US2942587A - Vapor generator - Google Patents

Description

June 28, 1960 j ass-mm "2,942,587 I 'f B Q B I Filed on. 25, 1957 {sh ets-sheet x INVENTOR. F G. 1 Charks 5.192112% ATTORNEY c. s. SMITH VAPORGENERATOR Jupe 28, 1960 Filed Oct. 25, 1957 3 Sheets-Sheet 2 INVENTOR. Czar/es Jls zzzz'i'fi ATTORNEY June '28, 1960 j 3 I VAPOR GENERATOR 1 .7 Filed 0th 25;1957- s Sheets-Sheets H 5 v INVENTOR.

C/zar/es S Saw/H1 OLA/pp ATTORNEY V superheater arrangement whereby United States O VAPOR GENERATOR Charles Smith, Westfield, N.J., assignor to TheBabcock & Wilcox Company, New York, N.Y., a corporation of New Jersey t Filed Oct. 25, 1957, Ser. No. 692,302

7'Claims. (Cl. 122--47s This invention relates generally to a vaporv generator including means for highly superheating the vapor produced therein. More particularly the invention is concerned with'a novel arrangement and construction of a vapor or steam superheater andreheater and their relationship to each other and to other components of the steam generator for producing superheated steam at substantially constant high temperature under conditions such prime movers is generallyproduced as a result of radiant absorption by a steam generator furnace wall tubes of heat given up by products of combustion within the furnace; the steam generated thereby being separated from the saturated water combined therewith as it issues from the wall tubes and the separated steam subsequently being superheated to a temperature in the order of 1050 F. or higher. Oftentimes economy dictatesthat steam which had been expanded in a prime mover be also reheated to substantially the initial superheat temperature and thereafter returned to the prime mover or turbine.

To attain superheat steam temperatures of the order required, it is sometimes necessary to position a portion of the superheater so as it will be exposed totfurnace radiation. However, this involves certain difficulties with reference to heat transfer rates and superheater metal temperatures. These difiiculties are particularly aggravated in furnaces fired by slag forming fuels operating at temperatures above the fusion temperatures of the which has passed through a prime mover is p'royidedfor incombustible particles in such fuels, since these particles at elevated temperatures tend to remain in a molten sticky condition which causes them to adhere to the radiant heat absorbing surfaces of the superheater and accumulate thereon.

An object of this invention is to effectively obtain high superheat temperatures by uniquely positioning a portion of the vapor superheating surfaces in a high temperature zone of a furnace in such a manner as to minimize attenu ating slagging tendencies and excessive superheater tube metal I temperatures. 7

Another object. of the invention is to provide a novel the desired high superheat, steam temperature is maintained substantially constant throughout widely varying load conditions.

These andother allied objects and advantages are obtained in a steam generator arrangement which generally comprises a Water cooled furnace section having a connected associated gas pass in communication with a steam cooled convection section. A plurality of vertically spaced furnace chamber is then separated from the saturated residual water issuing from the'furnace wall tubes in a separating drum positioned above the furnace.

According to this invention the saturated steam upon leaving the separating drum passes first through the wall tubes of the convection section where it becomes slightly superheated. This slightly superheated or dry steam then passes through a radiant section of a' primary superheater disposed in the high temperature zone of the furnace which is exposed to the radiant heat of the products of combustion flowing upwardly. through the furnace. Steam superheated in this radiantsection is then serially passed to a convection section of a primary superheater located in the relatively low gas temperature zone within the convection section where the steam is heated to an intermediate temperature. The intermediate heated steam is then finally heated by flowing thesame through a secondary superheater disposed in the gas pass which is adjacent the gas exit of the furnace. Reheating of steam by positioning a reheater for receiving the] same between theradiant priniarysection of the superheater and the secondary superheater in the vicinity ofthe furnace exit and the gas passentrance. q Accordingto this invention slag accumulation on the radiant superheater surfaces is minimized by flowing relatively cool steam therethrough and by arranging the superheating surfaces as. depending, vertically extending,

tube platens which comprise a plurality of return bend i.

tubes in which the laterally extending tube portions at the bends is provided with a minimum'radius. With this platen arrangement a minimum plane of radiant super- 1 heater surface is exposed transversely of the gas flow,

the transversely extending surfaces being the surfacesupon which the major portion of the slag generally tends to accumulate thereon due to the direct impingement of the entrained slag particles in the gas flow.

According to this invention superheat and reheat control under widely varying load conditions is attained by burner selection and manipulation and by regulably controlling the amount of gas flow over the convection section of the superheater, the latter being accomplished by a damper controlled bypass in the convection section.

A feature of this invention resides in the provision of initially flowing relatively cool, dry steam through radiant primary heat absorbing tubes exposed to the high temperature gases in the furnace.

Another feature of this invention resides in the posi- I accumulations of slag thereon and for maintaining the" metal temperatures thereof within tolerablelimits,

Other features and-advantages will be ,readily appare ent when consideredin view vof thedrawings and the descriptions thereof in which:

-Fig. 1 is a diagrammatic vertical section view of a vapor generating and superheating unit according tothis invention.

Fig.2 is a half plan section taken along line 2-2 of Fig. 1. r

Fig. 3 is a detailed side viewof a radiant superheater arrangement according to this invention.

Fig. 4 is a front view of a radiant superheater arrangement of Fig. 3.

Fig. 5 is a section view taken along line 5-5 of Fig. 3.

The illustrated vapor generating and superheater .unit comprises a steam generator intended for central station use having agenerating capacity of approximately 1,700,- 000 lbs. of steam per hr. with a total steam temperature at the superheater outlet in the order of 1050 Eat-2425 lbs. psi. Included also is a steam reheater for 0htaining a reheat steam temperature substantially of the same order as the high pressure steam temperature.

Referring to Fig. 1, the unit comprises anelongated furnace chamber having a laterally extending gas pass 11 adjacent the upper end thereof and a downwardly extending convection section 12 connected thereto. The furnace 10 is preferably of rectangular cross-section defined'by walls .13, 14 and 15 of steam generating tubes.

The front wall 13 includes a row of tubes 16 extending upwardly from a water supply drum 17 to an upper front wall header 18; the rear wall 15 includes a row of tubes 19 extending from water supply drum 20 to upper rear wall header 21, the lower portions of the tubes 16 and 19 in the front and rear wall being inwardly and downwardly inclined to form ahopper bottom 22 terminating in a slag opening 23. The side walls 14 each in; clude a row of tubes 24 extending between lowerwall headers 25, 26, 27 and upper side wall header 28. Upper headers 18, '21 and 28 in turn are connected to a steam and water separating drum .29 by suitable .conduits 3t), 31 and 32 respectively, through which steam generated in the furnace walls and residual 'water mixed therewith is discharged into the steam d1um29.

Aportion .of the rear walltubes 19A immediately below the laterally disposed furnace gas exit 33 arcinwardly bent to 'form a nose arch 34 in which the upper inclined portion 34A of the arch in effect constitutes an extension of the horizontally extending gas pass floor 35. Above the arch the tube portions 19A extend vertically upward, connecting into header 21. These tube extensions together with the vertically extending tube portions 19 of the rear wall form a screen 36 disposed as spaced platens (see Fig. 2) to provide openingstherebetween for the passage of the .gas from the furnace 10 to the lat? eral gas pass '11. A portion of the side wall tubes 24A in the vicinity of the arch 34 extend laterally and connect into a header 37 from which extend vertical tubes 38 to line the side walls of the gas pass 11.

The generatingunit shown is of the natural circulating type with feed waterb'eing supplied to the upper drum 29. Downcomers 39 and 48 interconnect drum 29 with the lower supply drums '17 and 20 to supply the Wall tubes 16, 19. The lower side wall headers 25, 26 and 27 and associated wall tubes 24 are also supplied from drums 17 and 28 by suitable conduits not shown.

Suitable burners 41 forfiring the unit are vertically disposed in the lower portion of the front wall 13. The hot gaseous products of combustion therefrom pass upwardly through the furnacecharnber 18, thence through the lateral gas pass 1'1 and finally downward through the convection section 12.

According to this invention the peripheral walls 14A, 42 and 43 of the convection section '12 form a 'f bi rd cage" i.e. are lined with steam cooling tubes. Saturated steam is delivered from the steam drum 29 through steam conduits 44 to a distributing header 45 disposed above and adjacent the gas exit 46 of the lateral 'gas pass .11. A row 47 of steam cooled tubes line wall --2 of the convection pass and connects the distributing header 45 to a ring header 48 disposed adjacent'the bottom of the convection .pass 12. A second row 49 of steam tubes, extending from header 45 and along the roof 5t? and opposite Wall 43 of the convection pass, connects into an opposite leg 48A -of the ring header 48. Dividing the convection pass 12 into a main gas pass 51 and a gas by-pass 52 is a third row 53 of steam tubes, spaced from Wall tubes 47 and extending from the distributing header 45 to the ring header 48. The convection section side wall tubes 54 connect the ring header 48 to upper con vection pass side wall headers 55, the steam going from the distributing header 45 down through tube rows 47, 49 and 53 to the ring header 48 and discharges therefrom into the side Wall tube rows 54.

A metallic wall 56 constituting an extension of the division wall tube row 53, divides the lower portion of the convection section 12 and terminates at dampers 57 and 58 which proportion the gases flowing through the respective gas passes as will be hereinafter described.

Steam from the side wall headers is delivered through conduit 59 to a header 68 disposed below the floor 35 of the gas pass 11. A row 61 of tubes connected into header 60 extend upwardly and rearwardly along the gas pass floor 35. Adjacent the exit of the gas pass, the floor tubes 61 extend vertically, and together with the upper portion of tube row 47, form a screen 62 across the exit 46 of the gas pass. Continuation of tubes 61 are'extended along the furnace roof 63 and connect with the uperheating tube banks.

According to this invention the superheating portion or tube banks of the unit comprises a radiant primary superheater section 64 disposed in the upper forward zone of high temperature of the furnace, a second portion 65 of the primary superheater disposed in the relatively cool temperature zone of the main gas pass 51 of the convection section and a secondary superheater 66 including a bank of tubes vertically disposed in the intermediate temperature zone of the lateral gas pass between the screens 36 and 62.

Referring to Figs. 1 to 4 the radiant superheater 64 comprises a plurality of relatively wide spaced tube banks or platens 67 transversing the width of the furnace. Each platen 67 includes an inlet header 68 positioned adjacent the front wall 13 and normal thereto and an outlet header 68A spaced therefrom and in alignment therewith. Spaced from and substantially coextensive with the aligned inlet and outlet headers 68, 68A is an intermediate header 69. Connecting the inlet and outlet headers 68, 68A to the intermediate header 69 are a plurality of depending return bend tubes 70, the tube portions 76C at the bend thereof being provided with a relatively small return bend radius, which e.g. is in the order of a radius to tube diameter ratio of 1 to 2 or the like. With this construction the amount of tube surface laterally exposed to direct impingement of the gases is reduced to a minimum and the possibility of slag build up thereon materially lessened.

As best seen in Figs. 3 and 4 a group 70A of these tubes connects the inlet header 68 with a portion of the intermediate header 69 and a second tube group 703 connects another portion of header .69 to the outlet header 68A. With this construction relatively cool steam delivered by roof tubes 61 by suitable conduits to the inlet header 68 of each platen 67 first flows from .the inlet header 68 through tube groups 70A to the intermediate header 69. The steam then flows longitudinally in header 69 and enters tube group 703 from whence it enters outlet header 68A. In the event of any unbalanced steam heating in the radiant superheater platens 67, the flowing of the steam longitudinally within header as it exits from tubes 70A enables some steam mixing to occur therein and the steam discharging therefrom will tend to approach an average ternperature. U

:Serially connected to the radiant superheater section 64 by means of conduits 72 and receiving .the steam heated therein is the convection primary .superheatersec tion-65 disposed in the-main'gas pass 51 of the convection section 12., As shown the convection superheater section 65 comprises vertically spaced banks .of multiple-loop horizontally disposed tubes. The radiantly heated steam from the platen'section 64 enters inlet headei 73, flows through section 65 and discharges therefrom at the outlet header 74,the steam at this point being'heated'to an' inter-' disposed in .the upper portion of the furnace .between the radiant superheater 64 and screen 36 in a zone where the B.t.u. absorption per lb. of steam flowing therethrough is maintained substantially constant throughout the load range of. the unit. regardless of the amount of excess air admitted into the furnace. As shown, the radiant reheater comprises serially connected spaced banks 80Zand 81 of multiple loop vertically disposed tubes. Each bank includes a plurality. of transversely spaced nested tube platens, theplatens of bank 80 being' spaced on widercenters than those in banks 81, see Fig.2. Steam to be reheated enters the reheater through an inletheader 82 andris discharged therefrom through header 83. ,The economizcr section includes serially connedted banks 84 and 85 of multiple-loop, horizontally arranged tubes;disposedrespectively in the main gas' pass: and bypass of the convection section12. Since constant steam temperature is the desirable operating' datum to be achieved,superheat andreheat' control according to this invention is attained by selective firing of burners and by gas bypassingaround the convection appreciated that the invention is not to' be taken ited to all of the details thereof as modifications and varispirit or scopeof theinvention.

ations thereof may be made without departing from the What'is'claimed isz" x V p 1. A vapor generating and heating unit comprising enclosing walls defining a setting having a radiant furnace chamber of uniform cross-sectional area and a lateral gas outlet in the upper portion of said setting, a convection section having a lateral gas inlet in the upper portion thereof, a laterally extending gas pass connecting said outlet with said inlet, means dividing said convection section intoa main gas pass and a gas by-pass,'a vapor drum positioned above said furnace chamber, a plurality of vapor generating tubes disposed in the'walls of said furnace chamber and connecting with said drum, means for burning fuel in thelower portion of said furnace chamber resulting in slagforming particles being carried in suspension in'acombustion gas flow path having an unobstructed longitudinal flow upwardly through said radiant furnace chamber, said gas thence flowing laterally through said lateral gas pass and downwardly through said connected convection section, damper means for separatelyproportioning the flowjof gas through said main gas pass. and by-pass, apluralityj of vapor heating tubes "disposed in the Walls of said convestion section,

said vapor heating tubes being connected for series vapor flow from said drum, a primary radiant vapor heater connected for series vapor flow from said vapor heating tubes, said primary radiant vapor heater including a plurality. of transversely spaced tube platens disposed in the. upper portion of said furnace in direct exposure to Y the radiant heat and slag forming particles in said furnace section 65 of the superheater. The arrangement is, such that at full loadwith maximum gas flowthrough the unit, the dampers are set so that the gas bypass52 is handling 20%of the total gas weightflowing through the convection section- 12 while at 40% loadthe bypass 52 is handling appromiately 5 %-,of the gas. Thus at lowloads the gas massflow over the convection section 65 is increased and as'load is increased the gas'mass flow thereoveris proportionately decreased. The combination of radiant superheater, which has a falling characteristic i.e. steam temperature drops as steam flow increases and convection superheater which has a rising characteristic i.e. steam temperature and steam flow increases with in creased firing rates when used in conjunction with controlled gas flow over the convection section of the superto the gas flow. Thus as the tube metal temperatures of the platens is maintained'relatively cool by the cold steam flowing therethrough, any of the slag particles impinging thereon will tend to become rapidly cooled thereby, causing them to become solidified and thereby rendering the .same incapable of sticking to the exposed surfaces of the superheater. p i p It will be further noted that the steam delivered to the radiant superheater is in a dry state, the vapor dryness resulting by flowing the saturated steam from the steam I drum through the fbirdrcage,which is in heat transfer relationship with the gases flowing through the convection section. Thus, any moisture in the steam is evapo- .rated'in the bird cage and any residual solids carried thereby deposited in the low temperature gas zone.

I While the instant invention has been disclosed with reference to a particular embodiment thereof, it is to be chamber and unshielded by any furnace structure, each of said tube platens-includes a plurality of vapor flow passes, each ofsaid passes being formed of a plurality of dependently supported side-by-side U-shaped tubes having closely spaced co-planar inlet and outlet leg portions connected by; a return bend, each of said leg portions having an upper portion and alower portion, said lower portiondepending into a zone of said 'furnace chamber wherein the gases flowing therethrough are at. a temperature above the fusion temperature of the gas en-' trained particles whereby the gas flow over said lower .potiron is 'in a direction parallel to the longitudinal axes of said superheated tubes, an inlet header, an out let header and a mixing header wherein said inlet and outletleg portions of one vapor pass serially connects the inlet header to said mixing header respectively .and said inlet and'outlet leg portions of another vapor pass serially connecting said mixing header to said outlet header respectively whereby the tubes of said radiant heater have only a relatively small amount of surface exposed to cross gas flow and slag impingement normal thereto, a primary convection vapor heater connected in series flow with said radiant heater, said primary convection heater being disposed in said main gas pass of said convection section, said dampers being arranged so as to increase the mass gas flow through said main gas pass and over said primary convection heater as load is decreased,"a secondary vapor heater connected in series tively low temperature vapor, an outlet header in alignmentwwith said. inlet header, an intermediate mixing header substantially co extensive and, disposed parallel with said aligned inlet and outlet headers, a plurality of radiantly heated vapor flow passes serially connecting said headers for vapor flow therethrough, eachof said as lini passes being adapted to be exposed to heating gases of substantially uniform temperature and including a plurality of depending U-shaped tubes each having closely spaced inlet and outlet leg portions connected at the lower ends thereof by a return bend, said tubes being disposed in side-by-side co-planar relationship to form atube platen wherein the inlet and outlet leg portions of one pass serially connect the inlet header to the mixing header, respectively, and the inlet and outlet leg portions respectively of another vapor pass serially connect the mixing header to said outlet header, each of said headers having its longitudinal axis lying in a plane parallel to the plane of said tubes.

3. A vapor generator comprising enclosing walls defining -a setting having a vertically elongated radiant furnace chamber of uniform cross-section, a liquid-vapor separating drum positioned at the top of said setting, a plurality of vapor generating tubesdisposed in the walls of said furnace chamber andconnected in communication with said drum, means for burning fuel in the lower portion of said furnace chamber resulting in slag forming particles being carried in suspension in combustion gases whereby said combustion gases have an unobstructed longitudinal flow path upwardly through said radiant furnace chamber, a radiant primary superheater arranged in the upper portion of said setting, said superheater including a plurality of dependingly supported U-shaped tubes having an upper portion and a lower portion, said lower portion depending into a zone of said furnace chamber wherein the gases flowing therethrough are at a temperature above fusion temperature of'the entrained particles therein, each of said superheater tubes having a downflow leg portion'and a closelyspaced upflow leg portion connected by a return bend at the lower ends thereof so that the gas flow over the lower portion of said superheater tubes is in a direction substantially parallel to the longitudinal axes of said superheater tubes, and each of said superheater tubes ibeingdirectlyexposed to both the radiation of the flame of said fuel burning means and the combustion gases containing entrained slag forming particles whereby only a minimum amount of the surface area of said' lower portion of the superheater tubes is subjected to cross-gas flow thereover, whereby said superheater tubes are unshielded by furnacestructure from the effects of slagging and of direct radiation of said 'flame and combustion gases.

4. A vapor generator comprising enclosing walls defining a setting having a vertically elongated radiant furnace chamber of uniform cross section, a liquid-vapor separating drum positioned at the top of said setting, a plurality of vapor generating tubes disposed in the walls of said furnace chamber and connecting with said drum, means for burning fuel in the lower portion of said chamber resulting in slag forming particles being carried in suspension in combustion gases whereby said combustion gases have an unobstructed longitudinal flow path upwardly through said radiant furnace chamber, a primary radiant superheater arranged in the upper portion of said setting, said superheater including a plurality of dependingly supported U-shaped tubes having an upper portion and a'lower'portion, said lower portion depending into a zone of said chamber wherein the gases flowing therethrough are at a temperature higher than the fusion temperature of the slag forming particles entrained therein, each of said superheater tubes having a downflow leg portion and a closely spaced upflow leg portion connected by a return bend at the lower :ends thereof, said return bend .having a radius to tube diameter ratio of substantially-.1 to 2,;saiddepending U-shapedtubes being disposed in .sideby-:side, co-planar relationship so that the gas flow over the lower portion of said superheater tubes is in a direction parallel to 'the longitudinal axes of said superheater tubes, and each of said superheater tubes being directly exposed to both the :radiation of the flame of said fuel burning means and the combustion gases containing the entrained slag forming particles so that only a minimum amount-of the surface area of said lower portion-of the superheater tubes is subjected to cross-gas flow thereov'er and to direct impingement of the gas entrained slag forming particles, and whereby none of the radiant heat of said furnace is shielded by furnace structure from acting directly on said exposed tubes.

SVA vapor generator comprising enclosing walls defining a setting including a vertically elongated radiant furnace chamber of uniform cross-section, a liquid-vapor separating drum positioned at the top of said setting, a plurality of vapor generating tubes disposed in the walls of saidfurnace chamber and connected in fluid circulation with said .drum, means for burning fuel in the lower portion of said chamber resulting in slag forming particles being carried in suspension in combustion gases whereby said combustion gases haveian unobstructed longitudinal flow path upwardly through said radiant furnace chamber, Ia primary'radiant superheater arranged in the upper p'ortionof said setting, said superheater including a plurality of dependingly supported U-shaped tubes having an upper portion and a lower portion, said lower portion depending into a zone of said chamber wherein the gases flowing therethrough are at a temperature above fusion temperature of the entrained particles therein, each of said superheater tubes having a downflow leg portion and a vclosely 'spaced'upflow leg :portionconnected by a return bend at the'lower ends thereof, said depending U-shaped tubesibeing disposed in side-by-side, co-planar relationship so that the gas flow over the'lower portion of said superheater tubes is in a direction parallel to the longitudinal axes of said superheater tubes, the upflow leg of one superheater tube being disposed adjacent the downflow leg of the next adjacent superheater tube, and-each of said superheater tubes being directly exposed to both the radiation of the flame of said fuel burning means and the combustion gases and to direct impingement of the gas entrained slag forming particles so that only a minimum amount of the surface area of said lower portion of the superheater tubes is subjected to cross-gas flow and to direct impingement of the gas entrained particles thereover whereby said superheater tubes being unshielded by furnace structure from the direct effects of slagging and furnace radiation.

6. A vapor generator having enclosing walls defining a setting including a vertically elongated radiant furnace chamber of uniform cross-section, a liquid-vapor separating drum positioned at the top of said setting, a plurality of'vapor generating tubes disposed in the walls of said furnace chamber and connecting with said drum in fluid circulation, means for burning fuel in the lower portion of said furnace chamber resulting in slag forming particles being carried in suspension in a combustion gas whereby said combustion gases have an unobstructed longitudinal flow path upwardly through the furnace chamber, a plurality of radiant primary superheater platen elements transversely spaced in the upper portion of said setting, eachof said platen elements including a first and second pass of dependingly supported return bend tubes having inlet and outlet leg portions connected by a return bendhaving a radius to tube diameter ratio of substantia ly l to 2 with said leg portions of each element lying in a common plane, each of said leg portions having an upper portion and a lower portion, said lower portion depending into a zone of said chamber wherein the gases flowing therethrough are at a temperature above the fusiontemperature of the entrained particles therein, and said .gas flow over the lower portion'of said superheater tubes being in a direction parallel with the longitudinal axes of said tubes, each of said superheater tubes being 'directly exposed to both the-radiation of the flame of said fuel burning means and the combustion gases containing the entrained slag forming particles, the arrangement being such that-only aminimum amount of the surface ea of saidlower portion of the superheater tubes is subjected to cross-gas flow and to direct impingement of the gas entrained particles thereover whereby said exposed surface area of said tubes is unshielded by furnace structure from the direct radiation effects of said flame and combustion gases, a common mixing header serially connecting the outlet leg portion of said first tube pass with the inlet leg portion of said second tube pass whereby any unbalanced heating of the vapor flowing in said first tube pass tends to be averaged out in said mixing header to thereby minimize any unbalanced heating occurring in said second tube pass, said header having its longitudinal axis extending in a plane parallel to the platen of said tubes whereby the relative position of said primary r-adiant superheater tubes to said gas flow and to the vapor flow therein cooperate to maintain said primary superheater substantially free of external slag accumulations while maintaining the tube temperatures of said radiant heater within tolerable temperature limits.

7. A vapor generator having a front, rear and connecting side walls defining a setting including a vertically elongated radiant furnace chamber of uniform cross-section, a liquid-vapor separating drum positioned at the top of said setting, a plurality of vapor generating tubes disposed in the Walls of said furnace chamber and connected in fluid circulation with said drum, means for burning fuel in the lower portion of said furnace chamber resulting in slag forming particles being carried in suspension in combustion gases wherein said combustion gases have an unobstructed longitudinal flow path upwardly through said furnace chamber, a radiant primary superheater comprising a plurality of platen elements spaced transversely in the upper portion of said setting, each platen element including an inlet header for receiving relatively low temperature vapor, an outlet header in alignment with said inlet header, an intermediate mixing header substantially co-extensive with and disposed parallel to said aligned inlet and outlet headers, and a plurality of radiantly heated vapor flow passes serially connecting said headers for vapor flow therethrough, each of said passes including a plurality of dependently supported return bend tubes having an inlet and outlet leg portion connected by a return bend having a radius to tube diameter ratio of substantially 1 to 2 with said leg portions lying in a common plane, each of said leg portions having an upper 10 portion and a lower portion, said lower portion depending into a zone of said furnace chamber wherein the gases flowing therethrough are at a temperature higher than the fusion temperature of the particles entrained in said gases and the gas flow over the lower portion of said superheater tubes is in a direction parallel to the longitudinal axes of said superheater tubes, each of said superheater tubes being directly exposed to both the radiation of the flame of said fuel burning means and the combustion gases containing the entrained slag forming particles, the arrangement being such that only a minimum amount of the surface area of the said lower portion of the superheater tubes is subjected to cross-gas flow and direct impingement of the gas entrained particles whereby said exposed surface area is unshielded by any furnace structure from the direct radiation effects of said flame and combustion gases, the inlet and outlet leg portions of the first superheater tube pass serially connects the inlet header to the mixing header respectively, and the inlet and outlet leg portions of the second tube pass serially connects the mixing header to said outlet header respectively so that any unbalanced vapor heating occurring in said first pass tends to be averaged out in said mixing header to thereby minimize any unbalanced heating occurring in said second tube pass, each of said headers having its longitudinal axis parallel to the plane of said tube platen whereby the relative position of said primary radiant superheater tubes with respect to said gas fiow and to the vapor flow therein cooperate to maintain said primary superheater substantially free of external slag accumulation thereon, while maintaining the tube temperatures of said radiant superheater within tolerably high temperature limits.

References Cited in the file of this patent UNITED STATES PATENTS 2,063,441 Kerr Dec. 8, 1936 2,685,279 Caracristi Aug. 3, 1954 2,798,464 Seidl July 9, 1957 2,835,479 Guarraia May 20, 1958 FOREIGN PATENTS 724,934 Great Britain Feb. 23, 1955 739,489 Great Britain Nov. 2, 1955

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