CA1247592A - Finned heat exchanger tubes and method and apparatus for making same - Google Patents

Abstract

"FINNED HEAT EXCHANGER TUBES AND
METHOD AND APPARATUS FOR MAKING SAME"

ABSTRACT

Improved method and apparatus for making finned tubing from difficult to work materials such as titanium and stainless steel uses spaced sets of finning discs on a plurality of arbors to form a tube against a mandrel having different diameters under each set of discs. By forming the tube fin tips to their final O.D. in a first disc set and by deepening their roots to bring the fins to their final height in a second disc set, tubes with higher fin counts and/or higher fin heights can be achieved than were formerly possible. The process of separately cold working the tips and roots also permits tubes to be made which are dimensionally identical to prior art tubes but with higher quality and productivity since tube stresses are greatly reduced. An improved titanium tube is also dis-closed which has at least 26 fins per inch, a fin height of at least 0.034" and an outside to inside surface area ratio of at least 3Ø

Description

:~Z~'7S~2 FINNED HEAT EXCHANGER TUBES AND
METHOD AND APPARATUS FOR MAKING SAME"

BA~KGROUND OF THE INVENTION

The invention relates to heat exchanger tubes and particu-larly to such tubes which are provided with fins. Finned tubes are used extensively in applications such as refrigeration and process-ing where it is desirable to maximize surface contact area and mini-mize tube length, weight and volume.
Materials for heat exchanger tub;ng vary widely depending upon their characteristics such as cost, corrosion resistance and fabricab;lity. In recent years, titanium has been receiving in-creased usage due to its excellent corrosion resistance in a variety of environments as well as due to ~ts increased availability and the decreased cost o~ welded tube relative to the extruded seamless tube formerly used. However, fabrication of finned tubing out of titan-ium is severely complicated by some differences in the mechanical and physical properties of titanium as compared with other materials, notably copper, aluminum and various nickel alloys. Most significant of these propertfes is the rate of work hardening. When metal is worked at a temperature below its crystallization temperature, its strength is increased while its ductility or ability to be defonmed without cracking is decreased. Continued deformation in this region of temperature can continue until a point is reached where fracture occurs. ~his fracture may be complete separation of the part into two or more pieces. However, such total separation is usually pre-ceded, except in the mDst brittle of metals, by localized cracking.
In a normal production operation, it is desirable to establish condi-tions such that any form of cracking rarely occurs. Thus, to take ~ '7 S~3 2 into account the many variables involved in a finning operation, such as tool wear and Yariations in dimensions, material and tem-perature, a total defornation significantly below the Yalues deter-mined by destructive tests is chosen.
There are several alternative methods of improving the workability of material so as to increase its ability to acsommodate more deformat;on without failure. These include increasing the working temperature of the mater;al and heat-treating the material between successive stages of deformation. As a general rule, the strength of a material decreases, and the ductility increases, with increasing temperature. However, with most metals and alloys, a point is reached, as temperatures increase, at which the material no longer work hardens. As rapidly as the material is defonmed, the metal relieves itself of the effect of the strain and a new strain-free, non-work-hardened structure is generated.
Heat-treatment is a broad term which covers any heating operation performed on a metal and its effects of course vary with each metal or alloy. Recrystallization is the heat-treatment of most significance in the present context. During recrystallization, old grains, which have accommodated deformation and, consequently, ha~e b~come strain-hardened, are replaced progressively through the formation of new yrains which are free of the effects of the previ-ous strain and are thus ready to accDmmodate as much strain as were the original grains before any deform3tion occurred. Another heat-trea~ment, knowm as recovery, involves the reduction Dr removal of work-hardening ~strain-hardening) without apparent, or at least major, motion of grain boundaries, that is, without major recrystallization.
Recovery will usually result in the ability of a metal to accept some ~24'75<3~2 more deformation prior to fracture, but not as much as would have been accv~modated had the material been fu11y recrystallized. While high temperature working and heat treatments do offer some techni-cal advantages, they are usually accompanied by increased costs due to increased equipment, labor, facility and other associa~ed compo-nents of manufacturing.
In the manufacture of finned tubing, the fins usually ex-tend in a helix along the length of the tube and are produced through use of fonming tools wh;ch deform the tube and force a por-tion of the metal radially outwardly to form fins while at the same time the I.D. of the tube is forced radially downward. The tools produce a continuous fin which normally has an outside diameter equal to or slightly less than the starting outside diameter of the tube.
Between each fin ~s a groove which is fonmed by the tooling and which defines the root diameter (R.D.) of the fin. The R.D. is smaller than the original diameter of the tube. In the conventional fin-form-ing operation, the forming is done using one or more sets of discs which force the tube against an internal mandrel pin which has a work surface with a constant diameter which is less than the I.D. of the starting tube. During the deformation, the amount oF work-hardening present in each portion of the work piece will vary widely. For exam-ple, there will be areas of high work-hardening near the outer dia-meter of the fin, with relatively low work-hardening effect in the tube wall under the fin. If one then assumes that the areas of high-est work-hardening which are produced near the outer diameter of the fin are the maximum achievable prior to failure, one may conclude that this configuration limits the fin dimensions which are possible without use of hot working, heat treatment, and/or metal removal pro-cedures.

~ 2~75~2 Historically, in the development of finned tubing, fin counts and Fin heights started with lower fin densities, such as 16 fins per inch (f.p.i.) and higher fin heights9 such as 0.050", espe-cially in the easy to fin materials such as copper, copper alloys and low carbon steels. Most probably, this situation prevailed more because of the ability of manufacturers to fabricate suitable dura-ble tools than because of the ability or inability of the material to withstand the work applied. At the present time, advances in tooling and in finning technology have allowed manufacture of prod-ucts with fin densities of double or more the aforesaid figure of 16 f.p.i. In the case of the easier to fin alloys, the prior fin heights have been held and even advanced to 0.060" or so. Obviously, the general goal of development work in connection with fin tubes is to max;mize heat transfer while minimizing tube length and cost.
Where higher fin counts and higher fin helghts can be ach~eved, it is obvious that the ratio Ao/Ai of the outside area to the inside area will be increased, thus increasing heat transfer and permitting less length of tubing to be used than is the case with a lower Ao/Ai ratio.
In the situation of the difficult to fin refractory alloys such as titanium and s~ainless steel, it had been felt necessary, in the past, to have fin walls under the fin, for titanium, of about 0.042" to produce fin densities of about 19 f.p.i. and fin heights of 0.035". Similar figures for stainless steel were 0.065" wall, 16 f.p.i. and 0.050" fin height. Later proposals were made to in-crease the fin density, such as to 26 f.p.i., for titanium, while decreasing the fin heigh~ to about 0.025" and reducing the wall thickness under the fin to about the same value. The last noted ~2~S~2 parameters increased the ratio of the outside tube surface area to the inside area as compared to the parameters formerly used. The aforementioned later proposals are at least generally embodied in U.S. Patent No. 4,366,859 issued to John M. Keyes. The Keyes patent emphasizes that fin heights should not exceed 0.033" ~or titanium, or 0.045" for stainless steel, and argues that "fin splits" will occur if these heights are exceeded. The Keyes patent shows the tubing as being finned on a mandrel having a single diameter work surface against which the tube is forced by one or more arbors carry-ing single sets of discs.
Another patent related to the finning of difficult to fin materials is Laing et al, U.S. Patent No. 3,795,125 which discloses a method of forming fins with a height of at least 0.100" on stain-less steel tubes. The fins are ~ormed in two completely separate .
fi ~ o~Qra~iQns through separate sets of discs with differing contours. The second finning operation produces both a substantial increase in the fin O.D. and a decrease in its R.D. but cannot be performed without an intermediate annealing operation. The technique . _ .. .
is time-consuming and costly. Also, it is not very practical when making the vast majority of tubes which require intermediate unfinned lands and plain ends due ~o the fact that there would be a non-pre-dictable varying amount of stretch of the tube between the separate finning passes. This situation would make it practically impossible to produce lands positioned within currently accepted dimensional specifications.
Two patents relating to the finning of easy to fin material such as copper are U.S. Patent No. 2,868,046 to Greene and U.S. Patent No. 3,383,893 to Counts. Each shows a disc arbor with spaced sets of ~LZ~ 3 ~

discs with the discs all being of different contours.

SUMMARY OF THE INVENTION

It is among the objects of the present invention to pro-vide a heat transfer tube of a difficult to form material such as titanium or its alloys which will provide an outside to inside sur-face area ratio which is at least 3.0 and higher than those previ-ously available. Another object is to provide such difficult to form tubes with combinations of fin densities and fin heights which are similar to those used for easy to fin materials and which exceed those previously thought possible with difficult to form materials.
A still further object is to provide an apparatus and method for finning a tube made from difficult to form material to provide com-binations of previously unattainable fin heights and/or fin counts in a single pass and in such a manner that the tube will not be over-stressed. Yet another object is to form tubes in a s~ngle finning pass which have combinations of fin heights and fin counts no greater than those previously obtainable but in a manner that reduces stresses in the wor~ and tools and improves quality and productivity.
The foregoing ~nd other objects and advantages are attained by the improved heat transfer tube of the present invention and by the apparatus and method which has been disclosed for making it.
The improved tubing has, in the case of titanium or alloys thereof containing at least 50% titanium, at least 26 f.p.i., a fin height ~f at least 0.034" and a rati~ between its outer and inner surface areas of at least 3Ø By way of comparison to a particu-lar, hereinafter defined, part number of a tube, "305028," made in accordance with the teaching of the aforementioned Keyes U.S. Patent ~Z~759~

No. 4,365,859, a tube made in accordance with the present invention was calculated to provide an improvement of about 28i' in the ratio Ao/Ai of the areas of the outside tube surface to the inside tube surface. As is well known in the tube art, part number "305028"
means that the tube has 30 fins per inch, a root diameter of 5 one-eighths of an inch and an average wall thickness under the fin of 0.028". According to the Keyes patent, the fin height can be a maxi-mum of 0.032," giving an Ao/Ai ratio of 3Ø However, in one example of a tube made in accordance with the present invention, a fin height of 0.047" was achieved, providing an Ao/Ai ratio of 4Ø When the rather unconventional ratio of the fin surface area after finning to the outside surface are~ prior to finning is considered, the im-provement in the value of the ratio as compared to Keyes can be cal-culated to be 32% (3.2 vs. 2.4). The latter ratio is the one dis-cussed in the Keyes patent as represent~ng a 26% improvement over the prior art. Obviously, the area ratio will vary, depending upon the particular part numbers being produced.
To achi~!ve the previously noted improvement in tube per-formance~ we prefer to make the tube in either 1, 2 or 3 starts, but not limited to either, by at least two disc arbors, and preferably three or four, which each have at leas~ two sets of discs separated by a relatively wide space. We also prefer to use a mandrel having a surface characterized in that the diameter of the mandrel pin is different under each of the spaced sets of discs. This could be achieved by steppinq or by a tapered sur~ace. Finally, we prefer to grind all of ~he d;scs to ~he same tip con~our so they differ only in their diamPters. The latter technique greatly minimizes tooling costs and also causes the fins to be formed in a much different man-~47592 ner than would ~e the case with the discs disclosed in the varioLIe aforementiDned patent~. In practicing the imprGved methDd! the tube i 5 first finned to obtain normal fin hei~hts with the first to~,ling set in coDperation with the larger diameter portior, of the mandrE~l. It i5 then additionally finned in the same pa~s, and without an intermediate annealing~ by a second toolir,cl set in cooperation with a smaller diameter portion of the mandrel The first too]ing set determines~ or at least su~stantially determines~ the final O.~. of the fin. The second tooling set does not change ~r at least does not appreciably change the O.~. bLIt it ~oes s~lb~tantially red~lce the ~.D.~ thu prDducing substantially higher fin heights than is normally possib]e witho~lt en-essive working nf the material or witho~lt an intermediate heat treatment. The tooling holders or di5c ar~ors which sLIpport the tooliny discs are set at angles relative to the tube in the usLIal manner so as to advance the work. They are also preferably geared together for rotation relative to each other and the tube so that when the tube has made a complete rotation~ it will have advanced through l~ or more pitches.
It will be appreciated that the foregoing apparatLIs and method will not move the fin tips up any more than is normal for low fin height finned tubes~ as discussed in the ~eyes patent~ and wil3 thus not overly work the tips~ It does~ however~ mo~e the F.~.
down in the second tooling set after the fin has been moved up in ~247S~2 the first set and thLls permits a t~be having the same fin count ac~ in ~eyes to be provided with a s~stantially higher fin height without developir~ ndesirable fin strEs5e5 Gr increa ing the wear and tear on the touling. However~ it should be recogni~ed~
that~ the C~.C~ ma imL~m ~pper- limit on fin heights diccLIcsed ~y ~eyeC i5 one which p~shes the material and tor.ling to a limit.
Th~s~ one co~lld e~pect that the quality cf the tubes produced and the productivity of the tooling would be somewhat compromised a5 compareti tc manufacturing a tube having a much lower fin height.
0 The in~ention also relates tu a novel method of achieving a high deyree of overall wor~ing of the tube material in order to achieve a fin configuration which would either not be otherwise obtainable by a single working operatiDn or would possibly compromise tube quality and~or productivity~
Additionally~ the added e~pense~ associated with either high temperature wor~ing and/or one or more recrystallization steps are avoided The inventinn provides a very substantial improvement in either the ratio of the Duter to inner s~rface areas ~o/Qi or in tube qu~,lity and~Dr productivity by ta~ing advantage of the fact that the tube defDrmatiDn during finning provide~ widely varying amounts of work hardening in different regions of the workpiece.
~s discussed in the keyes patent~ work hardenin~ at the fin tip would ~ppear to limit the overall fin height. However~ we have determined that the amount of wor~ hardening present in each portion of the workpiece will vary widely for reasons disc~lssed hereinafter in detail in connection with the accompanyina -s~2 drawings. The dr~ings will indicate that a ingle finr,ing pass forming the ma~:imum fin height prDpoced by ~eyes will prodL~ce areas of high worl hardening near the outer diameter of the fin~
but will prodL~ce relatively low wor~ harder,ing effect in the tube wal] ur,der the fin. Our invention ta~es advantage of the large volume of material in the tube which has much le~s than the critical amount of wor~ hardenir,g in it~ This i5 done ir, our proces~ by Lltili~ins the amount of deformation available withoL~t e~:ceeding the critical strain limit at any location.
In our process~ a ~econd fin-forming operation is performed subse~uent tu the first one in which both the root diameter of the groove and the inside diameter of the tLIbe are simL~lt~neou~ly reduced. This reduction is achieved through use of a second mandrel section who~e diameter is smaller than the diameter of the first mandrel section used in the first finning operation tu initially form the fin to a height and outer diameter within the limits proposed by ~eye~. As the disc~ on the tooling arbors press a~ainst the groove from the outside of the tube~ the pressLlre causes the tube I.D. to be reduced in diameter down to appro::imately the same diameter as the mandrel. There i 5 no significant change in the fin O.D. during this step. Thus~ the decrease in the groove diameter and the inside tube diameter lead to more sLlrface area in the fin~ thereby increa~ing the overall efficiency of the tube.
~lthough the process described herein teaches that sub~tantial increases in tube surface areas and~or in tube quality can be made in a single finning pass using two sets of ~Z4~755~

finning di~cs and twu ciifferer,t diameter mardrel sections and without heat treatmert steps~ it wc,LIld be wit~,in the scope of the present inventinn to use three or more sets of finning di~cs in combinaticn with three clr more different diameter ~,andrel sertionC to achieve a final desired tube shapen Furthermore~ the height of fin achieved by the variou~ sets of di~cs can be v~ried to optimi~e and or equali~e the overall work hardening for a particularly desired 4inal res~l]t of wor~ pattern~ within the wor~piece~ Also~ a lighter wall under the fin coLIld be achieved 0 than i~ possible with conventional techniqcle~ since less unit pres~ure needs t~ be applied~
The preceding description of the invention makes particL~lar mention of titanium and stainless steel as e amples of diffic~lt to form materials. H~wever~ we do not intend to limit the inventior, to these m~terials since its advantage5 would alsc~
be applicable to other difficult to form m~terials. For e~:ample~
copper~ although relatively easy to Form to fin height~ higher thar, previously thought possible in titanium or stainless ~teel~
could be difficult to fr~rm to higher fin heights.

Fig. 1 i5 a side view~ partially in ~ection, showing the relatiunship of the spaced sets Df finning cliscs and ~he varying diameter mandrel sectiDns to the tube a5 the tube ic: being formeci;
Fig. 2 is an enlarged view of a tube cross-~ection which schematic:ally indicates the wor~ hardening which might be present in the tubr- at region X ir~ Fig. 1 ~y means r~f lines which connect and define points rf equal deformatir~n and~ as a first appro~:imation~ points of equal strain hardening;
Fi~ ~ i 5 a Vi ew 5i milar to Fig~ ~ but schematically showinr~ the nature of the defDrmation which wDuld ta~e place in the tube cro~s-section at region Y in Fig. 1!
Fig. 4 is a view representing a summation of th~ strain hardenin~ effectG fro~, both the first finnin~ operation ~FicJ~ !~
and the se~ond finning opere~tion ~Fig.~ as they wDuld be embodied in the finished tube at region "~" Df Fig. li and Figs. 5a-Sc are enlarged~ partially bro~en away~ views showing the relationship between the tube cross-secti~n and mandl-el at regions "~J"~ "X" and "Y" in Fig~ 1~

DETAILE~ DE~,C~IFTION OF THE DR~WINGS

~eferring to Fig~ 1~ a tube indicated generally at 11 is lS shown in wor~ing relationship with respect to a mandrel holding rod 12 having a first larger diameter mandrel section 14 and a second smaller diameter mandrel section 1~ held thereon by a fastening member 1~ In Drder to provide relatively unif~rm finning pressure to the tube during forming~ a plurality of finning arbors 24 are located circumferentially around the tube on skewed a~:es in the usual fashion. The arbors are each provided with spaced sets 26 and 2~ of finning discs which are separated by a spacer member ~C) and retained on the arbor by fastener means such as a cup washer 32 and socket head screw ~4.

'75~32 Preferably, the individual fin discs 26a-26h and 28a-28-f all have their thicknesses tl and t2 equal to each other and they also all have their outer tip side and end contours 36 and 38 equal to each other. Thus, the 14 different discs shown differ only in that their outer diameters vary, thereby greatly facilitating their manufacture. Although they are not visible in the small scale of Fig. 1, the discs 28-28f are each separated by a thin shim or spacer member 40 which, together with the disc thickness t2, would cause the axial distance between adjacent discs, which is defined as the pitch, P2, of the second set of discs 28 to be slightly greater than the pitch, Pl, of the first set of discs 26.
This situation results in causng the high fins 11" produced in region "Y" of the tube by the second set of discs 28 to have a slightly greater pitch than the less high fins 11' produced in region "X" by the first set of pins 26. The fins 11' produced by disc set 26 are lengthened considerably in a vertical direction as they pass through the disc se-t 28.
However, the thickness oE their tips and their outer diameter remains substantially constant since the additional space between the fins in disc set 28 which is provided by the shims ~0 permits the Ein tips to move further into the V-shaped slots between the discs so that the roots of the ~ins will have their root diameters decreased from the dimension provided by disc 26h to the dimension provided by disc 28f.
The difference in pitch Pl and P2 between the disc sets 26, 28 and the stretch intro-duced in the tube 11 as it is worked causes the tube pitch to vary from a dimension Pl when it leaves the first disc set 26 to a larger dimension P2 when it leaves the second disc set 28. The stretching, or elongation of the tube reduces the amount of twisting which takes place as the tube is finned.

jb/ye .

7~

Figs~ 2 and ~ illustrate the fact that the pitch distance P~of the fins at regior, "X" of Fig. 1 is less thar, that at region "Y". They aleio ~chematice~lly reprecient the wor~
hardening present in the tut-e 11 as it is being formed in reglons "X" and "Y"~ respertively~ in Fig. 1. The lines ~ 4~ 6~ 8 and 1~:) which have be-en drawn are intended to connect and define pDints of equal defDrmation~ and a~ a first appro~imatiDn, points of equal strain hardening~ with higher n~mbers representing higher strecis. In Fig. ~ it can be seen that the areas lC) of ~0 high worl hardening are near the outer diameter of the fin. In Fig. 3~ it can be seen that the areas lO of high work hardening are in the area of the tube wall under the fins, and thus do not a4fect the fin tipe~ ~lthoLIgh small areas lO of high work hardening are also indicated immediately under the roots in lS Fig.~p this are~ is not aei critical with respe~t to being additionally worked as the fin tips would be~
Fig. 4 represents a summation of the strain hardening effects from both the first finning operation ~Fig. 2~ and the second finning operation tFig.~) and wo~lld be representative of the com~osite strain hardening effects pree-ent in the tube at region "Z" in Fig~ 1. It can be noted that the amount of wor~ing is more Llniform than for eitheer of the separe~te finnir,g operati Dns~ Thus~ the twD-step prDcess takes full advantage of the cold-worke~bility of the ~tarting material without e~pensive heat treatmer,t processes.
Figei. 5a~ 5b and Sc illustre~te the relative thicknescec;
Df the outer wall Df thce t~lbe 11 and the dimen_ions of the mandrel ~2~7~i92 and tube at locatiDns "W"~ "X" and "Y" in Fig. l. In Fig. 5a the tube has a wall thickness o-F ac before finninq. Following the _ first finning operation with the set of di~cs ~6 ~Fig. l)~ the wall thickness under the fin is reduced to the dimensior, a~b ~hile the fin tip ll~is wDrl~ed t~ ~ height of b~c ~ Follo~Jing the second finning operation with the set of discs ~8~ the wall thic~ne~s under the fin is reduced to the dimension "b" and the fin height is increaeed to the dimension b"c". Qs previoLIsly discussed~ the mandre] diameter i 5 al~o reduced from the relatively larger diatneter shown at 14 in Figs. l and 5b to the relatively smaller diameter shown at l~ in Figs. l and 5c.
In an e~ample of a tube made in accordance with the invention~ a starting tube having an O.D. of 0.747"~ a wall thic~ness Df C~.954" and an I.D. of 0.~4C~" was finned in a three arbor finning ~pparatus. The tube wa~ welded titanium Grade ~
which is a tube of essentially pu~e titanium. keferring to Figs.
Sa-Sc~ the first m~flndrel section 14 had an O.D. of 0.59C~"~ while the second mandrel section l6 had an O.D. of C~.58C~". The first set of discs 26 ~Fig. l) formed the fins ll~ ~Fig. 5b) so that the fin height b~c~ had a value of C~.C)3~" and an k.D. of C).6~
The second set of disc~ ~8 ~Fig. l) fDrmed the fins ll" ~Fig. 5c) so that the fin height b"c" had a value of C).C)47" and an k.D. of C).653". The final I.D. of the tube was C).5~7"~ producing ~ wall thickness under the fin of OrC)28ll. The tube I.D. i~ somewhat larger than the 0.~. of the mandrel sectiDn 16 since the tube has an inherent springback which prevent~ it from assuming the same dimension as the mandrel. The final fin pitch F2 ~Fig. ~) wa~ C

S9~

~ine per ir,ch ~s cDmparrd tG the F~ lLIe of 3~ fir.s ~er inch ~Fig. 2) prod~ced by the first set of di c ~6~ The di~erence in pitch is ~ reeL~lt of etretching of the tL~be ~nd ic accommodated in the second set of discs ~8 b~f pl~-iny ~hims 4(~
(Fig. 1) ha~ing a thickne-~ of aboL~t C1~ bet~Jeen e~ch Df the disc ~a-~8f.

Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A heat exchange tube made of titanium or an alloy thereof containing at least 50% titanium comprising at least one integral helical fin with a fin density of at least 26 fins per inch of tube, a fin height of at least 0.034" and a surface area ratio between its outer surface and its inner surface which is greater than 3.0, said at least one fin and the tube wall which is under said at least one fin being strain hardened to a greater extent in the tube wall under said at least one fin than in the tip of said at least one fin, said at least one fin being formed in n single pass, without intermediate annealing, by at least two spaced apart sets of finning discs, the strain hardening of the tip of said at least one fin being produced substantially completely by the first of said sets of finning discs and the strain hardening in the tube wall under said at least one fin being produced substantially completely by the last of said sets of finning discs.

2. A heat exchange tube according to Claim 1, wherein said tube has 26-60 fins per inch and a fin height in the range of 0.034-.075".

3. A heat exchange tube according to Claim 1, wherein said tube has 26-50 fins per inch and a fin height in the range of 0.034-.060".

4. A heat exchange tube according to Claim 1, wherein said tube has 26-40 fins per inch and a fin height in the range of 0.040-0.050".

5. A method of making a heat exchange tube from difficult to work materials such as titanium and stainless steel in a single finning pass comprising the steps of inserting a mandrel having at least a first larger diameter portion and a second smaller diameter portion inside a plain tube; moving the axes of a plurality of rotating disc carrying finning arbors toward said tube so that first and second sets of discs on said arbors, which are separated from each other by a spacer member, will sequentially force portions of said tube toward said first and second portions of said mandrel, said first set of discs serving to initially form the fins on said tube to at least approximately their final outside diameter and said second set of discs, whose discs are axially spaced so as to have a greater pitch than the discs of the first set, serving to reduce the root diameter of the fins previously formed by the first set of discs without substan-tially changing the outer diameter of the fins formed by said first set of discs, said greater pitch of said second set of discs causing an elongation of said tube and reducing its tendency to twist during finning.

6. A method of making a finned heat exchange tube according to claim 5, wherein all of the finning discs are of different diameters but have a constant thickness and also have identical contours at their tips.

7. A method of making a finned heat exchange tube according to claim 5, wherein said sets of finning discs are sized so that the first set will form fins of a height which is at about the normal limit for the particular material used to which the fin tips may be worked without splitting.

8. A heat exchange tube made of titanium or an alloy thereof containing at least 50% titanium comprising at least one integral helical fin with a fin density of at least 26 fins per inch of tube, a fin height of at least 0.034" and a surface area ratio between its outer surface and its inner surface which is greater than 3.0, said tube having a wall thickness under the fins which is no greater than the height of the fins, said at least one fin being characterized in that it is formed in a single pass without intermediate annealing by spaced apart sets of finning discs which sequentially cause the tip portion of said at least one fin to be formed to its final outer diameter and then cause the root to be formed to its final depth.