Heat exchangers comprising winglet tubes, winglet tubes and method for producing same

Abstract

Disclosed is a heat exchanger, in particular for motor vehicles, having a large number of flat tubes which contain indentations (“winglets”) and through which a fluid cooling medium can flow, and having corrugated fins which are associated with these flat tubes and to which environmental air or other media can be applied. In addition, there is disclosed a method for producing flat tubes of this type (“winglet tubes”) and for assembling these flat tubes into heat exchangers.

Description

This application claims priority from U.S. Provisional Application No. 60/506,754, filed Sep. 30, 2003, which is hereby incorporated by reference in its entirety, including the specification, drawings and claims.

BACKGROUND OF THE INVENTION

The invention relates to a heat exchanger, in particular for motor vehicles, having a large number of flat tubes which contain indentations (“winglets”) and through which a fluid cooling medium can flow, and having corrugated fins which are associated with these flat tubes and to which environmental air or other media can be applied. The invention further relates to a method for producing flat tubes of this type (“winglet tubes”) and for assembling these flat tubes into heat exchangers.

EP 0 030 072 B1 discloses a heat exchanger having a large number of flat tubes, through which coolant can flow, as well as corrugated fins which are associated with these flat tubes and to which environmental air can be applied. In this case, the flat tubes have indentations, with a very small indentation height. The indentations point inward on the flat faces of the tubes and are used to increase the robustness of the flat tubes. A heat exchanger such as this has the disadvantage that the coolant forms a hot core flow layer or stream within the flat tubes. This hot core flow is insulated from the flat tube walls by a cooler wall flow layer and exchanges little heat. As a result the amount of heat transferred between the core flow and the flat tube walls is decreased.

DE 1 96 54 367 A1 relates to the solution of a very different problem from the field of use of the present invention. It discloses a rectangular tube for an exhaust gas heat exchanger equipped with elongated winglets that point inward in the form of vortex generators. The vortex generators, which are each arranged in pairs in a V-shape, are formed from the solid material of the tube and are positioned such that they diverge in the main exhaust gas flow direction. The vortex generators are used to reduce deposits on the inner walls of the tubes of solids—such as carbon black—contained in the exhaust gases. No further details are given of the dimensions of the vortex generators.

In commonly assigned German Published Patent Application No. 10127084 A1, which is hereby incorporated by reference in its entirety, there is disclosed a heat exchanger manufactured from winglet tubes, in which the indentations extend essentially from one end to the other of the tubes. By providing sufficient spacing between perpendicularly transverse rows of indentations, it may be possible to provide end regions of the tubes that do not have indentations, in which case the tubes can be snuggly inserted into the header plate openings and brazed in a fluid-tight manner. However, design limits not only the number of different lengths of tubes that can be made (without using different indentation-producing apparatus) but also the density and/or configuration of the indentations. For example, in those embodiments of the commonly assigned application in which the rows of indentations are obliquely transverse to the tube axis, this option is not available at all.

If indentations exist in those regions of the tubes that are inserted into the header plate, excess brazing material is needed to secure the connections and/or leak failure points may remain after the brazing procedure. Although it is theoretically possible to provide specialized apparatus that can continuously stamp endless lengths of flat sheet material with intermittently applied patterns of indentations and thereby produce unstamped regions at selected locations, e.g., to accommodate different lengths of tubes, such a solution is completely impractical in terms of mass production costs, speed and flexibility to accommodate different tube lengths. Thus, if winglets are formed by indentations in tubes, the indentations may cause problems with respect to the forming of sufficiently fluid tight joints between the tube and the header. Specifically, depending on the dimensions of the indentation it may be difficult or impossible to form a sufficiently fluid tight joint by brazing and/or to obtain an acceptable heat exchanger.

SUMMARY OF THE INVENTION

Thus, one object of the present invention is to provide an improved heat exchanger tube of the type that contains winglets.

A further object of the invention is to provide an improved method for producing such improved heat exchanger tubes.

Another object of the invention is to provide an acceptable heat exchanger which comprises a fluid tight joint in the tube/header connection in which tubes contain winglets.

In accomplishing the foregoing objects, there has been provided according to one aspect of the invention a method for producing a flat tube for a motor vehicle heat exchanger, wherein the tube has a central portion and end portions, comprising: forming a pattern of indentations on a sheet, in areas that correspond to the central portion and at least one end portion of a tube to be produced; at least substantially flattening out at least a portion of the raised surface of at least one indentation in the area corresponding to the at least one end portion of the tube, to form an at least substantially flat region on the area of the sheet corresponding to the at least one end region; and forming the sheet into the shape of a tube having a substantially flat region at at least one end portion.

According to another aspect of the invention, there is provided a method for producing a heat exchanger for a motor vehicle, comprising:

• • (a) producing a flat tube by a method as defined above;
  • (b) inserting the substantially flat end portion of the tube into a header;
  • and (c) brazing to form a fluid-tight seal at the junction between the flat tube and header.

There has also been provided according to the invention both flat tubes and heat exchangers produced therefrom, where the flat tubes are made according to the method of the invention.

Further objects features and advantages of the invention will become apparent from the detailed description of preferred embodiments that follows, when considered together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows a three-dimensional partial view of a heat exchanger according to the invention, having fins, flat tubes and tube bases;

FIG. 2 shows a plan view of a first flat face, seen from the inside of the flat tube;

FIG. 3 shows a plan view of a second flat face, seen from inside the flat tube;

FIG. 4 shows a section illustration of a subregion of the flat tube, illustrated on a larger scale than in FIGS. 2 and 3;

FIGS. 5 and 6 show illustrations as in FIGS. 2 and 3 of a further embodiment;

FIGS. 7 and 8 show illustrations as in FIG. 2 or 3 of further embodiments;

FIG. 9 shows an illustration as in FIG. 7, but with further details added;

FIG. 10 shows an illustration as in FIG. 9, but with a modified geometry;

FIG. 11 shows an illustration as in FIG. 9, but with a modified geometry;

FIG. 12 shows an illustration as in FIG. 9, but with a modified geometry;

FIG. 13 shows a section illustration of a flat tube, with winglets arranged in a stepped form;

FIG. 14 shows a section illustration of a flat tube, with winglets arranged in a stepped form;

FIG. 15 shows a schematic of a method for producing winglet tubes according to a preferred embodiment of the present invention;

FIG. 16 shows a set of rollers including a “stamp out” roller in a variety of positions;

FIG. 17a, FIG. 17b, FIG. 17c and FIG. 17d each show exemplary welded tubes;

FIG. 18a, FIG. 18b and FIG. 18c each show exemplary folded tubes; and

FIG. 19 shows an hour glass tube made by the method according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As discussed in more detail below, the objects of the invention are accomplished according to the present invention by at least partially stamping out at least some of the indentations that have been previously made to produce the winglets, e.g., either completely or partially selectively flattening at least some of the winglets, from the end of tubes.

The term winglet is used in a broad sense to include any type of indentation that is of sufficient depth to materially affect the ability to manufacture a fluid-tight connection between the tube and header plate. The present invention is applicable to a broad array of types of winglets, including but not limited to the preferred embodiments that are illustrated in the drawings and described in more detail hereinafter. According to some of these preferred embodiments, the indentations are in the form of elongated winglets with a longitudinal axis, and the ratio between the height of the winglets and the height of the flat tubes ranges between approximately 0.05 to 0.5. Further, according to particularly preferred embodiments, the longitudinal axes of the winglets may be inclined at angles of approximately 10 degrees to 40 degrees to the direction of the tube longitudinal axis. Additionally, the adjacent winglets are preferably arranged in (i) an opposing direction and (ii) transversely with respect to the longitudinal axis of the tube. The winglets have a height, with respect to the cross-sectional dimension of the tubes, sufficient to increase the turbulence of the coolant flow, thereby, depending on the sizes of the winglets, causing either (i) vortices to be formed or, at least, (ii) the boundary layer to be broken up. This improves the exchange between the various coolant layers.

A further preferred aspect of the invention provides for the ratio between the height of the winglets and the height of the flat tubes to be approximately 0.05 to 0.25. Winglets with such dimensions function primarily to break up the boundary layer of the coolant flow, thereby ensuring improved exchange between the various coolant layers, with comparatively low pressure gradients.

Another preferred embodiment of the invention provides for the ratio between the height of the winglets and the height of the flat tubes to be approximately 0.25 to 0.5. Winglets with such dimensions deliberately produce longitudinal vortices due to their height and the elongated form. The winglets are inclined at angles relative to the tube longitudinal axis. These longitudinal vortices augment the thorough mixing of the individual coolant layers downstream because they move in a spiral shape in the tube longitudinal axis direction, and, thus, have transverse components in addition to the longitudinal movement.

An additional preferred aspect of the invention provides for the winglets to be arranged in winglet rows of, for example, at least three winglets which run transversely with respect to the tube longitudinal axis and are preferably essentially in straight lines. This aspect of the invention also provides, for example, a number of winglet rows arranged essentially in a straight line one behind the other in the direction of the tube longitudinal axis. This arrangement of the winglets, in the form of straight rows, allows the areas in which longitudinal vortices are produced to be defined accurately over the entire depth and width of the flat tube. Such an arrangement makes it possible to optimize the way in which the longitudinal vortices interact for specific coolant flow speeds or flow ranges and thereby enhance the thorough mixing. In this case, it has been found to be particularly advantageous for the ratio of (i) the distance between the winglet rows in the direction of the tube longitudinal axis to (ii) the length of the winglets to be approximately 1 to 10. It has further been found advantageous for the ratio of (i) the distance between the winglets, which are transverse with respect to the direction of the longitudinal axis of the tube to (ii) the length of the winglets to be approximately 0.1 to 0.9, preferably 0.2 to 0.8. In this context, the length of the winglets means the length projected transversely with respect to the tube longitudinal axis.

A further preferred embodiment of the invention provides for the capability to arrange the winglets on both flat faces of the flat tubes and for the respective winglet rows on the first flat face and on the second flat face to be arranged offset with respect to one another in the direction of the tube longitudinal axis. An arrangement of winglet rows such as this allows for mutual interference between the longitudinal vortices and, hence, further improvement in the thoroughness of mixing of the coolant layers. In addition, since the contact surface areas and hence the brazed surface areas are enlarged, the quality of the brazing between the flat tubes and the corrugated fins is improved. In this context it has been found to be particularly advantageous for the ratio between (i) the distance between the first flat face and the second flat face of the winglet rows in the direction of the tube longitudinal axis and (ii) the height of the winglets to be approximately 10 to 30.

Yet a further preferred embodiment of the invention provides for the winglet rows, which are adjacent in the longitudinal direction, to be arranged offset at an angle, “beta”, of approximately 10 degrees to 30 degrees, preferably at or about 20 degrees. The advantage of an arrangement offset in a manner such as this is that this results in the indentations forming a uniform pattern in the tube strip material. This is advantageous for production and for the fin-tube assembly, particularly its brazing, to be made more uniform. This can have a positive effect both on the strength of this joint and on the heat transfer, due to the homogenization of the heat flows.

Turning now to the figures, FIG. 1 shows a three-dimensional partial view of a preferred heat exchanger 10 for use in motor vehicles, comprising flat tubes 12 through which a liquid coolant 13 can flow. This coolant 13 carries heat from a propulsion unit (engine), which is normally included but has not been illustrated here for the sake of clarity, to the heat exchanger 10. The heat exchanger 10 dissipates this heat via corrugated fins 14 to the environmental air 15, or to other media. In this case, the corrugated fins 14 are each arranged between the flat tubes 12, and the flat tubes are each held by a tube base 16 at their ends. The tube base 16 in turn forms a part of a collecting tank, which is normally included but has not been illustrated here for the sake of clarity. The collecting tank is connected to the internal combustion engine via hoses.

The flat tubes 12 of the heat exchanger 10 have a relatively small flat tube internal height, “H”, for example 1 mm, as shown in FIG. 4, in comparison to a relatively large depth, “t”, FIG. 1. In this case, they have winglets 22 on both their first flat faces 18 and their second flat faces 20. The winglets 22 have a closed surface and are formed, for example, by rolling in the direction of the inside of the flat tubes 12. As illustrated in FIG. 2 and FIG. 3, the winglets 22 have an elongated form and are arranged in winglet rows 24 aligned transversely with respect to the tube longitudinal axis 13. A number of such winglet rows 24 are arranged one behind the other in the direction of the tube longitudinal axis 13. The ratio between (i) the distances, b, between the individual winglets 22 and (ii) the length, L, of the winglets (which is 3 mm, for example) is preferably, in this case, approximately 0.7, although this ratio may be in the range from 0.1 to 0.9, and preferably in the range from 0.2 to 0.8. The width of the winglets, “B”, is preferably 1.3 mm. The ratio between the distances, “C”, between the individual winglet rows 24 and the length, “L”, of the winglets is preferably approximately 4, although this value may be between 1 and 10.

The winglets 22 are preferably each inclined at an angle alpha=20 degrees to the tube longitudinal axis 13, although this angle may be between 10 degrees and 40 degrees. Winglets 22 which are, in each case, adjacent transversely with respect to the tube longitudinal axis 13 are preferably inclined in opposite directions. Two winglets are thus, in each case, arranged in pairs in a V-shape, with the two V-limbs diverging from one another in the direction of the tube longitudinal axis 13. The winglet height, “h”, is approximately {fraction (⅓)} of the flat tube height, “H”, and is preferably 0.2 mm, although this ratio may also be between 0.3 and 0.7, so that the sum of the respective winglet heights, “h”, of the first flat faces 18 and of the second flat faces 20 may be greater than the flat tube height, “H”. This is made possible because the individual winglet rows 24 and 24′ on the first flat faces 18 and on the second flat faces 20 are arranged offset with respect to one another. In this case, the ratio between (i) the distance between the winglet rows 24 on the two flat faces 18 and 20 and (ii) the winglet height, “h”, is approximately between 10 and 30.

In an alternate embodiment of the invention which is illustrated in FIGS. 5 and 6, there are gaps between the winglet rows 24 so that, for example, pairs of winglets 22 in the row 24 may each be at greater distances from one another than the two winglets in a pair. Adjacent winglet rows 24 are arranged offset with a gap in this embodiment.

Another embodiment of the invention illustrated in FIG. 7 provides for the winglet rows 24 not to extend at right angles to the tube longitudinal direction, although they do extend transversely with respect to the tube longitudinal direction, with the individual winglet rows 24 running parallel to one another. This results in the uniform distribution of contact points of the corrugated fins 14 with zones where the heat transfer is high and is not limited to individual fins, as in the case of an arrangement at right angles depicted in FIGS. 2 and 3.

A further embodiment of the invention, illustrated in FIG. 8, provides for the angle of inclination on the outermost winglet 22′ to be increased, thus improving the thoroughness of the mixing in the region of the narrow face of the flat tube 12, where it is not possible for any winglets to be arranged.

FIG. 9 shows another preferred embodiment corresponding to that in FIG. 7, with the winglet rows which are adjacent in the longitudinal direction being arranged offset at an angle, “β”, of 20 degrees to one another. The distance “C′” between the winglet rows in this case is preferably 6 mm. Alternatively, as shown in FIG. 10, it is also possible to use a geometry in which the winglets 22 are supplemented by winglets 22′ arranged between them. Furthermore, the winglets may also be split geometrically as shown in FIG. 11, with the winglets 22″ which are located in the outer area being arranged offset with respect to the winglets 22.

Combinations of the various embodiments are, of course, also contemplated. In this case, for example, the values relating to the tube may be related to one face of a beaded tube, separated by a longitudinal bead.

FIG. 13 shows an embodiment in which the winglets each have different heights, “h”, relative to one another, resulting in a rising stepped form seen from inside the tube. By this means the power density in the central area is further increased, with the height of the winglets extending overall within the range 10% to 80% of half the height, “H″”, of the flat tubes. A descending stepped form, illustrated as seen toward the inside of the tube in FIG. 14, is alternatively possible.

In order to produce tubes comprising winglets as discussed above, a preferred method provides that sheet metal may be stamped, for example, by one or more sets of rollers which comprise raised/recessed surfaces corresponding to a desired winglet configuration. Most preferably and efficiently, the roll-stamping step is a continuous process that produces endless lengths of flat sheet material having a continuous and continuously repeated pattern of indentations on the flat sheet material, i.e., which can be used as an intermediate material to produce tubes of any desired length. In positions along the sheet metal which correspond to the end regions of a selected final flat tube length, a roll with a “stamp out element” may be applied such that different geometric configurations are possible depending on the required tube length. Stamps having lengths of, for example, 10-100 mm may be used for the stamp out element. Preferably, individual tubes are formed by cutting through the central area of the stamped out region, to produce a stamped out region at the ends of the tube that has a length between about 5 and 50 mm, respectively.

Indentations may be formed on a metal sheet by feeding the metal sheet through at least one set of rollers, referred to below at the “first set of rollers”. At least one of the rollers in the first set of rollers may comprise at least one selectively raised surface which at least partially corresponds to an indentation to be formed on the metal sheet. As the metal sheet passes through the first set of rollers and the rollers rotate, the raised portion on the roller stamps the sheet metal thereby forming an indentation in the metal sheet. As the roller comprising the raised surface or surfaces continues to roll, it forms additional indentations periodically along the length of the metal sheet.

Each indentation formed as the metal sheet passes through the first set of rollers may or may not be in final form with respect to, for example, its shape and/or orientation. If an indentation or indentations are not in final form, the metal sheet may be fed through one or more further sets of rollers thereby, for example, progressively changing the shape of the indentation on the metal sheet. Such further rollers may comprise, for example, a raised surface and/or surfaces having a height greater than or an orientation different than a previous set of rollers. As a result, the rollers progressively increase the height and/or change the orientation of the indentations on the metal sheet. As an alternative to the further rollers or following the further rollers, a set of calibrating rollers may be used to set the maximum height for all or a portion of the indentations. Calibration may be accomplished, for example by passing the metal sheet through a set of rollers arranged so as to have a gap therebetween, the size of the gap correlating directly or indirectly to the maximum height of the indentation or indentations.

A given roller comprising a raised surface may preferably comprise a plurality of raised surfaces which may be different from each other with respect to shape and/or orientation or which may be substantially identical to each other. The indentations formed on the metal sheet may comprise, for example, one or more winglets, a steg and/or indentations for an hour glass tube.

Once the indentions in/on the metal sheet are in their final shape/orientation, the metal sheet may be selectively stamped in one or more regions to selectively substantially flatten all or a portion of the indentations in the region or regions. Thus, for example, it is possible that at the edges of the stamped region, a winglet is partially stamped, e.g., the stamp flattens half of a winglet. Preferably, the metal sheet is substantially flattened at regions along the length of the metal sheet which will correspond to regions at one or both ends of a tube that is to subsequently be formed from the metal sheet. By “at least substantially flattened” is meant that the previously formed indentations are either completely flattened or removed or that they are flattened or changed in shape to an extent that the remaining indentations do not materially adversely affect the ability to provide a fluid-tight seal between the tube and the header plate by brazing. It is preferred that the indentations are removed as completely as possible by the substantial flattening step.

Selective flattening may be accomplished, for example, by selectively pressing the metal sheet between two plates. Preferably, the metal sheet is passed between a set of rollers 40, 41 such as those depicted in FIG. 16. At least one roller 40 comprises a raised surface of varying length 42. This varying length corresponds to the length of the regions that are to be substantially flattened. The distance between the substantially flattened regions along the metal sheet may be changed by, for example, varying the speed at which the roller comprising the raised surface rotates and/or by varying the diameter of that same roller. If necessary, a counter-weight of radially lesser dimension 43 can be mounted diametrically opposite to the flattening portion, to provide for smooth rotation of the roller, while providing a clearance with respect to the flat sheet and/or the indentations. FIG. 16 shows a sequence of rotation of rollers 40, 41.

A substantially flattened region of a metal sheet which previously contained an indentation is different from a flat region which never contained an indentation. This is a result of the fact that the flattening process cannot completely remove the indentation, or at least not the molecular arrangement of the metal that was produced by the indentation procedure. As a result, the substantially flattened region normally contains a shallow indentation or at least localized deformations in the metal. Preferably, this profile does not materially adversely affect the subsequent brazing process. In one preferred embodiment, the profile has a depth of less than 0.1 mm. While higher indentation depths are possible, they are increasingly likely to cause problems and result in a block that is defective.

After at least substantially flattening regions of the metal sheet, the sheet is formed into a tube shape, welded along one end and/or clinched to form a tube, calibrated and then cut approximately through the central area of the substantially flattened region to form individual tubes. Cutting is performed at the substantially flattened regions so that the ends of each tube comprise a substantially flattened region. These substantially flattened ends are inserted into headers, and then a substantially fluid-tight/leakproof seal is formed between the tube and headers by brazing. It is possible to form the fluid-tight/leakproof seal because the tubes are substantially flat in the region of brazing.

According to one preferred embodiment, the tubes are cut at a position in the substantially flattened region with a tolerance of ±3 mm.

FIG. 15 shows a schematic of a method for producing winglet tubes according to a preferred embodiment of the present invention, which includes passing sheet metal 30 through stamping roller station 31, passing the stamped sheet metal to selective flattening station 32, passing sheet metal from selective flattening station 32 past a sensor 33 arranged with an optional offset relative to cutting station 34 and then over roller 35. A controller 36 and a servo regulator 37 may be provided for process control.

FIGS. 17 and 18 depict different types of welded tubes and folded tubes that may be formed with winglets and with substantially flattened or flat end regions according the process described above. Winglets may be formed on all or part of one or both sides. The tubes can be made from any type of material that is suitable, preferably from any metallic material, but also certain types of plastic materials are subject to hot or cold forming processes. Preferably, the tubes are made of aluminum or aluminum alloy. For the sake of simplicity, only some of the tubes are shown with winglets, and it is possible that the hour glass tube has only indentations that are continuously formed along the axis of the tube to form the hour glass profile.

FIG. 17a depicts an oval flat tube. FIG. 17b depicts an hour glass tube with a line, X, drawn through the center. FIG. 17c depicts a rectangular tube, and FIG. 17d depicts a steg tube.

FIG. 18a depicts a “clinched” rectangular tube. FIG. 18b depicts a “b-type” tube, and FIG. 18c depicts a “clinched” tube with an optional “steg.”

An hour glass tube may be formed, for example, by forming two parallel linear indentions along the length of a metal sheet, folding the metal sheet in half to form a tube shape with a rounded bottom to position the two linear indentations across from each other, welding the top of the sheet to form a rounded top for the tube and brazing to fill in the gap formed by the opening between the two indentations. As shown in FIG. 19, a portion of the linear indentation may be stamped out at the end regions 50 of the hour glass tube 51 such that, at the end region 52, the tube end is substantially the same as the ends of the tube labeled “flat tube (oval)”. As a result, a fluid-tight/leakroof seal may be obtained between the hour glass tube 51 and the header 53.

As an alternative procedure for realizing the advantages of the invention, a flat “stamping out” tool with a linear delivery hub inside the tube welding machine (“Rohrschweissmaschine”), i.e., the entire machine, may be used. This approach is less preferred because it does not permit as high of processing speeds.

In yet another possible method that can be employed according to the invention, stamping out is performed as an additional operation in the heat exchanger block assembly process (“Blockfügeprozess”). Thus, the ends of the tubes are subjected to a selective tube wall flattening procedure. This approach is likewise less preferred, since it includes higher costs and offers the possibility for decreases in process safety/certainty/reliability.

In view of this background, an object of the present invention was to produce a winglet tube with tube ends that are sufficiently smooth as to not interfere with the brazing connection, with the stamps/winglets removed, e.g., by being stamped out. This object is accomplished, preferably, without requiring additional steps in the block assembly process and/or core building/assembly process (“Blockfügprozess”) at low cost, without negatively influencing the processing time, without negatively influencing the time spent in the block assembly process and/or core building/assembly process, providing maximal flexibility for the tube length, and providing possible production speeds of 3 m/s or more.

As compared to the other possible solutions discussed in the background supra, the present invention offers a number of advantages. These advantages include comparatively low capital costs, high process speeds”, flexible/selectable positioning of the “stamped out” region such that tubes of varying length can be produced, easily variable stamp length by replacing the stamp out element on the roller with a different stamp out element, elimination of an additional operation for the block manufacturing process.

The disclosure of German Patent Application No. 100 29 998.9, filed Jun. 17, 2000 is hereby incorporated by reference in its entirety.

The foregoing embodiments have been shown for illustrative purposes only and are not intended to limit the scope of the invention which is defined by the claims.

Claims

1. A method for producing a flat tube for a motor vehicle heat exchanger, wherein the tube has a central portion and end portions, comprising:

forming a pattern of indentations on a sheet, in areas that correspond to the central portion and at least one end portion of a tube to be produced;

at least substantially flattening out at least a portion of the raised surface of at least one indentation in the area corresponding to the at least one end portion of the tube, to form an at least substantially flat region on the area of the sheet corresponding to the at least one end region; and

forming the sheet into the shape of a tube having a substantially flat region at at least one end portion.

2. A method according to claim 1, wherein said forming comprises a continuous procedure applied to an endless length of metal sheet, and the method further comprises cutting the tube in an intermediate portion of the substantially flat region, to form a tube having a predetermined length and a substantially flat region at at least one end portion.

3. A method according to claim 1, wherein the indentations comprise winglets.

4. A method according to claim 3, wherein the plurality of indentations comprise a plurality of winglets.

5. A method according to claim 1, wherein the step of forming indentations comprises forming two linear, parallel indentations and wherein the step of forming the metal sheet into a tube comprises forming an hour-glass tube.

6. A method according to claim 1, wherein the step of forming indentations comprises forming a steg.

7. A tube for a motor vehicle heat exchanger produced by the method according to claim 1.

8. A tube for a motor vehicle heat exchanger produced by the method according to claim 4.

9. A tube for a motor vehicle heat exchanger produced by the method according to claim 5.

10. A tube for a motor vehicle heat exchanger produced by the method according to claim 6.

11. A method for producing a heat exchanger for a motor vehicle, comprising:

(a) producing a flat tube by a method according to claim 1;

(b) inserting the substantially flat end portion of the tube into a header; and

(c) brazing to form a fluid-tight seal at the junction between the flat tube and header.

12. A method according to claim 1, comprising producing a plurality of flat tubes, inserting a plurality of substantially flat ends of tubes into a header, and brazing to form a plurality of fluid-tight seals at the junctions between the flat tubes and the header.

13. A heat exchanger produced according to the method of claim 11.

14. A motor vehicle comprising a heat exchanger according to claim 12.