Abstract:
The invention relates to an exhaust heat exchanger ( 1 ), in particular an exhaust cooler for motor vehicles with exhaust recycling, comprising a housing sleeve ( 2 ) for a coolant and a nest of tubes ( 3 ) with exhaust flowing through and coolant circulating around the above which are mounted on the housing sleeve by means of tube plates ( 4 ), whereby said nest of tubes, the tube plate and the housing sleeve form a closed force flow. According to the invention, a sliding seating ( 5 ) is arranged in the force flow, either in the housing sleeve or between a tube plate and the housing sleeve. The various expansions of the nest of tubes and of the housing sleeve are thus compensated for, such that unsupportable high loads do not occur in the components of the exhaust heat exchanger.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to an exhaust heat exchanger in particular for motor vehicles having an exhaust gas recirculation system (EGR), composed of a housing jacket for a coolant, and of a nest of pipes through which exhaust gas flows on the inside and around which coolant flows on the outside and which is held in the housing jacket by means of pipe plates, the nest of pipes, pipe plates and housing jacket forming an enclosed structure—such an exhaust heat exchanger has been disclosed by DE-A 199 07 163 by the applicant. 
   This known exhaust heat exchanger is an exhaust gas radiator such as is used in motor vehicles for recirculating exhaust gases in order to cool the hot exhaust gases. The exhaust gas radiator which is manufactured from stainless steel is essentially composed of a housing with a housing jacket through which a coolant flows, said coolant being removed from the coolant circuit of the internal combustion engine of the motor vehicle. A nest of pipes whose pipe ends are held by pipe plates which are themselves connected to the housing jacket is arranged in the housing jacket. The pipe ends are welded tightly to the pipe plates and the pipe plates are welded at the circumference to the housing jacket. In this respect the two pipe plates form, together with the housing jacket, what are referred to as fixed bearings. When this exhaust gas radiator operates, the pipes and housing jacket heat up to differing degrees because the exhaust gases flowing through the pipes have a higher temperature than the coolant flowing around the housing jacket. As a result, different degrees of expansion between the nest of pipes and the housing jacket occur, which leads to thermally induced stresses, i.e. compressive stresses in the pipes and tensile stresses in the housing jacket and flexural stresses in the pipe plates. The pipes of the nest of pipes, the pipe plates which hold the pipe ends, and the housing jacket thus form an enclosed structure in which the pipes are supported on the housing jacket by means of the pipe plates. In particular, in the case of exhaust gas coolers with a long length, such as are used in utility vehicles, the stresses which occur owing to the different degrees of expansion can lead to individual components failing or to the connection between the pipe plates being destroyed. 
   The object of the present invention is to reduce these thermally induced stresses, i.e. to decrease the resulting stresses in the components of the exhaust heat exchanger in order to achieve higher safety and a longer service life for the exhaust heat exchanger mentioned in the beginning. 
   SUMMARY OF THE INVENTION 
   The means of solving this object is proposed by a sliding fit being arranged within each enclosing structure, i.e. a fit between two components which can slide in relation to one another, that is to say what is referred to as a loose bearing, in contrast to a fixed bearing such as is present in the prior art of the generic type. Such a sliding fit compensates for the different degrees of expansion of the nest of pipes and housing, i.e. the abovementioned stresses do not occur at all. The sliding fit can be installed structurally at any desired location of the enclosing structure, it being necessary where possible to avoid the coolant and exhaust gas becoming mixed with one another, which could lead to damage to the engine. 
   According to one advantageous development of the invention, the sliding fit is arranged in the housing. This solution has the advantage that relatively large sliding surfaces are available and that there is no risk of coolant becoming mixed with the exhaust gas, or vice versa when there is a leakage due to the sliding fit. The housing jacket is divided transversely with respect to the direction of the force flux, i.e., coolant flow, and both housing parts are assembled in a telescopic fashion so that, when the nest of pipes experiences severe expansions, they can be pulled apart from one another without stresses occurring in the housing jacket, in the pipe plate or in the nest of pipes. 
   According to one advantageous development the sliding fit is composed of an outer ring and an inner ring between which a plastic sliding layer is arranged in order to improve the sliding properties. Both rings are pushed onto the end regions of the housing parts of the prefabricated sliding fit, and preferably bonded to said housing parts. The bonding avoids excessive application of heat and thus possible distortion of the components. The fitting on and bonding of the internal ring and outer ring is advantageous in particular when the housing jacket has a somewhat rugged contour: the surfaces of the inner and outer ring which slide one on the other can be configured as simple contours which can be sealed satisfactorily, for example, as a polygonal contour. 
   According to one advantageous development of the invention, the sliding fit is arranged between one of the two pipe plates and the housing. This solution thus provides a fixed bearing and a loose bearing for the nest of pipes. As a result, the nest of pipes can expand freely with respect to the housing jacket so that the abovementioned compressive stresses do not occur in the pipes and the abovementioned tensile stresses do not occur in the housing jacket. The pipe plate which is embodied as a sliding fit thus has a sliding surface which slides along an assigned sliding surface of the housing jacket and is sealed with respect thereto, preferably by means of O rings. 
   According to one development of the invention, a drainage, which is connected to the atmosphere, is provided between the O rings, i.e. between two O rings. This drainage provides the advantage that the coolant and exhaust gas cannot mix if an O ring or a corresponding seal fails because either the exhaust gas or the coolant escape to the outside through the drainage. 
   According to one advantageous development, the drainage is embodied as a slit in the housing, i.e. the housing is divided by a joint and is held spaced apart by means of spacer sleeves which are arranged on the circumference. If the seal fails, exhaust gas or coolant can be conducted away to the outside through the slit. 
   According to one advantageous alternative, the drainage is formed between two O rings as an annular groove in which the leakage fluid or the leakage gas collect and can escape to the outside via drainage openings which are arranged in the annular groove. This solution is structurally simple since the housing does not need to be divided. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention are illustrated in the drawing and will be described in more detail below. In said drawing: 
       FIG. 1 : shows a perspective view of an exhaust gas radiator with a sliding fit in the housing jacket, 
       FIG. 2  shows the exhaust gas radiator according to  FIG. 1  in a longitudinal section, 
       FIG. 2   a  shows a side view of the exhaust gas radiator according to  FIG. 2 , 
       FIG. 2   b  shows a section through the exhaust gas radiator according to  FIG. 2  in the sectional plane IIb—IIb, 
       FIG. 2   c  shows the sliding fit as an individual unit, 
       FIG. 3  shows a further embodiment of an exhaust gas radiator with the sliding fit between the pipe plate and housing jacket, 
       FIG. 4  shows a section through the exhaust gas radiator according to  FIG. 3  in the plane IV—IV, 
       FIG. 5  shows a modification of the exhaust gas radiator according to  FIG. 3  with the drainage groove, and 
       FIG. 6  shows a schematic view of the stresses in an exhaust gas radiator according to the prior art. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 1  shows a perspective view of an exhaust gas radiator  1  for a motor vehicle with an exhaust gas recirculation system (EGR). Such exhaust gas recirculation systems are used for recirculation cooling of the hot gases of an internal combustion engine (not illustrated) before they are combined with the intake air and fed to the intake tract of the internal combustion engine. The exhaust gas cooler  1  is composed of a housing jacket  2 , which holds in it a nest of pipes which are composed of exhaust gas pipes  3 . The ends of the pipes  3  are fastened to a pipe plate  4  which is itself welded to the housing jacket  2 . The housing jacket  2  has a sliding fit  5 , which is composed of an outer ring  6  and an inner ring  7 . 
   Firstly,  FIG. 6  shows the stress conditions in an exhaust gas radiator according to the prior art which is cooled by coolant. This schematic illustration corresponds to an exhaust gas radiator according to the prior art by the applicant which is mentioned at the beginning. Such a known heat exchanger  60  is composed of a housing jacket  61  which holds a nest of pipes which is composed of pipes  62  and whose ends are held in pipe plates  63 ,  64 . The pipes  62  are connected at both ends to the pipe plates  63 ,  64  in a secure and sealed fashion, for example, by means of welded connections. The pipe plates  63 ,  64  are securely connected to the housing jacket  61  at the circumference by means of welded connections  65 ,  66 . In this way, both pipe plates  63 ,  64  form two fixed bearings with the housing jacket  61 . When such an exhaust gas radiator  60  is operating, the hot exhaust gas flows through the pipes  62 , while coolant at a considerably lower temperature is applied to the inside of the housing jacket  61 . As a result, different degrees of expansion between the pipes  62  and the housing jacket  61  are produced. For this reason, compressive stresses, which are characterized by arrows and the letter C (compression) which are directed one against the other are formed in the pipes  62 . These compressive stresses continue further to the housing jacket  61  via the pipe plates  63 ,  64  and the welded connections  65 ,  66 , tensile stress, characterized by the letter T (tension) and arrows pointing away from one another, then building up in the said housing jacket  61 . The tensile stresses T and the compressive stresses C thus form an enclosed force flux or force flux ring over the pipe plates  63 ,  64  in which flexural and shearing stresses (not illustrated) occur. 
     FIG. 1  shows a perspective view of an exhaust gas radiator  1  for a motor vehicle with an exhaust gas recirculation system (AGR). Such exhaust gas recirculation systems are used for recirculation cooling of the hot gases of an internal combustion engine (not illustrated) before they are combined with the intake air and fed to the intake tract of the internal combustion engine. The exhaust gas radiator  1  is composed of a housing jacket  2  which holds in it a nest of pipes which are composed of exhaust gas pipes  3 . The ends of the pipes  3  are fastened to a pipe plate  4  which is itself welded to the housing jacket  2 . The housing jacket  2  has a sliding fit  5  which is composed of an outer ring  6  and an inner ring  7 . 
     FIG. 2  shows the exhaust gas radiator  1  according to  FIG. 1  in a sectional view, i.e. in a longitudinal section through the exhaust gas pipes  3  which are held at the ends in the two pipe plates  4  and  5 , i.e. are, for example, connected to the pipe plates  4 ,  5  by means of a welded connection. Said pipe plates  4 ,  5  are connected at the circumference to the housing jacket  2  in a secure and fluid-tight fashion by means of welded connections  6 ,  7 . The exhaust gas of the internal combustion engine (not illustrated) flows through the exhaust gas pipes  3 , and coolant, which is removed from the coolant circuit (not illustrated) of the internal combustion engine, flows around the exhaust gas pipes  3 , i.e. through the gaps  8  left between them. The connections for the inflow and outflow of the coolant for the housing jacket  2  are not illustrated for the sake of simplicity. The housing  2  is composed of two housing parts  2   a  and  2   b  which have a joint  9 . In the region of this joint  9 , the housing part  2   b  which is arranged to the right in the drawing has a smaller cross section than the housing part  2   a  which is illustrated to the left in the drawing. An outer ring  10  is attached to the housing part  2   a,  and an inner ring  11  is attached to the housing part  2   b.  The outer ring  10  and the inner ring  11  together form the sliding fit  5 , which is illustrated as a detail in  FIG. 2   c.    
     FIG. 2   c  shows the end regions of the housing parts  2   a,    2   b  in the region of the joint  9 , the end sides of the housing parts  2   a,    2   b  being spaced apart from one another by a gap s. The inner ring  11  is attached to the housing part  2   b  by bonding and the outer ring  10  is attached to the housing part  2   a  by means of a bonded connection. The outer ring  10  overlaps the inner ring  11  and forms with it a sliding fit  13 . A plastic layer  14  is securely attached to the internal surface of the outer ring  10  in the region of the sliding fit  13 . In contrast, the outside of the inner ring  11  is metallically smooth, for example ground. This results in a low-friction sliding pairing between the plastic layer  14  and the metallic surface of the inner ring  11  for the sliding fit  13 . The sliding fit  13  is sealed with respect to the outside, i.e. with respect to the atmosphere, by means of two O rings  15  so that coolant cannot escape to the outside. 
     FIGS. 2   a,    2   b  show the cross section of the exhaust gas radiator  1  as a view and as a section. It is apparent that the pipes  3  have a rectangular cross section and are at approximately equal distances  16  from one another. Owing to this arrangement of the pipes  3 , an approximately rectangular profile with shoulders  2   c  is obtained for the contour of the housing jacket  2   b.  The contour of the inner ring  11  is adapted to this somewhat rugged contour which is bent by the shoulders  2   c.  In contrast, the outer contour  11   a  of the inner ring is smoothed and has an approximately polygonal profile without severe curvatures, and this surface can therefore be manufactured relatively easily as a smooth surface and can be sealed with respect to the inner surface of the outer ring  10  using simple means such as O rings  15 . 
   The outer ring  10  and inner ring  11 , plastic sliding layer  14  and O rings  15  can be manufactured together as a prefabricated unit, i.e. as a prefabricated sliding fit  5 , and then connected to the housing parts  2   a,    2   b  by means of the bonded connection already mentioned. 
   When the exhaust gas radiator  1  is operating, the sliding fit  5  ensures that the housing  2  and the housing parts  2   a  and  2   b  can follow the relatively severe expansion of the pipes  3  by moving in relation to one another—thermal stresses and the excessive stresses of the components are thus avoided. 
     FIG. 3  shows a further exemplary embodiment of the invention for a sliding fit, i.e. an exhaust gas radiator  20  of which only the region of the sliding fit is represented as a detail. The exhaust gas radiator  20  has a housing jacket  21  which comprises a coolant region  22  and an exhaust gas region  23 . A pipe plate  24  in which exhaust gas pipes  25  are attached, for example by soldering or welding, is arranged inside the housing jacket  21 . The pipe plate  24  is adjoined by a hollow cylindrical region which holds in each case one O ring  29 ,  30  in each of two annular grooves  27 ,  28 . The cylindrical attachment  26  has an outer sliding surface  31  which bears in a sliding fashion against an inner surface  32  of the housing jacket  21  and thus forms a sliding fit  31 / 32  with the housing jacket  21 . The housing  21  is divided by a slot  33  between the two O rings  29 ,  30 . It thus has a left-hand housing part  21   a  and a right-hand housing part  21   b.  Both housing parts  21   a,    21   b  are held apart by a constant distance, i.e. the width of the slot  33 , by means of spacer sleeves (cf.  FIG. 4 ) distributed over the circumference and attachment eyelets  35 ,  36  which are provided on the housing parts  21   a,    21   b.  The attachment of eyelets  35 ,  36  and the spacer sleeves  34  are clamped to one another by means of screw or bolt connections (not illustrated). The slot  33  is thus connected to the atmosphere, i.e. the outside of the housing jacket  21 . 
     FIG. 4  shows a section along the sectional plane IV—IV in  FIG. 3 , i.e. through the region of the slot  33  and the spacer sleeve  34 . The cross section of the pipes  25  is circular here. 
   When the exhaust gas radiator  20  is operating, hot exhaust gases flow through the region  23  into the interior of the pipes  25 , around which coolant, which flows around the inside of the housing jacket  21  flows on the outside, i.e. in the coolant region  22 . Said housing jacket  21  is therefore at a lower temperature than that of the exhaust gas pipes  25 . The greater degree of expansion of the exhaust gas pipes  25  is compensated by the sliding fit  31 / 32 , i.e. the pipes can expand freely with respect to the housing jacket  21  by means of the pipe plate  24  and the cylindrical attachment  26 . The seal between the coolant region  22  and exhaust gas region  23  is provided by means of the O rings  29 ,  30 . If one of these O rings were to lose its sealing effect, coolant would leave the region  22  or exhaust gas would leave the region  23  and enter the slot  33  and pass from there to the outside and into the atmosphere. This prevents either exhaust gas entering the coolant region  22  or coolant entering the exhaust gas region  23  and thus causing damage. 
     FIG. 5  shows a modified exemplary embodiment of the exhaust gas radiator  20  according to  FIG. 3 , i.e. an exhaust gas radiator  40  with a continuous housing jacket  41  and a sliding fit  42  which corresponds to the sliding fit  31 / 32  of the exemplary embodiment according to  FIG. 3 . An annular groove  45 , which has a corresponding annular collar  46  (or an integral bead), is integrally formed between two O rings  43 ,  44 . The annular groove  45  is connected to the atmosphere via a drainage opening  47 . The drainage which has been described above for the exemplary embodiment according to  FIG. 3 , i.e. the conduction away of coolant or exhaust gas to the outside is thus possible in the same way. An advantage with this solution is that the housing  41  is in one piece and can thus be manufactured more easily.