Patent Publication Number: US-2015060028-A1

Title: Heat exchanger

Description:
TECHNICAL FIELD 
     The invention relates to a heat exchanger, such as in particular an exhaust gas evaporator, having a housing with a fluid inlet and a fluid outlet for a first medium, such as in particular exhaust gas, and having tubes which are arranged in the housing transversely with respect to the flow direction of the first fluid and through which a second medium can flow and the ends of which are arranged and connected in a fluidtight manner in a tube sheet at the inlet side and at the outlet side. 
     PRIOR ART 
     In the case of motor vehicles, there is a general trend towards reducing fuel consumption. A not inconsiderable proportion of the energy content of the fuel is transferred during combustion in the internal combustion engine to the hot exhaust gas, which often leaves the vehicle unused, and therefore a not inconsiderable proportion of the energy content is not used. 
     To further reduce the fuel consumption of vehicles, such as commercial vehicles or passenger vehicles, it is therefore expedient to recover some of the energy content of the hot exhaust gas in order to drive the motor vehicle. 
     There are various methods for this energy recovery currently being tested. Thus, there are attempts to recover the energy content electrically by means of thermoelectric elements. However, this is currently restricted to low powers, and therefore only about 1 kW is achieved by this means in the case of passenger vehicles. 
     This recovery can be accomplished thermally, i.e. the energy of the exhaust gas is used to heat the passenger compartment or to heat the engine and/or transmission. 
     In a variant that has been under discussion for some time, thermal energy is indeed taken from the exhaust gas, but the energy is returned to the engine in mechanical form. The method is based on a steam power process, in which a certain suitable medium is evaporated and superheated in an evaporator and expanded in an expander or in a turbine, thus producing mechanical energy. 
     Evaporation of the medium is accomplished by means of heating using the hot exhaust gas. To achieve as high as possible efficiency, it is expedient here if the medium can be raised to a relatively high pressure. In the case of water as a medium, it is possible here to achieve about 40-50 bar. When using organic refrigerants, pressures of up to about 30 bar are advantageous. 
     In what is referred to as an evaporator, the medium to be evaporated is heated to boiling temperature in a first step, then evaporated and finally superheated. This can take place at two different locations in a vehicle. Firstly, heat can be removed from the exhaust gas to evaporate the fluid which is to be evaporated in an evaporator, which is used instead of the exhaust gas cooler or in addition thereto. Secondly, the main exhaust gas flow can also be used as a heat source in order likewise to evaporate a fluid in what is referred to as a main exhaust gas evaporator. 
     In the vehicle air-conditioning sector, “tray evaporators” have been disclosed by WO 2011/051163 A2, wherein ribs are soldered in between pairs of trays and a row of such pairs of trays is connected in parallel with one another. In this case, a fluid flows through the pairs of trays and another fluid flows around these in a conventional manner. The fluid flowing through then evaporates in the trays when the exhaust gas flows around the trays. 
     Evaporators which consist of trays and ribs have a high power density which makes it possible to provide very compact high-performance evaporators, even for vehicles. However, the disadvantage is that such evaporators are relatively expensive to produce. 
     DESCRIPTION OF THE INVENTION, OBJECT, SOLUTION AND ADVANTAGES 
     It is the object of the invention to provide a heat exchanger which is simple and yet inexpensive to produce in comparison with the prior art and which has a good power density. 
     This object is achieved by means of the features of claim  1 . 
     A preferred illustrative embodiment discloses a heat exchanger, such as in particular an exhaust gas evaporator, having a housing with a fluid inlet and a fluid outlet for a first medium, such as in particular exhaust gas, and having tubes which are arranged in the housing transversely with respect to the flow direction of the first fluid and through which a second medium can flow and the ends of which are arranged and connected in a fluidtight manner in a tube sheet at the inlet side and at the outlet side, wherein the respective tube sheet has connected to it in each case a structure by means of which groups of tubes are connected to one another in such a way that an outlet of at least one tube is fluidically connected to an inlet of at least one other tube. It is particularly advantageous here if the respective outlet of one group of tubes is connected to the respective inlet of one group of tubes. 
     It is particularly advantageous if the structure comprises a deflection plate and a cover plate, wherein the deflection plate has openings which connect the outlets of one set of tubes to the inlets of the other set of tubes, and wherein the cover plate covers the deflection plate in a fluidtight manner. Thus, the deflection plate is connected to the tube sheet and has openings within which inlets and outlets of a predeterminable number of tubes are in fluid connection. 
     It is particularly advantageous if the deflection plate is placed on the respective tube sheet and is connected thereto, wherein the cover plate is placed on the respective deflection plate and connected thereto. 
     It is also expedient if the deflection plate is formed integrally with the respective tube sheet, wherein the cover plate is placed on the respective deflection plate and connected thereto. 
     In another illustrative embodiment, it is also advantageous if the deflection plate is formed integrally with the respective cover plate, wherein the deflection plate and the cover plate are placed on the respective tube sheet and connected thereto. 
     It is particularly advantageous if the tubes are arranged in rows, wherein the deflection plate deflects fluid between tubes in different rows. This means that the deflection plate deflects fluid from a first tube or from a group of first tubes into a second tube or into a group of second tubes, wherein the first tubes and the second tubes are preferably arranged in a different row of tubes. 
     It is particularly advantageous if the tubes are arranged in rows, wherein the deflection plate deflects fluid between tubes in one row. This means that the deflection plate deflects fluid from a first tube or from a group of first tubes into a second tube or into a group of second tubes, wherein the first tubes and the second tubes are preferably arranged in the same row of tubes. 
     It is also advantageous if the rows of tubes are arranged in segments, wherein the deflection plate deflects fluid from one segment into another segment. 
     It is furthermore expedient if a plurality of tubes is connected in parallel, at least in one segment. 
     It is also advantageous if a plurality of tubes connected in parallel is connected in series with one another, at least in one segment. 
     Further advantageous embodiments are described by the following description of the figures and by the dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in greater detail below on the basis of at least one illustrative embodiment with reference to the drawings, in which: 
         FIG. 1  shows a first illustrative embodiment of a heat exchanger according to the invention in a three dimensional view, 
         FIG. 2  shows a view of the heat exchanger from the side, 
         FIG. 3  shows a partial view of a header zone, 
         FIG. 4  shows a partial view of a header zone, 
         FIG. 5  shows a partial view of a header zone, 
         FIG. 6  shows a view of the heat exchanger core, 
         FIG. 7  shows a view of a front deflection zone of the heat exchanger, 
         FIG. 8  shows a view of a rear deflection zone of the heat exchanger, 
         FIG. 9  shows another illustrative embodiment in a view of a front deflection zone, 
         FIG. 10  shows another illustrative embodiment in a view of a front deflection zone, 
         FIG. 11  shows another illustrative embodiment in a view of a front deflection zone, and 
         FIG. 12  shows another illustrative embodiment in a view of a front deflection zone. 
     
    
    
     PREFERRED EMBODIMENT OF THE INVENTION 
       FIGS. 1 and 2  show a heat exchanger  1 , which is embodied as an exhaust gas evaporator in the illustrative embodiment in  FIG. 1 . In this case, there is a flow of a first fluid, in this case preferably exhaust gas, and of a second fluid, in this case a fluid to be evaporated, through the exhaust gas evaporator. The exhaust gas transfers heat to the fluid to be evaporated and evaporates said fluid. In this case, the heat exchanger  1  has a housing  2  having a fluid inlet  3  and a fluid outlet  4  for a first fluid. The exhaust gas flows through the housing from the inlet  3  to the outlet  4 , and a row of tubes  5  is arranged between the inlet  3  and the outlet  4 , preferably transversely to the flow direction  7  of the first fluid, and a second fluid can flow through said row. To improve heat transfer between the first fluid and the second fluid, ribs  6  which promote heat transfer are provided around the outside of the tubes  5  and between the tubes  5 . These ribs can be provided as corrugated ribs or as flat ribs or as turbulence generators. The tubes  5  for the through flow of the second fluid are preferably round tubes or flat tubes. The ends of said tubes are preferably also mounted in a fluidtight manner in tube sheets on both sides. In this case, the tubes  5  are preferably arranged by way of their ends  9  in a tube sheet  8  on the inlet side and the outlet side and are connected in a fluidtight manner. 
     The heat exchanger is connected to an inlet branch  10  to allow the second fluid to enter and to an outlet branch for discharge. Starting from the inlet, the fluid is distributed between a first number of tubes. The second fluid preferably flows in parallel through these tubes. The fluid is then deflected at the opposite ends of said tubes into a further number of different tubes. The second fluid flows through these, in turn. 
     To redirect the fluid, the respective tube sheet  8  has connected to it in each case a structure  12  by means of which groups of tubes  5  are connected to one another in such a way that an outlet  15  of at least one tube  5  is fluidically connected to an inlet  16  of at least one other tube  5 . 
     In this case, the structure  12  consists of at least one deflection plate  13  and one cover plate  14 , which are formed and arranged one on top of the other. Here, the cover plate  14  covers the deflection plate  13  in a fluidtight manner. The cover plate  14  is preferably welded or soldered or adhesively bonded to the cover plate  13  or even formed integrally therewith. 
     In this case, the deflection plate  13  has openings which connect the outlets  15  of one set of tubes  5  to the inlets  16  of the other set of tubes  5 . 
     In this case, the tubes  5  are inserted on at least one side in the tube sheet  8 , in openings  17 , where the tubes are soldered or welded to the plate. 
     It is possible to use aluminum, but particularly preferably stainless steel, as a material for the tubes and tube sheets. It is also possible for the entire heat exchanger to consist of aluminum or stainless steel. 
     The deflection plate  13  has openings or channel structures, which are suitable for connecting outlets of tubes to inlets of other tubes. 
     As an alternative to the separate formation of tube sheet  8 , deflection plate  13  and cover plate  14 , it may also be advantageous if the deflection plate is designed to give a single part with the tube sheet or if the deflection plate is designed as one part with the cover plate.  FIG. 4  shows that the deflection plate is designed to give a single part  18  with the tube sheet.  FIG. 5  shows that the deflection plate is designed to give a single part  19  with the cover plate. 
     In this case, the common part  18  or  19  is in each case placed on the other part  14  or  8  and connected in a fluidtight manner thereto. 
     In this case, the tube sheet can also be designed in such a way, being milled for example, that the tube sheet modified in this way so as to be multifunctional also additionally assumes the task of fluid distribution and acts as a combination of a tube sheet and a deflection plate. In that case, only one cover plate is mounted on and connected to the tube sheet. In a corresponding manner, part  19  can likewise act as a milled component which integrates the deflection plate and the cover plate. 
     It is furthermore also possible for the tube sheet and/or the deflection plate and/or the cover plate to be designed as a casting which has a corresponding structure with recessed integrated openings for distributing the medium. 
     The connection between the two or three elements, the tube sheet, the deflection plate and the cover plate, is advantageously accomplished by means of welding, soldering or screwing, and it is also possible to employ a combination of these connection options. For this purpose, it is also possible for the upper plate to have holes in order to connect the plates to one another by a welding method at particular points distributed over the surface. 
     Particularly to achieve a good soldered joint, the  3  plates can be fixed relative to one another and pressed against one another by means of riveting or tack welds, alternative means being spot welds, stamped features or screwed joints. 
     The deflection plate contains openings as structures in order to collect the medium from at least one tube and redistribute it to at least one other tube. The fluid to be evaporated is preferably collected in the openings from up to 4 or more tubes and is then redistributed to 4 or more tubes. During each collection and distribution of the fluid, thermal instabilities leading to nonuniform mass flow distribution in the tubes and hence to different temperatures and/or vapor contents are very largely compensated. It is thereby possible to compensate for instability effects that lead to considerable losses of performance. 
     This is also a significant advantage of the 2 sandwich sheets over the solution with tube bends and just one sandwich sheet. As a result, the mixing processes take place in both sheets. 
       FIG. 6  shows schematically a core  20  of the heat exchanger  1 , in which a multiplicity of tubes  5  is arranged. These tubes  5  are arranged between the distributor plates  21 ,  22 , which are designed as deflection zones, and are received there in tube sheets and deflection and cover plates. 
     As viewed in the exhaust gas flow direction  23 , the distributor plates  21 ,  22  are divided into individual segments  24 ,  25 ,  26 ,  27 ,  28  and  29 . A number of tube rows  30 ,  31  are in turn provided within a segment  24  to  29 . In the example in  FIG. 6 , two tube rows are provided for each segment. Ideally, a segment consists of just a few tube rows, e.g. of two tube rows in the exhaust gas flow direction, with the result that the temperature gradient across a segment is as small as possible and hence all the tubes are subjected to virtually the same exhaust gas temperature. However, it is also possible, depending on the working medium, for up to 6 tube rows to form a segment or for a plurality of segments to be connected to one another in parallel. 
     In the illustrative embodiment of  FIG. 6 , up to 4 tubes per tube row  30 ,  31  are furthermore connected in parallel perpendicularly to the exhaust gas flow direction. 
     As can be seen in  FIG. 6 , the second fluid flows in in the region of the tube ends  32  and is distributed between four tubes  5 . The fluid flows through these tubes to the ends of these tubes on the opposite side and there flows into zone  33 . The deflection zone  35  guides the fluid into the inlets of zone  34 , from where the fluid flows back to zone  36  through the tubes concerned. The fluid is then deflected by deflection zone  37  to zone  38  at the tube ends and distributed, with the result that the fluid then flows back through the tubes which lie below the first passage. 
     Thus, the fluid flows through the first segment in alternating passes and finally emerges from the segment in zone  39  and is diverted into the second segment  28  at crossover  40  from the first segment  29 . 
     The corresponding pass through the second segment  28  then takes place, until the fluid flows into the third segment  27  at crossover  41 . The corresponding pass through the third segment  27  then takes place, until the fluid flows into the fourth segment  26  at crossover  42 . The corresponding pass through the fourth segment  26  then takes place, until the fluid flows into the fifth segment  25  at crossover  43 . The corresponding pass through the fifth segment  25  then takes place, until the fluid flows into the sixth segment  24  at crossover  44 . The corresponding pass through the sixth segment  24  then takes place, until the fluid flows out of the sixth segment  24  at the outlet  4 . 
       FIGS. 7 and 8  again show the configuration of connections for the tubes at the front and rear deflection zone. It can be seen that four tubes in each case are connected in parallel and that fluid is deflected out of four tubes into four other tubes. In this case, the fluid enters tubes  5  on the front side according to  FIG. 7  and emerges from said tubes on the rear side. The tubes  5  are therefore also marked with complementary inlets and outlets in the front deflection zone according to  FIG. 7 , as in  FIG. 8 . 
       FIG. 9  shows a corresponding view of six segments  50  to  55 , which each have two tube rows. In this case, three tubes in each case are combined and connected in parallel to form a passage  56 . In the case of passage  56 , the fluid flows in and flows through the tubes to the rear deflection zone. There, the fluid is deflected from one tube row to the adjacent tube row. The fluid then flows through the next three tubes and is deflected in the front deflection zone into three further tubes in the same row of tubes. The fluid then flows through the tubes to the rear deflection zone. There, the fluid is again deflected from one tube row to the adjacent tube row. The fluid then flows through the next three tubes and is deflected in the front deflection zone into three further tubes in the same row of tubes. This continues until the fluid flows out of the tubes in zone  57  and is transferred into the next segment through crossover  58 . The crossover can preferably be integrated into the deflection plate or can be implemented by means of an external crossover for each tube. 
     The flow through the heat exchanger in  FIG. 9  reveals a difference with respect to the previous illustrative embodiment. In  FIG. 9 , the fluid is deflected in the front deflection plate from tubes in one row into tubes in the same row in accordance with opening  60 , while, in the rear deflection plate, the fluid is deflected from tubes in one row into tubes in a different row in accordance with opening or openings  59 . 
       FIG. 10  shows another illustrative embodiment in another view, wherein six segments  70  to  75  each have two rows  76 ,  77  of tubes. As can be seen, segments  71  and  72  are combined to form a common segment connected in parallel. The same applies to segments  73  and  74 . 
     Moreover, three tubes in each case are combined and connected in parallel to form a passage  78 . In the case of passage  78 , the fluid flows in and flows through the tubes to the rear deflection zone. There, the fluid is deflected from one tube row to the adjacent tube row through openings  79  in the deflection plate. The fluid then flows through the next three tubes and is deflected in the front deflection zone into three further tubes in the same row of tubes through the opening  80  in the front deflection plate. After this, the fluid flows through the tubes to the rear deflection zone. There, the fluid is again deflected from one tube row to the adjacent tube row. The fluid then flows through the next three tubes and is deflected in the front deflection zone into three further tubes in the same row of tubes. This continues until the fluid flows out of the tubes in zone  81  and is transferred into the next segment  71 ,  72  through crossover  82 . Crossover  82  can preferably be integrated into the deflection plate or can be implemented by means of an external crossover for each tube. 
     The flow through segments  71 ,  72  takes place in the same way as in segment  70 , although these segments are connected in parallel and the entry of fluid into zones  83  and  84  takes place in parallel. The fluid then flows through the tubes of segments  71  and  72  in the same way as through the tubes of segment  70 , before the fluid is discharged from the segment again in zones  85  and  86  and is transferred by means of crossover  87  into segments  73  and  74 , which are connected in parallel. In segments  73  and  74 , the fluid flows through as in segments  71  and  72 . The fluid is then collected from segments  73  and  74  and directed into the final segment  75 , where it flows through segment  75  as in the inlet-side segment  70  before it is discharged from the heat exchanger. 
       FIG. 11  shows another illustrative embodiment in another view, wherein six segments  90  to  95  each have two rows  96 ,  97  of tubes. As can be seen, segments  90  and  91  are combined to form a common segment connected in parallel. The same applies to segments  92 ,  93  and  94 , which are combined to form a common segment. 
     In segments  90  and  91  and in segments  92  to  94 , the fluid in each case flows through just one tube  98  parallel to one tube  99  of the other segment. Within the segment, the flow through the tubes  98  is exclusively serial. This continues as far as the center of the segment. There, the fluid flows out of the tubes  101 ,  102  of both segments. There, there is a mixing zone  100 , allowing the fluid from the first segment  90  to mix with the fluid from the second segment  91  before it is again distributed between tubes  103 ,  104  of the segments. 
     In the case of passage  98 , the fluid flows in and flows through a tube to the rear deflection zone. There, the fluid is deflected from one tube row to the adjacent tube row through an opening  105  in the deflection plate. 
     The fluid then flows through the next tube and is deflected in the front deflection zone into another tube in the same row of tubes through the opening  106  in the front deflection plate. After this, the fluid flows through the tubes to the rear deflection zone. There, the fluid is again deflected from one tube row to the adjacent tube row. The fluid then flows through the next tube and is deflected in the front deflection zone into another tube in the same row of tubes. This continues until the fluid flows out in the mixing zone  100 . In the second zone after the mixing zone there is a corresponding flow through the tubes. The fluid is then transferred to the next segment  92 ,  93 ,  94  through crossover  107 . Crossover  107  can preferably be integrated into the deflection plate or can be implemented by means of an external crossover for each tube. 
     The flow through segments  92 ,  93 ,  94  takes place in the same way as in segment  90 ,  91 , although all three of these segments are connected in parallel. The fluid then flows through the tubes of segments  92  to  94 , before the fluid is discharged from the segment again and is transferred to segment  95  by means of crossover  108 . In segment  95 , the fluid flows through as in segment  70  in  FIG. 10 , in which three tubes are in each case connected in parallel. The fluid is then discharged from the heat exchanger. 
       FIG. 12  shows another illustrative embodiment in another view, wherein six segments  110  to  115  each have two rows  116 ,  117  of tubes. As can be seen, segments  110  to  112  and  113  to  115  are combined to form a common segment connected in parallel. 
     In segments  110  to  112  and in segments  113  to  115 , the fluid in each case flows through just one tube  116  parallel to one tube  117 ,  118  of the other segment. Within the segment, the flow through the tubes  116 ,  117  or  118  is exclusively serial. This continues as far as the center of the segment. There, the fluid flows out of the tubes  119 ,  120 ,  121  of the three segments. There, there is a mixing zone  122 , allowing the fluid from the first segment  110  to mix with the fluid from the second and third segment  111 ,  112  before it is again distributed between tubes  123 ,  124  and  125  of segments  110 ,  111 ,  112 . 
     In the case of passage  116 , the fluid flows in and flows through a tube to the rear deflection zone. There, the fluid is deflected from one tube row to the adjacent tube row through an opening in the deflection plate. The fluid then flows through the next tube and is deflected in the front deflection zone into another tube in the same row of tubes through the opening in the front deflection plate. After this, the fluid flows through the tubes to the rear deflection zone. There, the fluid is again deflected from one tube row to the adjacent tube row. The fluid then flows through the next tube and is deflected in the front deflection zone into another tube in the same row of tubes. This continues until the fluid flows out in mixing zone  122 . In the second zone after the mixing zone there is a corresponding flow through the tubes. The fluid is then transferred to the next segment  113 ,  114 ,  115  through crossover  126 . Crossover  126  can preferably be integrated into the deflection plate or can be implemented by means of an external crossover for each tube. 
     In segments  113 ,  114  and  115 , the fluid flows through as in segments  110 ,  111  and  112 . The fluid is then discharged from the heat exchanger. 
     In the figures, the configuration envisaged for the deflection plate is rectangular. It can also be round, allowing it to be installed in a round cylindrical aperture in a housing or in a muffler. 
     To improve performance, gas-side ribs can be mounted on the tubes, see ribs  6  in  FIG. 2 . The gas-side ribs form the “secondary surface” for heat transfer and the tubes form the primary surface for heat transfer. The ribs  6  can be soldered to the tubes  3 , or a thermally conductive joint is achieved without the addition of solder during the process of soldering the overall evaporator. This can be achieved by means of a very tightly toleranced rim hole for the tube, which leads to a very narrow gap between the rib and the tube. A thermally conductive joint between the ribs and the tubes is thereby produced by means of diffusion processes during the high-temperature soldering process, even if there is no solder present. 
     A better bond between the ribs and the tubes, with or without solder, can be achieved through a combination of austenitic tubes and ferritic ribs. Ferrites expand less at high temperatures than austenites, with the result that the tubes are pressed against the ribs at the soldering temperature. In order to avoid the ribs coming away from the tubes during cooling, the rib can have small slots around the tubes. 
     The ribs have rim holes for the tubes, said holes having what are referred to as collars, which ensure the spacing between the ribs. As an alternative, it is also possible for the rib spacing to be ensured by raising spacers in the rib. The rib density here can be between 30 Ri/dm and 80 Ri/dm. The ribs can be punched and have cut and raised fins or can also have merely stamped-in structures, such as winglets, dimples or bosses, to improve performance. In particular, it is expedient to stamp structures into the ribs which guide the flow to the tubes in a controlled manner and thus enable greater heat transfer to be achieved at the 
     In this case, the rib thickness is 0.1 mm to 0.5 mm or preferably between 0.25 and 0.4 mm, this being advantageous for stainless steel as the rib material. 
     It is furthermore possible to make slots at the top and/or bottom in the plate assembly, allowing differential thermal expansion on the basis of different temperatures from the gas inlet to the gas outlet and ensuring that this does not lead to any damage. 
     The tube diameter of the tubes is preferably in a range of 3-20 mm, ideally in a range of 5-15 mm and preferably in a range of 6-10 mm. 
     Turbulence-generating structures can be introduced into the tubes, e.g. swirl generators, in order to promote heat transfer, particularly in the region where the fluid is superheated. 
     The tube can also be embodied as a rifled tube but in that case preferably has no external ribs. In particular, it is also possible to use tubes with very deep grooves which are of similar design to a bellows with relatively large tubing diameters in order to increase heat transfer on the gas side and, at the same time, to enable the differential thermal expansion between the tubes to be accommodated. In principle, different performance classes can be achieved if an evaporator consists of individual modules in the exhaust gas flow direction.