US3662716A

US3662716A - Furnance enclosure for natural circulation generator

Info

Publication number: US3662716A
Application number: US97700A
Authority: US
Inventors: William D Stevens
Original assignee: Foster Wheeler Inc
Current assignee: Foster Wheeler Inc
Priority date: 1970-12-14
Filing date: 1970-12-14
Publication date: 1972-05-16
Anticipated expiration: 1989-05-16
Also published as: JPS5632521B1; CA928177A; AU461157B2; FR2118589A5; AU3677371A; ES397904A1; GB1367479A; NL7117120A

Abstract

A natural circulation generator having a rectangular furnace enclosure. A plurality of parallel, upright, finned tubes welded together along their lengths make up the walls of the enclosure. Burners are provided in the front wall of the enclosure, in the lower portion thereof, providing a heat input to the finned tubes. In accordance with the present invention, the tubes have a first diameter in the lower portion of the furnace, and a second greater diameter in the upper portion of the furnace. The increased tube diameter in the upper portion of the furnace gives a higher circulation ratio in the generator, and in addition provides more heat absorption surface area which increases the throughput steam flow per hour-foot of furnace periphery. This enables construction of a natural circulation unit having increased capacity.

Description

Stevens 51 May 16, 1972 FURNANCE ENCLOSURE FOR NATURAL CIRCULATION GENERATOR inventor: William D. Stevens, North Caldwell, NJ.

Assignee: Foster Wheeler Corporation, Livingston,

Filed: Dec. 14, 1970 Appl. NO.: 97,700

US. Cl. ..l22/6 A, 122/235 C, 122/406, 165/147 Int. Cl ..F22d 7/00 Field 01 Search 1 22/6 A, 235, 235 A, 333, 406 R, 122/235 C; 165/146, 147

References Cited UNlTED STATES PATENTS 3,060,908 10/1962 Brister et a1 ..l22/235 X Primary Examiner-Kenneth W. Sprague Attorney-John Maier, [11, Marvin A. Naigur and John E. Wilson [57] ABSTRACT A natural circulation generator having a rectangular furnace enclosure. A plurality of parallel, upright, finned tubes welded together along their lengths make up the walls of the enclosure. Burners are provided in the front wall of the enclosure, in the lower portion thereof, providing a heat input to the finned tubes. In accordance with the present invention, the tubes have a first diameter in the lower portion of the furnace, and a second greater diameter in the upper portion of the furnace. The increased tube diameter in the upper portion of the furnace gives a higher circulation ratio in the generator, and in addition provides more heat absorption surface area which increases the throughput steam flow per hour-foot of furnace periphery. This enables construction of a natural circulation unit having increased capacity.

11 Claims, 3 Drawing Figures FINISHING SUPER HEATER OUTLETS REHEATER OUTLET EHEATER INLET PKTENTEnnmsmrz 3.662.716

Fl HIN UPER HE ER TLETS REHEATER OUTLET l E I REHEATE R INLET WILL/AM D. STEVENS Rmmno H- THOM As ATTORNEY INVENTOR.

FURNANCE ENCLOSURE FOR NATURAL CIRCULATION GENERATOR The present invention relates to natural circulation generators, and particularly to an improved furnace arrangement which enables the construction of natural circulation generators of higher capacity.

Natural circulation generators heretofore have been limited in size primarily because of the head available for circulation. The head or motive force in a natural circulation unit is dependent upon the difference in density between the flow in the downflow circuit and that in the upflow circuitry, minus losses from friction, shock, turbulence and other factors, which losses increase with increased capacity or size, Also, relatively high pressures are employed today in natural circulation units, further causing a reduction in the head available for circulation.

One measure of the circulation or head in a generator is the circulation ratio, or ratio of weight rate of water fed to the steam generator tubes divided by the weight rate of steam generated..When natural circulation is the sole motive force in the generator, variations in heat transfer or heat absorption in wall tubes of the generator, caused either by different heat intensities in the furnace periphery, or by the use of different lengths of tubes or circuits, require that the circulation ratio be high enough for the entire generator so that the least favorable furnace wall circuit receives an adequate water supply. This required margin of safety imposes a further limitation on the size of unit which can be built.

Still further, present day generators conventionally are topsupported. Limitations on the thickness of tube wall which can be used, and materials available, taken together with the need for adequate support, have heretofore limited the size of unit which can be built.

By utilizing a larger tube diameter size in the upper walls of the furnace, in accordance with the present invention, it is possible to obtain a lower mass flow rate for the circulating fluid in the upper walls and a correspondingly lower velocity head. Thus, friction and shock losses in the upper portion of the furnace which are directly proportional to the velocity head are respectively lower.

Accordingly, it is an object of the present invention to provide a generator design which enables building a natural circulation generator of greater capacity.

Another object of the present invention is to provide a natural circulation generator of greater capacity in which the likelihood of failure of tubes in the furnace wall circuitry is reduced.

Still another object of the present invention is to provide a natural circulation generator having a high circulation ratio in which the throughput steam flow per foot of periphery in the furnace walls of the generator is increased.

A still further object of the present invention is to provide a large capacity natural circulation generator in which conventional tube materials can be employed.

To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims, the following description and the annexeddrawingsetting forth in detail a certain illustrative embodiment of the invention, this being indicative, however, of but only one of the various ways in which the principles of the invention may be employed.

In such annexed drawing:

FIG. 1 is a section, elevation view illustrating a natural circulation generator in accordance with the concepts of the invention;

FIG. 2 is an enlarged, perspective view of a portion of the furnace enclosure tube wall of the generator of FIG. 1; and

FIG. 3 is an enlarged partial elevation view of a tube wall of the generator of FIG. 1 taken in zone B-B of the generator furnace enclosure.

Referring to the drawing, the vapor generator in accordance with the present invention is broadly indicated with the letter A, and comprises a vertically extending rectangular shaped radiant furnace area B having an upper gas exit C, and a convection area D which leads downwardly from the gas exit. A burner zone E occupies the furnace area of the generator immediately above hopper F. The flow of hot gases is upwardly in the furnace area from the burner zone, through the convection areas of the generator to the generator outlet G, and from there to a conventional air heater H for heat exchange between hot gases and incoming air for the burners.

The present invention is concerned primarily with the construction of the furnace portion B of the generator.

The vapor generator furnace comprises an upright, rectangular enclosure 12 defined by front and rear walls l4, 16. Side walls 18 (only one of which is shown) extend between the front and rear walls, the entire enclosure leading vertically from the bottom hopper F to an inclined roof 22. Beneath the roof 22, the rear wall is bent inwardly and thenrearwardly to provide a reverse arch 24, above which the wall is branched at the furnace exit C into parallel but spaced apart, open

screen tube panels

26, 28 which permit the flow of gas from the furnace area of the generator to the generator convection area.

In the convection area, frequently referred to as the heat recovery area, the generator comprises a generally, rectangular enclosure 30 separated into two gas passes 32, 34 by a division wall 36. The rearmost of the two gas passes houses the generator reheater section 38, the other of the gas passes containing superheater and

economizer sections

40, 42, respectively.

The upper zone 44 of the furnace, above the burner zone E, contains a plurality of J-shaped division wall panels 46 in the front of the furnace area, and a pendant finishing superheating section 48 above the arch 24 in the rear wall of the generator. The pendant finishing superheater is positioned immediately in front of the

screen panels

26, 28 leading to the convection area of the generator.

The flow of the fluid being heated in the generator is from the economizer 42 into the lower inlet headers 50 for the radiantly

heated walls

14, 16 and 18 of the generator enclosure. The flow in these walls is upwardly in generally parallel tubes of the walls into risers 52 at the top of the generator and from there into the steam and

water drums

54, 56. Liquid separated from the flow in the drums is recycled by downcomers 58 to the lower inlet headers of the enclosure, by natural circulation.

Vapor separated from the flow in the steam and water drums is transmitted by conduits 60 into an inlet header62 for the downwardly inclined roof 22 of the generator. At the end of the roof panel, a header 64 divides the fluid for flow into the walls of the convection enclosure 30 of the generator and into the division wall 36 of the convection area. These walls terminated in a lower header 66 at the bottom of the convection area, from which the flow is transmitted into the bank 40 of the primary superheating tubes. From there via additional headers and conduits, the flow is into the division wall panels 46, and pendant finishing superheating sections 48 in that order.

It is a feature of the invention that a membrane-type wall construction illustrated in detail in FIG. 2 is employed substantially throughout all of the furnace walls of the generator, except for the

screen tube panels

26 and 28 at the gas exit C, and except where the walls are penetrated, forinstance, by the burners and division wall panels. This membrane-type wall construction is obtained by welding together a plurality of finned tubes 68 along their lengths so that the enclosure is substantially gas-tight.

In accordance with the present invention, the furnace enclosure is divided into higher and lower temperature tube panel sections B and B", the higher temperature section B being roughly coextensive with the enclosure burner zone E, and extending from near the bottom of the furnace to an elevation above the burners 70; the lower temperature sections B" constituting the remainder of the furnace enclosure up to the roof and being roughly coextensive with the upper zone 44 of the enclosure. As shown in FIG. 3, the tube diameters are increased in the panel sections in a transition area 72 between the burner and lower temperature zones, being smaller in diameter in the burner zone and larger in diameter in the upper zone.

In a particular example, in a furnace about 41 feet by 91 feet in cross section, 36 burners are positioned in the front wall of the generator at four elevations. At an elevation about 20 feet above the uppermost row of burners, just below the elevation of the division wall panels and arch of the furnace, the transition from smaller diameter tubes to larger diameter tubes occurs. The smaller diameter tubes are 3 inches in outside diameter on 3% inch centerlines, and the larger diameter tubes are 3% inches in outside diameter, also on 3% inch centerlines, so that the centerlines for the tubes are in alignment for the full elevation of the furnace. At the furnace gas exit C, the tubes of the rear wall are divided into parallel spaced apart screen located on 7% inch centers, providing adequate space for the flow of gases from the furnace into the convection area.

It is apparent that to obtain increased capacity, it is necessary to increase the heat absorption surface in the furnace, requiring increased furnace height and periphery. It is contemplated that the unit will be oil fired, and oil firing has a relatively high heat release rate compared to coal firing (although less than gas firing). This will cut down on the furnace size somewhat, but not enough to attain the necessary circulation ratio. In addition, it has the disadvantage that the high heat release rate can cause overheating of the tube surface in the burner zone, requiring the use of expensive alloy tubing in this zone.

Employing smaller diameter tubing in the high heat intensity burner zone has the advantage that the relatively high flow in the tubes provides for better cooling in this zone. In addition, the smaller diameter permits the use of thinner walled tubing for better heat transfer and cooling of the tubes. As a result, plain carbon steel tubes can be used in this zone.

The use of large diameter tubing in the upper zone has the advantage of increasing the surface area for heat absorption, for a given furnace periphery, and to thereby obtain a relatively high throughput steam flow per foot of periphery, in the order of 20,000 lbs. per hour-foot in the unit described. It also reduces friction and other losses to insure a high circulation ratio in the generator, sufficiently high so that the least favorable circuit receives an adequate water supply.

The use of larger diameter thicker wall tubing in the upper zone of the furnace still further has the advantage of offering better support for the walls of the generator, permitting the generator to be top-supported.

Positioning the division wall and finishing superheater surfaces in the upper furnace has the advantage that it reduces gas temperature at the gas exit to within design limits. In this respect, it permits limiting the amount of surface area in the furnace perimeter to that necessary to attain the required circulation ratio. Location of these surfaces in a high heat intensity zone also facilitates control of superheat temperatures.

It should be understood that while the present invention is not limited to an all welded fin-tube construction, the use of different diameter tubes in the furnace of the fin-tube vapor generators becomes feasible, since if a conventional skin casing were employed, the wide gap at the bottom of the furnace between the tubes would make it difficult to seal the furnace.

As an example of the increased capacity attainable in a natural circulation generator by employing the concepts of the invention, the unit described above has a capacity of about 900 megawatts, as compared to a maximum of about 600 megawatts for previous units.

A latitude of modification, change and substitution is intended in the foregoing disclosure and in some instances some features of the invention will be employed with a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the spirit and scope of the invention herein.

What is claimed is:

l. A natural circulation generator of increased capacity comprising a rectangular furnace enclosure; a plurality of parallel upright tubes welded together along 5 their lengths forming the walls of the enclosure;

burners in at least one of the walls of the enclosure near the bottom of the enclosure;

gas exit means at the top of the enclosure;

the tube sizes in the lower portion of the enclosure being of one diameter, those in the upper portion of the furnace, above the burners, being of a larger diameter to provide an increased throughput steam flow per foot of furnace periphery, whereby the friction and shock losses are reduced in said upper portion and a higher circulation ratio is achieved in the entire circuit.

2. The generator of claim 1 wherein said burners are in the front wall of the furnace, the gas exit means being in the rear wall of the furnace further including division wall means at least in the upper portion of the furnace; and pendant superheater means in the upper portion of the furnace in front of said gas exit means. 3. The generator of claim 2 wherein said division wall means is steam cooled.

4. The generator of claim 2 wherein said division wall means is in the flow circuit of the generator immediately upstream of the pendant superheater means.

5. The generator of claim 4 including a convection zone connected with the furnace enclosure gas exit means, the convection zone housing reheater, superheater and economizer tube surface.

6. The generator of claim 2 wherein the furnace enclosure comprises front, rear and side walls, the tubes of the rear wall immediately below the gas exit means being bent inwardly to the furnace and then rearwardly to form a reverse arch penetrating into the furnace for deflection of hot gases across said division wall means.

7. The generator of claim 6 wherein said division wall means comprises .l-shaped panels of tubes having ends penetrating the front wall of the generator and the roof of the generator, the elevation of penetration of the front wall being above the area of transition in the enclosure wall tubes from the smaller diameter tubes to the larger diameter tubes.

8. The generator of claim 2 wherein said larger diameter and smaller diameter enclosure wall tubes are on vertically aligned centers.

9. The generator of claim 8 wherein said enclosure wall tubes are welded along their lengths to provide a gas-tight construction.

10. A natural circulation generator of increased capacity comprising a rectangular furnace enclosure;

55 a plurality of parallel, upright, finned tubes welded together along their lengths forming the walls of the enclosure;

gas exit means at the top of the enclosure;

the tube sizes in the lower portion of the enclosure being of one diameter, those in the upper portion of the enclosure being of a larger diameter;

the tube diameter in the lower portion of the enclosure being sufficiently small to permit the use of plain carbon steel tubing;

the tube diameter in the upper portion of the enclosure being sufficiently larger in diameter to obtain that circulation ratio and throughput steam flow per hour-foot of furnace periphery required;

further including division wall means at least in the upper portion of the furnace; and

pendant superheater means in the upper portion of the furnace in front of said gas exit means.

11. The generator of claim 10, the burner means being for 75 oil or gas firing.

Claims

1. A natural circulation generator of increased capacity comprising a rectangular furnace enclosure; a plurality of parallel upright tubes welded together along their lengths forming the walls of the enclosure; burners in at least one of the walls of the enclosure near the bottom of the enclosure; gas exit means at the top of the enclosure; the tube sizes in the lower portion of the enclosure being of one diameter, those in the upper portion of the furnace, above the burners, being of a larger diameter to provide an increased throughput steam flow per foot of furnace periphery, whereby the friction and shock losses are reduced in said upper portion and a higher circulation ratio is achieved in the entire circuit.

2. The generator of claim 1 wherein said burners are in the front wall of the furnace, the gas exit means being in the rear wall of the furnace further including division wall means at least in the upper portion of the furnace; and pendant superheater means in the upper portion of the furnace in front of said gas exit means.

3. The generator of claim 2 wherein said division wall means is steam cooled.

7. The generator of claim 6 wherein said division wall means comprises J-shaped panels of tubes having ends pEnetrating the front wall of the generator and the roof of the generator, the elevation of penetration of the front wall being above the area of transition in the enclosure wall tubes from the smaller diameter tubes to the larger diameter tubes.

10. A natural circulation generator of increased capacity comprising a rectangular furnace enclosure; a plurality of parallel, upright, finned tubes welded together along their lengths forming the walls of the enclosure; burners in at least one of the walls of the enclosure near the bottom of the enclosure; gas exit means at the top of the enclosure; the tube sizes in the lower portion of the enclosure being of one diameter, those in the upper portion of the enclosure being of a larger diameter; the tube diameter in the lower portion of the enclosure being sufficiently small to permit the use of plain carbon steel tubing; the tube diameter in the upper portion of the enclosure being sufficiently larger in diameter to obtain that circulation ratio and throughput steam flow per hour-foot of furnace periphery required; further including division wall means at least in the upper portion of the furnace; and pendant superheater means in the upper portion of the furnace in front of said gas exit means.

11. The generator of claim 10, the burner means being for oil or gas firing.