EP0489881B1 - Method for producing ribbed tubes - Google Patents

Abstract

A method for producing ribbed metal tubes designed mainly but not exclusively for heat exchangers since they are particularly suitable for heat transmission. According to the method, a strip of stainless or refractory metal or alloy is continuously rolled to form ribs thereon, whereafter it is bent and welded.

Description

The process which is the subject of the invention relates to the manufacture of fluted metal tubes, used mainly but not exclusively in heat exchangers, in order to improve the conditions of thermal transfer and therefore the thermal efficiency, all other things being equal. , while having the necessary characteristics of corrosion resistance and mechanical resistance.

The present invention also relates to sections of fluted tubes and a use of said sections according to claims 16 and 17 respectively.

Usually used to make these heat exchangers, circular section tubes in suitable grades of stainless steel or refractory steels or in refractory or special metals or alloys whose thermal conductivity is often poor. It is found that the heat exchanges between the fluids contained in the tubes and those circulating around them are limited by several factors such as in particular insufficient turbulence of the fluids, solid deposits formed on the walls of the tubes and oxide layers developed on these walls.

We know the possibility of increasing these exchanges by using grooved tubes. These grooves increase the exchange surface for a given tube diameter and at the same time reduce the thermal resistance of contact between the fluids and the wall of the tube thanks to the creation of a turbulence which prevents the formation of a layer of fluid. more or less static in contact with this wall.

Various means are known for making fluted tubes for such applications.

It is in particular possible to groove smooth tubes of circular section, without welds or welded, by means of suitable drawing dies.

These operations are however difficult to carry out, in particular in the case of stainless or refractory steels which have a relatively poor aptitude for cold drawing. It is often necessary to carry out several successive stretching operations, with lubrication, operations which lend themselves with difficulty to the production of grooved tubes of great unit length and with sufficiently deep groove profiles, the manufacturing cost being always very high.

Patent AT-A-238 670 describes a process for forming by cold rolling metal strips in order to obtain profiles comprising hard rods having mechanical characteristics superior to those obtained by hot rolling. The strips thus grooved can then be formed for example with a U-shaped section by means of forming cylinders of suitable shape. These U-shaped profiles can be welded two by two against each other to obtain tubes of rectangular section. The forming of the grooves can be carried out by means of a quarto having rolling cylinders provided with grooves of revolution over their entire width and also supporting cylinders which are grooved. Such fluted strips can be bent and welded to obtain tubes of circular section. The document does not indicate the conditions for guiding the strip, bending, facing the edges with a view to welding. No indication is given of the possibilities of controlling the bandwidth and the straightness of the scrolling.

We looked for the possibility of producing tubes for heat exchangers, grooved on one or two faces, of unit length which can reach ten meters or more having hard rods which can be relatively deep by a continuous process allowing to realize several sections of tubes of such unit length from the same tube. We also sought the possibility of avoiding intermediate heat treatments during the formation of the grooves and also the possibility of obtaining very precise dimensions with regard to the depth of the grooves, their profile, their pitch, the wall thickness of the tube and its straightness.

The process which is the subject of the invention is defined in claim 1 and makes it possible to achieve these results.

It consists in producing a very long tube, grooved on at least one face, in a metal or alloy belonging to the group comprising stainless or refractory steels, titanium or titanium alloys, the alloys having in their composition in elements of alloy combined at least 40% by weight of one or more of the following metals: Cr, Ni, Mo, Co, Ta Nb, Ti, Zr, Al and Hf. According to this method, a strip of a metal or alloy of the group thus defined and of rectangular section is subjected continuously to the following successive operations: engagement of the strip along a transformation axis through at least one pair of cylinders of forming of which at least one comprises a set of grooves, the tightening of these cylinders being sufficient to form corresponding grooves on the strip, an area of non-grooved margin being reserved in the vicinity of each of the two lateral edges, then engagement of the strip fluted through bending means which bend it transversely until it has a generally circular cross-section, the lateral edges being thus brought closer to each other, then passage to the right of welding means which carry out the welding to each other of the two edges of the strip, continuously, then passing to the right of cutting means which cut the tube into sections of length finished. According to the invention, precise guiding of the strip is ensured along the transformation axis thanks to the combination of two guiding means: The first guiding means disposed upstream of the forming means comprises a pair of smooth rolls between which the metal strip is engaged, one of these cylinders having guide flanges between which the strip passes, the distance between these flanges being equal to the width of the strip with little play and their height being substantially equal to the thickness of the strip so that, the two cylinders being brought together, the passage section between the rolls is substantially equal to the section of the strip and thus ensures precise guiding of this strip along the axis of transformation, band which then engages through the first means of forming.

The second guide means is arranged, after forming grooves on one or two faces of a metal strip, by passing through at least one pair of cylinders, at least one of which is grooved. This way has a pair of cylinders through which the metal strip is passed. At least one of the two cylinders is fluted and at least one is provided with lateral guide flanges whose spacing corresponds to the width of the fluted strip with little play. Advantageously, the height of these flanges guidance is determined so that, coming into abutment against the opposite cylinder, the passage width is such that the profile of the grooves formed on the strip corresponds to that of the grooves of the corresponding cylinder (s). Preferably, shoulders of determined height are produced on each side of the grooved zone of a cylinder of said pair of guide cylinders, over the width of the margin zone, to give this margin zone the thickness targeted.

When it is proposed to groove the metal strip on both sides, it is passed through at least one pair of cylinders, one of which comprises a set of raised grooves and the other a corresponding set of hollow grooves.

Preferably, these grooves on your cylinders are of revolution so as to produce undulations parallel to the transformation axis.

When it is proposed to groove the metal strip on one of its two faces only, it is passed through at least one pair of cylinders, one of which has a straight generator parallel to its axis of revolution and the other a fluted generator. Preferably, these grooves are of revolution. Preferably also, to produce grooves on one side of a metal strip, the latter is passed through at least two and advantageously 3 to 4 pairs of reduction cylinders each comprising a grooved cylinder.

The clamping pressure of the pair or pairs of groove forming cylinders can be adjusted so as to obtain, after forming the grooves, a grooved strip having a width identical to the initial width of the smooth strip. By working in this way at constant bandwidth, it is possible, on the same tube, to produce without modifying the setting on the line of the bending and welding means, at intervals smooth parts of smooth tubes, that is to say without grooves, having the same outside diameter as the tube in its grooved part, this by spacing the grooved cylinders at regular intervals for a given period, the strip then being able scroll freely. Such smooth parts of the same outside diameter as the grooved parts and located at the ends of the tubes after the cut to length thereof can be useful for mounting or assembling the grooved tubes on a tubular plate for example a heat exchanger plate. heat.

According to a particular embodiment of grooves on one or two faces of a metal strip, the grooved cylinder (s) used have hard rods inclined relative to the generators of an angle any less than or equal to ninety degrees.

Advantageously, before general bending of the grooved strip, the latter undergoes forming of the two margin zones by passing between rollers to give each of them a determined curvature for welding.

The general bending of the grooved strip is advantageously carried out by means of forming rollers, convex or concave generators which bend the strip transversely in passing, the final bending being able to be carried out through a die, or by rollers with circular grooves.

The welding means used, to weld the two edges of the metal strip to each other continuously, are preferably chosen from the group comprising: arc welding in a neutral atmosphere with consumable or non-consumable electrode, plasma welding, laser welding, electron beam welding, taking into account the nature of the metal or the alloy of the metal strip. For many uses, at least one heat treatment is carried out continuously, after welding, of the rod tube before and after cutting, depending on the nature of the metal or alloy of the strip.

The examples and figures below describe particular embodiments of the method according to the invention.

Figure 1 is a general diagram of the method of producing grooved tubes according to the invention.

FIG. 2 represents a pair of smooth guide cylinders for implementing the method according to the invention.

FIG. 3 represents a pair of grooved cylinders for implementing the method according to the invention.

FIG. 4 represents, enlarged, the grooves of the cylinders of FIG. 3.

Figure 5 shows, enlarged, the grooves of a pair of guide cylinders for the implementation of the following method the invention.

FIG. 6 represents the means for bending the grooved strip for implementing the method according to the invention.

FIG. 7 represents the welding means making it possible to obtain a grooved tube by the method according to the invention.

FIG. 8 schematically represents a rolling mill with 6 pairs of cylinders for producing, by the method according to the invention, a fluted tube on one side.

Figure 9 is a schematic view of a means for forming the margin areas of a fluted strip by the method according to the invention.

Example 1: The method according to the invention is implemented to produce a fluted tube on both sides, in the form of a set of corrugations parallel to its axis, from a strip, wound in a reel, of type 304L stainless steel (AISI) corresponding to AFNOR Z2CN18-10 standard.

Figure 1 is an overall diagram, not detailed, showing the successive stages of the process.

The metal strip 1, wound, is mounted on an unwinding device not shown which allows its end to be engaged along the arrow F1 along the Xo -Xo transformation axis. The strip 1 first engages in a rolling mill 2 comprising three pairs of cylinders 3, 10, 13 in series. The first pair 3, of guide, comprises smooth cylinders 4, 5 which are shown enlarged in Figure 2 in the plane of which are their axes X1 - X1 and X2 - X2. The upper cylinder 5 has a smooth generator parallel to its axis X2 - X2; the lower cylinder 4, also smooth, has at each end a circular rim 6, 7, of radial height "e" relative to the generator 9 parallel to the axis X1 - X1. This height "e" is substantially equal to the thickness of the strip to be laminated and the internal spacing "l1" of the flanges 6, 7 is substantially equal to the width of the strip, increased by a minimum clearance. There is preferably just the spacing of the cylinders 4, 5 so that the flanges 6, 7 come to bear against the cylinder 5. The second pair of cylinders 10 comprises, as shown in FIG. 3, two fluted cylinders 11 and 12 of which the axes X3-X3, X4-X4 are in the plane of this figure 3. The lower cylinder 11 is grooved hollow and the upper cylinder 12 in relief. Figure 4 shows, enlarged and in section, the working generators of the cylinders 11, 12 over a small part of their length. This Figure 4 shows grooves of revolution arranged so that the reliefs such as A1, A2 of the upper cylinder 12 correspond to the recesses such as B1, B2 of the lower cylinder 11, with a determined pitch. The double grooves thus produced on the walls of the strip 1 give it a wavy shape. Between the two cylinders 11, 12 is shown in dashes the section of the metal strip 1 which will be corrugated by bringing the cylinders closer over most of its width. On each side of the grooved zone, a margin zone is reserved, adjacent to the edge such as 1a of the strip 1, margin zone which is not grooved. During the formation of the grooves, the clamping pressure of the cylinders is adjusted, for example, so as to keep the width of the strip constant and the production of the double grooves is accompanied by a reduction in the thickness of the strip and a slight increase in length. The passage of the strip 1 through the last pair 13 of cylinders 14, 15 makes it possible to complete the production of the splines and at the same time ensures the guidance of the fluted strip 1. Figure 5, similar to Figure 4, shows in section a fraction of the length of the grooved generators of the cylinders 14, 15 engaged on the strip 1 to give it its final profile and regular thickness. We see the margin zone 17, adjacent to the rim 16, as well as the shoulder 16-1 which makes it possible to control the thickness of this margin zone. A similar margin zone is produced in the vicinity of the opposite edge of the strip, not shown, thanks to a second rim and a second shoulder of the same dimensions. The internal spacing of these flanges is determined to allow the passage of the grooved strip 1 with little play to ensure satisfactory guidance.

Beyond the rolling mill, the fluted strip is bent using initial bending means 18, 19 and 20 followed by a final bending means 21, as shown in FIG. 1. These means are shown more precisely in their principle in Figure 6 in which each of these means, shown along the Xo-Xo transformation axis, has undergone a rotation of 90 ° to reveal its mode of action on the fluted metal strip 1. The means 18 is a roller, with a convex generator 18-1, against which abuts the face of the grooved strip 1 which will become the inner face of the tube. The means 19 is a roller, with a concave generator 19-1 which comes to bear against the face of the strip which will become the outer wall of the tube. By controlling the tension of this strip and the distance between the two rollers, it is possible to exert sufficient stress on this strip to cause it to take the curvature of the generators of these rollers. The means 20 consisting of two rollers 20-1, 20-2, with concave generator, makes it possible to considerably accentuate the bending. Finally, the die 21 makes it possible to give the grooved strip 1 the desired circular section in order to make a tube of which the two edges are in contact with each other, this die being able to be replaced by a pair of rollers with circular grooves. At the outlet of this die 21 (FIG. 7), there is disposed a welding means 22 indicated purely schematically. This welding means can be any welding means making it possible to produce a continuous sealed weld. Arc welding in a neutral atmosphere can in particular be used using a consumable or non-consumable electrode (MIG or TIG process). A mandrel, not shown, can if necessary be placed inside the tube in line with the welding zone and connected upstream to an external support means, not shown. This mandrel protects the weld inside the tube by means of scanning the welding area with a neutral gas. Beyond the welding zone, the welded tube is driven along the arrow F2 by drive means not shown towards cutting means also not shown. Before or after cutting, continuous heat treatment means can be used to give the grooved and welded tube 1 particular characteristics.

By way of digital example, a fluted tube on both sides is produced by the method thus described from a strip of stainless steel Z2 CN 18-10 109.5 mm wide and 0.7 mm in diameter. thickness. After forming the grooves, the thickness of the strip is reduced to 0.62 mm, the overall thickness of the strip, including the grooves, then reaching 1.58 mm. This strip has 34 undulations and on each side, a margin zone of 2 mm wide. After bending and welding, a tube with an overall outside diameter of 35 mm is obtained.

Example 2 The method according to the invention is implemented for the production of a grooved tube, only on the outer face.

A strip wound in a 430 Ti stainless steel coil (AISI) is used, corresponding to the AFNOR Z2 CT18 standard. This strip must have a thickness greater than that which would be necessary for double grooves, of the same height, forming corrugations, because in the case of the present example, the height of the grooves is taken in the thickness of the strip. We must therefore perform a greater plastic deformation and in the case of the present example, the rolling mill used has 6 pairs of rolls instead of 3 as shown in the diagram in FIG. 8. As in the case of Example 1 the rolled metal strip 1 is mounted on an unwinding device, not shown, which enables its end to be engaged in the first pair of cylinders 32 of the rolling mill 31. This first guide pair comprises smooth cylinders having the same characteristics as the cylinders 4 and 5 of the first guide pair 3 of FIG. 1. The next four pairs of cylinders 33, 34, 35, 36 each have a smooth upper cylinder and a grooved lower cylinder. The width of the grooved zone is determined in the same way as that of the grooved zone of the cylinder 11 of FIGS. 3 and 4 in order to preserve on each side of the metal strip an area of non-grooved margin. The profile of the grooves and the tightening of each pair of cylinders are determined so that, at the outlet of the pair 36, the grooved strip has reached dimensions very close to the final dimensions targeted. The last pair of cylinders 37 plays the role of guiding pair in the direction of the means for bending and then welding the strip like the pair of cylinders 13 of Figure 1. The lower cylinder of this pair 37 is grooved in a similar manner to that shown for the lower cylinder 14 in Figure 5. As in the case of cylinder 14 this lower cylinder of the pair 37 comprises, each side of the grooved zone, a margin zone which extends to a guide rim, similar to the rim 16 of FIG. 5 and a shoulder, similar to the shoulder 16-1 which allows, in cooperation with the cylinder upper smooth, to control the thickness of the margin area. The initial and final bending operations then welding are carried out by means similar to those used in the case of Example 1. Additional means, after welding, can be implemented to ensure the tension of the strip in particularly during bending and then welding, as well as to calibrate it and heat treat it, if necessary, before the final cutting to the desired length.

By way of a digital example, a fluted tube on one side is produced by the method thus described, from a strip of Z2 CT 18 steel 108 mm wide and 1.75 mm thick. After rolling, the fluted strip obtained has an overall thickness of 1.75 mm and a lures cane depth of 0.65 mm. The elongation which depends mainly on the section of the grooves, reaches, in the case of the present example, about 30%. In the case of this example, the bending is carried out so that the smooth face of the strip is inside the tube and therefore the grooved face outside. For this, it suffices to put the smooth face of the strip in abutment against the roller with convex generator 18.1 of the bending means 18, the grooved face then coming into abutment against the concave generator 19.1 of the bending means 19.

Advantageously, before performing the initial bending of the fluted strips either on one face or on two faces, it is possible to perform, by means of forming 38 of the margin areas, a localized bending thereof. FIG. 9 shows such a means 38 that one can have upstream of the initial bending means 18. As shown in FIG. 9, this means comprises two pairs of rollers, arranged so as to be crossed each by a zone of margin of a grooved strip such as 1. Each pair of rollers comprises a roller with a convex generator 39, 41 and the other with a concave generator 40, 42 so as to curl the edges of the hollow strip on the side which will close the internal face of the tube, the radius of curvature being substantially equal to the radius of the tube to be formed.

For many applications, it is desirable to be able to easily provide a tight connection, resistant to pressure and temperature between the ends of each grooved tube and the other components of the fluid circuits in which they are inserted. It has appeared particularly advantageous to provide these tubes at their ends with non-grooved areas which allow the use of assembly means which are suitable for tubes without grooves, for example in the case of tubes connected to tubular plates.

For this, the metal strip is fluted by the method according to the present invention by adjusting the tightening of the pair or pairs of flute forming cylinders so as to keep the fluted strip a constant width, equal to the initial width. This or these pairs of cylinders are equipped with adjustment means, connected to remote control means, for example computerized, allowing the following operations to be carried out cyclically, perfectly reproducible, at determined time intervals, at the level of each pair of cylinders. : spacing of the cylinders to interrupt any permanent deformation of the strip, keeping the cylinders apart for a precisely determined time, bringing the cylinders together in order to resume grooving with a degree of tightening identical to that which was achieved before the start of the cycle.

As a numerical example, a batch of Z2CT18 type steel tubes for a heat exchanger is produced. These tubes, 4 m long and 35 mm in outside diameter, have a wall of about 0.65 mm thick and comprising grooves made on a strip 113 mm wide by means of a rolling mill with two grooving cages such as as that of FIG. 1, the cages 10 and 13 of which provide the grooving. Each of these cages is provided with adjustment means allowing, thanks to appropriate control means to carry out a sufficient spacing of the cylinders, of the order of 3 mm, in approximately 1 second and the return to the initial tightening at the same time. Maintaining 8 seconds in the open position makes it possible to create a non-grooved area of approximately 500 mm in length. It can be seen that the duration of the 8-second cycle corresponds to the passage of a strip length of approximately 500 mm. The computerized control means make it possible to execute this cycle at time intervals such that, after rolling and welding the strip, the centers of the non-fluted areas of the tube obtained are approximately at a distance from each other of approximately 4 m. It is thus possible to cut the tube at these media in order to have sections of 4 meters each comprising a non-fluted zone at each end of approximately 150 mm long.

The computerized control means make it possible to shift the spacing cycle and then to tighten each pair of cylinders so that, in the case for example of FIG. 1, the pair of cylinders 13 does not start to move apart until the 'instant when the start of the zone comprising a reduction in the height of the grooves made at the level of the pair of cylinders 10 engages in this rolling mill.

Advantageously, there is available beyond the last pair of cylinders 13 an additional pair, with a fixed spacing comprising, like the first guide pair 3, smooth cylinders, one of the two cylinders being provided with guide flanges. The distance between these edges and their height is determined so that the transverse play is small in order to ensure proper guidance, when the other cages are spaced, the fixed spacing of the cylinders allowing passage without deformation of the fluted or non-fluted zones .

Claims (17)

1. A process for the production of a tube which is fluted on at least one face, of a metal or alloy belonging to the group comprising stainless or refractory steels, titanium or titanium alloys, the alloys having in their composition, in respect of accumulated alloy elements, at least 40% by weight of one or more of the following metals: Cr, Ni, Mo, Co, Ta, Nb, Ti, Zr, Al and Hf, wherein a strip (1) of a natal or alloy of the group so defined, of rectangular section, is passed through at least one pair (10; 33) of shaping cylinders of which at least one (11, 12) comprises an assembly of flutes, the pressing action of said cylinders (11, 12) being sufficient to shape corresponding flutes on the strip (1), a non-fluted marginal zone (17) being reserved in the vicinity of each of the two lateral edges (1a), then the fluted strip (1) is engaged through bending means (18 to 21) which curve it transversely until the two lateral edges (1a) approach each other, welding means (22) continuously producing welding of the two edges (1a), cutting means then dividing the tube into portions of a given length, characterised in that, prior to engagement of the natal strip (1) through at least one pair (10; 33) of shaping cylinders (11, 12), it is passed through a first pair (3; 32) of smooth guide cylinders (4, 5) of which one (4) comprises guide edge portions (6, 7) which define between them a passage width 'l1' substantially equal to the width of the strip (1) plus a small clearance, the height 'e' of said edge portions being substantially equal to the thickness of the strip (1) so that, when the two cylinders (4, 5) are moved towards each other until the edge portions (6, 7) of the one (4) bear against the wall of the other (5), the passage section between said cylinders is substantially equal to the section of the strip (1) in order to guide same precisely along the transformation axis, and that after shaping of flutes by being passed through at least one pair (10; 33) of shaping cylinders (11, 12) the fluted strip, before being engaged through the bending means (18 to 21), passes through a second pair (13; 37) of guide cylinders (14, 15) of which one at least is fluted and of which at least one cylinder (14) is provided with lateral guide edge portions (16) whose spacing substantially corresponds to the width of the fluted strip (1) with a slight clearance in order to ensure highly precise guidance for the strip (1) which is fluted in that way in the direction of the bending means (18 to 21).
2. A process according to claim 1 characterised in that the height of the guide edge portions (16) is so determined that, when coming into a condition of abutment against the opposite cylinder (15), the passage width is such that the profile of the flutes formed on the strip (1) corresponds to that of the flutes of the corresponding cylinder or cylinders.
3. A process according to claim 1 or claim 2 characterised in that shoulders (16-1) of a given height are provided on each side of the fluted zone of a cylinder (14) of the pair (13; 37) of guide cylinders (14, 15) over the width of each marginal zone (17) to impart the intended thickness to them.
4. A process according to one of claims 1 to 3 characterised in that when an assembly of flutes is provided on each of the two faces of the strip (1) at least one pair of cylinders (10) is used, of which one (12) comprises flutes in raised relationship and the other (11) comprises corresponding flutes in recessed relationship.
5. A process according to one of claims 1 to 4 characterised in that the flutes are of revolution to produce flutes which are parallel to the axis of transformation (X_o-X_o) or not of revolution.
6. A process according to one of claims 1 to 5 characterised in that in order to flute the metal strip (1) only on one of its faces it is passed through at least one pair of cylinders (33) of which only one comprises a fluted surface.
7. A process according to claim 6 characterised in that the flutes are flutes of revolution or not of revolution.
8. A process according to claims 6 or claim 7 characterised in that the strip is passed through a plurality of pairs (33, 34, 35, 36) of shaping cylinders and a pair (37) of guide cylinders each comprising a fluted cylinder.
9. A process according to one of claims 1 to 8 characterised in that the fluted cylinder or cylinders comprises or comprise flutes which are inclined with respect to the generatrices at any angle less than or equal to ninety degrees.
10. A process according to one of claims 1 to 9 characterised in that prior to general bending of the fluted strip (1) the two marginal zones (17) thereof are subjected to a shaping operation by being passed between rollers (39, 40, 41, 42) to impart to each of said two marginal zones (17) a given curvature for the purposes of the welding operation.
11. A process according to one of claims 1 to 10 characterised in that general bending of the fluted strip (1) is effected by means of shaping rollers with convex (18-1) or concave (19-1) generatrices which curve the strip (1) transversely to its passage movement, final bending being effected by way of a die (21) or rollers with a circular groove.
12. A process according to one of claims 1 to 11 characterised in that continuous edge-to-edge welding of the fluted and bent strip (1) is effected by a process forming part of the group comprising arc welding in a neutral atmosphere with a consummable or non-consummable electrode, plasma welding, laser welding or electron beam welding.
13. A process according to one of claims 1 to 12 characterised in that after welding and after or before cutting at least one heat treatment is continuously effected on the welded tube.
14. A process according to one of claims 1 to 13 characterised in that the clamping pressure of the pair or pairs (10, 33, 34, 35, 36) of cylinders for shaping the flutes is regulated in such a way as to produce after shaping a fluted strip (1) of a width which is identical to the initial width of the smooth strip (1).
15. A process according to claim 14 characterised in that while operating with a constant strip width and without modifying regulation along the line of the bending means (18 to 21) and welding means (22) smooth tube portions, that is to say without flutes, are produced at regular intervals, being of the same outside diameter as the tube in its fluted part, by moving the fluted cylinders apart at regular intervals for a given time, the strip (1) then passing freely through.
16. Fluted tube portions of a given length comprising at each end a smooth part of an outside diameter which is substantially equal to the outside diameter of the fluted zone, which are produced by cutting up tubes of great length produced by the process according to claim 15.
17. Use of fluted tube portions according to claim 16 for the production of heat exchangers, the smooth ends of said portions being assembled by means of tube sheets.

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