US2988064A

US2988064A - Fluid heaters

Info

Publication number: US2988064A
Application number: US426829A
Authority: US
Inventors: Ronald E Zoller
Original assignee: Babcock and Wilcox Co
Current assignee: Babcock and Wilcox Co
Priority date: 1954-04-30
Filing date: 1954-04-30
Publication date: 1961-06-13
Anticipated expiration: 1978-06-13

Description

R. E. ZOLLER June 13, 1961 FLUID HEATERS 6 Sheets-Shee 1 Filed April 50. 1954 ATTORNEY June 13, 1961 R. E. zoLLER 2,988,064

FLUID HEATERS Filed April so, 1954 e sheets-sheet 2 INVENTOR ATTORNEY R. E. ZOLLER FLUID HEATERS June 13, 1961 6 Sheets-Sheet 5 Filed April 30. 1954 INVENTOR ozmfcif Zay/@2^ BY of/s,

ATTORNEY June 13, 1961 R. E. zoLLER 2,988,064

FLUID HEATERS ATTORNEY June 13, 1961 R. E. zoLLER 2,988,064

FLUID HEATERS Filed April so. 1954 e sheets-sheet 5 f l 1 l l l oooo 000000 INVENTOR fond/d5 Zai/er ATTORNEY June 13, 1961 R. E. zoLLER 2,988,064

FLUID HEATERS Filed April 30. 1954 6 Sheets-Sheet 6 oo oooooo oon@ o 0000011000000 oooooooooooo 00000000 OOO INVENTOR 85mm/a7 Zo//er ATTORNEY (latent Patented June 13, 1961 Free 2,988,064 FLUID HEATERS Ronald E. Zoller, London, England, assignor to The Babcock & Wilcox Company, New York, N.Y., a corporation of New Jersey Filed Apr. 30, 1954, Ser. No. 426,829 7 Claims. (Cl. 122-478) This invention relates in general to the construction and operation of fluid heating units, and more particularly, to vapor generating and superheating units which are primarily designed for marine propulsion use and in which the vapor generating surface is mainly concentrated in a vertically arranged tube bank extending between a pair of upper and lower drums and arranged to receive heating gases throughout the length of the bank from an integral iluid cooled furnace chamber laterally adjoining the tube bank. When vapor superheating surface is to be incorporated in such units, it has been proposed (U.'S. Patent No. 2,048,039) to arrange the superheating surface to extend part way of the tube bank length and to control the ilow of heating gas over the superheater by means of a transverse baille separating the tube bank section containing the superheater from the remaining section of the tube bank and utilizing a control damper in or leading to a common discharge duct at the upper end of the baille to proportion the heating gas flow through the parallel gas passes containing two tube bank sections.

The main object of the present invention is the provision of a vapor generating and superheating unit of the general character described in which the iluid heating tubes in diiferent sections of the tube bank are arranged, and the tube bank transverse baille construction, so that inspection and maintenance of this baille is facilitated. A more specific object is the provision of a transverse baille means in a unit of the character described which serves as a boundary for an access space extending between the gas passes and providing access for inspection or repair of all or substantially all parts of the baffle means.

The various features of novelty which characterize my 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 de-` scriptive matter in which I have illustrated and described embodiments of my invention.

The invention will now be described, by way of example, with reference to the accompanying partly diagrammatic drawings, in which:

FIG. l is a sectional side view of a two-drum integral furnace type marine boiler embodying the invention taken on the line `I-l of FIG. 4, a baille wall forming part of the boiler being omitted from this view;

FIG. 2 is a sectional side view of part of the boiler shown in FIG. l but taken on the line II--I'I of FIG. 4;

FIG. 3 is a plan View of the right-hand side of the boiler shown in FIG. l;

FIG. 4 is a sectional plan view taken on the line IV-IV of FIG. l, showing only those parts adjacent the section plane;

FIG. 5 is an enlarged sectional plan view of part of the baille wall illustrated in FIG. 4;

FIG. 6 is a sectional plan view, corresponding to FIG. 4, of a second form of two-drum integral furnace marine boiler indicating a modification ofthe construction of FIGS. l to 4;

FIG. 7 is an enlarged sectional plan view of part of the baille wall illustrated in FIG. 6; and

FIG. 8 is a sectional plan view, corresponding to FIG.

.4, of a third form of two-drum integral furnace marine boiler indicating a further modification of the constructions of FIGS. 1 to 4 and 6 to 7.

In FIGS. l to 5 of the drawings, the steam boiler in-Y cludes a horizontally arranged upper steam and water drum 1, a lower water drum 3 and a bank 5 of vertically inclined vapor generating tubes connecting the two drums. A side Wall header 7 parallel to drum 3 is connected by aligned bent wall tubes 9 to the upper drum 1. Tubes 9, which are spaced apart, are backed by refractory material 13, such as chrome ore, applied in plastic form, which also lls the intertube spaces, and so defining roof 15 and a side wall 17 of a combustion chamber 19. Those tubes 9 which are disposed near the frnot or tiring end of the combustion chamber have their lower parts, constituting wall 17, covered with refractory, as indicated at 21. The outside of the roof 15 and wall 17 is covered with heat insulating material 23 which also extends to cover the outward surfaces oi drum 1 and header 7.

The iloor 31 of combustion chamber 19 is of any suitable construction, and those parts of water drum 3 and header 7 which would otherwise be exposed to heat from the combustion chamber 19 are protected with

refractory material

33 and 35 respectively. The front and

rear end walls

41 and 43 of combustion chamber 19 are formed by dre-resisting brickwork 45 which is backed with heat insulating material 47, the bricks and insulation being suitably supported in known manner.

Front wall 41 is provided with ports 43 fitted with oil burners 49 arranged to discharge longitudinally of the combustion chamber 19 towards the rear wall 43. It will be seen from FIG. l that considerable gas flow spaces are provided both above and below the group of burners for the return flow of gases towards the front of the combustion chamber, so that the heating gases can enter the tube bank throughout its length and height.

The arrangement of the various inclined tubes which constitute the tube bank may be seen most clearly from FIGS. 1 and 4. The tube bank includes a screen 51 formed by three staggered rows of relatively large diameter inclined tubes and extending along one side of the combustion chamber from front wall 41 to rear wall 43. Beyond this screen, a baille wall 53 extending from the screen 51 transversely of the drums 1 and 3 to a side wall 59 of the boiler setting serves to divide the remainder of the tubes of the tube bank into two front and rear sections, 61 and 63 respectively.

Batlle wall 53 includes aligned inclined tubes 65 forming part of the tube bank 5, these tubes being spaced apart but being provided, as shown in FIG. 5, with radiallyextending studs 67 welded to their exterior, the studs extending into the spaces between adjacent tubes of the baille and serving to support refractory material 69, such as chrome ore, which is applied in plastic form so as to fill the intertube spaces, but to leave the opposite outward faces 71 of the tubes exposed. The baille wall includes a shaped sheet metal baille 73, suitably located by lugs welded to the end tube of the tubular part of the baille wall 53 and by lugs secured to the adjacent setting side wall 59, and serving to close the gap between those two parts.

The vapor generating tubes of section 63 of the tube bank 5 are arranged in two groups, 75 and 77, spaced apart and spaced respectively from the tube screen 51 and from the setting side wall 59 as shown in FIG..4. Each of these tube groups extends from the baille wall 53 to a setting rear wal-l 79, which itself extends from side wall 59 to the rear wall 43 of the combustion chamber 19 and is provided with access doors at 81, 83 and 85 to permit access to the

spaces

87, 89 and 91 respectively between the screen 51 and tube group 75, between the 3 two tube groups and between tube group 77 and setting side Wall 59.

The inclined vapor generating tubes of section 61 of the tube bank 5 are also arranged in two groups, 95 and 97, these groups being spaced apart and so arranged that they are staggered relative to the two groups of section 63, as shown in FIG. 4. Thus, group 97 is arranged nearer to setting side wall S9 than is the group 77, while group 95 is arranged opposite the space between

groups

75 and 77. Each of these

tube groups

95, 97 extends from the baie wall 53 to a setting front wall 99, which itself extends from side wall 59 towards front wall 41 of the combustion chamber but terminates short of wall 4,1.

Between setting front wall 99 and the adjacent end of the front wall 41 of the combustion chamber is provided an opening through which a superheater 105 may be inserted and removed. When in position, the superheater lies in space 107 between the tube group 95 and the tube screen 51. The superheater comprises two vertically inclined headers 109, 111 and a large number of horizontally extending U-shaped tubes 113 joining the two headers and forming the actual heat-exchange surface. These superheater headers are suitably divided by transverse diaphragms so as to obtain a flow of steam from header 109 through certain tubes 113 to header 111, then back through other tubes 113 to header 109, and so on. The nest of superheater tubes 113 is supported on several vertically spaced horizontally inclined bars 117 secured at their ends to two large bore tubes 119 and 121 forming part of tube bank 5 and disposed respectively in the tube screen 51 and in the tube group 95. The superheater is positioned close to the tube screen 51, thus permitting access by way of an access door 12.3, which closes the gap between the superheater header 109 and the adjacent end of setting rear wall 99, through space 107 to the bafe wall 53. A second access door 127 is provided in front setting wall 99 opposite the space 129 between

tube groups

95 and 97.

It will be seen that with this arrangement of the tubes which form the bank 5, substantially all parts of the stud tube bafe wall 53 are readily accessible for inspection and for repair of the refractory material 69.

A suitable door 133 joins setting side wall 59 to a Vlower water drum 3, to complete the boundaries of two parallel arranged gas passes 135 and 137 extending from the combustion chamber 19 respectively through the tube bank sections 61 and 63 upwardly to two damper controlled gas outlets 139 and 14@ which are described in more detail below.

The complete boiler is enclosed in a sheet metal casing, indicated diagrammatically in FIGS. l, 2, 3 and 4 by 141, spaced outwardly from the

walls

41, 43, 59, 79 and 99 and floor 31 referred to above. Cooling air enters the casing at the rear of the boiler and passes either under the floor 31 through a duct 143 or between the side wall 59 and the casing 141 to the front of the boiler where it is used as combustion air for the burners 49. The

passes

135 and 137 join in a common duct 143 extending upwardly from above the outlets 139, 141) from the passes, the pass 135 being provided at outlet 139 with dampers 145 for regulating the flow of gases through the pass and similar separately operable dampers 146 being provided for the pass 137, Duct 143, which leads to the ships smoke stack, contains an economizer 148 and has associated with it an economizer by-pass duct 147 provided with a damper 149 and extending upwardly from adjacent the outlet 141i of pass 137 only, the by-pass permitting the gas from the pass 137 to reach the stack without contacting the economizer.

Cradles 161 and 163 associated respectively with water drum `3 and side header 7 and suitably mounted on the structure of the vessel l'serve to support the weight 0f the drums and of the tubular parts of the boiler.

It will'be appreciated by those skilled in the art that many details unessential to an understanding of the in- `v'entionV have been omitted from the above description of an integral furnace type marine boiler, such as the provision of downcomer tubes leading to header 7 and to drum 3, and of means for separating steam from water in the steam and water drum 1.

During normal operation of the boiler described above, oil fuel supplied by the burners 49 is burned in the combustion chamber 19 and the hot products of combustion pass partly through pass containing the superheater 105 and the tube bank sections 61 and partly through pass 137 containing the tube bank sections 63 to the damper controlled outlets 139, and thence to the economizer 148 and the smoke stack.

During starting up of the boiler from cold, the dampers provided at the outlet 139 from the pass 135, which contains the superheater are closed, so preventing any flow of hot gases over the superheater and protecting it from overheating. At first, hot gases owing through pass 137 are diverted through the economizer by-pass duct 147 by the opening of by-pass damper 149 and the closure of main dampers 146, so avoiding excessive condensation on the economizer, but once the gases passing to the stack have reached a suitably high temperature, main dampers 145 are opened and by-pass damper 149 is closed, so causing all the hot gases to pass over the economizer.

Normally, the dampers controlling the flow of gases from the combustion chamber over the superheater will be kept closed until steam is generated in the boiler. This steam passes from drum 1 through the superheater and thence to its point of use, and once such a ow of steam has commenced, the dampers controlling the flow of hot gases over the superheater may be opened to a suitable extent.

The degree of superheat imparted to the steam may be varied by adjustment of the two sets of dampers controlling the gas flow respectively through pass 135 and through pass 137. Thus, a moderate steam temperature is obtained over a wide load range with both sets of

dampers

145, 146 open; a higher steam temperature may be obtained by partial closure of the dampers 146 associated with gas pass 137 and a lower steam temperature, by partial closure of the dampers 145 associated with gas pass 135. This is of considerable advantage in a marine installation using turbines, since usually steam of relatively low superheat is required when the astern turbines are in use.

It will be seen that, provided the dampers are operated in proper sequence, during starting up of the boiler the flow of gases through the economizer is prevented and it is an important feature of the construction that the dampers provided for the control of superheat serve the additional purpose of protecting `both the superheater and the economizer during starting periods. By the provision of the economizer by-pass and damper, the cleanliness of the economizer is promoted, since it is at low ratings when the boiler is relatively 'cold that the economizer is caused to become foul with soot.

It will normally prove advantageous to provide interlocking means between the three sets of dampers referred to above. A suitable arrangement would be effective to ensure that when the main `dampers of both gas passes are closed, the economizer by-pass must be open, lwhilst when the damper of Ithe economizer by-pass is closed, then the dampers of one of the gas passes must be fully open if the dampers of the other gas pass are partly or fully closed. Such interlocking of the three sets of dampers ensures that there is always an open passage between the combustion chamber and the stack.

The arrangement of the tubes of the tube bank 5 described above provides admirable facilities for cleaning the tubes of the tube bank and of the superheater while the bae-wall separating the two gas passes is readily accessible for inspection and repair. The boiler described above isparticularly compact and of light weight, it may be started rapidly .without damage, the temperature of superheat may be controlled and maintenance is facilitated.

'Ilhe embodiment of the invention illustrated in FIGS. 6 and 7 is also a two-drum integral furnace type marine boiler, and except for the arrangement of the various tubes in the tube bank 5, the associated superheater, the formation of the baille wall, and the provision of access doors, the above description of FIGS. l, 3 and 4 also applies to FIG. 6. Referring to FIG. 6, the tube bank 5 includes a tube screen 201 extending along one side of the combustion chamber from front wall 41 to rear wall 43 and formed by staggered rows of relatively large diameter inclined tubes, there being four such rows in front of the pass 137 but only three such rows in front of the superheater pass 135. Beyond this screen, a baille wall 203 extending transversely of the drums 1 and 3 with its plane nearer to wall 41 than to wall 43, serves to divide the remainder of the tubes of the tube bank 5 into two sections 205 and 2017, respectively nearer to and further from the combustion chamber front wall 41.

Part of baille wall 203 is formed by in'clined tubes 209 forming part of the tube bank 5, these tubes being spaced apart but being provided with means for closing the intertube spaces so as to provide, together with a shaped sheet metal baille 211 suitably secured to the end tube of the baille wall 203 and to the adjacent setting wall 59, a continuous baille between the setting side wall 59 and a point near to but spaced from the screen 201.

The baille is so formed as to include two lateral extensions 215, 217 from its front face, i.e. `from the side of the baille nearer the wall 41, each extension being formed by a pair of inclined tubes 225 extending parallel to the remainder of the tubes of the baille wall but displaced laterally with respect to the plane of the wall. The intertube spaces of the baille wall are closed by refractory material 227, such as chrome ore, which is ap plied in plastic form so as to till the intertube spaces but to leave the opposite outward faces 229 of the tubes exposed. The

tubes

209 and 225 are provided with radially extending studs 235 welded to their exteriors and extending into the spaces between adjacent tubes of the baille. The rear face of the baille wall includes a number of thin flat metal plates 239 (see FIG. 7) extending partway across the intertube spaces, each plate being welded along one edge to a

tube

209 or 225 and near its opposite edge to Ithe tip of at least one stud 235 arranged to intersect the plane of a corresponding plate 235. Advantageously, the plates are sectionalized in the direction of the tube axis to avoid undue stresses. These plates 239 together substantially close the spaces between the tubes and serve as a backing against which the refractory material 227 may be rammed and co-operate with the studs 235 in retaining the refractory material in position.

The vapor generating tubes of section 205 of the tube bank are arranged in two spaced

groups

243 and 245, nearer to and further from the combustion chamber respectively, each ygroup of tubes extending from the setting front wall 99 towards the baille wall 203 but terminating adjacent the tip of one of the lateral extensions 215, 217 of the baille, which are so arranged as to lie throughout their lengths adjacent the mid-widths of the

tube groups

243, 245 respectively. Thus the tubes of the two groups are spaced somewhat from the plane of the .baille wall, but the lateral extensions 215, 217 substantially prevent by-passing of the tube groups by gases ilowing through the gas pass. The tube group 243 is sep- -arated from the tube screen 201 by `an access `space 247 provided with an access door 249 in the front setting wall 99. A second access door 251 in wall 99 permits access to the space 253 between the

tube groups

243 and 245, and a further access door 255 in the setting side wall 59 permits access to space 257 bounded by baille wall 203, setting side wall 59 and tube group 245.

The vapor generating tubes of section 207 of the tube bank are arranged in three closely spaced

groups

261, 263 and 265, each group extending from the setting rear wall 79 up to the baille wall 203. Group 261 is the nearest group to the tube screen 201, from which it is separated by a space 267 into which extend the horizontally arranged U-shaped tubes 269 of a superheater 271. This superheater comprises two vertically inclined headers 273, disposed in the plane of the setting rear wall 79, into which the ends of the U-shaped tubes 269 are expanded. The straight legs of these tubes extend -as far as the baflle wall 203, so that the return bends actually extend beyond the baille wall into access space 247. A plate type tube support 281 for the `superheater tubes provides an extension of the baille wall 203, the plates being themselves supported on a large diameter downcomer tube 283 extending between the upper and lower drums.

The arrangement of tubes and superheater described above in connection with FIGS. 6 and 7 permits ready maintenance of the baille wall 203 due to the provision of the

access doors

249, 251, 255 and the

access spaces

247, 253 and 257. Once the downcomer tube 283 is expanded in position into the upper and lower drums, the superheater cannot be removed bodily. However, `due to the provision of the access space 247, it. is possible to cut out any defective superheater tube from the superheater headers 273 and to withdraw it forwardly through the access space 247 and access door '249. The operation of this form of boiler is similar to that of the boiler described above in connection with FIGS. l to 5; although a small part of the superheater heat exchange surface is exposed to hot gases when starting up the boiler from cold, this surface is so positioned that no overheating is likely to occur during the starting-up period.

The embodiment of the invention illustrated in FIG. 8 of tbe drawings is also a two-drum integral furnace type marine boiler, and except lfor the arrangement of the various tubes in the tube bank, the associated superheater, the formation of the baille wall and the provision of access doors, the above `description of FIGS. l, 3 and 4 also applies to FIG. S. The tube bank 5 in FlG. 8, includes a tube screen 301 extending along one side of the combustion chamber from front wall 41 to rear wall 43 and formed by three staggered rows of large bore inclined tubes. Beyo'nd this screen, an access space 303 defined by a wall 305 formed in part by inclined tubes 306 forming part of the tube bank 5 extends transversely of the drums 1 and 3 and serves to divide the remainder of the tubes of the tube bank 5 into two

sections

307 and 309, disposed in the gas passes 137 and 135 respectively nearer to and further yfrom the combustion chamber front wall 41.

The tubes 306 are spaced apart but are pro'vided with means 313 for closing the intertube spaces so as to provide a continuous wall. The means for closing the intertube spaces comprises studs, sectionalized thin llat met-al plates, and refractory material arranged and secured as described above in connection with the baille wall 203 of FIGS. 6 and 7, the plates being on the outer surface 315 of wall 305 with respect to access space 303.

The access space 303 extends from adjacent the setting side wall 59 towards the screen 301, but terminates in a tlat wall 317 spaced from the screen. The part 319 of the access space nearer the combustion chamber is narrower than the part 321 nearer the setting side wall 59. Sheet metal baille plates 325 extend between the setting side wall 59 and the adjacent ends of the tubular part of wall 305, while

access doors

327 and 329 in the setting side wall 59 and in the casing 141 respectively permit access to the space 303, whereby the wall 305 may be inspected and repaired by an operator fro'rn within the access space. The vapor generating tubes of section 307 of the tube bank are arranged in two closely spaced groups 331, 333 each extending from the setting front Wall 99 to the wall 305 which surrounds access space 303,

and the vapor generating tubes of section 309 are similarly arranged in two' closely spaced

groups

335, 337 each extending from the `setting-rear wall 79 to the wall 305.

The tube groups 331, 335 and the side parts ofthe wall 305 of the access space 303 are all arranged to terminate at about the same distance from the tube screen 301, so lleaving an open space 339, extending Vfrom setting front wall 99 to setting rear wall 79, into which the U- shaped tube 341 of a superheater 343 extend from the setting rear wall 79 as far as the wall `317 of the access space. The ends of the superheater tubes 341 are expanded into -two inclined headers 345 disposed outwardly of the setting rear wall 79, and the tubes are supported near their return-bends on supports 347 mounted on two large bore inclined tubes 349 which connect the drums 1 and 3. It `will be appreciated that, once the tubes 349 are expanded into po'sition into the upper Vand lower drums, the superheater cannot be removed bodily. However, due to the provision of the space 339, it is possible to cut out any defective superheater tube from the superheater headers 345 and to withdraw it forwardly through the space 339 and through aligned

access doors

351 and 353 provided respectively in front setting wall 99 and in the casing 141.

A series of vertically arranged plates 357 detachably fixed by metal clips to the return bends of the outermost superheater tubes `341 and to lugs secured to an adjacent tube of tube wall 317 form an extension of the baffle Iformed by the wall 305 of the access space 303. Im-

- mediately in front o'f the plates the spaces between three adjacent screen tubes are suitably closed by studs and refractory material as indicated at 359, in the manner described above in connection with the bafe wall 53 and illustrated in FIG. 5.

It will be seen that in this embodiment of the invention the baffle wall may be readily inspected and maintained, and the replacement of superheater tubes is facilitated.

It may be desirable for some of the superheater surface to extend into or through the gas pass 137, although this has the effect of reducing the effectiveness of the dampers in varying the `degree of superheat obtained. Thus, when the superheater tubes are supported adjacent the baffle wall, part of the tubes may extend into the gas pass 137 as has been described above with reference to FIG. 6; some of the superheater tubes may be extended across the gas pass 137 in order to' cool part of the setting front wall 99, for example in the boiler of FIG. 8; or the ends of a nest of superheater tubes located in the pass adjacent the rear wall 79 of FIG. 8 may extend across the gas pass 137 to superheater headers located outside the setting front wall 99.

The use o'f thin flat metal plates suitably secured to the inaccessible face of a baffle wall as illustrated in FIG. 7 considerably facilitates the maintenance of the refractory material filling the intertube spaces of the baffle wall and this type of intertube refractory support may be applied to the baflie wall 53 shown in FIGS. 2 and 4.

While in accordance with the provision of the statutes I have illustrated and described herein the best forms of the invention now known to me, those skilled in the art will understand that changes may be -made in the form of the apparatus disclosed without departing from the spirit of the invention covered by the claims, and that certain features of the invention may sometimes be used to advantage without a corresponding use of other features.

I claim:

l. A fluid heater comprising a housing including a furnace chamber and vertically extending bali-le means arranged todivide said housing into a pair of side-by-side parallel ow gas passes laterally adjoining said furnace chamber and arranged to receive heating gases from said furnace chamber, a bank of fluid heating tubes comprisof tubes in one gas pass being relatively arranged with respect to the group of tubes in the ygas pass at the opposite side of said baie means so as to expose substantially all,

portions of said baffle means to one or the other ofsaid gas passes, and access openings in said housing affording access `to said exposed baffle means portions.

2. A fluid heater comprising a housing including a furnace chamber and vertically extending baille means arranged to divide said housing into a pair of Vside-by-sidc parallel `flow gas passes laterally adjoining said furnace chamber and arranged to receive heating gases from said furnace chamber, a bank of fluid heating tubes comprising groups of tubes spaced longitudinally of each of said gas passes, the groups of tubes in one gas pass being relatively arranged with respect to the groups of tubes in the gas pass at the opposite side of said bae means so as to expose substantially all portions of said baflie means to one or the other of said gas passes, and access openings in said housing affording access to 'said exposed baille means portions. Y

3. A fluid heater comprising a housing including a furnace chamber and vertically extending bafe means arranged to divide said housing into a pair of side-by-side parallel low gas passes laterally adjoining said furnace chamber and arranged to receive heating gases from said furnace chamber throughout its length, a bank of iiuid heating tubes comprising groups of vertically arranged tubes spaced longitudinally of each of said gas passes, the groups of tubes in one gas pass being staggered with respect to the groups of tubes in the gas pass at the opposite side of said baille means so as to expose substantially all portions of said baille means to one or the other of said gaspasses, and access openings in said housing opposite to said exposed bafe means portions.

4. A lluid heater comprising a housing including a furnace chamber and vertically extending baille means arranged to divide said housing into a pair of side-by-side parallel flow gas passes laterally adjoining said furnace chamber and arranged to receive heating gases from said furnace chamber, a bank of fluid heating tubes comprising a row of vertically arranged screen tubesrseparating said furnace chamber from said gas passes, and groups of vertically arranged tubes spaced longitudinally of Veach of said Vgas passes, the groups of tubes in one gas vpass being staggered with respect to the groups of tubes in the gas pass at the opposite side of said bafde means so -as to expose substantially all portions of said bafe means to one or `the other of said gas passes, and access openings in said housing opposite to said exposed ballie means portions.

5. Asteam generating unit comprising a housing including a furnace chamber and vertically extending balile means arranged to divide said housing into a pairV of side-by-side p-arallel flow gas passes laterally adjoining said furnace chamber and arranged to receive heating gases from said furnace chamber, a. bank of steamrgenerating tubes comprising a row of vertically arranged screen tubesseparating said furnace chamber from said gas passes, and groups of vertically arranged tubes spaced transversely of eachof said gas passes, the groups of tubes in one gas pass being staggered with respect to the groups of tubes in the gas pass at the opposite side of said baie means so as to expose substantially all portions all portions of said baille means to one or the other of said gas passes, a group of steam superheater tubes arranged to receive the steam generated and positioned in one ot' said gas passes between said screen tubes and the remaining groups of tubes in said gas pass, and access openings in said housing opposite to said exposed baiiie means portions.

6. A fluid heater as claimed in claim 1 in which a group of tubes in one of said gas passes is spaced from said baffle means to provide a space therebetween -for access to said bae means, a vertically elongated lateral extension on said baffle meansterminating adjacent said spaced group of tubes, and an access opening in said 9 housing aiording access to said space between said tube group and baie means.

7. A ud heater as claimed in claim 1 in which said baie means comprises spaced upright tubes arranged in a U-shaped row and means closing the intertube spaces, and an access opening in said housing aiording access to the interior of said baiiie means.

References Cited in the file of this patent UNITED STATES PATENTS 10 Stillman July 21, 1936 Rowand Apr. 16, 1940 Bailey Dec. 15, 1942 Mayo et al. Feb. 9, 1943 Qvarfordt Feb. 26, 1946 Bailey Nov. 4, 194-7 FOREIGN PATENTS Great Britain Feb. 24, 1943