Abstract:
An exhaust gas heat exchanger, in particular for use in a motor vehicle, having at least one first flow channel that conducts a first fluid, which first flow channel is accommodated in respective tube sheets at end areas of the first flow channel. A housing surrounds the first flow channel and forms a second flow channel for a second fluid that flows through the housing and flows around the first flow channel. Pipe sheets are inserted into the housing such that the first flow channel is sealed off from the second flow channel. A first diffuser conducts the first fluid into the first flow channel and a second diffuser conducts the first fluid out of the first flow channel. A first shielding element has a first passage and a first spacing element is placed onto a tube sheet from the side facing away from the first flow channel.

Description:
This nonprovisional application is a continuation of International Application No. PCT/EP2013/059897, which was filed on May 14, 2013, and which claims priority to German Patent Application No. DE 10 2012 208 100.3, which was filed in Germany on May 15, 2012, and which are both herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an exhaust gas heat exchanger, particularly for use in a motor vehicle, having at least one first flow channel conducting a first fluid, which is taken up in its end regions in each case in a tube sheet, having a housing, which surrounds the first flow channel, whereby the housing has an inlet opening and an outlet opening and forms a second flow channel for a second fluid, whereby a second fluid is able to flow through the housing and the second fluid is able to flow around the first flow channel, the tube sheets are inserted in the housing such that the first flow channel is sealed off from the second flow channel, having a first diffuser, which conducts the first fluid into the first flow channel, and a second diffuser, which conducts the first fluid out of the first flow channel. 
     Description of the Background Art 
     Exhaust gas heat exchangers are used in motor vehicles today to reduce the exhaust gas temperature and thereby to greatly prevent nitrogen oxide and particulate emissions. In this regard, part of the exhaust gas is branched off downstream of the internal combustion engine and then passed through a suitable exhaust gas heat exchanger. 
     The cooled exhaust gas is then combined with fresh air drawn in by the internal combustion engine and again supplied to the internal combustion engine. Temperatures of up to 700° Celsius occur at the inlet of the exhaust gas heat exchanger. 
     Exhaust gas heat exchangers are known, for instance, from DE 10 2007 011 184 A1, that include exhaust gas-conducting tubes that are inserted on both sides into plates and are connected to these, for instance, by soldering or welding and the tubes are surrounded by a housing. The exhaust gas is thereby passed into the tubes via diffusers connected to the plates. In so doing, a coolant flows through the housing, as a result of which the coolant flows around the exhaust gas-conducting tubes. 
     A heat transfer occurs between the exhaust gas in the tubes and the coolant flowing around the tubes, as a result of which the exhaust gas is cooled. 
     A disadvantage of the prior art is that sometimes boiling processes in the coolant occur in exhaust gas heat exchangers of the described type. This adversely impacts the service life of the exhaust gas heat exchanger per se and can have a negative effect on the chemical composition of the coolant. Boiling processes can occur in particular in the inlet region, because here the hot exhaust gas directly strikes the plate of the exhaust gas heat exchanger and the plate of the exhaust gas heat exchanger is in direct contact with the coolant. 
     Furthermore, condensation of the exhaust gas can occur on the exit side of the exhaust gas heat exchanger. The condensing condensate can enter the combustion chamber with the exhaust gas returned to the internal combustion engine and there lead to increased corrosion and further damage to individual components. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an exhaust gas heat exchanger in which no boiling processes occur and furthermore the formation or the conducting of condensate from the outlet side of the exhaust gas heat exchanger into the intake area of the internal combustion engine is ruled out. 
     In an embodiment, an exhaust gas heat exchanger, particularly for use in a motor vehicle, includes at least one first flow channel conducting a first fluid, which is taken up in its end regions in a tube sheet, having a housing, which surrounds the first flow channel, whereby the housing has an inlet opening and an outlet opening and forms a second flow channel for a second fluid, whereby a second fluid is able to flow through the housing and the second fluid is able to flow around the first flow channel, whereby the tube sheets are inserted in the housing such that the first flow channel is sealed off from the second flow channel, having a first diffuser, which conducts the first fluid into the first flow channel, and a second diffuser, which conducts the first fluid out of the first flow channel, whereby the exhaust gas heat exchanger has at least one first shielding element, which has at least one first passage, and at least one first spacing element, which is placed on a tube sheet from the side facing away from the first flow channel. 
     This is particularly advantageous, because the shielding element forms a thermal spacing between the first fluid through the heat exchanger and the tube sheet and the first flow channels. As a result, the heating of the heat exchanger is positively influenced. 
     It is also advantageous if the passage projects into a first flow channel. As a result, the fluid flowing through the shielding element is conveyed directly into the first flow channels. Direct contact between the throughflowing fluid and the tube sheet does not occur thereby. 
     In an embodiment, the outer contour of the passage can substantially coincides with the inner contour of the first flow channel. 
     This is advantageous in order to be able to position the shielding element on the tube sheet such that the passages can project into the first flow channels to be able to convey the fluid directly into the interior of the first flow channels. 
     The outer cross section of the passage can be smaller than the inner cross section of the first flow channel. 
     It is advantageous, furthermore, if the outer contour of the passage does not contact the inner contour of the first flow channel. 
     Because of the smaller outer cross section of the passage in comparison with the inner cross section of the flow channel, the passage can be spaced apart from the flow channel such that both are not in direct contact, which is beneficial for the thermal isolation. 
     It is also expedient, if the spacing element spaces apart the first shielding element relative to the tube sheet. In this way, the thermal isolation between the tube sheet and the shielding element is also increased, which contributes overall to an advantageous temperature distribution in the heat exchanger. 
     It is advantageous, furthermore, if the first shielding element has an at least partially circumferential upstanding edge region, with which the shielding element is supported on the first diffuser. 
     This is used particularly for the permanent positioning of the shielding element in the interior of the heat exchanger. 
     It is also expedient, if a second shielding element is fixedly connected to the second diffuser at least at one location. 
     The thermal isolation effect of the shielding element can be achieved on both sides of the heat exchanger by the arrangement of a second shielding element particularly in the second diffuser. The arrangement of a second shielding element on the second side of the heat exchanger can be additionally advantageous for the separation of condensation water that forms within the scope of cooling of the fluid flowing through the heat exchanger. 
     In an embodiment, the second diffuser can have in a side wall a first opening, which is arranged between the inlet opening, facing the housing, of the second diffuser and the connecting site of the second shielding element to the second diffuser. 
     The condensation water already mentioned above, which condenses on the walls of the first flow channels and runs through the air gap between the second shielding element and the first flow channels out of the first flow channels, can be conducted out of the heat exchanger through this opening in the diffuser and thus removed from the fluid flow of the first fluid. 
     It is also expedient, if the second diffuser has a water separator placed downstream of the second shielding element in the flow direction of the first fluid. 
     The water separator can also filter out moisture that flows with the first fluid through the heat exchanger out of the fluid flow. As a result, advantageous fluid properties are achieved, particularly if the first fluid is an exhaust gas, which is again supplied to an internal combustion engine at least in part after flowing through the heat exchanger. 
     In an embodiment, the second shielding element in the at least partially upstanding edge region can have a second opening, which is arranged between the downstream water separator and the inlet opening, facing the housing, of the second diffuser. 
     The condensation water condensing on the water separator can be advantageously removed from the heat exchanger through an opening thus arranged. 
     An exhaust gas heat exchanger, according to an embodiment, can include a plurality of first flow channels, having a first shielding element and a second shielding element, which quantitatively have a number of passages, corresponding to the plurality of the first flow channels, and a plurality of spacing elements, whereby in each case a shielding element is placed on one of the tube sheets from the side facing away from the housing. 
     An embodiment of an exhaust gas heat exchanger in this form is particularly advantageous for use in the exhaust gas line of a motor vehicle. Thermal isolation of the tube sheet and the first flow channels is advantageously achieved by the first shielding element on the inflow side of the heat exchanger, and the generated condensation water is advantageously removed from the fluid flow by the second shielding element on the outflow side of the heat exchanger. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein: 
         FIG. 1  shows a sectional cut through a shell and tube heat exchanger of the invention with an additionally inserted shielding element; 
         FIG. 2  shows a schematic illustration of a detail of an exhaust gas heat exchanger in which the coolant boils in the inlet area; 
         FIG. 3  shows a detailed view of the exhaust gas heat exchanger shown in  FIG. 1 , with an illustration of the gas flow and the coolant flow in the exhaust gas heat exchanger; 
         FIG. 4  shows a schematic illustration of a detail of an exhaust gas heat exchanger, in which the throughflowing exhaust gas condenses in the outlet area; 
         FIG. 5  shows a detailed view of the outlet area of an exhaust gas heat exchanger of the invention, with an illustration of the gas flow and the coolant flow in the exhaust gas heat exchanger; and 
         FIG. 6  shows a sectional cut through an exhaust gas heat exchanger of the invention, with an illustration of the outlet side of the exhaust gas heat exchanger with a discharge opening for the condensate forming in the outlet area. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a sectional cut through an exhaust gas heat exchanger  1 . Exhaust gas heat exchanger  1  includes a plurality of flow channels  2  through which a fluid, in this case an exhaust gas, flows. 
     Flow channels  2  are taken up at their ends in tube sheets  3 . Flow channels  2  taken up in tube sheets  3  are surrounded by a housing  4  in such a way that housing  4  with tube sheets  3  produces a tight connection, which separates flow channels  2  from a flow channel  10  in the interior of the housing. 
     Housing  4  has an inlet opening and outlet opening, not shown in  FIG. 1 , as a result of which a second fluid can flow through housing  4 . A flow thereby flows around flow channels  2  conducting the first fluid. The heat transfer occurs between the first fluid, which flows within flow channels  2 , and the second fluid, which flows through housing  4  and thereby in flow channel  10  around flow channels  2 . 
     A diffuser  5  is inserted in tube sheet  3 , which, depending on the flow direction of the fluid, supplies or removes the fluid in flow channels  2 . The inflow area of exhaust gas heat exchanger  1  is illustrated in the partial section shown in  FIG. 1 . 
     The exhaust gas flows into diffuser  5  and is distributed there among flow channels  2  and flows through heat exchanger  1  along them. 
     In order to prevent a direct striking of the inflowing exhaust gas on tube sheet  3 , a shielding element  6  is integrated in addition in heat exchanger  1  shown in  FIG. 1 . 
     Shielding element  6  has a plurality of passages  7  and a plurality of spacing elements  8 . It is inserted with passages  7  in flow channels  2  and sits with spacing elements  8  on tube sheet  3  of heat exchanger  1 . It is important here according to the invention that passages  7  are designed such that they are not in direct contact with the inner surfaces of flow channels  2 . Shielding element  6  furthermore has a laterally upstanding edge region  9  that reproduces the inner shape of diffuser  5  and thereby makes it possible to support shielding element  6  laterally on diffuser  5 . 
     It is achieved by shielding element  6  that the hot, inflowing exhaust gases do not strike tube sheet  3  and flow channels  2  directly, but first strike shielding element  6 , which because of its structural design is spaced apart from tube sheet  3  and flow channels  2 . 
     An air space, which has an isolation effect with respect to the exhaust gas temperature, is formed between shielding element  6  and tube sheet  3 , as well as flow channels  2 , because of the spacing apart by spacing elements  8 . 
     Passages  7  correspond in their outer contour to the inner contour of flow channels  2 . The cross-sectional opening of passages  7 , however, is configured smaller than the internal cross section of tubes  2 , so that direct contact of the passages with the wall surfaces of flow channels  2  is prevented. 
     In the illustration shown in  FIG. 1 , shielding element  6 , which preferably is made of a material that withstands the high temperatures of the exhaust gas flow, has a number of spacing elements corresponding to the spaces between flow channels  2 . Thereby, shielding element  6  is supported on tube sheet  3  in the area between flow channels  2 . Shielding element  6  is laterally supported in addition on the inner surfaces of diffuser  5 . 
     In alternative embodiments, the design of spacing elements  8  and the number of spacing elements  8  can deviate from the embodiment shown here. Preferably, the smallest possible contact surface is to be created between shielding element  6  and tube sheet  3 . In alternative embodiments, it is also conceivable to support shielding element  6  totally on diffuser  5 , so that no additional support of shielding element  6  relative to the tube sheet is necessary. This would further increase the isolation effect of shielding element  6 . This could be achieved, for example, by a circumferential groove on the inner surface of the diffuser into which the shielding element is inserted. 
       FIG. 2  shows a detailed view of a flow channel  2 , through which the exhaust gas flows  11  and two flow channels  10 , through which coolant flows  12 . It is shown in  FIG. 2  how areas in which the coolant begins to boil form within flow channels  10  of the coolant. These boiling areas  13  form primarily on the side of exhaust gas heat exchanger  1  that faces the inlet side of the exhaust gas. 
     Tube sheet  3  and the connected walls of flow channels  2  or  10  heat up by the striking of the hot exhaust gas on tube sheet  3 . The temperature of the exhaust gas is thus transferred over the bridge of tube sheet  3  directly to the coolant, which flows in the interior of flow channels  10 . If the temperature of the exhaust gas is correspondingly high enough and in addition there is a poor throughflow of flow channel  10 , this can lead to the coolant beginning to boil. 
     The chemical property of the coolant can be changed by the boiling. Thus, for instance, silicates, which are dissolved in the coolant, are destroyed, which leads to a greater corrosive stress of the materials, particularly the aluminum materials of an exhaust gas heat exchanger  1 . 
       FIG. 3  shows an illustration that is similar to  FIG. 2 . Shielding element  6 , analogous to the illustration in  FIG. 1 , which contributes to preventing or eliminating boiling regions within the coolant, is shown additionally in  FIG. 3 . 
     Shielding element  6 , as already described in  FIG. 1 , is supported via spacing elements  8  on tube sheet  3 . In addition, passages  7  project into flow channels  2  without touching these in so doing. An isolation effect arises due to the small contact surface of shielding element  6  to tube sheet  3  or to the walls of flow channels  2 . 
     The high temperatures of the exhaust gas, which analogous to the arrow  11  in  FIG. 2  flows through exhaust gas heat exchanger  1 , thus no longer directly affect tube sheet  3  or the walls of flow channels  2  leading to a thermal relief for this part. As already noted in the description of  FIG. 1 , particularly the thermal decoupling of shielding element  6  from tube sheet  3  or the walls of flow channels  2  is of great importance. 
     The inlet side of exhaust gas heat exchanger  1  was described in  FIGS. 1, 2 , and  3 . 
       FIG. 4 , in contrast, shows a sectional cut through an exhaust gas heat exchanger  1  and thereby in particular the outlet side, where the exhaust gas, flowing through the exhaust gas heat exchanger, exits flow channels  2  into diffuser  5  and ultimately leaves exhaust gas heat exchanger  1 . 
     The employed reference characters correspond to those used in the previous figures. New elements are labeled by correspondingly new reference characters. 
     The structure of shielding element  6  on the outlet side is also largely identical to the structure of the inlet side in  FIG. 1  or  FIG. 3 . As a departure from the design on the inlet side, shielding element  6  is now fixedly connected at least at one location  18  to diffuser  5 . 
     Advantageously, this connecting site is arranged in the bottom area of exhaust gas heat exchanger  1 . Shielding element  6  in this case is connected to the inner wall of diffuser  5  such that it is spaced apart from the inner wall of diffuser  5 , especially in the diffuser area inserted in tube sheet  3 . This makes it possible that the condensate, forming in the interior of flow channels  2  in the vicinity of the outlet region, can collect in hollow space  24 , generated between shielding element  6  and the inner surface of diffuser  5 . 
     The exhaust gas flowing through exhaust gas heat exchanger  1  is greatly cooled along flow channels  2  by the coolant flowing around flow channels  2 . Because of this cooling, it can occur that the exhaust gas condenses particularly in the end region, facing the outlet side, of flow channels  2 . Because these condensate droplets, should they flow unimpeded further into diffuser  5 , could again enter the internal combustion engine via a possible exhaust gas recirculation, where they can cause corrosion and further damage, the outflow of the condensate out of exhaust gas heat exchanger  1  is to be avoided. 
     Shielding element  6  fulfills this function on the outlet side of exhaust gas heat exchanger  1 . In contrast to the inlet side, where shielding element  6  is used as thermal isolation of tube sheet  3  and the wall of flow channels  2 , shielding element  6  on the outlet side of exhaust gas heat exchanger  1  now functions as a separating element, which conveys the arising condensate out of exhaust gas heat exchanger  1 . 
     The same as on the inlet side, passages  7  project into flow channels  2  and shielding element  6  is furthermore supported on the tube sheet via spacing elements  8  that are in contact with tube sheet  3 . Likewise, passages  7  are spaced apart from the inner surfaces of flow channels  2 . 
     The condensate forming especially on the inner walls of flow channels  2  can now run out of flow channels  2  through the gaps forming between passages  7  and the inner surfaces of flow channels  2 . The condensate therefore collects on the side, facing flow channels  2 , of shielding element  6  and accordingly runs downwards there. 
     Hollow space  24 , which can take up condensate that collects and runs down on shielding element  6 , forms by the spacing apart of the bottom upstanding edge region of shielding element  6  from the inner wall of diffuser  5 . The condensate is thus effectively removed from the exhaust gas flow. 
     In order to remove still more condensate present in the exhaust gas, an additional water separator  16  is shown in  FIG. 4 . Said separator is arranged in the interior of diffuser  5  and is placed downstream of shielding element  6  in the direction of the exhaust gas flow. 
     Water separator  16  is substantially a perforated sheet. The condensate in the exhaust gas flow condenses on water separator  16  and then like the condensate occurring on shielding element  6 , runs downward on water separator  16  and there, via an opening  19  provided in shielding element  6 , also runs into hollow space  24  formed between shielding element  6  and the inner wall of diffuser  5 . 
     Diffuser  5  has an opening  17 , which is located in the outer wall of diffuser  5  in the area of hollow space  24 . The condensate collected on water separator  16  and shielding element  6  can run out of exhaust gas heat exchanger  1  via this opening  17 . A condensate collector  20 , which is connected via a line to opening  17  of diffuser  5 , is shown outside the exhaust gas heat exchanger. 
     The condensate collected on shielding element  6  and on water separator  16  flows into condensate collector  20 . Condensate collector  20  shown in  FIG. 4  has in its interior a float valve  21 , which depending on the fill level of condensate collector  20  releases the condensate via a discharge opening  22 . 
     Optionally, condensate collector  20  may have a ventilation opening that is in communication with diffuser  5  via a line  23 . Over said opening, residual amounts of exhaust gas conveyed via opening  17  out of diffuser  5  into condensate collector  20 , can again be conveyed back to diffuser  5 . 
     The collecting unit, shown here outside diffuser  5 , for the condensate can also be completely omitted in alternative embodiments of the exhaust gas heat exchanger, provided some other removal device for the condensate is provided. For example, it would be conceivable to return the condensate via a nozzle that atomizes the condensate extremely finely, back into diffuser  5 , where it is then supplied again to the combustion process of the internal combustion engine within the scope of exhaust gas recirculation. 
       FIG. 5  shows a detailed view similar to  FIG. 2 . In contrast to  FIG. 2 , the outlet side of the exhaust gas heat exchanger is now shown here. 
     It can be seen that in flow channel  2  conducting the exhaust gas, particularly in the end region, a condensation region  14  forms where liquid condensate condenses from the exhaust gas flow on the inner walls of flow channel  2 . As a result, condensate droplets  15  can form that are taken along with the exhaust gas flow. 
       FIG. 6  also again shows the arrangement of  FIG. 4  with the difference that shielding element  6  is now shown in addition here. As in the figures already described above, shielding element  6  is supported via spacing elements  8  on tube sheet  3  of exhaust gas heat exchanger  1 . Passages  7  in  FIG. 6  also project into flow channels  2  of the exhaust gas heat exchanger, without touching them, however. 
     Apart from condensation regions  14 , exhaust gas flow  11  is also illustrated, which runs along flow channels  2  through shielding element  6 . 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.