US3007459A

US3007459A - Forced flow vapor generating unit

Info

Publication number: US3007459A
Application number: US685119A
Authority: US
Inventors: Paul H Koch
Original assignee: Babcock and Wilcox Co
Current assignee: Babcock and Wilcox Co
Priority date: 1957-09-20
Filing date: 1957-09-20
Publication date: 1961-11-07
Anticipated expiration: 1978-11-07
Also published as: GB899359A; NL126467C; BE571390A; NL231525A

Description

Nov. 7, 1961 P. H. KOCH FORCED FLOW VAPOR GENERATING UNIT 4 Sheets-Sheet 1 Filed Sept. 20, 1957 lllllll lll INVENTOR.

Paul H. Koch AT TORNEY Nov. 7,1961

P. H. KOCH FORCED FLOW VAPOR GENERATING UNIT Filed Sept. 20, 1957 4 Sheets-Sheet 2 v INVENTOR.

Paul H. Koch 011W AT TORNEY Nov. 7, 1961 Filed Sept. 20, 1957 P. H. KOCH FORCED FLOW VAPOR GENERATING UNIT.

FIG.2

4 Sheets-Sheet 3 INVENTOR.

Paul H. Koch ATTORNEY Nov. 7, 1961 P. H. KOCH 3,007,459

FORCED FLOW VAPOR GENERATING UNIT Filed Sept. 20, 195'? 4 SheetsShet 4 52 INVENTOR.

Paul H. Koch X E3 BY ATTORNEY United States This invention relates in general to forced flow fluid heating units and more particularly to the construction and arrangement of the fluid heating walls for a oncethrough forced circulation fluid heating unit.

Heretofore, the tubular heating surface for the boundary walls of a forced flow fluid heating unit has been so arranged that the fluid to be heated passes successively through the tube panels or sections of each boundary wall. This tube arrangement has given difficulty in that the fluid temperature continuously increases as it flows in series through the boundary walls. This change in fluid temperature creates differential expansions in the walls of a magnitude resulting in high fatigue stresses in the wall forming components. It was thought that parallel flowing of fluid to all boundary walls would provide a solution to the differential expansion problem, however the tube inside diameter required to maintain adequate fluid circulation velocity was so small, about A" I.D., as to render this arrangement economically impractical in respect of the cost of manufacturing to the close tolerances needed to minimize marl-distribution of fluid to the parallel flow paths and functionally impractical in respect of the relatively high susceptibility to plugging of a tube of this size.

In accordance with the present invention the bound my wall fluid heating surface of a unit of the general character described is so proportioned and arranged that the distribution of flow to all fluid flow paths is substantially even; that in any plan cross-section of the gas flow chamber the average temperature of the tubes in each boundary wall is about the same and the maximum temperature differential between adjacent tubes is below a predetermined critical limit, thereby maintaining differential expansions in the walls within safe limits; and that the tubes are of suflicient inside diameter to prevent plugging and to provide adequate fluid circulation velocities. More specifically, the invention is concerned with the provision of an improved construction of a forced circulation fluid heating unit comprising walls including fluid heating tubes defining a vertically elongated gas flow chamber, each well being horizontally divided into a plurality of vertically adjacent tubular sections. Each tubular section includes groups of upwardly extending closely spaced parallel tubes connected for series flow of fluid to corresponding vertically adjacent tube groups to form parallel flow fluid passages receiving heat from the gases flowing through the chamber. Each tube group comprises three tubes connected for series flow of fluid and arranged for upflow and downflow of fluid. This particular routing and arrangement of the tubes affords a pressure drop of a magnitude suflicient to assure substantially uniform flow of fluid to the parallel flow fluid passages; permits the use of tubes having an inside diameter sufficiently large to preclude plugging due to solids deposition or other foreign matter and to assure safe fluid circulation velocities at all loads; and gives an average temperature of about the same value in each wall of the chamber inany horizontal cross-section of the chamber. While it is expected that heat and fluid flow distribution will be substantially uniform, header means are provided to receive and mix the fluid discharged from each of the tube groups of one of the tubular sections of each wall and flow the mixed fluids to the tube groups of the vertically adjacent tubular section of each wall,

atent ice The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated and described.

Of the drawings:

FIG. 1 is a partially diagrammatic sectional elevation of a once-through forced flow steam generator embodying the invention;

FIG. 2 is a diagrammatic representation of the vaporizable fluid flow path within the steam generator of FIG. 1;

FIG. 3 is a fragmentary sectional elevation showing a representative pair of parallel flow fluid passages in the rear wall.

FIG. 4 is a plan section taken on the line 4-4 of FIG. 3; a

' FIG. 5 is a plan section taken on the line 5-5 of FIG. 3; V

FIG. 6 is a sectional elevation taken on the line 66 of FIG. 3;

FIG. 7 is a fragmentary diagrammatic sectional elevation showing a modification to the boundary wall tube arrangement; and

FIG. 8 is a fragmentary diagrammatic sectional elevation showing another modification to the boundary wall tube arrangement.

In the drawings the invention has been illustrated as embodied in a top-supported forced flow once-through steam generating unit designed for furnace operation under internal gaseous pressures above atmospheric pressure and for the production of superheated steam at pressures and temperatures in excess of the critical pressure of 3206 psi. and the critical temperature of 705 F., a unit of this general construction being disclosed and claimed in a co-pending application 'of Paul H. Koch et al., Serial No. 595,163, filed July 2, 1956, which has been assignedto the same assignee, The Babcock & Wilcox Company, as the present application.

It will be understood that the fluid heating wall constructions illustrated and hereinafter described can also be advantageously used in once-through and recirculating forced flow fluid heating units designed for sub-critical pressures and temperatures and for furnace operation under internal gaseous pressures at or below atmospheric pressure.

The forced circulation steam generating unit illustrated includes a vertically elongated setting of substantially rectangular horizontal cross-section having a furnace chamber A and a superjacent convection gas cooling chamber B defined in most part by a front wall 10, a rear wall 12 and opposing side walls 14 (of which only one is shown). Each boundary wall includes fluid heating tubes, the arrangement of which will be hereinafter described, having their intertube spaces closed by metallic webs 13, as shown in FIGS. 4 and 5, to provide a gastight enclosure and their outer faces covered by suitable insulation material. The lower portion of the furnace chamber is formed by a hopper having inclined front 15 and rear 16 wall portions converging downwardly from the lower ends of the front and

rear walls

10 and 12, respectively, to define a rectangular throat passage 17 for discharging ash or other solids into an ash pit, not shown, positioned therebelow.

The furnace chamber A is fired by two vertically spaced rows of horizontally extending burners 18 arranged to direct fuel and air in mixing relationship into the furnace chamber through corresponding burner ports 19 in the lower portion of the front, rear and side walls.

Upright gas-tight battles 20 and 21, each of which includes fluid heating tubes having their intertube spaces closed by metallic webs, cooperate with the

enclosure walls

10, 12 and 14 to define three parallel gas passes 22, 23 and 24 in the chamber B. Gas passes 22 and 24 are separated from the furnace chamber A by

screens

25 and 26, respectively, formed by fluid heating tubes projecting downwardly and outwardly from the baflles 20 and 21, respectively. The parallel gas passes are occupied by economizer surface and steam superheating and reheating surface in the form of return bend horizontally arranged tubes connected to headers of which most are arranged externally of the chamber B. The upper central portion of the gas pass 24 is occupied by a first primary superheater 27; the central portion of the gas pass 22 by a second primary superheater 28, the lower portion of all gas passes by a secondary superheater 29, which also includes a section extending subjacent the entrances to the parallel gas passes; the central portion of the gas pass 23 by a first stage reheater 30; the central portion of the gas pass 24 by a second stage reheater 31; and the upper portion of all gas passes by an economizer 32.

Dampers

33, 34 and 35 control gas flow through the gas passes 22, 23 and 24, respectively. Breeching 36 receives gases flowing from the parallel gas passes and flows them to other heat exchange apparatus, not shown. Thus, the gaseous products of combustion passing from the furnace chamber A are regularly divided into three parallel streams beyond the

screens

25, 26 and simultaneously flow over the superheating, reheating, and econornizer surface located in the respective passes.

The steam generator setting is top-supported by structural steel members including upright members 37, cross beam 38, and beams 39 from which hangers 40 support all walls.

For the sake of clarity, FIG. 2 diagrammatically shows the flow path of the vaporizable fluid to, through and from one of the parallel flow passages in both the front and rear walls and 12, the fluid flow path through all parallel flow passages in these walls and in the side walls 14 being essentially the same. Feed water at a pressure of 4500 p.s.i. is supplied by a feed pump, not shown, to the economizer 32 wherein it is partially heated. The heated fluid then flows through a downcomer 41 and supply tubes 42 to a rear wall fluid distribution or supply header 43. The rear wall 12 is horizontally divided into

tubular sections

44, 45, 46 and 47 comprising

tube groups

48, 49, 50 and 51, respectively. Each of the tube groups comprises upwardly extending tubes connected for series flow of fluid to the vertically adjacent group of tubes to forma once-through fluid flow passage. Each of the tube groups of

sections

44, 45 and 46 comprises three tubes connected for series flow of fluid and of which two tubes (48A, 48B in

group

48, 49A, 49B, in

group

49, 50A, 50B in group 50) are arranged for upflow of fluid and one tube (48C in

group

48, 49C in

group

49, 50C in group 50) for downflow of fluid. Upflow tube 50B of group 50 is trifurcated at its discharge end to provide parallel fluid flow through the tubes of group 51.

Fluid discharging from the header 43 passes through

tubes

48A and 48C to a fluid drain header 52, then flows through

tubes

48B, 49A and 49C to a combination fluid drain and collecting header 53. The fluid discharges from the header 53 to a fluid enthalpy equalization header 54 wherein the fluids are mixed so that the enthalpy, and thereby the temperature, of the fluid will be substantially uniform in passing to the vertically adjacent tubular section. After mixing in the header 54, the fluid discharges to a combination fluid drain and distributing header 55, then flow through

tubes

4913, 50A and 50C to a second fluid drain and collecting header 56, a second fluid enthtalpy equalization header 57 and a second drain and distributing header 58. From the header 58 the fluid flows through tube 50B and the tubes of group 51 to a header 59 at the top of the rear wall 12. Thereafter the fluid passes through a fluid enthalpy equalization header 60 and then in parallel flow paths through downcomers 86, screen supply headers 61 by way of supply tubes 62,

screen tubes

25, 26 and

baflie tubes

63, 64 to a header 87. While not shown in FIG. 2, the fluid then flows through the primary superheater 27, the primary superheater 28, and the secondary superheater 29 to a turbine, not shown. The fluid after having been partially expanded in the turbine passes through the reheater 30 and then to an intermediate stage of the turbine. After the intermediate expansion stage, the fluid flows through the reheater 31 and then returns to the turbine for final expansion. The described fluid flow path is the same for the front wall 10 illustrated in FIG. 2 and for the tube groups in the side walls 14.

FIG. 3 shows a pair of once-through parallel flow fluid passages in the rear wall 12, each flow passage in this wall and all other boudary walls being basically the same. Each boundary wall is horizontally divided into vertically adjacent

tubular sections

44, 45, 46 and 47, each section being substantially planar and comprising upwardly extending closely spaced parallel tubes. Each wall includes metallic webs 13 between and welded to adjacent tubes, as shown in FIGS. 4 and 5, to provide a gas-tight enclosure covered by suitable insulation material on the outer side thereof. It will be understood that the walls may also be formed, for example, by tubes having their intertube spaces closed with refractory material and/or metallic studs or by tangent tubes, with metallic casing secured to the outer faces of the tubes in either construction. Corresponding sections on all walls extend upwardly to substantially the same level, the tubes of section 44 extending to about the top of the hopper, the tubes of section 45 to a point superjacent the top row of burners, the tubes of section 46 to about the gas entrance to the chamber B, and the tubes of section 47 to the top of chamber B. The height of each tubular section is set so that in the event of unbalanced heat and fluid flow the maximum differential temperature between adjacent tubes will not exceed a predetermined critical limit beyond which undue thermal stresses on the wall forming components would be expected to occur. This limit was set at about F.

With particular reference to the rear wall 12 tube construction and arrangement,

tubular sections

44, 45, 46, and 47 comprises

tube groups

48, 49, 50 and 51, respectively, the tube groups of each of these sections being arranged in a row in side by side parallel relationship and connected for series flow of fluid with corresponding vertically adjacent groups to form, in effect, once-through parallel flow fluid passages receiving heat from the gases flowing through the furnace chamber A and chamber B. correspondingly located tubes of each vertically adjacent group are arranged to lie in a plane extending substantially normal to the planes of the tubular sections.

Each group of tubes 48 in the tubular section 44 comprises three tubes connected for series fluid flow and of which two tubes are arranged for upflow of fluid and one for downflow. Tube 48A has its inlet end connected to the horizontally disposed lower rear wall header 43 and a return bend tube portion 48D connects the outlet end of tube 48A to the inlet end of tube 48C. The outlet end of tube 480 and the inlet end of tube 48B are connected to the horizontally arranged drain header 52 disposed superjacent the header 43. Tube 49A is a continuation of tube 48B and has its outlet end connected to the inlet end of tube 49C by a return bend tube portion 49D. As shown in FIG. 6, the discharge portion of the tube 49C is bent outwardly from the plane of tubular section 45 at a point immediately above the tube portion 48D and connected to a header 53A which constitutes one leg of a continuous horizontally arranged drain and collecting header 53 disposed externally and about the periphery of the furnace chamber A' and receiving fluid from the downflow tube of all boundary wall tube groups in the tubular sections corresponding to the tubular section 45 of the rear wall 12. The discharge ends of the continuous header 53 are connected to a vertically arranged fluid enthalpy equalization header 54 adjacent the juncture of the front and side walls wherein the fluids are mixed to a substantially uniform temperature and discharged to the inlet ends of a drain and distributing header 55 of like con struction and arrangement as the header 53 and disposed superjacent thereto. The header 55 distributes the mixed fluid to tube groups 50 and to the tube groups of the tubular sections of all other walls corresponding to the tubular section 46 of the rear wall 12. Again with reference to the rear wall tube arrangement, tube 493 has its inlet end connected to leg 55A of the continuous header 55 and enters the wall 12. at a point immediately above tube; portion 48D. Tube 50A is a vertical continuation of tube 493 and has its outlet end connected to the inlet end of tube 500 by a return bend tube portion 50D. The discharge portion of tube 50C is bent outwardly from the plane of tubular section 46 at a point immediately above tube portion 49D and connected to a header 56A which constitutes one leg of a continuous header 56 of the same character as the header 53A and receives fluid from the downflow tube of all boundary wall tube'groups in the tubular sections corresponding to the tubular section 46 of the rear wall 12. tinuous header 56 are connected to afluid enthalpy cqual ization header 57 at the juncture of the rear and side walls of like arrangement and function as the header 54. The mixedfluid is discharged from the header 57 to the inlet ends of a and distributing header.58 of like arrangement and construction as the header 56 anddisposed superjacent thereto. Once more with'reference to the rear wall tube arrangement, tube 503 has its inlet end connected to leg 58A of the continuous header 58 and enters the wall 12 at a point immediately above tube portion 49D, The outlet end of tube 50B is trifurcatcd from which upright tubes 51 extend to the header 59. All the headers in the embodiment of FIG. 3 and those of FIGS. 7 and 8 include suitable drain connections, such as at 85 in FIG. 6.

By way of example, and not of limitation, in the unit illustrated in FIG. 1 all boundary wall tubes are 1'' OD. x .18" thick on 1% centerlines up to the point of trifurcation beyond which the tubes arelVs" OD. x .22" thick on 1%" centerlines up to the top of chamber B. The lengths of the

tubular sections

44, 45, 46 and 47 are approximately 34', 31', 78' and 49', respectively.

FIGS. 7 ancl8 schematically illustrate modifications to the-wall tube arrangement of FIGS. 2-and 3. In each embodiment the fluid heating wall, of which only a part is shawn,-is of substantiallythe same character as the wall of FIGS. 2 and 3, except for the tube routing and header locations.

FIG.-7 shows a pair of once-through parallel flow fluid passages. As indicated by the arrows in'FIG. 7, in one of the parallel flow fluid passages the yaporizable fluid enters-a fluid distribution or supply header 70 and then successively passes through upflow tube, 71A, downflow tube 71B, drain header 72, upflow tube 710, drain and collecting header 73, fluid enthalpy equalization header 74, drain and distributing header 75, upflow tube 76A, downflow tube 76B, drain header 77 and upflow tube 76C to the next group of tubes. Flow in the other passage is in the same order.

The discharge ends of the con- 1 The FIG. 8 embodiment also illustrates a pair of onccthrough parallel flow fluid passages. As indicated by the arrows in FIG. 8, in one of the parallel flow fluid passages the vaporizable fluid enters a fluid distribution or supply header 78 and then successively passes through upflow tube 79A, downflow tube 79B, drain header 80, upflow tube 79C, upflow tube 81A, downflow tube 81B,

- 6 drain and collecting header 82, fluid enthalpy equaliza tion header 83, drain and distribution header 84, and up flow tube 810 to the next group of tubes. Flow in th other passage is in like order.

While in accordance with the provisions of the statute I have illustrated and described herein the best form of th invention now known to me, those skilled in the art wil understand that changes may be made in the formo the apparatus disclosed without departing from the spiri of the invention covered by the claims, and that certai1 features of the invention may sometimes be used to ad vantage without a corresponding use of other features.

What is claimed is:

1. In a forced circulation fluid heating unit, a wall sub ject to high temperature heating gases including a pai of vertically adjacent tubular sections; each tubular sec tion comprising laterally adjacent groups of upwardly ex tending closely spaced tubes which are rigidly secured t1 each other along at least a portion of the lengths thereof each group including an initial upflow tube, a second up flow tube, and a downflow tube positioned between an next adjacent to said upflow tubes and connected for serie flow of fluid from said initial upflow tube and to sail second upflow tube; the tubes of one tubular section bein; longitudinally aligned with the tubes of the other section so that the tube surface presented to the heating gases ex tends substantially throughout the height of the tubula sections; means for interconnecting the second, 'upflox tubes of said one tubular section to the initial upflow tubes of the other tubular section; header' means com municating with and receiving and mixing fluids flowin from the downflow tubes of the tube-groupsof said on tubular section and distributing the mixed fluids to cor respondingly located upflow'tub'esot the tube groups 0 ai o he u ular SQFliQ l; n means PP Y vaporizablefluid of substantially the'same enthalpyi parallel flowrelation to the initial upflow tubes of sai one tubular section; the height of the tubes of each tubula section being set so that in the event of unbalanced he: and fluid flow distribution to the tubes the maximum dil ferential temperature between laterally adjacent tubes wi not exceed a predetermined limit beyond which undu thermal stresses on the tubes would occur.

2. In a forced circulation fluid heating unit, a wa subject to high temperature heating gases including pair of vertically adjacent tubular sections; each tubule section comprising laterally adjacent groups of uprigl closely spaced parallel tubes which are rigidly secured t each other along at least a portion of the lengths thereo each group including an initial upflow tube, a second u flow tube and a downflow tube positioned between an next adjacent to said upflow tubes along the lengths then of and connected for series flow of fluid from said initi: upflow tube and to said'second upflow tube; the tubes one tubular section being longitudinally aligned with tl: tubes of the other section so that the tube surface prl sented to the heating gases extend substantially througl out the height of the tubular sections; means for inte connecting the second upflow tube of each tube group i said one tubular section to the initial upflow tube of ti correspondingly located tube group of the other tubul: section; header means communicating with and receivir and mixing fluids flowing from the downflow tubes of ti tube groups of said one tubular section and distributir the mixed fluids to correspondingly located upflow tub of the tube groups of said other tubular section; an means for supplying a vaporizable fluid of substantial the same enthalpy in parallel flow relation to the initi upflow tubes of said one tubular section; the height of t] tubes of each tubular section being set so that in t1 event of unbalanced heat and fluid flow distribution the tubes the maximum differential temperature betwel laterally adjacent tubes will not exceed a predetermim limit beyond which ilndue thermal stresses on the tub would occur.

3. In a forced circulation fluid heating unit, a Wall subject to high temperature heating gases including upper and lower vertically adjacent tubular sections; each tubular section comprising laterally adjacent groups of upright closely spaced parallel tubes which are rigidly secured to each other along at least a portion of the lengths thereof; each group including an initial upfiow tube, a second upflow tube, and a downflow tube positioned between and next adjacent to said upfiow tubes and connected for series flow of fluid from said initial upflow tube and to said second upfiow tube; the tubes of the lower tubular section being longitudinally aligned with the tubes of the upper section so that the tube surface presented to the heating gases extends substantially throughout the height of the tubular sections; means for interconnecting the second upfiow tube of each tube group of the lower tubular section to the initial upflow tube of the overlying tube group of the upper tubular section, header means communicating with and receiving and mixing fluids flowing from the downflow tubes of the tube groups of said lower tubular section and distributing the mixed fluids to correspondingly located upfiow tubes of the tube groups of said upper tubular section; and means for supplying a vaporizable fluid of substantially the same enthalpy in parallel flow relation to the initial upflow tubes of said lower tubular section; the height of the tubes of each tubular section being set so that in the event of unbalanced heat and fluid flow distribution to the tubes the maximum differential temperature between laterally adjacent tubes will not exceed a predetermined limit beyond which undue thermal stresses on the tubes would occur.

4. In a forced circulation fluid heating unit, a wall subject to high temperature heating gases including a plurality of vertically adjacent tubular sections; each tubular section comprising laterally adjacent groups of upright closely spaced parallel tubes which are rigidly secured to each other along at least a portion of the lengths thereof; each group including an initial upfiow tube, a second upflow tube, and a downflow tube positioned between and next adjacent to said upfiow tubes along the lengths thereof and connected for series flow of fluid from said initial upflow tube and to said second upflow tube; the tubes of each tubular section being longitudinally aligned with the tubes of the other sections so that the tube surface presented to the heating gases extends substantially througl out the height of the tubular sections; means for interconnecting the second upfiow tubes of each tubular section to the initial upfiow tubes of the next vertically adjacent tubular section; header means communicating with and receiving and mixing fluids flowing from the downflow tubes of the tube groups of the lowermost tubular section and distributing the mixed fluids to correspondingly located upfiow tubes of the tube groups of the next vertically adjacent tubular section; and means for supplying a vaporizable fluid of substantially the same enthalpy in parallel flow relation to the initial upfiow tubes of the lowermost tubular section; the height of the tubes of each tubular section being set so that in the event of unbalanced heat and fluid flow distribution tothe tubes the maximum difierential temperature between laterally adjacent tubes will not exceed a predetermined limit beyond which undue thermal stresses on the tubes would occur.

5. In a forced circulation fluid heating unit, a wall subject to high temperature heating gases including upper and lower vertically adjacent tubular sections; each tubular section comprising laterally adjacent groups of upwardly extending closely spaced tubes which are rigidly secured to each other along at least a portion of the lengths thereof; each group including an initial upfiow tube, a second upfiow tube, and a downflow tube positioned between and next adjacent to said upflow tubes and connected for series fiow of fluid from said initial upfiow tube and to said second upfiow tube; the tubes of the lower tubular section being longitudinally aligned with the tubes of the upper section so that the tube surface presented to the heating gases extends substantially throughout the height of the tubular sections; means for interconnecting the second upfiow tubes of the lower tubular section to the initial upflow tubes of the upper tubular section; a header directly connected to the outlet end of the downflow tube of each tube group of the lower tubular section and to the inlet end of the second upfiow tube of each tube group of the lower tubular section; header means communicating with and receiving and mixing fluids flowing from the downflow tubes of the tube groups of said lower tubular section and distributing the mixed fluids to correspondingly located upflow tubes of the tube groups of said upper tubular section; the communication between said header means and the downflow tubes of the tube groups of the lower tubular section being provided by said header, the second upfiow tubes of the tube groups of the lower tubular section, the initial upflow tubes of the tube groups of the upper tubular section, and the downflow tubes of the tube groups of the upper tubular section; and means for supplying a vaporizable fluid of substantially the same enthalpy in parallel flow relation to the initial upfiow tubes of the lower tubular section; the height of the tubes of each tubular section being set so that in the event of unbalanced heat and fluid flow distribution to the tubes the maximum difierential temperature between laterally adjacent tubes will not exceed a predetermined limit beyond which undue thermal stresses on the tubes would occur.

6. In a forced circulation fluid heating unit, a wall subject to high temperature heating gases including upper and lower vertically adjacent tubular sections; each tubular section comprising laterally adjacent groups of upwardly. extending closely. spaced tubes which are rigidly secured to each other along at least a portion of the lengths thereof; each group including an initial upflow tube, a second upfiow tube, and a downflow tube positioned between and next adjacent to said upfiow tubes and connected for series flow of fluid from said initial upflow tube and to said second upflow tube; the tubes of the lower tubular section being longitudinally aligned with the tubes of the upper section so that the tube surface presented to the heating gases extends substantially throughout the height of the tubular sections; means for interconnecting the second upfiow tubes of the lower tubular section to the initial upfiow tubes of the upper tubular section; a header directly connected to the outlet end of the downflow tube of each tube group of the lower tubular section and to the inlet end of the second upfiow tube of each tube group of the lower tubular section; header means communicating with and receiving and mixing fluids flowing from the downflow tubes of the tube groups of said lower tubular section and distributing the mixed fluids to the initial upflow tubes of the tube groups of the upper tubular section; the communication between said header means and the downflow tubes of the tube groups of the lower tubular section being provided by said header and the second upfiow tubes of the tube groups of the lower tubular section; and means for supplying a vaporizable fluid of substantially the same enthalpy in parallel flow relation to the initial upfiow tubes of said one tubular section; the height of the tubes of each tubular section being set so that in the event of unbalanced heat and fluid flow distribution to the tubes the maximum differential temperature between laterally adjacent tubes will not exceed a predetermined limit beyond which undue thermal stresses on the tubes would occur.

7. In a forced circulation fluid heating unit, walls forming an upright gas flow chamber; means supplying high temperature heating gases to said chamber for flow therethrough; each of said walls including upper and lower vertically adjacent tubular sections; each tubular section comprising laterally adjacent groups of upright closely spaced parallel tubes which are rigidly secured to each other along at least a portion of the lengths thereof; each group inclpding an initial upflow tube, a second upfiow tube, and a downflow tube positioned between and next adjacent to said upflow tubes and connected for series flow of fluid from said initial upfiow tube and to said second upflow tube; the tubes of the lower tubular section of each wall being longitudinally aligned with the tubes of the upper section of the corresponding wall so that the tube surface presented to the heating gases extends substantially throughout the height of the tubular sections; means for interconnecting the second upflow tubes of the lower tubular section of each wall to the initial upflow tubes of the upper tubular section of the corresponding wall; header means communicating with and receiving and mixing fluids flowing from the downfiow tubes of the tube groups of said lower tubular section and distributing the mixed fluids to correspondingly located upflow tubes of the tube groups of said upper tubular Section; and means for supplying a vaporizable fluid of substantially the same enthalpy in parallel flow relation to the initial upflow tubes of the lower tubular section of each wall; the height of the tubes of each tubular section being set so that in the event of unbalanced heat and fluid flow distribution to the tubes the maximum differential temperature between laterally adjacent tubes will not exceed a predetermined limit beyond which undue thermal stresses on the tubes would occur.

8. In a forced circulation fluid heating unit, walls forming an upright gas flow chamber; means supplying high.

temperature heating gases to said chamber for flow therethrough; each of said walls including upper and lower vertically adjacent tubular sections; each tubular section comprising laterally adjacent groups of upright closely' spaced parallel tubes which are rigidly secured to each other along at least a portion of the lengths thereof; each group including an initial upflow tube, a second upflow tube, and a downflow tube positioned between and next adjacent to said upfiow tubes and connected for series flow of fluid from said initial upflow tube and to said second upflow tube; means for draining each of said tubes; the tubes of the lower tubular section of each wall being longitudinally aligned with the tubes of the upper tubular section of the corresponding wall so that the tube surface presented to the heating gases extends substantially throughout the height of the tubular sections; means for interconnecting the second upflow tube of each tube group of the lower tubular section of each wall tothe initial upflow tube of the overlying tube group of the up per tubular section of the corresponding wall; header means communicating with and receiving and mixing fluids flowing from the downflow tubes of the tube groups of said lower tubular section and distributing the mixed fluids to correspondingly located upflow tubes of the tube groups of said upper tubular section; and means for supplying a vaporizable fluid of substantially the same enthalpy in parallel flow relation to the initial upflow tubes of the lower tubular section of each wall; the height of the tubes of each tubular section being set so that in the event of unbalanced heat and fluid flow distribution to the tubes the maximum differential temperature between laterally adjacent tubes will not exceed a predetermined limit beyond which undue thermal stresses on the tubes would occur.

References Cited in the file of this patent UNITED STATES PATENTS 1,925,026 Austin Aug. 29, 1933 1,959,866 Jacobus May 22, 1934 FOREIGN PATENTS 911,264 Germany May 13, 1954 770,456 Great Britain Mar. 20, 1957 78,458 Netherlands Sept. 15, 1938