Abstract:
Heat exchanger for recovering heat from the exhaust gases of a motor vehicle, in which the exhaust gases circulate between two manifolds delimited by respective caps and by the end walls of a cylindrical casing, passing in tubes which are elongated in the axial direction in the casing. The caps are produced by stamping from a thin sheet metal, which leads to a reduction in weight and in cost and allows the caps to remain fitted snugly by elasticity on the ends of the casing, until brazed.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of heat exchangers. 
     BACKGROUND OF THE INVENTION 
     It is known to provide heat exchangers for heat transfer between a first fluid and a second fluid, comprising: 
     a central casing of generally axisymmetric cylindrical shape formed by two end walls and a cylindrical peripheral wall equipped with inlet and outlet piping for the first fluid; 
     a bank of tubes which are elongated parallel to the axis of the casing, the end regions of each tube passing, in fluid-tight fashion, through apertures formed in said end walls, and the outer faces of the tubes delimiting, within the casing, a chamber for the circulation of the first fluid; and 
     two axisymmetric annular caps respectively capping the two ends of the casing and connected in a leak tight way at their periphery, so as, with the end manifolds which communicate with each other by means of tubes for the circulation of the second fluid, to define the central apertures of the caps constituting respectively an inlet aperture and an outlet aperture for the second fluid. 
     Such exchangers are used particularly for recovering heat from the exhaust gases of motor vehicles, the first fluid being a fluid in circulation for recovering the heat, and the second fluid consisting of the exhaust gases. 
     In these known exchangers, the various elements are assembled together so as to be fluid-tight by brazing, and the end caps are solid molded components. The material used is stainless steel, in order to allow brazing. These pieces are re-worked at the machining stage in order to obtain a good fit between the assembly and the other components and to obtain the correct surface finish necessary for brazing. The material and the manufacturing technique make the pieces heavy and expensive. 
     Moreover, the caps have no elasticity and have to be fitted over the casing with clearance. They cannot be held on the casing by friction, and tack welds have to be formed in order to hold the pieces together until brazing. 
     Finally, the absence of elasticity of the caps severely limits the permitted oval shaping of the casing. 
     An object of the invention is to remedy the drawbacks set out above. 
     SUMMARY OF THE INVENTION 
     According to a first aspect the present invention provides a heat exchanger for heat transfer between a first fluid and a second fluid, comprising: 
     a central casing of generally axisymmetric cylindrical shape formed by two end walls and a cylindrical peripheral wall equipped with inlet and outlet piping for the first fluid and by two end walls; 
     a bank of tubes which are elongated parallel to the axis of the casing, the end regions of each tube passing, in fluid-tight fashion, through apertures formed in said end walls, and the outer faces of the tubes delimiting, within the casing, a chamber for the circulation of the first fluid; and 
     two axisymmetric annular caps respectively capping the two ends of the casing and connected in a leak tight way to their periphery, so as, with the end walls of manifolds which communicate with each other by means of tubes for the circulation of the second fluid, to define central apertures of the caps constituting respectively an inlet aperture and an outlet aperture for the second fluid, wherein the caps are made from stamped sheet metal. 
     The stamped sheet metal caps are lighter and less expensive than the molded and machined caps of the state of the art. Their elasticity facilitates assembling the caps the casing and holding them in the assembled state before brazing. 
     These effects are further reinforced by a slight oval shaping, obtained naturally by the stamping from thin rolled sheet metal. The oval shape and sheet metal material make it possible simultaneously to obtain an appropriate diametrical clearance between the casing and the cap, and light clamping in a particular radial direction, allowing the assembly to be held together. 
     Optional supplementary or alternative characteristics of the invention are set out below: 
     at least one of the caps include, at its axial end turned away from the casing, a radially internal cylindrical portion defining axial inlet or outlet piping for the second fluid. 
     the outer face of the axial piping is brazed to the inside of a through hole formed in a plate-type fixing flange which extends perpendicularly to the axis of the casing. 
     the cap includes, close to the axial piping, a portion oriented substantially radially which is brazed onto the face of the flange turned towards the casing. 
     at least one of the caps includes a snug-fitting cylindrical portion covering an end region of said peripheral wall. 
     said snug-fitting cylindrical portion is adjacent to a flared portion situated at the axial end of the cap turned towards the casing, able to facilitate the fitting of the cap snugly on the casing. 
     said snug-fitting cylindrical portion is connected, going away from the casing, to a portion of progressively decreasing diameter forming an abutment for the end of the casing and defining a path of variable cross-section for the second fluid between the casing and the inlet or outlet aperture. 
     According to another aspect, the invention provides a method of assembling a heat exchanger comprising the steps of: 
     assembling a central casing of generally axisymmetric cylindrical shape formed by two end walls and a cylindrical peripheral wall equipped with inlet and outlet piping for the first fluid; 
     assembling a bank of tubes which are elongated parallel to the axis of the casing, the end regions of each tube passing, in fluid-tight fashion, through apertures formed in said end walls, and the outer faces of the tubes delimiting, within the casing, a chamber for the circulation of the first fluid; and 
     assembling two axisymmetric annular caps respectively capping the two ends of the casing and connected in a leak tight way to their periphery, so as, with the end walls of manifolds which communicate with each other by means of tubes for the circulation of the second fluid, to define central apertures of the caps constituting respectively an inlet aperture and an outlet aperture for the second fluid, wherein the caps are made from stamped sheet metal and wherein at least one of the caps exhibits a cylindrical snug-fitting portion covering an end region of said peripheral wall in which method: 
     after having assembled the casing and the tubes, the caps are fitted snugly over the end regions of the casing, and the assembly thus obtained is brazed. The external diameter of the casing, the internal diameter of the snug-fitting cylindrical portion and the elasticity of the caps is such that the caps fit snugly with friction holding them in place before the brazing. 
     The method according to the invention may include at least some of the following features: 
     the casing and/or the caps exhibit a slightly oval shape and have an external diameter and an internal diameter respectively; the external diameter having a minimum value less than the minimum value of the internal diameter and having a maximum value greater than the maximum value of the internal diameter and the internal diameter of said snug-fitting portion being greater than the external diameter of the casing. 
     the snug-fitting movement of the caps is continued until their portion of progressively decreasing diameter comes into abutment on the end of the casing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The characteristics and advantages of the invention will be set out in greater detail in the description below, by referring to the attached drawings. 
     FIG. 1 is a view in axial section of a heat exchanger according to the invention for recovering heat from the exhaust gases of a motor vehicle. 
     FIG. 2 is a half-view in axial section of a stamped sheet metal cap belonging to this exchanger. 
     FIG. 3 is an enlarged detail of FIG.  1 . 
     FIG. 4 is a view in axial section of the cap and of a fixing flange brazed to it. 
     FIG. 5 is an enlarged detail of FIG.  4 . 
     FIGS. 6 and 7 are explanatory diagrams relating to the snug fitting of a cap on the casing of the heat exchanger. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the various figures, like reference numerals refer to like parts. 
     The heat exchanger represented in FIG. 1 comprises, as is known, a central casing of axisymmetric cylindrical shape consisting of a cylindrical peripheral wall  1  and two circular end walls  2 ,  3 . Inlet and outlet piping  4 ,  5  for the fluid for cooling the engine of the vehicle are formed on the wall  1 , at diametrically opposed positions, one near the wall  2  and the other near the wall  3 . Circular holes distributed over the surface of the walls  2  and  3  receive the ends of tubes  6  which are elongated parallel to the axis A of the casing. Two annular caps  7 ,  8  axisymmetric about the axis A, respectively cap the ends of the casing and are terminated, on the side away from the casing, by axial piping  9 ,  10  brazed respectively to flanges  11 ,  12  intended for fixing the heat exchanger onto the vehicle. The walls  1 - 3 , the tubes  6 , the caps  7 ,  8  and the flanges  11 ,  12  are assembled together by brazing so as to be leak tight to the fluids. The exhaust gases enter via the piping  9  into the manifold  13  delimited by the wall  2  and the cap  7 , travel within the tubes  6  so as to reach the manifold  14  delimited by the wall  3  and the cap  8 , and exit through the piping  10 . The cooling fluid enters through the piping  4  into the casing  1 - 3 , where it sweeps over the outer surface of the tubes so as to pick up the heat from the gases which are circulating within them, then exits through the piping  5 . The circulation of the fluid and the gas may also take place in the opposite direction to what has just been described. 
     According to the invention, the caps  7 ,  8  are produced by stamping from thin rolled sheet metal. In addition to the reduction in weight and cost, this manufacturing technique allows the caps to be held in place temporarily on the ends of the casing by elasticity until they are brazed as described in detail below. 
     It is seen in FIG. 2 that the cap  8 , which is identical to the cap  7 , comprises a cylindrical portion  15  which tightly surrounds the end region of the cylindrical wall  1  of the casing. The cylindrical portion  15  is adjacent to a flared portion  16 , with a circular-arc cross-section, which extends as far as the axial end  17  of the cap which is turned towards the casing. Going away from the flared portion  16 , the portion  15  is connected to a frustoconical portion  18  the diameter of which decreases in proportion to the distance from the casing. The frustoconical portion  18  is connected in turn, by a rounded feature, to a portion  19  extending substantially in a radial plane. Finally, the portion  19  is connected by a rounded feature  20  to the piping  10 , which constitutes the smallest-diameter part of the cap and which extends as far as the axial end of the cap opposite the casing, where it defines a gas inlet or outlet aperture  21 . 
     The flared portion  16  facilitates the insertion of the end of the casing into the cap, this insertion being continued until the outer face  3 a of the end wall  3  of the casing comes into abutment on the frustoconical portion  18  of the cap, as shown in detail in FIG.  3 . The frustoconical portion  18  also serves for guiding the gas between the inlet or outlet piping  10  and the ends of the tubes  6 , which lie within a surface area substantially greater than the working cross-section of this piping. 
     FIGS. 4 and 5 show the link between the cap  8  and a fixing flange  22  which is different from the flanges  11  and  12  which are represented in FIG.  1 . The flange  22  has the shape of a plate extending parallel to a radial plane, traversed by a central cylindrical aperture  23  in which the piping  10  is housed and by two threaded holes  24 , symmetric with one another with respect to the axis A. The threaded holes serve for fixing the heat exchanger onto a support which is not represented. As can be seen more particularly in FIG. 5, the flange  22  is in contact with the outer face of the radial portion  19  of the cap via the flange face  25  turned towards the casing, and with the outer face of the piping  10  via the wall of the aperture  23 . It is brazed to these two surfaces, which ensures excellent mechanical stability of the assembly, particularly with regard to the vibration from the engine of the vehicle. An annular chamber  26  is formed facing the rounded feature  20 . 
     The technique of manufacturing the caps by stamping from thin sheet metal entails forming a slight oval shaping, which facilitates the assembling of the exchanger according to the invention, as will be explained by referring to FIG.  6 . In the figure, the hatched circle diagrammatically represents the axisymmetric cylindrical casing  1 ,  2 , of diameter d 1 , and the non-hatched ellipse represents the internal perimeter of the snug-fitting cylindrical portion  15  of a cap, of minimum diameter d 2m  and of maximum diameter d 2M , the differences in diameter being represented with exaggeration for greater clarity. The value of d 1  lies between those of d 2m  and d 2M , such that, during the fitting, the minimum diameter of the cap increases, as indicated by the arrows F 1 , while its maximum diameter decreases, as indicated by the arrows F 2 . When fitting is complete, the casing bears radially on the cap, at two diametrically opposed places, according to the arrows F 1 , thus holding the cap by friction, while a clearance remains between the two components in the direction of the arrows F 2 , because the internal perimeter of the cylindrical portion  15  is greater than the external perimeter of the casing. 
     Although a practically axisymmetric casing and of an oval-shaped cap are the most frequent embodiments, the invention functions equally well when the casing is oval in shape, whether the cap is itself oval or axisymmetric, given the double condition that the maximum diameter of each of the two interacting cylindrical surfaces is greater than the minimum diameter of the other, and that the internal perimeter of the cylindrical portion  15  is greater than the external perimeter of the casing. 
     FIG. 7 illustrates the case where these two elements are oval in shape, the casing more so than the cap. The relationship d 1m &lt;d 2m &lt;d 2M &lt;d 1M  exists, the letters m and M indicating respectively a minimum diameter and a maximum diameter, and the numbers  1  and  2  being allocated respectively to the external diameter of the casing and to the internal diameter of the snug-fitting portion cylindrical  15  of the cap. The arrows F 1  marking the expansion of the cap are then oriented in the direction of the maximum diameters, and the arrows F 2  indicating the contraction of the cap, in the direction of minimum diameters, in contrast to the case of FIG.  6 .