US3796195A

US3796195A - Circuit arrangement for once through vapor generator

Info

Publication number: US3796195A
Application number: US00247068A
Authority: US
Inventors: W Gorzegno
Original assignee: Foster Wheeler Inc
Current assignee: Foster Wheeler Inc
Priority date: 1972-04-24
Filing date: 1972-04-24
Publication date: 1974-03-12
Anticipated expiration: 1991-03-12
Also published as: JPS5138361B2; AU5482473A; ES414504A1; AU465725B2; CA971444A; JPS4941702A

Abstract

A once-through vapor generator in which the roof and heat recovery area rear wall comprise a single continuous downflow pass feeding the generator primary superheater. An outlet header for the pass, at the lower end of the pass, also serves as the inlet header for the primary superheater, flow from the header being upwardly in the superheater. A plurality of diaphragms at spaced intervals along the axis of this common header divide the header into a plurality of separate sections which in turn form in the downflow pass and superheater a plurality of narrow-width parallel down-up flow circuits. The invention prevents the occurrence of recirculation of flow at low loads within the roof and heat recovery area rear wall. A plurality of mix headers may be employed in each down-up flow circuit, at a midpoint in elevation in the heat recovery area rear wall. This reduces the possibility of recirculation within a particular down-up flow circuit in large sized units having a large difference in elevation between the roof inlet header and the lower common header.

Description

Matted @tates Fatent [191 Gorzegno 1 Mar. 12, 1074 CIRCUIT ARRANGEMENT FOR ONCE THROUGH VAPOR GENERATUR Walter P. Gorzegno, Florham Park, NJ.

[75] Inventor:

[73] Assignee: Foster Wheeler Corporation,

Livingston, NJ.

22 Filed: Apr. 24, i972 [21] Appl. No.: 247,068

Primary Examiner-Kenneth W. Sprague Attorney, Agent, or Firm-John Maier, Ill

[ 5 7 ABSTRACT A once-through vapor generator in which the roof and heat recovery area rear wall comprise a single continuous downflow pass feeding the generator primary superheater.-An outlet header for the pass, at the lower end of the pass, also serves as the inlet header for the primary superheater, flow from the header being upwardly in the superheater. A plurality of diaphragms at spaced intervals along the axis of this common header divide the header into a plurality of separate sections which in turn form in the downflow pass and superheater a plurality of narrow-width parallel downup flow circuits. The invention prevents the occurrence of recirculation of flow at low loads within the roof and heat recovery area rear Wall.

A plurality of mix headers maybe employed in each down-up flow circuit, at a midpoint in elevation in the heat recovery area rear Wall. This reduces the possibility of recirculation within a particular down-up flow circuit. in large sized units having a large difference in elevation between the roof inlet header and the lower common header.

6 Claims, 8 Drawing Figures PM'ENYEBW I 2 I874 SPEET 3 OF 5 so I I K 58 PAIIIEIIII I 2 IIIII 33% 195 SHEET 5 [IF 5 CIRCUIT CHARACTERISTICS OF DOWN-UPFLOW ROOF. HRA REARWALL AND PRIMARY SUPERHEATER FOR START UP CONDITIONS (TEN DIAPHRAGMED SECTIONS IN PARALLEL FLOW) TOTAL PRESSURE DROP. ROOF INLET HEADER TO PRIMARY 'SUPERHEATER OULET HEADER --u- FLOW CIRCUIT CHARACTERISTICS OF A DOWNFLOW RooF AND HRA REAR WALL FOR sTART UP CONDITIONS A-=50% LESS ABSORPTION B AVERAGE ABSORPTION C- 50% GREATER ABSORPTION D- 200% GREATER ABSORPTION (EXTREME ABSORPTION UPSET) CIRCUIT ARRANGEMENT FOR ONCE THROUGH-I VAPOR GENERATOR BACKGROUND OF THE INVENTION The present invention relates to a forced flow vapor generator, and particularly to improvements in the flow circuitry thereof.

The invention is particularly applicable to a supercritical once-through vapor generator of the Benson design, and will be described with reference thereto, although it will be appreciated that the invention has broader application, such as with the Sulzer design or with a recirculation type of generator.

A generator with which the present invention can be employed is described in prior Gorzegno et al. US. Pat. No. 3,556,059, patented Jan. 19, l971, assigned to assignees of the present application. The vapor generator of this patent is provided with a plurality of successive passes, in series, for the flow of the fluid being heated. The first three passes of the generator define the generator furnace enclosure and part of the heat recovery area enclosure. The fourth pass constitutes the roof and heat recovery area rear wall, the flow from this pass then being into the generator primary superheater and finishing superheater in that order.

The arrangement of surfaces in the generator is such that the roof and the heat recovery area rear wall form a single continuous downflow pass between an upper roof inlet header and a lower outlet header located in the plane of the rear wall. The primary superheater is located physically above the lower header, so that the latter can thus be employed as the inlet header for th superheater, as well as the outlet header for the downflow pass; i.e., the header serves as a common header for both surfaces.

The use of a single downflow pass defining the generator roof and heat receovery area rear wall is desirable as it is a less costly construction of this portion of the generator. In order for the rear wall to be an upfiow pass, it would be necessary to terminate the roof pass in a roof outlet header, at the roof elevation of the gen: erator. It would then be necessary to connect this header through two outside unheated downcomers and feeders to feed the inlet header for the upflow primary superheater, The upflow heat recovery area rear wall could befed by downcomers which connect to other heat recovery area walls, but this still would require connecting feeders. The elimination of a roof outlet header, two additional downcomers and connecting feeders represents a large reduction in the cost of the generator.

The design of the roof and heat recovery area rear wall pass, and its location in the generator circuitry, insure a stable circuit characteristic at all normal loads. The pass, at full load, has a pressure drop of at least 50 psi, with an inlet orifice pressure drop of at least psi. In addition, the pass is located upstream of the start-up bypass and pressure reducing station so that it receives at all times the full flow in the generator, which is at least the minimum 25 percent start-up flow. Pressure within the pass is at about full generator pressure, for instance about 3,550 psi for a supercritical unit.

However, there is the possibility that during the transient periods of start-up and very low loads, conditions in the generator, for instance a fluid enthalpy unbalance at the roof inlet header, or an extreme firing unbalance, could subject sections of the pass to extreme absorption input variations. A much lesser density of the fluid in those tubes in a section having a much higher heat input could, despite the friction component pressure difference between the roof inlet header and lower header, cause a reverse flow or recirculation in the tubes and overheating of the tubes. For instance, a larger heat input at one side of the generator, as compared to a much smaller input in the opposite side, could result in a sufficient difference in the densities of the fluid in the respective sides to cause a reverse upward flow of the low density fluid, despite the friction pressure drop or pressure difference between the roof inlet header and lower pass outlet header.

Accordingly, it is a principal object of the present invention to provide an improved vapor generator flow circuitry in the heat recovery area of the generator by which capital costs of the generator are reduced.

It is further an object of the invention to provide a downflow circuit in the heat recovery area of the generator in which flow in the circuit is stable.

It is also an object of the present invention to provide a stable downflow circuit in the heat recovery area of the generator, in place of the conventional generator downcomers employed in this area, which is suitable for even large sized units.-

It is still further an object of the present invention to provide a method for improving the stability of a downflow circuit in the heat recovery area of the generator which can be employed with existing units.

Other aspects of the invention, objects and advantages thereof will become apparent as the following description proceeds.

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 annexed drawings setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but several of the various ways in which the principles of the invention may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS The figures herein illustrate preferred forms of the vapor generator embodying the present invention, in which:

FIG. 1 is an elevation, partial section view of a .vapor generator in accordance with the concepts of the present invention;

FIG. 2 is a schematic flow diagram showing the sequence of passes of a part of the vapor generator of FIG. .1;

FIG. 3 is an enlarged, partial, section, elevation view showing a part of the heat recovery area of the vapor generator of FIG. 1, illustrating the concepts of the present invention;

FIG. 4 is an elevation view of the generator heat recovery area taken along the rear side of FIG. 3;

FIG. 5 is an enlarged, detailed, section view of a part of the heat recovery area illustrated in FIG. 4;

FIG. 6 is a rear, elevation view of the heat recovery area illustrated in FIG. 5;

FIG. 7 is a section view taken along line 7-7 of FIG. 6; and

FIG. 8 is a graph showing circuit characteristics of the tube circuits of FIG. 3 illustrating advantages of the invention.

SUMMARY OF THE INVENTION Referring to FIG. 1, the vapor generator comprises an upright, rectangular, furnace enclosure 12 extending from a lower hopper 14 to a roof l6. Burners 18 immediately above the hopper provide a heat input into the fluid cooled tubes of the enclosure. Near the top of the enclosure, below the roof 16, the tubes of the rear wall 20 are branched at 22 to provide a gas pass 24 leading to the generator convection area 26.

In the convection area, frequently referred to as the heat recovery area, the generator is provided with a plurality of banks of superheating and reheating tubes, plus banks of economizer tubes. The only bank illustrated is primary superheater 28 being the bank with which the present invention is primarily concerned. The convection or heat recovery area 26 also is provided with a plurality of panels of tubes which form the heat recovery area enclosure and which also divide the heat recovery area enclosure into at least two gas passes or cells one of which houses the primary superheater. Only the roof l6 and rear wall 30 of the enclosure are illustrated, as these are the portions of the enclosure with which the present invention is concerned.

The roof 16 is comprised of a plurality of parallel tubes connected to an inlet header 32 which is located near the front upper corner of the genrator, parallel to the generator front wall. The tubes emanating from the header 32 extend downwardly and then into the plane of the roof. The latter is sloped in a slightly downward direction towards the rear wall 30. At the intersection of the plane of the rear wall and the plane of the roof, the tubes are then bent downwardly to define the rear wall, and in this way, the roof and rear wall tubes form a continuous net downflow pass, identified with the numeral 33, terminating in a lower pass header 34 positioned well below the elevation of the roof.

The primary superheater 28 comprisies a plurality of multilooped horizontally disposed spaced tubes which extend across the depth ofa convection pass of the generator adjacent to the rear wall 30. The particular configuration of the primary superheater is not critical, except that the tubes of the superheater are connected at their lower ends with the roof-heat recovery area outlet header 34, and at their upper ends with headers 36 disposed above the roof 16. Thus the header 34 serves as a common header for both the downflow pass 33 and the superheater 28.

Further details of the circuit are illustrated in the schematic flow diagram of FIG. 2. Flow from the generator enclosure walls is through a fan-mix arrangement 38 into downcomers 40 to surfaces of the heat recovery area including vestibule side walls 42, convection area enclosure front and side walls 44, and a convection area partition wall 46. The flow from these walls is collected in the inlet header 32 which feeds the roof 16 and heat recovery area rear wall panel 30, identified as downflow pass 33, the latter leading to outlet header 34. The primary superheater panels 28 are shown schematically connected between the header 34 and superheater outlet headers 36. The flow from the primary superheater headers 36 is then in succession through a platen superheater (not shown) and a finishing superheater (not shown) to a point of use. As illustrated in prior U.S. Pat. No. 3,556,059, a start-up bypass system may be employed between the platen superheater and the finishing superheater to bypass the point of use during start-up of the generator. A pressure reducing station, functioning to reduce the pressure of the flow to a pressure more suitable for the start-up of the turbine, may be positioned in the bypass system. Accordingly, the circuitry illustrated in FIG. 2 is at the full operating pressure of the generator. In addition, the circuit is located in the generator to receive the full flow in the generator which is at least the minimum 25 percent flow during start-up.

It is also a characteristic of the circuitry of FIG. 2 that the downflow pass 33 will have a substantial pressure drop, for instance a full load pressure drop of at least about 50 psi, plus an inlet orifice drop of at least about 10 psi, which will insure a stable circuit or pass characteristic for all normal loads. Despite this, it is apparent that the downflow pass may be sensitive to certain transient conditions at start-up andlow loads, such as a fluid enthalpy unbalance at the roof inlet header 32, or an extreme firing unbalance resulting in extreme absorption input to the pass, which may cause certain tubes in the pass to recirculate and overheat at the furnace roof boundary location. In other words, a large heat input at one side of the generator, as compared to a much'smaller heat input for instance in the center of the generator, will cause the fluid in the tubes having the greater heat input to have a much less density. This difference in heat inputs and densities may be sufficient to cause the lower density fluid to reverse in direction and flow upwardly, back to inlet header 32, despite the difference in friction pressure in the circuitry between

headers

32 and 34. The down flow of the higher density fluid and up flow of the lower density fluid would thus establish an unwanted natural circulation loop.

In accordance with the concepts of the present invention, referring to FIG. 4, the outlet header 34, which is also the inlet header for the primary superheater 28, is sectioned, along its length, into a plurality of successive short header sections 47. Preferably this is accomplished by positioning a plurality of impervious diaphragms 48 at spaced intervals within the header. In the embodiment illustrated, the generator is approximately 50 feet in width, and the outlet header is divided into approximately 10 sections e achof about 5 feet in length:

In that the outlet header 34 of the roof heat recovery area rear wall circuit is also the inlet header of the primary superheater, diaphragming the header 34 forces all of the flow from a particular five foot wide section of the downflow pass to enter a corresponding section of the superheater. This establishes in these two surfaces a plurality of narrow-width, parallel, down-up flow circuits which extend the full distance between the roof inlet header 32 and the primary superheater outlet headers 36.

This is diagrammatically illustrated in FIG. 2, the multiple circuits being indicated by the parallel dashed lines identified with the numeral 50. As shown in FIG. 2, the dashed lines extend in a continuous fashion from the inlet header directly to the superheater outlet headers, with no reverse flow.

Accordingly, the flow in this part of the generator will be controlled by the circuit characteristics of the multiple down-up flow circuits, rather than by the circuit characteristic of the single wide down-flow pass between the roof header 32 and rear wall header 34.

Referring to FIG. 8, upper and lower graphs are shown which illustrate the concepts of the invention.

The lowergraph of the figure contains a set of curves which give the circuit characteristics for the downflow pass alone when the lower header 34 is not diaphragmed. The upper graph of FIG. 8 contains a set of curves which give the circuit characteristics of the individual down-up flow passes when the header 34 is diaphragmed.

Both sets of curves were plotted at start-up or low load transient conditions. Specifically, the conditions selected were an inlet fluid enthalpy, at the roof inlet header 32, of approximately 500 BTUs per pound (a sub-cooled condition); percent minimum flow during start-up; and full generator pressure of 3,550 psi. In both sets of curves, the abcissa or x axis represents the flow in the pass. In the lower set of curves, the ordinate or y" axis gives the total pressure drop which occurs, or pressure difference which exits, between the roof inlet header and the heat recovery area rear wall outlet header 34. In the upper set of curves, the ordinate or v" axis gives the pressure drop which occurs, or pressure difference which exists, between the roof inlet header and the superheater outlet headers 36.

Referring to the lower set of curves (when the header is not diaphragmed) point b is that point representing an eixsting total pressure drop for a flow of 25 percent of full flow, for average absorption conditions. This system total pressure drop is represented by line E in FIG. 8. The curve line B shows the change in total pressure drop, with average absorption conditions. which occurs in the pass with increased or decreased flow through ,the pass. Line B intersects the abcissa at point b. By

comparison, curves A andC represent abnormal conditons, namely50 percent less absorption and 50 percent greater absorption, respectively. These curves show that for the existing total pressure drop in the downflow pass, there is less flow with 50 percent greater absorption, as indicated by the location of point c, and greater flow with 50 percent less absorption, as indicated by the location of point a. These flow changes occur because of the relative variation of hydrostatic head and friction pressure drop in the circuit for the absorption and flow conditions imposed. Yet with 50 percent less absorption or 50 percent greater absorption, the curves indicate that there is still a positive flow in a downward direction in the pass, at the pressure drop in the pass, as evidenced by the intersection of curves A and C with the abcissa at points a and 0.

Curve D represents the circuit characteristics which would exist with extreme absorption upset, for instance 200 percent greater absorption. As shown, this curve does not intersect the abcissa, indicating that at the total pressure drop existing there will be no downward flow. In fact, a reverse flow will exist (in the tubes subjected to 200 percent greater absorption), the amount of the reverse flow being indicated by point d on the reverse side of the ordinate line or y axis. Point d is the point of intersection of the abcissa with a line D" representing recirculated flow. If total pressure drop for the system is increased from E to F then downward flow for condition D would be re-established.

Referring to the upper curves of FIG. 8 (when the header 34 is diaphragmed) horizontal line X represents an existing total pressure drop in the down-up circuit comprised of the roof, heat recovery rear wall pass, and primary superheater for average flow condition B" and average absorption B. The intersection of curve B with line x' at b indicates the flow that ocurs with average absorption, namely B". This is the same flow that would occur for average absorption conditions in the roof-heat recovery area rear wall alone, without a diaphragmed header, as shown by vertical line B.

Curves A and C, for 50 percent less absorption and 50 percent greater absorption, intersect horizontal line x at points a and c, indicating that in the down-up flow circuits, there is the compensating effect of more flow with greater absorption and less flow with less absorption. Even with 200 percent greater absorption, curve D shows that there is a significant increased flow in the down-up flow circuits (as compared to the reverse flow of the lower set of curves with this absorption). This increased flow with greater absorption is the result of the driving force stemming from the heat input into each up-flow superheater portion of the individual down-up flow circuits.

A principal advantage of the invention should be apparent from the curves of FIG. 8. Because the greater part of absorption upsets in a generator will be between different sections of the unit, such as a center section versus an end section, arranging the circuitry in accordance with the concepts of the invention, better circuit characteristics and more stable flow is obtained.

An embodiment of the present invention is illustrated in FIG. 5, 6 and 7. In a large gas or oil fired generator, the difference in elevation between the common outlet header 34 and the roof inlet header 32 may be about 55 feet. By comparison, the same elevation for larger coal fired steam generators may be as much as to feet, because of the larger size of these generators. With this much greater difference in elevation, extreme absorption differences tube-tube may result in recirculation within each diaphragmed header section, in particular with a very low enthalpy input flow into the roof header 32.

This recirculation is prevented by employing mix headers for each down-up flow section. Preferably these headers are employed at a midpoint in elevation in the roof-heat recovery area rear wall panel to mix the fluid within each section and to re-introduce an orifice pressure drop in the succeeding down-flow portion of the section.

Referring to FIGS. 5, 6 and 7, the mix header arrangement is illustrated. These mix headers are disposed outside of the rear Wall 30, horizontally across the face of the wall. Two

headers

52 and 54 are employed for each section, the headers being at spaced elevations along the wall. Alternate tubes of each section are bent outwardly at a first elevation 56' and are connected with feeders 57 which lead to the upper header. The header is provided with a longitudinally extending diaphragm 58 with an opening 60 in the center (FIG. 7), and the flow is from one side of the diaphragm to the other through the opening; Return feeders 62 transmit the flow back into tubes in the plane of the rear wall along the same centers occupied by the alternate tubes feeding the header.

The lower header 54 is fed by the remaining alternate tubes of the rear wall section, this header also being provided with a diaphragm 58 and mixing occurring in the same way as with the flow in the upper header.

It is apparent that the fluid in any one section entering the lower outlet header 34 will be essentially at the same temperature for all of the tubes of the section, by virtue of the mix headers. In addition, the generator is top supported, and the use of two mix headers, each connected with alternate tubes of the rear wall panel, retains the integrity and strength of the panel. The integrity of the panel is further aided by vertically offsetting the mix headers of one section with those of an adjacent section, as shown in FIGS. 4 and 6.

An advantage of this aspect of the invention is that it can be applied readily to existing units. it is a simple matter to modify the rear wall panel to include the mix headers.

Other advantages and embodiment within the scope of the following claims will be apparent to those skilled in the art.

What is claimed is:

l. A once-through vapor generator comprising:

means defining a furnace enclosure and a heat recovery area, said heat recovery area including a downflow panel which forms a wall of the area, the tubes of the wall being closely spaced, parallel and welded together along their lengths to form a gastight construction;

an inlet header and an outlet header means for said downflow panel;

further upflow heat absorption surface in said heat recovery area at an elevation above said outlet header means, the outlet header means being connected directly with said further upflow surface; said downflow panel being of sufficient width and I having sufficient heat absorption surface whereby recirculation could occur in the panel;

said outlet header means being divided into a plurality of sections which establish in the downflow panel and further upflow heat absorption surface a plurality of adjacent parallel down-up flow circuits each having a downflow portion arid an upflow portion and each being of substantially smaller dimension, width-wise, than said panel, wherein the circulation in the downflow portion of each circuit is considerably enhanced by the driving force in such cicuit upflow portion.

2. A once-through vapor generator comprising:

an enclosure including a roof and a heat recovery area rear wall forming a single continuous downflow panel, the tubes of the panel being parallel and closely spaced together;

an inlet and an outlet header for said downflow panel;

a primary upflow superheater positioned in the generator heat recovery area above said outlet header; said downflow panel being of sufficient width and having sufficient heat absorption surface whereby recirculation could occur in the panel;

means connecting said outlet header directly with the primary upflow superheater so that the outlet header also serves as the inlet header for the superheater; and

means partitioning said outlet header into a plurality of sections which establish in the downflow panel and superheater a plurality of adjacent, parallel, narrow'width, down-up flow circuits, each having a downflow portion and an upflow portion and each being of substantially smaller dimension, width-wise, than said panel wherein the circulation in the downflow portion of each circuit is considerably enhanched by the driving force in such circuit upflow portion.

3. The generator of claim 2 wherein said partitioning means comprises a plurality of diaphragms disposed in the outlet header at regularly spaced intervals along the axis of the header.

4. A once-through vapor generator comprising:

an inlet and an outlet header for said downflow panel;

means connecting said outlet header directly with the primary upflow superheater so that the outlet header also serves as the inlet header for the superheater;

means partitioning said outlet header into a plurality of sections which establish in the downflow panel and superheater a plurality of adjacent, parallel, narrow width, down-up flow circuits, each having a downflow portion and an upflow portion and each being of substantially smaller dimension, width-wise, than said panel wherein the circulation in the downflow portion of each circuit is considerably enhanced by the driving force in such circuit upflow portion; and

further including mix headers for each of said downup flow circuits at mid-elevation in the downflow panel.

5. The generator of claim 4 wherein the tubes of the down-flow panel are welded together along the lengths thereof to form a gas-tight enclosure; said generator being top supported, further including two mix headers for each down-up flow circuit at spaced elevations in the downflow panel, alternate tubes of each downflow panel being connected to the upper mix header of the mix header pair, the remaining tubes being connected to the lower header of the mix header pair, thereby defining a substantially continuous load bearing tube surface.

6. The generator of claim 4 wherein said mix headers include a diaphragm means longitudinally disposed therein, an opening in said diaphragm means located axially in about the center of the diaphragm means, and means communicating the downflow panel tubes with opposite sides of the diaphragm means whereby the fluid flow in said tubes is through said opening causing mixing thereof.

Claims

1. A once-through vapor generator comprising: means defining a furnace enclosure and a heat recovery area, said heat recovery area including a downflow panel which forms a wall of the area, the tubes of the wall being closely spaced, parallel and welded together along their lengths to form a gastight construction; an inlet header and an outlet header means for said downflow panel; further upflow heat absorption surface in said heat recovery area at an elevation above said outlet header means, the outlet header means being connected directly with said further upflow surface; said downflow panel being of sufficient width and having sufficient heat absorption surface wHereby recirculation could occur in the panel; said outlet header means being divided into a plurality of sections which establish in the downflow panel and further upflow heat absorption surface a plurality of adjacent parallel down-up flow circuits each having a downflow portion and an upflow portion and each being of substantially smaller dimension, width-wise, than said panel, wherein the circulation in the downflow portion of each circuit is considerably enhanced by the driving force in such cicuit upflow portion.

2. A once-through vapor generator comprising: an enclosure including a roof and a heat recovery area rear wall forming a single continuous downflow panel, the tubes of the panel being parallel and closely spaced together; an inlet and an outlet header for said downflow panel; a primary upflow superheater positioned in the generator heat recovery area above said outlet header; said downflow panel being of sufficient width and having sufficient heat absorption surface whereby recirculation could occur in the panel; means connecting said outlet header directly with the primary upflow superheater so that the outlet header also serves as the inlet header for the superheater; and means partitioning said outlet header into a plurality of sections which establish in the downflow panel and superheater a plurality of adjacent, parallel, narrow width, down-up flow circuits, each having a downflow portion and an upflow portion and each being of substantially smaller dimension, width-wise, than said panel wherein the circulation in the downflow portion of each circuit is considerably enhanched by the driving force in such circuit upflow portion.

4. A once-through vapor generator comprising: an enclosure including a roof and a heat recovery area rear wall forming a single continuous downflow panel, the tubes of the panel being parallel and closely spaced together; an inlet and an outlet header for said downflow panel; a primary upflow superheater positioned in the generator heat recovery area above said outlet header; said downflow panel being of sufficient width and having sufficient heat absorption surface whereby recirculation could occur in the panel; means connecting said outlet header directly with the primary upflow superheater so that the outlet header also serves as the inlet header for the superheater; means partitioning said outlet header into a plurality of sections which establish in the downflow panel and superheater a plurality of adjacent, parallel, narrow width, down-up flow circuits, each having a downflow portion and an upflow portion and each being of substantially smaller dimension, width-wise, than said panel wherein the circulation in the downflow portion of each circuit is considerably enhanced by the driving force in such circuit upflow portion; and further including mix headers for each of said down-up flow circuits at mid-elevation in the downflow panel.