US2800116A - Steam generating and superheating unit with recycled gas flow - Google Patents

Description

July 23, 1957 A. E. RAYNOR ET AL 2,800,116

STEAM GENERATING AND SUPERHEATING UNIT WITH RECYCLED G AS FLOW Tlql.

Filed Oct. 20, 1954- l VENTORS Inn/w? RAY/v01? H AND ATTORNEY 5 Sheets-Sheet l July 23, 1957 A. E. RAYNOR EI'AL 2,800,116

STEAM GENERATING AND SUPERHEATING UNIT WITH RECYCLED GAS FLOW Filed Oct. 20, 1954 5 Sheetsg-Sheet 2 lNV 'ro JE L. 4mm: 5. $2w539 w y H. wmvo ATTORNEY y 1957 A. E. RAYNOR EIAL 2,800,116

STEAM GENERATING AND SUPERHEATING UNIT WITH RECYCLED GAS FLOW Filed Oct. 20, 1954 5 Sheets-Sheet'S mvzmons 4? THUR E. Aa Y/VOI? w. 1. 0 vvmv 0 ATTORNEY y 1957 A. E. RAYNOR ETAL 2,800,116

STEAM GENERATING AND SUPERHEATING UNIT WITH RECYCLED GAS FLOW Filed 001;. 20, 1954 5 Sheets-Sheet 4 I i I I I g I i 192 i /24 i 1 l l INVENTOR5 July 23, 1957,.

STEAM GENERATING AND SUPERHEATING UNIT WITH RECYCLED GAS FLOW Filed 001-. 20, 1954 A. E. RAYNOR ETAL, 2,800,116

5 Sheets-Sheet 5 Fig.5.

INVENTORS IRTHUR E. R4 Y/VOP WILL ll OWIND BY fi ATTORNEYW STEAM GENERATING AND SUPERHEATING UNIT WITH RE Y L D GA FLOW Arthur E, Raynor, Rockville Centre, N. Y., and Will H. Rowand, Short Hills, N. J., assignors to The Babcock 9a Wilcox Company, New York, N. Y., a corporation of New Jersey Application October 20, 1954, Serial No. 463,418 7 Claims. (01. 122-240) This invention relates to high capacity steam generating and superheating units. It is more particularly concerned with improvements in such units operating at high furnace temperatures, high steam temperatures, and high steam Pr ssures, and having a high ratio of steam generating surface to furnace or furnace chamber volume.

A steam generating and superheating unit exemplifying the invention includes a single boiler setting having a twin or duplex furnace arrangement with separately tired furnaces or combustion chambers separated by a common wall of steam generating tubes. The remaining walls of the combustion or furnace chambers separated by this wall have their boundaries defined by other steam generating tubes connected into the circulation of the unit. The furnaces, or furnace sections, are fired by fuel burning means preferably disposed adjacent the lower parts of the separate furnace chambers with the gases flowing upwardly therethrough on opposite sides of the division wall and then over the surfaces of separate convection sections. One of these sections preferably includes a reheater, as well as some economizer and superheater surface, while the other includes other superheater and economizer surfaces, In a preferred embodiment of the invention the gases exiting from the different gas flow paths join beyond the convection sections and pass over an air heater dis.- posed at one side of that part of the setting including the furnace chambers and the convection sections. This side m be co i e e s a s e co nec he wo si es of the setting along which the opposite sets of fuel burning means are arranged,

The pertinent arrangement of elements promotes better mixing of recirculated gases ahead of the superheater elements because of the smaller throat Width just ahead of he per e tert als mi imiz unbalatlsed a temperatures across the unit, and minimizes the amount of uppo in s e re ed o a un t f a c ta n capac ysf rab the Opposite mes? sha b rs; teearated y the divisi n all, ce gases ro independent se s of .0 burn n cy on u aces na c amb r d p nd & h site s d o ach t'the first me t p s'd furna chambers which ma be n d e s c dar f r ace hamb r u al y There s a P mary.

f coal rn n y nes teha s nto ea h primar furnace chamber.

F r nt f s a h a n nd or nt l of he absorption of the steam generating furnace. wall tubes, over a' wide load range, as well as for promoting the effecting of optimum combustion by operation of the fuel burning means at highest optimum temperatures, partially ooled urnac a e a e hdra n fr m os ions, o a posi n n t gas o pa h d ns eam sfthe s e st sn ction nd are at qs u d a on o were s cted o itions with reference to gas flow. Preferably, for maintaining an optimum temperature of superheated steam as the load on the unit decreases, an increasing amount of recycled gases are caused to flow into the primary furnace chambers and thence through the secondary chambers and then over the convection sections to increase the mass flow of gases and thereby relatively increase the heat absorption of the convection section. At the high loads, or in the upper part of the load range, the temperature of the gases at the gas entrances of the convection section may be decreased 'or held at a permissible value by the introduction of controlled amounts of recycled gases as tempering gases, at positions just ahead of the convection sections.

For increasing the elfectiveness of the introduction of tempering gases just ahead of the convection sections, the division Wall has some of its tubes (i. e. one-third) bent to the right of the division wall and some of the remainder of the division wall tubes bent oppositely to the left of that wall to define a tempering gas duct, or tempering gas entry chamber. Some of the bent-out parts of the tubes may have inter-tube flat studs welded thereto to better form this tempering gas duct. In this way the tempering gases are introduced at the opposite sides of each furnace section, to promote optimum mixing of the recirculated gases with the other gases.

The invention will be concisely and clearly set forth in the claims appended hereto, but for a better understanding of the invention, its advantages, and uses, recourse should be had to the following description which refers to the accompanying drawings in which a preferred embodiment of the invention is shown.

Of h ,rawings:

Fig. 1 is mainly a vertical section through a steam generating and superheating unit, on the line 1-1 of Fig. 2; 1

Fig. 2 is a vertical section at right angles to the plane of the Fig. 1 section and taken on the dual plane section line 22 of Fig. 3;

Fig. 3 is a plan section on the dual level section line -3 f F Fig. 4 is a vertical section on the dual plane section line 4...4 of Fig. 3; and

Fig 5 is a fragmentary vertical section through one of the convection sections, on the line 5--5 of Fig. 3.

Fig. 1 shows two opposite secondary furnace chambers 10 and 12 separated by a division wall 14 consisting of wall aligned steam generating tubes extending from the lower header 16 to the upper headers 18 and 29. The upper part of this division wall also separates the gas passes 22 and 24 for the separate convection sections. The gases flowing through the furnace section 10 originate in a coal burning cyclone 26, having steam generating tubes in its boundaries, and otherwise of a type indicated in the U. s. patent to Bailey et al. 2,357,301. There be three of these cyclones, including the other cyclones 2,8

chamber 34 and then upwardly along the right hand wall 1 40 of t secondary fu a e hamb r The m be further extended 'alongtthe right hand wall 42 of the gas (v pass 24 to the header 44 which is appropriately connected to the steam and water drum 46.

At the opposite side of the unit there is a similar arrangement of cyclones 48, 50 and 52 discharging combustion products into the primary furnace chamber 54 for flow therefrom across the screen 56, and thence into the secondary furnace chamber 12:.

The floor and outer walls of the primary furnace chambers 34 and 54- preferably include steam generating tubes connected at their lower ends to the lower headers 58 and 60, and extending upwardly therefrom for appropriate connection to the drum 46.

Gases flowing upwardly from the secondary furnace chamber pass over the banks of tubes 62 and 64 constituting a part of the secondary, or high temperature superheater. The gases then pass in succession over the banks of tubes 65-69 constituting the primary, or low temperature superheater. Beyond this superheater the gases pass over the bank of tubes 70, constituting the first section of the economizer. They continue past thegas flow control dampers 72 into the breeching or ductwork 74 leading to the air heater having two sections 76 and 78, each including a bank of tubes through which the gases flow.

From the other secondary furnace chamber 12'the gases flow upwardly across the banks of tubes 82 and 84 constituting the other section of the secondary or high temperature superheater, and thence across the banks of tubes 86-88 constituting the steam reheater. Following the reheater the gases pass over the banks of tubes 90-92 constituting the second section of the economizer. From the economizer the gas flow continues past the gas flow control dampers 94 to the ductwork component 74, and thence to the air heater including the sections 76 and 78.

The separate gas flows from the separate secondary furnace chambers 10 and 12 are combined in the ductwork 74 for flow through the air heaters. From the air heaters the gases enter the breechings or ductwork components 100 from which the gases flow to a stack.

Air, to support combustion in the cyclones passes from an appropriate forced draft fan, or fans, to the air inlet ducts 102 and 104 (Fig. 4), thence through connected ductwork 196 and 108, and then through a first pass 110, as indicated by the arrow 112 across the gas conducting tubes of the section 78 of the air heater, whence they continue then upwardly as indicated by the arrow 114 to the second pass 116 in which they flow to the right transversely of the lower part of the tubes of the air heater section 78. These parts of these tubes are separated by a division plate 118 (Fig. 2 and Fig. 4) from the upper parts of the tubes between which the air passes in a third pass as indicated by the arrow 120. From this third pass the gases pass downwardly through one or more ducts 122 to the air inlets of the respective cyclone coal burners. The ductwork or chamber 124 provides for the turning of the air stream as it emerges from the pass 112, and its entry into the pass 116, and similar ductwork or chamber 126 provides for the passage of gases from the pass 116 through the last pass 120.

It is to be understood that the arrangement of air heater components just described is duplicated on the other side of the unit, with the ducts 128 directing heated air to the different cyclones 26, 28 and 30.

Feedwater is conducted from an appropriate source to the inlet header 139 or the first economizer 70. From this economizer the feedwater flows from the outlet header 132 through an appropriate conduit 134 to the inlet header 136 of the second economizer, including the banks of tubes 9092. From the outlet header 138 of the second economizer the heated feedwater flows through appropriate connections to the steam and Water drum 46.

Steam and water mixtures fed into the drum.46 by all of the steam generating tubes are subject to steam and water separation (preferably by cyclone separators of the type shown in the Rowand et al. Patent 2,289,970), with the separated steam passing through tubes 14% to the inlet header 142 of the primary superheater. Thence the steam passes through the return bend tubes forming the bank of tubes 6569 and to the outlet header 144. From this outlet header part of the steam passes through the tube 145 to the inlet header 146 of a part of the secondary superheater, including the banks of tubes 62 and 64. The remainder of the steam from the header 144 passes through the tube 150 to the inlet header 152 of the banks of tubes 82 and 84 constituting the other part of the secondary superheater.

From the outlet ends of the serially connected return bend tubes constituting the banks of tubes 82 and 84, the superheated steam passes through the outlet headers 154 and 156, andthence to a point of use. Similarly at the other side of the unit the steam superheated in the serially connected return bend tubes constituting the banks of tubes 62 and 64 of the first part of the secondary superheater passes from those tubes to the outlet headers 158 and 160 whence the steam may pass to junction with the steam exiting from the headers 154 and 156, and then pass to a point of use.

Convection superheaters and reheaters such as those shown in the drawings tend to give too high a steam temperature at loads above their control point loads. They also tend to give too low steam temperature at loads below their control point loads, and inasmuch as it is highlyimportant that these inherent tendencies be overcome in order that a predetermined superheat temperature and a predetermined reheat temperature may be maintained over a wide load range, the invention includes means for compensating for these inherent tendencies.

For eifecting a predetermined superheat temperature recirculated gas system which withdraws partially cooled combustion gases from a position downstream of the economizer and effects the entry of the returned gases into the secondary furnace chambers at a position'just ahead of the secondary superheater. Effective mixing of the returned gases at this position with the gases issuing directly from the fuel burning means not only promotes superheat and reheat control by regulation of the gas temperature and gas mass flow over the convection surfaces, but it also permits the operation of the fuel burning means at gas temperatures which would otherwise be too high under some conditions of operation, for safe contact with the convection surfaces. This return of partially cooled combustion gases and the mixing of such gases with the other furnace gases at positions just ahead of the convection surfaces may be referred to as gas tempering. Such returned gas flow for gas tempering purposes will be usually at a maximum at high load and high rates of fuel burning, and a reduction of this tempering effect may be combined with a change of the amount of spray attemperation as a load decreases, to be effective in the over-all steam temperature control.

At low loads, or in the lower part of the load range the returned combustion gases are caused to enter the primary furnace chambers for flow through and along walls of those chambers, before entering the secondary furnace This recirculated gas flow has a double ef-' chambers. fect toward maintaining a predetermined steam temperation surfaces, and the other part of the effect is the reduction of heat transfer to the heat absorbing surfaces of the furnace chambers. I

Another part of the over-all superheat control system involves separately regulable sets of dampers 72 and 94, for the separate gas flows through the gas passes leading from the secondary combustion chambers and across the convection surfaces.

Referring to Figs. 4 and 2 of the drawings the partially cooled combustion gases are withdrawn from a ductwork component 162 in the path of the gases leading from the outlets of the separate convection gas passes to the air heater. Communicating with the ductwork component 162 s a d 1 a n to the nle 1 f a circulated gas fan, the outlet 168 of which is connected o th t mpe n ga utle chambe .180 and 1 an i h th l t d as out et c am ers 84 nd 6 by ductwork components '170176. These ductwork components are appropriately dampered as at 193 and 192, and otherwise, in order that recirculated gas ,flow to the chambers 184 and 186 may be shut .ofi or approp iately controlled throughout a desired range, and in order that a tempering gas flow to the chambers 180 and 182 may be shut off or appropriately controlled throughout .a desired load range.

For effective entry of tempering gas at a position just ahead of the convection sections and for effective mixing of the recycled and unrecycled gases the tubes of the division wall 14 are so constructed and arranged to provide an additional tempering gas outlet chamber or duct 200 at a level adjacent to the level of the tempering gas outlet chambers 180 and 182. This tempering gas outlet chamber 200 may be defined by bending some of the division wall tubes 14 to the right out of their wall forming align.- ment, and then returning them to that alignment, others of the division wall tubes being bent to the left in a similar manner. The bent out portions of the division wall tubes may have inter-tube stud plates welded thereto to sufficiently define the duct or chamber 200, with distributed openings for the exit of gases. This chamber or duct 2,0,9 extends through the wall of the secondary furnaee cha n.- ber ng t Observer in Fi and s cehn ct d to a vertical duct 202 comunicating with the ,outlet of the re,- c cu at s gas n t ough h d c erk ccmpene ts 1 2- T duct 202 is pp op ia ely slampe ed a 204 r the n r f emp in a .flcw to th o tlet duct of chamber 200.

For the flow of tempering gas from the chambers 180 and 182 into the secondary furnace chambers '12 and 10 some of the wall tubes along the inner sides .of these chambers 180 and 182 are bent out of their wall align.-

ment to provide openings through which the tempering gas may flow.

The recirculated gases entering the chambers 184 and 186 flow therefrom downwardly-into the primary furnace chambers 34 and 54. This flow may take place in openings provided between the headers 210 and 212 and adjacent steam generating tube sections 214 and 216 which are integral with the tube sections forming the primary furnace chamber screens 36 and 5.6. The headers 210 and 212 are connected to the lower headers .or drums 58 and 60, respectively, by steam generating tubes 220 and 222. Some of the tubes 220 and' 222 may :be bent out of their wall forming alignment to provide openings through which the recirculated gases flow into the primary furnace chambers 34 and 54. The headers 210 and 212 for these tubes are appropriately connected into the cinculation of the unit, preferably by vdirect connections to the drum 46.

6 a d '1! ate h P m furnace ch rs 4 a d 55- As the load and firing rate decrease from the rnanirnum, the flow and entry of the tempering gases will be decreased and the flow of recirculatedgases through the chambers 184 and 186 will be increased to a manirnum at a predetermined low load valne.

It is contemplated that the flow of tempering gases to the upper parts of the secondary furnace chambers 10 and 12 shall be automatically controlled from registered and recorded variables, such as gas temperatures at the upper parts of the secondary furnace chambers, representations of load and steam temperatures. Similarly, it is contemplated that the flow of recirculated gas from the outlet chambers 184 and 186 shall be automatically controlled from registered representations of load and steam temperatures.

Ga fl hroug the a P ss 2. eadin u wardly r the s c d ry fur ace cham e 12 and e e the eheater n l i th beaks c t bes 8 y e $942: trolled by operation .of the dampers 94 at the'ontlet of this gas p The n o .c t ese damper as we s the control c t e damp rs .2 a he outlet o the as p 24 leading r m he s nda y furnace cham e 19. 10? y m y be su h s o p e nt as fl w ve the ehe t r u ing s n up f h unit when o s a s throu h the reheater, for the purpose of protecting the reheater, but may also be such as to limit-all attemperation to superheated steam only. In this event, and when the dampers 72 an 74 a au cmeti ll op r t d eh cf t p e mnept in ue ce in such operat o i l be hesteam tea per tu at m adj cen the cht et .ct th reh tere. the Pu p s of periodi al emoving a umulaon o cl s arch the he t absehp w su face in the c daw .fhr ae c ambers 9 and 1.2 and in the et n s t eh et eet ble se t lowers are .QBQ 'MQQ hreush p nin s indi a ed t 2s -2s4 in Fi 11 eus t l t bl we s ind cated a 236. haerahle hr ug in in the side a 2. 8 in .F ss- 2 and 5- e pp site l 2 i i dicat d a havin .a s mila arrangement cf .Iet a Iib e sect lowe s 24, operates h r r s Be s o he t in u n ce hst h es o t l t t d uni i h ts d isi n el f generating tubes, h r e se h nsehsiqh cf t echvecticn c i and t e s cchdsr utne e chambers across which t .retractiblc sec blowers a e epera et i m e ly reduced, in e de h t sect hle ers e educcd l gths may b s d: Th inc ease t arai b y c t n incr asin t e lite cf the see! lo ec o thei s er e ileh th- The l nge h blowers 'used in the illustrated unit have a length .of the order of 20 feet. Referring to the disclosure of 'Fig 2, it will be noted that the air heater including the sections 76 and 78 is spaced sufliciently from the wall 226 of the secondary furnace chamberand the convection section to permit the operation of the soot blowers 240 "between the fa ing walls of the air heater and the furnace and convection section component of the unit.

The lateral spacing of the soot blowers 236 operated through openings in the wall 228 is such that they do not place any substantial limitation upon the location of downcomers intermediate the length of the steam water drum 46. The downcomer 250, together with the end downcomers 252 and 254 leaddirectly downwardly frornthe drum 46 to the level of the lower headers or drum 16, 58 and 60. The intermediate downcomer 250 is joined directly to the lower drum 16, and the downcomers 252 and 254 are integral with the lower drums 6t) and 58, respectively.

The drum 46, with its depending downcomers 250 2 52 and 254, as well as the remainder of the main components of the unit are pendently supported from the top of steelwork including the uprights 300-408 joined at their upper ends by horizontals 310-3 17. The beams, or

horizontal elements of the steelwork 317 are preferably disposed in pairs along the top of the 'steelwork and in the direction of the length of the drum 46, with the drum straps 320 disposed between the elements 317, of each of said pair. In this way, and with the arrangement of elements shown and described, the drum may be supported at a plurality of positions distributed lengthwise of the drum, to the end that drum bending stresses are materially reduced, or minimized.

Fig. 2 of the drawings indicates the furnace walls. 226 and 228 as lined by rows of steam generating tubes. The tubes 330, along the wall 228 have their lower ends connected to the lower header 332 which is preferably in appropriate connection with one or more of the lower drums or headers 16, and 60. The upper ends of these steam generating wall tubes are connected to the upper header 334 which, in turn, is connected to the drum 46 by the tubes 336. At the opposite side of the furnace chambers there is a similar arrangement of tubes and headers including the lower header 340 from which the wall tubes 342 extend along the wall 226 to the upper header 344. The latter is appropriately connected to the drum 46. The lower headers 332 and 349 may be directly connected by floor tubes 346.

Although the invention has been shown and described with reference to the specific details of one embodiment, it is to be understood that the invention is not to be taken as limited to all of the details shown and described. It is rather to be taken as a scope commensurate with the scope of the subjoined claims.

What is claimed is:

1. In a high capacity steam generating and superheating unit of the radiant type, a furnace chamber having steam generating tubes along its walls, fuel burning means for producing high temperature gases for passage through the furnace chamber, a convection section including a tubular steam heater receiving gases from the furnace chamber, means normally conducting steam to the steam heater, a division wall including steam generating tubes dividing the furnace chamber into two sections with portions of said tubes outlining a recycle gas outlet chamber or duct communicating with both sections of the furnace chamber at a position remote from the fuel burning means, the division wall so dividing the furnace chamber that the sections of the latter are subject to parallel flow of gases from the fuel burning means, the steam heater having different parts exposed to the gas flows from the different furnace sections, and a recycled gas system including a fan with its inlet communicating with the gas flow path downstream of the steam heating means and its outlet connected with said chamber or duct.

2. In a high capacity steam generating and superheating unit of the radiant type, a furnace chamber having steam generating tubes along its walls, fuel burning means for producing high temperature gases for passage through the furnace chamber, a convection section including a steam heater receiving gases from the furnace chamber, means normally conducting steam to the steam heater, a division wall including steam generating tubes dividing the furnace chamber into two sections with oppositely bent out portions of said division tubes near the gas outlet of the furnace chamber outlining a recycled gas outlet chamber or duct, the division wall so dividing the furnace chamber that the sections of the latter are subject to parallel flow of gases from the fuel burning means, the steam heater having different parts exposed to the gas flows from the different furnace sections, and a recycled gas system including a fan with its inlet connected with the gas flow path downstream of the steam heating means and it's outlet connected with said chamber or duct.

3. A high capacity steam generating and superheating unit operating at high steam pressures, walls including steam generating tubes defining a furnace chamber, a convection section including a convection steam, heater receiving furnace gases from the furnace chamber, means normally conducting steam to the steam heater, fuel burning means producing combustion gases for passage through the furnace chamber and then through the con-, vection section, a division wall including steam generating tubes dividing the furnace chamber into two separate furnace chamber sections, the division wall so dividing the furnace chamber that the sections of the latter are subject to parallel flow of gases from the fuel burning means, the steam heater having dilferent parts exposed to the gas flows from the different furnace sections, some of said division wall tubes being bent outwardly from one side of the division wall for a short vertical length and then returned to the wall alignment below that position, others of the division wall tubes being bent in the opposite direction from said first bent division wall tubes and re turning to the wall alignment at a similar lower position, said bent portions of the division wall tubes outlining a chamber or outlet duct extending into the furnace chamber sections from an opening in a wall of the furnace chamber section, and a recycled gas system including a fan and an inlet duct leading to the inlet of the fan from a position in the gas flow path downstream from the steam heating means, said recycled gas system also including outlet duct means leading from the outlet of the fan to positions along the furnace chamber walls confronting said recycle gas outlet chamber formed by the bent division wall tubes and at a position remote from the fuel burning means, and means for varying the flow of recycled gases through said system to decrease said flow from a maximum at maximum load.

4. A high capacity steam generating and super-heating unit operating at high steam pressures, walls including steam generating tubes outlining a furnace chamber, a convection section including a convection steam heater receiving furnace gases from the furnace chamber, fuel burning means producing combustion gases for passage through the furnace chamber and then through the convection section, means normally conducting steam to the steam heater, division wall including steam generating tubes dividing the furnace chamber section into two tubes dividing the furnace chamber section into two separate furnace chamber sections, the division wall so dividing the furnace chamber that the sections of the latter are subject to parallel flow of gases from the fuel burning means, the steam heater having different parts exposed to the gas flows from the different furnace sections, some of said division wall tubes being bent outwardly from one side of the division wall for a short vertical length and then returned to the wall alignment below that position, said bent portions of some of the division wall tubes combining with other division wall tubes to outline a chamber or outlet duct extending into the furnace chamber sections from an opening in a wall of the furnace chamber, and a recycled gas system including a fan and an inlet duct leading to the inlet of the fan from a position in the gas flow path downstream from the steam heating means, said recycled gas system also including outlet duct means leading from the outlet of the fan to positions along the furnace chamber walls confronting said recycled gas outlet chamber formed by the bent division wall tubes and at a position remote from the fuel burning means, and means for varying the flow of recycled gases through said system to decrease said flow from a maximum at' maximum load.

5. In a high capacity steam generating and super-heating unit of the radiant type, a furnace chamber having steam generating tubes along its walls, fuel burning means for producing high temperature gases for passage through the furnace chamber, a convection section including a steam heater receiving gasesfrom the furnace chamber, means normally conducting steam to the steam heater, a division wall including steam generating tubes dividing the furnace chamber into two sections with portions of said tubes remote from the fuel burning means defining a recycled gas duct, the division wall so dividing the furnace chamber that the sections of the latter are subject 2,soo,1 1a

to parallel flow of gases from the fuel burning means, the steam heater having different parts exposed to the gas flows from the different furnace sections, a recycled gas system including a fan with its inlet connected with the gas flow-path downstream of the steam heating means and its outlet connected with said recycled gas duct, the recycled gas duct having openings for recycled gas flow into each furnace section, and means in each of the furnace walls opposite the division wall connected with the recycled gas system outlet and having openings through which recycled gas flows toward the division Wall.

6. A vapor generating and superheating unit having Walls defining a furnace chamber and a superjacent convection section arranged to receive heating gases therefrom, vapor generating tubes lining walls of said furnace chamber, vapor superheating tubes arranged in said convection section, means providing a heating gas flow through said furnace chamber and convection section, vapor generating tubes arranged to define a vertically extending division Wall dividing the furnace chamber into two side-by-side sections having a parallel flow of heating gases therethrough, a gas recirculation system including a fan with its inlet communicating with said convection section downstream of said vapor superheating tubes, portions of said division wall tubes being bent to define opposite sides of a gas outlet duct opening to both sections of said furnace chamber, and conduit means connecting the outlet of said fan to said gas outlet duct.

7. A vapor generating and superheating unit having walls defining a furnace chamber and a superjacent convection section arranged to receive heating gases therefrom, vapor generating tubes lining Walls of said furnace chamber, vapor superheating tubes arranged in said convection section, means providing a heating gas flow through said furnace chamber and convection section, vapor generating tubes arranged to define a vertically extending division wall dividing the furnace chamber into two side-by-side sections having a parallel flow of heating gases therethrough, a gas recirculation system including a fan with its inlet communicating with said convection section downstream of said vapor superheating tubes, portions of said division Wall tubes being bent to define opposite sides of a gas outlet duct opening to both sections of said furnace chamber, conduit means connecting the outlet of said fan to said gas outlet duct, and means in each of the furnace chamber walls opposite said division wall connected to said fan outlet and arranged to discharge recirculated gas towards said division wall.

References Cited in the file of this patent UNITED STATES PATENTS {BERTIFICAT OF CORRECTION Patent No. 2,800,116 Arthur E. Raynor et al. July 23, 1957 Y It is hereby certified that error appears .in the printed specification of the above ni 'znbered patent requiring correction and that the said Letters Patent should 'rogad as corrected below.

001mm 8, line 38, after "heater," insert a---; line 40, strike out "tubes dividing the furnace chamber eection into two":

Signed and sealed this 17th day of June 1958.

(SEAL) Attest:

KARI- AXLINE ROBERT c. WATSON Atteeting Officer Conmissioner of Patents

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