US 3776018 A
Tubing with a longitudinal axis and inner fins extending inclined to the axis and being indented to form multitudinous baffles on the fins, and a method of producing such tubing from a round inner-fin tube blank, by partially flattening the blank for interpress of opposed fins, and subsequently expanding the partially flattened blank by internal fluid pressure.
Description (OCR text may contain errors)
United States Patent 1 1 French * Dec 4, 1973  TUBING WITH INNER BAFFLE FINs AND  References Cited METHOD OF'PRODUCING IT UNITED STATES PATENTS  Inventor: Fred W. French, Morris, Conn. 3,662,582 5 1972 French 72/367 1,951,394 3 1934 Chase et al. 165 177 s g Nm'flnda Metal lndllstl'les, e 3,422,518 1 1969 French 29 1573 A Bellingham, Wash. 3,596,495 8 1971 Huggins 29 1573 A 3,603,384 9 1971 Huggins 29/1573 A Notice: The portion of the terms of this i patent subsequent to May 16,
1989 has been disclaimed.
Filed: Feb. 29, 1972 Appl. No.: 230,220
us. 01. 72/367, 113/118 R, 29/1573 A, 165/177 Int. Cl 13210 53/06 Field of Search 29/1573 AA, 157.3 A, 29/1573 v; 113/118 R, 118 A; 72/367;
Primary Examiner-Richard J. Herbst Attorney-Waiter Spruegel ABSTRACT 12 Claims, 18 Drawing Figures PATENTEU EB 41975 3 776 018 SHEETEOFQ TUBING WITHINNER .BAFFLE FINS AND METHOD OF PRODUCING IT This invention relates to tubing in general, and to finned tubing in particular.
The type of tubing with which the present invention is concerned is provided with inwardly extending fins, or so-called inner fins, on its peripheral wall, and is used especially, though not exclusively, for heatexchange purposes. Such known tubing is formed in different ways, including brazing or otherwise joining inserted fins. to the round wall of a tube, but preferably by displacement in known manner of metal from the wall of a tube into inner fins on the tube wall, with the inner fins in such tubing extending uninterruptedly along the tube wall either axially or helically of the tube. Tubing of this type is well known for its heatexchange properties which vary from good to excellent, depending on the inner-fin pattern and size, the particular heat-exchange application, and other factors. However, while tubing of this type fully meets a great many different heat-exchange and other requirements, there are limits to its applicability for certain specific I purposes, such as, for example, high fluid turbulence or agitation, owing to inadequate dispersing effect on passing fluid of the inner fins by virtue of their continuity and also parallel extension.
It is the primary aim and object of the present invention to provide tubing of this type of which the inner fins are arranged so as to be fully applicable for the aforementioned exemplary and also other specific purposes. This is achievedby resorting to tubing of which the inner fins extend helically in the first place to force passing fluid into a primary spiral flow path along the tube wall, and the inner fins themselves, rather than being uninterrupted throughout, are frequently interrupted over their longitudinal extent to providea multitude of spaced baffle-like formations, or baffle fins,
whose heat-exchange and also flow dispersing effects on passing fluid are far beyond the capability of uninterrupted fins.
It is another object of the present invention to provide tubing of this type of which the aforementioned baffle fins may be arranged in a great variety of difierent patterns in the matter of their helix angle, their individual length and height, and their spacing side-by-side andalso end-to-end, thereby to afford as widely varying effects of the baffle fins on passing fluid.
It is a further object of the present invention to provide tubing of this type the peripheral wall of which may be round or of any other cross-sectional shape, thereby to bring widely different proportions of passing fluid within contact reach of the baffle fins to meet most exacting heat-exchange and other requirements, including fluid dispersion and flow diversion.
Another object of the present invention is to devise a method of producing tubing of this type, by starting with a round tube blank with inner fins which extend helically and uninterruptedly along the tube wall, and reforming the uninterrupted inner fins into multitudinous baffle fins by pressure-indenting the fins at recurring intervals for displacement of fin metal at the indentations and formation of the baffle fins between successive indentations. This method thus avails itself of tube blanks provided with inner fins in any of the aforementioned different ways, and especially by preferred displacement of metal from the peripheral wall of a round tube into integral inner fins thereon, with the inner fins serving as blank formations for their reformation, by pressure-indentation, into the multitudinous bafile fins.
A further object of the present invention is to provide, in the aforementioned method of producing tubing of this type, for pressure indentation of the inner fins in the starting tube blank by the fins themselves. To this end, the round tube blank is partially flattened from two opposite sides to such anextent that theperipheral tube wall will be fonned into two opposite flat wall sections and opposite return wall sections joining the latter, and the fins on the opposite flat wall sections are at their crossings interpressed, as disclosed inmy copending application Ser. No. 38,132, filedMay I8, 1970, now US. Pat. No. 3,662,582, dated May 16, 1972 with the partially flattened tubeblank thenbeing expanded, preferably by internal fluid pressure, to separate the interpressed fins for bringing the formed bafflefins into functional exposure in the tube. The. partial flattening of the round tube blank is thus a key feature of the method which lends to the baffle fins certain unique characteristics, and also secures numerousadvantages To begin with, it is the nature of thus interpressing the fins at their crossings that the ensuing indentations in the fins are of a depth approximately equalto one-half the extent of the interpress of the fins, wherefore the endwise spaced multitudinous baffle fins emerge, not from thetube wall, but from the remaining uninterrupted bases or roots of the fins. The fins thus have in the expanded tube unique blending characteristics and effects of continuous and interrupted fins. The step of partially flattening the round tube blankisan exceedingly simple and efficient one, affording a choice of thus flattening an entire tube blank, either simultaneously between two opposite platens, or progressively, yet rapidly, between companion pressure rolls. Further, with the fins in the round tube blank being preferably of the same height and thickness throughout and being also parallel to each other throughout theirhelical extent, the indentations at the crossings of all fins, formed by their equal interpress thereat, are identical in depth, width and shape, wherefore the effect of the fins on passing fluid in the ex panded tube is most uniform peripherally throughout the tube. Also, the extent of partially flattening the round tubular blank may vary widely to afford as widely different extents of interpress of the fins at their crossings from a barely interpressed minimum to a maximum at which the tips of the fins on either of the flat tube wall sections extend to the other flat tube wall section, whereby the height of the baffle fins in relation to the overall height of the fins may be varied as widely, with ensuring widely varying effects of the baffle fins on passing fluid in the expanded tube. Further, theopposed fins on the flat tube wall section will cross each other at twice their original helix angle, even though these fins are, in the course of partially flattening the round tube blank, displaced from helical disposition on the round tube wall into straight parallel disposition on the opposite flat tube wall sections. Also, with this angle of the crossing fins beirtgpreferably other than degrees, displacement of metal of the fins at and near their crossings in the course of interpressing the fins, will follow a path of least resistance to such displacement and, accordingly, will be predominantly to one and the same side of the fins. This displacement of tin metal has the added effect of at least partly twisting the baffle fins in each helical row out of their orderly linear succession in the row, with these partly twisted baffle fins having further enhanced heat-exchange and flow dispersing effects on passing fluid in the expanded tube.
It is another object of the present invention to provide, in the method of producing tubing of this type, for expansion by internal fluid pressure of the partly flattened tube blank, not just to separate the interpressed fins, but also simultaneously to deform the tube wall, if not directly into, then at least close to, any one of many different cross-sectional shapes. This involves for most final tube shapes, in addition to internal fluid pressure for tube expansion, the use of an outer form or a shaping die, or both.
It is a further object of the present invention to apply tubing of this type to duplex tubing in which an inner tube extends in an outer tube with the present baffle fins, of which the baffle fins are press-fitted on the outer periphery of the inner tube. In such a duplex tube, any fluid passing through the outer tube, rather than being confined in its flow to the helical channels between the fins, is afforded communication peripherally throughout the outer tube by the baffle fins, with the baffle fins having their described and still further enhanced effects on the passing fluid with ensuing beneficial results also on fluid passing through the inner tube.
Further objects and advantages will appear to those skilled in the art from the following, considered in conjunction with the accompanying drawings.
In the accompanying drawings, in which certain modes of carrying out the present invention are shown for illustrative purposes:
FIG. 1 is a cross-section through a round tube blankwhich is to be formed into final tubing embodying th invention;
FIG. 2 is a longitudinal section through the tube blank taken on the line 22 of FIG. 1;
FIG. 3 illustrates an intermediate step in an also featured method of forming the tube blank into final tubmg;
FIG. 4 is a cross-section through the tube blank in an intermediate stage of its formation into final tubing;
FIG. 5 is a longitudinal section through the tube blank taken on the line 55 of FIG. 4;
FIG. 6 illustrates another step in the method of forming the tube blank into final tubing;
FIG. 7 is a cross-section through final tubing formed by the disclosed method;
FIG. 8 is a longitudinal section through the final tub.- ing taken on the line 88 of FIG. 7;
FIG. 9 is an enlarged view of a part of the final tubing encircled by dot-and-dash lines in FIG. 8;
FIG. 10 is a fragmentary section through the final tubing as taken on the line 10--10 of FIG. 9;
FIG. 11 is a cross-section through a tube blank in a modified intermediate stage of its formation into final tubing;
FIG. 12 illustrates a step in the method of forming a tube blank into final tubing of a different cross-section;
FIG. 13 is a cross-section through the tube blank taken on the line 13-13 of FIG. 12;
FIG. 14 illustrates a final step in shaping the tube blank of FIG. 13 into final round cross-section;
FIG. 15 is a longitudinal section through the round final tubing as taken on the line 15-15 of FIG. 14;
FIG. 16 illustrates a step in the method of forming a tube blank into final tubing of non-round cross-section;
FIG. 17 illustrates a modified step in the method of forming a tube blank into final tubing of non-round cross-section; and
FIG. 18 is a cross-section through modified tubing which also embodies the invention.
Referring to the drawings, and more particularly to FIGS. 1 and 2 thereof, the reference numeral 20 designates a tube blank which is a starting blank for the formation of tubing with featured inner baffle fins. The tube blank 20, which may be of any suitable metal, has an annular peripheral wall 22 and a plurality of inner fins 24 which longitudinally extend helically on the peripheral wall 22 in one direction. The fins 24 are in this instance formed integrally with the peripheral wall 22, and the inner-fin tube blank 20 is preferably and advantageously formed from an initial non-finned round tube in accordance with a method disclosed in my prior US. Pat. No. 3,422,518, dated Jan. 21, 1969, pursuant to which the fins 24 extend at the same uniform helix angle throughout, and they are of the same height and thickness and preferably also equally spaced from each other.
In forming the inner-fin tube blank 20 into the featured tubing with inner baffle fins (FIGS. 7 to 10), the blank 20 is, in an initial. step of an also featured method, partly compressed from opposite sides for interpress of opposed fins 24 at their crossings (FIG. 4). This initial step of the featured method is disclosed in my aforementioned copending' application, now US. Pat. No. 3,662,582. More particularly, the blank 20 is partially flattened into oblong cross-section with opposite flat, and preferably parallel, wall sections 26 and opposite return wall sections 28 which join the flat wall sections 26, with the blank 20 being in this instance partially flattened to an extent at which the fins on the flat wall sections 26 are at their crossings interpressed over their entire heightwise extent (FIG. 4). The blank 20 is thus partially flattened in any suitable manner, such as simultaneously between flat platens (not shown), or progressively, yet rapidly, between rotary companion rolls 30 and 32 (FIG. 3). In thus partially flattening the blank 20, the fins 24 will be displaced from their original helical disposition on the round peripheral wall 22 into rectilinear disposition on the flat wall sections 26 (FIG. 5), with the rectilinear fins on either flat wall section extending parallel to each other and inclined to the axis x of the partially flattened blank at their helix angle A, wherefore the fins on the opposite flat wall sections are inclined to each other at twice their helix angle. With adjacent fins being also equally spaced from each other, the fins on the opposite flat wall sections 26 cross each other at regularly recurring intervals and in a uniform pattern (FIG. 5), and by virtue of their interpress are indented at their crossings c, with the indentations i in each fin (FIGS. 8 to 10) being characteristically of a depth approximately equal to one-half of the extent of interpress of the fins, and with the depth of the indentations i being approximately equal to one-half the fin height because the interpress of the fins is in this instance over the full fin height (FIG. 4). In thus interpressing the fins at their crossings 0, metal of the fins will be displaced for the formation of the indentationsi in these fins at and near their crossings 0. Such displacement of fin metal will occur depthwise of the fins and also sidewise of the fins. The fin metal thus displaced sidewise or laterally of the fins will always seek a path of least resistance. Thus, if the fins on the opposite flat wall sections were to cross each other at right angles, the lateral displacement of fin metal would be about equal on both sides of each fin. However, with the fins on the opposite flat wall sections 26 crossing each other at the angle 2A, which is an acute angle, the path of least resistance to lateral displacement of fin metal at and near the crossings is predominantly to one and the same side s of the respective fins (FIG. 5).
As a result of indenting the fins on the opposite flat wall sections 26 at their crossings 0, there are formed on each of these fins endwise spaced baffles b of a height equal to the depth of the indentations 1'. Further, by reason of the explained lateral displacement of fin metal at and near the crossings c predominantly to one and the same side of the respective fins, ensuing from the interpress of the fins, the baffles b thus formed are at least in part also twisted out of the linear succession of the baffles on each fin, with these twisted baffle parts 34 (FIGS. 5 and 8) projecting into the flow channels 36 formed by adjacent fins.
In the partially flattened blank 38 (FIG. 4), the fins 24 on the opposite flat wall sections 26 are interpressed and their respective baffles b, though finish formed, are interlocked over their heightwise extent. In order to bring the baffles b into at least partial, but mostly into complete, functional exposure in the interior of the blank, the method further contemplates as another featured step expansion of the partially flattened blank 38 for separation of the fins at least partly from their interpress, but mostly for their complete separation from each other. This is accomplished preferably by internal fluid pressure in the blank. To this end, the partially flattened blank 38 is at one end sealed in any suitable manner, and the same is with its other, open end 40 connected with a header 42 (FIG. 6) which is formed in parted sections 44 and 46 with interposed compression seals 48 and 50 at their partition, with these header sections 44, 46 forming a chamber 52 with which the open end 40 of the partially flattened blank 38 is in communication. The header sections 44, 46 are removably clamped together with sufficient free to compel the compression seals 48 and 50 to seal the chamber 52 and therewith communicating blank end 40 from the outside. Also communicating with the chamber 52 is an inlet conduit 54 through which to introduce fluid f under pressure, preferably a liquid, into the chamber 52 and, hence, also into the partially flattened blank 38 for expansion of the latter with ensuing separation of the interpressed fins. However, expansion of the partially flattened blank 38 in this fashion is preferably and advantageously controlled for simultaneously forming the desired final tubing 2 which in this instance is a tube of oblong cross-section with two predominant opposite flat wall sections 56 and short return wall sections 58 which jointhe flat wall sections 56 (FIG. 7), with these flat wall sections 56 being spaced so that the baffles b of the respective fins thereon are in this instance spaced apart for their full operational exposure in the interior of the tubing. To the end of thus controlling the expansion of the partially flattened blank 38 for its formation into the final tubing 1, there are provided on opposite sides of the blank 38 flat parallel platens 60 and 62 against which this blank is expanded into form-fit therewith. The pressure of the fluid is thereafter released, and the tube 1 removed from the header 42 and from the platens 60, 62. On then cutting off the opposite ends of the tube, there remains a full length of finished tubing t of uniform cross-section throughout (FIG. 7), with this tubing being usable for heatexchange and other purposes involving the passage of a fluid therethrough.
FIG. 8 illustrates in top plan view the disposition of the fins 24 on one of the flat wall sections 56 of the tubing t together with the baffles b on these fins, as they are in their entirety exposed to fluid passing through the tubing. Besides the sharply delineated multitudinous baffles b with their readily comprehensible dispersing and other effects on passing fluid, another easily noticeable and outstanding characteristic of these baffles lies in their end parts 34 which are twisted out of the linear succession of the end-to-end arranged baffles and project into the primary flow channels 36 between adjacent fins 24 to act as highly effective deflectors of passing fluid from each of these flow channels into adjacent ones. Also, the multitudinous baffles b are arranged in as orderly and uniform a pattern (FIG. 8) as are the interpressed fins on the opposite flat wall sections 26 of the partially flattened blank 38 (FIG. 5), meaning that the baffles b are of identical length and are inclined to the axis xz of the tubing t at the same uniform angle, and their twisted ends 34 project to one and the same side of the fins on which the baffles are formed, and all of these twisted baffle ends 34 are, moreover, baffle ends which lead in one direction axially of the tubing, in this instance in the direction of the arrow 66. Accordingly, the effect of the baffles b on fluid passing through the tubing t in either direction is most uniform on the fluid throughout the tubing, with their effect on fluid passing through the tubing in one direction being, moreover, different from their effect on fluid passing through the tubing in the opposite direction, as is self-evident from FIG. 8, wherefore the tubing even affords a choice of either of two opposite fluid flow directions through the tubing for different ef- V fects of the baffles on the fluid. The baffles on the fins on the flat wall section of the tubing opposite to the flat wall section 56 shown in FIG. 8 (with one of these baffles being shown in dot-and-dash lineo in FIG. 8) act in concert with the baffles shown in FIG. 8 for optimum dispersing and other effects of all baffles on passing fluid. Moreover, with the tubing t being of the exemplary oblong cross-section in which the baffles on the fins on the opposite flat wall sections 56 are in close proximity to each other (FIG. 7), virtually the total of fluid passing through the tubing; is within effective reach of all the fins and their baffles for their maximum effect on the fluid. This tubing is thus particularly effective for heat-exchange purposes as well as other pur poses involving far-reaching flow diversion of fluid passing therethrough.
The baffles b on the fins in the tubing t have further unique and advantageous structural characteristics not shown in FIG. 8. Thus, each baffle b is in top plan view of the shape shown at an enlarged scale in FIG. 9, with each baffle being over its entire lengthwise extent, and also over most of its heightwise extent, curved at 68-out of alignment with the remaining fin 24 on which it is formed, with this curvature being :morepronounced in one end part of the bafile, i.e., its end part 34 which projects into the adjacent flow channel 36. This curved formation of the bafiles b springs from the earlier interpress of the fins on the opposite flat wall sections 26 of the partially flattened blank 38 (FIGS. 4 and 5),
whereby the ensuing lateral displacement of fin metal at and near the crossings of these fins predominantly to one and the same side of the fins apparently affects each baffle over its length in distorting it into this characteristic curved formation. Of course, this curved formation of the baffles b further enhances their flow diversion effect on passing fluid.
FIG. shows a baffle b in side elevation at the same enlarged scale in which the same is shown in top plan view in FIG. 9. Thus, FIGS. 9 and 10 illustrate the characteristic indentations i in the fins as obtained by pressure-indentation of the fins at their tips, with these characteristic pressure-induced indentations i being clearly perceptible on actually formed tubing t, and being never mistaken for indentations that could be formed by machining the fins for removal of fin metal thereat. FIG. 10 further shows bafl'les b of the exemplary height of approximately one-half the fin height, with the baffles emerging from the remaining continuous root or base 70 of the original fin, wherefore the continuous fin roots or bases and the spaced baffles thereon also have unique blending characteristics and effects of continuous and interrupted fins.
While for the formation of the described exemplary tubing t the fins on the opposite flat wall sections 26 of the partially flattened blank 38 are interpressed over the full height of the fins (FIG. 4), it is, of course, fully within the purview of the invention to partially flatten an inner-fin tube blank for interpress of the opposite fins over less than the fin height. Thus, FIG. 11 shows a partially flattened inner-fin tube blank 38a of which the fins 24a on the opposite flat wall sections 26a are interpressed over part of the fin height, in this instance over the distance d. In subsequently expanding the partially flattened blank 38a for separation of the interpressed fins and preferably also for its simultaneous formation into tubing of desired final cross-sectional shape, the emerging baffles on the fins will have the same general structural characteristics as the baffles b in the described tubing t of FIGS. 7 to 10, except that the depth of the indentations in the fins and, hence, the height of the baffles is less than one-half the fin height, and is approximately one-half the distance d. Of course, the effect of these exemplary baffles or less height on passing fluid is different, at least in degree, than is the effect on passing fluid of the higher bafiles in the tubing of FIGS. 7 to 10, and it is within the purview of the invention to vary the effect of baffles on passing fluid within a particularly wide range by varying the interpress of the fins on the opposite flat wall sections of partially flattened blanks anywhere between bare interpress and full interpress of the fins. The featured method of forming inner-fin tube blanks into tubing with inner baffle fins lens itself to the attainment of still far more widely different effects of the baffles on passing fluid, by simply resorting to starting tube blanks with inner fins of widely different helix angles of the fins, or widely different fin heights, or widely different numbers of fins, for example.
While it is advantageous for many purposes to expand a partially flattened blank into tubing of oblong or many other non-round cross-sectional shapes, it is also contemplated to expand such partially flattened blanks into tubing of round, or near-round, crosssection, with such tubing having the unique inner baffle fins with their advantageous effects on fluid passing through the tubing to meet many exacting heatexchange and other requirements of tubes which must be round for one reason or another, such as required passage of fluid therethrough at optimum volumetric flow rates, for example. To this end, a partially flattened inner-fin tube blank, such as the blank 38 of FIG. 4, for example, is closed at one end, and is with its other end connected with a header (FIG. 12) which may be the same as, or similar to, the described header 42 in FIG. 6, whereupon fluid j under pressure is admitted into the header and, hence, also into the partially flattened blank to expand the same. In thus expanding the blank, the same is free to expand naturally into the tube tb of a cross-sectional shape like or similar to that shown in FIG. 13, which may be said to be nearly round and may be adequately round for some applications. What prevents the blank 38 from expanding into true round section are its short, curved return wall sections 28 (FIG. 4) which even under the highest permissible pressure of the expansion fluid in the blank fail to open up sufficiently, with the result that these return wall sections form the opposite ridges 82 on the tube tb. At that, the tube tb is in FIG. 13 shown considerably outof-round for clearer perception, whereas a similar tube actually expanded came considerably closer to being round, with the opposite ridges on the tube being much less pronounced.
To form the near-round tube tb into true round section, it is merely necessary to draw the tube through a round shaping die 84 (FIG. 14). The unique arrangement of the baffle fins in the round tube tb is shown in FIG. 15.
As already mentioned, the partially flattened innerfin tube blank may be expanded into tubing of many different cross-sectional shapes. To this end, different expediences may be resorted to for deforming, and controlling the deformation of, an expanded tube blank into the desired cross-sectional shape. Thus, a partially flattened blank may by internal fluid pressure be expanded into the oval intermediate tube ti shown in FIG. 16, whereupon the same may be passed between forming rolls for its formation into the tube to of exemplary rectangular cross-section. There may be series of pairs of companion rolls for stepwise configuration of the tube from its initial oval shape into its final rectangular shape, with the pairs of end rolls 86, 88 and 90,92 finishing the tubing formation. As an alternative to rolling the intermediate oval tube ti into the rectangular tube tc, the intermediate tube ti may be drawn through a shaping die 94 (FIG. 17) for its formation into the rectangular tube tc.
It is, of course, understood that in partially flattening an inner-fin tube blank for interpress of the fins and, hence, formation of the baffles thereon, fin lengths along the return wall sections of the blank, such as the short, curved wall section 28 of the blank 38 in FIG. 4, for example, will not be interpressed with any other fin and thus lack baffles. However, the number of baffles thus lacking is insignificant and constitutes but a minute fraction of the multitude of formed baffles in a tube. In any event, the overall effect of the baffles in a tube on passing fluid is for all practical intents and purposes the same as though there were not even one missing baffle.
Tubing with the featured inner baffle fins is also highly advantageous in forming duplex tubing (FIG. 18). Thus, there is introduced in the round tube th of FIG. 14, for example, a tube 102, with the baffles b of the outer tube tb resting on the peripheral wall 104 of the inner tube 102 (FIG. 18). Preferably, the interprojected tubes tb and 102 are then drawn through a die 106 for firmly pressing the outer tube tb with its baffles b against the inner tube 102, which completes the formation of the duplex tubing 100. Duplex tubing of this type is highly advantageous for heat-exchange purposes in particular, in any fluid passing through the outer tube, rather than being confined in its flow to the helical channels between the fins, is afforded communi cation peripherally throughout the outer tube, and is even forcefully compelled into diversionary flow peripherally throughout the outer tube, with the baffles having, moreover, their described characteristic effects on the passing fluid with ensuing beneficial results on fluid passing through the inner tube.
What is claimed is:
1. Method of forming tubing, which comprises providing a round metal tube blank having an axis, an annular peripheral wall about said axis, and inner metal fins on said wall extending helically thereon in one direction and being spaced from said axis; partially compressing the blank from two opposite sides for interpress of opposed fins at their crossings; and expanding the partially compressed blank to separate the fins from their interpress.
2. Method of forming tubing, which comprises providing a round metal tube blank having an axis, an annular peripheral wall about said axis, and inner fins on and integral with said wall and extending helically thereon at the same helix angle and parallel to each other, with said fins being of equal height and spaced from said axis; partially flattening the blank from two opposite sides into oblong cross-section with two opposite flat wall sections at a spacing at which the fins on one flat wall section are at their crossings with the fins on the other flat wall section interpressed with the latter fins; and expanding the partially flattened blank to separate the interpressed fins.
3. Method of forming tubing as in claim 2, in which the partially flattened blank is expanded by internal fluid pressure. I
4. Method of forming tubing as in claim 2, in which the blank is partially flattened to have the opposite flat wall sections extend parallel to each other.
5. Method of forming tubing as in claim 2, in which the blank is partially flattened for interpress of the fins on the opposite flat wall sections at their crossings over the entire height of the fins.
6. Method of forming tubing as in claim 2, in which the blank is partially flattened for interpress of the fins on the opposite flat wall sections at their crossings over less than the height of the fins.
7. Method of forming tubing as in claim 2, in which the helix angle of the fins in the blank is other than 45,
so that the fins on the opposite flat wall sections cross each other at an angle other than 90.
8. Method of forming tubing as in claim 2, in which the blank is partially flattened progressively between rotary companion rolls.
9. Method of forming tubing as in claim 2, in which the partially flattened blank is by internal fluid pressure expanded with its peripheral wall into near annular cross-sectional shape.
10. Method of forming tubing as in claim 9, in which the' expanded blank is drawn through a round shaping die for formation of its peripheral wall into annular cross-sectional shape. i
11. Method of forming tubing as in claim 2, in which the expanded blank is drawn through a die opening of other than circular cross-section.
12. Method of forming tubing as in claim 2, in which the expanded blank is passed between rotary companion rolls for formation of its peripheral wall into a cross-sectional shape other than round.
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