US3227142A

US3227142A - Steam generator configurations

Info

Publication number: US3227142A
Application number: US243926A
Authority: US
Inventors: Bell Alan; Lawrence Arthur Charles
Original assignee: Foster Wheeler Inc
Current assignee: Foster Wheeler Inc
Priority date: 1961-12-11
Filing date: 1962-12-11
Publication date: 1966-01-04
Anticipated expiration: 1983-01-04
Also published as: FR1347938A; BE625968A; DE1401666A1; CH388356A; SE304758B; CH384006A

Description

Jan. 4, 1966 A. BELL ETAL STEAM GENERATOR CONFIGURATIONS 2 Sheets-Sheet 1 Filed Dec. 11, 1962 INVENTOR A L AN BEL L BY ARTHUR CHARLES LAWRENCE ATTORNEY Jan. 4, 1966 A. BELL ETAL STEAM GENERATOR CONFIGURATIONS 2 Sheets-Sheet 2 Filed Dec. 11, 1962 III! iNvENToR BELL A TTORNfY ARTHUR CHARLES LAWRENCE. 5 fiz'fl/M/zym United States Patent Molesey, Surrey, "England, nssignors to Foster Wheeler Corporation, New York,.N.Y.,.a corporation of New York a u FiledDec. I1, 1962; Ser. No. 243,926 I (,laii'rns priority, application-Great Britain; Dec." 11, H61, 4 4;2'41/61 44, 242 /61', 44,243761 l 7-Cliairns. (Chi-22: 3

This invention relates to water'tube steam generators which the heating fluid is a clban, high-temperature liquid such as the-primary coolant from a nuclear reactor.

One object in the design of such generators is to provide the maximuiir heating surface in a minimum volume, to keep the. overall volumesmall for a given output and to simplify as muchas possible the/fabrication and con.- struction which, having. regard to essential requirements, are necessarily somewhat complicated.

The invention has been made with those considerations in mind.

The steam generator in accordance with the invention comprises an upright cylindricalshell through which the heating fluid is circulated and which. contains a coaxial downcomer, and rises connected to the lower end of the d'owncomer communicating with a steam and water drum at the upper end of the dhwncomer. Each risei comprises an inlet tube and an. outlet tube joined by a bundle of substantially straight intermediate tubes of smaller diameter having at each end a polygonal header, the arrangement beingv such that. the intermediate tubes are regularly and uniformly distributed over the crosssection of the shellfso as toform. an annular composite bundle closely surrounding the downcomer and occupy ing. the greater part ofthe volume ofthe shell.

Preferably, the headers are hexagonal. The hexagonal shape is advantageous in that a number ofhexagons can be arranged side-by-side soas substantially to fill a circle or an annulus.- If, howcveninthe arrangement described above, the headers are juxtaposed and interfit'ted, there will be no space left for circulation of theheating fluid.

in one plane and the other halt in a slightly lower plane. The same applies with other polygonal shapes such as a triangle, quadrangleand certain other shapes which may be regular or irregular.

In a preferred form of the invntion tlie outlet tubes of the risers are connected to an annular collecting riser surrounding the downcomer .at its upper end and leading to the steam and Water drum. provides, in combination with the above tube bundle and header arrangement-,a very compact design. 4 The invention is particularly advantageous if the steam and water drum is-made spherical instead of being, as is usual, cylindrical. A cylindrical drum: can, howinches diameter and nearly 10,090 intermediate riser tubes of about /8 inch diameter. The shell can be made of considerable length-60: feet or more-and therefore a very large heating surface and' a relatively small volume for a given output can be provided.- It also enables a large proportion of the constructional work to be done in the factory rather than on the site Thus, the tube bundles can be prefabricated and can be provided, in

the factory, with inlet and; outlet tubes so that the only This difliculty is easily. overcome by staggering. the tube bundles vertically so that half the headers at one end he Thus, it is possible to arrange within a cylindrical oh-site Work-left to beiperformed is thewelding of the latter tubes to the" down'comer and the collecting riser. That welding, it will be' appreciated", can be eifected before the downcomer is encased in the shell. It is also an important advantage that substantially the.whole of the'h'eating surface is provided with a large set of identical elements. Consequently, there can' be' a substantial amount otlmass productionwhich considerably'cheapens the cost of manufacture.

Aiioth'er important advantage is that it is'easy, to provide'for an'even distributionof the tubesover the whole cross-"sectionof'the shell. To ensure this, it may be necessary to adopt a simple expedient which'will be described further'on. I

The use'ofalarge number of headers in place of a single tube plate is a significant advantage. If, as will generally be the case, the generator is to'operate at a substantialpressure such as 10001b./sq. in., the tube plate Wouldhlave to have a thicknessof the order of 15 inches whereas the headers need only have that thickness which is required to" give them adequate rigiditysa'y about '1 inch. Moreover, a'tubeplate extending over the whole crosssectional area of the shell would obstruct flowo'f the heating fluid. The inlet and outlet for that fluidwould there'fore havetobe arranged between the tubeplatesand there would, in consequence, be ineffective lengths'of tubing at each end of the shell adding un necessarily to the bulk of the plant and also -to the cost.

In the steam and water drum there are the usual centrifugal separatorsfor separating steam from water and di'iersfor the separated steam.-

For the purpose of superheating the steam, the following featurespreferablyare provided in accordance with the invention.

(a) The' steam and water drum is bat-lied so that a passage is formed bounded externally by theshell of the drum for the saturated steamleaving the d'riers enroute to supeiheating tubes, the latter being withinthe shell of the generator; This has the advantage that the shell of the rumis heated evenly lbythe steam during starting up of the genera-tor; It' has the further advantage of eliminating external pipe connections between the'drum and theshell.

(b*)" An annularpassage is providedroundthe downcoiner establishing communication forthe steam between the drum and the-superheating tubes in the-shell. This hasthe advantage ofavo'iding the necessity of more than one piercing-of the upper end of the shell'and also of eliminating external pipeconnections between the drum and the? shell;

(e91 There preferably are provided within the shell superheatin'g tubeswhic'h line the shell and surround the steam generating tubes therein. This disposition of the superheating tubes is advantageous in that a large number of-suehtubescanbeprovided without. enlarging the. diametenof-the shell toany substantial extent. (d) A superheated steam outlet isprovided at the lower end of the shell WhlChdS an extension of the downcomer. The advantage of this is that access-tothe inside ofthe doiwncomer'is preserved;

(e) Asuperheat'e'd steanr outlet may be provided which passes thioughtheend of the shell and is free to expand, a"flex-ibleseal beiiig' provided between the outlet and the shell which is not s-uhjected to the steam pressure.

Arieiiample of a'steam generator in accordance with FIGURE 1 isa somewhat diagrammatic sectional elevation view of the generator in accordance with the invention;

. 3 FIGURE 2 is a vertical section on an enlarged scale through a tube bundle header;

FIGURE 3 is a plan view of FIG. 2;

FIGURE 4 is an elevational view showing the disposition of the tubes relatively to the header; and

FIGURE 5 is a plan view showing another shape of the header.

The steam generator shown in the drawings has a cylindrical shell having an inlet 12 and an outlet 14 for a heating fluid which can be liquefied sodium or other organic fluid from a nuclear reactor.

Within the shell and coaxially with it is a downcomer 16 which proceeds from a spherical steam and water drum 18. Surrounding the downcomer there is a bank of steam generating tubes which are heated by the heating fluid which passes through the shell.

Each tube in the steam generating bank is in the form of a riser made up of an inlet tube 20 and an outlet 22 and an intermediate tube bundle 24. The inlet tubes 20 are welded to and communicate with the downcomer at its lower end and the outlet tubes 22 communicate with and are welded to the wall of an annular riser 26 which leads to the steam and water drum 18.

Each bundle 24 is made up in this particular case of nineteen small tubes 28 which are welded to top and bottom headers 30. These headers are shown in FIGS. 2 and 3, FIG. 2 being a section view taken along line 2-2 of FIG. 3. As will be seen therein, the tubes 28 are welded to one side of these headers, while the inlet or

outlet tubes

20, 22 are connected to the other side by means of adaptors 32.

The headers are hexagonal in plan and the adaptors are, of course, hexagonal at the large end and circular at the small end where they are welded to the inlet and outlet tubes.

There are nineteen tubes 28 in each bundle and the hexagonal headers are such that they can be juxtaposed and interfitted so as to cover substantially the whole of the cross section of the shell which it is desired to utilize. If they were juxtaposed in a single plane, however, the circulation of the heating fluid from inlet to outlet would be impeded. Accordingly, the headers in each of the two sets are staggered as shown in FIG. 1 so that half of them lie in one plane and the other half in a slightly lower plane.

In operation, feed water is delivered into the steam and water drum 18 through an inlet 36. It is discharged into the downcomer 16 through a ring main 37. The downcomer is closed at the lower end so that the water leaves the lower end through the inlet tubes 2t) of the risers. A'steam and water mixture is delivered from the outlet tubes of the risers into the annular collecting riser 26 surrounding the downcomer and thence into the steam and water drum. Therein, the mixture is separated in the centrifugal separators 38, the steam thus separated being passed to dryers 4-0 before leaving through the saturated steam outlet 42, and the water being returned to the downcomer 16.

The steam and water drum internals, that is to say the water main 37, the separators 38 and the driers 40 are enclosed by batfie plates which define with the shell of the drum a passage 52 for the steam which leaves the driers. This passage 52 which is bounded by the shell of the drum leads to an annular passage or space 54 bounded by the outer wall of the collecting riser 26 and a tube 56. The tube 56 is drilled and has butt-welded to it tubes 44 which serve for superheating the steam and which pass downwards through the shell 10 and close to its inner wall. The superheating tubes of which there may be several rows form a cylinder closely surrounding the steam generating tubes 28. A large number of them may be provided without resulting in any substantial enlargement of the diameter of the shell.

It will be noted that there are no external pipe connections between the steam and water drum and the shell.

If such connections were provided, large expansion loops would be necessary and the compactness of the whole plant would suffer. As the steam passage 52 is bounded by the shell of the drum, that shell will, when the plant is started up, be brought up to its working temperature rapidly and evenly.

At the lower end, the superheating tubes 44 are directed inwards and are welded to a tube 60 which is an extension of the downcomer tube 16. As shown here, which is the preferred arrangement, the tube 60 is an integral extension of the downcomer 16 which is closed by a closure or door 62 which can be opened or removed when desired as, for example, if a steam generating tube has failed and its end has to be plugged. The superheater tubes communicate with the tube 60 through holes in the wall of the latter and the superheated steam is drawn off from the lower end of the tube.

The steam outlet tube 60 passes through the bottom end of the shell. It must be free for axial movement with expansion and contraction of the downcomer and provision has to be made for an expansion joint between it and the shell. This is efiected in the example shown in the drawing by the provision between the tube 60 and the shell of a bellows piece 66. It is important to note that as the tube 60 passes through the end of the shell, the bellows piece can be arranged so that it is not subjected to the steam pressure. It is subjected on one side to the pressure within the shell which is moderate-say 270 p.s.i.g. and can be subjected on the other side to atmospheric pressure by the provision of a sliding sleeve 64 having grooves or the like in its inner surface.

The arrangement thus described is very compact and lends itself to the construction of steam generators of very large output. Substantially the whole of the available space within the shell 10 is occupied by steam generating tubes which, in spite of their closeness, can be assembled into bundles and the bundles fixed in their final position relatively easily. Each riser formed by a bundle 24 and inlet and

outlet tubes

20, 22 can be prefabricated in the factory and delivered to the site where the only work remaining to be done is to weld the inlet tubes to the downcomer and the outlet tubes to the steam riser 26. It will be appreciated that the composite risers can be all identical in spite of the staggering of the headers 30 referred to above.

The provision of an annular collecting riser surrounding the downcomer also contributes to the compactness. Such a collecting riser also has the advantage particularly with a spherical steam and water drum of contributing to a smooth flow of the steam and water mixture produced in the steam generating tubes.

The annular riser surrounding the downcomer has the added advantage that all the circulation needs can be satisfied without necessitating more than one piercing of the upper end of the shell.

Another material advantage is that the steam generating tubes, being connected to the collecting riser at the upper end and to the downcomer at the lower end, are free to expand with the downcomer without restraint by the shell.

As will be appreciated from an inspection of the drawing, the arrangement shown therein has the further advantage, due to the provision of a collecting riser which surrounds the downcomer, that the steam and water drum is supported by the shell of the generator and that there are no external pipe connections between the shell and the drum.

Although it is preferred to use a spherical steam and water drum, this drum can, if desired, be of the usual or more conventional cylindrical form, with its axis horizontal or, preferably, vertical. A spherical drum has the big advantage over a cylindrical drum that the thickness of its shell can be reduced by one half.

The provision of the annular steam passage 54 for dried steam to be superheated also contributes very materially to the compactness of the plant and avoids the is effected by the shell 10;

Asatypical example, asteam generator in accordance with the invention, with hineteen tubes each bundle, can have within a shell 8 feet diameter and 60 feet in length, having a central downcemer 1 4 inches in diameter, 8,598 steam generating tubes, withsorhe 1,200 superheating tubes and nearly l fl'iflfltl ste'am generating tubes if no superheat'iirg tubes are provided.

It will he appreciat'edthat with the arrangement described above, the steamlgeneratin'g tubesand the superheatin'g tubes, if any, ea-n be evenly distributed over the cross section bruissl'iell. This ensures best utilization of the heating fluid and uniform heat transfer in the various tubes.

To ensure completely even-distribution, it may be necessary to adopt an expedient which is illustrated in FIG- URE 4; The wall thickness of theadaptor 32a must be su'fiicient' to enable it 'towithstand the internal pressure which may be substantial, for instance 2,500 psi. In practice, if as shown in FIGURE" 4, the tubes 28 are pitched -as clos'ely' as' pdss'i'bl'eg the-thickness t of the adaptor will be greater than'hal f he'distance' x between neighbearingtubes. Conseqr'teiitly, if the tubes are to be completely uniformly distributed overthe cross-section of the shell, the headers 3% would havelto overlap. This, however,- can be avoided by'the expedientofproviding spacers 33 at intermediate points along-Ithe length of the tubes "slightly bending the tubes: as shown in FIGURE '4. Further, thiscanbe done-without putting a set inthe tubes as the latter can quite easilybeisprung' to the desired extent. This is important as it ensuresthat all the tubes can be provided as identical straight lengths needing normechanical operationv to be performed on: them and therefore of minimum cost otproductiom V The hexagonal shape of the headers-v is advantageous in that the hexagon isa si'mple regular geometrical figure a number of whiclrcanbe juxtaposed to cover the Wh0l6 surface of the circumscribing. circle- There may, howshown in FIGURE '5 t'o'which can bejoined twelve tubes and which is interfittingwith otherheaders of identical shape. There are, of course, other shapes which canbe used such asthesqua're.

Although the invention has been describedwith reference to specificstructure", "it will be'understood" by those .skilled in the art that many modifications can'be made without the spirit and scope oftheinvention, as defined in the following claims.

What is claimed is:

1. A vapor generator comprising an upright shell; a vapor-liquid'drum at the upper end of said shell; means for circulating a heating fluid through said shell; a downcomer within said shelb coaxial therewith, said downcomer leading from the interioriof'tsaid vapor-liquid drum toapointadjacentthe bottom of said shell; risers connected to the lower end of said downcomer each comprising an inlet tube, an outlet tube, polygonal headers for said inlet and outlet tubes, and a plurality of straight intermediate tubes of smaller diameter than said inlet and outlet tubes extending between said inlet and outlet headers, the arrangement of said risers being such that said intermediate tubes are regularly anduniformly distributed over a cross-section of the shell occupying a greater part of the volumn of said shell in the annulus surrounding said downconier; a collecting riser in the form .of an annulus surrounding the upper end of said downcomer 6 and communicating with said vapor-liquid drum, said riser outlet tubes being connected to said collecting riser such that the circulation needs of the generator are satisfied with only a single piercing of the upper end of said shell and of the vapor-liquid drum.

2. A vapor generator according to claim 1 wherein said vapor-liquid drum is spherical in shape.

3. A vapor generating and superheating arrangement comprising. an upright shell; a vapor-liquid drum at the upper end of said shell, said drum including means defining. a vapor space and a liquid space in the drum; means for circulating a heating fluid through said shell; a downcomer With-in said shell coaxial therewith leading'from said drum liquid space to a point adjacent the bottom of the shell; risers connected to the lower end of said downcomer each comprising an inlet tube, an outlet tube, headers for said inlet and outlet tubes having a polygonal plan configuration, and a plurality of straight intermediate tubes of smaller diameter than said inlet and outlet tubes extending between said inlet and outlet tube headers; a

collecting riserin the form of an annulus surrounding the upper end' of said downcom'er and communicating with said drum vapor space, said riser outlet tubes being connectcd to said collecting riser; a plurality of superheat'er tube bundles within said shell, outlet means for said superheater tube bundles and inlet means adapted to convey heating arrangement are satisfied with only a single piercing of the upper end of said shell and said vaporliquid drum.

4. A. vapor generating and superheating arrangement according to' claim 3 whereinsaid superheater tube bundles comprise each an inlet tube connected to said annular passageway, an outlet tube, headers for said inlet and outlet tubes having a polygonal plan configuration, and a plurality of straight intermediate tubes of smaller diameterthan said inlet and outlet tubes extending'between said inlet and outlet tube headers.

5. A vapor generating and superheating arrangement according to claim 4 wherein said superheater tube bundles occupy an annularspace adjacent said shell and surrounding said risers, the arrangement being such that said riser and superheater intermediate tubes. are regularly and uniformly distributed over the cross-section of the shell occupying a greater part of the volume of the shell.

6. A vapor generating and superheating arrangement according to claim 4 wherein said superheater tube bun- .dle outlet means comprises a tubular duct which is an extension of said downcomer passing through the bottom of the vapor generator shell, said arrangement further including removable closure means separating the downcomer passageway from said tubular duct through which access is obtained to the downcomer, and means sealing said tubular duct with said upright shell.

7. A vapor generating and superheating arrangement according to claim 6 wherein said last-mentioned means comprises a. cylindrical expansible member extending be tween said tubular duct and said shell adapted to permit :expansion of the downcomer and said duct relative to the shell.

8. A vapor generating and superheating arrangement according to claim 3 wherein said drum further includes baflle means defining with the drum an annular vapor passage communicating with said vapor space and with said annular passageway surrounding the downcomer, said bafile and annular vapor passage encompassing the vapor space of the drum, said generator further including drier means between said vapor passage and the drum vapor space.

9. A vapor generating and superheating arrangement according to claim 8 wherein said vapor-liquid drum is 7 spherical in shape, said bafile'being Substantially concentric with the drum.

10. A vapor generator comprising an upright shell, a vapor-liquid drum at the upper end of said shell, said drum including means defining a vapor space and a liquid space in the drum; means for circulating a heating fluid through said shell; a downcomer within said shell coaxial therewith leading from said drum liquid space to a point adjacent the bottom of the shell; risers connected to the lower end of said downcorner each comprising an inlet tube, an outlet tube, headers for said inlet and outlet tubes having a polygonal plan configuration, and a plurality of intermediate tubes of smaller diameter than said inlet and outlet tubes extending between said inlet and outlet tube headers; a collecting riser in the form of an annulus surrounding the upper end of said downcorner and communicating with said drum vapor space, the riser outlet tubes being connected to said collecting riser; said headers being staggered in separate vertical planes to obtain circulation of the heating fiuid around the headers and further being dimensioned so that from a plan view the edges of adjacent headers overlap and being arranged to substantially fill from a plan view the area of said shell surrounding the downcorner; said generator further including spacing means at intermediate points along the lengths of said intermediate tubes arranged to slightly bend the tubes whereby the tubes are regularly and uniformly distributed over the cross-section of the shell.

11. A vapor generator comprising an elongated cylindrical upright shell; a vapor-liquid drum at the upper end of said shell having a vapor space and a liquid space; means for circulating a heating fluid through said shell; a downcorner within said shell coaxial therewith, said downcorner leading from the liquid space of said vaporliquid drum to a point adjacent the bottom of said shell; a collecting riser in the form of an annulus surrounding the upper end of said downcorner and communicating with the vapor space of said vapor-liquid drum; and a plurality of risers connected to the lower end of said downcorner and to said collecting riser arranged so as to be regularly and uniformly distributed over the cross-section of the shell occupying a greater part of the volume of said shell in the annulus surrounding said downcorner, said downcorner and collecting riser extending between said shell and the vapor-liquid drum with only a single piercing of the upper end of the shell and of the vapor-liquid drum for the circulation needs of the generator.

12. A vapor generator according to claim 11 wherein said vapor-liquid drum is spherical in shape.

13. A vapor generator according to claim 12 wherein said vapor-liquid drum includes bafile means defining with the drum an annular vapor passage in communication with said vapor space, and vapor drier means disposed between vapor space and said vapor passage; said generator further including a plurality of superheater tubes within said upright shell and means including an annular passageway surrounding said downcomer and collecting riser and in fluid communication with said drum vapor passage, said annular passageway occupying with the downcorner and the collecting riser the single piercing of the upper end of the shell and of the vapor-liquid drum.

14. A vapor generator of the shell and tube type comprising a vapor-liquid drum including means defining a liquid space and a vapor space therein; a central downcomer passing through the bottom of the drum and leading from said liquid space; vapor-liquid separating means arranged to receive a vapor-liquid mixture and to transmit vapor to said vapor space and liquid to said liquid space; and an annular riser encompassing said downcomer and leading to said vapor-liquid separating means; said riser and downcorner passing through the bottom of the drum with only a single piercing of the drum for the circulation needs of the generator.

15. A vapor generator according to claim 14 wherein said drum further includes baflies means defining with the drum an annular space surrounding the liquid space and the vapor space of the drum, drier means disposed between said space and said vapor space, and an annular passage encompassing said downcorner and said annular riser in fluid communication with said drum annular space.

16. A vapor generating and superheating unit of the shell and tube type comprising:

a shell in which heating fiuid circulates,

inlet means for liquid entering in the unit including a downcorner within the shell coaxial therewith leading from above the shell to a point adjacent to the bottom of the shell,

vapor generating tubes connected at one end to the lower end of the downcorner,

superheater tubes for superheating the vapor generated in the vapor generating tubes, means for connecting the other end of the vapor generating tubes to one end of the superheater tubes,

an outlet means including a tubular duct in the form of an extension to said downcorner leading through the bottom of the shell for the superheated vapor to leave the unit, the other end of the superheater tubes being connected to said tubular duct, and

a movable closure means separating said downcorner from the inside of said tubular duct by which access can be had to the inside of the downcomer.

17. A vapor generating and superheating unit according to claim 16 including a cylindrical expansible member extending between said tubular duct and said unit shell to permit expansion of the downcorner and duct relative to the shell.

References Cited by the Examiner UNITED STATES PATENTS 1,741,121 12/1929 Winslow 29157.4 1,934,021 11/1933 Wafter 29157.4 2,518,270 8/1950 Barr 122332 2,862,479 12/1958 Blaser et a1. 122-34 2,946,116 1960 Norris et a1 29157.4 3,018,764 1/1962 Huet 12234 3,097,630 7/1963 Kingon et al. 12234 3,103,206 9/1963 Halvorsen et al. 122-34 FOREIGN PATENTS 1,197,675 6/ 1959 France. 1,243,428 9/ 1960 France.

KENNETH W. SPRAGUE, Primary Examiner.

FREDERICK L. MATTESON, JR., Examiner.

PERCY L. PATRICK, ROBERT A. OLEARY,

Assistant Examiners.

Claims

1. A VAPOR GENERATOR COMPRISING AN UPRIGHT SHELL; A VAPOR-LIQUID DRUM AT THE UPPER END OF SAID SHELL; MEANS FOR CIRCULATING A HEATING FLUID THROUGH SAID SHELL; A DOWNCOMER WITHIN SAID SHELL COAXIAL THEREWITH, SAID DOWNCOMER LEADING FROM THE INTERIOR OF SAID VAPOR-LIQUID DRUM TO A POINT ADJACENT THE BOTTOM OF SAID SHELL; RISERS CONNECTED TO THE LOWER END OF SAID DOWNCOMER EACH COMPRISING AN INLET TUBE, AN OUTLET TUBE, POLYGONAL HEADERS FOR SAID INLET AND OUTLET TUBES, AND A PLURALITY OF STRAIGHT INTERMEDIATE TUBES OF SMALLER DIAMETER THAN SAID INLET AND OUTLET TUBES EXTENDING BETWEEN SAID INLET AND OUTLET HEADERS, THE ARRANGEMENT OF SAID RISERS BEING SUCH THAT SAID INTERMEDIATE TUBES ARE REGULARLY AND UNIFROMLY DISTRIBUTED OVER A CROSS-SECTON OF THE SHELL OCCUPYING A GREATER PART OF THE VOLUMN OF SAID SHELL IN THE ANNULUS SURROUNDING SAID DOWNCOMER; A COLLECTING RISER IN THE FORM OF AN ANNULUS SURROUNDING THE UPPER END OF SAID DOWNCOMER AND COMMUNICATING WITH SAID VAPOR-LIQUID DRUM, SAID RISER OUTLET TUBES BEING CONNECTED TO SAID COLLECTING RISER SUCH THAT THE CIRCULATION NEEDS OF THE GENERATOR ARE SATISFIED WITH ONLY A SINGLE PIERCING OF THE UPPER END OF SAID SHELL AND OF THE VAPOR-LIQUID DRUM.