US2243402A - Tubular furnace structure and method of forming same - Google Patents

Description

y 1941- J. E. TRAINER ETAL v 2,243,402

TUBULAR FURNACE STRUCTURE AND METHOD OF FORMING SAME Filed Feb. 9, 1938 3 Sheets-Shet 1 14 W/J/ W I 'III/ INVENTOR5 me: E Trainer Isaac Harter Jizhn PROjEI ATTORNEY.

May 27, 1941. q. E. TRAINER ETAL 2,243,402

TUBULAR FURNACE STRUCTURE AND METHOD OF FORMING SAME Filed Feb. 9, 1938 3 Sheets-Sheet 2 Fig? [6 I Jamesf. Tra z [saac Harrier f fa/9n PPoger May 27, 194-1.

I J. E. TRAINER EIAL TUBULAR FURNACE STRUCTURE AND METHOD OF FORMING SAME 3 Sheets-Sheet 3 Filed Feb. 9, 1938 r Fig [8 0 l w 0 y 3 INVENTORS (James E. Trainer Isaac Harier ,JbbnP Eager ATTORNEY.

"ditiom, the use of-wholly desirable wall construction Patented m 21, 1941 TUBULAR FURNACE STRUCTURE AND METHOD OF FOBMING'SAME James E. Trainer, New York, N. Y.,

Fain-lawn, Ohio, Isaac Hatter, and John P. Roger, Barber-ton,

Ohio, assignors to The Babcock a Wilcox Company, Newark, N. 1.,

a corporation of New Jersey Application February 9, 1938, Serial No. 189,518 14 7 (Cl. 29157.4)

This invention relates to hollow metallic struc- It is also concerned with a novel method of manufacturing such structures.

- One object of the invention is to aiford an organization of such structures forming a furnace water wall of superior efliciency, durability, and scope of utility.

A further object is to provide hollow metallic metallic wall inwardly fluid flowing through the elements.

Another object is to closures must have considerable resistance to ce gases or the nongaseous particles carried in suspension by the gases. Such particles ex-v ist,forinstance,inashof coalinacoalburning furnace, and particularly in a pulverized coal horizontal headers. These water walls differ in the means for closing the spaces between them, and in the mean for providing their furnace faces.

To meet the wide range of service conditions as to temperature of furnace, temperature of.

wall face, heat absorbing capacity per square foot of wall, corrosiveness of furnace constituents such as'ash, and ash removal in the molten or the solid state, a considerable variety of water wall constructions have' been :proposed, each havin'g some specialfeature oLadaptability as an encasing wall or a partition This invention presents a universal water wall element adapted to be advantageously used in difierent walls which have different characteristics as to their heat transfer efiects. The element is of low cost, and by meansof its use various water wall conditions may be met, conditions varying as to heat absorption and the stabilization of deposits upon the water wall face.

The universal water out invention is of a S ngle piece of forged or rolled steel. It has a flat face and is hollow so as to constitute a conduit for cooling water and is of such construction that, when several of the elements are assembled in a row, the'flat faces are-located in the same plane so as to form a flat water wall face while the water conduits may. be connected in parallel to headers at their ends. The cross section of the water wall conduit may have any form, but when it is of and its gas outlet passages. The latter 7 condition reaches its maximum severity in large volume high capacity furnaces for steam boilers, and experience has shown that water cooling of the walls of such boilers is a necessity. Experlencehas also shown that, under many conreiractory wall conand that the most is a combination of water cooling elements with the refractory- It has been proposed, when water cooling is employed in such constructionsto use riser tubes structlons is unsatisfacto tocarrythewaterandsteamiormeihandthew' tubes are normally set vertically with the tubes paced'from each other and extending between,

c rcular-cross section, it is especially suitable for time afford adequate cross section of metal to conduct heat from either or both faces, and maintain the temperature "of the metal face receiving heat and located farthest from the water at a suillciently low value.

The invention of the elements being promoted byforming the water wall elements initially in a rolling mill in much-the same manner as steel shapes. The desirable characteristics of the'rolled elements include such ductility that they resist cracking under stress, especially the stresses due to local difierencesin wall element of the presinternal cleaning and advantageous from the standpoint of pressure conditions. The walls of temperature. The use of such a single piece of forged steel minimizes the number of joints and avoids joints across heat flow paths, imposing thermal resistance which makes the metal remote from the water hotter than it would be without such joints. This construction also reduces fabrication and assembly cost.

The forged steel elements used in the construction of the illustrative water wall elements are characterized by flat faces at the opposite sides of an intervening web. This web is shaped, as by bending, to bring its edges together and the fiat faces into alignment. The flanges, or those portions of the elements presenting the flat faces, are then fusion welded along their line of junction.

A water wall-formed by a row of the universal elements set with their fiat faces in alignment may include gas seals to limit the leakage of gas between adjacent elements, and the wall thus formed would be of all-bare cold metal on both sides, one side of the wall being flat and the other substantially wavy or corrugated. 'When it isv desirable to limit the heat absorption or gas cooling on either side of the wall, refractory mate- 5 rial maybe added to provide thermal resistance, and by the use of such material in varying degrees, any kind of wall heat-absorbing condition may be provided with no change in the primary or universal wall elements. In service, when the flat face of the wall is exposed to hot gases and is absorbing heat therefrom, especially when this side of the wall is uncovered or bare, the metal of the edges of the flat faces will be hotter than that at the middle, but the temperature diiferences may be limited by the cross section of the metal conducting heat from the face of the wall. Such excess of temperature of the edges, although permissible in some cases, will result in the tendency for these edges to expand more than the colder parts of the metal, and as the flat face is on one side of the center of the con-' duit, this condition will cause the element and the wall to bend due to stresses in the metal. In some cases, such curvatures of the wall are of no consequence, and the material being of forged steel and being ductile, the stresses may do no harm. However, such bending can be limited or prevented and the expansion stresses rendered negligible ,by cutting the projecting edges of the flat faces at intervals along thelengths of the universal elements and in the planes of heat flow, such cuts have substantially no effect on the heat flow, or the metal temperatures.

In referring to a furnace wall or water wall 55 herein, the word wall is intended to include any boundary of the furnace or combustion chamber, including a bottom wall or floor, or oft) passages for gases leaving the furnace, and any dividing furnace boundary separating one part from another. In using the word water in connection with the water wall or boiler, it is not intended that the word water shall be any limitation, but that the word may' be taken as indicative of any suitable fluid for the process described. By a. water wall element, it is intended to refer to any single hollow metal element or metal enclosing fluid passage that is provided with the characteristic features of this invention.

In the accompanying drawings,

Fig. 1 is a transverse section of a single-piece rolled metallic blank (such as may be produced in a rolling mill) from which the illustrative water wall element may be constructed.

Figs. 2, 3, 4, and '5 are transverse sections of the rolled shape, or blank, shown in Fig. 1, indicating successive steps in the process by which the single piece water wall element is formed from the rolled shape and prepared for fusion welding.

Fig. 6 is an enlarged transverse section of the welding groove at the edges of the water well element, indicating the first welding operation.

Fig. '7 is a transverse section on an enlarged scale showing the effect of converging forces exerted on opposite sides to exert pressure on the metal at the first stage weld while it is hot as a result of the welding.

Fig. 8 is a transverse section indicating the completion of the deposition of weld metal between the edges of the flat face of the rolled steel shape or blank.

Fig. 9 is a partial transverse section indicatin the weld seam after excess weld metal is removed to produce smooth faces.

Fig. 10 is a. transverse section through the illustrative hollow .water wall element showing drilled holes which are formed as a part of the means for limiting expansion stresses when the element is in service in a wall enclosing or dividing hot gases.

Fig. 11 is an elevation of the flat face side of the water wall element showing the arrangement of the drilled holes indicated in Fig. 10.

Fig. 12'is an elevation of the water wall element, indicating the spaced transverse stressrelieving cuts and their relationship to the drilled holes indicated in Fig. 11.

Fig. 13 is an elevation of the illustrative water wall element, showing how it is joined to a tube section of circular. cross section and uniform wall thickness for connecting the wall element into a fluid circulation, as, for example, through a header not here shown.

Fig. 14 is a cross section of the water wall element taken at the line "-44 of Fig. 13 in the cuts indicated in Fig. 12.

,Fig. 15 is a transverse section of a tube connected to the illustrative water wall element, as shown in Fig. 13.

Fig. 16 is a view in the nature ofa vertical section indicating a part of a boiler to which the illustrative water wall is applied.

Fig. 17 is a section taken on the line l|-l| of Fig. 16 and showing a furnace wall dividing two parts of a furnace gas space in which different temperatures and wall heat absorption conditions prevail.

Fig. 18 is a transverse section through the junction indicated in Fig. 19.

Fig. 19 is a longitudinal, or vertical, section taken on the line Iii-I9 of Fig. 18.

Fig. 20 is a perspective view of the junction indicated in .Fig. 19.

Fig. 21 is a perspective view of the end of the water wallqelement, showing the recess which is formed to facilitate the formation of the Junetion with the tube, as indicated in Fig. 19.

Fig. 22 is a view in the nature of an elevation indicating a step by which the Fig. 19 junction is formed.

The water wall element I0 is shown in cros: section in Fig. 10 in the form which has a circular hollow section or cylindrical interior. It consists of a web portion l2 connecting flat-facet flanges l4 and IS formed in one piece, prefer ably of rolled steel. These portions, together form the water passage l8 and a wide flat see the section of tion of wall face presented by the aligned flat surfaces of the-flanges. The latter have ample cross section to 'afiord a suflicient area of heat flow path from all parts of the flat face inwardly to the hollow interior and the water flowing therein, to limit the temperature of the outer edges of the flanges to the desired low values.

This is due to the indicated shapes and arrange-' ment including the cross section of the flanges which are thick enough to present a wide avenue of heat travel, and are of tapering cross section from the most remote points or tips of the flanges to the, points of heat conduction to the circulating water.

When a number of the illustrative water wall elements are arranged in row formation, the faces of the flanges combine to form a substantially continuous fiat smooth face, each wall element forming an integral metallic body formed of a single piece of the same In forming this body the present invention comprehend the forming of each tubular element from a rolled shape, bending the latter to tubular form and then welding the adjacent edges to complete a tubular enclosure.

The relied steel structural shape made for the construction of the illustrative water wall tube is indicated in Fig. l of .the drawings.- It includes the similar flat-faced non-symmetrical flanges l4 and 16 of non-uniform thickness connected by a web H which is of uniform thickness. In rolling the Fig. 1 blank, the flanges are formed with surfaces ABC and DEF on one side which conform to the ultimate interior contour of the tube, and there is thus predetermined, in the rolling of the blank, enough of the ultimate interior tube contour to permit the bending of the web to produce the remainder without irregular surfaces at the margins of the web. As indicated in Fig. 1, each of said surfaces includes 130 degrees of a circular arc of a radius of the desired final cross section, or together,

more than two-thirds of the cross section of the cylindrical interior. The web l2 extending between the points F and A, and joining the flanges, is reversely bent with respect to the curvature of the surfaces ABC and DEF, and. of predetermined length. All of I the subsequent bending of the rolled blank takes place in the web, thereby minimizing .undesirable stresses.

As shown particularly in Fig. l, the flange surfaces ABC, and DEF are formed to the same true metal throughout.

no twisting of the blank ed by the forming process except that the flanges may be slightly twisted throughout the length of the blank. -'Ihis twist will not, however, result in a stress whichexceeds the elastic limit of the metal. When the fo or bending process is carried out by means of dies acting upon the entire length of the blank at once, there is portions. weld grooves or recesses and 21 are formed, the web I! is bent as indicated in Fig. 3 to bring the flanges l4 and [6 to positions near the ultimate interior cylindrical surface. In this step of the process, the web I2 is substantially flattened, as clearly indicated in the drawings.

Further bending of the web I! is indicated in Figs. 4 and 5, in the latter of which the edges complished by along the entire adjacent radii so as to ultimately form a part of the inner circumferential surface of the tube. The

opposite sides, KG and MB, of the flanges are formed as two irregular surfaces of predetermined contour so related to the remaining surfaces of the flanges as to provide enough metal t) for heat conduction from the flat faces to the face of the hollow interior to limit the maximum temperature of the metal.

After the Fig. 1 blank is rolled, the marginal portions of the blank nearlthe points C and D are cut away to form the welding groove recesses indicated at 20 and 22 in Fig. 2. These recesses are so formed that they weld groove with a dam at its bottom. Thereafter, the blank is subjected to a forming or bending process in which it may be bent by means of dies acting upon the entire length of the section at once until the edges of the opposite flanges come together. In either case, the work of the forming process is performed only and entirely upon the web l2 of the blank, and the shape of the flange portions is in no way affect- .erably pressed into contact by of the flanges at, c and D are brought into reg- 'ister so that the grooves 20 and 22 combine to Also, the flat faces l6 are brought into the form the entire weld groove. of the vflanges I l and alignment indicated.

The next; step in the manufacture of the illustrative water wall element consists in uniting the tongues 24 and 26 near the points 0 and D" of the flanges. These tongues are united in a preliminary welding operation which may be acan electrical welding process or by gas welding. Alternatively, resistance welding, or arc welding, may be employed. In any event, a continuous preliminary weld is formed cated by the numeral 28 in Fig. 6 of the drawings. When the tongues 24 and 26 are heated .to a welding temperature by an acetylene flame or other welding means, these tongues are prefa considerable portion of the ultimate cross This is indi- Fig. 7 of the external force which causes a weld to be Positioned within the section or contour of the interior. cated by position of the weld 30, in drawings.

' Subsequently, the protruding part of the weld metal is removed and the contour of the water wall element finished. This action involves an elimination of parts of the preliminary weld which may contain impurities, or otherwise be unsatisfactory.

After. the interior surface of the illustrative water wall element and the outside face of the weld 32 the weld is a single piece metal structure of identical metal everywhere except the weld, and this may be made substantially or actually identical also by selection of weld metal and heat treatment. that part of the flat face that is .coldest in service because the metal has least thickness and the heat path is shorter from thewall face to the water in the tube than elsewhere in the flat face.

To minimize the stress effect of the expansion of the hottest metal that is farthest from the water in the tube relative to the coldest, the continui y of the hot face is interrupted by spaced transverse cuts, each of which endsin a round hole.

Figs. 10 and 11 of the drawings indicate holes 34 and 36 which are drilled separately through the heavy flanges at distances surface *of the tube suitable for the internal pressure and, as shown, about equal to the thickness of the web l2. These holes are spaced at predetermined short distances along the length of the tube as particularly indicated in Fig. 11.

length of the weld groove indi-,

are finished, that section of the element This weld is located along from the interior.

Thereafter, the heavy sections of the flanges are cut through in the plane of the heat flow scribed herein are used in the construction of' the walls of a furnace, enclosing its furnace or gas outflow passages, or dividing the gas space by a wall, such as the wall indicated in Figs. 16 and 17, the ends of the wall elements should be bent in order that proper communication may be established with such co-operating elements as the header 4! and the drum M of the Fig. 16

boiler. The illustrative water wall elements, unlike tubes which have-uniform wall thickness,

will not bend about neutral axes jco-incident with the axes of passageways through the tubes, but will rather, bend about neutral axes parallel to and considerably removed therefrom. Furthermore, the particular construction of the flanges of the illustrative wall elements imposes other limitations making. their bending somewhat difficult. when these difllculities are severe, they.

may be overcome by the method indicated in Figs. 13, 14, and 15. In this method, the flanges l4 and ii of an element of cylindrical in rior are cut away from the end portion ll to change that end portion to a tube'which is of a uniform wall thickness. Thereafter, a tube 43 preferably of the same size as the end portion 4| is united with that portion by such a weld as that indias a plastic or semi-plastic over the curved faces of the wall elements and around the studs. This refractory material forms a furnace face presenting a part of the surface of the high temperature combustion chamber 52, the refractory material serving to promote high gas temperatures in the interest of complete and rapid combustion by regulating and minimizing heat absorption to obtain high temperatures. This also promotes slag fluidity and stabilizes the thickness of deposits on that side of the wall. The Fig. 17 wall, along its opposite face, presents a bare flat smooth metallic surface to promote a high rate of heat absorption in the gas space 54 in which it is desired to cool the gases and to chill suspended slag so it will freeze and can be readily separated. The smooth cold face resists slag adhesion to the maximum degree, even when sticky, and so stabilizes deposits diiferently than on the other 'side. It also forms cleavage planes so that adhering slag will fall of its own weight.

The invention may also be embodied in a division wall presenting, on one side, an all-metallic surface, and, on the opposite side, a surface which is partly metallic and partly of ce ramic refractory material. The fluid-cooled metal of the tubes may be exposed between zones of ceramic refractory material between adjacent tubes. and the refractory material may be held in position by inter-tube studs which are similar to some of the studsv shown in Fig. 17.

. The exact construction of this type of wall may be varied in some paiticulars'as to the arrangement, length, and positioning of the studs while the wall itself still has the characteristics of such a partially studded wall, including the alternate zones of fluid cooled metal and refractory material exposed to the heat of the furnace opposite their flat faces formedby the flanges 0 gases.

It is considered unnecesmry, for the purposes .of this invention, to refer to the detail construccated at 45. Each wall element may have each of its ends provided with a tube extension in t is m These extensions 43 may be read tion of such a boiler as that indicated in F1 g. 16, bent in a manner usual with this type of tube but some of the elements of the construction s: g g gs be joined to headers by shown will receive reference. The boiler furnace The upper level straight section of the wall fi gz g g g gggfgg m ggf i-fi 23:15: :tstszfsaits s ste pness-:3; g as; i rac o m eri an co ru h while the lower portion of the wall elements be 1 g t b tn 1 scri ti i t-h Fl 17 tween the drum or header M and the position ner n e y e e p on o e g. R, consists of ordinary tubes of uniform wall thickness, 0 the Sections e these the drum 44 is connected to the header 66 by lower tube extensions of the water wall elements floor tubes :3, are bent, as indicated at 50, to Provide P85 8 Furnace gases pass out of the chamber 52 at of sumcient flow area for the flow of furnace position near the drum 44 a b t n spaced trams: mastitis 212.52.23.22 gr e"... 3: W reaer owuwar 'ne 1:" have tube extensions between the header 4! and p be and straight section of the wall ill. other walls 10 and l2,- similarly including wall The illustrative wall tube elemen -51 ml h mtubes and connected into the circulation of the a n a ar urnace c am r spaces, such as 52 and 54, in which widely The cooler? gases turn at the bottom of th t ferent gas temperature and wall heat absorpchamber and then passv upwardly across banks tion conditions prevail. Fig. 17 indicates a wall of convection heated tubes 18, 80, and 82. construction empl'ying the water wall elements Another method which may be employed in of this invention under such circumstancealdzr the joining of wall tubes to the wall elements this construction, metallic studs 58 are, we at their ends is indicated in Figs. 18-22, incluto the curved outside faces of the wall elements sive. In this process an arcuate groove or recess is cut in the end of the water wall element, materialbeing thereby removed from the flanges so that a circular section 92 is presented at the end of the element. Thereafter, a cylindrical 5 tube 93 of the same diameter and same wall thickness is subjected to an expanding operanect the header 86 with the header 42, and

I4 and I6. These studs are distributed 'over the surfaces of the elements, as clearly indicated in Fig. 17, and the face of the furnace wail opposite the flanges is completed by the high temperature ceramic refractory material it installed wall. Tubes defining the walls 62 and 84 con- 1 wherein heat is absorbed by the wall ill, and

Thereafter,

tion so that it is provided with an expanded end 95 which will fit closely clearly indicated in with reference to certain particular embodiments, it is to be understood that it is not limited to all of the details thereof, but it is rather to be interpreted as of a scope which would cover various modifications thereof, within the compas of the subjoined claims.

We claim: 1. In the manufacture of tubular furnace wall elements, rolling a billet to produce a structural pe of the general nature of an I-beam but havingheavier and unsymmetrical flanges connected by a thin web, said flanges having wide outer surfaces, bending the web only of said shape to form a substantially tubular body with said outer surfaces presented'substantially tangentially tosaid body at the same side thereof,

and welding lar body.

2. In a method of manufacturing a tubular element for fluid cooled furnace walls, rolling a steel billet to produce a structural shape including heavy flange portions connected by a web the flangeportions being thicker than the web throughout their entire crosssectional extent, bending the web only to bring the outer faces of the flanges into alignment in tangential position, and fusion welding the flanges together to complete the element.

3. A rolled metallic element connecting flange portions which present cylindrical surfaces toward thesame side of the element, the web being reversely curved with respect to the cylindrical surfaces and the combined areas of said cylindrical surfaces being greater than the area of one side of said web.

4. A rolled metallic structural element comsaid flanges to complete the tubuprising a thin web joining flange portions more than four times as thick as the web, said flange portions presenting like cylindrical surfaces on comprising a web 5 furnace, a unitary metallic element adapted to absorb radiant'heat and to be cooled by interior circulation, the same comprising a tube portion and integral facing extensions thereon surrounding and covering the inner side only of the tube and facing the fire but terminating to leave uncovered the outside of the tube, and having such ample thickness of metal along the paths of heat conduction from the exposed face of the tubes as to afford high conduction rates from all exposed portions to the medium circulating in the tubes and thus to avoid destructive congestion of heat, said unitary element consisting of a rolled shape having the integral facing extensions formed during the rolling of the metal from which the element is formed and subsequently welded together after the rolled shape is bent to tubular form.

8. In the manufacture of a. boiler furnace wall structure adapted to form a high temperature furnace face on one side and a low temperature the same side of said element, said web being reversely curved with respect to said cylindrical surfaces.

5. A rolled metallic structural element comprising a thin web connecting flange portions which comprise more than three-fourths of the total metal of the element, said flange portions presenting on the same side of said element cylindrical surfaces of the same radius of curvature,

said web being reversely curved with respect to said cylindical surfaces.

furnace face on the other side, rolling a, billet to form a shape of the general nature of an I- beam with heavy flanges presenting cylindrical surfaces joined by a web of reverse curvature,

bending the web until its inner surface forms a substantial continuation of said cylindrical surfaces and until corresponding edges of the flanges are brought into confronting relationship, and welding the flanges together to complete the structure. a

9. A tubular furnace wall element formed entirely of. forged steel and comprising a tube portion of uniform wall thickness, and facing extensions integral with the tube portion along one side thereof and having a wall thickness sufliciently greater than the wall thickness of the tube portion along the paths of heat conduction from their exposed faces' as to afford high conduction rates to a medium circulating through the element, said extensions being joined by weld metal to complete the element. 7

- 10. The method of manufacture of a furnace wall tube of non-uniform wall thickness which comprises rolling of a structural steel blank having a web of uniform thickness connecting flanges corresponding sidesof which have surfaces which conform to the ultimate contour of the inner surface of the tube and together form .more than fifty per cent of that surface, bending the web sisting of flanges of non-uniform wall thicknesseach presenting a surface rolled to conform to more than one-fourth of the ultimate inside contour of the tube, and a web of uniform thick- 6. In the wall or lining of a fluid cooled b0i1- er furnace, a unitary metallic element adapted to absorb radiant heat and terior circulation,

to be cooled by inthe same comprising a tube portion and facing extensions integrally formed therewith by rolling, said integral extensions stir rounding and covering the inner side only of the tube and facing the fire but terminating to leave uncovered the outer side of the tube, said extensions each having across sectional thickness near the tube portion of at least several times that of the wall of the tube portion.

7. In the wall or lining of a fluid cooled boiler flanges into confronting relationship, welding the inner parts of said confronting portions to ness connecting the flanges and having a reverse curve relationship to each of such surfaces.

12. Method of manufacturing a furnace wall tube which comprises rolling a structural steel shape with a web of uniform wall thickness connecting flanges having surfaces rolled to conform to more thanflfty per cent of the ultimate in side contour of the tube, bending the web only to bring corresponding marginal portions of said form a tube enclosure and simultaneously pressing said flanges together to force at least a part of the weld to positions within the ultimate inside contour of the tube, completing the welding of said flanges by deposition of weld metal between them, removing the excess weld metal projecting within the ultimate inside contour of the tube, forming tangential bores extendin through the flange portions at spaced positions and at distances from the inside surface of the tube equal to the thickness'of said web, making stress-relieving saw cuts from the outer edges of the tube into and communicating with said bores, cutting away the excess metal of the flanges at an end of the tube so that said end becomes of uniform steel thickness throughout, and welding a tube section to the tube end from which the excess metal has been cut away.

13. A tubular structure for fluid cooled furnace walls constructed entirely of forged'steel and comprising a tubular part having a wall of uniform thickness, and integral extensions forming tangential furnace faces at one side of the structure, said extensions formed by bodies of metal much thicker than said wall throughout their entire cross-sectional extent and Joined by weld metal deposited to form a longitudinal weld seam which completes the structure.

14. A tubular elemen adapted for use as a furnace element and comprising a tube portion of uniform wall thickness, and. rolled metal facing extensions integral with the metal of the tube portion and of gradually increasing thickness greater than the wall thickness of the tube portion along the paths of heat conduction from their exposed faces to afford high conduction rates to a medium circulating through the element, the minimum thickness of the extensions being greater than the thickness of the tube portion, the portions of the extensions adjacent their junctions with the tube portion being of gradually increasing thickness radially of the element and the external surfaces formed by those portions of the extensions being substantially tangentialto the tube portion, the opposite longitudinal marginal portions of the extensions being joined by weld metal to complete the tubular element.

JAMES E. TRAINER. ISAAC HAR'I'ER. .IOHNP. ROGER.

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