US2404826A - Heat transfer apparatus and method of manufacture - Google Patents

Description

July 30, 1946. g ss 2,404,826

HEAT TRANSFER APPARATUS AND METHOD OF MANUFACTURE Filed June 3,1942- /ll///////////!////////// I 1N VENTOR.

Perry Cassz'dy i ATTORNEY.

Patented July 30, 1946 HEAT TRANSFER APPARATUS AND METHOD OF MANUFACTURE lerry R. Cassidy, Darien, Connassignor to The- Babccck & Wilcox Company, Jersey City, No.1

T T OFFICE aucorporation of New J ersey Application June 3, 1942, Serial No. 445,588

11 Claims;

The invention" herein disclosed relates to heat' exchang apparatus particularly adapted to the transfer of heat between fluids through the wall of a metallic tubular element.

When heat is to be transferred from a fluidoutside of a tube to a fluid inside of a tube, or vice versa, there is a resistance to the flow of heat at the boundary of each fluid and in the material of the tube, the cumulative efiect of these thermal resistances determining the heat transfer rate fora given temperature excess of one fiuid over the other. In manycascs of practical importance, heat is transferred from a gaseous fluid outside of the tube to a liquid within the tube, the liquid remaining as such or boiling to a vapor; and under these conditions, the boundary of gas outside of the tube affords a much greater resistance to heat flow than the tube metal and-insidefiuid boundary combined, andacts to control and limit the rate of heat transfer.

Therefore, when the controlling thermal resistance isin the boundary of a gas outside of a tube, and it is desiredto increase the rate of heat transfer, it is common practice to increase the external surface area of the tube, and thus the gas boundary area, by providing metallicextensions on the outside of the tube the form of fins, studs or pins; the manner of constructing such extended surface units generally involving the attachment of a solid piece of metal to the tube by means of a pressure joint or bythe continuous metallic juncture such as results from soldering, brazing or welding.

When fluid carrying tubes are used in the walls' ers for the desired ligament strength.- The spaces between such wall tubes generally been closed by metallic extensions on individual tubes, with or without the addition of refractory material,,to provide an exposed surface bridging the space between tubes, from which surface ,heat is conducted through the, extension, met-alto the metal of the tube and thence to the fluid within the tube.

A purpose of the present invention is toprovide a heat exchange unit especially adapted to the transfer of heatbetw-cen fluids of different, temperatures, one fluid being carriedwithin a: metallic tubular memherand the exterior t r-- face of the member being extended to provide increased area of exposure to the other fluid. Certain particular embodiments of the invention are contemplated whereby a plurality of suchunits may be disposed in the path' of a moving fluid, either gaseous or liquid, witlrthe entireextension surface swept by the movingfluidat minimum resistance to the flow 'ofsuch fluid; Other *embodiments utilizing difierentzforms of extensions 7 may conveniently be arranged to form an inrpervious' wall structure; associated'with'a furnace for example, where ordinarily onlyaportion of the extension surface is exposed directly to'a source of heat.

For "any condition of service; the arrangement of the extension is such that-heat received 'atits; exposed outer surface, forexample, is conducted through the material of the extension to its juncture with the tubular memberwithout excessive temperature differential betweenthe extreme outermost portion of the extension and its innermost portion -or root: This involves suitable attachment of 'the'extension to the tubular base portion to provide thermal contact of sufiicient' area and conductivity to avoid'overheating the part farthest from-the. tube, th attachment being also made'in such manner as to'prevent excessive stresses-being setup in the 'tube due to differential thermal expansion and contraction of themetal of the tube and'that of the'extension.

In the furtherance of these objects'itis' proposed to provide a composite form of extension wherein different materialscare combined, bothof which are conductors otheat'butusuallyin difierent degrees, with one morereiractory-to heat and serving as ashieldbrjacketforthe other.

In most instances, it'will probably be found most suitable to form th outer shield Qr'jacket of relatively thin metal; the selection ofthe'kind of metal being determ'inedinpart by-the character of service for which the heat "exchange unit is intended, and in partby its :cost: Accordingly it is desirable to form thesheath or'jacket of a temperatures insideandoutside ofthe tubular member as affecting th direction of *heat flow. As a heat transfer medium it is proposed to use a material diiferentfrom that of th sheet metal q jacket, selected for ease or economy of applica- 3 tion, and of suitable heat carrying capacity for the temperature conditions under which it must function in service.

The filler material may be solid at ordinary temperatures and remain solid during operation at higher temperatures, metallic or non-metallic, and may be shaped to fit the jacket and the tube before the jacket is attached to the tube, or it may be molten and poured into the jacket space after the jacket is attached to the tube. It may 4 pansion to the same end. To avoid excess stresses without compensating coefiicients of expansion the sheet metal jacket may be so formed as to yield under stresses within the elastic limit.

When a liquid is used as the filler and it remains liquid, or when a solid is used that becomes liquid, in operation, the expansion and contraction conditions may be met, by forming the sheet metal jacket so that it yields to changes have metallic fusion or alloy attachment'to'the tube water walls, the copper being fitted as a solid block or poured molten.

The filler material may be solid at ordinary temperatures and become molten during operation at higher temperatures, and metallic such as lead, or non-metallic, organic or inorganic, including diphenyl and such substances known as bath salts for immersion heat treatment of metals to and at a given high temperature.

vThe filler materialmay be liquid at ordinary temperatures and remain liquid during operation at higher or lower temperatures, such as mercury for example, or an oil, or for very low temperatures abrine.

The filler material may be liquid at ordinary temperatures and freeze to a solid during operatio'n'at lower temperatures or boil to a vapor during' operation at higher temperatures.

' The filler material may also be composite, for examplaa liquid in which solid particles are submerged, the liquid being any one of the liquid media heretofore described and the solids having generally a'higher thermal conductivity as in the case of metals, especially copper and aluminum, and preferably being in such form as to provide a compact mass with relatively little liquid'betweein the solid particleswhen the jacket space is filled. Solids for this purpose include metal balls; or metals in granular form such as filings or cutting tool chi s, or non-granular particles of metals that cutting tools do notchip. Such metallic or even non-metallic solid particles of "greater conductivity than that of the liquid around'them increase the heat carrying capacity of the .iacket filler over that of liquid alone. It is less than for a filling of the same metal alone but cheaper and easier to apply.

The difference in the materials of the sheet metal jacket and its filler, results in differences in ph sical properties includingthat of thermal expansion, and in addition, their relative posi tions result in'difi'ere'nces intemperature, mean and" actual local values. My construction makes it ossible to avoid damage due to such differences and in s me cases to advantageously utilize the difierences.

When the filler is a material that remains solid in operation, and the heat flow is inward, the filler material may be selected to have a sufficiently higher coefficient of thermal expansion than that of the jacket so that in spite of a lower mean tem erature. it will expand more than the iacket and insure close contact between the filler and tube on one side and between the filler and jacket elsewhere for good thermal conduction and without imposing excess stresses. On the contrary, when the heat flow is outward, and the mean temperature of the filler is higher than that'of the jacket, .a different material may be used with the same or a lower coeffici nt of exin the volume of the filler while keeping its metal stress within the elastic limit. Also, a space may be left unfilled above the liquid to reduce the maximum pressure exerted by the filler at its highest temperature excess over that of the jacket, when thermal conditions are such as will not overheat the jacket around the unfilled space.

With a filler that is liquid in operation the heat transfer between the tube and the jacket is not limited to pure conduction as is the case of a solid filler, or packed solid particles submerged in a liquid, because convection circulation is set up in the liquid causing upward flow next to the hotter metal face and downward flow next to the colder one, to carry the heat in the moving liquid from the hotter to the colder face. The jacket may also be formed so as tobe flexible to limit internal pressures.

A filler material that vaporizes duringoperation, a liquid that boils, or a solid that sublimes, also conducts heat from a hotter to a colder face of its enclosure, the vapor forming on the hotter face and condensing on the colder face. A space above the liquid may preferably be left in this case to limit maximum pressures and for the same purpose the jacket form may be a yielding one.

Those skilled in the art are referred to the accompanying drawing for a more detailed dis closure of the manner in which the invention tion, thus facilitating manufacture of the apparatus and contributing to low cost of production either in small or large quantities. The method of their manufacture is also pertinent to the installation and/or replacement of such apparatus in the field, the latter being a factor of considerable importance in reducing the time required for restoring normal operating conditions, particularly when the installation is remote from the place oforiginal manufacture.

The accompanying drawing, and the struc' tures shown therein, may be briefly described as follows:

Fig. 1, in sectional elevation, shows a steam boiler unit embodying certain features of the invention; 7

Fig. 2 is a fragmentary transverse section along line 22, for example, of Fig. 1;

Fig. 3 is a front elevation of parts shown in Fig. 2;

FigsJl and 5 are fragmentary transverse sections similar to Fig. 2 showing modifications;

Fig. 6 is a fragmentary transverse section along line 6-6, for example, utilizing parts shown in Fig.2; and r Fig. '7 shows an additional modified form of the invention, in section along line 1-1, for example, of Fig. 1.

In detail, the boiler unit of Fig. 1 includes a omizer M and boiler? section l6;

combustion'chambersA repi-esented as bein'g fired with pulverized fuel burners B and defined by wallscC, each or which is illustrative of an :embodirnent'utilizing fiu-i'd cooling elements constructed and arranged as more specifically described hereinafter. The hot-gaseous products of combustion resulting from the combustion of fuel within the chamber A are discharged through the opening D 'formed between'spa'ced fiuid'cooling elements 'oi'the inner wall C which partitions the combustion chamber A from the upflow open gas pass E, the gases continuing in their flow through the opening '1" abovev the upperendiof avsecond fluid cooled partition wall G downwardly through a second open gas pass Hidisposed :between the fluid cooled walls Gand Kc Gases leaving the lower portionof pass. H areLdirected' across a row of spaced tubes in and thence upwardly through the gas passage M across other rows of spaced fluid heating tubes forming: for example, the superheater -l2, .econ- In the arrangement shown, all of the fluid cooling elements associated with the combustion chamber A "and with the gas passes E andiH'are'so constructed and. arranged ast'o constitute a'considerable portion of the boiler heating'surface available'for the generationofisteam or other vapor.

The walls. I8, 20 and '22 constituting outer boundaries'of the furnace chamber A may be constructed as shownin Figs. 2 and 3 to'include a plllrality of wall cooling elements- 24 arranged at suitable "spacings in a row across the width of the wa1l,each elementcomprisin'g a tubular member 2 6 l having its ends suitably connected to headers 28 and 30, for example, for'the circulation of boiler 'fiuid therethrough. Each wall element 24 is preferably compositely formed to provide a 'greater a'rea of exterior surface than is afforded by the tubular base member 25.

shown 'inFig. 2; for-an outer. furnace wall l8, for example,a plurality of extensions 32 may besecured to each tubular member'26 insymmetrical arrangement'to provide a substantially continuous planar surface 34 offset from the center line of the row, I preferably ini substantially tangential relation to'the convex-cuter surfaces of the tubes 26. A small clearance space 35may be left between adjacent extensions 32 on adjacent tubes. tensions may conveniently be filled'with afsuitable ceramic refractory material 3%"app1ied in a plastic or'semi-plastic condition witha suitable backing of heat insulating material 49. Individual extensions 32 can be made in relatively f 5- beef low carbon steelfpreferably in plate 'form of about A" thickness, and the filler material lfiof copper, as one example of a suitable filler as heretofore described.

If the operating temperatures shouldbe such that a different relationship is desired. between the thermal expansion of the jacket andthat of the. filler, an adjustment'could be made b modifying the composition of one or the other of the materials, for example, instead of using a low carbon steelforthe'jacket as describedjan alloy The spacesto the 'rear of the exof steel might beprferable, with alower or higher coefficie'nt of expansion, as required,

The jacket Mmay be formed as'a box with one side open and with edges 48 andfiilat the perimeter of the'sid'e' opening adapted to be weldedto the wall of the tube? 26'as shown. Plates 52for1ning the ends'oi the box provide edges 54 'at the perimeter of the'side'opening alsofor welding to thetubewall. After the'box has been formed, irom'a'single plate or from a plurality of plates joined togethenthe box may be filledwith: scrap ooppenpreierabl of high purity, and thecopper integrallybrazed with the insidesurface of 'the box and fusedto form? a solid blocki'of metal within the boxp'rojectin'g beyond the edges 48, 50,54 'a siiilicient 'di'stance"to allow for machining to a concave surface accurately fitting the outside diameter of thetube'EB, the filler material still extendingasmall 'distancebeyond the welding edges. The contact'a'rea of the tube maybe thoroughly cleaned, by brushing orbysand-blasting, and the composite extension suitably clamped against the tube in position for welding. The as sembly with the tube may be completed by fillet welding'aIo-ng the edges 48, 50, 54 as shown, the slight shrinkage of the welds tending to draw theblock iof'filler material 46 against the tube wall and to maintain close thermal contact throughout the entire area of contact.

'It willbe noted that the walls of the jacket are relatively thinand of substantially uniform thickness throughout, athic'kness not exceeding the thickness of thetube metal generally being sufficient. As seen in 'Fig. 2, such walls represent a relatively small cross-section of jacket metal in comparison with cross-section of the entire e'xtension, the'cros's-sectional area of jacket metal beingless than the cross-sectional area of the space within 'the jacket and consequently less than the cross-sectional area/of the heat conducting-filler.

Various filler materials 43, and methods ofapplication'have already been discussed. For example, it has been stated that a'rnetal such as copper could be'poure'd into the box in'a molten condition, in which case the box or jacket 46 could'be'welded to the tube in an upright position,with its upperendopen, and the molten copper poured into the box through its open end, whereupon if'desiredthe upper end plate 52 could be welded to the sides of the box and to the tube to complete the assembly. The same general method of procedure may be followed for other molten metals, and for such other materials as have" been mentioned of a iiuidor semi-fluid character. It will be understood that when the. filler material is .a'solid at ordinary temperatures, a block of the material may be preformed and then assembled with a jacket of conforming contour against the'tube, the filler block preferably projecting beyond the edges of the jacket and the jacket edges being Welded to the/tube asbefore. The extension 3'2 is of desirable formin that it presents a smooth surface toward the heating zone andin conjunction-with adjacent similar extensions forms a closure for the space between tubes 28 and provides a substantially' continuous planar wall surface. The edge portions'et of the box are bent out of the plane of the surfaceM to provide an angular recess 58 of at least with the wall of the tube 25 to accommodates, deposit of-weld metal sufficiently large i or strength withat forming a projection beyond the plane of the surface 34. 'Th rearward surface- 6d of the oxtension 32 divergessufficiently from the plane of the forward surface-34 to form a similar welding recess 62 of at least 90 between the longitudinal edge portions 50 and the tube wall. Thus from its extreme outer edge portion 56 advancin inwardly toward the tube wall, the cross section of extension 32 is expanded so that the flller material 45 engages the tube throughout an are greater than 30, with a total are of engagement of at least 45, and preferably from about 50 to 60, for the entire composite extension 32 including the welds along longitudinal edges 48 and 50.

According to Fig. 4, each composite extension 64 presents a planar surface 34 preferably arranged in tangential relation to the exterior wall of tube 26 as in Fig, 2, the edge portions 48, 50 and 54 of the jacketing member 66 being similarly arranged and'welded to the tube wall. The rearward portion 63 of the jacket, normally the unheated portion, is formed with one or more bends or corrugations 10 to provide flexibility, such corrugations being also formed if desired in the portion 12 of the jacket on the side adjacent a similar block on the adjacent tube. The pocket 14 formed between complementary corrugations on adjacent blocks may be utilized to retain plastic refractory material for sealing the joint between adjacent blocks. The heat conducting filler 46 may be one of the several materials heretofore described, a material of fluid or semi-fluid consistency being especiall suitable because of its ability to yield to whatever form is required by the flexing of the corrugated wall of the jacket 66. A pluralityof extended surface elements such as shown in Fig. 4 may be associated in a wall structure, if desired, with suitable refractory and backing materials 38 and 40 applied as in Fig. 2.

In Fig. 5,each composite extension 18 has a jacket portion 18 of exterior form similar to the jacket 44 of Fig, 2, and similarly welded to the tube 26 along edges 48, 50 and 54. In this embodiment, metallic projections 89 are provided within the jacket 18 and secured either to the tube 26, or to the jacket as shown, to project a substantial distance into the mass of filler material 46 as a means of increasing its heat conducting capacity, Such projections 80 in the form of bars or rivets, for example, may be conveniently welded to a selected area of the jacket 18 when made of sheet metal before. the jacket is shaped to final form. The elements of Fig. may also be incorporated in a wall structure similar to Figs. 2 and 3, as will be understood.

Fig. 6 illustrates an embodiment utilizing composite extensions 32 to form a Wall structure having exposed planar surfaces 34 at opposite sides, suitable for use where heat is applied to both surfaces as in the case of the partition wall C, for example, dividing the combustion chamber A from the adjacent open gas pass E. Individual extensions 32 may be the same as described as for Fig. 2, or may be of the form shown in either Fig. 4 or Fig. 5, with a suitable filler 46 within the jacket 44 as already indicated. With the blocks 32 attached to'both faces of the partition wall in this manner, the spaces between oppositely arranged series of blocks 32 may be filled with a suitable refractory cement-38 to provide a seal against gas leakage from one face of the wall to the other.

'When the extended surface elements are arranged in a row to form a wall, as in Figs. 2 to 6 inclusive, it will be understood that suitable provision may be rnade to maintain them in their relative positions, either by tying adjacent tubes together or by tying the tubes to some stationary structure as in the case of outside boundary walls Fig. '7 illustrates compositely formed extended surface elements particularly adapted for the transfer of heat by convection, a. suitable application of such elements being in the gas flue M as the tubular heating surface of a unitsuch as asuperheater, reheater or economizer, wherein heat is transferred from a hot gaseous fluid externally of the element to a fluid, either gaseousor liquid, within the tubular base member, 84'.- However, in other instances, the direction of heat; flow may be reversed as previously explained from a hot fluid within the tubular member 84 to a cooler fluid, either gaseous or liquid, externally of V having its exterior surface'extended by means of composite extensions 86 each comprising a metal jacket 88 welded to the tube 84 and containing a suitable heat conducting filler 46 in intimate thermal contact with the tube wall. The jacket 88 is generally V-shaped in cross section and may be formed as a box in the manner described for previous embodiments, with the side toward the tube open, and with longitudinal edges 90 and other free edges welded to the tube wall. There may be a plurality of separate extensions 86 along each tube as shown for extensions 32 in Fig. 3. The composite extensions 86 are shown attached to each tube in pairs, at diametrically opposed locations, at the upstream and downstream sides, although other arrangements of a plurality ofextensions may be used as well as a single extension circumferentially of each tube. It will be understood also that the tubes in successive rows may be in alignment relative to fluid flow thereover, or may be in staggered relation, as desired.

While in accordance with the provisions of the statutes I have illustrated and described herein the-best forms of my 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 certainfeatures of my invention may sometimes be used to advantage without a corresponding use of other features.

I- claim: 1. The method of extending the external surface-of a tubular element of circular cross section which comprises, forming relatively thinmetal into a hollow, box-like jacket having one side open to extend partially around said element, filling the space within said jacket with molten metal to provide a molecular bond between said filler metal and the metal of said jacket, sufficient molten metal being introduced to overfill the jacket space to an extent providing an extension of the filler metalbeyond the edges of the jacket surrounding the open side after complete solidification, shaping said filler metal extension to a surface conforming to the external contour of the tubular element, clamping the filled jacket against said tube'with the shaped extension surface in contact with the conforming tube wall surface and with the jacket edges spaced therefrom, and welding said jacket edges to the tube wall to maintain close thermal contact between the filler metal and tube. I 7

2. A furnace wall unit comprising a fluid-conducting metal tube having means integral therewith forming extended heating surface for exposure toward said furnace, said means comprising extensions arranged oppositely on saidtube and 2 providing. heat absorbing. surface substantially in a plane tangent to said tube, each extension being compositely-tformcd to include a .sheet metal .j ackethavingoppositely disposed uniformlycthinwalls in diverging relation towardsaid tube and terminating in edges. spaced L circumferentially adjacentsaid tube through an ;arc of at-leastBOf, one oflsaid. walls having an exposed surfaceportion in saidplane and havingits ad-= joining inner edge portion bent to,extend approximatelyradially of saidtube. to form a weld.-

ing. recess below the. plane; of said exposed jacket surface,.filler.materialwithin said j acket-having a heat transfer capacity. greater than .the;material ofsaidjacket, said fillermaterial having. a..cir-=.

cumferential .dimension of engagement. with said tube several times the thicknes of said. jacket walls. and an average. circumferential dimension throughout. its radial extent: also greater than said wall thickness, andmeans for unitingsaid edgerportions with said tube including weld-metal fused-within said recess to approximately thetangential position of said. exposed heating surface.

3;: The method of extending the. external surfaceof atubular elementsofcircular cross. section which comprises, forming.- relatively; thin metal ,into a hollow box-like jacket. having one id pen. to. extend. partially; around.;said. ement. introducingim talin asolidstateinto the spacewithin said jacket, transforming said .metal into; a .molten mass.- having. fusion engagement with the Walls of said jacket; effecting solidification of said mass to provide a block of metalcompletely filling .saidjacket space While maint i ingits metallicbondwith said Walls, said .filler sition into a box-likejacket having its walls arranged todefinean openingin a side thereof adapted to embrace an arcuate area of saidtubularelement, the space within said jacket being of minimum cross section at a. location farthest from said opening and of progressively greater cross section toward its maximum cross section adjacent said opening, effecting solidification of molten metal withinsaid jacket to form a metal filler having a substantially continuous metallic juncture with the walls of. said jacket and providing; an exposed surface beyond. the surrounding jacket edges, said filler metal having a heat conductivity of the order of copper and thereby greater than the heat conductivitylof thejacket metal, shaping said exposed surface to asurface conforming to the contour of said arcuate area, positioning said. jacket adjacent said tubular element with said shaped surface in contact throughout with said arcuate area and with said jacket edgesspaced therefrom, and welding; said TJI 5, heat transfer capacitygreater than the ma- 1:0 therewith providing extended heat transfer surface; said means. comprising extensions arranged oppositely onsaidgtub'e, and providingheat transfernsurface substantialdy in aplane. tangentv to said tube, each extension being compositely formed to include a sheet metal jacket; having oppositely; disposed uniformly. thin. wallsin diverging {relation toward said tube andterminating. in; edges. spaced :circumferenti ly. adjacent saidtube; one. oflsaid wallsilhaving a-n exposed surfacerportionin said plane and..havirrg its ad:- joining: inner edge; portion; bent .to; extend approximately radially-of, said :tube ,tojorm .a welding reces'sbelowthe planeof saidexposedjacket surface;..fillery material. within. said, jacket having a; heat; transfer; capacity, greater than; the material ;of said jacket, .said .filler material having a. circumferential dimension of engagement with saidtube; several. times the thickness. of said jacket. walls-and an average circumferential dimension throughout its radial extent also greater than said. wall thickness-and means for uniting said 1edge portionswith said .tubeinclucling weld metal fused. within said recess. to approximately .the tangential position of said exposed heat transfersurface.

6.- A heattransferunit comprisinga fluid conducting. metal. .tube havinggneans integral therewith providing extended heat transfer surface, -said means comprising an extension .on said tube providing heat transfer. surface substantially in a-plane tangent to said tube, said, extension beingcompo-sitely formed .to include a sheet metal jacket having oppositely disposed uniformly thin walls-in .divergingrelation toward said, tube and terminating inedgesspaced circumferentially adjacent said'tube, one of saidwalls having an exposedsurface portion in said, plane and having its adjoining inner edge portion bent to extend approximately radially of said tube to form a welding recess below the plane ofsaid exposed jacket surface, filler material within said jacket having a heat transfer capacity reater than the material ofsaid jacket, said filler materiahhavinga circumferential dimension of engagement with said tube several times the-thickness of said jacketswalls and an. average circumferential dimension throughoutitsradial extent also greater than said wall thickness, and means for uniting said edge portions with saidtube including weld metal fused Within .said recess to approximately the tangential position of said expcsedheat transfer surface. I

'7. A heat transfer unit comprising a fluid-conductingtmetal tube having means integral therewith providing extended heat transfer surface, said means comprising extensions arranged oppositely on said tubeand each providing heat transfer surface substantially in a plane tangent to said tube, each extension. being compositely formed to include asheet metal jacket having oppositely disposed uniformly thin walls in diverging relation toward. said tube and terminating in edges spaced circumferentially adjacent said tube, one ofsaidwalls having an exposed surface portion in said plane and having its adjoining inner edge portion bent toextend approximately radially of said tube to form a welding recess below theplaneof said exposed jacket surface,-.filler.material within said. jacket having terial of-said jacket, said. filler. material having a circumferential dimension of engagement with said tube several times the thickness of said jacket walls and an average circumferential di- 11 inens'i'on throughout its radial extent also greater than said wall thickness, and means for uniting said edge portions with said tube including weld metal fused within said recess to approximately the tangential position of said exposed heat transfer surface.

8. A furnace wall unit comprising a fluid-conducting metal tube having an extension thereon providing extended heat transfer surface substantially in a plane tangent to said tube at the furnace side thereof, said extension comprising a sheet'metal jacket formed'as a box having one side open and having edges surrounding said open side secured to said tube to define a closed compartment adjacent said tube, the enclosing walls of said compartment thereby including the portion of said tube embraced by said jacket, said jacket having oppositely disposed uniformly thin side walls extending in diverging relation toward said tube and terminating in circumferentially spaced edges adjacent said tube, one of said side walls having an exposed surfac portion in said cumferential dimension throughout its radial extent correspondingly greater than said side wall thickness, and means for uniting said edge portion with said tube including weld metal fused within said recess to approximately the tangential position of said exposed heat transfer surface.

9. A heat transfer wall unitcomprising a fluid conducting metal tube having an extension thereon providing heat transfer surface substantially in a plane tangent to said tube, said extension comprising a sheet metal jacket formed as a box having one side open and having edges surrounding said open side secured to said tube to define a closed compartment adjacent said tube, and a heat conducting medium fluid at operating temperatures substantially filling said compartment, said jacket having oppositely disposed uniformly thin side walls extending in diverging relation toward said tube and terminating in circumferentially spaced edges adjacent said tube, one of said side walls having an exposed surface portion in said plane and having its adjoining inner edge portion bent to extend approximately radially of said tube to form a welding recess below the plane of said exposed jacket surface, said side wall being formed with projections thereon extendin into said medium partially across the space toward the opposite of said walls, said heat conducting medium having a circumferential dimension of engagement with said tube several times the thickness of said diverging side walls and an average circumferential dimension throughout its radial extent correspondingly greater than said side wall thickness, and means for uniting said edge portion with said tube including weld metal fused within said recess to approximately the tangential position of said exposed heat transfer surface. a a

10. A heat transfer wall unit comprising afluid conducting metal tube having an extension thereon providing heat transfer surface substantially in a plane tangent to said tube, said extension comprising a sheet metal jacket formed as a box having one side open and having edges surrounding said open side secured to said tube to define a closed compartment adjacent said tube, said jacket having oppositely disposed uniformly'thin side walls extending in diverging relationtoward said tube and terminating in circumferentially spaced edges adjacent said tube, one of said side walls having an exposed surface portion'i'n said plane-and having its'adjoimng inner edge'portion bent' to extend approximately radially of said tube to form a welding recess below the plane of said exposed jacket surface, the other ofsaid walls having corrugations therein throughout its thickness, a heat conducting medium fluid at opwith said tube several times the thickness of said diverging side walls and an average circumferential dimension throughout its radial extent correspondingly greater than said side wall thickness, and means for uniting said edge portion with said tube including weld metalfuse'dwithin said recess to approximately the tangential position of said exposed heat transfer surface. f

11. In a furnace wall adapted for exposure to heating gas zones of different pressures atopposite sides, a row of spaced elongated heat transfer units each comprising a metal tube of curvilinear cross section for conducting fluid to be heated, and means for extending the exterior surfaces of said tubes to provide substantially continuous heat absorbing surfaces in planes substantially tangent to said tubes at opposit sides of said row, said means at each side comprising metal jackets secured in pairs to each of said tubes and a heat conducting material substantially filling said jackets, said jackets each having uniformly thin side Walls extending in diverging relation toward one of said tubes and terminating in circumferentially spaced edges adjacent said tube, said jackets having certain of said side walls in said tangent planes for substantially bridging the spaces between adjacent tubes while the remaining side walls diverging therefrom define substantially closed intertube spaces intermediate said planes, one of said walls of each jacket having an exposed surface portion in one of said planes and having its adjoining inneredge'portion bent to extend approximately radially of said tube to form a welding recess below the plane of said exposed jacket surface, means for uniting said edge portions with said tubes including weld metal fused within said recesses to approximately the tangential positions of said exposed heat absorbing surfaces, and refractory material filling said intertube spaces for sealing said wall against gas leakage from one of said zones to the other, said heat conducting material having a circumferential dimension of engagement with each tube sev-

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