Abstract:
A heat exchanger for cooling batteries in hybrid or electric vehicles comprises a plurality of spaced apart, discrete heat exchanger panels, each having a coolant inlet manifold section, a coolant outlet manifold section, and a plurality of coolant flow passages extending between the inlet and outlet manifold sections. The inlet and outlet manifold sections of the discrete panels are connected by tubes to define continuous coolant inlet and outlet manifolds, each having a coolant opening. The flow of coolant through discrete panels may be balanced by providing the fluid flow passages of the panels with various cross-sectional areas and/or hydraulic diameters, depending partly on the proximity of each panel to the coolant opening. In an embodiment, where the panels are formed from pairs of stamped plates, variation of the cross-sectional area and/or hydraulic diameter of the coolant flow passages may be achieved by deliberately offsetting the plates during assembly.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of U.S. Provisional Patent Application No. 61/933,547 filed Jan. 30, 2014, the contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a heat exchanger for battery thermal management, the heat exchanger comprising discrete panels with integrally formed manifold sections, wherein the manifold sections of the panels are joined by tubes, and enhancements are provided for ensuring balanced flow distribution through the panels, especially in sequences of panels fed by a common coolant supply channel. 
       BACKGROUND 
       [0003]    Rechargeable batteries such as batteries made up of many lithium-ion cells can be used in many applications, including for example in electric vehicle (“EV”) and hybrid electric vehicle (“HEV”) applications. Such batteries can generate large amounts of heat that needs to be dissipated. 
         [0004]    In a typical construction of such batteries, individual battery cells are sandwiched between heat exchanger panels having coolant circulation passages. The panels are connected to externally formed manifold structures which feed coolant to each of the heat exchanger panels, the connections between the panels and the manifold structures typically being mechanical connections sealed by gaskets or O-rings. The large number of mechanical joints in conventional battery construction can be problematic in terms of reliability and manufacturability of the heat exchanger. 
         [0005]    There is a need for a simplified construction of heat exchangers for rechargeable batteries while improving manufacturability, reliability and efficiency of the heat exchanger. 
       SUMMARY 
       [0006]    According to an embodiment, there is provided a heat exchanger, comprising: a plurality of discrete heat exchanger panels, each of the heat exchanger panels having an inlet manifold section, an outlet manifold section, and a plurality of fluid flow passages extending between the inlet and outlet manifolds; at least one inlet manifold tube, wherein each said inlet manifold tube connects the inlet manifold sections of an adjacent pair of said discrete heat exchanger panels, wherein an inlet manifold of the heat exchanger comprises the inlet manifold sections of the discrete heat exchanger panels and the at least one inlet manifold tube; at least one outlet manifold tube, wherein each said outlet manifold tube connects the outlet manifold sections of an adjacent pair of said discrete heat exchanger panels, wherein an outlet manifold of the heat exchanger comprises the outlet manifold sections of the discrete heat exchanger panels and the at least one outlet manifold tube; an inlet opening provided in said inlet manifold; and an outlet opening provided in said outlet manifold. 
         [0007]    According to an embodiment, The heat exchanger according to claim  1 , wherein each of the discrete heat exchanger panels comprises a pair of stamped plates, each having a plurality of open channels, wherein the plates are joined together face-to-face to define said inlet manifold section, said outlet manifold section, and said plurality of fluid flow passages. 
         [0008]    According to an embodiment, wherein the stamped plates are identical. 
         [0009]    According to an embodiment, the inlet and outlet manifold sections of the discrete heat exchanger panels are parallel to one another and each have a pair of open ends. 
         [0010]    According to an embodiment, the fluid flow passages are substantially perpendicular to the inlet and outlet manifold sections. 
         [0011]    According to an embodiment, the discrete heat exchanger panels each have a pair of flat, opposed faces which are traversed by said fluid flow passages. 
         [0012]    According to an embodiment, the discrete heat exchanger panels each have a pair of opposed, axially-extending edge portions in which said inlet and outlet manifold sections are provided. 
         [0013]    According to an embodiment, the discrete heat exchanger panels have a pair of opposed, transversely-extending edge portions. 
         [0014]    According to an embodiment, the transversely-extending edge portions of adjacent pairs of said discrete heat exchanger panels are axially spaced apart. 
         [0015]    According to an embodiment, the heat exchanger has a longitudinal axis, and wherein the inlet manifold and the outlet manifold are parallel to the longitudinal axis. 
         [0016]    According to an embodiment, the inlet and outlet openings are provided at the same end of the heat exchanger. 
         [0017]    According to an embodiment, the inlet and outlet openings are provided at opposite ends of the heat exchanger. 
         [0018]    According to an embodiment, the at least one inlet manifold tube and the at least one outlet manifold tube are cylindrical. 
         [0019]    According to an embodiment, the inlet and outlet manifold sections each have open ends, and wherein each of the open ends is cylindrical and is sized to receive one end of one of the tubes, wherein a sealed connection is provided between said open end and said one end of said tube. 
         [0020]    According to an embodiment, the sealed connection is a brazed connection. 
         [0021]    According to an embodiment, the at least one inlet manifold tube and the at least one outlet manifold tube each have a wall thickness which is greater than a thickness of material from which the panels are formed. 
         [0022]    According to an embodiment, one or more flow restrictions are provided in at least a first heat exchanger panel of the plurality of discrete heat exchanger panels, said one or more flow restrictions producing a reduced cross-sectional area and/or hydraulic diameter in the first heat exchanger panel. 
         [0023]    According to an embodiment, said one or more flow restrictions of the first heat exchanger panel are provided in at least some of the fluid flow passages, the inlet manifold section, and/or the outlet manifold section. 
         [0024]    According to an embodiment, said one or more flow restrictions of the first heat exchanger panel are provided in the at least one inlet manifold tube and/or the at least one outlet manifold tube. 
         [0025]    According to an embodiment, each said flow restriction is in the form of a depression. 
         [0026]    According to an embodiment, each said depression is in the form of a crimp, a dimple, or a rib. 
         [0027]    According to an embodiment, the number and/or size of the depressions is varied in different fluid flow passages of the first heat exchanger panel, so as to provide different flow restrictions in two or more of the fluid flow passages of the first heat exchanger panel. 
         [0028]    According to an embodiment, a second heat exchanger panel of the plurality of discrete heat exchanger panels is adjacent to the first heat exchanger panel, wherein the first heat exchanger panel is proximal to at least one of the inlet opening and the outlet opening, and the second heat exchanger is distal to at least one of the inlet opening and the outlet opening. 
         [0029]    According to an embodiment, the second heat exchanger panel is free of said flow restrictions. 
         [0030]    According to an embodiment, the second heat exchanger panel is provided with one or more flow restrictions, and wherein the cross-sectional area and/or the hydraulic diameter in the second heat exchanger panel is greater than that in the first heat exchanger panel. 
         [0031]    According to an embodiment, the plurality of discrete heat exchanger panels includes a first heat exchanger panel and a second heat exchanger panel; the first and second heat exchanger panels each comprise a pair of stamped plates, each having a plurality of open channels, wherein the plates are joined together face-to-face to define said inlet manifold section, said outlet manifold section, and said plurality of fluid flow passages; each of the stamped plates has a pair of opposed, axially-extending edge portions in which open channels are defined for said inlet and outlet manifold sections, a central portion in which open channels are defined for said fluid flow passages, and a pair of opposed, transversely-extending edge portions; wherein at least one of the transversely-extending edge portions and/or at least one of the axially extending edge portions of each of the stamped plates is provided with one or more indexing features which provide different degrees of axial alignment of the stamped plates in the first heat exchanger panel relative to the second heat exchanger panel; and wherein said different degrees of axial alignment provide the fluid flow passages of the second heat exchanger section with a greater cross-sectional area or hydraulic diameter than the fluid flow passages of the first heat exchanger section. 
         [0032]    According to an embodiment, the indexing features are provided in each of the stamped plates and comprise at least a first set of indexing holes and a second set of indexing holes provided in the transversely-extending edge portions of the stamped plates; and alignment of the first set of said indexing holes in the first stamped plate with the first set of said indexing holes in the second stamped plate results in substantially complete axial alignment of the plates, such that there is substantially no offset of the open channels for the fluid flow passages in the first stamped plate relative to the open channels for the fluid flow passages in the second stamped plate; alignment of the second set of said indexing holes in the first stamped plate with the second set of said indexing holes in the second stamped plate results in axial misalignment of the plates, such that there is a partial offset of the open channels for the fluid flow passages in the first stamped plate relative to the open channels for the fluid flow passages in the second stamped plate. 
         [0033]    According to an embodiment, the first and second stamped plates include a third set of said indexing holes, wherein: alignment of the third set of said indexing holes in the first stamped plate with the third set of said indexing holes in the second stamped plate results in axial misalignment of the plates, such that there is a partial offset of the open channels for the fluid flow passages in the first stamped plate relative to the open channels for the fluid flow passages in the second stamped plate; and wherein the partial offset produced by alignment of the third sets of indexing holes produces a partial offset which is different from the partial offset produced by alignment of the second sets of indexing holes. 
         [0034]    According to an embodiment, each said set of indexing holes includes at least one indexing hole in each of the transversely-extending edge portions of each of the stamped plates. 
         [0035]    According to an embodiment, the first and second stamped plates each have an axial axis of symmetry. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]    The invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
           [0037]      FIG. 1  is a perspective view of a heat exchanger according to an embodiment, in a flat state; 
           [0038]      FIG. 2  is an axial (longitudinal) cross-section along line  2 - 2 ′ of  FIG. 1 ; 
           [0039]      FIG. 3  is a transverse cross-section along line  3 - 3 ′ of  FIG. 2 ; 
           [0040]      FIG. 4  is an enlarged, partial, axial cross-section through the first panel of the heat exchanger, along line  4 - 4 ′ of  FIG. 2 ; 
           [0041]      FIG. 5  is an enlarged, partial, axial cross-section through the second panel of the heat exchanger, along line  5 - 5 ′ of  FIG. 2 ; 
           [0042]      FIG. 6  is an enlarged, partial, axial cross-section through the third panel of the heat exchanger, along line  6 - 6 ′ of  FIG. 2 ; 
           [0043]      FIG. 7  is a perspective view of a stamped plate of the heat exchanger of  FIG. 1 ; 
           [0044]      FIG. 8  is an axial (longitudinal) cross-section through a panel of a heat exchanger according to another embodiment; 
           [0045]      FIG. 9  is an enlarged, partial, axial cross-section along line  9 - 9 ′ of  FIG. 8 ; and 
           [0046]      FIG. 10  is an enlarged, partial, axial cross-section along line  10 - 10 ′ of  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION 
       [0047]      FIG. 1  illustrates a heat exchanger  10  according to a first embodiment. The heat exchanger  10  comprises three heat exchanger panels, labelled  12 ,  14  and  16 . Although heat exchanger  10  is shown as having three panels, it will be appreciated that the heat exchanger  10  may comprise more than three panels or, alternatively, the heat exchanger  10  may include only two panels. 
         [0048]    The heat exchanger panels  12 ,  14 ,  16  are discrete structures, meaning that they are separately formed. The formation of panels  12 ,  14 ,  16  as discrete structures simplifies the maintenance of acceptable dimensional tolerances during manufacturing, and makes it possible to use conventional tooling for heat exchanger manufacture. 
         [0049]    The following description relates specifically to the first heat exchanger panel  12 . However, except as otherwise noted in the following description, all three heat exchanger panels  12 ,  14 ,  16  are identical, and therefore the following description of the first heat exchanger panel  12  applies equally to the second and third heat exchanger panels  14 ,  16 . 
         [0050]    The first heat exchanger panel  12  includes integrally formed inlet and outlet manifold sections  18 ,  20  and a plurality of fluid flow passages  22  extending between the inlet and outlet manifold sections  18 ,  20 . The inlet and outlet manifold sections  18 ,  20  each have a pair of open ends. 
         [0051]    In the illustrated embodiment, the inlet and outlet manifold sections  18 ,  20  have a substantially circular cross-section, as shown in the transverse cross-section of  FIG. 3 . However, it will be appreciated that the cross-sections of the inlet and outlet manifold sections  18 ,  20  are not necessarily circular in all embodiments, and that the cross-sectional shapes of the inlet and outlet manifold sections  18 ,  20  may vary along their lengths. 
         [0052]    The heat exchanger panel  12  has a pair of flat, opposed faces which are traversed by fluid flow passages  22 . The passages  22  may have a flattened shape as shown in  FIG. 4 . This flattened shape of fluid flow passages  22  is advantageous as it maximizes the area of the fluid flow passages  22  which is in engagement with the article to be cooled, such as a battery cell, and helps to minimize thickness of the heat exchanger  10 . It will be appreciated that the passages  22  do not necessarily have a flattened shape. Rather, the passages  22  may have a circular cross-section, or any other convenient cross-sectional shape. Although the passages  22  are shown in the drawings as being straight, it will be appreciated that this is not essential. Rather, the passages  22  may be have a non-linear configuration, such as curved, wavy or serpentine shape so as to conform to the profile of the surface of the article being cooled. 
         [0053]    As shown in the cross-section of  FIG. 4 , adjacent fluid flow passages  22  are separated by recessed areas  24  through which no fluid may flow. The fluid flow passages  22  have open ends which are in flow communication with the interiors of the inlet and outlet manifold sections  18 ,  20 . 
         [0054]    In the illustrated embodiment, the heat exchanger panel  12  is rectangular or square, having a pair of opposed, axially-extending edge portions  26 ,  28  in which the inlet and outlet manifold sections  18 ,  20  are formed, respectively. The heat exchanger panel  12  also has a pair of opposed, transversely-extending edge portions  30 ,  32 . The inlet and outlet manifold sections  18 ,  20  are parallel to one another and parallel to the longitudinal axis of heat exchanger  10 . 
         [0055]    As shown in  FIG. 1 , heat exchanger panels  12 ,  14 ,  16  are connected together by a plurality of tubes  34 . More specifically, tubes  34  join together the inlet manifold sections  18  of the three exchanger panels  12 ,  14 ,  16  so as to form a continuous inlet manifold  36  extending throughout the entire length of heat exchanger  10 . Similarly, tubes  34  join together the outlet manifold sections  20  of the three heat exchanger panels  12 ,  14 ,  16  so as to form a continuous outlet manifold  38  extending longitudinally throughout the entire length of heat exchanger  10 . When connected together, the adjacent heat exchanger panels  12 ,  14 ,  16  are spaced apart along the longitudinal axis, being separated by spaces  70 . The tubes  34  connecting the inlet manifold sections  18  are sometimes referred to herein as the “inlet manifold tubes”, and the tubes  34  connecting the outlet manifold sections  18  are sometimes referred to herein as the “outlet manifold tubes”. 
         [0056]    In the illustrated embodiment, the heat exchanger  10  includes an inlet opening  40  at one end of the inlet manifold  36 , and also includes an outlet opening  42  at one end of the outlet manifold  38 . In the illustrated heat exchanger  10 , the inlet and outlet openings  40 ,  42  are both formed at the same end of the first heat exchanger panel  12 . The inlet and outlet openings  40 ,  42  may be provided with respective inlet and outlet fittings  72 ,  74  to connect the heat exchanger  10  to a coolant circulation system (not shown) which may include a heat exchanger such as a radiator to remove heat from the coolant. Also, the opposite ends of the inlet and outlet manifolds  36 ,  38  provided in the third heat exchanger, are sealed. For example, the opposite ends of manifolds  36 ,  38  may be sealed by plugs  76 , or they may be crimped shut. 
         [0057]    Alternatively, the inlet and outlet openings  40 ,  42  may be provided at opposite ends of the heat exchanger  10 . In this regard, one of the openings  40  or  42  may be provided in heat exchanger panel  12 , while the other opening  40  or  42  may be provided in the heat exchanger panel  16  at the opposite end of heat exchanger  10 . It will be appreciated that this alternate arrangement may have an impact on the flow distribution of coolant flowing through the fluid flow passages  22  of the panels  12 ,  14 ,  16 . 
         [0058]    The tubes  34  connecting heat exchanger panels  12 ,  14 ,  16  are typically of cylindrical shape and are constructed to withstand any forming or support loads that the heat exchanger  10  may encounter in the process of assembling the battery, and in the final application in the vehicle. For example, the tubes  34  may be constructed so as to be bendable, for which purpose they may have a wall thickness which is greater than the thickness of the plate material making up the heat exchanger panels  12 ,  14 ,  16 . Accordingly, the heat exchanger panels  12 ,  14 ,  16  and tubes  34  will typically be separately formed and subsequently connected together to form heat exchanger  10 . Furthermore, the formation of discrete heat exchanger panels  12 ,  14 ,  16  makes manufacturing easier and less expensive, and permits tighter (or more stringent) dimensional tolerances to be maintained. As used herein, the term “cylindrical” refers to articles such as tubes having a cross-section which is circular, oval, elliptical or of similar shapes. 
         [0059]    For the purpose of joining with tubes  34 , the open ends of the inlet and outlet manifold sections  18 ,  20  may be formed as cylindrical sockets  44  which are sized to closely receive the ends of the tubes  34 . Such a construction is disclosed in commonly assigned, co-pending U.S. patent application Ser. No. 13/423,385 entitled “Battery Cell Cooler”, and published as US 2012/0237805 A1 on Sep. 20, 2012, the contents of which are incorporated herein by reference. 
         [0060]    Once the tubes  34  are inserted into sockets  44 , the assembly may be brazed or welded so as to form sealed metallurgical connections between the heat exchanger panels  12 ,  14 ,  16 , thereby providing the brazed or welded assembly shown in  FIG. 1 . With heat exchanger  10  in the flat condition of  FIG. 1 , the tubes  34  connecting heat exchanger panels  12 ,  14 ,  16  are substantially straight. The provision of integrally formed manifold sections and the connection of panels  12 ,  14 ,  16  by metallurgical joints avoids the need for mechanical joints between the panels and manifolds, thereby improving reliability. As used herein, references to “brazing” include other means for forming metallurgical joints, including welding, soldering, etc. 
         [0061]    The heat exchanger panels  12 ,  14 ,  16  are each formed from a pair of stamped plates  46  which are assembled in face-to-face relation by brazing. In the illustrated embodiment, all of the stamped plates  46  making up the heat exchanger panels  12 ,  14 ,  16  are identical, which further simplifies manufacturing. However, it will be appreciated that the plates  46  are not necessarily identical to one another. In another embodiment, the stamped plates  46  making up panels  12 ,  14 ,  16  are non-identical mirror images of one another. 
         [0062]      FIG. 7  illustrates a rectangular, stamped plate  46  of heat exchanger  10  in isolation. The stamped plate  46  has a plurality of open channels which combine with corresponding channels of a second, identical plate  46  to form the inlet manifold section  18 , the outlet manifold section  20  and the plurality of fluid flow passages  22 . 
         [0063]    As shown in  FIG. 7 , the stamped plate  46  has a pair of opposed, axially-extending edge portions  48 ,  50  in which open-ended channels  52 ,  54  are formed. These channels  52 ,  54  will combine with channels  52 ,  54  of another plate  46  to form the inlet and outlet manifold sections  18 ,  20 . Furthermore, the stamped plate  46  includes a central portion  56  in which open channels  58  are defined for said fluid flow passages  22 . The open channels  58  are in the form of flat-topped ribs separated by flat portions  78  which braze to flat portions  78  in a second stamped plate  46  to form the recessed areas  24  between adjacent fluid flow passages  22 . 
         [0064]    Due to the simple design of plate  46 , material can be pulled from the featureless, axial edge portions  48 ,  50  of plate  46  during formation of channels  52 ,  54 . This permits the formation of larger channels  52 ,  54 , and consequently the formation of manifolds  36 ,  38  with larger cross sectional areas. Increasing the cross-sectional area of the manifolds  36 ,  38  provides lower pressure drop and more even flow distribution of coolant through the fluid flow passages  22  along the length of the heat exchanger  10 , and thus allows the heat exchanger  10  and its manifolds  36 ,  38  to be elongated by adding more heat exchanger panels similar to panels  12 ,  14 ,  16 , where necessary. 
         [0065]    It can be seen from  FIG. 3  that the socket  44  is formed to be as close to cylindrical as possible, in order to form a tight fit and reliable seal with the outer surface of tube  34 . However, the material of plates  46  at the inner and outer edges of channels  52 ,  54  will inevitably have a bending radius which results in the formation of small gaps  80 ,  82  at the respective inboard and outboard edges of the manifold section  18 /manifold  36 . The size of these gaps  80 ,  82  should be minimized by minimizing the bending radius at the edges of the channels  52 ,  54 . The gap  82  at the outboard edge can be minimized during formation of plates  46 , as discussed above, because material from the outer, free axial edge  48  is pulled into this area. However, this is not possible at the inboard edge due to the presence of the fluid flow passages  22 . In order to avoid excessive strain and thinning at the inboard edges of channels  52 ,  54 , a discontinuity such as a hole or slit  84  (see  FIGS. 3 and 7 ) may be provided adjacent to the inboard gap  80 , to allow for material to be pulled into the region of gap  80  so as to minimize the size of gap  80  and reduce thinning in this area. As shown in  FIG. 7 , slits  84  may be formed with circular inner ends to avoid the tendency of cracks to propagate from slits. Although the above discussion of gaps  80 ,  82  focuses on the sides of plates  52  in which the open channels  52  for inlet manifold section  18  are formed, the same gaps  80 ,  82  exist at the other side of plate  46 , in which the open channels  54  for outlet manifold section  20  are formed, and the above discussion applies equally to the outlet manifold sides of plates  46 . 
         [0066]    The stamped plate  46  also includes a pair of opposed, transversely extending edge portions  60 ,  62 , the features of which will be discussed below. 
         [0067]    When two stamped plates  46  are brought together in face-to-face relation, the plates  46  will be aligned with one another in the transverse direction as precisely as possible, within acceptable tolerances, such that the open channels  52  will combine to form inlet manifold section  18 , and the open channels  54  will combine to form outlet manifold section  20 . The stamped plates  46  may be assembled in a braze fixture (not shown) to ensure the desired transverse alignment, or the stamped plates  46  may first be assembled in an assembly fixture and then placed into a simplified braze fixture. 
         [0068]    It can be seen from the cross-section of  FIG. 6  that precise axial alignment of a pair of stamped plates  46 , within acceptable tolerances, will result in substantial axial alignment of the open channels  58  in the stamped plates  46  to form fluid flow passages  22  in which the transversely-extending edges of the open channels are in substantially opposed alignment. In other words, in  FIG. 6  there is substantially no axial offset of the open channels  58  relative to one another. This precise alignment without offset will provide the fluid flow passages  22  with maximum cross-sectional area and/or largest hydraulic diameter, and will maximize fluid flow through the fluid flow passages  22 . 
         [0069]    Due at least in part to the relatively small diameter of the integrally formed inlet and outlet manifolds  36 ,  38 , it will be appreciated that the flow distribution of coolant through the heat exchanger panels will not be uniform. In this regard, the flow rate of fluid through heat exchanger  10  will decrease with increasing distance from the inlet and outlet openings  40 ,  42 . Conversely, the flow rate will be highest in those regions which are closest to the inlet and outlet openings  40 ,  42 . Therefore it can be seen that the locations of the inlet and outlet openings  40 ,  42  will determine the regions of heat exchanger  10  that will experience higher or lower flow rates. In the illustrated embodiment, where the inlet and outlet openings  40 ,  42  are both provided in the first heat exchanger panel  12 , there will tend to be a higher coolant flow rate through the first heat exchanger panel  12  than through the third heat exchanger panel  16 , and the flow rate through the second heat exchanger panel  14  will be intermediate between the flow rates through the first and third heat exchanger panels  12 ,  16 . 
         [0070]    Embodiments in which the inlet and outlet openings  40 ,  42  are located at opposite ends of the heat exchanger will exhibit a different flow distribution, with the highest flow rates being in the heat exchanger panels where the inlet and outlet openings  40 ,  42  are located. In such an embodiment, it may be desired to restrict the flow through both end panels, while maintaining maximum cross-sectional area and/or hydraulic diameter in the middle panel(s). 
         [0071]    In order to provide substantially balanced flow distribution throughout all three heat exchanger panels  12 ,  14 ,  16 , it may be desirable to restrict the flow through the second heat exchanger panel  14  relative to the third heat exchanger panel  16 , and also to restrict the flow through the first heat exchanger panel  12  relative to the second heat exchanger panel  14 . 
         [0072]    The present embodiment provides variable flow restriction in the heat exchanger panels while maintaining the benefits of constructing the heat exchanger panels  12 ,  14 ,  16  from identical stamped plates  46 . This is accomplished by intentionally introducing axial offset or misalignment of identical stamped plates  46  during assembly, to adjust the hydraulic diameter and/or cross-sectional areas of the fluid flow passages in the heat exchanger panels  12 ,  14 ,  16 . For example, the stamped plates  46  in the third heat exchanger panel  16  may be in substantial axial alignment as described above, the stamped plates  46  in the second heat exchanger panel  14  may be assembled with a first amount of offset or axial misalignment, and the stamped plates  46  in the first heat exchanger panel  12  may be assembled with a second amount of offset or axial misalignment, which is greater than the first amount of axial misalignment. In this way, the flow restriction through the first heat exchanger panel  12  is greater than the flow restriction through the second heat exchanger panel  14 , and the flow restriction through the second heat exchanger panel  14  is greater than the flow restriction through the third heat exchanger panel  16 . By selecting the amount of offset or axial misalignment, it is possible to achieve a substantially uniform flow distribution in each of the heat exchanger panels  12 ,  14 ,  16  making up heat exchanger  10 . 
         [0073]    The variable axial misalignment of the stamped plates along the longitudinal axis is provided during assembly of the heat exchanger panels  12 ,  14 ,  16 . For example, each of the stamped plates  46  may be provided with a plurality of indexing holes in their transverse edge portions  60 ,  62 . In the illustrated embodiment, each stamped plate  46  is provided with twelve indexing holes, with six holes being provided in each of the transverse edge portions  60 ,  62 . This arrangement permits the identical plates  46  to be flipped over or flipped and rotated by 180 degrees so as to bring them into face-to-face relation for assembly, while providing three different degrees of alignment or offset. It will be appreciated that this arrangement where the plates can be brought into alignment by flipping, with or without rotation, ensures proper assembly of plates  46 , but may require plates  46  to have an axis of symmetry along the axial direction. In embodiments where rotational alignment of the plates is either not desired or not needed, such that the plates are brought into alignment only by flipping them over, it will be appreciated that the axially symmetrical pattern of indexing holes on stamped plates  46  is unnecessary, and it may be desired to reduce the number of indexing holes in the plates. For example, the number of indexing holes could be reduced to six, two holes for each degree of alignment. 
         [0074]    Although one arrangement is shown in the drawings, it will be appreciated that there are numerous other ways to provide axial alignment/offset between plates  46 , and that these alternate arrangements may require different plate configurations. The specific plate configurations will depend partly on the number of degrees of alignment/offset required, and the amount of space available for holes or other indexing features. For example, the plates  46  may include indexing features other than indexing holes, such as notches in the edges of the plates. Also, it will be appreciated that the indexing features may be provided in the axial edge portions  48 ,  50  of plates  46 , in addition to or instead of being provided in the transverse edge portions  60 ,  62 . Also, the number of holes can be reduced where the plates  46  are not required to be symmetrical along the axial direction, or where an assembly fixture registers with specific features of the plates and produces the alignment/offset. For example, an assembly fixture may include an edge locator such that the transverse edges of plates  46  are used to provide alignment/offset. 
         [0075]    To achieve the required three degrees of axial alignment in heat exchanger  10 , three sets of indexing holes are provided, each set consisting of four holes, with two holes in each of the transverse edge portions  60 ,  62 . Referring to  FIG. 7 , each stamped plate  46  has a first set of indexing holes  64 , a second set of indexing holes  66 , and a third set of indexing holes  68 . 
         [0076]    Each set of indexing holes will produce a specific axial alignment of the two stamped plates  46  during assembly of the heat exchanger panels  12 ,  14 ,  16 . In this regard, with the first set of indexing holes  64  in alignment with one another, the stamped plates  46  will be axially aligned within acceptable tolerances, resulting in the configuration shown in  FIG. 6  having maximum flow and minimum pressure drop through passages  22 . 
         [0077]    Similarly, when the holes  66  of the second set are aligned with one another, there will be a first amount of offset in the plates  46  as in the second heat exchanger panel  14  shown in  FIG. 5 . 
         [0078]    Lastly, when the holes  68  of the third set are aligned with one another, there will be a second amount of offset in the plates  46  as in the first heat exchanger panel  12  shown in  FIG. 4 , the second amount of offset being greater than the first amount of offset. 
         [0079]    Accordingly, it can be seen that the provision of multiple sets of indexing holes in the transverse edge portions  60 ,  62  of the stamped plates  46  permits the degree of alignment and/or misalignment to be controlled. 
         [0080]    During assembly of the heat exchanger panels  12 ,  14 ,  16 , the plates  46  are placed in assembly or brazing fixtures (not shown) having pins which will be received in the indexing holes. In this regard, a first fixture will have a set of pins which are arranged to be received in the first set of holes  64 , a second fixture will have a set of pins which are arranged to be received in the second set of holes  66 , and a third fixture will have an arrangement of which are arranged to be received in the third set of holes  68 . In the illustrated embodiment each fixture will have four pins. However, as mentioned above, the number of holes for each alignment position can be reduced to two, in which case each fixture will have only two pins. In this way, heat exchanger panels of variable offset can be manufactured from identical plates  46 . 
         [0081]    There are various ways in which the heat exchanger  10  may be assembled. In one embodiment, the parts may be assembled in an assembly/brazing fixture on the (moving) belt of a braze furnace by the following sequence of steps: insert the lower plate  46  of panel  12  into the fixture; place inlet and outlet fittings  40 ,  42  into the ends of open channels  52 ,  54 ; insert the lower plate  46  of panel  14  into the fixture, in spaced, side-by-side relation relative to the bottom plate  46  of panel  12 ; place a pair tubes  34  into the channels  52 ,  54  of the two side-by-side bottom plates  46 ; place the top plate  46  of panel  12  over the corresponding bottom plate; repeat above steps until assembly is complete; and then braze the parts together in the braze furnace. It will be appreciated that the portions of the fixture in which the panels  12 ,  14 ,  16  are assembled will include pins which will be received in the indexing holes to produce the desired degree of offset. 
         [0082]    In another embodiment, the parts may be assembled in an assembly fixture by the same assembly steps as discussed above, and then compressed and mechanically locked together. The assembled structure is then removed from the assembly fixture and placed in a simplified brazing fixture to be brazed in a furnace. 
         [0083]    In another embodiment, the plates  46  are first brazed together to form the discrete panels  12 ,  14 ,  16 . The panels  12 ,  14 ,  16  are then placed in an assembly/brazing fixture and combined with the tubes  34 , inlet and outlet fittings  72 ,  74  and plugs  76 . This assembly is then subjected to a secondary brazing/welding/joining process, such that the panels  12 ,  14 ,  16  are joined together to produce heat exchanger  10 . The assembly/brazing fixture for each panel  12 ,  14 ,  16  may have features, such as pins which align with the indexing holes of panels  12 ,  14 ,  16 , and ensure that the panels are located in the correct positions. 
         [0084]    It will be appreciated that the number of indexing holes provided in the plates  46  can be varied according to the number of heat exchanger panels to be assembled into the heat exchanger. It will also be appreciated that more or fewer than four indexing holes may be provided in each set of holes. For example, as discussed above, each set of holes may comprise two holes. 
         [0085]    Although the illustrated embodiment uses misalignment or offset to provide balanced flow, it will be appreciated that other methods can be used to perform this flow balancing. For example, flow balancing can be achieved by providing flow obstructions in at least one of the fluid flow passages  22 , in the inlet manifold section  18 , and/or in the outlet manifold section  20  of at least one of the sections  12 ,  14 ,  16 . Furthermore, flow obstructions may be provided in one or more of the tubes  34  connecting adjacent sections  12 ,  14 ,  16 . 
         [0086]      FIGS. 8-10  shows a heat exchanger  100  according to a second embodiment. Heat exchanger  100  has a number of elements which are similar or identical to elements of heat exchanger  10 . Like elements of heat exchangers  10  and  100  are therefore described with like reference numerals, and the above descriptions of these elements in heat exchanger  10  also apply to heat exchanger  100 , unless otherwise indicated below. 
         [0087]    Although heat exchanger  100  comprises a plurality of heat exchanger sections connected together by tubes  34 , only a portion of one section  14  is shown in  FIGS. 8 to 10 . For the purpose of illustration, heat exchanger  100  is shown as having four different types of flow obstructions. However, it will be appreciated that heat exchanger  100  may include any one or more of the flow obstructions shown in  FIGS. 8 to 10 . These flow obstructions may be used instead of, or in combination with, the misalignment or offset described above in order to provide balanced flow throughout the heat exchanger  100 . 
         [0088]      FIG. 8  includes a pair of flow obstructions in the tube  34  connecting the heat exchanger section  14  to another section (not shown). In the illustrated embodiment, these flow obstructions comprise an opposed pair of depressions  102 ,  104  which reduce the cross-sectional area of the tube  34 , partially restricting flow therethrough. The depressions  102 ,  104  may be in the form of ribs, dimples or crimps, and may be produced by placing the tube  34  in or on a die and striking its opposite side with a punch, for example, either before or after assembly of the heat exchanger  100 . Although two opposed depressions  102 ,  104  are shown, it will be appreciated that the tube  34  may be provided with only one depression  102  or  104 . Also, it will be appreciated that the depressions  102 ,  104  may be of different size and/or shape, and are not necessarily opposite to one another, and/or may be longitudinally spaced from one another. Also, depressions  102  and/or  104  may be formed at any point in the circumference of the tube  34 . This type of flow obstruction may be particularly useful where the inlet and outlet for the coolant are located at the same end of the heat exchanger, as in heat exchanger  10  described above. 
         [0089]      FIGS. 8 and 9  also show a second type of flow obstruction, comprising a depression  106  provided in the inlet manifold section  18 . As shown in  FIG. 9 , the depression  106  reduces the cross-sectional area of the inlet manifold section  18 , partially restricting flow therethrough. The depression  106  may be in the form of a rib, dimple or crimp and may be produced by striking the inlet manifold section  18  with a tool. In this embodiment, the depression  106  is provided in the open channel  52  of the top plate  46  (shown in  FIG. 9 ), and may be formed either before or after two plates  46  are joined to form the second heat exchanger section  14 . Although only a single depression  106  is provided in the inlet manifold section  18 , it will be appreciated that the inlet manifold section  18  may be provided with a plurality of depressions  106 , which may be of the same or different size and/or shape, may be opposite to one another or may be longitudinally spaced from one another along the length of the inlet manifold section. Also, as shown in  FIG. 9 , the centre of the depression  106  is at about 45 degrees from the horizontal, although it will be appreciated that the location of the depression  106  about the circumference of inlet manifold section  108  is variable. Also, although the depression  106  is shown as being provided in the inlet manifold section  18 , one or more similar depressions  106  may be provided in the outlet manifold section  20  (not shown in  FIG. 8 ). This type of flow obstruction may also be particularly useful where the inlet and outlet for the coolant are located at the same end of the heat exchanger, as in heat exchanger  10  described above. 
         [0090]      FIGS. 8 and 9  also show a third type of flow obstruction, comprising a plurality of depressions  108  and  110  formed in the individual fluid flow channels  22 . As shown in the partial sectional view of  FIG. 9 , the fluid flow channels  22  are provided with an opposed pair of depressions  108 ,  110  which reduce the cross-sectional area of the fluid flow channel  22 , partially restricting flow therethrough.  FIG. 8  shows only three of the fluid flow channels  22  being provided with these depressions, however, it will be appreciated that the depressions  108 ,  110  will typically be provided in all the fluid flow channels  22  of section  14 . It will also be appreciated that neighbouring heat exchanger sections  12  and/or  16  may also be provided with similar depressions  108 ,  110 , although the amount of flow restriction produced by the depressions  108 ,  110  in other sections  12 ,  16  do not necessarily provide the same flow restriction as the depressions  108 ,  110  in section  14 , thus providing the ability to balance flow throughout heat exchanger  100 . This type of flow obstruction is useful for various inlet/outlet configurations, and can be used where the coolant inlet and outlet are located at the same end or at opposite ends of the heat exchanger. 
         [0091]    Depressions  108 ,  110  may be in the form of ribs, dimples or crimps and may be produced by placing section  14  in or on a die and striking its opposite side with a punch, for example, either before or after assembly of the section  14 . In the illustrated embodiment, the depressions  108 ,  110  are in the form of a longitudinally extending ribs or troughs. Although two opposed depressions  108 ,  110  are shown, it will be appreciated that each channel  22  may be provided with only one depression  108  or  110 . Also, it will be appreciated that the depressions  108 ,  110  may be of different size and/or shape, and are not necessarily opposite to one another, but rather may be transversely spaced from one another. 
         [0092]      FIGS. 8 and 10  also show a fourth type of flow obstruction, comprising a plurality of depressions  112  formed in the individual fluid flow channels  22 . As shown in the partial sectional view of  FIG. 10 , the fluid flow channels  22  are provided with a plurality of transversely spaced depressions  112  in the top wall of channel  22 , partially restricting flow therethrough.  FIGS. 8 and 9  show only three of the fluid flow channels  22  being provided with these depressions  112 . However, it will be appreciated that the depressions  112  may typically be provided in all the fluid flow channels  22  of section  14 . 
         [0093]    Depressions  112  are in the form of dimples produced by striking the section  14  with a tool, either before or after assembly of the section  14 . Although depressions  112  are formed only in the top wall of channel  22 , it will be appreciated that each channel  22  may be provided with only depressions  112  in the top and/or bottom wall of channel  22 . It will be appreciated that the depressions  112  may be of different size and/or shape. 
         [0094]      FIG. 8  shows that the channels  22  of section  14  may be provided with different numbers of depressions  112 , thereby providing different amounts of flow obstruction in the fluid flow channels  22  of section  14 . In this regard, the number of depressions  112  decreases in the direction of fluid flow (to the right in  FIG. 8 ), thereby providing a lesser amount of flow restriction. It will be appreciated that each fluid flow channel  22  may instead be provided with the same number of depressions  112 , and/or the number of depressions  112  in the channels  22  of adjacent heat exchanger sections  12  and  16  may be the same as or different. This type of flow obstruction is also useful for various inlet/outlet configurations, and can be used where the coolant inlet and outlet are located at the same end or at opposite ends of the heat exchanger. 
         [0095]    According to an embodiment, the flow obstructions described above may be provided instead of, or in combination with, the misalignment or offset described above. For example, all three heat exchanger panels  12 ,  14 ,  16  can be assembled with substantially complete alignment, such that all the fluid flow passages  22  initially have maximum cross-sectional area/hydraulic diameter as in  FIG. 6 . The passages  22  in the first and second panels  12 ,  14  can then be struck by tooling to restrict the passages  22  and reduce their cross-sectional area/hydraulic diameter, with the degree of restriction being greater in the first panel  12  than in the second panel  14 . Thus, it is possible to provide flow balancing in panels  12 ,  14 ,  16  without providing the plates with indexing features. 
         [0096]    Also, it will be appreciated that the depressions in the heat exchanger sections  12 ,  14  and  16  may have many different forms, and may be formed in numerous different ways. For example, as stated above, the passages  22  may be struck by tooling after the panels  12 ,  14 , and/or  16  are assembled, either before or after brazing, so as to bring about the required degree of deformation to restrict at least some of the passages  22 . Alternatively, the plates  46  themselves may be struck by tooling before being assembled to form panels  12 ,  14  and/or  16 , either during or after the stamping of the open channels  52 ,  54 ,  58 . The deformation may take a variety of forms, including ribs which may extend across or along the passages  22  and/or the channels  58  from which they are formed, and locally reduce the depth of channel(s)  58  and/or the cross-sectional area of the passage(s)  22  in which they are provided. Alternatively, the deformation may take the form of discrete bumps or dimples which locally reduce the depth of channel(s)  58  and/or the cross-sectional area of the passage(s)  22  in which they are provided. It will be appreciated that ribs or dimples in mating pairs of plates  46  may either align or not align with one another. Where the ribs or dimples of mating plates  46  align with one another, they may produce a local flow restriction which is greater than a flow restriction which is produced where the ribs or dimples do not align. Thus, it is possible to provide an arrangement of ribs and/or dimples which are formed in the plates  46  before assembly of panels  12 ,  14  and/or  16 , the ribs and/or dimples being located such that the ribs and/or dimples in one plate  46  align with those in a mating plate  46  when the plates  46  are flipped over to bring them into position for assembly; and such that the ribs and/or dimples do not align when the plates  46  are flipped and rotated to bring them into position for assembly. It will be appreciated that alignment of the ribs and/or dimples in opposed plates  46  may bring them into contact with one another during assembly, such that they produce an obstruction extending throughout the entire height of the fluid flow passage(s)  22 . 
         [0097]    Although the invention has been described in connection with certain embodiments, it is not restricted thereto. Rather, the invention includes all embodiments which may fall within the scope of the following claims.