US3545409A

US3545409A - Offset mix tubes

Info

Publication number: US3545409A
Application number: US822622A
Authority: US
Inventors: Fred W Young
Original assignee: Babcock and Wilcox Co
Current assignee: Babcock and Wilcox Co
Priority date: 1969-05-07
Filing date: 1969-05-07
Publication date: 1970-12-08
Anticipated expiration: 1987-12-08
Also published as: IT943107B; FR2042438A1; GB1295757A; FR2047370A5; DK125809B; SE352941B; BE750098A

Description

i United States Patent 3,162,179 12/1964 Strohmeyer,Jr.....Q

Fred W. Young West Point, Mississippi May 7, 1969 Dec. 8, 1970 The Babcock & Wilcox Company New York, New York a corporation of New Jersey lnventor Appl. No. Filed Patented Assignee OFFSET MIX TUBES 8 Claims, 13 Drawing Figs.

U.S. Cl. 122/6, 122/406, 122/510 Int. Cl. F22b 37/66 Field of Search 122/6(A), 235(A), 406, 510

References Cited UNITED STATES PATENTS 3,307,524 3/1967 Ambrosewl 3,358,650 12/1967 Olsonetal Primary Examiner-Kenneth W. Sprague Attorney-J. Maguire 122/510 l22/5l0X ABSTRACT: A forced flow vapor generator wherein the PATENTEDBEB 8&870 3545409 SHEEI BF 5 OFFSET MIX TUBES This invention relates in'general to the construction and operation of a forced flow fluid-heating unit, and more particularly to, improvements in the construction and arrangement of the furnace fluid-heating circuits of a forced circulation once-through steam generating and superheating unit.

The present invention is directed to improvements in the furnace fluid-heating circuitryof a forced flow vapor generator of the type disclosed in U.S. Pat. application Ser. No. 447,699 wherein the upright boundary walls of the furnace are subdivided, into a plurality of separate continuous upflow fluid-heating passes with the parallel flow tubes of one of the fluid-heating passes being interlaced and coextensive with the parallel flow tubes of another of the heating passes, and with special provisions for supporting and interconnecting the tubes of the fluid heating passes to provide a serial flow of fluid successively through the respective fluid-heating passes and for mixing the fluids to equalize fluid enthalpies as they flow from one furnace-heatingpass to anotherL In furnace walls of prior units of the character described, the extensive web plate arrangements used for scaling in past constructions proved costly and time consuming both in manufacture and maintenance. In service, the former constructions have been vulnerable to high temperature corrosion. Because of this vulnerability, past constructions have had to include a gas tight vestibule to contain the flue gas in the and a second group of upwardly extending laterally spaced fluid-heating tubes of circular cross section throughout most of their lengths rigidly united by metallic webs to form an upper portion of thewall. Headers are provided for receiving and mixing fluid flowing from the tubes of the first group and distributing the fluids to the tubes of the second group. Some of the tubes of the first and second tube groups are bent out of the plane of the wall at first and second levels, respectively, for connection to the .headers and interlaced with and laterally spaced from each otherin the plane of the wallbetween the first and second levels, with-the second level being subjacent the first level; Provisions for supporting the wall include metallic webs disposed in the spaces between and having their longitudinal edges weld united to the interlaced tube portions of the first and second groups of tubes, thereby transmitting the load of the first group to the second group of tubes, and with the second group of tubes being top supported from steelwork. The bent tubes in the plane of the wallin the vicinity of the first and second levels are of oval cross section and laterally contiguous and rigidly united in the plane of the wall to each other and to tubes of circular cross section of the first and second tube groups.

In the drawings:

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

FIG. 2 is an enlarged sectional side view of a portion of the fluid collection, mixing and distribution system shown in FIG.

FIG. 3 is an enlarged sectional side view of an alternative arrangement of a fluid collection, mixing and distribution system;

FIG. 4 is a diagrammatic perspective view showing the interlaced portion of the furnace wall tubes associated with the arrangement of FIG. 2; i

FIG. 5 is a diagrammatic perspective view showing the in- FIG. 7 is a sectional side detail viewtaken along line 7-7 of FIG. 6;

FIG. 8 is a sectional plan detail view taken along line 8-8 of FIG. 6;

FIG. 9 is a sectional plan detail view taken along line 9-9 of FIG. 6;

FIG. 10 is a sectionalfront detail view showing a gas tight construction of interlaced furnace wall tubes of FIG. 3;

FIG. 11 is a sectional side detail view taken along line 11-11 of FIG. 10;

FIG. 12 is a sectional plan detail view taken along line 12-12 ofFIG. 10; and i 5 FIG. 13 is a sectional plan detail view taken along line 13-13 ofFlG. 10. t

In the drawings the invention hasbeen illustrated as embodied in a top-supportedforced flow once-through steam generator intended for central station use. The particular unit illustrated is designed to produce a maximum continuous steam output of 9,300,000 pounds per 'hour and a total temperature of l003 at the superheater outlet and at a reheat stage based on feedwater being supplied to the unit at a temperature of 550 F. 1

The main portionsof the unit, as illustrated in FIG. I, include an upright furnace chamber 10 of substantially rectangular horizontal cross section defined by front wall 11, rear wall 13, sidewalls 14, a roof 16, and a hopper 17 and having a gas outlet 18 at its upper end opening to a horizontally extending gas passage 19 of rectangular vertical cross section formed by a floor 21 and extensions of the furnace roof l6 and. sidewalls 14. Gas passage 19 communicates at its rear end with the upper end of an upright gas passage 22 of rectangular horizontal cross section formed by a front wall 23, a rear wall 24, sidewalls 26, and an extension of the roof of the gas passage 19.

ries with respect to gas flow while gas passage 22 is occupied in thedirection of gas flow by a primary superheater 33 and an economizer 34.

In the normal operation of the fluid-heating unit, combustion air and fuel are supplied to the burners 27 and the fuel is burned in the lower portion of the furnace. Heating gases flow upwardly through chamber 10 to the furnace outlet, thence to gas passage 19, and then pass successively over and between the tubes of secondary superheater 28 and reheater 29 in gas passage 19 and over and between the tubes of primary superheater 33 and economizer 34 in gas passage 22, and then discharge to another heat trap, not shown, before entering to the stack. It will be understood "that in accordance with well-known practice, each of the superheater and reheater sections extends across the full width of its corresponding gas passage and is formed for serial flow of steam by multiple looped tubes.

The vapor generating setting is top members including upright members and crossbeams 90, from which hangers 95, of which only a few are illustrated, support all walls and convection surfaces.

Feedwater at high pressure is supplied by a feed pump, not shown, to economizer inlet header 25, then passes through economizer 34 to outlet header 30 from which it flows through a downcomer 35 for flow to the furnace boundary wall fluid-heating circuitry, and the fluid-heating circuitry of the convection gas passes 19 and 22, which will be hereinafter described. 1

Each of the upright boundary walls of gas passages 19 and 22 are of gastight. construction and include parallel tubes,

front wall 23 having tubes 71 extending between inlet and

outlet headers

72 and 73, rear wall 24 having tubes 74 extending between inlet and

outlet headers

76 and 77, each sidewall 26 supported by structural having tubes 78 extending between inlet and

outlet headers

79 and 81, and each sidewall of gas passage 19 having tubes 82 extending between inlet and

outlet headers

83 and 84. Floor 21 is lined by a row of tubes 86 having their inlet ends connected to header 57 and their outlet ends to header 73, with header 73 being connected for fluid flow to headers 88 by a row of screen tubes 89. Headers 7 2, 76, 79, 83 and 57 are connected for parallel supply of fluid from a conduit 69, while

headers

77, 81, 84 and 88 are arranged for discharge to another collector header 91, from which fluid passes to the primary superheater via conduit 92. From primary superheater 33 the partly superheated vapor passes to secondary superheater 28, within which the vapor receives its final superheating before passing to a high pressure turbine (not shown), partially expanded steam from the high pressure turbine passes through reheater 29, and from thence to the reheat pressure turbine wherein final expansion takes place.

Each of the upright boundary walls of furnace 10 is formed by upwardly extending parallel tubes arranged to provide three upflow fluid-heating passes and having their intertube spacing closed by metallic webs welded to adjacent tubes to provide a gastight construction. The first two fluid heating passes comprise the first group of tubes and form the lower portion of each furnace wall and the third fluid-heating pass comprises the second group of tubes and forms the upper portion of each furnace wall. Special header provisions are made for mixing the heat absorbing medium intermediate its flow from one pass to another, the mixing system from each of the fluid-heating passes being specifically for the purpose of keeping the wall tube temperature differentials across the width of the furnace wall panel to a minimum. With differences in furnace cleanliness as well as in the flow rates in the multiple parallel fluid flow paths it is possible to develop temperature differences between adjacent tubes of a magnitude sufficient to induce high stresses in the tubes and in the metallic webs therebetween. By limiting the total absorption in a fumaccheating pass, the degree of thermal imbalance within the tubes comprising the parallel flow paths is also limited. Accordingly, the boundary wall of the furnace is designed so the temperature at a particular level of furnace height or elevation differs by no more than l F. from the calculated average fluid temperature of all furnace wall tubes at that level; so the calculated maximum temperature differential between adjacent tubes is below a predetermined critical limit to minimize fluid flow imbalances; and so the tubes of each fluid-heating pass are sufficient in flow area to provide an adequate circulation rate. Further, all heated tubes of the furnace boundary walls are arranged for upflow of fluid, since flow stability within heating passes having their tubes so arranged is markedly improved over that condition where flow circuitry has heated downflow as well as upflow tubes. In other words, flow imbalances for the same average and upset heat absorption conditions are considerably less severe with all upflow tubes than with both upflow and downflow circuitry within a heat absorbing zone.

The front wall 11 comprises first pass upflow tubes 37A, second pass upflow tubes 37!} disposed in spaces between first pass upflow tubes 37A, and third pass upflow tubes 37C. Rear wall 13 includes first pass upflow tubes 38A, second pass upflow tubes 38B situated in the spaces between tubes 38A, and third pass upflow tubes 38C, some of which form a screen extending through gas passage 19 and the remainder form a nose arch 41. Each sidewall 14 first pass upflow tubes 39A, second pass upflow tubes 39B located in the spaces between tubes 39A, and third pass upflow tubes 39C.

First pass upflow tubes 37A, 38A and 39A of the front, rear and sidewalls of furnace have their inlet ends connected to inlet headers 44 associated with the corresponding walls and their upper ends connected to intermediate outlet headers 49 associated with the corresponding walls, with inlet headers 44 being supplied with fluid from conduit 35. Fluid passing through initial upflow tubes 37A, 38A and 39A is collected in first pass outlet headers 49, and then passed through conduit 51 to second pass inlet headers 52 arranged to supply fluid to second pass upflow tubes 37B, 38B and 398. The second pass upflow tubes have their upper ends connected to second pass outlet headers 53 located superjacent first pass outlet headers 49.

The third

pass upflow tubes

37C, 37C, 38C, 38C, 39C, 39C extend from third pass inlet headers 57, associated with the corresponding walls, to the top of the furnace,

tubes

37C, 37C having their discharge ends connected to third pass outlet header 58, some of the

tubes

38C, 38C having their discharge ends connected to screen tube inlet header 73 and the remainder to screen tube outlet header 88, and tubes 39C, 39Chaving their discharge ends connected to headers 63, with third pass inlet headers 57 of the front and sidewalls being connected by conduits 55 to supply fluid from corresponding second pass outlet headers 53. Headers 53 are connected for discharge of fluid to inlet headers 59 for wingwalls 15, the outlet tubes of which discharge to headers 62.

From the above description it is evident that tubes 37A, 38A and 39A constitute the first fluid heating pass of the furnace, tubes 37B, 38B and 398 the second heating pass, and

tubes

37C, 37C and 38C, 38C and 39C, 39C the third fluid heating pass. The tubes of the first-and second-heating passes are substantially coextensive within each respective all and have their intertube spaces closed by metallic webs 103, weldunited to the tubes along substantially their entire parallel lengths so that each web is welded to one of the tubes of the first-heating pass and to one of the tubes of the second-heating pass. Tubes of the third fluid-heating pass, with the exception of screen tubes, have their intertube spaces closed by metallic webs 104, weld-united to the tubes along substantially their entire lengths. Tubes of the third fluid-heating pass of each upright wall, except for the rear wall 'of the furnace, are coplanar along almost their entire length within their respective walls and are also coplanar with the tubes of the associated first and second fluid-heating passes of the cor responding wall.

Since the construction and arrangement of the fluid collection, mixing and distribution systems and their associated tubes are substantially the same in all walls, it will suffice to describe the front wall system fluid-heating passes.

Referring to FIGS. 2 and 4 there are shown sectional views of front wall 11 with the discharge portion of first pass tubes 37A bent outwardly from the plane of the wall at about the second level 102A and then extending horizontally and downwardly for radial connection to first pass outlet header 49 wherein the fluids discharging from the first fluid-heating pass are collected and mixed to insure uniform temperature of the fluid entering the front wall second fluid-heating pass via exterior conduits 51. The discharge portions of second pass tubes 37B are bent outwardly from the plane of the wall 11 at the first level 101 and extend horizontally and downwardly for radial connection to second pass outlet header 53 wherein the fluids discharging from the second fluid-heating pass are collected and mixed to neutralize the differences in amount of heat picked up in the second-heating pass from whence the steam-water mixture is passed to third pass inlet header 57 via exterior conduits 55 for unifonn distribution to the

parallel flow tubes

37C and 37C comprising the front wall third fluidheating pass. lnlet portions of third pass tubes 37C extend vertically, then horizontally, and then bend upwardlyto enter the front wall 11 at the second level 102 from whence they extend upwardly in the plane of the wall and in contiguous relation with the adjacent portion of first pass tubes 37A and the second pass tubes 37B. At the about second level 102A which is the location where first pass tubes 37A begin to bend out of the plane of the wall, these third pass tubes 37C extend upwardly in the plane of the wall between the second pass tubes 37B and in axial alinement with the vertical run of first pass tubes 37A. Inlet portions of the remaining third pass tubes 37C extend vertically, then horizontally from third pass inlet header 57 and then bend upwardly to enter the front wall 11 atthe about first level 101A from whence they extend upwardly in the plane of the wall and in contiguous relation with the ad jacent portion of second pass tubes 37B and the third pass tubes 37C which entered the plane of the wall at level 102. At the first level 101 which is the location where second pass tubes 37B begin to leave the plane of the wall, these remaining third pass tubes 37C extend upwardly in the plane of the wall between the third pass tubes 37C and alinement with the second pass tubes 37B. Metallic webs 103 close the spaces between the first and

second pass tubes

37A and 378 up to the second level 102, while metallic webs 104 close the spaces between the

third pass tubes

37C and 37C above the first level 101. The wall intertube spaces intermediate the first and second levels are closed by metallic webs 105 except at the points where

tubes

37A,37B, 37C and 37C bend out of the plane of the wall. Wall seals are provided at these points by metallic fillet bars 106 and web plates 107 suitably shaped to fit the intertube spaces. A

Thus at the locations where mixing of the fluids occur as they flow from one furnace fluid-heating passto another, the tubes of the several fluid-heating passes are interlaced and rigidly weld-united by the webs and bars therebetween to provide a gastight structure. The tubes bending out of the plane of the wall in the vicinity of the first and second levels 101 and.

102 have an ovalized cross section as shown in FIGI8 and are arranged contiguously with their minor axes lying in the plane of the wall.

Referring to FIGS. 6, 7, 8 and 9 there are shown detail views of front wall 11 in the vicinity of the second level 102. It should be borne in mind that the arrangement at first level 101 is substantially the same. The wall portions of second pass tubes 37B are circular in cross section while passing through the vicinity of second level 102. The inlet'portion of the third pass tube 37C bends upwardly to enter the wall at the second level 102 and bends laterally in the planeof the wall at the about second level 102A to vertically aline itself with the first pass tube 37A. The discharge portion of the first pass tube 37A bends laterally in the plane of the wall at the second level 102 to fit in contiguous fashion between the

tubes

37B and 37C and then bends outwardly from the plane of the wall at the about second level 102A. As shown in FIG. 7 the ovalization of the cross-sectional area of the third pass tube 37C commences just prior to where the tube bend entering the plane of the wall is tangent to the second level 102 and terminates at the about level 102A. The ovalization of the cross-sectional area of the first pass tube 37A commences just prior to the second level 102 and terminates after the tube bend as the tube leaves the plane of the wall.

Metallic webs 103 close the spaces and weld-unite the first and second pass tubes 37A and 373 up to the second level- 102, while metallic webs 105 close the spaces and weld-unite the

tubes

37B and 37C from the about second level 102A and the about first level 101A. Wall seals are provided at the points where

tubes

37A and 37C bend out of the plane of the wall, these consist of welded metallic fillet bars 106 and web plates 107.

The alternative arrangement of the fluid collection, mixing and distribution system has each of the upright boundary walls of the furnace formed by upwardly extending parallel tubes arranged to'provide two, upfiow fluid-heating passes. The first fluid-heating pass comprises the first group of tubes and forms the lower portion of each furnace wall and the second fluidheating pass comprises the second group of tubes and forms the upper portion of each furnace wall. I

Since the construction and arrangement of the fluid collection, mixing and distribution systems and their associated tubes are substantially the same inall walls, the construction and arrangement of only the front wall system and its associated fluid-heating passeswill be described.

Referring to FIGS. 3 and 5 there are shown sectional views of front wall 11 with the discharge portion of the'first pass tubes 37A bent outwardly from the plane of the wall at about the second level 102A and then extending horizontally and downwardly for radial connection to the first pass outlet header 53A. The remaining first pass tubes 37A are bent outwardly from the plane of the wall at the first level-101 and extend horizontally and downwardly for radial connection to the first pass outlet header 53A wherein the fluids discharging from the firstfluid-heating pass are collected and mixed to neutralize the differences in heat absorption by the fluid. The fluid is then passed to second pass inlet header 57A via conduit 55A for uniform distribution to the parallel flow tubes of the front wall extend vertically fluid-heating pass. Inlet portions of second pass tubes 37B extend vertically and then horizontally and bend upwardly to enter the front wall 11 at t the second level 102 and then extend upwardly in the plane of the wall and in contiguous relation with the discharge portion of the first pass tubes 37A and the remaining first pass tubes 37A. At the about the second level 102A and the position where the first pass tubes 37A leave the plane of the wall, the second pass tubes 37B extend upwardly in the plane of the wall between the remaining first pass tubes 37A and in alinement with these first pass tubes 37A. Inlet portions of the remaining second pass tubes 37B'extend vertically and then horizontally from second pass inlet header 57A and bend upwardly to enter the front wall 11 at the about first level 101A and then extend upwardly in the plane of the wall and in contiguous relation with the discharge portion of the first pass tubes 37A and the second pass tubes 37B. At the first level 101 and the position where the first pass tubes 37A leave the plane of the wall, the second pass tubes 37B extend upwardly in the plane of the wall between alternating second pass tubes 37B and in alinement with the first pass tubes 37A. Metallic webs 103 close thespaces between the

first pass tubes

37A and 37A up to the second level 102, while metallic webs l04 close the spaces between the second pass tubes 37B, 378' above the first level 101. The wall intertube spaces intermediate the first and second levels are closed by metallic webs 105 except at the points where

tubes

37A, 37A, 37B and 37B bend out of the plane of the wall. Wall seals are provided at these points by metallic fillet bars 106 and web plates 107 suitably shaped to fit the intertube spaces, as shown in FIGS. 10 and 12.

Referring to FIGS; l0, l1, l2 and 13 there are shown detail views of front wall 11 in the vicinity of the second level 102 for the alternative arrangement of the fluid collection, mixing and distribution system. It should be borne in mind that the arrangement at first level 101 is substantially the same. The wall portions of the first pass tubes 37A are circular. in cross section while passing through the vicinity of the second level 102. The second pass tube 378 bends upwardly to enter the wall at the second level 102 and then bends laterally in the plane of the wall at the about second level 1021A to become alined with the first pass tubes 37A. The discharge portion of the first pass tube 37A bends laterally in the plane of the wall at the second level 102 to fit in contiguous fashion between the

tubes

37A and 37B and then bends outwardly from the plane of the wall at the about'second level 102A. The ovalization of the crosssectional area of the second pass tubes 37B commences just prior to entering the plane of the wall 11 and terminates at the about second level 102A. The ovalization of the cross-see tional area of the first pass tube 37A commences just prior to the second level 102 and terminates where the tube bend leaves theplane of. the wall adjacent the about second level 102A. Metallic webs 103 close the spaces and weld-unite the

first pass tubes

37A and 37A up to the second level 102, while metallic webs 105 close the spaces and weld-unite the second pass tubes 378 above the about second level 102A. Wall seals are provided at the points where

tubes

37A and 37A" and 37B and 37B bend out of the plane of the wall, these consist of metallic fillet bars 106 and web plates 107.

In the main embodiment, the weight or load of the front wall below the first and

second levels

101 and 102 is transferred through the first and

second pass tubes

37A and 37B and metallic fillet bars 106 and

webs

105 and 107 into the

third pass tubes

37C and 37C, with the latter tubes in turn transferring the load to the steelwork by wayjof hangers 95. The actual load transfer through fillet bars 106, webs 10S and 107 to the

tubes

37C, 37C, is accomplished by shear loading rather than by tension. In the alternative arrangement, the weight or load of the front wall below the first and

second levels

101 and 102 is transferred through the

first pass tubes

37A and 37A and metallic fillet bars 106 and

webs

105 and 107 into the

second pass tubes

37B, 37B with the latter tubes in turn transferring the load to the steelwork by way of hangers 95. As in the case of the main embodiment the actual load transfer through fillet bars 106,

webs

105 and 107 to the tubes 37B is again accomplished by shear loading, rather than by tension. For both of the above arrangements, fillet bars 106 and

webs

105 and 107, cooled by the fluid flowing in the tubes of wall 11 are of sufficient proportions to assure that the load induced shear stresses are of relatively low order of magnitude.

By way of example and not of limitation, and with reference to the main embodiment the tubes of the first and second fluid-heating passes combining to form the lower portion of the furnace enclosure are 1% inches 0.D. spaced on 2 inch centers, the first and second fluid-heating passes each having 332 tubes in the front wall, 332 tubes in the rear wall, and 153 tubes in each sidewall. The tubes of the front, rear and sidewalls of the third fluid-heating pass are 1% inches O.D. spaced on 2 inch centers, which the exception of the front screen tubes which are 74 in number and 2 31/32 inches CD. on 18 inch centers. The rear wall up to and including the nose section and the front wall have 664 tubes while each sidewall has 306 tubes.

It will be understood that the number of tubes in the first and second fluid-heating passes need not be identical and can be varied to satisfy mass flow requirements for tube metal temperatures limits, considering the different fluid enthalpies, and consequent different fluid heat transfer properties, in the respective fluid-heating passes.

I claim:

1. In a forced circulation fluid-heating unit, upright walls forming a furnace for flow of heating gases, means supplying high temperature-heating gases to the furnace, one of the walls including a first group of upwardly extending rigidly united laterally spaced fluid-heating tubes of circular cross section throughout most of their lengths forming a lower portion of said one'wall, a second group of upwardly extending rigidly united laterally spaced fluid-heating tubes of circular cross section throughout most of their lengths forming an upper portion of said one wall, means supplying fluid to tubes of the first group, header means communicating with and receiving and mixing fluids flowing from tubes of the first group and distributing mixed fluids to tubes of the second group, some of the tubes of the first and second tube groups being bent out of the plane of the wall at first and second levels, respectively, for connection to the header means, and interlaced with each other in the plane of the wall between the first and second levels, the second level being subjacent the first level, some of the tubes of the first and second tube groups being bent out of the plane of the wall at about the second and first levels, respectively, for connection to the header means, and means for supporting said one wall including metallic webs weld-united to interlaced tube portions of the first and second groups of tubes and transmitting the load of the first group of tubes to the second group of tubes, and means for top supporting the second group of tubes, the bent tubes in the plane of the wall in the vicinity of the first and second levels being of oval cross section and laterally contiguous and rigidly united in the plane of the wall to each other and to tubes of circular cross section of the first and second groups.

2. A forced circulation fluid-heating unit according to claim 1 wherein some of the tubes of the first group comprise a first fluid-heating pass, the remaining tubes of the first group comprise a second fluid-heating. pass, and all of the tubes of the second group comprise a third fluid-heating pass.

3. A forced circulation fluid-heating unit according to claim 2 wherein in the plane of thewall, the tubes of the first fluidheating pass are interlaced with the tubes of the second fluidheating pass up to the second level, and the tubes of the third fluid-heating pass are interlaced with the tubes of the first and second fluid-heating passes between the first and second levels.

4. A forced circulation fluid-heating unit according to claim 1 wherein the tubes of the first group comprise a first fluidheating pass and the tubes of the second group comprise a second fluid-heating pass.

5. A forced circulation fluid-heating unit according to claim 4 wherein in the plane of the wall of tubes of the first fluidheating pass are interlaced with the tubes of the second fluidheating pass between the first and second levels.

6. A forced circulation fluid-heating unit according to claim 1 wherein the tubes of the second group have their longitudinal axes alined with the longitudinal axes of the tubes of the first group, each pair of axially alined tubes of the first and second groups being bent out of the plane of the wall at positions overlapping and contiguous to each other for connection to the header means.

7. A forced circulation fluid-heating unit according to claim 5 wherein the overlapping portions of the tubes of each pair of axially alined tubes are weld-united to each other.

8. A forced circulation fluid-heating unit according to claim 1 wherein the ovalized bent tubes have their major axes extending normal to the plane of the wall.