[1]Uni'ted States Patent Office 2@826@672 P a t e n t e d M a r . 1 1 , 1 9 5 8 2,826,672 METHOD OF WFLDING FINS TO TUBES Jbbn W. Brown, Jr., Lake%,ood, and Arvid C. K. Nihlen, Oberfiii, Ohio, assi-,nors to Browia Firt,7t!-De Corrpa.-iy, Elyria, Ohio, a corporation of Ohio Application February 16, 1955, Serial No. 488,586 16 Claims. (Cl. 219-81) 5 10 This invention relates t6 a method and apparatus for resistance welding metal fin members to the exterior of metal tubes for prodticing finned tubes. The tubes are ad@pted particularly for use as heat exchan-,e cond-Liits, alth6ugh they may al@o be u ed as strtictural members. Finned tubes of the general type to which the present inventioii relates are disclosed and claimed in the prior @atents of John W. Br6wn, Jr., Nos. 2,261,136 and2,226,137. An apparatus for mdking such finned tubes is disclosed and claimed i-@i the prior patent of John W. Browii, Jr., No. 2,298,249, and a method for making such finned tubes is disclosed in the prior patent of John W. Brown, Jr., No. 2,298,250. All of these patents are owned by the assignee of the present application. In the mantifacture of heat ex@hangers it is frequently desirable to augment the area of the fluid conductin-. tubes employed in the heat exdhangers by securin- longiiudinally extending fins to the exterior of the tubez. The pri6r Brown patents referred to above disclose finned tubes having desirable characteristics from the standpoint of heat iransfer, and the method a.-d apparatus patents disclose and claim methods and apparatus that have been widely used in the production of longitudinaily finned heat exchanger tubes. According to the method of the prior Brown patents, the bases of two oppositely d;sposed finned members, which may have any desired cross-section such as a U shape or an L shape, are welded simultaneously to the tube by electric resistance weldin- to produce substantially identical rows of welded areas. The welding operation is carried out with opposite pairs of welds made by the use of two roller electrodes pressed against the bases of the two oppositely disposed finned members. The current flows throtigh the tube from the weld in one fin base to the weld in the other fin base, and the presstire necessary to effect the weld is exerted by the welding electrodes. T@is method has been highly successful but is somewhat limited in its application because the pressure exerted by the welding electrodes is resisted solely by the strength of the tube. This fact has made it impossible to weld successfully light gauge tubing and has required, even with 12 gau.-e tubing having a thickness of more than one-tenth of an inch and an outside di-,meter of two inches, that the welding operation be carried out at a relatively high speed and that the welds bb made intermittently so as to preserve the strength of the tube as much as possible during the welding operation. It is, accordingly, a general object of the present invention to provide aii improved method and apparatus for welding fins to tubes which will not be subject to the difaculties and limitations of prior methods and by means 65 of which fins can be welded to the exterior of tubes of sbch large diameter and light gauge that the tubes themselve have insufficient strength to resist the pressures required in the usual resistance welding operation. An- 70 other object is to provide such a method for making finned iubes *hich can be carried out rapidly and efficiently. 15 20 25 30 35 40 45 50 55 2 Another object is to provide stich a method in which adequate paths for the transfer of heat between the bases of the finned members and. the wall of the tube are provided. Further objects and advantages of the invention will become apparent from the following description of preferred forms thereof, reference being made to tne accompanying drawings. Briefly, our invention conten-iplates a metliod and apparatus for the production of finned tubes, of the type embodying longitudinally extending fins having bases which are welded to the exterior of the tubes, in which the diametrically opposite fin members are welded to the tubes simultaneously and the tubes are internally supported during the welding so that they can resist the pressure exerted by the electrodes, by means of a metallic member which has the property of substantially conformii3g to the interior wall of the tube regardless of reasonable variations in tube diameter and out-of-roundness of the tube, and of changing its external diameter when it is changed in length. In its simplest form, the member consists of a coil spring and in the preferred form of the invention the tube on which the fins are to be welded is slipped over an elongated coil spring while the spring is stretched to reduce its diameter. The spring is then released so that its convolutions firmly engage and support the interior of the tube; the fins are then welded onto the tube, preferably in an apparatus such as shown in the prior Brown Patent No. 2,298,249. Referring now to the drawings, Figure 1 is a perspective view of a section of finned tubing made according to the ;nvention. Fi.-ure 2 is a fragmentary elevation with parts i-@i crosssection to illustrate somewhat diagrammatically tl-le @-nanner in wliich the weldirig operation is carried out. Figure 3 is a transverse sectional view taken as indicated on line 3-3 of Figure 2. Figure 4 is an elevation showing a portion of a weldingmachi,.i-- adapted to @arry out the inethod and provided Nvith apparatus for supporting and guiding the tube and fins tlirough t@ic machine. Figure 5 somewhat dia.-rammatically illustrates a preferred form of apparqtus for placing the coil sprin.- within the tubes. Figure 6 is a fra.-mentary longitudinal section showin.a modified form of spri@ig for supporting the tube. Figure 7 is a simildr fragmentary section sho,,ving a still furtlier niodified forn of sprin.- for supporting the tube. Figure 8 is a seciional view of a supportin.- member embodying a plurality of split rings whir-ii are c-,imm-,d outwardly agains' the inner surfaces c)f the tube. Figure 9 is a transverse sectional view showin.- another type of supporting nien@ ber in which resilient split rings normally expand oiitwqrdly iiito engagement with the tube and are cammed inwardly to be released from the tube. Figure 10 is a 16ngitudinal sectional view somev,,hat diagrammatically iilllsttating the welding of fins onto the exterior of a tub@ wh,@ch has previously bden provided with internal fins as described in our co-p@--ndin.- a-oplication 60 Serial No. 488,560, filed of even date herewith, and Figure 11 is a tt,,insverse section tal@en albng line 11-11 nf Figure 10@ . As sh6wn i-,i Fil-Ilre 1, bf th-@ drawin.-s, a finned tube made a-- cording to i preferred form c-f the inve!-ition cilay comprise ,) tube T to wh:.ch:a n7iriber of fini@-cl membef-S 11 arc seclired. in this forrn of the inventio@-i the finned mep.,tbers are show@i as beiiig of ch,@innel or U section, and each fi-.ined member comprises a base portion 12 that is Nvelddd to the exterior of the tube and t@vo finned p6ttibn 14 f6rmed integraffy with the base, The base and the fins rreferably are of sub.-,tantiilly the same thick-
[2]3 ness and after the welding operation, the base portions conform closely to the exterior of the tube throughout a substantial pqrt of their width. As indicated in the drawi.ig, the @ube T may be of quite thin wall thickness. So far as appears at present, the wall thickness is Eiiziitcd o.nly by th.- 'Lhickiaess that wo7,ild be required to E7ake a proi)er weld in a conventional snot welding operation i-ivolvinelectrodes enga.-ing directly on opposite sides of the weld. in any -vent there is no difficulty in we',din.fins having a thickness of .031" to t@abin.- ",3,4" i;@ eler and hav;ng a thickness of .031". The sLi7en,- La of sticli ttibir.@- by itself is entirely inadequate to wichstand the press@.ires ncrrnally required for resistance weld,'@ng operations without substantial deformation and such deformation prevents the welding operatio-n froi-ii carried oLt properly. In order to eliminate the difliculties that result the weakness and ready deformability of light wafled t,,iLe, we preferably carry out the welding operation as shown diagr.zir.mal,ical',y in Figures 2 and 3 whereiia th,- ttibe -is stipported itil@ernally by a coil spring 15. The n,,rmal external di,-meter of the coil si)rin.- is slightly gr@-ater than the ir)-ternal diameter of the tube T. The spriii@- is ins--rted ir@ the tube by first stretching the sdring u-@it,.1 its external diameler is less than the internal diameter of the tubp- aid thon sl@ippin@- the tube over the sprilig whil-- it is in th-- stretched condition. Thereafter the sprin.- is rele--ised, whereupon it ext)ands olitwardly a.-ainst the internal s-Lirface of the tlibe exerti-@ig a pressure thereupon. If desired, the tube may be vibrated as the tensicii o.1 the spring is being released to assist the - convolutions of the si)r-ing to seat themselves against the internal wall of the tlbe. The spring is selected to have, in conjunct;on with ',he tube. sufficient strength to withstand the we,diiig pressures 'without undue distortign. The pitch and the diameter of the spring both contribute to the srpport of tiie tube. A spring with a smaller pitch, i. e., rriore tlirr@s p.-r uia:t o' len.-th, can have smaller diameter wi,_-e for the sam-, streligth as a spring with fewer turns per unit of ler@.-th and lar.-er diameter .,Yire. However, the convolutions of the spring should be spaced closely enough to prevent su@bstantial dei'iection of the ttibe wall in the zoncs between adjacent convo.utions. In the exaniple ,-iven the spring was co@-nposed of wire about 3,@2" ill diarrieter, and the pitch of the spring was approxiniatelv one inch when the spring was in place within Llic ti-ibe. This gave comoletely adea ,i.iate support:against a weldin.pressl-,re of several hundred pounds. Ilo@x--ver ' for lighter gau.-C tubing, closer spacing of the convolutions may b-- reoi-iired and lar,-er diameter tubing of &le same wall 'hickness probabiy would require a sprin.@ composed of heavier vvire. Tt is to be noted that each convol-Lition of the sprin.presses oiitwardly more or less independently of the other convolutioris so long as variations in diameter or shape between adjacent convolutions are not excessive. Tubing of the sort ordinarily employed iii the man-,ifacture o@' heat exchangers, such as seamless tubing, fi-equen' ly varies slightly in diameter and is often of slight'@y oval rather than circular cross-section. Furtherriore, the major or long diameter of the oval cross-section does not ordinarily stay in the same plane but rotates longitudinally of the tubing. The resilient stipport, however, is adapted to conform itself to the variations in the tubing of the order that occur in ordina.-y tubing manufactilrin,operations. Furthermore, the resilient spring siipport functions to support the tube wall against external pressure adequately even though the tubing may not be straight but may be curved somewhat. Thus, the resilient spring member substantially conforms to and is able adequately to support the tubiilig against the welding pressure regardless of variations in diameter, out-of-roundness, or lack of straightness of the tubing within the usual tubing manufacturing tolerances. 21826,672 - 4 ures 2 and 3, engage the bases 12 of upper and lower finned mernbers 11, and the welding cLirr4@nt fiows throiigh th-. bases of the finned menbers and into the tub@- adjacent the welded area, the welds bein.- irad-- as tho ass.@,.,mbly of tube, spring and finned r-nembers is being prog@-essed between the rotating weld-Ing electrodes. Between the welded areas the current fiows th@- o-ag@i the wall of the ii,,be and the sprin.-, iinless the sormg is composed o@L insula'Led wir.- or is otherwise insulafed from the tuibe. 1() T-@cr so,@-.c c,,)evations it may be desirabe to insulate the wirc: aii@' thzreby to prevent any likelihood of 't)iirning th-@ intericr of the tube where the wire o-'L tl-.@e spring -ngages it. In other instances it may be Su-fRcient to make the spring of a rather roc-,r conductor; @lor examdle, 15 -,i stf@inl,ess spring may be used where fins ai7e be-Ing to tubes of ord-inary low carbon steel, tic tube ha,,,ing slfflcient conductivity to carry the current bef,;jcen the bases of the fins. In other instaic.- s, is fGr exar@aple, when welding fins onto light gaugesla-inless steel tubi,-ig, 20 it i-@iay be des-rable to utilize a sprin.- comDGs--d of a good condlct-nr, such as beryllium copper, ;n which case a substantial portion of the w@-lding current @.vill f'@ow threugh the spring. The welding operation can be carried out advanta25 geously iii apparatus of the type shov,,n in Fi-ure 4 of the drawings and described in greater detail in the pri@,r Brown Pa'cer.'L No. 2,298,249. The apparatus includes a con-,,entio@qal welding riiachiae having a frame 20, only a portion of which is show;i. Electrode rolls '@6 aiid 17 30 forrii a part of 'Lhe ir@achine. Thcse rg-ils are saitably driven and are siipporl,ed fr--,-i th@- fraw--@ by brack,-ts 23 and '@4, v@hich are provid,d with neans, such as air crerated plungers (not shown), for urg-'-qg tho -,Iectrodes towards each other and thereby supplyirg the pressure re35 ql,;red to carry out tl-,e we'idiiig operat,o-Ai. It is this pressure that r@iust be res@@sted by t.,ie combiiied @@ength o-L the tube wall and the internal siipportii.- inen-iber is. Current is stipplied to the electrodes by i7c,,zaEs olr a conventional welding transforrier, preferably vyilh a thyra40 'Lron or like contr,,-l for timing and controlling the current. In order to gcicle the tubc a-,id fns through the ii-iachiiie, 'Lhe jig indicated -en.- rally at '-@5 is pref@rably eriployed. The I-lig is supported by irarr@e irembc-- 26 of the i@@icidi,-g @@,3 rachine and compris.-s a frame mad,- up of end plates 27 and a pluraiity of longitiidinal menab-,rs 23, ,xzhich are d @ 10 preferably we'@ded to th-@ en p'ates to ' rna a rigid structure. A plural-ity of rollers are rrounted upon sti-'@tabl-, bearings carried by the lotigitudino.1 membt,rs and 50 these rollers ar,- arranged to gu@de t'--e tube a-.id fins through '@le machine and to provide for conve-iiient indexing of the ',ube Lor the vielding of a plurality of pairs of iqns thereto. The rollers include Iwo pairs ol' fm ali.@nmerit r--Ils 30 Z,5 that @'unct.;on to -u;de the ins and posit@'on Llie-I -oroperl3, for the welding operation, T-'ir.-c pai.-s of c..-@iterir@g rolls 32 and 3-l' are employ.-d. The rolls 32 ara disposed adjaceiit the al@-'@gnmen" rolls 30, vvh;Ac t-ic rolls 32' are pref-erably placed as close as p-,ssible to the weld60 .;ng zone. indexing rolls 34 at the cids c@f '@he j'ig are angularly spaced j'rom 'Lhe ali,@nme-@it rolls 30 by amount eqlal to the qngle between adjacent pairs ot@ fln members. The operation and construct-'@On of the @ig is d-cscribed 63 in greater detail in th-@ aioresaid Paten', ZTo. 2,29' .'@49. For purposes of ',he present anplication, -'@t -wi'jl be s-,ifflcient to say that 'the ali.-n@neiit ro@lis ind roll@f; ' hirl etigage wit -iqnned w-eirbers, -,vhile t@@ic tube centeri@-irolls enga,@e 'che outer surface cf the tube cli.-cc',Iy. Tht7( cl@etrodes, of course, engage w;thin the fir@s as shcwn Figure 3. The several rolls fu@-iction to g,-,ide th-@ tu',-,-@ anC, fins throu,-h the apparatus as ihe asQ.e:-bly is bei@,iprogressed by the driven electrode rolls. The control for the welding curren'c rqay be adjusteci In operation the welding electrodes 16 and 17, Fig- 7s in any desired manner to produce ffic character of weld
[3]5 required. Preterably, the Nvold consisis of a seiies of over'iap pi-tig spots with scal!6ped edges to provide an excell,-nt path for heat to flo-;i7 between thd tube or wall and finned bqse as described in prior Pateit No. 2,261,13 7. The speed and pressur-. 6f the operation r) may b-- adjusted in any conventional manner to produce a good weld; 'che tube with its iiiternal support has sufficient stre-iigth to resist the weld;ng pressure so long as the pressure is maintai-@ied withiii the range ordinarily i.itil@'@ Zed in welding like materials of like thicknesses. , f) Thiis, witq the tube stipported by th-, inner resiliem member, no particular care need be exercised in the weldin.@ proceeiu .re other than the ordinary care reqtiited to obtain good welds. The operation can be carried out rapidly, pairs of fins being welded on the tube simulta- 15 neously until the desired number of fins have been welded to the tube. After the welding operation has been con-ipleted, the spring is stretched i6 reduce its external diameter ar@d release its frictional engagement witn the interior of the tube, and thereafter it is removed from the tube. 20 Figure 5 illustrates a suit@ble apparatus for iiiserting the spriii- within the ttibe and for removing the spring ther-- from after t.ic welding operation has been complel,ed. This app,@irattis comprises a frame 35 supported on legs 'o'6 and havin- a rod 37 anchored at one eiid as 25 at 38. At the other ciid of the frame 35, 'Lliere is a fluid pressure cyli-,lder 40 having a piston 41 aild piston rod 42. Piston rod 42 is provided with an eye 43 at its end, and rod 37 is provided with a sirqilar eye 45 at its end nearest tlae eye 43. The coil spring 15 is hooked to the two eyes, and iluid pressure is supplied to the cylinder 40 through the conduit 46, the tube T having previously been slipped over the rod 37. The action of the cylinder 40 stretches the sprin,- IL5 ar@d reduces its dian-i-,ter. Then, with the spring clor,- 35 gated E,.nd cO-.itracted, the tube T is slipped over the sprin.- 15. The pressure in the hydraulic cylilder is then released, permitting Lh-- spring to contract longitudinally and expand diametricallv into firm engagoweiit with the interior of the tube. if desired, the +ube may be vibrated 40 as the pressure in the cylinder is gradually released, so that the convolutions of the spring can better seat themselves against the inncr surface of the ti-lbe. After the spring has contracted to its maximum extent, it is disconnected from the eyes 43 and 45 and the assembly is passed throtigh the welding apparatus. After @iie welding o-ocrati6n 'tias been completed, the assembl y is rettirned to the applratus shown in Figure 5, the spring is hooked to the eyes 43 and 45, and the cylinder actuated to stretch tiie sprin.- so that it will be released 50 from enga,@em--nt with the -ann-@r surface of the tube. The-ii the tiibe is slipped back over the rod 37, the spring is disconnected, the tlibe is removed, and the apparatus is ready for another operatioil. Modified forms of the invention are shown in Figures 55 6 to 10, inclusive. In Fig@,ire 6 the tube T is suppotted by a sprin.- 50 which is similar to spring 15 except that it is made of square cross-section wire. Sprilig 50 is used iii the same nianner as spring 15. Figure 7 shows a furth-Ir modification in which the resilient supportin.- member is made tip of two interm--shed sprin.-s, the convolutions of one spring bein.- shown at 51 and the convolutions of the other spring being shown at 52. This type of spring is useful where 65 consider able support is required apd it is desired to have the convolu'Lions spaced rather -,Iosely tog-,tber witneut, however , req,.iiring an tindue stretching and lengthening of the spring to obtain ti%e needed reduction in diameter thereof for inserlion within the t@,ibe, as woil-ld be neces- 70 sary if a sing'le spring with closely spaced convollitions were used. Figure 8 illustrates a different type or resilient internal support for the tube. Here the tube T is supported by a series of resili-.nt split rings 54 which normally spring 75 6 ', 5) j67' 2 6 inwardly but of engagemen'E wilh the interi6r of the tube; i. e., their @@iorriial externa'l diameter is less than the internal diamet-@r of tl-le tub-,. Th-- split rings 54 are su@ported on a central arbor or i--aiadrel 55, whicil may take the form oll a rod or a tube 2,lid which carries on it a series of solid ca-nniing rin;,,s 96. -1@',ings 56 are beveled aL oiie end as at 57 to correspond t6 the bevel 58 of the split rings and have radial surfaces 59 to abut the radial surfaces 60 of the split riiigs. Ntits 61 and 62 are disposed at opposite ends of the arbor 55. Nut 61 functio-,is a ,itop against which the assembly inay be drawn up by ti,,,hten'i-ig nut 62 to reduce the leiigth of the assembly of split rings aiid camini-ii@. rin.-s t6 urge the camming ri-@igs toward each other and cani th@ split rilngs !34 outwardly against i'@ie inside of the tube. Preferably, the -@iat 6'. acts on fac adjace@nl cammin-.i ring 56 through coriipres--icn spring 64 and ttirust bearing 65. With this arrangemeiit, Nvhen the nut 62 is loosened so th,,)'@ the calnir@ilig rings do Pot exert any substantial pressure on the split rings, the split rings tak-e their normal contracted diameter and the tiibe can be readily slipped over the assembly. After the tube is pdsitioned over tiie assembly the nut 62 is tightened up. Tl-.e force exerted by the n,@,t through the spring 64 causes the cammir@g rings to cam the split rings outwardly into firm supporiin.- en.@a.-ement with the interior surface of the tube, thus enabling the inner assembly to support the tube against exterpal pressures such as the pressures exerted by the welding rolls. It will be noted that the split rings 54 can expand vary,@ng amounts to compensate for variations in diaincter of th@ '@ube and that the radial surfaces 59 and 60 of the cammiii.- rings and split rings, respectively, permit adiacen', spl.it rings to move independently of eacl@i other along with their associated camminTinos, there being a clearance as at 66 betvieen the arbor 55 and the cammin.- zings 56 to permit this action to take place. Thus, the riiigs can accominodate themselves individually to reasonable variations and irregularities in the tube diameter. The spring 64, codstantly urges the camming rings in a direction to force the s-olit ring outwardly. The presence of the sprin,@ giv-,s a resilient action to the assembly, so that if, for example, the tuoe should expand tinder the influence of heat, 'Lhe spring will continue to force the camming rin.-s out,@vardly and cause them to expand and inaintain their f@rm co-@itact with the interior of the tube. Another modiiication of the invention is shown in Fi.-Ure 9. -'Llere the tlibe T is internally supported by a series of split rings 70 vvhich are normally of a greater diameter than the interiial diameter of the tube and thus normally resilieiitly support the tube. These rings are mounted on an arbor 71 along with intermediate cammin.- rings 72; Split rings 70 have camming surfaces 73 and 74 oii opposite sides thereof which are engaged by camming slirfaces 75 and 76, respectively, on opposite ends of the camriiing rings 72. In this case there are iiuls 77 and 79 at either end of Ctie arbor 71, and either of thes-@ ca@l b,- used to change the length of the assembly. S'nortening of the assembly of split rings and camming rings in this case causes the spl:it rings to be constricted inwardlv, thereby reducing their extertial diameter so that the asseinbl@y cait be inserted within a tube. After the assembly has been inserted within the tube, the pressure exerted by the nuts 77 and 73 is released, and the split rings 70 are perw@itted to expand outwardly into engagement with the internal surface of the tube to siipport it ,igai-@ist external pressures. The rin,-S, as before, can con_fo@-_m themselves individually to the internal surface of the tube, there bein- ample clearance as indicated at 79 between the rings and the outer surface of the a@-bor 71. After the nuts 77 and/or 78 have been released so that the tube is prot)erly supported, the welding oper-
[4]-'7 ation is carried on as before, with the rina 70 support@s ing the tubing agains" the welding pressures. After the weldin,- operation is completed, the nuts 77 a-@ld 73 ol, either of them- are tightened toward each otlier, thereby camming tlle split rings 70 i-nwardly and releasing them froin e-@igagemen', with the - inter-'@or of 'Lhe tube, so 'Lhqt tl,ie assembly can be removed 'rom the ilibe and used agai:i i-n another weldin.@ operation. Fi,,ures 10 atid 11 illustrate a m--tho@-I of making the tube having f.;iis on both the inside and t.he ou'sid---. 'lere the tube T' is provided ivith external fins 30 and internal fins 31. The f@ns, in -,ach case, are preferqbly formed of channel section fin inembers, and the interpal fins are preferably sec-Lired i-@i place in the man-tier described in our copending application Serial l@,To. 4'og,5.@-@o, filed of even date herewith. Accord;Tig to that application, fns are brazed to the iihier surfaces of a tu..be by holding them i-@i plac-. by a resii'ient -member, s@,ich as the coil spring 32, v,,hich is first contracted diametrically so that the spring, with a series of fi@-s ii, posilion aro@and it, can be inserted withir@ a tube ard is then perm;'Lted to expand diametrically to hold the finned members j'irmly in place against the interi--r of the tube. A brazing metai is stippli,--d to the stirfac.-s to be joined together ar@d the asserrbly heated to -@'Use the brazing -netal and bond the 'ins to 'he, iiiner s@,iri'ace of the tube. Accordin.- to the inventio-ii of our a.-oresa;d- co-pendi-ng application, this com-pletes the tube with t'iie inlerpal fins, ai3ct ordinarily the spring sur,.port;n.- is then remove,:i from the asse-idbly. According to the presert i-tive-,-it3on, however, the spring support;n,- riember is in plac@, and the fins @9@O a@--, resistance v@elded to the exteric-r of the tube by passin- the assen-ibly between the welding eleelrodes 16 and 17 in the manner heretofore described, this orieration be-Ingillistrated diagrar-rirp.atically in Figure 10. After the weldin.- operation is con-pleted, the supporting srring 02 rnay be removed, leaving the tube with internal ard external iins frmly bonded thereto. This operat:oi is useful where it is desired to provide ferro,is tubes with inside and outsid-- fins. The @q-is are secured to th.- ;_nner stirface pref@,-r2bly by copper braziiig in a protective atinosphere, wliile the fins are secured to the outer surface by the res,'@staBce welding rnethod itist described. FroT@i the fore-cin.- descriptioa of preferred fornis of the invention, it will be ev@dent that 'Lhe invention Drovides a rriethod q-nd apparatt,.s vihere'oy fins can be w-,ided to ti@e extericr of light @-fl@ici-.ntly an-,' exped@tiously. The -.nternal s-apport of the tibes is such that proper support is regardi,@ss of variations @n tub.- diameter a-@id out-of-rou-.ijr,@-ss ,iilh normal comn@-ercial standards. The suppor@ir,- mem@ b-.rs, particularly when 'they take 'the forr@i of c.-il sprin.-s, are inexper@sive, and @'t is a sirr@ple matter to asser-b'@e the tubes and the siipporti-iig me-mbers and to remcve the supportmembers from the tubes afcr the vield,.rg operation @ias taket: -place. The efflcient cf the tubes nakes it i-ioss@'ble to select proper weldinfz procedures to obtain th@ best tvne o'L' welds for th-- partici-ilar service -,Yithout reqtiirir,g the modificat-'@b.,i of the weldipq procedures because --LI the nat-Lire of tti-, tLi7t3c, tube at all. times bei.n,- adequsitely supporled agaiiist weldi@ig pressure that m@ ay be req-uired@ Varioiis ci'iaill-es and n@ in t.ie invention ,,iii'i loc arpalcill to 'hos,@ skilied in th-- The essential characteristics of the inventio@l are defined by the llt)peiaded We
