HEAT EXCHANGER AND METHOD FOR THE MANUFACTURING THEREOF

Abstract: The invention relates to heat exchangers and methods for the manufacture thereof. More specifically, the invention relates to fully brazed heat exchangers which are resistant to an aggressive medium, due to brazing joints (5) between cover plates (1, 2) and duct plates (3) being protected by a coating resisting the aggressive medium. The protective coating may consist of a metal such as tin or silver, whereas the cover plates and duct plates are made of acid-resistance stainless steel and the solder is copper solder. According to the method, a coat-forming material is circulated through the brazed heat exchanger, and is subsequently discharged, whereupon a coating is formed at the brazing joints (5).

Description: Heat exchanger and method for the manufacturing thereof TECHNICAL AREA The present invention relates to heat exchangers and methods for manufacturing thereof. More specifically, the invention relates to fully brazed heat exchangers having an improved durability through protection of the brazed joints by means of a coating. Within the refrigeration industry, a substantial demand has arisen for heat exchangers able to withstand ammonia, as the use of freons has been successively prohibited. The inven- tion provides, in a simple and unexpensive manner, leak-proof and resistant heat exchangers that can be utilized together with ammonia. STATE OF THE ART The fully brazed heat exchangers of today are manufactu¬ red from sheet steel and brazed together with copper solder. Copper is presently the most durable and least expensive soldering material. When switching from freons to ammonia, however, copper can no longer be used, as it is dissolved by ammonia. This problem has been approached in different ways. One method is to use nickel instead of copper as the brazing material. Nickel is however less durable and more expensive than copper. Another method is welding the plates together instead of using the brazing method. However, welding is a more expensive and complicated method than brazing. The present invention solves the problem in a simpler and less costly way, by applying a protective coating on top of the brazed joints. SUMMARY OF THE INVENTION The present invention thus provides a heat exchanger for the ciculation of an aggressive medium, comprising cover plates and duct plates made of materials resistant to the medium. The plates are brazed together using a less resistant material. According to the invention, existing brazing joints are protected by a coating resistant to the aggressive medium. The coating may consist of a material which is able to diffuse into the brazing material, preferably a metal, e.g. tin or silver. The invention also provides a method for the manufacture of such a heat exchanger. According to the invention, a coat- forming material is circulated through the brazed-together heat exchanger during a period of time long enough for wetting the brazing joints to be protected. The excessive coat-forming material is then removed, whereby a coating is left at the brazing joints. The invention is set forth in further detail in the accompanying claims. BRIEF DESCRIPTION OF DRAWINGS The invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a side view of the heat exchanger according to the invention; Fig. 2 is a top view of said heat exchanger; Fig. 3 shows a cross sectional view along the line III in Fig. 2; and Fig. 4 is a detail sectional view showing a brazed joint with the coating applied. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The drawings show one embodiment form of the present in¬ vention. The heat exchanger comprises a top cover plate 1 and a bottom cover plate 2. The top cover plate has four connection ports 4 for medium inlet and outlet. Between the cover plates, a number of duct plates are stacked against each other so as to form a plurality of ducts inside which the medium can circu¬ late. Conventionally, the heat exchanger is manufactured by placing copper foil (not shown) between the individual duct plates 3 and between the cover plates and their respective adjoining duct plates. The whole stack is held together under pressure and brazed in a vacuum oven. However, as mentioned previously, copper cannot be expo- sed to ammonia as it will then dissolve. As copper is a rela¬ tively inexpensive and durable material, it is however desir¬ able to continue using copper. According to the invention, the brazed joints are thus protected by a coating. Preferred coating materials are silver and tin. The method for manufacturing the heat exchanger according to the invention is described in detail below. When the assem¬ bly of cover plates 1, 2 and duct plates 3 has been brazed together with the copper solder, a coat-forming material is poured in through the connection ports 4, until the unit is completely filled. There must be no air left in the material. The coating material is allowed to circulate until all internal surfaces have been wetted. The material is then emptied out. As illustrated in detail by Fig. 4, a small drop 7 of the coating material is then left close to the copper solder 6. The coating material is held there by the capillary forces. Other internal surfaces of the unit will have only a very thin coat¬ ing. This means that the coating material is used efficiently. The thin coating does not fulfill any specific function and any cracks forming in it will be of no importance, as the plates themselves are sufficiently resistant to ammonia. One of the alternatives is to tin coat the brazed joints. Tin is a material adhering very strongly to copper due to its diffusion into the copper, and tin also has an excellent resis¬ tance to ammonia. First, the heat exchanger is pickled in the conventional way. Thereafter, the heat exchanger is heated and evacuated to a low vacuum in order to facilitate the penetra¬ tion of the tin. Fluid tin is poured in and allowed to circu¬ late. After circulating for an appropriate length of time, the tin is discharged. Preferably, the tin is blown out by hot nitrogen to push out excessive coating material. The tin drops remaining at the brazing joints are then allowed to solidify, to form a tin coating. Part of the tin diffuses into the brazing material, i.e. the copper, creating a very strong bond. Another alternative is to silver plate the brazing joints by using a water solution of silver nitrate as the coat-forming material. This material is allowed to circulate in the same way and is then discharged, whereupon drops of the solution remain at the brazing joints. Some silver ions are exchanged against copper ions of the brazing material, and are thus diffusing into the copper. The water is then vaporized, preferably by heating the unit, and silver is precipitated at the brazing joints. To further reinforce the bond between the silver and the brazing material, the heat exchanger may subsequently be heated to about 800 °C. This increases the diffusion of silver into the brazing material and a very strong bond is achieved. The present invention thus provides several advantages compared to the previously known art. Thanks to the fact that copper can still be used, a very durable product is achieved, yet having a favourable manufacturing cost. The coating material is collected at the brazing joints where it is best needed, which also entails a low consumption of coating material. Neither does said material entail any weakening, as it deposits on top of the existing unit surfaces without affecting them. It is preferred that the cover plates and duct plates are manufactured from acid-resistant stainless steel and that copper solder is used, but the invention as such can also be applied to other materials. The invention is solely limited by the accompanying claims.

Claims: CLAIMS 1. Heat exchanger for the circulation of an aggressive medium, comprising cover plates (1, 2) and duct plates (3) made from a material resistant to the medium, the plates being brazed together with a less resistant material (6) , character¬ ized in that existing brazing joints (5) between said plates are protected by a coating (7) which has at least partially diffused into the brazing material and which withstands the aggressive medium. 2. Heat exchanger according to claim l, characterized in that the protective coating consists of silver. 3. Heat exchanger according to claim l, characterized in that the protective coating consists of tin. 4. Heat exchanger according to any one of the preceding claims, characterized in that the cover plates (l, 2) and the duct plates (3) are made from acid-resistant stainless steel and that the solder (6) is copper solder. 5. Method for the manufacture of heat exchangers for circulation of an aggressive medium, comprising the brazing together of cover plates (1, 2) and duct plates (3) made from a material resistant to the medium with the use of a solder (6) of less resistant material, characterized in that a liquid coat-forming material is circulated through the brazed-together heat exchanger during a period of time long enough for wetting the brazing joints (5) to be protected, that the coat-forming material is allowed, at least partially, to diffuse into the brazing material, and that the excessive coat-forming material is subsequently removed, whereby a coating (7) is left at the brazing joints (5) . 6. Metod according to claim 5, characterized in that the coat-forming material is a water solution of silver nitrate which is brought to circulate through the heat exchanger during said period of time, whereupon it is discharged, and in that the water in the remaining solution at the brazing joints (6) is vaporized to precipitate silver. 7. Method according to claim 6, characterized in that the water is vaporized by heating the heat exchanger. 8. Method according to claim 7, characterized in that the heat exchanger is aftertreated by heating it to 800 °C. 9. Method according to claim 5, characterized in that the coat-forming material is tin, the heat exchanger being heated and tin in liquid form brought to circulate through the heat exchanger during said period of time and subsequently being discharged, whereupon the remaining tin at the brazing joints (6) is allowed to solidify to form a tin coating. 10.Method according to claim 9, characterized in that the heat exchanger is evacuated to a low vacuum before the tin is introduced and brought to circulate, the tin subsequently being blown out of the heat exchanger by means of hot nitrogen gas.

Inventors: PERSSON LARS, PERSSON, LARS
Applicants: PERSSON LARS, PERSSON, LARS
Application year: 1995
Main classification: B23K1
IPC codes: B23K1, C23F15, F28D9, F28F19