METHOD FOR PRODUCING A HEAT EXCHANGE TUBE

Abstract

A method for producing a heat exchange tube having an inner turbulence insert for a heat exchanger may include providing an austenitic heat exchange tube and the turbulence insert. The method may also include inserting the turbulence insert into the austenitic heat exchange tube, brazing the turbulence insert and the austenitic heat exchange tube via induction brazing, and pressing the austenitic heat exchange tube onto the turbulence insert at least one of during the brazing and after the brazing.

Claims

1. A method for producing a heat exchange tube having an inner turbulence insert for a heat exchanger, comprising: providing an austenitic heat exchange tube and the turbulence insert; inserting the turbulence insert into the austenitic heat exchange tube; brazing the turbulence insert and the austenitic heat exchange tube via induction brazing; and pressing the austenitic heat exchange tube onto the turbulence insert at least one of during the brazing and after the brazing.

2. The method according to claim 1, further comprising applying a brazing alloy as at least one of a brazing foil and a brazing paste onto at least one of an inner side of the austenitic heat exchange tube and the turbulence insert.

3. The method according to claim 1, wherein the pressing of the austenitic heat exchange tube onto the turbulence insert is via at least one roller pair.

4. The method according to claim 1, wherein at least one of: the austenitic heat exchange tube is composed of an austenitic stainless steel; and the turbulence insert is composed of at least one of a ferritic stainless steel and an austenitic stainless steel.

5. The method according to claim 1, wherein the brazing takes place subject to a protection-gas atmosphere.

6. A heat exchanger comprising at least one heat exchange tube affixed in an associated tube sheet via a laser-weld connection, the at least one heat exchange tube being produced by: providing an austenitic heat exchange tube and a turbulence insert; inserting the turbulence insert into the austenitic heat exchange tube; brazing the turbulence insert and the austenitic heat exchange tube via induction brazing; and pressing the austenitic heat exchange tube onto the turbulence insert at least one of during the brazing and after the brazing.

7. The heat exchanger according to claim 6, wherein a wall thickness of the at least one heat exchange tube is smaller than 0.5 mm.

8. The heat exchanger according to claim 6, wherein the at least one heat exchange tube is a shaped and laser-welded steel strip.

9. The heat exchanger according to claim 6, wherein the at least one heat exchange tube includes at least one of at least one longitudinal bead and at least one transverse bead.

10. The heat exchanger according to claim 6, wherein at least one of: a longitudinal-end region of the at least one heat exchange tube laser-welded to the associated tube sheet is free of a brazing alloy; and the heat exchanger is designed as an exhaust-gas heat exchanger.

11. The heat exchanger according to claim 6, wherein a longitudinal-end region of the at least one heat exchange tube laser-welded to the associated tube sheet is free of a brazing alloy.

12. The heat exchanger according to claim 7, wherein the at least one heat exchange tube includes at least one of at least one longitudinal bead and at least one transverse bead.

13. The method according to claim 1, wherein the at least one of during the brazing and after the brazing includes both during the brazing and after the brazing.

14. The method according to claim 1, wherein the austenitic heat exchange tube is composed of an austenitic stainless steel.

15. The method according to claim 1, wherein the turbulence insert is composed of a ferritic stainless steel.

16. The method according to claim 1, wherein the turbulence insert is composed of an austenitic stainless steel.

17. A method for producing a heat exchange tube having an inner turbulence insert for a heat exchanger, comprising: providing an austenitic heat exchange tube and the turbulence insert; inserting the turbulence insert into the austenitic heat exchange tube; brazing the turbulence insert and the austenitic heat exchange tube subject to a protection-gas atmosphere via induction brazing; pressing the austenitic heat exchange tube onto the turbulence insert via at least one roller pair at least one of during the brazing and after the brazing.

18. The method according to claim 17, further comprising applying a brazing alloy onto at least one of an inner side of the austenitic heat exchange tube and the turbulence insert.

19. The method according to claim 18, wherein the brazing alloy is a brazing foil.

20. The method according to claim 18, wherein the brazing alloy is a brazing paste.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Thereby, the figures schematically show

[0024] FIG. 1 is a cross-sectional illustration through a heat exchange tube produced according to the invention,

[0025] FIG. 2 shows an apparatus for carrying out the production method according to the invention,

[0026] FIG. 3 is a cross-sectional illustration through a possible embodiment of a heat exchanger according to the invention.

DETAILED DESCRIPTION

[0027] In accordance with FIG. 1, a heat exchange tube according to the invention 1 comprises a turbulence insert 2 arranged on the inside. Such a heat exchange tube 1 according to the invention is used in a heat exchanger 3, for example, an exhaust-gas heat exchanger 4 (compare FIG. 3). According to the invention, now, the heat exchange tube 1 is made of an austenitic material, for example, a stainless steel, whereas the turbulence insert 2 is made of a preferably ferritic material, for example, also a stainless steel. The difference between the two materials can thereby only be in their magnetizability. By designing the turbulence insert 2 out of a preferably ferritic material and designing the heat exchange tube 1 out of an austenitic material, in particular, a completely new production method for the heat exchange tube 1 can be made possible, which, in particular, avoids the disadvantages known from prior art, such as a decrease in strength due to soft annealing in a brazing furnace for example.

[0028] The heat exchange tube 1 according to the invention is produced as follows:

[0029] Initially, the austenitic heat exchange tube 1 and the preferably ferritic turbulence insert 2 are provided, wherein, then, the turbulence insert 2 is pushed into the heat exchange tube 1. Now, a brazing of the turbulence insert 2 to the heat exchange tube 1 takes place by means of induction brazing, for which, in particular, an inductor 5 (compare FIG. 2), for example, an inductor coil 6, is used. Thereby, in FIG. 2, the induction coil 6 is arranged in such a way that the heat exchange tube is moved through it. Thereby, inductors 5 are also conceivable that are arranged above and below and designed as flat coils. Before, during and/or after the induction brazing, a pressing of the heat exchange tube 1 against the inner turbulence insert 2, whereby the brazing connection to be produced can be affixed in a reliable manner. The pressing of the heat exchange tube 1 against the turbulence insert 2 can thereby take place by means of a roller pair 7 (compare FIG. 2) for example. Up until this point, the turbulence inserts 2 are also affixed within the heat exchange tube 1 by means of brazing, wherein, however, the brazing took place in a brazing oven, in which both the heat exchange tube 1 as well as the turbulence insert 2 were heated for hours, thereby experiencing a decrease in strength which was not to be underestimated due to soft annealing. This can now be reliably avoided by means of induction brazing according to the invention since a heat input required for brazing only takes place locally over a short period without the long-term heating of the turbulence insert 2 or of the heat exchange tube 1. A soft annealing does not take place here at all. A considerably quicker cooling of the heat exchange tube 1 after brazing has been completed, which can naturally still be supported in an optimal manner that a roller pair 7 are cooled, for example, being arranged after the inductor 5. For this purpose, a corresponding cooling device is arranged in the roller pair 7 and connected to this.

[0030] In the case of inductive heating of the heat exchange tube 1, the electromagnetic alternating field, which is incorporated by means of the inductor, is partially or fully shielded in the interior space of the tube by the preferably ferritic material according to the invention so that heating the brazing alloy only takes place via contact with the heated heat exchange tube 1. In the case of an austenitic stainless steel, which is ferromagnetic and has a relatively poor electrical conductivity, this penetration depth is approx. 1 mm or more in the case of frequencies of 50 to approx. 200 kHz, which are usual for inductive brazing. In the case of a wall thickness of the heat exchange tube 1 of a maximum of 0.5 mm, thereby, a direct heating of the brazing alloy or of the turbulence insert 2 is possible. Due to the design of the turbulence insert 2 according to the invention made of a preferably ferritic material (steel), the power coupling of the alternating field is further shifted into the direction of the turbulence insert 2, which further reduces the temperature differences between the heat exchange tube 1 and the turbulence insert 2, and, thereby, the inner stress levels.

[0031] The heat exchange tube 1 according to the invention is used, for example, in an exhaust-gas heat exchanger 4, wherein the individual heat exchange tubes 1 are affixed on their respective longitudinal ends 8 in an associated tube sheet 9 or in passages arranged in this respectively. A fluid-tight fixation of the heat exchange tubes 1 in the associated passages of the tube sheet 9 takes place by means of laser welding, for the purpose of which a laser-welding device 10 is used. By means of this, the production of a heat exchanger block 11 with heat exchange tubes 1 and a longitudinal-end side of the same arranged tube sheets 9 can take place in a conventional way, whereby a comparably robust heat exchanger block 11 can be created without having to fear a decrease in strength, which has taken place up until this point in the brazing furnace.

[0032] In order to avoid a negative influence of a brazing alloy 12, which is applied onto an inner side of the heat exchange tube 1 and/or onto the turbulence insert 2, for example, as a brazing foil 13 or as a brazing paste 14, in view of the laser-weld connection 17, preferably, it is provided that a longitudinal-end region 8 of the heat exchange tube 1 welded to the tube sheet 9 is free of brazing alloy. By means of this, in particular, an entry of the nickel-container brazing material to the welding region and, at the same time, also the negative influence associated therewith can be avoided.

[0033] Brazing with the induction-brazing method and/or the laser welding used according to the invention can thereby take place subject to a protection-gas atmosphere, which, in particular, prevents an undesired entry of oxygen to the brazing connection or to the laser-weld connection 17.

[0034] With the heat exchange tube 1 produced by means of induction brazing and the tube sheets 9 connected to it by means of laser welding, in particular, the soft annealing of the steel in the brazing oven causing the decrease in strength up until this point can be avoided, whereby the entire heat exchanger block 11 and, in particular, also the tube sheets 9 have a higher level of strength over the long term. Due to the laser-weld connection 17 of the heat exchange tubes 1 into the passages of the tube sheets 9, an exclusively local heat input can also be achieved, which, in turn, does not result in soft annealing of the tube sheet 9 or the heat exchange tubes 1 and, by means of this, also does not cause the decrease in strength caused due to this up until this point in the brazing oven. By means of the induction brazing method according to the invention and the laser welding, which is also according to the invention, the extensive thermal load can be considerably reduced when joining the turbulence insert 2 in the heat exchange tube 1 and when joining the heat exchange tube 1 to the related tube sheet 9, whereby the steel microstructure and thereby, also the strength can be maintained. In order to further increase the stiffness of the heat exchange tubes 1, these can also have longitudinal beads 15 (compare FIG. 1) or transverse beads 16.

[0035] Thereby, the method according to the invention can particularly be used as a continuous production process, whereby very low lead times and costs for the production of the heat exchange tube 1 with a turbulence insert 2 inductively brazed into it can be achieved in comparison to furnace brazing. Furthermore, a tried and tested tube bundling technology with heat exchange tubes 1 laser-welded into the tube sheets 9 and their known advantages can be maintained.

[0036] Thereby, it is conceivable that the provided heat exchange tube 1 is open on its longitudinal side so that the turbulence insert 2 together with a brazing foil 13 can be better inserted/pressed. After inserting the turbulence insert 2, the heat exchange tube 1 is sealed by means of a laser-welding process and then brazed by means of induction brazing. Thereby, the brazing foil 13 can be attached on both sides of the turbulence insert 2, for example, above and below, which makes the production process considerably easier since the problem up until this point has been pressing the brazing foil 13 together with the turbulence insert 2 in a clean manner. This is done with a brazing foil 13, which is folded on the front around the turbulence insert 2, meaning a long brazing foil 13, which is longitudinally laid around the turbulence insert 2 a single time. That has the advantage that the brazing foil 13 initially completely seals the heat exchange tube 1 on one side a single time and has to be melted completely during the brazing process. Thereby, unnecessary brazing alloy 12 is also located in the heat exchange tube 1.