US2834326A

US2834326A - Vapor generating and superheating unit, and method effected thereby

Info

Publication number: US2834326A
Application number: US306329A
Authority: US
Inventors: Willem F C Schaap
Original assignee: Babcock and Wilcox Co
Current assignee: Babcock and Wilcox Co
Priority date: 1952-08-26
Filing date: 1952-08-26
Publication date: 1958-05-13
Anticipated expiration: 1975-05-13

Description

May 13, 1958 w. F. c. SCHAAP 2,834,326

ERATING AND SUPERHEATING UNIT, AND

VAPOR GEN METHOD EFFECTED THEREBY 7 Sheets-Sheet 1 Filed Aug. 26, 1952 M5 FIG. 3.

INVENTOR Egg/ emf USE/142a A'TTORNEY May 13, 1958 w. F. c. SCHAAP 2,834,326

VAPOR GENERATING AND SUPERHEATING UNIT, AND

METHOD EFFECTED THEREBY Filed Aug. 26, 1952 7 Sheets-Sheet 3 F/GS.

INVENTOR ATTORNEY M y 1953 w. F. c. SCHAAP 2,834,326

VAPOR GENERATING AND SUPERHEATING UNIT, AND

METHOD EFFECTED THEREBY 7 Sheets-Sheet 4 Filed Aug. 26. 1952 INVENTOR FCSchaa v 'A'II'TORNEY May 13, 1958 VAPOR GENE Filed Aug. 26, 1952 F. C. SCHAAP RATING AND SUPERHEATING UNIT, AND

METHOD EFFECTED THEREBY 7 Sheets-Sheet 5 INVENTOR 'A'T'roRNEY Filed Aug. 26, 1952 May 13, 1958 w. F. c. SCHAAP 2,834,326

VAPOR GENERATING AND SUPERHEATING UNIT, AND

METHOD EFFECTED THEREBY '7 Sheets-Sheet 6 iNVENTOR Mf 222 FCJchaa v ATTORNEY Filed Aug. 26, 1952 May 13, 1958 w. F. c. SCHAAP 2,834,326

I VAPOR GENERATING AND SUPERHEATING UNIT, AND

METHOD EFFECTED THEREBY 7 Sheets-Sheet '7 F762. 33 oooocfqg o 0- INVENTOR ATTORNEY United States atent Ofitice 2,834,326 Patented May 13, 1958 VAPOR GENERATING AND SUPERHEATING UNIT, AND METHOD EFFECTED THEREBY Willem F. C. Schaap, London, England, assignor to The Babcock & Wilcox Company, New York, N. Y., a corporation of New Jersey Application August 26, 1952, Serial No. 306,329

13 Claims. (Cl. 122-481) This invention relates to a method of generating and superheating a vapor under substantial pressure, superheating the vapor mainly by heat radiantly transmitted to a relatively quiescent zone from a furnace or combustion chamber and the combustion products passing therethrough.

More specifically, the method involves the superheating of the vapor by heat radiantly transmitted from the products of combustion effected by the burning of an ash bearing fuel such as pulverized coal, the superheating being effected in such a relatively quiescent zone laterally of the main combustion products flow that there is a minimum of interference with continuous operation and long life of the apparatus for effecting the method. Continuity of operation, or maximum availability, is effected as a result of the manner in which the radiant superheating is effected, the depositions of solids from the combustion gases being minimized.

The invention also involves a vapor generating and superheating unit for effecting the illustrative method. This unit includes a furnace which, in one embodiment is vertically elongated, and has a radiant superheater disposed in a recess or pocket at one wall, at one side of the main fiow of combustion products. The furnace is fired by a burner, or burners, supplied with air and an ash bearing fuel, such as pulverized coal. The furnace gas outlet is at the upper part of the furnace, and preferably opposite the furnace wall in which the radiant supcrheater is recessed. In this arrangement an upwardly inclined bafiie or arch may be advantageously provided, projecting into the furnace (or combustion chamber) at a position between the radiant superheater and the fuel buming means. i

The illustrative unit may be operated at such high pressures and temperatures that improved efficiency is attained. The availability of the unit is also increased by minimizing slag or ash or other deposits which result from the high temperature furnace operation, this temperature being greater, in some instances, than the fusion temperature of the ash or slag resulting from the combustion. Such incombustible residue has components which undergo appreciable change during the process of combustion as well as those which can become plastic or sticky at reduced temperatures. The former can give rise to potentially condensable vapors (i. e. the vapors of metallic salts) in the gases, vapors readily condensable on the vapor heating surfaces under some conditions. When such conditions include low temperatures of these surfaces (although the gas temperatures may be high) such vapors freeze before they reach the heat exchange surfaces (i. e. the radiant superheater tubes) and a dry deposit, easily removed by soot blowers is the worst that occurs. If, however, the temperature of the tubes is high and the temperature of the gases correspondingly high, these vapors cannot freeze before they contact the tubes and direct condensation upon the tubes may result.

In such considerations is thought to lie the relative immunity from fouling of low temperature vapor generating tubes of a vapor generator, compared to the degree of foulingof higher temperature tubes of a superheater receiving the generated vapor.

Detailed study of gas flow over convection surfaces introduces such factors as the depression of the temperature of inert solid particles (carried by the gases) to a value below that of the temperature of the surrounding gases, due to the proximity cold aspect, the interchange of radiation between these particles and the tube surfaces and between the emissive components of the gases and the same tube surfaces.

Thus, in the case of a vapor generating unit in which the vapor heater is preceded by a deep boiler bank, the hot gases are encouraged to throw out their troublesome vapors by condensation on dust nuclei, and the superheater is kept relatively free from fouling. However, the constant demand for higher steam or vapor temperatures has necessitated the increasing reduction of cold surface ahead of the superheater, and the assistance gained by the freezing out of troublesome vapors has practically been lost.

Wide spacing of the superheater tubes largely eliminates bridging-over of the tubes by deposits thereon, but the tubes are still susceptible to fouling. This is largely, if not wholly, avoided by the present invention by providing radiant superheater surfaces in a comparatively quescent or stagnant zone, relative to the movement of combustion products. This zone is preferably at one side of the main flow of combustion products so that the superheater surfaces will not be swept by dust laden gases to any substantial degree. The superheater surfaces, disposed in a recess of a wall of a combustion or furnace chamber defined by vapor generating tubes, may be considered as embraced by a relatively cool enclosure whereby freezing of the condensable vapors is promoted at a position in advance of the superheater surfaces.

The invention is concisely set forth in the sub-joined claims, but for a complete understanding of the invention its advantages and uses, recourse should be had to the following description which refers to similarly characterized components of preferred embodiments of the invention shown in the accompanying drawings.

In the drawings:

Fig. 1 is a sectional side elevation of an illustrative vapor generating and superheating unit in which an embodiment of the invention is incorporated;

Fig. 2 is a fragmentary horizontal section on the line IIII of Fig. 1, showing the arrangement of the superheated platens;

Pig. 3 is a sectional side elevation of a vapor generating unit having a furnace fired by superposed and spaced burners and incorporating the invention;

Fig. 4 is a sectional side elevation on the line IVIV of Fig. 5 of another vapor generating unit in which the invention is involved;

Fig. 5 is a sectional elevation on the line V-V Fig. 4;

Fig. 6 is a sectional plan on the line VI-VI of Fig. with some details omitted for the sake of clarity;

Fig. 7 is a fragmentary plan, on an enlarged scale, a part of the structure shown in Fig. 6;

Fig. 8 is a sectional side elevation of an embodiment of the radiant superheater;

Fig. 9 is a fragmentary plan from the plane of the line IX-IX of Fig. 8; and

Fig. 10 is a fragmentary plan similar to Fig. 3, but showing a further embodiment of the radiant superheater.

Fig. 1 shows a cyclone burner of furnace 1 of a construction similar to that described in U. S. pat nt to Bailey et al. 2,357,301, September 5, 1944. It discharges combustion gases and the slag from an ash bearing fuel through the opening 4 in the water tube and refractory wall 5. The combusion products are received by the primary furnace chamber 3 having a slagging fioor 2. The cyclone (or primary furnace) is of horizontally elongated, and of substantially circular cross section, its circular wall being formed by closely spaced studded tubes covered by high temperature refactory material. Each tube has a semi-circular bent portion and adjacent tubes have their bent portions oppositely arranged to form the circumferential wall of the cyclone. The lower ends of these wall tubes terminate in the lower cyclone header 6 and their opposite ends terminate in the upper cyclone header 7. The latter is preferably connected by tubes 8 to the lower drum 20 of the circulatory system of the unit.

A vertically elongated secondary furnace chamber 12 extends upwardly from the primary furnace chamber 3 from which it is separated by a group of vapor generating tubes 13, the upper parts of which are associated with refractory material to form a reflecting arch 16 for the primary furnace chamber. The intermediate parts 14 of these tubes are disposed at a moderate inclination to form the slag screen 15 across the path of combusion products passing from the primary furnace chamber 3 to the secondary furnace chamber 12. Other screen tubes 17 augment the slag screen 15 and have upper inclined parts 18 extending to the header 22. The

tubes

13 and 17 have their lower ends connected to the drum 20 and the upper ends of tubes 13 are connected to the header 21. The headers are connected by appropriate risers to the vapor and liquid separating drum 30.

The floor 2 of the primary furnace chamber 3 and the roof 24 of the secondary furnace chamber 12, and the walls of these chambers are lined with vapor generating tubes some of which directly connect the lower drum 20 to the upper drum 30, as shown. The connections for the floor tubes are indicated at 26 and similar connections 27 are shown for the wall tubes lining the rear wall 28. The side wall tubes (not shown) are connected to the drums through the intermediacy of upper and lower side wall headers and appropriate circulators.

The front wall of the primary furnace chamber 3 is continued upwardly as the front wall of the secondary furnace chamber 12 which may be otherwise termed a radiant pass or radiation chamber. Above the level of the header 21 this front wall is formed across its width with a rearwardly or inwardly projecting arch 31 which serves to deflect furnace gases and other combustion prod ucts such as suspended solids away from a bay or recess 32 above the arch. A radiant superheater 33 is disposed in the relatively quiescent zone so formed in the bay or recess 32.

The radiant superheater is formed by a number of vertically arranged tube panels or platens 38 widely horizontally spaced across the width of the front wall (see Fig. 2). Each platen consists of several U-shaped tube lengths with their inlet legs connected to the inlet header 40 and their outlet legs connected to the outlet header 41. The inlet header 40 is connected by conduits 43 to the vapor space of the upper drum 30 while the outlet header 41 is connected by pipes 68 to the convection superheater described below.

Aligned with each platen 38 is an upright vapor generating tube 45 which, over its lower part, constitutes one of the tubes cooling the front wall 5 of the lower part of the secondary furnace chamber 12. These tubes extend upwardly through the roof 24, and thence to the drum 30.

The upper parts of alternate tubes 27 lining the rear wall 28 are continued vertically, as indicated at 55, across a lateral furnace gas outlet 56, constituting the inlet of the lateral gas pass 58 leading from the upper end 59 of the secondary furnace chamber 12, While the upper parts of the remaining tubes 27 extend rearwardly and then upwardly as indicated at 60 across the outlet of the gas pass 58. intermediate parts of some of these tubes extend along the upwardly inclined wall forming the bottom of gas pass 58.

Downcomer tubes 61, disposed mainly exteriorly of the furnace chamber 12 connect the drums 20 and 30 and have their upper ends 62 disposed in a gas turning chamber 63 beyond the outlet of gas pass 58.

After passing through the line 43 to the radiant superheater, and after superheating therein, steam (or vapor) passes through the line 68 to the inlet header 66 of the pendent primary convection superheater 64. It is further superheated therein and then passes from the outlet header 67 through the line 70 to the inlet header 71 of the secondary convection superheater 65. It is still further superheated therein, and then passes from the outlet header 72, and through the line 73, to a point of use. For superheat control, an attemperator (not shown) may be interposed relative to the

headers

67 and 71.

The furnace gases turn downwardly in the gas turning chamber 63 and then flow downwardly through the gas pass 75, which contains an economizer 76 formed by tubes 77 connected at their lower ends to the inlet header 78 and having their upper parts lining the rear wall 79 and the roof 80 of the chamber 63 and connected at their upper ends to the water space of the drum 30. Other vapor generating tubes line the side walls of the lateral gas pass 58 and the down pass 75. and are appropriately connected to the drums 20 and 30 of the circulating system of the illustrative vapor generating unit.

In the operation of the illustrative vapor generating and superheating unit, owing to the disposition of the cyclone furnace or furnaces and the gas path through the radiation chamber or secondary furnace chamber 12, there is a tendency for the gases to lay-pass the radiant superheater zone in the upper region of the radiation chamber. Moreover, the gases are further directed by the arch 31 which, with the associated radiation chamber walls, forms a bay or recess constituting a relatively quiescent radiation chamber zone. Gases eddying in this zone will be substantially cooled and the parts of the radiant superheating surfaces nearest the stream of gasentrained particles passing through the radiation chamber are substantially screened from direct contact therewith by the provision of the series of vapor generating tubes 45, aligned with the radiant superheater tube platens.

The Fig. 3 vapor generating and superheating unit includes a vertically elongated radiation chamber or furnace chamber 100, fired by pulverized fuel burners 101 arranged at vcrtically spaced positions along the front wall 102 and adapted to operate with the removal of ash in solid form through the throat 110 of the hopperbottom furnace.

The furnace chamber front wall 162 is formed approximately midway of its height and across its full width with a rearwardly extending arch 103, and a radiant super-heater 105 is disposed in the bay 104 formed above the arch. This radiant superheater is similar in construction to the radiant superheater 33 described in connection with the Fig. 1 unit.

The part 108 of the furnace chamber below the level of the arch 103 is considerably enlarged in a forward direction and is provided with the hopper-like bottom 109 terminating in the throat 110 for the downward passage of dry or solidified ash into an ash pit. The roof 111 and four walls of the furnace chamber are lined with vapor generating tubes connected into the circulating sys tem of the unit. Thus, tubes 112, connected at their lower ends to an inlet header 113, line the front wall 102 and terminate at their upper ends in the vapor and liquid separating drum 114, while tubes connected at their lower ends to inlet header 116, line rear wall 117. Certain of the tubes 112, denoted by 112; are continued vertically upwards above the arch 103 in alignment with the platens of the superheater, constructed as described above in connection with Figs. 1 and 2. The upper ends 118 of alternate tubes are continued upwardly across a lateral gas outlet 119 constituting the entry to a lateral gas pass 120 leading rearwardly from an upper part 121 of the furnace chamber 100 and, above the roof 111, are connectetd to the drum 114. The upper ends 125 of the remainder of the tubes 115 are bent rearwardly to line the floor 126 of the lateral gas pass 120, then upwardly across the rearward end of the gas pass 120 to the roof 111, along which they extend in a forward direction as far as the front wall 102, beyond which they are curved upwardly and connected to the vapor and liquid drum 114. The

headers

113 and 116 are connected into the fluid circulation system of the vapor generating uni-t by downcomers such as those indicated in part at 129, which downcomers are connected at their upper ends to the drum 114.

A secondary convection superheater 130 disposed in the lateral gas pass 120 comprises a number of U-shaped tube parts 131 arranged with their return bends near the top of the unit and extending vertically downwardly to a location adjacent the bottom of the lateral gas pass,

the lower ends of these tube parts being connected by the horizontal tube parts 132 extending across a gas turning chamber 133 disposed at the rearward end of the gas pass 120. Their ends are connected to inlet and

outlet headers

134 and 135 disposed outside the chamber 133 and through which the superheater may be drained.

A primary convection superheater 136 is disposed in a gas pass 140 extending downwardly from the chamber 133 and is connected to inlet and

outlet headers

141 and 142 disposed outside the gas pass, while below this in the gas pass is a tubular economizer 143 suitably connected into the feed water supply system to the drum 114.

A pipe 145 connects the vapor space of the drum 114 to inlet header 146 of the radiant superheater 105, and a second pipe 147 connects outlet header 148 of this superheater to a header 149 positioned above the rearward end of the lateral gas pass 120. A number of tubes 150 extend from the header 149, alternate tubes extending vertically downwards across the rearward end of the lateral gas pass 120 to line the front wall 151 of the gas pass 140 to a level below that of the economizer 143 and then across the gas pass to a header 152. The remainder of the tubes 150 line the roof 153 of the chamber 133 and the rear wall 154 of this chamber and of the gas pass 140 down to the levelof the header 152, to which they are connected. Further tubes (not shown) line the side walls of the lateral gas path 120 and the gas pass 140, and are connected into the circulation system of the steam generating unit. Header 152 is connected by pipes to the inlet header 141 of the primary convection superheater 136. interposed in a connection 161 between the outlet header 142 of the primary superheater 136 and the inlet header 134 of the secondary convection superheater 130 is an attemperator 162, which may be of any suitable type. The outlet header 135 of the secondary superheater 130 is connected by a pipe 163 to a point of use of the superheated vapor.

In the operation of the vapor generating and superheating unit described above, the radiant superheater 105 is located in a quiescent zone and is protected from direct contact with the stream of gas-entrained particles passing through the radiation or furnace chamber 100, as has been described above with respect to Figs. 1 and 2.

In the vapor generating and superheating unit shown in Figs. 4, 5, 6, and 7, a vertically elongated radiation of furnace chamber 208 is fired by downwardly directed pulverized fuel burners and is adaptedto operate with removal of the ash in solid form.

The front wall 182 of the furnace chamber extends vertically for the full height of the furnace chamber 180,

but the side wall 183 is formed at approximately its midheight and across its full width with an inwardly extending arch 184 (Fig. 5), and in the bay 185 so formed above the arch, a radiant superheater 186 is disposed, similar in construction to the radiant superheater 33 described above in connection with Figs. 1 and 2. The opposite side wall 193, is similarly formed with an arch 194, and provided with a radiant super-heater 196 disposed in the bay 197 above the arch. The rear wall 198 (Fig. 4) of the furnace chamber is provided, at a level above that of the

arches

184, 194, but below a lateral gas pass 199 extending rearwardly from an upper part 200 of the furnace chamber, with a forwardly extending arch 201 extending across the full width of the rear wall and projecting approximately one third of the distance between the front and rear walls. It will. be seen from Fig. 6 that the lateral gas pass 199 is of somewhat lesser width than the distance between

arches

184, 194.

The part 208 of the furnace chamber below the

arches

184, 194 is considerably enlarged in asideward direction, and is provided with a hopper-like bottom 209 terminating in a throat 210 for the downward passage of solidified ash into an ash collecting compartment (not shown). The roof 211 and the four walls of the furnace chamber are lined with fluid heating tubes connected into the fluid circulating system of the unit. Thus tubes 212 connected at their lower ends to an inlet header 213 line the front wall 182 and terminate at their upper ends in a drum 214, while tubes 215 connected at their lower ends to an inlet header 216 line rear wall 198. The upper ends 217 of tubes 215 adjacent the sides of the rear wall 198 are continued vertically upwards from the forward edge of the arch 201, and,-above the roof 211, are connected to the drum 214. The upper ends 218 of alternate tubes 215 throughout the remainder of the width of the rear wall 198 are continued upwardly across a lateral gas outlet 219 constituting the entry of the lateral gas path 199, and, above the roof 211, are also connected to drum 214. The upper ends 225 of the remainder of the tubes 215 are bent rearwardly to line the floor 226 of the latter gas pass 199, then upwardly across the rearward end of the gas pass to the roof 211, along which they extend in a forward direction as far as the front wall 182, beyond which they are curved upwardly and connected to the drum 214.

Theheaders

213 and 216 are connected intothe fluid circulation system of the unit by downcomers such as those indicated in part at 229, which downcomers are connected at their upper ends to the liquid space of the drum 214. The side wall 183 is lined with fluid heating tubes. 230 terminating at their upper and lower ends, respectively, in

headers

231 and 232 connected into the circulation system of the unit, certain of the tubes, 230, being continued vertically above the arch 184, respectively, in alignment with tube platens of the radiant superheater 186 (see Fig. 7). The opposite side wall 193 is similarly lined by fluid heating tubes 234 connected to upper and

lower headers

235 and 236, with certain of the tubes, 234', in alignment with tube platens of the radiant superheater 196.

in the lateral gas pass 199 is arranged a secondary convection superheater, and, at the rearward end of the pass, is arranged a gas turning chamber, and below that a gas pass containing a primary convection superheater and an economizer; the arrangement is similar to that described above with reference to Fig. 3, and the same numerals have therefore been used to designate corre spending parts. Since there are two radiant superheaters in the present embodiment, these have their inlet headers 245 connected in parallel to the drum 214 by pipes 246 and their outlet headers 247 connected to the header 149 positioned above the rearward end of the gas pass 199 by pipes 248.

In this embodiment, two sets of pulverized fuel burners, indicated by 250, 251, are respectively arranged to discharge through the lower faces of

side wall arches

184, 194 in a downward direction towards the hopper bottom 209.

During operation of the vapor generating and super heating unit, hot gases produced in the enlarged lower part 208 of the radiation or furnace chamber 130 rise upwardly into the upper part 200 of the chamber and thence pass into the gas pass 199 and to the gas pass 140. Owing to the provision of the

side wall arches

184, 194 and the size and location of. the lateral gas outlet, there is but little tendency for the hot gases to spread sidewardly into the

bays

185 and 197 above the

side wall arches

184, 194, respectively. The two

radiant superheaters

186, 196 are thus located in relatively quiescent zones of the furnace chamber and they are protected from direct contact with the stream of gas-entrained particles passing through the radiation or furnace chamber 180 by the provision of the series of fluid heating tubes (not forming parts of the superheater) aligned with the tube platens. Gases eddying in such quiescent zones will be substantially cooled by transfer of heat to the adjacent fluid heating tubes lining the walls of the radiation chamber.

Figs. 8 and 9 illustrate an alternative construction of radiant superheater which may be used in any of the three embodiments of the invention described above, but which is illustrated as applied to the embodiment of Fig. 1. Each of the superheater tubes comprises a pair of

upright tube lengths

261 and 262 connected at their upper ends by a return bend 263 and respectively joined at their lower ends to vapor outlet and

inlet headers

264 and 265 through which the tubes may be drained. The tube lengths 261 are arranged in a plane parallel to and spaced at short distance from the row of steam generating tubes 26 of the furnace front wall 5, while the tube lengths 262 are arranged in groups to form tube panels or platens 266 which extend normally to the front wall 5. As has been described with reference to Fig. l, steam generating tubes 45 associated wtih the front wall of the radiation chamber 12 are continued upwardly from the arch 31, one in alignment with each of the tube platens 266.

Fig. illustrates the provision of a tubular fluid heater cleaner element 270, arranged to discharge cleaning fluid onto the exposed surfaces of the tubes of superheater 271, suitably of the type described above with reference to Figs. 1 and 2. It will be seen that the tube platens 272 are in this arrangement spaced from the furnace front wall 5 in order to permit free movement of the cleaner element 270.

It will be seen that in each of the embodiments of the invention described above, each bay containing a radiant superheater is sectionalized by the tube platens and particles contained in the gases within any one section are intensively cooled by radiation to the adjacent tube panels and to the tubes in the adjacent wall, or, in the construction of Figs. 8 and 9, the tube lengths of the superheater arranged in a plane parallel to the adjacent wall.

While in accordance with the provisions of the statutes. I have illustrated and described herein the best form and mode of operation 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 my claims, and that certain features of my invention may sometimes be used to advantage without a corresponding use of other features.

What is claimed is:

1. In a vapor generating and superheating unit, means including vapor generating wall tubes defining a vertically elongated furnace, burner means firing the furnace, the furnace having a lateral gas outlet in its upper portion, the furnace wall opposite the gas outlet being recessed outwardly away from the main path of combustion elements through the furnace, a radiant superheater disposed within said recess, a lateral gas pass leading from the gas outlet, another gas pass receiving the furnace gases from the lateral gas pass, a primary convection superheater in said other gas pass, a drainable dog-leg secondary superheater including return bend tubes with upright parts across gas flow in the lateral gas pass and transversely related parts drainably disposed across gas fiow in said other gas pass, headers disposed at the ends of the transversely related parts, and inter-superheater tubular connections whereby vapor flows from the radiant superheater to the primary convection superheater and then to the secondary convection superheater.

2. In a vertically elongated vapor generating and superheating unit, means including vapor generating wall tubes defining a combustion chamber, fuel burning means at the lower part of the combustion chamber providing the combustion chamber with high temperature combustion products, means providing a gas outlet at the upper part of the combustion chamber, a wall construction having its upper part presenting a recess or pocket opening at one side toward the main flow of combustion products from the fuel burning means to said outlet, radiant superheater tubes associated as tubular platens dividing said recess into a plurality of sub-pockets opening at said one side toward the main flow of combustion products to said outlet, some of the vapor generating tubes having parts arranged in close formation and in wall alignment to constitute a tubular wall of each of the sub-pockets at the side opposite said one side of said pocket.

3. The combination of claim 2 further characterized by vapor generating tubes each aligned with a platen and disposed between the platen and the main flow of combustion gases.

4. The combination of claim 3 further characterized by a downfiow gas pass beyond said gas outlet, a lateral gas pass connecting said gas outlet and the inlet of the downfiow gas pass, both of said gas passes being disposed at the side of the combustion chamber opposite the radiant superheater, and a convection superheater having upright tube lengths disposed across gas flow in the lateral gas pass and having other tube lengths disposed horizontally across downfiow gases in the downfiow gas pass, means for connecting the radiant superheater and the convection superheater in series as to vapor flow, and a header connected to the other tube lengths whereby the convection superheater may be drained.

5. The combination of claim 2 further characterized by said tubular platens being formed by corresponding legs of return bend tubes, and associated tube platens pre sented by the other legs of said return bend tubes, the associated tube platens extending transversely relative to said tubular platens.

6. The combination of claim 2 further characterized by the combustion chamber wall portion forming said recess being disposed opposite said gas outlet.

7. The combination of claim 2 further characterized by said tubular platens being formed by corresponding legs of U-shaped superheater tubes, with the mid-portions of the U-tubes aligned to form roofs of the sub-pockets and the remaining legs of the U-tubes aligned in wall formation at the side opposite said one side of each of the sub-pockets and between said tubular platens and said vapor generating tubes arranged in wall alignment.

8. In a vapor generating and superheating unit, a vertically elongated furnace with the lower parts of its upright walls including upright and closely arranged vapor generating tubes, fuel burning means for firing the furnace at its lower part, means forming a furnace gas outlet leading laterally from the upper part of the furnace, one of the walls of the furnace having its upper part so constructed and arranged as to provide a recess at one side of the main path of gas flow from the fuel burning means to said outlet, a radiantly heated vapor superheater in said recess including upright vapor heating tubes arranged in widely spaced upright platens with a plurality of tube sections in each platen aligned in a plane transverse to the furnace face of the recessed wall, the platens being horizontally spaced apart at least several tube diameters, and sections of vapor generating tubes with each section aligned with the tubes of a platen and disposed between the platen and the main flow of combustion gases.

9. The combination of claim 8 further characterized by a furnace wall opposite said one wall having its upper part so constructed and arranged as to provide a recess at the side opposite said one side of the main path of gas flow from the fuel burning means to said outlet, and a radiantly heated vapor superheater in said last named recess.

10. The combination of claim 8 further characterized by an inclined baflie formed by one of the furnace walls and its tubes and extending into the furnace at a position below the radiant heater.

11. The combination of claim 8 further characterized by an arrangement of the tubes of the radiant heater as a plurality of tube platens distributed in widely spaced relationship across the back wall of the recess, each platen consisting of a plurality of tubes in wall alignment in a plane transverse to the plane of said back wall.

12. The combination of claim 8 further characterized by the lining of the recess by the tubes of the radiant superheater.

13. In a high pressure vapor generating and vapor superheating unit, a furnace having vapor generating wall tubes in which substantially all of the vapor of the unit is generated, fuel burning means normally providing high temperature gases for passage through the furnace, a radiant superheater including tubes receiving the generated vapor, the superheater including contiguous tubes arranged in a plurality of groups with the tubes of each group arranged in a row as a platen, the platens projecting into the furnace from a wall thereof and so widely spaced across that wall that successive platens combine with the intervening furnace wall portions to define pockets in which the heat received by the pocket defining tubes is predominantly or substantially all radiantly transmitted heat, said pockets being open toward the interior of the furnace, and a plurality of more than four furnace wall tubes arranged along the third or middle wall of each pocket.

References Cited in the file of this patent UNITED STATES PATENTS 1,747,011 Kerr Feb. 11, 1930 1,782,096 Keenan, Jr. et al Nov. 18, 1930 1,826,029 Smith Oct. 6, 1931 2,213,185 Arrnacost Sept. 3, 1940 2,599,316 Boyer June 3, 1952 FOREIGN PATENTS 13,646 Switzerland Dec. 21, 1896 212,686 Great Britain Mar. 20, 1924 373,860 Great Britain June 2, 1932