Patent Publication Number: US-11029101-B2

Title: Reverse header design for thermal cycle

Description:
FIELD OF THE INVENTION 
     The invention relates to a heat exchanger, and more specifically to a header of a tank of the heat exchanger, wherein the header includes at least one portion with a reversed structure for accommodating thermal cycling of the heat exchanger. 
     BACKGROUND OF THE INVENTION 
     Heat exchangers typically include a centralized plurality of heat exchanger tubes or passageways connected at each respective end thereof to one of a first header tank and a second header tank. The plurality of heat exchanger tubes forms a heat exchanger core of the heat exchanger for transferring heat energy between two different heat exchanging fluids. The header tanks each typically include a surface that acts as a header having tube openings for receiving end portions of the heat exchanger tubes therein. The header of each of the header tanks is then coupled to a corresponding casing that acts as a fluid reservoir aiding in distributing or collecting a fluid flowing through the heat exchanger tubes. 
     Heat exchangers may be susceptible to damage due to thermal cycling when various different components of the heat exchanger thermally expand relative to each other as the temperature of the different components is increased or decreased depending on the desired operation of the heat exchanger. For example, heat exchangers may be especially susceptible to failure at the joint formed between each of the heat exchanger tubes and each of the headers. Typically, an end portion of each tube is received through a collar defining one of the tube openings of one of the headers in order to form a joint between an outer surface of the tube and an inner surface of the collar. A rigid and fluid tight connection may be formed at this joint by means of brazing, as one non-limiting example. However, this rigid connection leads to increased stresses at the joint between the tube and the collar of the header when the joint attempts to accommodate the relative thermal expansion occurring between the header and the tube. Repeated cycling of these thermal stresses may accordingly lead to a failure at one of the tube and header joints, thereby causing leakage of the heat exchanging fluid from the corresponding header tank. 
     It may also be the case that certain portions of the header are especially susceptible to the type of failure discussed hereinabove. It is not uncommon for slight temperature variations to exist at different regions within the heat exchanger as a result of the form and configuration of various components of the heat exchanger such as the header tanks, the headers, and the heat exchanger tubes. This in turn leads to some of the tube and header joints being exposed to substantially different maximum and minimum temperatures during use of the heat exchanger. 
     For example, in one type of heat exchanger configuration one of the two header tanks includes a partition dividing the header tank into two independent chambers to cause the heat exchanging fluid to follow a substantially U-shaped path as the heat exchanging fluid passes in order through a first one of the chambers of a first header tank, a first set of heat exchanger tubes, an opposing second header tank, a second set of the heat exchanger tubes, and a second one of the chambers of the first header tank. It has been discovered that such heat exchanger configurations may be especially susceptible to failure of the tube and header joints formed to one or both sides of the dividing partition due to the difference in temperature of the heat exchanging fluid when passing through the tubes formed to either side of the dividing partition. 
     Alternatively, in some other configurations the heat exchanging fluid enters one of the header tanks through an associated fluid port, passes through the heat exchanger tubes, and then exits an opposing second one of the header tanks through an associated fluid port. It has been discovered that such heat exchanger configurations may be especially susceptible to failure at the tube and header joints adjacent end portions of each of the headers, and especially when the fluid port of the corresponding header tank is formed adjacent one of the end portions of one of the headers. 
     It would therefore be desirable to produce a heat exchanger having a header that better distributes the stresses formed at the tube and header joints thereof due to the effects of thermal cycling. 
     SUMMARY OF THE INVENTION 
     Compatible and attuned with the present invention, an improved header configuration for distributing the stresses caused by thermal cycling has been surprisingly discovered. 
     In an embodiment of the invention, a header for a header tank of a heat exchanger comprises a header wall defining a tube receiving portion having a plurality of longitudinally spaced tube openings formed therethrough. The tube receiving portion includes a planar portion and an adjacent offset portion. The planar portion is disposed on a first plane and the offset portion has a variable distance from the first plane as the offset portion extends away from the planar portion with respect to a longitudinal direction of the header. 
     In another embodiment of the invention, a header tank for a heat exchanger comprises a casing having a hollow interior and a header coupled to the casing. The header comprises a header wall defining a tube receiving portion having a plurality of longitudinally spaced tube openings formed therethrough. The tube receiving portion includes a planar portion and an adjacent offset portion. The planar portion is disposed on a first plane and the offset portion has a variable distance from the first plane as the offset portion extends away from the planar portion with respect to a longitudinal direction of the header. 
     In yet another embodiment of the invention, a heat exchanger comprises a first header tank including a hollow first casing and a first header. The first header comprises a header wall defining a tube receiving portion having a plurality of longitudinally spaced tube openings formed therethrough. The tube receiving portion includes a planar portion and an adjacent offset portion. The planar portion is disposed on a first plane and the offset portion has a variable distance from the first plane as the offset portion extends away from the planar portion with respect to a longitudinal direction of the first header. A second header tank includes a hollow second casing and a second header. A plurality of heat exchanger tubes extends between the first header tank and the second header tank. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above, as well as other objects and advantages of the invention, will become readily apparent to those skilled in the art from reading the following detailed description of a preferred embodiment of the invention when considered in the light of the accompanying drawings: 
         FIG. 1  is an elevational cross-sectional view of a heat exchanger according to the present invention; 
         FIG. 2  is an elevational cross-sectional view of a header of the heat exchanger of  FIG. 1 ; 
         FIG. 3  is a front elevational view of the header of  FIG. 2 ; 
         FIG. 4  is a cross-sectional view of the header as taken through lines  4 - 4  of  FIG. 3 ; 
         FIG. 5  is a cross-sectional view of the header as taken through lines  5 - 5  of  FIG. 3 ; 
         FIG. 6  is a cross-sectional view of the header as taken through lines  6 - 6  of  FIG. 3 ; 
         FIG. 7  is an elevational cross-sectional view of a header tank having a header according to another embodiment of the invention; and 
         FIG. 8  is a fragmentary cross-sectional view of a header tank having a header according to yet another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description and appended drawings describe and illustrate various embodiments of the invention. The description and drawings serve to enable one skilled in the art to make and use the invention, and are not intended to limit the scope of the invention in any manner. In respect of the methods disclosed, the steps presented are exemplary in nature, and thus, the order of the steps is not necessary or critical. 
       FIG. 1  illustrates a heat exchanger  10  according to an embodiment of the invention. The heat exchanger  10  may be used for any heat exchanging application such as forming an evaporator or a condenser of an air conditioning system, a radiator of a cooling system, or a charge air-cooler of a turbocharger system, as non-limiting examples. The heat exchanger  10  may be configured to pass any type of fluid therethrough, including a refrigerant or a coolant, as non-limiting examples. The fluid passed by the heat exchanger  10  may be configured for exchanging heat energy with a flow of air passing through the heat exchanger  10  in a direction arranged substantially perpendicular to a plane generally defined by the heat exchanger  10 , but any form of secondary heat exchanging fluid may be used without departing from the scope of the present invention. 
     The heat exchanger  10  includes a first header tank  12 , an oppositely arranged second header tank  14 , and a heat exchanger core  16  extending between the first header tank  12  and the second header tank  14 . The heat exchanger core  16  is formed by a plurality of spaced apart and parallel heat exchanger tubes  20 . The heat exchanger tubes  20  may be any form of heat exchanger tubes, including extruded tubes or folded flat tubes, as non-limiting examples. The heat exchanger core  16  may further include surface area increasing features  18 , such as corrugated fins, disposed between adjacent ones of the heat exchanger tubes  20  in order to increase a heat exchange capacity of the heat exchanger  10 . 
     The first header tank  12  includes a hollow first casing  30  and a first header  50 . The first casing  30  defines a manifold for distributing or recombining a first fluid passing through each of the heat exchanger tubes  20 . The first casing  30  includes a foot  32  extending around a perimeter of a header opening  31  of the first casing  30 . The foot  32  generally forms an outwardly flanged portion of the first casing  30 . The foot  32  may generally include a substantially rectangular cross-sectional shape as the foot  32  extends around the perimeter of the header opening  31 . The foot  32  may be divided into a first foot segment and an oppositely arranged second foot segment meeting at each of two opposing ends of the first casing  30 . Further, the first casing  30  may include a first wall segment and an oppositely arranged second wall segment, wherein the first wall segment extends from the first foot segment to a spine of the first casing  30  while the second wall segment extends from the second foot segment to the spine. The first and second wall segments may each be substantially arcuate in shape to form a first casing  30  having a substantially semi-circular or semi-elliptical cross-sectional shape. 
     The first casing  30  may include a plurality of longitudinally spaced crimp structures (not shown) having a substantially semi-cylindrical shape. Each of the crimp structures may be an integrally formed structure projecting from one of the foot segments and a corresponding one of the wall segments. Each of the crimp structures may include a substantially semi-circular cross-sectional shape for allowing a corresponding structure to be bent or deformed to match the semi-circular shape of each of the crimp structures. The first casing  30  may further include a plurality of spaced apart ribs (not shown) formed on an outer surface thereof with each of the ribs extending from one of the crimp structures disposed on the first foot segment to an opposing one of the crimp structures disposed on the second foot segment. The ribs may be added to the first casing  30  in order to re-inforce the first casing  30  against deformation due to thermal expansion when receiving the first fluid at an elevated pressure therein and other stresses applied to the casing  30 . 
     In the embodiment shown in  FIG. 1 , the first casing  30  includes a partition  33  dividing an interior of the first casing  30  into a first chamber  35  and a second chamber  36 . The partition  33  may be an insert received in the first casing  30  arranged on a plane substantially perpendicular to a longitudinal axis of the first casing  30 . The partition  33  extends across the first casing  30  to prevent direct fluid communication between the first chamber  35  and the second chamber  36  with respect to the heat exchanging fluid circulated within the first casing  30 . 
     The first casing  30  includes a first fluid port  44  providing fluid communication between the first chamber  35  of the first casing  30  and the remainder of a fluid system conveying the first fluid therethrough. The first fluid port  44  may form an inlet or an outlet of the first casing  30  depending on a direction of flow of the first fluid through the heat exchanger  10 , and especially in cases where the heat exchanger  10  is configured to be passable bi-directionally to accommodate multiple different modes of operation of the associated fluid system. 
     The first casing  30  further includes a second fluid port  45  providing fluid communication between the second chamber  36  of the first casing  30  and the remainder of a fluid system conveying the first fluid therethrough. The second fluid port  45  may similarly form an inlet or an outlet of the first casing  30  depending on a direction of flow of the first fluid through the heat exchanger  10 . 
     The first fluid port  44  and the second fluid port  45  are each shown as a cylindrical conduit intersecting the first casing  30  and arranged substantially parallel to a direction of extension of the heat exchanger tubes  20 , but it should be understood that the first fluid port  44  and the second fluid port  45  may have any cross-sectional shape and any orientation relative to the first casing  30  without departing from the scope of the present invention. The fluid ports  44 ,  45  are also shown as intersecting each respective chamber  35 ,  36  of the first casing  30  at a central region thereof, but the fluid ports  44 ,  45  may alternatively intersect the first casing  30  at any suitable position for causing the flow configuration of  FIG. 1  without departing from the scope of the present invention. 
     The first casing  30  may be formed from a polymeric material such as a rigid plastic material suitable for withstanding the internal pressure of the first fluid when passing through the first casing  30 . The first casing  30  may accordingly be formed in a suitable molding operation, as one non-limiting example. However, it is understood other materials can be used as desired without departing from the scope of the invention. 
     The second header tank  14  includes a hollow second casing  130  and a second header  150 . The second casing  130  includes substantially similar structure to the first casing  30  except the second casing  130  of the present embodiment is devoid of any fluid ports for communicating the first fluid to a remainder of the associated fluid system. Instead, the second casing  130  is used as a turn-around for the first fluid to cause the first fluid to follow a substantially U-shaped flow path when flowing through the heat exchanger  10 . 
     The first header  50  is shown in isolation in  FIGS. 2-6  to better illustrate the features thereof. The first header  50  generally includes a header wall  52  contoured to define each of a tube receiving portion  54 , a coupling portion  56 , and a connecting portion  58  of the first header  50 . The header wall  52  includes an outer face  55  facing towards the second header tank  14  and an inner face  57  facing towards the first casing  30  while also defining a portion of each of the first chamber  35  and the second chamber  36  of the first header tank  12 . The first header  50  extends in a longitudinal direction thereof from a first end  61  to a second end  62 . The first header  50  further includes a first longitudinal side  63  and an opposing second longitudinal side  64  separated from each other by a width direction of the first header  50 . The contoured portions of the header wall  52  are further disposed on different planes of the first header  50  separated from each other in a height or depth direction of the first header  50 , wherein the height or depth direction is arranged perpendicular to each of the longitudinal direction and the width direction. 
     The coupling portion  56  of the first header  50  includes a trough  70  and a plurality of crimping walls  80  extending from an outer portion of the trough  70  in the height direction of the first header  50 . The trough  70  extends circumferentially around a perimeter of the tube receiving portion  54  of the header wall  52  and is configured to receive the foot  32  of the first casing  30  therein. The tube receiving portion  54  may include a substantially rounded-rectangular or rectangular perimeter shape, as desired, hence the trough  70  may similarly extend circumferentially in a substantially rounded-rectangular or rectangular perimeter shape while circumscribing the tube receiving portion  54 . However, the tube receiving portion  54  and the surrounding trough  70  may include any suitable longitudinally extending perimeter shape while remaining within the scope of the present invention, such as an elongated elliptical shape, as one non-limiting example. 
     In the illustrated embodiment, the trough  70  includes four of the crimping walls  80  extending therefrom, with one of the crimping walls  80  corresponding to each of the first end  61 , the second end  62 , the first longitudinal side  63 , and the second longitudinal side  64  of the first header  50 . The crimping walls  80  are configured to be inwardly deformed relative to the foot  32  of the first casing  30  when the foot  32  is received within the trough  70 , thereby coupling the first header  50  to the first casing  30 . A distal end of each of the crimping walls  80  may include an outwardly flared portion  81  configured to aid in locating the foot  32  of the first casing  30  when received within the trough  70  of the first header  50 . The crimping walls  80  may further include one or more openings  82  formed therein and spaced from each other about the circumferential direction of the trough  70 . The openings  82  may be provided to aid in inwardly deforming the crimping walls  80  towards the foot  32  at spaced intervals about the circumference of the trough  70  to provide an interference fit therebetween, as desired. As mentioned earlier, the foot  32  of the first casing  30  may include a plurality of semi-circular crimp structures forming a surface about which the crimping walls  80  are deformed for forming the interference fit. 
     The trough  70  is shown in  FIG. 4  as including a substantially arcuate semi-circular cross-sectional shape, but it should be understood that any substantially concave surface or structure may form the trough  70  without necessarily departing from the scope of the present invention. The trough  70  may alternatively be formed by a substantially planar surface arcing upwardly to either side of the planar surface, as one non-limiting example. Regardless of the shape of the trough  70 , the trough  70  defines a seal engaging surface  72  extending circumferentially about the perimeter of the first header  50 . In the provided example, the seal engaging surface  72  is formed by a lowermost portion of the semi-circular cross-sectional shape of the trough  70  as shown from the perspective of  FIG. 4 . 
     The seal engaging surface  72  is configured to engage a sealing element  5 , wherein the sealing element  5  is configured to be compressed between the trough  70  of the first header  50  and the foot  32  of the first casing  30  when the first header  50  and the first casing  30  are coupled to each other via a method such as the crimping described hereinabove. The sealing element  5  may include substantially the same perimeter shape as the trough  70  while further including a strip  6  extending between opposing side surfaces of the sealing element  5 , wherein the strip  6  is configured to engage a surface of the partition  33  facing towards the first header  50  when the first header  50 , the first casing  30 , and the partition  33  are in the assembled configuration in order to prevent fluid communication between the first chamber  35  and the second chamber  36 . 
     The tube receiving portion  54  of the header wall  52  includes a substantially planar portion  66  disposed on a first plane P 1  of the first header  50  and an offset portion  68  deviating from the first plane P 1  of the first header  50 . The first plane P 1  is defined by the longitudinal and width directions of the first header  50  to cause the first plane P 1  to be arranged perpendicular to the direction of extension of the heat exchanger tubes  20 . The first plane P 1  is spaced in the height direction of the first header  50  from a second plane P 2  of the first header  50  defined by the circumferentially extending seal engaging surface  72  of the trough  70 , wherein the first plane P 1  and the second plane P 2  are arranged parallel to each other. 
     The connecting portion  58  of the first header  50  forms a wall extending between the trough  70  and the tube receiving portion  54  about a perimeter of the first header  50 . As shown in  FIGS. 2 and 3 , the offset portion  68  of the tube receiving portion  54  curves away from the first plane P 1  defined by the planar portions  66   a ,  66   b  before eventually being arranged on and parallel to the second plane P 2  defined by the seal engaging surface  72  of the trough  70 . As best shown in  FIG. 6 , the transition from the first plane P 1  to the second plane P 2  causes the connecting portion  58  to reduce in slope relative to the second plane P 2  until the connecting portion  58  merges into the co-planar seal engaging surface  72  and tube receiving portion  54  along a central region of the offset portion  68 . 
     In the provided embodiment, the planar portion  66  of the tube receiving portion  54  is divided into a first planar portion  66   a  and a second planar portion  66   b , wherein the offset portion  68  is disposed intermediate the first and second planar portions  66   a ,  66   b . The first planar portion  66   a  extends from a position adjacent the first end  61  of the first header  50  toward a central portion of the first header  50  including the offset portion  68  while the second planar portion  66   b  extends from a position adjacent the second end  62  of the first header  50  toward the central portion of the first header  50  including the offset portion  68 . A length of the first planar portion  66   a , the offset portion  68 , and the second planar portion  66   b  may be dependent on a configuration of the first header tank  12 , including a positioning of any one of the fluid ports  44 ,  45  or the partition  33 , as desired. 
     As best shown in  FIGS. 2-6 , the tube receiving portion  54  further includes a plurality of tube openings  85  formed therein and extending through the header wall  52  from the outer face  55  to the inner face  57  thereof. The tube openings  85  may be substantially rectangular or rounded-rectangular in shape with a longitudinal dimension extending in the width direction of the first header  50 . The tube openings  85  may be substantially evenly spaced from each other with respect to the longitudinal direction of the first header  50 , but any spacing may be used without necessarily departing from the scope of the present invention. 
     Each of the tube openings  85  is formed through a distal end  97  of a corresponding tube projection  86  of the tube receiving portion  54 . Each of the tube projections  86  is formed by a portion the header wall  52  bent or curved away from one of the first plane P 1  defined by the first and second planar portions  66   a ,  66   b  or the curvilinear shape formed by the offset portion  68  as it curves away from the first plane P 1 . Each of the tube projections  86  includes a base  96  wherein the header wall  52  first curves away from the surrounding planar or curvilinear surface of the tube receiving portion  54 . A height of each of the tube projections  86  is accordingly measured between the base  96  and the distal end  97  thereof with respect to the height direction of the first header  50 . 
     The tube projections  86  may be divided into a plurality of first tube projections  86   a  projecting from the planar portions  66   a ,  66   b  of the tube receiving portion  54  and a plurality of second tube projections  86   b  projecting from the offset portion  68  of the tube receiving portion  54 . The tube openings  85  may similarly be divided into a plurality of first tube openings  85   a  formed through the first tube projections  86   a  of the planar portions  66   a ,  66   b  and a plurality of second tube openings  85   b  formed through the second tube projections  86   b  of the offset portion  68 . 
     In the illustrated embodiment, a planar surface defining each of the planar portions  66   a ,  66   b  is shown as substantially surrounding each of the first tube projections  86   a  about an entirety of a perimeter thereof, including being present between adjacent ones of the first tube projections  86   a  ( FIG. 4 ) as well as being present between each of the first tube projections  86   a  and the laterally disposed connecting portion  58  ( FIG. 5 ). However, in some embodiments, the first tube projections  86   a  may extend laterally to merge at least partially with the connecting portion  58  of the first header  50  to each lateral side of the first tube openings  85   a , thereby resulting in the planar portions  66   a ,  66   b  being present only between adjacent ones of the first tube projections  86   a . Either configuration may be used without departing from the scope of the present invention. 
     As mentioned above, the offset portion  68  of the tube receiving portion  54  curves away from the first plane P 1  defined by the planar portions  66   a ,  66   b  until the offset portion  68  is arranged on and parallel to the second plane P 2  defined by the seal engaging surface  72  of the trough  70 . Specifically, with reference to the inner face  57  of the header wall  52 , the offset portion  68  includes a pair of substantially convex surfaces  74  where the offset portion  68  initially curves away from each of the planar portions  66   a ,  66   b  and a centrally located concave surface  75  formed between the convex surfaces  74 . Each of the convex surfaces  74  of the inner face  57  corresponds to a concave surface  76  of the outer face  55  while the concave surface  75  of the inner face  57  corresponds to a convex surface  77  of the outer face  55 . For clarity, the contour of the offset portion  68  of the tube receiving portion  54  is hereinafter primarily described by reference to only the convex surfaces  74  and the concave surface  75  of the inner face  57 , as opposed to referring to the concave surfaces  76  and the convex surface  77  of the outer face  55 . The transition from the planar portions  66   a ,  66   b  to the convex surfaces  74  and then to the concave surface  75  causes the offset portion  68  to include a curvilinear and substantially arcuate profile from the perspective of  FIG. 2  absent sharp changes as the first header  50  extends longitudinally. 
     As best shown in  FIG. 2 , each of the first tube projections  86   a  projects away from the inner face  57  of the header wall  52  in a first direction parallel to the direction of extension of the heat exchanger tubes  20  and hence the height direction of the first header  50 . Each of the first tube projections  86   a  includes a common height relative to the planar portions  66   a ,  66   b  wherein the distal end  97  of each of the first tube projections  86   a  is disposed on a third plane P 3  of the first header  50  spaced from the first plane P 1  of the first header  50  in the height direction thereof, wherein the third plane P 3  is spaced from the first plane P 1  in a direction opposite the spacing of the second plane P 2  from the first plane P 1 . The first tube projections  86   a  projecting in the first direction corresponds to the first tube projections  86   a  projecting inwardly towards an inner surface of the first casing  30  when the first header  50  is coupled to the first casing  30 . 
     In contrast to the first tube projections  86   a , the second tube projections  86   b  include a variable height and a variable direction of extension as the offset portion  68  progresses inwardly toward a central region thereof from each of the surrounding planar portions  66   a ,  66   b . More specifically, the second tube projections  86   b  include a maximum height adjacent each of the planar portions  66   a ,  66   b  while projecting inwardly in the first direction in similar fashion to the first tube projections  86   a . As the offset portion  68  progresses inwardly toward the central region thereof, a height of each of the second tube projections  86   b  successively decreases relative to the inner face  57  while still projecting in the first direction. The second tube projections  86   b  eventually reverse in direction relative to the height direction of the first header  50  to project in an outward direction from the outer face  55  towards the second header tank  14  while the height of each of the second tube projections  86   b  successively increases as the offset portion  68  progresses inwardly towards a central region thereof. The offset portion  68  of the tube receiving portion  54  accordingly includes a transition of a direction of projection of the second tube projections  86   b  that includes a maximum extent of projection in a first direction towards the first casing  30  adjacent the planar portions  66   a ,  66   b  and a maximum extent of projection in a second direction towards the second header tank  14  at a central region of the offset portion  68  arranged on the third plane P 3 . 
     The change in the direction of projection of the second tube projections  86   b  may occur at each of the transitions between the centrally located convex surface  75  and each of the outwardly located concave surfaces  74  of the inner face  57 . For example, as shown in  FIG. 2 , the height of each of the second tube projections  86   b  successively decreases along each of the concave surfaces  74  when progressing inwardly towards the convex surface  75  with the second tube projections  86   b  projecting in the first direction towards the first casing  30 . In contrast, the height of each of the second tube projections  86   b  successively increases along the convex surface  75  when progressing inwardly towards the center of the convex surface  75  disposed on the third plane P 3  while the second tube projections  86  project in the second direction towards the second header tank  14 . As shown in  FIG. 7 , the distal end  97  of each the second tube projections  86   b  formed immediately to either side of the center of the concave surface  75  of the inner face  57  is disposed on a fourth plane P 4  spaced from and arranged parallel to the second plane P 2 , wherein the fourth plane P 4  is spaced from the second plane P 2  in a direction opposite the spacing of the first plane P 1  from the second plane P 2 . 
     In the illustrated embodiment, the second header  150  includes the same structure as the first header  50  while arranged symmetrically thereto, hence each of the features described with reference to the first header  50  may be aligned with a corresponding feature of the second header  150  with respect to the longitudinal directions thereof. For example, the second header  150  includes a tube receiving portion  154  including a first planar portion  166   a , a second planar portion  166   b , and a centrally located offset portion  168 , wherein each of the features is aligned with a corresponding feature of the first header  50 . 
     As shown in  FIG. 1 , a first end portion  21  of each of the heat exchanger tubes  20  extending beyond the first header  50  is disposed in the first header tank  12  while a second end portion  22  of each of the heat exchanger tubes  20  extending beyond the second header  150  is disposed in the second header tank  14 . In the provided embodiment, each of the heat exchanger tubes  20  includes the same length to cause those first and second end portions  21 ,  22  passing through each of the offset portions  68 ,  168  of each of the headers  50 ,  150  to have an increasing length when progressing towards the center of the corresponding offset portion  168 . A length of a portion of each of the heat exchanger tubes  20  disposed between the first and second headers  50 ,  150  accordingly decreases when progressing towards the center of the corresponding offset portion  68 ,  168 . However, in other embodiments, the heat exchanger tubes  20  may be selected to include variable lengths corresponding to the shapes of the first and second headers  50 ,  150  in order to cause each of the end portions  21 ,  22  to have a common length disposed within each respective header tank  12 ,  14 , as desired. 
     Assembly of the heat exchanger  10  includes coupling the first and second headers  50 ,  150  to the first and second casings  30 ,  130  in the manner described hereinabove, assembling the heat exchanger core  16  into the configuration of  FIG. 1 , inserting the opposing first and second end portions  21 ,  22  of each of the heat exchanger tubes  20  into each of the opposing first and second header tanks  12 ,  14 , and then coupling the heat exchanger core  16  together while also coupling the heat exchanger core  16  to each of the opposing first and second headers  50 ,  150 . The coupling of the heat exchanger core  16  and the first and second headers  50 ,  150  may be accomplished by a brazing process, thereby forming a metal bonded joint at each intersection of each of the heat exchanger tubes  20  with the first and second headers  50 ,  150 . However, other forms of metal bonding, such as welding, may also be used without necessarily departing from the scope of the present invention, so long as a fluid tight seal is formed at each joint between the heat exchanger tubes  20  and the first and second headers  50 ,  150 . The heat exchanger core  16  and each of the first and second headers  50 ,  150  may accordingly be formed from the same metallic materials or from two complimentary metallic materials suitable for undergoing a metal bonding process such as brazing. The heat exchanger core  16  and the first and second headers  50 ,  150  may be formed from aluminium or alloys thereof, as one non-limiting example. It should be appreciated by one skilled in the art that the first and second headers  50 ,  150  may be coupled to the first and second casings  30 ,  130  via alternative means to those described herein while still appreciating the benefits of the present invention as described hereinafter. 
     In use, the first fluid enters the first header tank  12  through the first fluid port  44  where the first fluid is distributed to a first set of the heat exchanger tubes  20  extending into the first chamber  35 . The first fluid then passes through the first set of the heat exchanger tubes  20  while exchanging heat energy with a second fluid passing between the heat exchanger tubes  20 . The first fluid is then recombined within the second header tank  14  before passing through a second set of the heat exchanger tubes  20  while again exchanging heat energy with the second fluid. The first fluid is again recombined within the second chamber  36  of the first header tank  12  before exiting the heat exchanger  10  through the second fluid port  45 . The heat exchanger  10  accordingly includes a substantially U-shaped flow configuration with the partition  33  forming a boundary between the opposing flows of the first fluid through the heat exchanger tubes  20 . 
     During the passage of the first fluid through the heat exchanger tubes  20 , the heat exchanger tubes  20  may be cooled or heated over a given period of time, thereby causing the heat exchanger tubes  20  to increase or decrease in length due to the effects of thermal expansion and contraction over the same period of time. Varying conditions regarding the operation or specific configuration of the heat exchanger  10  may cause different ones of the heat exchanger tubes  20  to experience the first fluid at varying temperatures or to have varying degrees of heat transfer across each of the heat exchanger tubes  20 . Furthermore, a change in the mode of operation of the associate fluid system may cause the conditions within the heat exchanger tubes  20  to change drastically upon a reversal of the operational mode or the like, such as when the heat exchanger  10  is passable bi-directionally such that the first fluid has varying characteristics depending on the direction of flow thereof. 
     In some circumstances, the first fluid has a different temperature when reaching some of the heat exchanger tubes  20  than others due to a varying distance between the corresponding fluid port  44 ,  45  associated with distributing the first fluid to the heat exchanger tubes  20  and each of the end portions  21 ,  22  of the heat exchanger tubes  20 . In other circumstances, the first fluid may flow through some of the heat exchanger tubes  20  while having a different pressure, flow rate, degree of turbulence, or the like in comparison to other heat exchanger tubes  20 , thereby varying the heat transfer across the different heat exchanger tubes  20 . Still, in other circumstances, the flowing of the first fluid through the heat exchanger tubes  20  including two or more passes of the first fluid through the heat exchanger tubes  20  causes the first fluid to have a different temperature when encountering the downstream arranged heat exchanger tubes  20 , such as when flowing to either side of the partition  33  in the U-shaped flow configuration illustrated in  FIG. 1 . One skilled in the art should appreciate various other conditions or flow configurations causing the varying temperature of the different heat exchanger tubes  20  in addition to those described herein. 
     The expansion or contraction of the heat exchanger tubes  20  accordingly applies a stress at each metal bonded joint formed between each of the heat exchanger tubes  20  and each of the first and second headers  50 ,  150  that tends to separate or bring together the opposing first and second headers  50 ,  150 . The first and second headers  50 ,  150  may further experience a bending moment as a result of the expansion or contraction of the heat exchanger tubes  20  relative to each of the first and second headers  50 ,  150 , which introduces additional stresses to the first and second headers  50 ,  150 . A repeated change in the temperature of each of the heat exchanger tubes  20  may further cause the stresses to be cycled, and may even cause the stresses to be cycled in opposing directions depending on the variability of the temperature of each of the heat exchanger tubes  20 . Those heat exchanger tubes  20  subject to the greatest difference in temperature during use of the heat exchanger  10  or those heat exchanger tubes  20  having the greatest difference in temperature from the remaining heat exchanger tubes  20  may accordingly be the heat exchanger tubes  20  most likely to fail as a result of the thermal cycling as the greatest stresses will likely be present at these locations. 
     The present invention prevents the above mentioned failure from thermal cycling by better distributing the stresses experienced by the heat exchanger tubes  20 , the first and second headers  50 ,  150 , and each of the metal bonded joints formed therebetween. Specifically, the curvilinear shape of the offset portion  68  better distributes the stresses experienced by the first and second headers  50 ,  150  in comparison to a header having all tube to header joints formed on a single plane. The gradually curved shape of the offset portion  68  allows for the stresses experienced within each of the headers  50 ,  150  to be distributed across a greater length of each of the headers  50 ,  150  while also varying a plane on which the stresses act on each of the headers  50 ,  150 . The curvature of each of the offset portions  68 ,  168  further allows for each of the headers  50 ,  150  to be flexed in a desired manner when experiencing the thermal cycling, thereby causing a reduced stress to be carried through each of the metal bonded joints which in turn reduces a stress experienced by each of the heat exchanger tubes  20 . The reversal of the direction of projection of the second tube projections  86   b  when progressing inwardly further promotes the distributing of the stresses within the offset portion  68  without introducing sharp changes in direction that could introduce undesirable stress risers within each of the headers  50 ,  150 . Lastly, the varying of the distance between the opposing headers  50 ,  150  as a result of each of the offset portions  68 ,  168  extending towards each other actually causes those heat exchanger tubes  20  engaging each of the offset portions  68 ,  168  to have a decreased length between the opposing metal bonded joints thereof, which actually causes each of the heat exchanger tubes  20  to experience less thermal expansion or contraction due to the reduced dimension present between the opposing metal bonded joints. As such, a variation of the length between the opposing metal bonded joints due to thermal expansion or contraction is reduced in a manner reducing the stresses carried by each of the metal bonded joints. 
       FIG. 7  illustrates a header tank  212  having a header  250  according to another embodiment of the invention. The header tank  212  includes a casing  230  having a fluid port  244  disposed adjacent a first end of the casing  230 . The header  250  includes a tube receiving portion  254  having a planar portion  266  and an offset portion  268 . As can be seen in  FIG. 7 , the planar portion  266  and the offset portion  268  include substantially identical structure to the planar portion  66  and the offset portion  68  of the first header  50  illustrated in  FIG. 2 , with the only exception being that the offset portion  268  only increases in distance from a plane defined by the planar portion  266  when progressing away therefrom without a symmetrically arranged portion having a decreasing distance from the plane before returning to the plane. The offset portion  268  is accordingly formed at an end of the header  250  with an end of the offset portion  268  merging into the plane of a trough  270  of the header  250 . 
     The header  250  accordingly appreciates the same benefits as the first and second headers  50 ,  150  while simply moving the improved stress distribution from a centrally located region of the header  50  to an end region of the header  250 . As mentioned above, one possible configuration of a heat exchanger causing variable temperature distributions may include a varying distance between an associated fluid port and each of the heat exchanger tubes in fluid communication therewith. The header tank  212  accordingly illustrates one possible configuration wherein the offset portion  268  is selected to be arranged adjacent or in alignment with the corresponding fluid port  244  due to the temperature of the first fluid passing through the header tank  212  potentially being maximized or minimized when entering the fluid port  244  to cause the header  250  to be most susceptible to thermal cycling adjacent the fluid port  244 . 
     The header tank  212  may be used in conjunction with a secondary header tank (not shown) arranged substantially symmetric relative to the header tank  212  in both the vertical and horizontal directions from the perspective of  FIG. 7  to form a heat exchanger having a flow configuration wherein the first fluid enters the heat exchanger adjacent a corner of the heat exchanger before exiting the heat exchanger adjacent an opposing corner of the heat exchanger. 
       FIG. 8  illustrates a header  350  having an offset portion  368  according to another embodiment of the invention. The offset portion  368  of the header  350  is substantially identical to the previously disclosed offset portion  68  of the first header  50  except that the offset portion  368  includes a centrally located tube projection  386  and tube opening  385  as opposed to a pair of tube openings straddling a center of the offset portion  368 . An associated partition  333  may accordingly be caused to engage an associated sealing element  305  to either side of the centrally located tube projection  386 . The header  350  otherwise includes the same benefits as described herein with reference to the first header  50 . 
     The various different header configurations disclosed herein may be adapted for any type of heat exchanger having any type of flow configuration, as desired. In some embodiments, a single header may include multiple offset portions spaced from each other in the longitudinal direction of the header. For example, an end of the header may include an offset portion similar to that disclosed in  FIG. 7  while an internally located offset portion may be similar to that disclosed in  FIG. 2 . The header may include one of the offset portions corresponding to each feature of the heat exchanger likely to cause variations in thermal expansion thereof, such as each of the fluid ports associated with the header or each of the partitions changing a direction of flow of the first fluid. The disclosed header configurations may also be used in conjunction with a traditional header configuration including a planar array of tube openings, as desired. 
     From the foregoing description, one ordinarily skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications to the invention to adapt it to various usages and conditions.