Patent Publication Number: US-2007095503-A1

Title: High density corrosive resistant gas to air heat exchanger

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This patent application is a continuation-in-part of U.S. patent application Ser. No. 11/236,072, filed Sep. 27, 2005. 
    
    
     TECHNICAL FIELD  
      The present disclosure relates generally to gas to air heat exchangers, and more specifically to cooling potentially corrosive gases, such as engine exhaust, in an envelope with relatively tight spatial constraints.  
     BACKGROUND  
      In recent times, when an engine included a turbocharger and exhaust gas recirculation, it might be only the incoming air that is compressed via the turbocharger before being combined with recirculated exhaust gas that is supplied to the engine. Such an engine, for example, is shown in co-owned U.S. Pat. No. 6,526,753. More recently, there have arisen reasons for adding the exhaust gas to the incoming air before passing the combined mixture through the turbocharger for compression. The compressed exhaust gas/air mixture often needs to be cooled before being supplied to the engine intake. Because the exhaust gases can contain corrosive constituents, such as sulfuric and/or nitric acid, the wetted surfaces of the cooler can, and often will, corrode over time. After a prolonged period, the fluid isolation between the cooling tubes and the air fins can be undermined, and in more extreme situations, the inlet or outlet tank can become corroded leading to holes allowing the hot exhaust gases to vent to atmosphere.  
      Some heat exchanger applications have additional problematic constraints. For instance, the spatial envelope available in an over the road truck can severely limit the space available for inclusion of a necessary gas to air heat exchanger. When relying on construction techniques according to the conventional wisdom these spatial constraints can become even more acute. Typically, a heat exchanger will include tubes, air fins and heads all constructed from a similar material that are joined together in a conventional well known brazing process. However, when corrosion resistance is a substantial issue, combined with severe spatial constraints, the conventional wisdom in some instances will suggest that the cooling demands of a given engine system in a specific application, such as an over the road truck, simply cannot be met in the space available. Completely redesigning the remaining portion of the engine to gain additional volume for a gas to air heat exchanger is too expensive an option for realistic consideration, in many cases.  
      While certain heat exchanger materials can provide adequate corrosion resistance in engine exhaust environments, such as stainless steel, such materials are often accompanied by a trade off in terms of increasing weight. Certain materials having relatively higher corrosive resistance are also often characterized by relatively lower cooling performance, requiring very thin wetted wall thickness and/or larger size and complexity to achieve heat exchange efficiency similar to that of less corrosive resistant materials. The relatively tight spatial constraints, corrosive conditions, and the need to minimize weight and complexity of exhaust gas coolers have together provided substantial challenges to engineering acceptable exhaust gas coolers for internal combustion engines.  
      The present disclosure is directed to overcoming one or more of the problems set forth above.  
     SUMMARY OF THE INVENTION  
      In one aspect, the present disclosure provides a gas to fluid heat exchanger including a core having a plurality of tubes in heat transfer contact with a plurality of fins, the tubes being fluidly isolated from the fins. A plurality of turbulators are disposed within the tubes, each including at least one base material having a relatively low corrosive resistance. The heat exchanger further includes a brazing material having a relatively high corrosive resistance coating the turbulators and attaching the turbulators to the tubes.  
      In another aspect, the present disclosure provides an engine system having an engine housing and a gas passage fluidly connected to the engine housing. A gas to fluid heat exchanger is fluidly positioned within the gas passage and includes a core having a plurality of tubes and a plurality of fins in heat transfer contact with the tubes. A plurality of turbulators are disposed within the tubes and include at least one base material having a relatively low corrosive resistance, and are coated with a brazing material having a relatively high corrosive resistance which attaches the turbulators to the tubes.  
      In still another aspect, the present disclosure provides a method of making a gas to fluid heat exchanger including coating at least one base material of a turbulator having a relatively low corrosive resistance with a brazing material having a relatively high corrosive resistance. The method further includes placing the turbulator within a tube of a heat exchanger core, and attaching the turbulator to the tube at least in part via the brazing material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic of an engine system according to the present disclosure;  
       FIG. 2  is a schematic illustration of a gas to air heat exchanger according to the present disclosure;  
       FIG. 3  is a sectioned view looking into one of the tubes for the heat exchanger of  FIG. 2 ;  
       FIG. 4  is a partial sectioned corner view of a mechanical attachment between a head and tank portion of the heat exchanger of  FIG. 2 ; and  
       FIG. 5  is a sectioned end view of a heat exchanger with a tube having a turbulator therein, according to the present disclosure. 
    
    
     DETAILED DESCRIPTION  
      Referring now to  FIG. 1 , an engine system  10  includes a plurality of combustion cylinders  12  and at least one turbocharger  14 . In the illustrated example, two turbochargers  14  include a pair of compressors  18  in series as well as a pair of turbines  20  in series, which are fluidly connected to exhaust manifold  16  in a conventional manner. Engine system  10  includes a gas to air heat exchanger  28  fluidly connected between a compressor outlet  22  and an engine intake  24  via a hot gas passage  26  and a cooled gas passage  30 , respectively. Engine system  10  also includes an exhaust gas recirculation system  34  fluidly connected between an engine exhaust  36  and a compressor inlet  21 . In particular, the exhaust gas recirculation system  34  includes an exhaust gas recirculation passage  39  fluidly connected to supply passage  31  via an EGR control valve  40 . Ambient air is drawn into supply passage  31  past an air filter  32  and through a valve  33  so that, along with EGR control valve  40 , the relative amounts of exhaust gas and fresh air supplied to the engine can be controlled via an electronic control module (not shown) in a conventional manner. The engine also includes one or more exhaust aftertreatment devices  35  positioned in exhaust passage  36 , which may include a particle trap, an oxidation catalyst and the like. Exhaust passage  36  eventually terminates in a tail pipe  38 .  
      Referring now in addition to  FIG. 2 , gas to air heat exchanger  28  includes a core  60  and an inlet tank  61  with an inlet  62  fluidly connected to hot gas passage  26 , and an outlet tank  63  with an outlet  64  fluidly connected to cooled gas passage  30 . Hot gases entering inlet  62  enter an inlet manifold area  73  and travel through a plurality of tubes  67  into outlet manifold area  75 . The hot gases traveling through tubes  67  exchange heat, via a heat transfer surface of tubes  67 , with air traveling in a direction in and out of the page and past air fins  66  in a conventional manner.  
      In order to meet tight spatial constraints while having superior heat transfer capability in the face of potentially corrosive gases, gas to air heat exchanger  28  includes a number of unique features. By putting an appropriate amount of the appropriate material in the right locations, gas to air heat exchanger  28  can provide adequate heat exchange while avoiding many of the problems associated with corrosive gases, and do so in a tight spatial envelope. From one perspective, this is accomplished by making the minimum wetted wall thickness of tanks  61  and  63  thicker than the minimum wetted wall thickness of tubes  67 , which have a greater thickness than the minimum wetted wall thickness of air fins  66 . Using this strategy, and realizing that the air fins need not be substantially corrosive resistant, they can be made of a relatively thin highly thermally conductive material, such as thin sheeting made predominantly of copper. Although not preferred, air fin material could also be cuprobrazed copper, and less preferably a suitable stainless steel alloy, such as 409 stainless steel. Those skilled in the art will appreciate that the air fin material can include any of a variety of materials exhibiting thermal conductive properties typical of the materials just identified. In any instance, the air fin material should be more thermally conductive than a tube material for tubes  67 .  
      Like air fins  66 , tubes  67  must have substantial thermal conductivity, but resistance to corrosion is also an important consideration. Those skilled in the art will recognize that the more thermally conductive a material is, generally the lower its ability to resist corrosion, and vice versa. With this in mind, tubes  67  might be made of stainless steel, with that being chosen in order of preference from 409 stainless steel, 304 and possibly even 316 stainless steel. Apart from stainless steel, tubes  67  might also be constructed from a suitable corrosive resistant material such as titanium, nickel plated aluminum, or possibly even nickel plated steel. Given these examples, those with ordinary skill in the art will recognize a family of materials that could be used for tubes  67  that have significant corrosive resistance, yet retain sufficient thermal conductivity for use in a heat exchanger application. As shown in  FIG. 3 , tubes  67  may or may not include internally brazed turbulators  78 , which if included, may be made of a material similar to that of its surrounding tube  67 . The tube material should be more corrosive resistant than the air fin material.  
      As in a conventional heat exchanger, the gas to be cooled is isolated from air fins  66  by attaching heads  69  and  70  at opposite ends of tubes  67 . Like turbulators  78 , heads  69  and  70  are preferably made from a material similar to that of tubes  67 , to ease the attachment between the two. Thus, in one specific example, heads  69  and  70 , as well as tubes  67  and turbulators  78 , if any, would all be made from a common stainless steel material and then brazed to one another with a high temperature brazing material  71  in a conventional manner. Some suitable high temperature brazing alloys include 613 nickel based alloys, nickel plating alloys, and possibly even Bnix alloys. After brazing together the tubes  67 , heads  69 ,  70  and any turbulator  78 , air fins  66  are fitted between tubes  67  and attached to the tubes in a relatively low temperature brazing process that facilitates good heat transfer between tubes  67  and air fins  66 . Some suitable low temperature alloys might include OKC 600, nickel plating alloys, and copper based alloys. Those skilled in the art will appreciate that based upon these example brazing alloys, a number of different alternatives would be available without departing from the scope of the present disclosure.  
      Inlet tank  61  must take into account other considerations, including but not limited to, corrosive resistance and cost considerations as well as high temperatures. With these considerations in mind, tank  61  could be constructed from aluminum, such as a cast aluminum alloy with relatively thick walls that can tolerate expected corrosive concentrations and durations without allowing corrosive holes to develop. Outlet tank  63  could be made of a like material, or further weight and cost savings might be achieved by employing some other material, such as a non-metallic composite since the gases arriving at outlet area  75  are much lower in temperature than those entering inlet tank  61 . Referring to  FIG. 4 , in order to mate the inlet and outlet tanks  61 ,  63  to the respective heads,  69  and  70 , a mechanical attachment is preferably used by crimping the respective heads around an exposed flange on the respective tanks  61  and  63 . Another mechanical attachment might include conventional fasteners, such as bolts or screws. In order to prevent gas from escaping, a suitable o-ring seal is positioned between the respective tank  61 ,  63  and its counterpart head  69 ,  70 . Thus, heat exchanger  28  is preferably constructed by first employing a high temperature brazing process to assemble the heads  69 ,  70 , tubes  67 , and interior turbulators  78 , if any, in a high temperature brazing process using a suitable brazing alloy. Next, the highly thermally conductive air fins are attached to the tubes, which are typically made from a different and relatively thinner material than that of the tubes  67  via a low temperature brazing process. The heat exchanger is then completed by mechanically attaching, such as via a crimping process, the external tanks  61  and  63  with an o-ring seal positioned there between.  
      As an alternative or supplement to the aforementioned materials and material placement in a heat exchanger, certain of the heat exchanger components having desired heat exchange properties may be coated with suitably corrosive resistant materials. Referring to  FIG. 5 , there is shown a heat exchanger  128  according to another embodiment of the present disclosure. Heat exchanger  128  is contemplated to be applicable to engine systems in a manner similar to that of heat exchanger  28  discussed above. For instance, heat exchanger  128  might be positioned between hot gas passage  26  and cool gas passage  30  in engine system  10  of  FIG. 1 .  
      Heat exchanger  128  may include a core, having a plurality of tubes  167 , one of which is shown via a sectioned end view in  FIG. 5 . Tube  167  may be configured to thermally contact one or more air fins (not shown), similar to tubes  67  shown in  FIG. 2 . Heat exchanger  128  differs from other heat exchangers described herein, primarily in that turbulator  178  which is positioned within tube  167  may comprise a heat transfer/exchange surface  186  that is coated with a relatively highly corrosive resistant coating. The coating, which may comprise a corrosive resistant brazing material  184 , may be applied to heat exchange surface  186  to provide corrosive resistance to gases and condensed gases passing through tube  167 . Coating turbulator  178  with brazing material  184  serves the dual purposes of providing substantial corrosive resistance, while also attaching turbulator  178  to tube  167 . This strategy allows turbulator  178  to be constructed from a relatively good heat transfer base material such as copper, having a relatively low corrosive resistance, without subjecting the base material (copper) to the corrosive exhaust gas environment. In a typical embodiment, a plurality of turbulators similar to the single turbulator  178  shown in  FIG. 5  will be positioned within a plurality of tubes similar to tube  167  to provide a heat exchanger core having a configuration similar to heat exchanger  28 , described above, but with some or all internal surfaces coated with corrosive resistant brazing material.  
      In a related embodiment, tube  167  may itself be constructed from a relatively highly effective heat transfer material such as copper, also protected from the corrosive environment within tube  167  via a coating such as brazing material  184 . It is contemplated that turbulator  178  may comprise at least one base material, predominantly copper, but might also include another base material such as a solder or similar material used in connecting turbulator  178  to tube  167 . In other words, while it is contemplated that application of brazing material  184  to turbulator  178  will serve dual purposes of attachment to tube  167  and protection from the corrosive environment, some additional material might be used in the attaching and/or coating process. It is further contemplated that the base material of turbulator  178 , identified herein via numeral  179 , may have a first thickness T 1 , and brazing material  184  may have a second thickness T 2  that is less than the first thickness T 1 . In one specific embodiment, the thickness of brazing material  184  may be in the range of about 0.05 millimeters to 0.10 millimeters.  
      In still other embodiments, turbulators  178  might comprise predominantly copper, but tubes  167  might comprise a relatively highly corrosive resistant material such as stainless steel, absent coating  184 . Those skilled in the art may recognize that in certain applications, the heat transfer effectiveness of a relatively highly corrosive resistant material such as stainless steel may be optimized by manufacturing heat exchanger materials to have a particularly thin wetted wall thickness. To this end, a wall  180  of tube  167  might be relatively thicker or thinner, depending upon the material chosen for tubes  167 . Where tube  167  is predominantly copper, coated with brazing material  184 , wall  180  may be relatively thicker. Where stainless steel tubes are used, wall  180  might be relatively thinner.  
      Manufacturing of heat exchanger  128  may take place via a brazing process, wherein brazing material  184  is applied to coat all of the desired surfaces. A fluent material such as a brazing slurry might be sprayed or otherwise applied to all of the surfaces to be protected from the corrosive environment prior to brazing the heat exchanger components together. One suitable thermal spray technique for application of brazing material is taught in U.S. Pat. No. 7,032,808. Whether brazing material is applied to surfaces  182  of tubes  167  will depend upon the material selected for tubes  167 . In related embodiments, air fins (not shown) may be attached to tubes  167  via brazing material  184 . Thus, the entire heat exchanger  128  might be dipped, sprayed, etc. with brazing material  184  prior to attaching the respective components via heating in a brazing furnace, if desired. It should be understood that the terms coated, coating, etc. as used herein are intended to mean that the subject brazing material substantially or entirely covers the surfaces of heat exchanger  128  which are to be protected from the corrosive exhaust gas environment. Thus, turbulators  178  are coated with brazing material in the embodiment of  FIG. 5 , meaning that the brazing material provides a corrosive resistant barrier between the turbulator base material  179  and the corrosive environment inside tube  167 .  
      Similar to the embodiments described above, air fins included in heat exchanger  128  will generally need not be particularly corrosive resistant, and will thus typically be made from an air fin material that has a relatively high heat exchange capacity to optimize operation of heat exchanger  128 . Use of the presently described process and construction techniques can enable manufacturing of heat exchanger  128  in a minimal number of steps, for example via a single heating/brazing step, wherein the air fins are attached to tube(s)  167  via brazing material, and optionally wherein heads (not shown) of heat exchanger  128  are also connected tubes  167  via brazing material.  
      In selecting a brazing material that may also serve as a coating on surfaces of turbulator  178  and/or tube  167 , several factors must be considered. Where copper is used as the tube material and/or turbulator material, it will of course be desirable to utilize a copper compatible brazing material. The brazing material, however, will need to have a corrosive resistance higher than that of copper. Suitable brazing pastes, slurries, foils, etc., are available from a variety of commercial sources. The brazing filler material mentioned above, OKC 600, may serve as a suitable corrosive resistant material for coating the respective surfaces of heat exchanger  128  and also attaching turbulators  178  to tubes  167 . Other, suitably corrosive resistant, for example acidic corrosive resistant, materials may be used as the coating/brazing material without departing from the spirit and scope of the present disclosure. Suitable brazing materials are disclosed in U.S. Pat. No. 5,378,294, for example.  
     INDUSTRIAL APPLICABILITY  
      The gas to air heat exchanger  28  according to the present disclosure finds potential application where corrosive gases need to be cooled with, but isolated from, air, and this cooling must be done in a relatively tight spatial constraint. For instance, in some work machines, such as over the road trucks, engines have evolved to include turbocharging and exhaust gas recirculation upstream from the compressor. When this occurs, the mixture of incoming air and exhaust gases must often need to be cooled prior to entry into the engine so that the engine can better function to achieve good efficiency and low emissions. Prior to such an engine evolution, the same over the road truck might have had a simple air to air aftercooler that did not need substantial corrosive resistance since there may not have been exhaust gas recirculation. Even where exhaust gas recirculation has been used, typically the exhaust gases were added to the intake downstream from the air cooler. The spatial envelope available for intake gas cooling, however, has remained about the same, leading to the need for a high density corrosive resistant gas to air heat exchanger of a type described in this disclosure.  
      The gas to air heat exchanger of the present disclosure seeks to address the needs of specific portions of the heat exchanger with materials having specific properties and in specific quantities (wall thicknesses) necessary to perform needed functions. For instance, the air fins need not necessarily be corrosive resistant but should be made from a highly thermally conductive material, such as one predominantly made of copper so that good heat transfer can occur from air fins  66  to air passing through heat exchanger  28 . Tubes  67 , on the other hand, also need substantial heat exchanging capabilities, but this must be tempered with the need for corrosive resistance, either through selection of suitable tube material, or via the coating strategy described herein. Although air fins  66  can be made extremely thin, tubes  67  generally have a wetted wall thickness thicker than that of air fins  66 . The respective heads  69  and  70  attached to opposite ends of tubes  67  should have a thickness at least on the order of that of the tubes and are preferably made of the same materials, for ease in attaching the two during a high temperature brazing process. Core  60  can be completed with a low temperature brazing process when attaching the relatively thin predominantly copper air fins  66  to the outer surfaces of tubes  67 . Although tanks  61  and  63  could also be made from stainless steel, substantial cost savings can be achieved by making them from a less expensive material, such as cast aluminum, and possibly a composite for the cooler outlet tank  63 . However, because aluminum has less corrosive resistance than stainless steel, the walls of tanks  61  and  63  would generally have to be thicker than that of tubes  67  and head  69  and  70  so that the inevitable corrosion to the wetted inner surface of the tanks could be tolerated over the expected life of the heat exchanger  28  without holes developing. Any problems associated with attaching aluminum and/or composite tanks to the stainless steel heads of core  60  may be remedied via a mechanical attachment process, such as by crimping and extension of the heads about a flange on the respective tank  61  and  63 . Before doing so, a suitable o-ring seal is positioned between the tanks and heads to inhibit leakage of corrosive gases from heat exchanger  28 . Thus, the present disclosure brings a unique combination of heat exchanger features together, and assembles them in a unique way to arrive at a cost effective heat exchanger that can tolerate corrosive gases, and cool the same in a relatively small spatial volume.  
      With regard to the embodiment shown in  FIG. 5 , heat exchangers may be manufactured from particularly effective heat exchange base materials, yet protected from the corrosive environment of hot, acidic condensed exhaust gases. These goals may be achieved without sacrificing weight, increasing complexity or expanding the spatial envelope within which the heat exchanger is positioned in an engine system. In embodiments wherein the tubes are made from predominantly copper, coating the tubes with corrosive resistant brazing material can provide for heat exchangers with a relatively fewer number of conventionally sized and configured tubes than in designs using other tube materials such as stainless steel. Moreover, the ability to attach all or virtually all of the components of a heat exchanger core, while providing a corrosive resistant coating, enables a relatively simple and efficient manufacturing process.  
      It should be understood that the above description is intended for illustrative purposes only, and is not intended to limit the scope of the present disclosure in any way. Thus, those skilled in the art will appreciate that various modifications might be made to the presently disclosed embodiments without departing from the full and fair spirit and scope of the present disclosure. For instance, while it is contemplated that the heat exchangers described herein are well suited to use in acidic corrosive environments associated with EGR equipped engines, the present disclosure is not thereby limited. Salt water environments may result in corrosion of heat exchanger materials via introduction of salt laden air or water into fluid passages of a heat exchanger. Heat exchanger performance and corrosion resistance may be addressed in such situations in a manner similar to that described herein regarding exhaust gas environments, namely, through the proper selection and placement of heat exchanger materials and/or coating of corrosion sensitive surfaces of the heat exchanger. Further, while air will typically be used as a cooling fluid, some other fluid such as water or engine coolant might be used in the heat exchangers described herein without departing from the full and fair scope of the present disclosure. Other aspects, objects, and advantages of the disclosure can be obtained from a study of the drawings, the disclosure and the appended claims.