Patent Publication Number: US-9403204-B2

Title: Heat exchanger assembly and method

Description:
BACKGROUND OF THE INVENTION 
     Emission concerns associated with the operation of internal combustion engines (e.g., diesel and other types of engines) have resulted in an increased emphasis on the use of exhaust gas heat exchange systems with such engines in vehicular and non-vehicular applications. These systems are often employed as part of an exhaust gas recirculation (EGR) system in which a portion of an engine&#39;s exhaust is returned to combustion chambers via an intake system. The result is that some of the oxygen that would ordinarily be inducted into the engine as part of its fresh combustion air charge is displaced with inert gases. The presence of the inert exhaust gas typically serves to lower the combustion temperature, thereby reducing the rate of NO x  formation. 
     In order to achieve the foregoing, it is desirable for the temperature of the recirculated exhaust to be lowered prior to the exhaust being delivered into the intake manifold of the engine. In many applications employing EGR systems, exhaust gas recirculation coolers (EGR coolers) are employed to reduce the temperature of the recirculated exhaust. In the usual case, engine coolant is brought into heat exchange relation with the exhaust gas within the EGR cooler in order to achieve the desired reduction in temperature. The use of engine coolant provides certain advantages in that appropriate structure for subsequently rejecting heat from the engine coolant to the ambient air is already available for use in applications requiring an EGR system. 
     In some applications, however, the temperature to which recirculated exhaust must be lowered in order to achieve the desired reduction in the rate of NO x  formation is lower than, or appreciably close to, the temperature at which the engine coolant is regulated by the engine&#39;s thermal management system. In such cases, a second EGR cooler may be employed to extract from the recirculated exhaust that portion of the desired heat load which cannot be readily transferred to the engine coolant at its regulated temperature. This second EGR cooler (frequently referred to as a “low temperature EGR cooler” or “LT EGR cooler”) commonly receives either a flow of coolant from a separately regulated coolant loop, or a portion of the regular engine coolant loop which has been cooled to a lower temperature. 
     Packaging the LT EGR cooler along with an EGR cooler (sometimes referred to as the “high temperature EGR cooler” or “HT EGR cooler”) can be problematic due to space constraints. Placing both EGR coolers into a common casing can help to ease these packaging issues, but can make it more difficult to accommodate the differences in thermal expansion between the exhaust gas conveying tubes in the EGR coolers and the casing. Such thermal expansion differences have been known to lead to premature failure of the heat exchanger. 
     Although applications involving EGR cooler connections (to other EGR coolers and/or other structures) illustrate the design challenges described above, such challenges exist in other heat exchanger applications as well—some of which involve heat exchangers outside of exhaust gas recirculation technology. Based upon these and other limitations of conventional heat exchanger connection designs, improved heat exchanger connections and connection methods continue to be welcome in the art. 
     SUMMARY OF THE INVENTION 
     In accordance with some embodiments, of the present invention, a heat exchanger includes a casing having a proximal end and a distal end, with a fluid flow path extending from the proximal end to the distal end. The heat exchanger further includes a plurality of heat exchange tubes defining a first section of the fluid flow path extending from the proximal end, and another plurality of heat exchange tubes defining a second section of the fluid flow path extending to the distal end. A third section of the fluid flow path fluidly connects the first section to the second section, and includes at least one sealing plate. The heat exchange tubes defining the first section are rigidly attached to the casing at the proximal end, and are structurally decoupled from the casing at their opposite ends. The heat exchange tubes defining the second section are rigidly attached to the casing at the distal end, and are structurally decoupled from both the casing and the heat exchange tubes defining the first section at their opposite ends. 
     Another feature of the present invention includes a casing having a pocket containing at least a portion of the sealing plate. The pocket is defined by a planar wall that provides a sealing surface for a fluid-tight seal between the casing and the sealing plate, and by one or more peripheral walls that bound the outer periphery of the planar wall. The pocket may be further defined by another planar wall that is parallel to and spaced apart from the first planar wall. This second planar wall can provide a sealing surface for a fluid-tight seal between the casing and a second sealing plate. 
     In some embodiments, the third section of the fluid flow path includes a group of one or more cylindrical flow conduits rigidly attached to the heat exchange tubes defining the first section, and a group of one or more cylindrical flow conduits rigidly attached to the heat exchange tubes of defining the second section. At least one of the groups extends at least partially into the pocket in the casing. As one feature, fluid-tight seals extend around one or more of the cylindrical flow conduits and allow for movement in the axial direction relative to the casing. The first and second groups of cylindrical flow conduits may be separated from one another in order to accommodate thermal expansion differences between the heat exchange tubes and the casing. 
     In some embodiments of the present invention, the heat exchanger includes a second fluid flow path passing over the heat exchange tubes defining the first section, and a third fluid flow path passing over the heat exchange tubes defining the second section. The second and third fluid flow paths are sealed off from the first fluid flow path by at least some of the fluid-tight seals in the third section of the first fluid flow path. In some cases the second and third fluid flow paths are not in fluid communication with one another within the heat exchanger. 
     In some embodiments of the invention the heat exchanger may be used as an EGR cooler, with a recirculated exhaust gas flowing along the first flow path, a first flow of coolant flowing along the second flow path, and a second flow of coolant flowing along the third flow path. In some cases one of the flows of coolant may be at a lower temperature than the other flow of coolant. 
     In accordance with some embodiments of the present invention, a heat exchanger includes a casing having a proximal end and a distal end, with a fluid flow path extending from the proximal end to the distal end. The heat exchanger further includes a first plurality of heat exchange tubes defining a portion of the fluid flow path including the proximal end, and a second plurality of heat exchange tubes defining a portion of the fluid flow path including the distal end. A flow transitioning structure defines the fluid flow path between the distal end of the first plurality of heat exchange tubes and the proximal end of the second plurality of heat exchange tubes, and structurally decouples the distal end of the first plurality of heat exchange tubes from the proximal end of the second plurality of heat exchange tubes. 
     In some embodiments, the casing includes a pocket containing at least a portion of the flow transitioning structure. The pocket is defined by a planar wall that provides a sealing surface for a fluid-tight seal between the casing and the flow transitioning structure, and by one or more peripheral walls that bound the outer periphery of the planar wall. The pocket may be further defined by another planar wall that is parallel to and spaced apart from the first planar wall. This second planar wall can provide a another sealing surface for another fluid-tight seal between the casing and the flow transitioning structure. 
     Other objects, features, and advantages of the invention will become apparent from a review of the entire specification, including the appended claims and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a heat exchanger according to an embodiment of the present invention. 
         FIG. 2  is a perspective view of a heat exchanger core for use in the heat exchanger of  FIG. 1 . 
         FIG. 3  is a perspective view of a tube and insert for use in the heat exchange core of  FIG. 2 . 
         FIG. 4  is a perspective view of a casing section of the heat exchanger of  FIG. 1 . 
         FIG. 5  is another perspective view of the casing section of  FIG. 4 . 
         FIG. 6  is a partially exploded and partially cut-away perspective view of a portion of the heat exchanger of  FIG. 1 . 
         FIG. 7  is a cut-away perspective view of a sealing plate for use in the heat exchanger of  FIG. 1 . 
         FIG. 8 a    is a sectional detail view of the heat exchanger of  FIG. 1 , taken along lines VIII-VIII of  FIG. 1 . 
         FIG. 8 b    is a sectional detail view of the heat exchanger of  FIG. 1  according to an alternative embodiment of the present invention, also taken along lines VIII-VIII of  FIG. 1 . 
         FIG. 9  is a schematic representation of an engine system including a heat exchanger embodying the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Before any embodiments of the present invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings. 
     An embodiment of a heat exchanger  1  according to the present invention is shown in  FIGS. 1-8   a , and includes a heat exchanger  1  providing a flow path  8  for a fluid to pass through the heat exchanger  1 , wherein the flow path  8  extends from a proximal end  11  of the heat exchanger  1  to a distal end  12  of the heat exchanger  1 . The flow path  8  is enclosed within a casing  2 , which can comprise multiple casing sections  3 . As further shown in  FIG. 1 , the casing  2  of the illustrated embodiment additionally encloses flow paths  9  and  10 , along which one or more fluids can be passed through the heat exchanger  1  so as to be placed in heat exchange relation with a fluid passing along the flow path  8 . 
     Although  FIG. 1  shows the flow paths  8  and  9  to be in counter-current flow orientation, it should be understood that in some applications, other flow orientations (such as, for example, concurrent flow), may be preferred or equally suitable. Similarly, although flow paths  8  and  10  are depicted as being in concurrent flow orientation, it should be understood that in some applications, other flow orientations, such as, for example, counter-current flow, may be preferred or equally suitable. 
     The fluid flow paths  8 ,  9  and  10  of the illustrated embodiment are at least partially defined by first and second heat exchange cores  4  and  5 , shown generically in  FIG. 2 . Each heat exchange core  4 ,  5  of the heat exchanger  1  shown in  FIG. 1  has a construction as shown in  FIG. 2  (although adapted in length as needed to match the casings  3   a ,  3   b  into which the cores  4 ,  5  are received, as necessary). Each of the cores  4 ,  5  include a bundle of parallel heat exchange tubes  6  extending between a first header  7  and a second header  20 . Ends of the tubes  6  are sealingly attached to the headers  7 ,  20 , such as by brazing, welding, or in any other suitable manner. Cylindrical flow conduits  21  are provided at that end of the core  5  where the tubes  6  are attached to the header  20 , to at least partially define another portion of the fluid flow path  8  downstream or upstream of the portion defined by the heat exchange tubes  6 . It should be understood that, although the exemplary embodiment depicts four of the cylindrical flow conduits  21 , the number of flow conduits present in a given application may be less than or more than four, without limitation. 
     The heat exchange cores  4 ,  5  further may include one or more baffles  40  arranged along the length of either or both heat exchange cores  4 ,  5 . Such baffles  40  can provide benefit during assembly of the heat exchange cores  4 ,  5  by maintaining desired spacing between the tubes  6 . In some embodiments, the baffles  40  can define a tortuous portion of the flow path  9  or  10  over the outer surfaces of the heat exchange tubes  6  in order to increase the rate of heat transfer between fluids traveling over and through the tubes. Alternatively or in addition, fluid flow plates (not shown) can be included between adjacent heat exchange tubes  6  in order to direct a fluid flowing along the flow path  9  or  10 . 
     In some embodiments, the heat exchange cores  4 ,  5  can include spring plates  36  around one or more of the outer surfaces of the bundles of tubes  6 . The utility of these spring plates  36  will be discussed in detail below. In some cases, one or more of the spring plates  36  can be attached directly to one or more of the baffles  40 . Alternatively or in addition, one or more of the spring plates  36  can be attached to straps  39  wrapped around one or more of the heat exchange tubes  6 , and/or other structure located adjacent, between, or around the heat exchange tubes  6 . 
     It should be readily apparent to those having skill in the art that the heat exchange tubes  6  can take many different forms. In some embodiments, such as that shown in  FIG. 2 , the tubes  6  can be flat tubes having first and second opposing substantially flat and long walls connected with relatively short (and in some cases, arcuately shaped) walls. In other embodiments, the tubes  6  can have a more rectangular shape, as shown in  FIG. 3 . In still other embodiments, the tubes  6  can have a circular cross-sectional shape, or can be constructed from two or more stacked plates. Also, in some embodiments, one or more of the heat exchange tubes  6  include an insert  41  (shown in  FIG. 3 ) floating within or bonded to the inner walls of the tubes  6  to improve the rate of heat transfer to or from fluid traveling through the tubes  6 . 
     While the cores  4  and  5  for a given heat exchanger  1  may be identical to one another in some cases, it should be understood that there is no requirement for them to be identical. In some cases, the cores  4 ,  5  can differ in a variety of ways, including but not limited to tube length, tube size, number of tubes, arrangement of tubes  6 , and the like. 
     Turning now to  FIGS. 4 and 5 , certain aspects of a casing section  3  will be discussed. Although the specific casing section  3  shown in  FIGS. 4 and 5  corresponds to the casing section  3   b  in  FIG. 1 , it should be understood that certain features shown in  FIGS. 4 and 5  can similarly be found in the casing section  3   a  of  FIG. 1 . 
     The casing section  3  of  FIGS. 4 and 5  includes a first opening  37  at a first end of the casing section  3 , and a second opening  38  at a second end opposite the first end. The opening  37  can be sized to accommodate the entirety of a core  4  or  5  (in some cases, without the header plate  7 ). The opening  38  can be smaller than the opening  37 , and can be sized to at least accommodate the one or more cylindrical flow conduits  21  of a core  4  or  5 . 
     The illustrated casing section  3  further includes a plurality of fastening locations  26  at the second end. These fastening locations  26  can be located in a flange  17  at the second end. While the specific fastening locations  26  shown in the accompanying figures are depicted as circular through-holes, it should be understood that any other assembly features suitable for assembling casing sections can be similarly substituted. For example, the fastening locations  26  can, in some cases, take the form of pins, V-band grooves, blind threaded holes, etc. 
     The casing section  3  can include a pocket  16  at the second end. In some embodiments, the pocket  16  is defined by a planar wall  14  in which the opening  38  is located, and by one or more walls  15  bounding the outer periphery of the planar wall  14 . In other embodiments, the pocket  16  can be defined by other portions of the casing while still providing a recess open to and facing away from the rest of the casing section  3 , and can be wider, thinner, deeper, or shallower as desired. Additionally, the casing section  3  may optionally include a groove  27  at the second end, with the opening  38  at least partially enclosed by the groove  27 . In those embodiments in which both a pocket  16  and a groove  27  are present, the groove  27  can encircle the pocket  16 , as shown in  FIG. 5 . 
     In some embodiments, the casing section  3  includes one or more of the following: an inlet  33  to receive a fluid traveling along the flow path  9  into the heat exchanger  1 ; an outlet  34  to remove a fluid traveling along the flow path  9  from the heat exchanger  1 ; an inlet  31  to receive a fluid traveling along the flow path  10  from the heat exchanger  1 ; and an outlet  32  to remove a fluid traveling along the flow path  10  from the heat exchanger  1 . A casing section  3  can also include a flow conduit  54  to allow a fluid traveling along one of the flow paths  9 ,  10  to transfer from the casing section  3  to another casing section  3  without exiting the heat exchanger  1 . Such a flow conduit  54  can, if present, be advantageously disposed within the boundaries of the groove  27 , if present. 
     Heat exchange cores  4  and  5  can each be assembled into respective ones of the casing sections  3   a  and  3   b , as shown in  FIG. 6 . A core  4  or  5  can be inserted into a casing section  3  by passing the core  4 ,  5  through the opening  37  of the respective casing section  3   a ,  3   b , starting with the cylindrical conduits  21 , until the header  7  of the core  4 ,  5  reaches the casing section  3   a ,  3   b , respectively. Spring plates  36  assembled to outer surfaces of the core  4 ,  5  can be used to locate the core  4 ,  5  within the casing section  3   a ,  3   b  by engaging with, and sliding along, one or more inner surfaces  13  of the casing section  3   a ,  3   b . The spring plates  36  can have a suitable compliancy such that they can deform to allow for contact between all of the spring plates  36  and their corresponding adjacent walls  13 . This allows the core  4 ,  5  to be firmly contained within the respective casing section  3   a ,  3   b  in order to withstand shock and/or vibration loadings that may be experienced during operation of the heat exchanger  1 , even when the inner surfaces  13  of the casing sections  3   a ,  3   b  are uneven, and/or have varying surfaces resulting from production variations and manufacturing tolerances (e.g., in casting processes). 
     Once the heat exchange core  4 ,  5  is so assembled into the respective casing section  3   a ,  3   b , the header  7  of the core  4 ,  5  can be fastened to the end of the casing  4 ,  5  in a leak-tight fashion. In some embodiments, this fastening is achieved through the use of mechanical fasteners, such as, for example, bolts that extend through holes  57  found in the header  7  and into corresponding threaded holes  56  in the end of the casing  3   a ,  3   b . A gasket (not shown) can be placed into a groove  55  or can be otherwise installed at another suitable feature at the mating face of the casing  3   a ,  3   b  either during or prior to assembly in order to effect a leak-free joint between the header  7  and the casing  3   a ,  3   b . In other cases, a leak-free joint can instead be achieved by welding the header  7  to the casing  3   a ,  3   b  along the entire periphery of these elements. 
     It should be appreciated that assembling the core  4 ,  5  into the casing section  3   a ,  3   b  as described allows for the location of cylindrical flow conduit(s)  21  of the core  4 ,  5  to vary within the casing section  3   a ,  3   b , since that location will be dictated by the bearing of the spring plates  36  on the inner casing walls  13 . 
     A sealing plate  18  (shown in greater detail in  FIG. 7 ) is assembled onto the end of the heat exchange core  4 ,  5  by insertion of the cylindrical flow conduits  21  through corresponding apertures  28  in the sealing plate  18 . A sealing gasket  22  (shown in  FIGS. 8 a  and 8 b   ), such as an O-ring, can be placed into a groove  29  located within each of the apertures  28  in order to achieve a fluid-tight seal between the cylindrical flow conduits  21  and the sealing plate  18 . In embodiments in which a plurality of cylindrical flow conduits  21  are used, excellent registration between the sealing plate  18  and the cylindrical flow conduits  21  can be achieved, owing to the unitary construction of both the sealing plate  18  and the end portion of the core  4 ,  5  containing the cylindrical flow conduits  21 , despite the variable location of the core  4 ,  5  within the casing  3   a ,  3   b.    
     When the casing section  3   a ,  3   b  includes a pocket  16  as described above, the sealing plate  18  can advantageously be received into the pocket  16  such that assembly of the sealing plate  18  does not increase the overall length of the heat exchanger  1 . The pocket  16  can be larger than the sealing plate  18  so that a sufficient clearance gap is provided between the peripheral walls  15  of the pocket and the sealing plate in order to allow for variability in the location of the cylindrical flow conduits  21  within the pocket  16 . 
     The heat exchange cores  4  and  5  can both be assembled into respective casing sections  3   a ,  3   b  as described above, and the casing sections  3   a  and  3   b  can be joined together at the fastening locations  26  of the casing sections  3   a ,  3   b . As shown in  FIG. 8 a   , assembling of the casing sections  3   a  and  3   b  can mate the end faces  35  (see also,  FIG. 5 ) of the flanges  17  of the casing sections  3   a ,  3   b  against one another. A sealing gasket  24  can also be provided in a groove  27  of at least one of the casing sections  3   a ,  3   b  in order to create a leak-tight seal between the casing sections  3   a ,  3   b , or can otherwise be retained in place between the casing sections  3   a ,  3   b  for this purpose. Also shown in  FIG. 8 a    are additional gaskets  23  located in grooves  30  found in at least one face of the sealing plate  18 . Assembly of the casing section  3   a  to the casing section  3   b  can cause the sealing plates  18   a  (assembled to one of the cores  4 ,  5 ) and  18   b  (assembled to the other of the cores  4 ,  5 ) to contact each other and compress the gaskets  23  against the walls  14  in order to create a leak-tight seal. The fluid-tight seals created by the gaskets  22 ,  23 , and  24 , alone or in combination, can prevent fluid communication between fluids traveling along the flow paths  8 ,  9 , and  10 , and can similarly prevent leakage of those fluids out of the heat exchanger  1 . 
     Since the location of the cylindrical flow conduits  21  of each of the cores  4 ,  5  can be allowed to vary relative to the casing section  3   a ,  3   b  into which the core  4 ,  5  is assembled, the apertures  28  of the sealing plate  18   a  may not be directly aligned with the apertures  28  of the sealing plate  18   b . However, such non-alignment will not result in the loss of sealing between the fluid streams. 
     Once the heat exchanger  1  is so assembled, a continuous flow path  8  is defined from the proximal end  11  of the heat exchanger  1  to the distal end  12 . The flow path  8  includes a first (upstream) section defined by the tubes  6  of the core  4 , extending from the inlet header  7  of the core  4  to the outlet header  20  of the core  4 , and further includes a second (downstream) section defined by the tubes  6  of the other core  5 , extending from the outlet header  20  of the core  5  to the inlet header  7  of the core  5 . A third intermediate section of the heat exchanger  1  is defined by a flow transitioning structure  59  fluidly connecting the upstream and downstream sections just described. The flow transitioning structure  59  extends from the header  20  of the first core  4  to the header  20  of the second core  5 . 
     In some embodiments, the ends of the tubes  6  at both the proximate end  11  and the distal end  12  of the heat exchanger  1  are rigidly attached to the casing  2  by the attachment of the headers  7  to the casing sections  3   a  and  3   b . In other words, this attachment between the tube ends  6  and headers  7 , and the casing  2  is substantially inflexible, and does not permit relative movement between the tube ends  6  and headers  7  and the casing  2 . In a similar way, in some embodiments, the flow transitioning structure  59  is rigidly attached (or is relatively inflexible, and does not permit relative movement) at either end to the ends of the tubes  6 , by way of the headers  20 . In contradistinction, the two ends of the flow transitioning structure  59  are flexibly connected to one another (indirectly through the sealing plates  18   a ,  18   b ) and to the casing  2 , and/or are permitted to shift or otherwise move (in at least one direction, and/or at least during thermal expansion of the tubes  6  with respect to the casing  2 ) based upon the manner in which the flow transitioning structure  59  is assembled. Since the gaskets  22  provide a sliding seal for the cylindrical flow conduits  21  (as is required to enable assembly of the sealing plate  18  over the cylindrical flow conduits  21 ), and the cylindrical flow conduits  21  of core  4  can be separated from those of core  5  by a gap  58 , the tube ends attached to the header  20  of either core are not prevented from displacing some amount in the tube-axial direction, and stresses at the tube-to-header joints by such displacement can be reduced or eliminated. 
     The flexible joint and/or relative movement enabled by the transitioning structure  59  described above can be especially beneficial in applications where a large thermal expansion differential exists, either between the tubes  6  of core  4  and the tubes  6  of core  5 , or between the tubes  6  of either core and the casing  2 , or both. Such thermal expansion differences have been known to cause premature failure of heat exchangers by causing high stresses, especially at tube-to-header joints. Consequently, the life of a heat exchanger  1  constructed according to some embodiments of the present invention can be beneficially enhanced. 
     Another embodiment of a heat exchanger  1  according to the present invention is illustrated in  FIG. 8 b   . In the embodiment of  FIG. 8 b   , the pocket  16  (see also,  FIG. 5 ) is found only in one of the casing sections ( 3   b ). A casing section  3   a ′ lacking the pocket  16  has replaced the previous casing section  3   a . Additionally, the sealing plate  18   a  has been replaced with a larger sealing plate  19 , and the gasket  23  found in the previous sealing plate  18   a  has been replaced with a similar gasket  25 . In the embodiment of  FIG. 8 b   , the sealing plate  19  is included in the joint between the flanges  17  of the casing sections  3   a ′ and  3   b . The seal between fluid flowing along the flow path  8  and fluid flowing along the flow path  10  in this embodiment can be provided solely by the gaskets  22 . The new gasket  25  can prevent leakage of fluid flowing along the flow path  10  to the outside of the heat exchanger  1 . 
     In some embodiments, the heat exchanger  1  can be provided as an EGR cooler for use in an EGR system  60 , shown in  FIG. 9 . The EGR system  60  can include an engine  43  having an intake manifold  44  and an exhaust manifold  45 , a compressor  47  coupled to an expander  46 , and an EGR valve  51 . A portion  49  of the hot, pressurized exhaust flow produced by the engine  43  is directed from the exhaust manifold  45  to the expander  46 . The exhaust flow  49  is expanded to a lower pressure in the expander  46 , and the energy derived thereby is used to compress a fresh combustion air flow  48  in the compressor  47 . The compressed air flow  48  is directed from the compressor  47  to the intake manifold  44 . 
     With continued reference to the embodiment of  FIG. 9 , another portion  50  of the hot, pressurized exhaust flow produced by the engine  43  is recirculated, by way of the EGR cooler  1  and the EGR valve  51 , from the exhaust manifold  45  back to the intake manifold  44 , where it is combined with the compressed air flow  48 . The recirculated exhaust flow  50  passes through the EGR cooler  1  along the flow path  8  (described above), is cooled by a first coolant flow  52  passing through the heat exchanger  1  along the flow path  10  (described above), and is further cooled by a second coolant flow  53  passing through the heat exchanger  1  along the flow path  9  (also described above). 
     In some embodiments of the EGR system  60  according to the present invention, the coolant flows  52  and  53  can be recombined at some point in the system. In still other embodiments, the coolant flows  52  and  53  can belong to segregated coolant flow circuits. Also, in some embodiments, the coolant flow  53  enters the EGR cooler  1  at a lower temperature than does the coolant flow  52 , or the coolant flow  52  enters the EGR cooler  1  at a lower temperature than does the coolant flow  53 . 
     In some embodiments, the coolant flows  52  and  53  both comprise a conventional engine coolant such as water, ethylene glycol, propylene glycol, other coolant, or any mixture of these coolants. Also, either or both of the coolant flow  52  and  53  can comprise a working fluid for a Rankine cycle waste heat recovery system. 
     Various alternatives to the features and elements of the present invention are described with reference to specific embodiments of the present invention. With the exception of features, elements, and manners of operation that are mutually exclusive of or are inconsistent with each embodiment described above, it should be noted that the alternative features, elements, and manners of operation described with reference to one particular embodiment are applicable to the other embodiments. 
     The embodiments described above and illustrated in the figures are presented by way of example only and are not intended as a limitation upon the concepts and principles of the present invention. As such, it will be appreciated by one having ordinary skill in the art that various changes in the elements and their configuration and arrangement are possible without departing from the spirit and scope of the present invention.