Brazed Heat Exchanger and Manufacturing Process

Abstract

A brazed heat exchanger, for example a heat exchanger to be used in an air-conditioning system, preferably as a condenser, includes flat tubes extending between a pair of header tubes and fins arranged between the flat tubes. The components are produced from aluminum alloys, and are brazed together using an AlSi braze alloy. The aluminum alloys have a zinc content of no greater than 0.5% before brazing, and zinc from the aluminum alloys diffuses into the braze joints to result in braze joints having an average zinc content of no greater than 0.1%.

Claims

1. A brazed heat exchanger comprising: a pair of spaced apart header tubes constructed of a first aluminum alloy; a plurality of flat tubes constructed of a second aluminum alloy, each of the flat tubes extending between the pair of spaced apart header tubes and having a first end joined to the first header tube and a second end joined to the second header tube by way of braze joints; and a plurality of fins constructed of a third aluminum alloy arranged between the flat tubes and joined to broad sides of the flat tubes by way of braze joints; wherein each of the first, second, and third aluminum alloys have an average Zn content of no greater than 0.05% before the creation of the braze joints.

2. The brazed heat exchanger of claim 1, wherein the braze joints have an average Zn content of no greater than 0.1%.

3. The brazed heat exchanger of claim 2, wherein the Zn content of the braze joints is not uniformly distributed throughout the braze joints.

4. The brazed heat exchanger of claim 1, wherein the third aluminum alloy has a electrochemical corrosion potential between −750 mV and −700 mV.

5. The brazed heat exchanger of claim 1, wherein the third aluminum alloy is less noble than the second aluminum alloy.

6. The brazed heat exchanger of claim 5, wherein the difference in electrochemical corrosion potential between the second and third aluminum alloys is no greater than 20 mV.

7. The brazed heat exchanger of claim 1, wherein the first aluminum alloy is a long life alloy comprising a core layer and a surface layer formed by silicon diffusion during the course of brazing.

8. The brazed heat exchanger of claim 7, wherein the surface layer is less noble than the core layer.

9. The brazed heat exchanger of claim 8, wherein the difference in electrochemical corrosion potential between the surface layer and the core layer is no greater than 20 mV.

10. The brazed heat exchanger of claim 1, wherein the braze joints are formed from an AlSi braze layer applied to the pair of headers and to the plurality of flat tubes or the plurality of fins or both.

11. The brazed heat exchanger of claim 10, wherein the AlSi braze layer has an average Zn content of no greater than 0.05% before the creation of the braze joints.

12. The brazed heat exchanger of claim 11, wherein the Si content of the AlSi braze layer is between 6.8% and 12%.

13. The brazed heat exchanger of claim 1, wherein the plurality of fins have a material thickness between 30 and 100 μm.

14. The brazed heat exchanger of claim 13, wherein the plurality of flat tubes have a wall thickness of no greater than 0.20 mm.

15. A method of manufacturing a heat exchanger comprising: assembling a pair of header tubes, a plurality of flat tubes, and a plurality of fins to form a heat exchanger core assembly, each of the header tubes, flat tubes, and fins being formed from an aluminum alloy having an average Zn content of no greater than 0.05%; heating the heat exchanger core assembly to a brazing temperature sufficient to melt and flow an AlSi braze coating applied to the pair of headers and to the plurality of flat tubes or the plurality of fins or both; maintaining the heat exchanger core assembly at the brazing temperature for a period of time sufficient to create a Si diffusion surface layer on the header tubes; and cooling the heat exchanger core assembly to create braze joints between the pair of header tubes and the plurality of flat tubes, and between the plurality of flat tubes and the plurality of fins.

16. The method of claim 15, further comprising diffusing Zn from the aluminum alloys into the braze joints to form local areas of Zn enrichment within the braze joints.

17. The method of claim 16, wherein the step of diffusing Zn results in an average zinc content of the braze joints of not greater than 0.1%.

18. An air-conditioning system including a condenser, the condenser comprising, a plurality of tubes, each of the plurality of tubes including two opposing broad sides and two opposing narrow sides, two opposing ends, and a tube wall having a first long life aluminum alloy layer and a first braze layer; two headers, each header including a header wall having a second long life aluminum alloy layer and a second braze layer; and a plurality of fins, wherein each of the plurality of tubes is arranged between the two headers, such that each of the two opposing ends of each of the plurality tubes is brazed to one of the headers, wherein at least one of the fins is located between and brazed to two of the plurality of tubes by the first braze layer of each of the two of the plurality of tubes, and wherein each of the first long life aluminum alloy layer, the second long life aluminum alloy layer, the first braze layer, and the second braze layer have an average Zn content of no greater than 0.05%.

19. The condenser of claim 18, wherein each of the plurality of tubes further comprises at least two sheet strips, and one of the at least two sheet strips is a tube insert defining ducts inside of the tube.

20. The condenser of claim 19, wherein at least one of the two sheet strips includes the first long life aluminum alloy layer and the first braze layer.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0028] FIG. 1 is a simplified partial, cross-sectional view of select components of a heat exchanger according to an embodiment of the invention prior to brazing.

[0029] FIG. 2 is the same as FIG. 1, but after brazing.

[0030] FIG. 3 is a diagrammatic representation of the material structure through a braze joint of the heat exchanger of FIG. 2.

DETAILED DESCRIPTION

[0031] Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings.

[0032] FIG. 1 shows part of a single tube 1 of a heat exchanger, in particular a flat tube, in cross-section along one of two narrow sides of the flat tube 1, having two fins 2 arranged on two broad sides 1a, 1b thereof, on the top and on the bottom. FIG. 1 also shows only one of two tube ends 1c, which sits in an opening 30 in a header tube 3 (shown only partially and in partial cross-section). The second header tube 3 is located in an identical manner at the other flat tube end 1c (not shown). FIG. 2 shows roughly the same image, but after the conclusion of a CAB brazing process in a brazing furnace, or after the formation of corresponding brazed joints 10. FIG. 3 shows a section from a cross-section of a brazed joint 10 in purely abstract form, which includes regions 11, 12 that form during brazing and have a relatively high and a relatively low Zn concentration, respectively.

[0033] The flat tubes 1 and the fins 2 form a block consisting of alternating flat tubes 1 and fins 2, in which flat tubes 1 are joined to fins 2. The opposing tube ends 1c of the flat tubes 1 are located in the openings 30 in the header tubes 3, which extend perpendicular to the flat tubes 1.

[0034] The brazed heat exchanger is intended for use in an air-conditioning system in a motor vehicle, preferably as a condenser. A refrigerant flows through the flat tubes 1 of the block. Cooling air flows through the fins 2 of the block perpendicular to the plane of the figures.

[0035] The flat tubes 1 of the exemplary embodiment have been produced from two or from three sheet strips coated with a braze coating or layer 5—one of the sheet strips being formed into a duct forming tube insert, as is shown and described in the DE document cited second in the introduction.

[0036] Embodiments in which the flat tubes have been produced from a single sheet strip are also possible.

[0037] In further exemplary embodiments, which are not shown, extruded multi-chamber flat tubes 1 are present. In this case, an AlSi braze layer 5 has been applied to the flat tubes 1 by a spraying method, for example. In other embodiments of this type, the flat tubes 1 remain without a braze coating or layer 5, which is then located on the fins 2. In these cases, the braze required for the connections 10 of the flat tube ends 1c in the openings 30 is present in a sufficient quantity as an AlSi braze layer 5 on the header tubes 3.

[0038] All component parts of the heat exchanger consist of suitable aluminum alloys. The AlSi braze coating/layer 5 which has been modified in respect of a Zn content but is otherwise known per se is located on the surface of the header tubes 3 and of the flat tubes 1. The fins 2 in the exemplary embodiment do not have a braze coating/layer 5.

[0039] The average Zn content in the aluminum alloy of the fins 2, of the flat tubes 1, of the header tubes 3 and in the AlSi braze layer 5 on the flat tubes 1 and the header tubes 3 is 0.00 to ≦0.05% before the brazing.

[0040] By contrast, an average Zn content of 0.00-max. 0.1% is present in the brazed connection joints 10, as depicted in FIG. 2.

[0041] The Zn content in the brazed joints 10 is not uniformly distributed. There are local regions 11 having a relatively high Zn concentration and other regions 12 having a relatively low Zn concentration. It cannot be ruled out that some regions 11 having a relatively high Zn concentration also have a Zn content lying slightly above 0.1%. The formation of the local regions 11, 12 is also not foreseeable in terms of their local distribution in the brazed joints 10 and in terms of their shape and size, as depicted in FIG. 3. Since the average Zn content thereof has been reduced to a significantly, the susceptibility of the heat exchanger to corrosion has been reduced considerably.

[0042] The aluminum alloy of the fins preferably consists of modified AA 3003 or AA 3103. The fins 2 have an electrochemical corrosion potential of approximately in the range of −750 to −700 mV.

[0043] The electrochemical corrosion potentials are determined, for example, by a method described in ASTM-G69 (American Society of Testing and Materials).

[0044] The header tubes 3 consist of an aluminum long life material. They have a core layer (not shown) of modified AA 3003 or AA 3103 and a silicon-containing surface layer, with an electrochemical voltage potential difference of 0.00 to +20 mV between the aforementioned layers.

[0045] The aluminum alloy of the flat tubes 1 consists of long life material of modified AA 3003, and it has a voltage potential difference of 0.00 to +20 mV with respect to the fins 2.

[0046] The different voltage potentials are formed by corresponding alloying constituents, such as for example Cu, Mn or Mg. These and other alloying constituents are known to hide behind the aforementioned alloy classifications AA. The silicon diffusion already mentioned above likewise contributes to the reduction of the voltage potential through the formation of precipitations.

[0047] The addressed values of the voltage potentials refer to a state after the brazing.

[0048] The AlSi braze layer 5 is of the AA 4343 or AA 4045 type. It has an Si content of approximately 6.8-12%.

[0049] The fins 2 have a thickness of between 30 and 100 μm.

[0050] The flat tubes 1 are normally significantly thicker; they have a wall thickness of 0.20 mm or smaller.

[0051] In the course of the brazing process, zinc present as an impurity of the aluminum alloys and in the AlSi braze coating(s)/layer(s) 5 is enriched in the brazed connection joints 10. An average Zn content of up to approximately 0.1% is present in the brazed joints 10 after the brazing process.

[0052] Various alternatives to the certain features and elements of the present invention are described with reference to specific embodiments of the present invention. With the exception of features, elements, and manners of operation that are mutually exclusive of or are inconsistent with each embodiment described above, it should be noted that the alternative features, elements, and manners of operation described with reference to one particular embodiment are applicable to the other embodiments.

[0053] The embodiments described above and illustrated in the figures are presented by way of example only and are not intended as a limitation upon the concepts and principles of the present invention. As such, it will be appreciated by one having ordinary skill in the art that various changes in the elements and their configuration and arrangement are possible without departing from the spirit and scope of the present invention.