Title of Invention

A METHOD OF BRAZING OF FLATTENED FORMED TUBES WITH A SHEET SUBSTRATE IN HEAT EXCHANGER PANELS

Abstract

The invention relates to a method of brazing of flattened formed tubes (3) with a sheet substrate in radiation panels, cryogenic panels, solar shields and other heat exchanger panels (5) comprising the following steps of laying of guiding rails (2) as per the required tube layout on the base plate (1), placing a packing shim (4) of suitable thickness between said guide rails (2) on base plate (4), placing said flattened formed tube (3) wrapped with fluxed brazing (filler material) foil (9) between said guide rails (2), placing a sheet substrate (5) treated with flux on the joint surface above the flattened formed tube (3) characterized by placing thermal stabilizer (8) on the sheet substrate as per tube layout between the clamp pads (6), clamping the thermal stabilizers and sheet substrate (4), with the base plate (1) using clamp pads (6) and clamps (7), heating gradually from one end focusing on the thermal stabilizer with a conventional flame torch (10) and progressively advancing from end to end to complete the brazing.

Full Text

This invention relates to the field of metal joining process. This new process is employed for joining metals without melting the parts to be joined, but with an intermediate material that melts and makes a joint, called filler material. This filler material is usually an alloy of silver when the process is called o o brazing (carried out at 650 C - 700 C) and when the filler material is an alloy of lead and tin, it is called soldering, o carried out at about 200 C). Heat exchangers, which involve predominantly non-ferrous and dissimilar materials, often involving very thin sections and intricate configuration involve brazing. Further whenever heat is transferred through the joined contact surface, the bracing becomes a crucial process that decides the product performance and quality. Applications that involve conveying of a fluid heat source or heat sink through tubing, attached to plate surfaces for effective surface heat transfer as in radiation panels, cryogenic panels, solar shields, etc., employ tube to sheet braz ing. Brazing of tubes to plates or "sheets is usually carried out employing a torch panel brazing) brazing is executed inside a furnace which will uniformly provide the melting energy for the filler metal for objects that have to work under vacuum environment, the joining process also has to be carried out under vacuum. Specially built vacuum furnaces are employed for such applications, preferably referred to as vacuum brazing. Pior art A prior art search on 'Brazing fixtures', 'flat tube type heat exchangers' etc., gave a result as follows: Among these the following patents are from the above search list closes to the topic being discussed. 1. USPAT#5,685,075 Method of brazing of flat tube of laminated heat exchanger. This patent deals with a methodology of making 'flat tubes' by brazing of formed metal sheets at edges. Further unlike the invention described above this patent talks about fabricating of brazed panel laminated heat exchangers using furnace brazing. 2. USPAT#6,3B2,501 Bl: This is about producing joint using furnace brazing. 3. USPAT#6,198,068 Bit This deals with brazing using plasma forming device. Torch flame brazing using conventional techniques are limited to localized small area of joining only. When large area is to be joined improper heatinput, heat loss due to conduction etc., impose restrictions on process capability and resultant joint quali ty. Furnace brazing is the alternative for such cases, which involve a large brazed joint area and often for mass production. Furnace brazing employs gradual and uniform heating of the entire job, which includes parts to be brazed, the filler metal interposed in the form of s foil or paste with flux and holding fixtures that have to supply clamping force continuously on the joint as in the case of panel heat exchanger joints. Since the brazing process itself lasts just a few minutes (enough for the foil or paste to melt and re-solidify making the joint) the whole set up needs to be cooled quickly. Failure to do so may result in improper joint as the un-solidified brazing filler material can flow out of the joint area. This is one of the serious limitations of furnace brazing, where quick temperature changes are not possible. Brazing of tubes to thin substrates result in frequent thermal distortion / warping of the substrate sheet. Hence, positive clamping arrangement is essential to ensure joint contact and therefore joint quality. Often the clamping devices may turnout to be heavier than the parts to be clamped together. Further, the clamping devices and fixtures absorb 3 lot of heat energy and retapn it over a long period. On cooling of brazing joints the clamping devices continue to supply heat energy to the joint location. This can lead to severe loss of joint quality. To overcome the above problem, very expensive and exotic materials with low specific heat and low thermal conductivity like carbon (graphite) blocks are often used to clamp the large surfaces and tubes together during brazing. Yet another difficulty encountered is the perfect flatness/surface requirements to achieve the proper fit over the entire joint area. Proper profile matching, machining and grinding of the surfaces to achieve exactingflatness, surface requirement of the joining surfaces are essential. Further, containing the thermal expansion during furnace heating, differential expansion due to different material chemistry and section thickness, need to design intricate jigs and fixtures etc. make furnace brazing highly skill intensive, complex and much more expensive overall. Thus, the three major limitations of furnace brazing using vacuum are the need for a vacuum environment, a large furnace that can accommodate and heat the whole setup, appropriate clamping arrangements, exacting surface preparation etc., that together make brazing of large surfaces of tubes and sheets a very technologically challenging task. The invention relates to a brazing process and set up therefore than can effectively produce large (joint) surface area of brazing without needing vacuum or furnace or any exotic fixturing materials. Specifically this relates to the brazing set up and methodology employed by which very large area to be joined can be brazed with conventional and simple oxyacetylene torch along with specific clamping methodologies and brazing temperature controlling features. Description of th* invention This innovative brazing configuration and procedure results in highly consistent and reliable quality of brazed joints over a large surface area. Since it employs an innovative copper thermal stabilizer, regulated quantity of heat is made available for just melting the brazing filler material. The copper stabilizer, owing to its high theraml conductivity ensures that excess or over heat is appropriately controlled. In this 'indirect heating* method, since the flame does not contact the filler material, it causes no chemical contamination . It also obviates the need for vacuum furnaces. The unique flattened tube ensures excellent contact surface area and therefore a very sound brazing joint. Specifically configured clamping arrangement ensures that clamps are well removed from the heat zone of the heating flame (torch) hence thermal expansion induced slackening of clamps (that may result in unsound joints) is avoided. The clamping arrange also ensure against thermal warping of the brazed panel. Very large panels, whose dimensions may be higher than normal furnace sizes can be easily, brazed using this method. The methodology of heating being local and spot focused, excessive thermal distortions in the adjoining areas is avoided. This is specially very critical when the materials are stainless steel sheets and thin walled tubes are to be brazed on them as usually executed in the manufacture of solar shields, cryogenic panels, radiation panels etc. Since it does not require any furnace or enclosure to be evacuated, the set up and methodology is extendable to any shape and size. Possibility of being carried out an normal shop floor area, by standard workman the whole process is highly economical and success rate for all joints in the first attempt is very high. Accordingly to the invention there is provided a method of brazing of flattened formed tubes with a sheet substrate in radiation panels, cryogenic panels, solar shields and other heat exchanger panels comprising the following steps of laying of guiding rails as per the required tube layout on the base plate, placing a packing shines of suitable thickness between said guide rails on base plate, placing said flattened formed tube wrapped with fluxed brazing filler foil between said guide rails, placing a sheet substrate treated with flux on the joint surface above the flattened formed tube characterized by placing thermal stabilizer on the sheet substrate as per tube layout between the clamp pads clamping the thermal stabilizers and sheet substrate, with the base plate using clamp pads and clamps, heating gradually from one end focusing on the thermal stabilizer with a conventional flame torch and progressively advancing from end to end to complete the brazing. The invention is described in more details with reference to the enclosed drawings; wherein Fig. 1 shows a typical large sized brazed heat exchanger panel Fig. 2 shows a sectional view of typical tube to plate brazing arrangement Fig. 3 shows a schematic end view of panel brazing in progress with torch flame heating the thermal stabilizer. With reference to the drawings, the set up for continuous brazing as described above has following critical components. A base plate (1) large enough to accommodate the parts to be joined and rigid enough to hold and retain all the guide rails (2) the flattened formed tubes (3) sheet substrate (5) clamping pads etc. The base plate (1) is placed on a convenient surface easily accessible for the workmen. Guide rails (2) are fixed on to the base pkte as per the shape of the formed tubes (3). The material of base plate (1) and guide rails (2) should be chosen keeping in mind the contamination requirements of the final product and ease of welding etc. The flattened formed tubes (3) are then housed amidst the guide rails (2) with the fluxed brazing filler foil (9) appropriately wrapped over the tubes (3). This soft metal packing shines (4) are introduced in between the base plate (1) and the flattened tubes (3) to adjust height. The sheet substrate (5) is then placed over the flattened formed tubes (3) as per joining requirement and held in place by the clamping pads (6) and clamps (7). Thermal stabilizers (8) made of high conducting material like silver or copper, appropriate sized are placed. just. above the tubes 3) and held fast by the1 clamps (7). Heat energy is supplied using a conventional oxy-acetylenes torch The heat source is applied from one end of the flattened formed tube (3) and is gradually traversed along the tube contour. The thermal stabilizers (8) should be so shaped and placed that it guides the traverse of the flame. The clamp (7) shall be retained till such time the entire job cools sufficiently after the completion of brazing process so as to minimize the clearance of thermal distortion. The duration of the flame heating decides the brazing temperature for a given thickness of the chosen thermal stabilizer (8) and shall be varied according to the material of tube (3) and sheet substrate (5) in question. Also according to the material chemistry of tubes 3), sheet substract (5) and filler material foil (9) suitable brazing flux should be applied on all surfaces that take part in the brazing joint. WE CLAIM 1. A method of brazing of flattened formed tubes (3) with a sheet substrate (5) in radiation panels, cryogenic panels, solar shields and other heat exchanger panels comprising the following steps of laying of guiding rails (2) as per the required tube layout on the base plate (1), placing a packing shines (4) of suitable thickness between said guide rails (2) on base plate (1), placing said flattened formed tube (3) wrapped with fluxed brazing filler foil (9) between said guide rails (2), placing a sheet substrate (5) treated with flux on the joint surface above the flattened formed tube (3) characterized by placing thermal stabilizer (8) on the sheet substrate as per tube layout between the clamp pads (6), clamping the thermal stabilizers and sheet substrate (5) with the base plate (1) using clamp pads (6) and clamps (7), heating gradually from one end focusing on the thermal stabilizer with a conventional flame torch (10) and progressively advancing from end to end to complete the brazing. 2. A method of brazing of flattened formed tubes as claimed in claim 1, wherein said thermal stabilizer (8) are made of highly conducting materials tube silver or copper, and having a minimum thickness of 6-8 m m. 3. A method of brazing of flattened formed tubes (3) as claimed in claim 1, wherein said base plate (1) clamp pads (6), clamps (7), guide rails (2) are made of materials resistant of heat and corrosion tube stainless steels etc. 4. A method of brazing of flattened formed tubes as claimed in claim 1, wherein said brazed joint so produced is suitable for vacuum environment applications, meeting the out-gassing requirements. 5. A method for brazing of flattened formed tubes as claimed in claim 1, wherein said tubes have a large span, complicated routing occupying a large over all size, involving very long brazing contact area. 6. A method for brazing flattened formed tubes as claimed in claim 1, wherein said non-flat faced tubes is brazed after suitable flat surface preparation operations tube grinding or suitably forming the sheet substrate and thermal stabilizer (8) to match the tube surface contour. 7. A method for brazing flattened formed tubes as claimed in claim 1, wherein the tube layout spans in more than one plane and the brazing is carried out progressibly from one plane to another. 8. A method for brazing flattened formed tubes as claimed in claim 1, whereas the final brazed tube sheet panel can be rolled or formed or bent to required shapes and needs with the tube to sheet substrate bonding remaining intact. 9. A method for brazing flattened formed tubes as claimed in claim 1, wherein size variation on the flattened formed tube (3) height/or flatness of the sheet substrate (5) is easily tolerated by appropriate local clamping force variation choice of packing shines (4) thickness. 10. A method for brazing flattened formed tubes as claimed in claim 1, whereas the need for furnace brazing owing to the large size of the tube and substrate is obviated. A method of brazing of flattened formed tubes substantially as herein described and as illustrated in the drawings.

Documents:

447-del-2004-abstract.pdf

447-del-2004-claims.pdf

447-DEL-2004-Correspondence Others-(02-01-2012).pdf

447-del-2004-correspondence-others.pdf

447-del-2004-correspondence-po.pdf

447-del-2004-description (compelete).pdf

447-del-2004-drawings.pdf

447-del-2004-form-1.pdf

447-DEL-2004-Form-15-(02-01-2012).pdf

447-del-2004-form-19.pdf

447-del-2004-form-2.pdf

447-del-2004-form-3.pdf

447-DEL-2004-GPA-(02-01-2012).pdf

447-del-2004-gpa.pdf