WO1999036200A1

WO1999036200A1 - A welding method, a heat exchanger tube, and an apparatus for the manufacturing of a heat exchanger tube

Info

Publication number: WO1999036200A1
Application number: PCT/DK1999/000019
Authority: WO
Inventors: Lars Skytte JØRGENSEN
Original assignee: Aalborg Industries A/S
Priority date: 1998-01-16
Filing date: 1999-01-14
Publication date: 1999-07-22
Also published as: ZA99245B; AU1961299A; TW425323B

Abstract

An apparatus for the manufacturing of a heat exchanger tube comprising a tube fitted with an exterior helical fin, comprises means (13) for rotating and axially displacing a tube (2) about a tube axis, means (16) for dispensing a strip (4) under controlled tension and in edge orientation towards a position of engagement with the tube surface, and means (20) for emitting a laser beam (21) towards a bottom edge surface of the strip at a position adjacent the position of engagement. The laser beam is aimed tangentially against the nip (8) where the strip contacts the tube surface. The bonding is effected without melting the parts. The invention provides an apparatus, a method, and a finned tube.

Description

A welding method, a heat exchanger tube, and an apparatus for the manufacturing of a heat exchanger tube

The present invention relates to a method for the manufacturing of a heat exchanger tube with an exterior helical fin. The invention further relates to a heat exchanger tube with an exterior helical fin, and to an apparatus for the manufacturing of a heat exchanger tube .

Heat exchanger tubes of the general kind comprising a tube fitted with fins or ribs provide the advantage of an enlarged surface area active for exchanging heat and also provide a guiding of an external cross flow of a fluid which takes part in the heat exchange. The fins exchange heat with the exterior medium and they transfer heat to or from the tubes by way of contact between the fins and the tube surface.

The invention relates to tubes with an exterior fin which comprises a length of strip material applied in a helical pattern about the tube, wherein the strip contacts the tube by one edge so as to extend in a generally annular and radial arrangement, and where the strip is secured to the tube surface by welding. The welding serves the purpose of structural attachment of the fin and the purpose of enhancing heat transfer from the fin to the tube, and vice versa.

The structural attachment of the fin necessitates only partial bonding, however, in the interest of optimum heat transfer, welding of the fin throughout the length may be desirable.

Optimum heat transfer is of particular concern in case of heat exchangers subjected to high exterior temperatures and to high rates of heat transfer. For applications where the temperature outside the tubes exceeds the temperature inside the tubes, the fins will during service be subjected to higher temperatures than the tubes and will therefore be likely to expand thermally more than the tubes. In this way, high ambient temperatures will tend to loosen the fins from the tube surface, creating openings which intercept the intended path of heat transfer, and perhaps causing fracturing of bondings .

European patent application No. 0 303 074 A2 describes a method for the manufacturing of a heat exchanger tube of metal with a helical, welded fin. By this method a naked tube section is placed in a lathe and spinned while a metal strip is dispensed from a strip dispenser displaced axially along the tube so as to wind the strip in a helical pattern around the tube. The fin and the tube are heated by means of a laser beam. The laser beam is directed in an acute angle from the tube axis towards the fin and the tube in the area where these components meet. The laser beam melts the tube surface, and the laser beam forms a channel transversely through the strip close to the edge, whereby the edge region of the strip is melted. The laser beam may also reflect in the tube surface whereby to create added heating of the strip edge.

The inventor has found that a solid metal surface tends to reflect a substantial proportion of an impinging laser beam, thereby effectively repelling a substantial part of the energy. On the other hand, a molten bed of metal reflects a smaller proportion of the laser beam, and a plasma of evaporated metal tends to reflect even less. This may be the explanation why laser beam welding is generally perceived as requiring a very concentrated beam in order to succeed. A less concentrated laser beam is generally not capable of heating a material such as steel to a sufficient temperature for a welding to get started.

The laser beam heating effect necessarily takes place from the surface. Melting a part of the surface necessitates a high intensity of laser beam irradiation, however, once melting has commenced the rate of energy absorption rapidly increases, in case of concentrated laser beams usually to a point where part of the molten bed of metal evaporates and gives off a plasma. The molten bed and the plasma heat up adjacent areas in order to melt them. However, this method is very likely to form a pin hole or a channel in the specimen along the direction of the laser beam. The plasma of evaporated metal absorbs and scatters the laser beam energy with the result of heating up the immediate surroundings and ambient equipment and of disturbing the rate of heat input to the weld.

In a set-up where a strip is to be welded by the edge by means of a laser beam impinging onto the strip side, the only way to achieve a welding across the bulk of the strip is to make the laser beam create a channel or a pin hole which extends all the way through the strip, and slowly scanning the laser beam along the length of the strip, whereby the edge is melted.

The varying degrees of absorption in the medium, and thus the tendency for the laser beam to form pin holes, makes the process difficult to control. The difficulties may result in flaws in the welding, in burning away parts of the fin, in creating thermal degradations of the fin and of the tube, and in forming pin hole leaks in the tube. The plasma given off dissipates a substantial amount of heat to surrounding portions of the tube as well as to surrounding portions of the manufacturing set-up.

US patent US-A 5 343 015 describes a method for the manufacturing of a heat exchanger tube fitted with a helical fin. The fin is wound onto the tube and secured by a high frequency welding method, wherein high frequency electric power is applied by means of a pair of electrodes, one electrode contacting the tube and another one contacting the fin at a point close to the point where the fin contacts the tube. This publication suggests using a laser beam for preheating the tube surface so as to supplement the heat induced into the tube by the high frequency electrical current flow.

This prior art method necessitates proper balancing and control of two methods of providing heat input, both of which methods are inherently complicated. The high frequency electric current input requires moving electrodes for contacting the strip and the tube, and is incapable of effectively bonding the ends of the strip. The strip tends to receive too much heat and tends to be melted excessively at the surface areas. Inputting additional heat by means of a laser beam is complicated due to varying absorption factors of the surface of the tube as explained above. Furthermore the area of the tube surface heated by the laser beam must precisely match the track of the strip to be wound onto the tube, raising complications regarding geometrical alignment. Practical experiments with this method have shown that a substantial heat input to the parts is required and that much energy is lost to the surroundings with the result that the overall energy efficiency is low. Excessive heating of the tube may lead to recrystallizations which degrade the tube structure. GB patent specification No. 1 359 149 explains a method for applying a helical fin onto a heat exchanger tube, by which a tube section is fitted in a lathe and rotated while a strip is dispensed from a strip dispenser displaced axially along the tube section so as to wind the fin. The fin ends are bonded by gas welding. The intermediate fin section is not welded but merely tensioned around the tube by means of the bondings of the ends .

The invention in a first aspect provides a method as recited in claim 1.

By this method the edge surface of the strip to be brought into contact with the tube is heated by means of a laser beam, which laser beam is oriented so as to heat the edge surface over an area which is generally symmetric about a median of the edge surface. In this context, the median is a geometric line running in the edge surface and midway between the strip side surfaces. This provides a uniform heating of the region to be welded and reduces the power requirement, since no portions of this region are heated to excessive temperatures. The edge surface is heated to a fusion temperature, i.e. to a temperature whereby it is possible to achieve a bonding by keeping the strip pressed against the tube surface for a suitable time.

The fusion welding may be effected without melting or evaporating any part of the contacting surfaces. This avoids the creation of a plasma, which is generally associated with laser welding methods of the prior art, and which is associated with the disadvantages explained above .

The fusion temperature must be sufficient to activate the respective surfaces in order that they may engage in a bonding contact. In the case of steel, the surfaces may be activated by a combination of heating and pressing, whereby part of the surfaces are actually forged or strained. The forging or the strain breaks up brittle surface coatings, such as oxides, thereby exposing active areas of metal. In the case of steel, the favoured range of temperatures extends from about 700°C to about 1550°C. The lower limit is the point of onsetting formation of austenitic steel whereas the high end of the range is marked by the melting point. Austenitic steel may be forged. The pressure required to forge the steel is high at temperatures within the lower end of the range and gradually declines with rising temperatures. A surface temperature about 1100°C has been found to strike a favourable balance between heat input and power input for the mechanical working.

The method according to the invention is well suited for implementation in a continuous process line, wherein the zone to be heated moves continually along the strip and along the surface of the tube. The method is easily adapted to various sizes of tubes and to tubes with different pitch between the fins, e.g. finned tubes with a narrow pitch, which are difficult to manufacture by methods of the prior art. The heating of the tube surface may be effected by a method similar to the method of heating the strip edge, or it may be effected by any other method known in the art.

According to a preferred embodiment, the tube surface within a region adapted for being contacted by the strip is heated to a fusion temperature by directing a laser beam onto the tube surface.

Preferably the heating of the strip edge surface as well as heating of the tube surface is effected by means of a laser beam which is directed onto the nip, where the strip meets the tube in such way that heat power input by the laser beam is distributed among the strip edge and the tube. This method simplifies control in using one and the same heat source for heating both parts entering into the welding process. Tuning the balance of heat power input between the strip and the tube, respectively, is possible by tuning the aim of the laser beam relative to the nip, e.g. to offset the axis of the laser beam closer towards one of the parts which might require a higher proportion of heat power.

Aiming the laser beam towards the nip has the advantage that laser beam energy impinging on one of the surfaces to be not absorbed but rather reflected will be reflected towards the surface of the opposite leg in the nip with the possibility of being absorbed to advantage there. By the geometry of the nip the angles of incidence are steeper upon each reflection, therefore the proportion of energy likely to be absorbed rises on each reflection. This effectively secures that the energy is not lost.

According to a preferred embodiment, the laser beam is adapted to irradiate a spot which extends along the tube axis for a sufficient length to allow for wrinkling of the edge of the strip due to the strip being wound onto the tube while still ensuring proper heating of a suitable region of the strip edge. This eases the requirements for alignment and therefore simplifies setting up the line, and provides for enhanced stability of process parameters.

In the process of winding the strip, reshaping the linear strip into a ring-like shape necessitates lengthening one edge of the strip and shortening the opposite edge. The dimensional changes may be accommodated by longitudinal tensioning the peripheral edge, longitudinal compressing the inner edge, and by transverse wrinkling the inner edge. Transverse incisions might also be formed in part of the strip to allow for dimensional changes. Wrinkling of the strip may be avoided by proper guiding, i.e. by forcing deformations into the strip to work it into a flat annular shape, however, such guiding requires an energy input in the process, therefore there is an interest in a welding method which will adapt to some wrinkling of the strip inner edge.

Making the laser beam irradiate a spot which is wider than the strip necessarily involves some loss of energy, however, this loss has been found acceptable in view of the overall savings in energy consumption achieved by the invention.

According to a preferred embodiment, the laser beam is emitted through a focusing device adapted to converge the beam towards a focal point, whereas the focusing device is placed at a distance from the tube axis selected to offset the focal point from the respective regions of laser beam impingement. This smears out the area effectively covered by the laser beam. Defocusing the laser beam avoids the creation of any points of very high laser beam power surface density which might otherwise give rise to melting, plasma formation, and the creation of pin holes.

Fusion without melting reduces energy dissipation into the surroundings and avoids certain difficulties associated with a molten bath, e.g. dynamic instabilities on scanning the weld too fast. This makes the process suitable for very high welding speeds. According to a preferred embodiment, the strip is pressed against the tube surface with a pressure sufficient to forge a deformation in parts of the respective contacting, heated regions, whereby brittle coatings, such as oxides, are caused to break up. This provides for higher quality continuous welding, and it reduces the requirements to a surface cleaning of the components prior to welding.

According to a preferred embodiment, a section of strip is wound onto the tube, and the section is terminated by shearing the strip on the fly while continuing rotating the tube until the strip tail end has been joined with the tube. In this way the tail of a fin section is welded merely by continuing the welding process with no special precautions other than shearing the strip.

According to another preferred embodiment, the winding of a finned tube section commences by starting the laser gun, advancing a strip leading end to contact the tube surface and keeping the strip leading end pressed against the tube for a length of time sufficient to allow it to form a bonding with a tube. Thus the only special precaution required to start up the welding of a section is the advancing of the leading end to contact and shortly keeping it pressed against the tube.

The invention, in a second aspect, provides a heat exchanger tube as defined in claim 14.

This provides a finned tube, of which the fin is well bonded without the manufacturing process having created any recrystallization or excessive heating of the tube, and therefore without degrading the structural properties of the tube. This heat exchanger tube is suitable for being manufactured by very effective methods . The invention, in a third aspect, provides an apparatus as recited in claim 18.

This apparatus is very effective in the manufacturing of a finned heat exchanger tube of high quality. The apparatus is well suited for handling a variety of kinds of tubes, e.g. finned tubes combining different kinds of materials, such as finned tubes where the fin material differs from the tube material.

Preferred embodiments of the apparatus appear from the apparatus subclaims .

Further features and advantages of the invention will appear from the appended description of preferred embodiments given with reference to the drawings on which:

Fig. 1 illustrates a lay-out plan of an apparatus according to the invention,

Fig. 2 illustrates a section through parts of the apparatus according to the invention by a plane which is perpendicular to the axis of the tube mounted in the apparatus,

Fig. 3 illustrates a detail from Fig. 2 as viewed with a viewing direction along the axis of the laser beam,

Fig. 4 illustrates a plan view of a heat exchanger tube according to the invention, and

Fig. 5 illustrates a detail from Fig. 1 in an enlarged view.

All figures are schematic and not neccesarily to scale and illustrate only those parts which are essential in order to enable those skilled in the art to understand and practice the invention, whereas other parts are omitted from the drawings for the sake of clarity.

Throughout the drawings identical references have been used to designate identical or similar items.

Reference is first made to Fig. i for a description of the apparatus according to the invention. Fig. 1 is a simplified plan view illustrating the basic lay-out. Thus Fig. 1 illustrates a tube 2 fed from the right in the Figure towards the left through a tube drive mechanism 13. The tube drive mechanism engages the tube by means of a set of motor driven friction wheels or rollers. The axes of the rollers are slightly inclined in such way that the tube is advanced in the axial direction while being spinned about the axis in order that a point on the tube surface describes a helical track. This type of tube drive mechanism is considered known in the art.

The apparatus of Fig. 1, which is designated as a whole by the reference 27, further comprises a metal strip dispenser 16 which includes a reel 17 with feedstock metal strip, a guide roller 18 for guiding the strip and various accessories, such as strip guides, means for forming partial transverse slits in the strip

(serrations) , strip cutter, strip straighteners, a controlled strip brake, adjustments, monitoring equipment, etc. The accessories are considered to be comprised in the state of the art and are not shown in Fig. 1.

The purpose of the strip dispenser is to uncoil the reel to pay out a continuous length of strip 4 with a controlled tension to allow the strip to be taken to the surface of the tube and bonded to the surface of the tube in order that the strip on the motion of the tube will provide the helical fin 3.

To the left, Fig. 1 symbolically illustrates a tube support 14 which provides a bearing for steadying the tube so as to keep the axis of the tube steady.

Fig. 1 further illustrates a laser gun 20 which is adapted for emitting a beam of laser radiation 21 generally aimed towards the point where the strip meets the tube surface.

Reference is now made to Fig. 2 for an illustration of the situation where the strip meets the tube. Fig. 2 illustrates a section by a plan which is oriented perpendicular to the tube axis, and which generally includes the region where the strip meets the tube. Fig. 2 illustrates how the metal strip 4 extends linearly from the guide roller 18 (refer to Fig. 1) to the point of contact 25 where the metal strip bottom edge 5 meets the outside of the tube. In the set-up illustrated in Fig. 2, the linear portion of the metal strip extends horizontally and meets the tube surface at the tube topmost point. The region where the strip and the tube meet is referred to as the nip 8.

The metal strip has the character of a flat ribbon with a generally rectangular cross section. The strip is fed to the tube in a generally vertical orientation in order that the strip touches the tube by a lower edge surface 5. The metal strip passes a strip guide 19 which engages the metal strip sides 7. At the upper edge, at a position adjacent the nip 8, the metal strip is engaged by a pressure roller 15 which urges the strip against the tube. The rotation of the tube combined with the action of the pressure roller, strip guides and the tensioning of the strip, forces the strip into a deformation which takes it into a peripheral relation in order to place the strip in a helical pattern wherein the strip extends generally peripherally in a radial orientation so as to provide the fin 3.

The laser gun 20 emits a laser beam 21 which is aimed towards the nip 8, tangentially to the tube and from a direction which deviates from the strip bottom edge by a small angle v (ref . Fig. 2) .

Reference is now made to Fig. 3 which illustrates the strip and the tube in the same area, Fig. 3 illustrating a view as seen in a direction along the laser beam axis. Fig. 3 generally illustrates the cross section of the laser beam by a plane perpendicular to the beam axis, as taken at a position adjacent the nip 8. Fig. 3 illustrates the fin bottom edge 5 and the fin sides 7.

A geometric line midways between the strip edges is referred to as the edge median 6. The beam cross section defines a circular spot 22 which covers part of the strip bottom edge and part of the tube outside. Those areas of the strip and of the tube which are affected by the laser beam are referred to as the edge heated region 23 and the tube heated region 24, respectively. The edge heated region 22 is generally symmetric about the median 6 of the strip edge surface.

The laser gun is provided with aiming adjustments (not shown) adapted for permitting aiming of the laser beam precisely on the nip, or possibly towards some other point offset from that point as selected. Reference is now made to Fig. 5 for a description of some geometrical aspects about the set-up. Fig. 5 basically illustrates a part of Fig. 1 to an enlarged scale in order to clarify certain aspects about aiming the laser gun.

Thus Fig. 5 illustrates how the strip extends in a straight leg from the guide roller 18 to the point of contact 25 in an oblique direction relative to the tube axis. The direction of the strip is selected to match the pitch of the helical strip, which is controlled by the tube drive mechanism 13 (refer Fig. 1) . This angle may be e.g. 5° from a plane perpendicular to the tube axis, depending on the pitch.

The laser gun 20 produces a laser beam which is focused on a focal point 26. Thus the laser beam may be perceived as a bunddle of laser rays, emanating from various points across the laser gun aperture A and all aimed at the point 26.

According to the invention the laser gun is aimed so as to direct the beam towards the nip, however, the laser gun is spaced from the nip by a distance adapted to ensure that the laser beam is intentionally defocused relative to the regions where the heating effect is desired. In the preferred embodiment illustrated in Fig. 5, the nip effectively falls short of the focal point. The result is that the laser beam affects a spot of a finite transverse dimension, while extreme area concentration of laser beam energy is avoided within the field of interest.

The general axis of the laser beam 21 deviates from the direction of the metal strip median by an angle sufficient to avoid any rays of the converging beam striking the metal strip at points too far away from the nip to be of useful purpose. The guide roller 18 controls the metal strip straight leg precisely and facilitates the design of the apparatus by permitting the strip dispenser to clear the laser gun.

The transverse dimension of the laser beam spot may be adapted as appropriate by suitable design of the laser gun and by varying the spacing of the laser beam from the nip as appropriate according to geometric considerations, which will appear to those skilled in the art.

One preferred embodiment comprises a laser gun with an aperture of a diameter of 50 mm and a focal length of 400 mm. The rays of the laser beam are aimed towards the focal point from a range of directions within approximately plus or minus 3.5° from the beam axis. The beam axis is adjusted to deviate approximately 4° from the direction of the strip straight leg. In this case arranging the laser gun to aim on the nip with the focal point offset by 28 mm beyond the nip ensures a laser spot diameter of approx. 3.5 mm, roughly equivalent to 10 mm² (referred to a plane perpendicular to the beam axis) .

In a test run, a C0₂ laser gun was used outputting a laser beam at 10.6 micrometer wave length with a beam power of 10 kW. The power density on the spot may be estimated at 1 kW per mm². This is expected to be below the point, where melting or plasma formation in steel can be offset. Experiments have found this power density well suited for welding by the method according to the invention.

This provides optimum conditions for a fast process, yielding an excellent end product wherein any thermally induced degrading influences on the components have been kept to an absolute minimum. Thus with a metal strip of steel with a cross section of 16 x 1 mm strip welds have been made to a speed of 115 m per minute. Heating of the surfaces was estimated to a temperature of 1100°C. The pressure roller applied a force of 90 kp onto the strip. Samples of the end product have been examined by appropriate methods and have revealed that even by these high speeds the end product was in fact superior to the products achieved by methods of the prior art.

Reference is now made to Fig. 4 for a description of a heat exchanger tube. Fig. 4 illustrates a heat exchanger tube 1, comprising a finned section 11 delimited by naked tube sections 12. Thus the fin comprises a finite length of strip, of which the ends by the sequence of manufacturing are referred to as the lead end 9 and the tail end 10.

In order to manufacture this tube section, a lead end of metal strip is advanced into contact with the tube surface. The lead end is advanced so far as to be engaged under the pressure roller 15. At this stage the laser gun is turned on but on a comparatively low level of power, and the tube drive mechanism 13 is started. The tube and the pressure roller immediately pull forward the strip lead end. The speed of the tube drive mechanism as well as the laser power are ramped up in an appropriate coordinated pattern, and a high quality welding is obtained after but a very short lead end. The process is so easily controlled that the high quality weld may be achieved to a uniform result even during a phase of ramping up the speed of the tube drive mechanism as well as the power of the laser gun.

Once a suitable tube section has been provided with fins, the metal strip is sheared transversely on the leg between the guide roller 18 and the nip 8. The shearing is preferably made by a shearing device adapted for moving forward along with the advance of the metal strip during a phase of shearing. Once the cut is completed, the reel 17 immediately stops while the tail end of the strip simply continues, the pressure roller 15 providing appropriate guiding to ensure that also the tail end is adhered. Proper bonding of the tail ends has been achieved even at full speed in the process.

Although specific embodiments have been described above it is emphasized that the invention may be exercised in several ways and that the explanation given above exclusively serves to clarify the invention and not to limit the scope of protection conferred, which is exclusively defined by the appended claims.

Claims

Claims:

1. A method for the manufacturing of a heat exchanger tube with an exterior helical fin, the method comprising: -rotating a tube about a longitudinal axis,

-heating a region of a bottom edge surface of a strip to a fusion temperature by directing a laser beam onto the region, the laser beam being oriented so as to heat the edge surface over an area, which is generally symmetric about a median of the edge surface,

-advancing the strip longitudinally to feed it onto the tube surface under tension with the bottom edge of the strip engaging the tube exterior surface, while shifting the tube axially relative to the point of engagement between the strip and the tube in such way that the strip is shaped to form a helix about the tube in which the strip extends radially,

-keeping the strip pressed against the tube surface for a time sufficient to allow the formation of a fusion bond between the strip and the tube.

2. The method according to claim 1, wherein the heating of the region of the edge surface is tuned to stay below the limit, where melting of the strip edge surface may occur.

3. The method according to claim 1, wherein the tube surface within a region adapted for being contacted by the strip is heated to a fusion temperature by directing a laser beam onto the tube surface.

4. The method according to claim 3, wherein the heating of the strip edge surface and of the tube surface is effected by means of a laser beam, which is directed onto the nip, where the strip meets the tube, in such way, that heat power from the laser beam is distributed among the strip edge and the tube.

5. The method according to claim 4, wherein the laser beam is directed along an axis, which runs generally tangentially towards the tube and in a small angle to the longitudinal axis of the strip being fed onto the tube.

6. The method according to claim 1, wherein the laser beam is adapted to irradiate a spot, which extends along the tube axis for a sufficient length to allow for wrinkling of the edge of the strip due to the strip being wound onto the tube, while still ensuring proper heating of a suitable region of the strip edge.

7. The method according to claim 1, wherein the laser beam is emitted through a focusing device adapted to converge the beam towards a focal point, while the focusing device is placed at a distance from the tube axis selected to offset the focal point from the respective regions of laser beam impingement .

8. The method according to claim 1, wherein the strip is pressed against the tube surface with a pressure sufficient to forge a deformation in parts of the respective contacting, heated regions, whereby brittle coatings such as oxides are caused to break up.

9. The method according to claim 1, wherein the strip is pressed against the tube surface by feeding the strip onto the tube under tension.

10. The method according to claim 1, wherein the strip is pressed against the tube surface by the action of a guide roller.

11. The method according to claim 1, wherein the axial shifting of the tube relative to the strip is implemented by axially shifting the tube, while keeping the position of feeding the strip onto the tube stationary.

12. The method according to claim 1, wherein a section of strip is wound onto the tube, the strip section being terminated by shearing the strip on the fly, while continuing rotating the tube, until the strip tail end has been joined with the tube.

13. The method according to claim 1, wherein the winding of a finned tube section commences by initiating laser irradiation and advancing a strip leading end to contact with the tube surface and keeping it pressed against the tube for a length of time sufficient to allow it to form a bonding with the tube.

14. A heat exchanger tube comprising a tube fitted with an exterior fin formed from a strip, which is placed in a helical pattern and so as to extend radially from the tube, the strip being bonded along one edge surface to the tube surface, by way of the strip edge surface and a corresponding region of the tube surface having been heated to a fusion temperature substantially without having been melted, and brought together and kept pressed together for a time sufficient to allow them to form a fusion bonding.

15. The heat exchanger tube according to claim 14, wherein the fin is elastically tensioned against the tube surface in a state of thermal equilibrium.

16. The heat exchanger tube according to claim 14 and comprising a finned section delimited by naked sections, within which finned sections the strip has been joined with the tube by a uniform fusion bond with no particular provisions to bond the strip ends.

17. The heat exchanger tube according to claim 14 and manufactured by the method according to any of the claims 1 through 13.

18. An apparatus for the manufacturing of a heat exchanger tube comprising a tube fitted with an exterior helical fin, the apparatus comprising:

-means for rotating a tube about a tube axis, -means for dispensing a strip under controlled tension and in edge orientation towards a position of engagement with the tube surface,

-means for providing controlled axial displacement of the tube relative to the position of engagement, -means for emitting a laser beam towards a bottom edge surface of the strip at a position adjacent the position of engagement, the laser beam being oriented and adapted so as to heat the edge surface to a fusion temperature over an area, which is generally symmetric about a median of the edge surface,

-means for keeping the strip pressed against the tube surface for a time sufficient to allow the formation of a fusion bond between the strip and the tube.

19. The apparatus according to claim 18, and comprising means adapted for emitting a laser beam adapted for heating an area of the tube surface to a fusion temperature.

20. The apparatus according to claim 18, wherein the means for keeping the strip pressed against the tube surface comprises a bonded leading section of strip in cooperation with brake means for controlling the tension in the strip being dispensed.

21. The apparatus according claim 18, wherein the means for keeping the strip pressed against the tube comprises a guide roller adapted for urging the strip against the tube surface.

22. The apparatus according claim 18, comprising means for implementing the method according to any of the claims 1 through 13.