FORMING HEAT EXCHANGERS

Abstract: A method of forming a heat exchanger from aluminium or aluminium alloy components. The method comprises forming the components, at least one of which is a tubular component (10), assembling the components, and brazing the components together. Said tubular component (10) is formed by bending a sheet bringing opposite edge portions (10a) of the sheet into overlapping relationship with one another, the gap (12) between said overlapping portions extending along said component. A bead (14) containing particles of brazing flux in a solvent is applied to the tubular component (10) along the exposed end (12a) of said gap, and an adhesive agent (16) is applied on top of said bead (14) so that the flux material is encapsulated. During the brazing the tubular component (10) is orientated so that the flux flows into said gap (12) and enables a brazed joint to be formed between the overlapping portions (10a).

Description: FORMING HEAT EXCHANGERS This invention is concerned with forming heat exchangers, in particular for use in vehicles. It is common practice to form heat exchangers by assembling components, many of which are tubular, and brazing the components together. The components are made of aluminium or aluminium alloy and are usually clad with a eutectic filler alloy of aluminium and silicon. In order to remove the surface oxides, brazing flux is applied to the joints. The brazing flux may be applied by dipping the components into an aqueous slurry containing the brazing flux, eg as described in US patent number 3,951,328. In the brazing operation, the filler alloy and the flux melt and form brazed joints. Many heat exchangers incorporate tubes which are of an elongated oval shape in transverse cross-section, ie the width of this cross-section in one direction is many times the width in a direction normal to said one direction. Such tubes are conventionally formed from a sheet of aluminium or an alloy thereof. The sheet is bent into a tubular shape by bringing opposite edges of the sheet together. These edges are then joined together to seal the tube. Conventionally, the metal sheets have been seam- welded to achieve a reliable seal but this is a difficult process. Attempts have been made to seal the jointε of such tubes by brazing. The technique involves cladding the aluminium with a eutectic filler alloy before it is bent to form the tube, bending opposite edges into the interior of the tube to give two parallel flanges which are adjacent to one another, and applying a flux-containing slurry to the exposed end of the gap between the flanges. The flux is intended to flow down the gap during brazing so that the brazed joint extends across said flanges. This process, however, results in a high failure rate. It is an object of the present invention to provide a method of forming a heat exchanger which enables tubular components to be formed with reliable brazed joints. The invention provides a method of forming a heat exchanger from components which have at least a surface layer made of aluminium or an alloy thereof, the method comprising forming the components, at least one of which is tubular, assembling the components, and brazing the components together, wherein said tubular component is formed by bending a sheet into a tubular shape by bringing opposite edge portions of the sheet into overlapping relationship with one another, the gap between said overlapping portions extending along said component, characterised in that the method also comprises applying a bead containing particles of brazing flux, the bead being applied to the tubular component along the exposed end of said gap, and applying an adhesive agent on top of said bead, the liquid adhesive agent being such that it dries to leave a solid residue which adheres to the component and retains the flux in position and said solid residue can be driven off by heating, the tubular component being orientated during brazing so that the flux which melts in the brazing flows into said gap and enables a brazed joint to be formed between the overlapping portions. The term "adhesive agent" is used herein to denote any substance which causes the particles to adhere to a component and hence includes some substances which are not normally considered to be adhesives. In a method in accordance with the invention, the bead provides a reservoir of flux ensuring a plentiful supply for forming a reliable brazed joint along the tubular component. The adhesive encapsulates this flux so that the component can be handled before brazing without loss of the flux. The bead may comprise dry particles but is preferably in the form of a paste consisting of particles dispersed in a solvent which can be driven off by heating without leaving a significant residue. The requirement for the paste is that it should not flow significantly away from the gap. The solvent may be, for example, toluene, isopropanol, methylether etone, or methylene chloride. The bead may, for example, comprise 15 to 20 % by weight of solvent. The bead may be about 3mm in width and at least Imm in depth. The bead may contain between 2 and 20 milligrammes of particles per centimetre of the gap. Preferably, said overlapping portions of the tubular component extend parallel to one another into the interior of the component. This form of tubular component is known per se and provides a large area for the brazed joint. In order to provide additional space for the bead, said sheet may be bent in such a way that said exposed end of said gap is at the bottom of a groove which accommodates said bead. In order to avoid the use of sheet which has been clad with eutectic filler alloy, the bead may also contain eutectic or near eutectic filler alloy. For example, the bead may contain particles of the eutectic or near eutectic alloy which are at least partially coated with flux. WO 94/29072 describes a method of brazing using such particles and WO 94/17941 describes a method by which such particles may be manufactured. Alternatively, the particles may comprise particles of filler alloy and particles of flux mixed together. Preferably, said bead and said liquid adhesive agent contain the same solvent. The solvent used may be toluene and the component may be dried, eg by infra-red heaters to drive off the solvent. In order to enable the tubular component to be brazed to another component of the heat exchanger, preferably, after the application of the bead and either before or after the application of the liquid adhesive agent, further particles containing brazing flux are applied in dry form to the tubular component for use in forming a brazed joint with another component, the further particles being secured to the component by the liquid adhesive agent. The further particles may also contain a eutectic or near eutectic filler alloy, eg the further particles may comprise particles of the type described above, having eutectic or near eutectic alloy coated at least partially with flux, or the further particles may be a mixture of eutectic alloy particles and flux particles. The further particles may be applied by sprinkling them on to the surface of the tubular component. If the further particles are applied after the liquid adhesive agent, they stick to the adhesive or, if they are applied before the agent, they are encapsulated thereby. The liquid adhesive agent may be applied by spraying, preferably by hydraulic spraying (also known as airless spraying) . There now follows a detailed description, to be read with reference to the accompanying drawing, of a method of forming a heat exchanger which is illustrative of the invention. The drawing is a diagrammatic cross-sectional view of a tubular component of a heat exchanger made by the illustrative method. The illustrative method is a method of forming a heat exchanger from components which include tubular components such as the component 10 shown in the drawing. The components are made from aluminium which has not been clad with filler alloy. The illustrative method comprises forming the components, in the case of the components 10 by bending sheet aluminium which is 320 microns thick. The illustrative method also comprises assembling the components, and brazing the components together. The tubular component 10 is formed by bending a sheet into the elongated oval tubular shape shown in the drawing by bringing opposite edge portions 10a of the sheet into overlapping relationship with one another. Specifically, the overlapping portions 10a extend parallel to one another into the hollow interior of the component 10. The gap 12 between said overlapping portions 10a is very narrow as the portions 10a are close together. This gap 12, however, extends along the component 10 and requires to be sealed to make the component water-tight. The tubular component 10 has substantially flat horizontal upper and lower surfaces, 10b and 10c respectively. The upper surface 10b has the portions 10a projecting downwardly from a central region thereof and, at this point, the sheet is bent so that it forms a groove 13 projecting downwardly and leading to an exposed upper end 12a of the gap 12 which is at the bottom of the groove 13. The illustrative method also comprises applying a bead 14 in the form of a paste containing particles of brazing flux in a solvent which can be driven off by heating. Specifically, the bead 14 comprises particles of near- eutectic filler alloy with a composition of 11 wt% silicon and 89 wt% aluminium coated with flux having a composition of 40 to 50 wt% of potassium fluoride and 50 to 60 wt% of aluminium fluoride. The particles are dispersed in 15 wt% of toluene. The bead 14 is applied to the tubular component 10 along the exposed end 12a of the gap 12 by being extruded into the groove 13 to a width of 3mm. The bead stands slightly proud of the groove and has a depth of about 2mm. The bead 14 contains about 5.7 milligrammes of particles per centimetre length of the gap 12. Next, in the illustrative method further particles 15 containing brazing flux are applied in dry form to the upper surface 10b of the component 10. The particles 15 are of the same type as the particles in the bead 14 and are for use in forming a brazed joint with another component of the heat exchanger. The particles 15 are sprinkled over the surface 10b from a vibrating sieve. The illustrative method also comprises applying a liquid adhesive agent 16 over the surface 10a on top of said bead 14. The liquid adhesive agent is such that it dries to leave a solid residue which adheres to the component 10 and retains the bead 14 in position and said solid residue can be driven of by heating. The liquid adhesive agent 16 also serves to secure the further particles 15 to the surface 10b. Specifically, the liquid adhesive agent 16 comprises an acrylic resin dissolved in toluene. The acrylic resin is preferably polymethylmethacrylate. The agent 16 is applied by hydraulic spraying. Clearly, the spray must be gentle enough to avoid significant displacement of the particles 15 on the surface 10b. After the application of the liquid adhesive agent 16, the component 10 is passed through a drying oven in which the solvent in the agent 16 and in the bead 14 is driven off by the application of infra-red heat. The component 10 can now, if desired, have more particles 15 secured to its lower surface 10c by inverting the component 10, sprinkling particles 15 on to the surface lOe, and spraying on further liquid adhesive agent 16 which is then also dried. The component 10 can now be handled without significant loss of flux and is assembled with the other components of the heat exchanger. The assembled heat exchanger is then inserted into a brazing furnace. In the furnace, the components are joined by brazed joints and, during this brazing, the tubular component 10 is orientated with the surface lOb uppermost so that the flux which was in the bead 14 melts and flows into said gap 12 enabling a brazed joint to be formed between the overlapping portions 10a. It is found that the illustrative method results in heat exchangers having reliably sealed tubular components 10.

Claims: CLAIMS A method of forming a heat exchanger from components which have at least a surface layer made of aluminium or an alloy thereof, the method comprising forming the components (10) , at least one of which is tubular, assembling the components, and brazing the components together, wherein said tubular component is formed by bending a sheet into a tubular shape by bringing opposite edge portions (10a) of the sheet into overlapping relationship with one another, the gap (12) between said overlapping portions extending along said component, characterised in that the method also comprises applying a bead (14) containing particles of brazing flux, the bead being applied to the tubular component along the exposed end (12a) of said gap, and applying an adhesive agent (16) on top of said bead (14) , the liquid adhesive agent being such that it dries to leave a solid residue which adheres to the component and retains the flux in position and said solid residue can be driven off by heating, the tubular component (10) being orientated during brazing so that the flux which melts in the brazing flows into said gap (12) and enables a brazed joint to be formed between the overlapping portions (10a) . A method according to claim 1, characterised in that the bead (14) is in the form of a paste containing the particles in a solvent which can be driven off by heating without leaving a significant residue. A method according to claim 2, characterised in that the paste contains 15 to 20 % by weight of solvent. A method according to any one of claims 1 to 3, characterised in that said overlapping portions (10a) of the tubular component extend parallel to one another into the interior of the component (10) . A method according to claim 4, characterised in that said sheet is bent in such a way that said exposed end (12a) of said gap is at the bottom of a groove (13) which accommodates said bead (14) . A method according to any one of claims 1 to 5, characterised in that the bead (14) also contains eutectic or near eutectic filler alloy. A method according to claim 6, characterised in that the bead (14) contains particles of the eutectic or near eutectic alloy which are at least partially coated with flux. A method according to any one of claims 1 to 7, characterised in that said bead (14) and said liquid adhesive agent (16) contain the same solvent. A method according to any one of claims 1 to 8, characterised in that, after the application of the bead (14) and either before or after the application of the liquid adhesive agent (16) , further particles (15) containing brazing flux are applied in dry form to the tubular component (10) for use in forming a brazed joint with another component, the further particles being secured to the component by the liquid adhesive agent. A method according to claim 9, characterised in that at least some of said further particles (15) comprise a eutectic or near eutectic filler alloy coated at least partially with flux.

Inventors: O'NEILL CHRISTOPHER GERALD, O'NEILL, CHRISTOPHER, GERALD
Applicants: NEILL CHRISTOPHER GERALD O, T & N TECHNOLOGY LTD, O'NEILL, CHRISTOPHER, GERALD, T & N TECHNOLOGY LIMITED
Application year: 1996
Main classification: B23K1
IPC codes: B23K1, F28D1