is admitted through thefeedwater inlet nozzle 33 through the feedwater duct 35 into the downcomer cham ber 62 from whence it passes to the bottom of the vapor generating section 24 to the outer annular chamber 64, thereby substantially filling the chamber as indicated by the position of the liquid level 32. Heat from the heating fluid is transferred to the vaporizable liquid through the heating surface of the tubes 28 whereby a portion of the liquid is transformed into vapor and circulation due to the thermal siphonic action generated during the transformation of a portion of the liquid into vapor is initiated. Vapor and liquid mixture emerges from the liquid space 34 and enters the flow passage created between the vessel wall 14 and the downcomer trough 74 and also between the trough wall 74 and the depending annular plate 72 thereby causing the mixture to undergo an abrupt change of direction in flowing toward the vapor space 36. Because of this abrupt change of direction, centrifugal force effects the removal of the liquid entrained in the vapor discharging it into the downcomer trough 68. The separated vapor flows through the opening formed by the cylindrical plate 72 into the vapor space 36 and thence through the outlet nozzle 38 to a point of intended use. The liquid discharged from the vapor mixture is discharged from the trough 68 through the opening 80 from whence it passes through the duct 82 into the downcomer chamber 62 where it mixes with the incoming feedwater to be again circulated through the unit. In flowing through the duct 82 the velocity of the liquid is maintained sufiiciently low due to the dimensioning of the duct in order that undue fluid pressure drop is prevented.

By means of the invention there is therefore provided a novel shell and tube type vapor generator which design achieves maximum unit efficiency in a minimum amount of space and at a lower cost of manufacture. The ductwork that is necessary in the vapor generating section of the unit for maximum circulation efficiency is so designed and arranged that it requires the elimination of a minimum amount of heating surface and that permits the provision of a tube sheet support within the unit without requiring the removal of additional heating surface to accommodate the support. The presence of the tube sheet support in the manner described reduces the amount of thickness required for the tube sheet and therefore materially reduces the cost of unit fabrication.

In FIGURE 4 there is shown a slightly modified form of vapor generator constructed in accordance with the invention. In this arrangement the vapor separating apparatus, here indicated as 86, takes the form of a plurality of centrifugal separators 88 of known construction, in which the vapor and liquid mixture is spun in order to separate the liquid from the mixture and to discharge it downwardly into the downcomer trough 68. In the arrangement shown the separators 88 are mounted upon an annular plate 90 that is attached between the top of the cylindrical trough wall 74 and the inner surface of the vessel wall 14. Mounting the separators 88 is accomplished by means of an inlet tube 92 attached to the plate 90 and to the inlet end of the separators.

The operation of this form of vapor generator is substantially the same as that described above with the exception that instead of the steam flowing through a tortuous path in order to effect gravity separation of the liquid it is passed through the separators 88 wherein the bulk of the liquid is removed by spinning action that occurs therewithin.

It will be understood that various changes in the details, materials, and arrangements of parts which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.

What is claimed is:

1. A vapor generator comprising: wall means forming a vertically elongated pressure vessel of circular cross section; a tube sheet dividing said vessel into a vapor generating section and a heating fluid section; a body of liquid in said vapor generating section having a level defining an upper vapor space and a lower liquid space; a vapor outlet from said vapor space; a tube bundle including parallel rows of vertically elongated U-tubes immersed in said body of liquid, said tubes being disposed with their bends uppermost and with their ends attached to said tube sheet in communication with said heating fluid section; means for passing a heating fluid through said tubes; means dividing said heating fluid section into inlet and outlet portions; axially disposed conduit means spaced inwardly from said vessel wall to form an inner downcomer chamber and an outer annular chamber containing said rows of tubes, said conduit means having its lower end spaced from said tube sheet to establish communication between said inner and outer chambers; separator means communicating with said outer annular chamber operative to discharge vapor to said vapor space and liquid to said liquid space; a downcomer trough positioned in said vapor space and being adapted to receive liquid discharged from said separator means; and a vertically elongated, generally rectangular duct disposed between adjacent rows of said U-tubes, having its upper end communicating with said trough and its lower end communicating with said downcomer chamber, said duct being of sufficient width to provide sufficient cross-sectional area for limiting the velocity of liquid flowing therethrough.

2. A vapor generator as recited in claim 1 wherein said downcomer trough includes: an annular wall spaced inwardly from said vessel Wall and a generally conical bottom open to said duct.

3. A vapor generator as recited in claim 2 including: annular plate means disposed above said downcomer trough having its outer peripheral edge attached to said vessel wall; and cylindrical plate means depending from the inner peripheral edge of said annular plate means and being telescopically received within said annular wall to form a reversing hood separator.

4. A vapor generator as recited in claim 2 including: annular plate means having its outer peripheral edge attached to said vessel wall and its inner peripheral edge attached to the top of said annular wall; and centrifugal separator means mounted upon said annular plate means.

5. A vapor generator as recited in claim 4 wherein said centrifugal separator means includes a plurality of centrifugal separators spaced about the circumference of said annular plate means.

6. A vapor generator as recited in claim 1 wherein said rectangular duct extends substantially diametrically transverse said pressure vessel and said vapor generator further includes: axially disposed tube sheet support means supportingly attached between said tube sheet and the Wall forming the bottom of said pressure vessel; and generally flat plate means cooperating with said tube sheet, said tube sheet support means and said pressure vessel wall to divide said heating fluid section into inlet and outlet portions, said flat plate means including a pair of diametrically opposed plates in substantial vertical alignment with said rectangular duct.

7. A vapor generator as recited in claim 6 wherein said tube sheet support means comprises an elongated structural cylinder.

8. A vapor generator as recited in claim 7 wherein said axially disposed conduit means is cylindrically formed and said tube sheet support means is located in axial alignment therewith.

9. A vapor generator as recited in claim 6 wherein said tube sheet support means comprises a structural member having cross-sectional dimensions that are substantially coincident with the cross-sectional dimensions of said conduit means and in axial alignment therewith.

10. A vapor generator as recited in claim 6 wherein said downcomer trough includes: an annular wall spaced inwardly from said vessel wall and a generally conical bottom open to said duct.

11. A vapor generator as recited in claim 10 including: annular plate means disposed above said downcomer trough having its outer peripheral edge attached to said vessel wall; and cylindrical plate means depending from the inner peripheral edge of said annular plate means and being telescopically received within said annular wall to form a reversing hood separator.

12. A vapor generator as recited in claim 10 including: annular plate means having its outer peripheral edge attached to said vessel wall and its inner peripheral edge attached to the top of said annular wall; and centrifugal separator means mounted upon said annular plate means.

13. A vapor generator as recited in claim 12 wherein said centrifugal separator means includes a plurality of centrifugal separators spaced about the circumferences of said annular plate means.

References Cited by the Examiner UNITED STATES PATENTS 3,071,119 1/1963 Ammon et al. l2234 3,114,353 12/1963 Sprague 12234 3,129,697 4/1964 Trepaud 12234 KENNETH w. SPRAGUE, Primary Examiner.

Claims

1. A VAPOR GENERATOR COMPRISING: WALL MEANS FORMING A VERTICALLY ELONGATED PRESSURE VESSEL OF CIRCULAR CROSS SECTION; A TUBE SHEET DIVIDING SAID VESSEL INTO A VAPOR GENERATING SECTION AND HEATING FLUID SECTION; A BODY OF LIQUID IN SAID VAPOR GENERATING SECTION HAVING A LEVEL DEFINING AN UPPER VAPOR SPACE AND A LOWER LIQUID SPACE; A VAPOR OUTLET FROM SAID VAPOR SPACE; A TUBE BUNDLE INCLUDING PARALLEL ROWS OF VERTICALLY ELONGATED U-TUBES IMMERSED IN SAID BODY OF LIQUID, SAID TUBES BEING DISPOSED WITH THEIR BENDS UPPERMOST AND WITH THEIR ENDS ATTACHED TO SAID TUBE SHEET IN COMMUNICATION WITH SAID HEATING FLUID SECTION; MEANS FOR PASSING A HEATING FLUID THROUGH SAID TUBES; MEANS DIVIDING SAID HEATING FLUID SECTION INTO INLET AND OUTLET PORTIONS; AXIALLY DISPOSED CONDUIT MEANS SPACED INWARDLY FROM SAID VESSEL WALL TO FORM AN INNER DOWNCOMER CHAMBER AND AN OUTER ANNULAR CHAMBER CONTAINING SAID ROWS OF TUBES, SAID CONDUIT MEANS HAVING ITS LOWER END SPACED FROM SAID TUBE SHEET TO ESTABLISH COMMUNICATION BETWEEN SAID INNER AND OUTER CHMBERS; SEPARATOR MEANS COMMUNICATING WITH SAID OUTER ANNULAR CHAMBER OPERATIVE TO DISCHARGE VAPOR TO SAID VAPOR SPACE AND LIQUID TO SAID LIQUID SPACE; A DOWNCOMER TROUGH POSITIONED IN SAID VAPOR SPACE AND BEING ADAPTED TO RECEIVE LIQUID DISCHARGED FROM SAID SEPARATOR MEANS, AND A VERTICALLY ELONGATED, GENERALLY RECTANGULAR DUCT DISPOSED BETWEEN ADJACENT ROWS OF SAID U-TUBES, HAVING ITS UPPER END COMMUNICATING WITH SAID TROUGH AND ITS LOWER END COMMUNICATING WITH SAID DOWNCOMER CHAMBER, SAID DUCT BEING OF SUFFICIENT WIDTH TO PROVIDE SUFFICIENT CROSS-SECTIONAL AREA FOR LIMITING THE VELOCITY OF LIQUID FLOWING THERETHROUGH.