Patent Publication Number: US-9901966-B2

Title: Method for fabricating flattened tube finned heat exchanger

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to heat exchangers and, more particularly, to flattened tube and fin heat exchangers and the fabrication of same. 
     BACKGROUND OF THE INVENTION 
     Heat exchangers have long been used as evaporators and condensers in heating, ventilating, air conditioning and refrigeration (HVACR) applications. Historically, these heat exchangers have been round tube and plate fin (RTPF) heat exchangers. However, all aluminum flattened tube and fin heat exchangers are finding increasingly wider use in industry, including the HVACR industry, due to their compactness, thermal-hydraulic performance, structural rigidity, lower weight and reduced refrigerant charge, in comparison to conventional RTPF heat exchangers. 
     A typical flattened tube and fin heat exchanger includes a first manifold, a second manifold, and a single tube bank formed of a plurality of longitudinally extending flattened heat exchange tubes disposed in spaced parallel relationship and extending between the first manifold and the second manifold. The first manifold, second manifold and tube bank assembly is commonly referred to in the heat exchanger art as a slab. Additionally, a plurality of fins are disposed between each neighboring pair of heat exchange tubes for increasing heat transfer between a fluid, commonly air in HVACR applications, flowing over the outer surface of the flattened tubes and along the fin surfaces and a fluid, commonly refrigerant in HVACR applications, flowing inside the flattened tubes. Such single tube bank heat exchangers, also known as single slab heat exchangers, have a pure cross-flow configuration. In an embodiment of flattened tube commonly used in HVACR applications, the interior of the flattened tube is subdivided into a plurality of parallel flow channels. Such flattened tubes are commonly referred to in the art as multichannel tubes, mini-channel tubes or micro-channel tubes. 
     Double bank flattened tube and fin heat exchangers are also known in the art. Conventional double bank flattened tube and fin heat exchangers, also referred to in the heat exchanger art as double slab heat exchangers, are typically formed of two conventional fin and tube slabs, one disposed behind the other, with fluid communication between the manifolds accomplished through external piping. However, to connect the two slabs in fluid flow communication in other than a parallel cross-flow arrangement requires complex external piping. For example, U.S. Pat. No. 6,964,296 shows a flattened tube and fin heat exchanger in both a single slab and a double slab embodiment with horizontal tube runs and vertically extending fins. U.S. Patent Application Publication No. US 2009/0025914 A1 shows a double slab flatted tube and fin heat exchanger wherein each slab has vertical tube runs extending between a pair of horizontally extending manifolds and includes corrugated fins disposed between adjacent tubes. 
     SUMMARY OF THE INVENTION 
     A method is provided for fabrication of large, multiple slab flattened tube and fin heat exchangers. The disclosed method facilitates high volume semi-automated production. 
     In an aspect, a method is provided for assembling a flattened tube heat exchanger having a first tube bank and a second tube bank. The method includes: arraying a first plurality of flattened heat exchange tube segments in parallel spaced relationship; installing at least one spacer clip on a longitudinally extending edge of each heat exchange tube segment of the first plurality of flattened heat exchange tube segments; and arraying a second plurality of flattened heat exchange segments in parallel spaced relationship with each second heat exchange tube disposed in alignment with a respective one of the first heat exchange tube segments and engaging the at least one spacer clip installed on the respective one of the first heat exchange tube segments. The method further includes: mounting a first manifold to the respective first ends of each of the first plurality of flattened heat exchange tubes, mounting a second manifold to the respective second ends of the first plurality of flattened heat exchange tubes, mounting a third manifold to the respective first ends of each of the second plurality of flattened heat exchange tubes, and mounting a fourth manifold to the respective second ends of the second plurality of flattened heat exchange tubes, thereby forming a final assembly. The method further includes metallurgically bonding the plurality of first and second heat exchange tube segments to the respective manifolds. The metallurgical bonding may be accomplished by brazing the final assembly in a brazing furnace. 
     In an aspect, a method is provided for assembling a flattened tube finned heat exchanger having a first tube bank and a second tube bank. The method includes forming a tube array by: arraying a first plurality of flattened heat exchange tube segments in parallel spaced relationship; installing at least one spacer clip on a longitudinally extending edge of each heat exchange tube segment of the first plurality of flattened heat exchange tube segments; and arraying a second plurality of flattened heat exchange segments in parallel spaced relationship with each second heat exchange tube disposed in alignment with a respective one of the first heat exchange tube segments and engaging the at least one spacer clip installed on the respective one of the first heat exchange tube segments. The method further includes inserting a folded fin between each set of neighboring parallel first and second aligned flattened heat exchange tube segments to form a partially assembled fin and tube pack. The method further includes forming a final assembly by: mounting a first manifold to the respective first ends of each of the first plurality of flattened heat exchange tubes, mounting a second manifold to the respective second ends of the first plurality of flattened heat exchange tubes, mounting a third manifold to the respective first ends of each of the second plurality of flattened heat exchange tubes, and mounting a fourth manifold to the respective second ends of the second plurality of flattened heat exchange tubes. The method further includes metallurgically bonding the folded fins to the first and second heat exchange tube segments and the plurality of first and second heat exchange tube segments to the respective manifolds. The metallurgical bonding may be accomplished by brazing the final assembly in a brazing furnace. 
     In an aspect, the method includes limiting a depth of insertion of the respective ends of the first and second heat exchange tube segments into a respective one of the manifolds by disposing an insertion depth control rod in each manifold, and positioning each insertion depth control rod so as to extend parallel to a longitudinal axis of the manifold in which it is disposed and to oppose the direction of tube insertion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a further understanding of the disclosure, reference will be made to the following detailed description which is to be read in connection with the accompanying drawings, where: 
         FIG. 1  is a diagrammatic illustration of an exemplary embodiment of a multiple tube bank, flattened tube finned heat exchanger as disclosed herein; 
         FIG. 2  is a side elevation view, partly in section, illustrating an embodiment of a fin and flattened tube assembly of the heat exchanger of  FIG. 1 ; 
         FIG. 3  is a top plan view of the heat exchanger of  FIG. 1 ; 
         FIG. 4  is side perspective view, partly in section, illustrating placement of an embodiment of a spacer clip as installed during assembly of the multiple bank heat exchanger of  FIG. 1 ; 
         FIG. 5  is side perspective view, partly in section, illustrating placement of another embodiment of a spacer clip as installed during assembly of the multiple bank heat exchanger of  FIG. 1 ; 
         FIG. 6  is side perspective view, partly in section, illustrating placement of another embodiment of a spacer clip as installed during assembly of the multiple bank heat exchanger of  FIG. 1 ; 
         FIG. 7  is side perspective view, partly in section, illustrating placement of still an embodiment of a spacer clip as installed during assembly of the multiple bank heat exchanger of  FIG. 1 ; 
         FIG. 8  is a side perspective view, partly in section, illustrating another method of spacing the forward and aft tubes during assembly of the multiple bank heat exchanger disclosed herein; 
         FIG. 9  is a plan view, partly in section, illustrating assembly of the respective manifolds and tube banks during fabrication of the multiple bank heat exchanger as disclosed herein; 
         FIG. 10  is a plan view, partly in section, illustrating one method for assembly of an external fluid flow connection between the manifolds at the right side of the multiple bank heat exchanger illustrated in  FIG. 9 ; 
         FIG. 11  is a plan view, partly in section, illustrating another method for assembly of an external fluid flow connection between the manifolds at the right side of the multiple bank heat exchanger as illustrated in  FIG. 9 ; and 
         FIG. 12  is a side elevation view, partly in section, of a manifold wherein a stepped insertion depth control rod has been positioned. 
     
    
    
     DETAILED DESCRIPTION 
     There is depicted in  FIG. 1  in perspective illustration an exemplary embodiment of a multiple bank flattened tube finned heat exchanger  10  in accordance with the disclosure. The first heat exchanger slab  10 - 1  includes a first manifold  102 , a second manifold  104  spaced apart from the first manifold  102 , and a first tube bank  100  connecting the first manifold  102  and the second manifold  104  in fluid communication and including a plurality of heat exchange tube segments  106 , including at least a first and a second tube segment. Similarly, the second heat exchanger slab  10 - 2  includes a first manifold  202 , a second manifold  204  spaced apart from the first manifold  202 , and a second tube bank  200  connecting the first manifold  202  and the second manifold  204  in fluid communication and including a plurality of heat exchange tube segments  206 , including at least a first and a second tube segment. The first and second heat exchanger slabs  10 - 1 ,  10 - 2  are juxtaposed in generally adjacent relationship with the first manifold  102  of the first heat exchanger slab  10 - 1  and the first manifold  202  of the second heat exchanger slab  10 - 2  disposed at the refrigerant inlet side  12  of the heat exchanger  10  (i.e. the left side of the heat exchanger  10  as viewed in  FIG. 1 ) and with the second manifold  104  of the first heat exchanger slab  10 - 1  and the second manifold  204  of the second heat exchanger slab  10 - 2  disposed at the refrigerant outlet side  14  of the heat exchanger  10  (i.e. the right side of the heat exchanger  10  as viewed in  FIG. 1 ). Although a dual slab heat exchanger construction is depicted in  FIG. 1 , the design can be extended to multiple slabs with no limitation, primarily dictated by economics and available footprint. Also, a different number of refrigerant passes can be considered within each heat exchanger slab, primarily dictated by the refrigerant side pressure drop. 
     In the embodiment depicted in  FIG. 1 , the first manifolds  102 ,  202  and the second manifolds  104 ,  204  extend along a vertical axis. The plurality of heat exchange tube segments  106  extend longitudinally in spaced parallel relationship between and connect the first manifold  102  and the second manifold  104  in fluid communication. Similarly, the plurality of heat exchange tube segments  206  extend longitudinally in spaced parallel relationship between and connect the first manifold  202  and the second manifold  204  in fluid communication. It is to be understood, however, that one or both of the tube banks  100  and  200  may comprise one or more serpentine tubes having a plurality of heat exchange tube segments extending in longitudinally spaced parallel relationship and interconnected by return bends to form a serpentine tube connected at its respective ends between the respective first and second manifolds of the tube banks. 
     Referring now to  FIG. 2 , there is depicted, partly in cross-section, a plurality of tube segments  106 ,  206  of the dual slab arrangement of the multiple bank heat exchanger  10  shown in  FIG. 1  disposed in spaced parallel relationship, with a folded fin  320  disposed between each set of adjacent tube segment  106 ,  206 . In the depicted embodiment, each of the heat exchange tube segments  106 ,  206  comprises a flattened heat exchange tube having a leading edge  108 ,  208 , a trailing edge  110 ,  210 , an upper flat surface  112 ,  212 , and a lower flat surface  114 ,  214 . The leading edge  108 ,  208  of each heat exchange tube segment  106 ,  206  is upstream of its respective trailing edge  110 ,  210  with respect to air flow through the heat exchanger  10 . The interior flow passage of each of the heat exchange tube segments  106 ,  206  of the first and second tube banks  100 ,  200 , respectively, may be divided by interior walls into a plurality of discrete flow channels  120 ,  220  that extend longitudinally the length of the tube from an inlet end of the tube to the outlet end of the tube and establish fluid communication between the respective headers of the first and the second tube banks  100 ,  200 . In the embodiment of the multi-channel heat exchange tube segments  106 ,  206  depicted in  FIG. 2 , the heat exchange tube segments  206  of the second tube bank  200  have a greater width than the heat exchange tube segments  106  of the first tube bank  100  to provide an additional degree of flexibility for the refrigerant side pressure drop management. Also, the interior flow passages of the wider heat exchange tube segments  206  may be divided into a greater number of discrete flow channels  220  than the number of discrete flow channels  120  into which the interior flow passages of the heat exchange tube segments  106  are divided. 
     The second tube bank  200  of the second (rear) heat exchanger slab  10 - 2 , is disposed behind the first tube bank  100  of the first (front) heat exchanger slab  10 - 1 , with respect to the flow of air, A, through the heat exchanger  10 , with each heat exchange tube segment  106  directly aligned with a respective heat exchange tube segment  206  and with the leading edges  208  of the heat exchange tube segments  206  of the second tube bank  200  spaced from the trailing edges  110  of the heat exchange tube segments of the first tube bank  100  by a desired spacing, G. In the embodiment depicted in  FIG. 2 , the desired spacing, G, is established by an open gap, thereby providing an open water/condensate drainage space between the trailing edge  110  and the leading edge  208  of each set of aligned heat exchange tube segments  106 ,  206  along the entire length of the heat exchange tube segments  106 ,  206 . The ratio of the flattened tube segment depth and gap G is defined by thermal and drainage characteristics and may range between 1.2 and 6.0, with the optimum residing between 1.5 and 3.0. 
     The flattened tube finned heat exchanger  10  disclosed herein further includes a plurality of folded fins  320 . Each folded fin  320  is formed of a single continuous strip of fin material tightly folded in a ribbon-like fashion thereby providing a plurality of closely spaced fins  322  that extend generally orthogonal to the flattened heat exchange tubes  106 ,  206 . Typically, the fin density of the closely spaced fins  322  of each continuous folded fin  320  may be about 18 to 25 fins per inch (about 7 to 10 fins per centimeter), but higher or lower fin densities may also be used. In an embodiment, each fin  322  of the folded fin  320  may be provided with louvers  330 ,  332  formed in the first and third sections, respectively, of each fin  322 . The louver count and louver geometry may be different within each section of the fins  322  and may be related to the respective flattened tube depth. 
     The depth of each of the ribbon-like folded fin  320  extends at least from the leading edge  108  of the first tube bank  100  to the trailing edge of  210  of the second bank  200  as illustrated in  FIG. 2 . Thus, when a folded fin  320  is installed between a set of adjacent heat exchange tube segments in the assembled heat exchanger  10 , a first section  324  of each fin  322  is disposed within the first tube bank  100 , a second section  326  of each fin  322  spans the spacing, G, between the trailing edge  110  of the first tube bank  100  and the leading edge  208  of the second tube bank  200 , and a third section  328  of each fin  322  is disposed within the second tube bank  200 . In an embodiment (not shown) of the flattened tube finned heat exchanger  10 , with respect to the first tube bank  100 , the leading portion  336  of each folded fin  320  may extend upstream with respect to air flow through air side pass of the heat exchanger  10  so as to overhang the leading edges  108  of the flattened tube segments  106  of the first tube bank  100 . The ratio of the flattened tube segment depth (leading edge to trailing edge) to fin depth (leading edge to trailing edge) is defined by thermal and drainage characteristics and in an embodiment is positioned between 0.30 and 0.65, inclusive, and in another embodiment resides between 0.34 and 0.53, inclusive. Similarly, the ratio of the fin overhang to the flattened tube segment depth is defined by thermal and drainage characteristics and ranges between 0 and 0.5, inclusive, and in an embodiment is between 0.13 and 0.33, inclusive. 
     Heat exchange between the refrigerant flow, R, and air flow, A, occurs through the outer surfaces  112 ,  114  and  212 ,  214 , respectively, of the heat exchange tube segments  106 ,  206 , collectively forming the primary heat exchange surface, and also through the heat exchange surface of the fins  322  of the folded fin  320 , which forms the secondary heat exchange surface. In the multiple bank, flattened tube finned heat exchanger  10  disclosed herein, because the fins  322  of the folded fin  320  span the spacing, G, the ratio of the surface area of the primary heat exchange surface to the surface area provided by the secondary heat exchange surface may be selectively adjusted without changing the width of the tube segments or the spacing between parallel tube segments. Rather during the design process, the depth of the spacing, G, may be increased to increase the surface area provided by the folded fin  320 , thereby decreasing the ratio of primary to secondary heat exchange surface, or may be decreased to decrease the surface area provided by the folded fin plate  320 , thereby increasing the ratio of primary to secondary heat exchange surface. The ratio of primary heat exchange surface to secondary heat exchange surface may also be decreased by increasing the overall fin depth by increasing the distance by which the leading portion  336  of the folded fin  320  extends upstream with respect to air flow, A, beyond the face of the heat exchanger  10  and/or by reducing the number of flatted tube rows forming the tube banks of both the heat exchanger slabs. 
     In accordance with an embodiment of the method disclosed herein for fabrication of a multiple bank heat exchanger, to maintain during assembly of the heat exchanger the proper spacing, G, between the tube banks  100  and  200 , at least one spacer clip  40  is disposed between each set of aligned forward tube segments  106  and rear tube segments  206 . Typically, a plurality of spacer clips  40  may be disposed between disposed between each set of aligned forward tube segments  106  and rear tube segments  206 , the plurality of clips  40  being disposed at longitudinally spaced intervals, for example, such as illustrated in  FIG. 3 . When installed, each spacer clip  40  maintains a distance between the trailing edge  110  of each tube segment  106  of the first tube bank  100  and the leading edge  208  of each tube segment  206  of the second tube bank  200  equal to the desired spacing, G, through the fabrication process. The number of clips  40  disposed along the longitudinal length of a tube segment  106 ,  206  depends upon the length of the tube segment, In general, the longer the tube segments, the greater the number of clips  40  used. In an embodiment, the ratio between the spacing between clips  40  to the length of the heat exchanger tube segments may range between 1 to 2 and 1 to 8. 
     Various embodiments of the spacer clip  40  are illustrated in  FIGS. 4-7 . In the embodiment depicted in  FIG. 4 , the spacer clip  40  comprises a generally rectangular body  42  having a single groove  44  extending inwardly in an end face  46  of the body  42 , the groove  44  having a depth and a width. In the embodiment depicted in  FIG. 5 , the spacer clip  40  comprises a generally rectangular body  42  having multiple grooves  44  extending inwardly in an end face  46  of the body  42 , each groove  44  having a depth and a width. Such a clip forming a comb-like shape can extend over the entire heat exchanger height encompassing all the tubes. In this case, two fin strips will be positioned between the adjacent tubes on both sides of the comb-like clip. In the embodiment depicted in  FIG. 6 , the spacer clip  40  comprises a generally rectangular body  42  having a single groove  44  extending inwardly in each of the opposite end faces  46 ,  48  of the body  42 , each groove  44  having a depth and a width. In the embodiment depicted in  FIG. 7 , the spacer clip  40  comprises a generally rectangular body  42  having multiple grooves  44  extending inwardly in each of the opposite end faces  46 ,  48  of the body  42 , each groove  44  having a depth and a width. Once again, such a clip forming a twin comb-like shape can extend over the entire heat exchanger height encompassing all the tubes. Similarly, two fin strips will be positioned between the adjacent tubes on both sides of the twin comb-like clip. In the embodiment, the twin comb-like shape can represent an intermediate tube sheet where the grooves become holes through which the tubes are inserted during the assembly process. 
     When installed during assembly of the heat exchanger  10 , each spacer clip  40  receives a leading edge or a trailing edge of a respective one of the heat exchange tube segments  106 ,  206 . The width of each groove is sized relative to thickness of the respective heat exchange tube segments  106 ,  206  to ensure a snug interference fit of the respective heat exchange tube segment into the groove  44 . The depth of each groove  44  is sized relative to the width of the respective heat exchange tube segments  106 ,  206  to receive at least a substantial extent of the width of the respective heat exchange tube segment  106 ,  206 . The spacer clips  40  remain in position throughout the fabrication process and following completion of the fabrication process. 
     In the embodiments depicted in  FIGS. 4 and 5 , a second heat exchange tube segment  206  (i.e. the aft tube segment) is received in each groove  44  of each spacer clip  40  and the trailing edge  110  of the aligned first heat exchange tube segment  106  (i.e. the forward tube segment) abuts against the opposite end face  48  of the body  42  of the spacer clip  40 . In these embodiments, the distance between the base of each groove  44  and the end face  48  is equal to the desired spacing, G, to be maintained between the trailing edge  110  of the first heat exchange tube segment  106  (i.e. the forward tube segment) and the leading edge  208  of the second heat exchange tube segment  206  (i.e. the aft tube segment). 
     In the embodiments depicted in  FIGS. 6 and 7 , a second heat exchange tube segment  206  (i.e. the aft tube segment) is received in each groove  44  in the end face  46  of the body  42  of each spacer clip  40  and the trailing edge  110  of the aligned first heat exchange tube segment  106  (i.e. the forward tube segment) is received in each groove  44  in the opposite end face  48  of the body  42  of the spacer clip  40 . In these embodiments, the distance between to base of each groove  44  in the end face  46  of the body  42  and the base of each groove  44  in the end face  48  of the body  42  is equal to the desired spacing, G, to be maintained between the trailing edge  110  of the first heat exchange tube segment  106  (i.e. the forward tube segment) and the leading edge  208  of the second heat exchange tube segment  206  (i.e. the aft tube segment). 
     In an embodiment of the method disclosed herein for fabricating the flattened tube heat exchanger  10 , the first and second tube banks are assembled to form a multiple bank tube array. A first plurality of flattened heat exchange tube segments, for example the second (aft) heat exchange tube segments  206  forming the second tube bank  200 , are arrayed in parallel spaced relationship with their trailing edges  210  lying in a common plane. At least one spacer clip  40 , and generally multiple spacer clips  40  disposed at longitudinally spaced intervals, are installed on a longitudinally extending leading edge  208  of each heat exchange tube segment  206  in the array of flattened heat exchange tube segments forming the second tube bank  200 . The first tube bank  100  is then assembled by arraying a second plurality of flattened heat exchange segments  106  in parallel spaced relationship with each heat exchange tube segment  106  disposed in alignment with a respective one of the heat exchange tube segments  206  and engaging the at least one spacer clip  40 , or engaging each of the multiple spacer clips  40 , as the case may be, installed on the leading edge  208  of the respective one of the heat exchange tube segments  206 . 
     After the multiple tube bank assembly has been assembled, a folded fin  320  may be inserted between each set of neighboring parallel first and second aligned flattened heat exchange tube segments to form a partially assembled fin and tube pack. As noted previously, each folded fin  320  defines a plurality of fins  322  each of which extends continuously at least from the leading edges  108  of the heat exchange tube segments  106  of the first tube bank  100  to the trailing edges  210  of the heat exchange tube segments  206  of the second (aft) tube bank  200 , and may, if desired, overhang the leading edges  108  of the heat exchange tube segments  106  of the first (forward) tube bank  100 . 
     The final assembly of the multiple bank flattened tube finned heat exchanger  10  is constructed by: mounting the manifold  102  to the respective first ends of each of the plurality of flattened heat exchange tube segments  106  forming the first tube bank  100 , mounting the manifold  104  to the respective second ends of the plurality of flattened heat exchange tube segments  106  forming the first tube bank  100 , mounting the manifold  202  to the respective first ends of each of the plurality of flattened heat exchange tube segments  206  forming the second tube bank  200 , and mounting the manifold  204  to the respective second ends of the plurality of flattened heat exchange tube segments  206  forming the second tube bank  200 . The method further includes metallurgically bonding the folded fins  320  to the first and second heat exchange tube segments  106 ,  206  and the plurality of first and second heat exchange tube segments  106 ,  206  to the respective manifolds  102 ,  104  and  202 ,  204 . The metallurgical bonding may be accomplished by brazing the final assembly in a brazing furnace. 
     In a variation of the above described method, the folded fins  320  may be inserted into the assembled array of spaced parallel heat exchange tubes  206  forming the second tube bank  200  before assembling the first tube bank  100  in alignment with the second tube bank  200 . In this variation, after the spacer clips  40  are installed on a longitudinally extending leading edge  208  of each heat exchange tube segment  206  in the array of flattened heat exchange tube segments forming the second tube bank  200 , a folded fin  320  is inserted in the space between each set of neighboring heat exchange tube segments  206  in the array of flattened heat exchange tube segments forming the second tube bank  200 . Then, each of the heat exchange tube segments  106  forming the first tube bank  100  is installed in alignment with a respective one of the heat exchange tube segments  206  forming the second tube bank  200  and in engagement with one or more spacer clips  40 , thereby forming a tube and fin pack comprising an array of aligned forward heat exchange tube segments  106  and aft heat exchange tube segments  206  with a folded fin  320  disposed therebetween in an alternating arrangement, for example, as illustrated in  FIG. 1 . 
     Referring to  FIG. 8 , in another embodiment of the method disclosed herein for fabrication of the multiple bank flattened tube finned heat exchanger  10 , the spacer clips  40  are eliminated. In this embodiment, to maintain the proper spacing, G, between the tube banks  100  and  200  during assembly of the heat exchanger, a spacer tab  50  is cut in the fold between fins  322  of the folded fin  320  abutting upper surface of the aligned heat exchange tube segments  106 ,  206 . The spacer tab  50  is cut on three sides and bent back along its uncut base downwardly to provide a support surface on which the trailing edge  110  of the first heat exchange tube segment abuts when placed in assembly during the fabrication process. The cut in the fold of the fin is located such that the spacer tap  50  when bent back positions the trailing edge  110  of the first heat exchange tube segment  106  (i.e. the forward tube segment) at a distance from the leading edge  208  of the second heat exchange tube segment  206  equal to the desired spacing, G. It is to be understood that in practice, it would not be necessary to cut a spacer tab  50  in every fold of the folded fin  320 . Rather, spacer tabs  50  would be cut in selected folds at longitudinally spaced intervals along the length of the folded fin. 
     In this embodiment, after the heat exchange tube segment  206  are arranged in spaced, parallel arrangement on their respective trailing edges on a work surface to form an array of flattened heat exchange tube segments forming the second tube bank  200 , a folded fine  320  is inserted in the space between each set of neighboring heat exchange tube segments  206  in the array of flattened heat exchange tube segments forming the second tube bank  200 . Each folded fin has precut therein at least one spacer tab  50  as herein before described. Then, each of the heat exchange tube segments  106  forming the first tube bank  100  is installed in alignment with a respective one of the heat exchange tube segments  206  forming the second tube bank  200  and seated on the support surface of the spacer tabs  50 . The spacer tabs  50  are precut in selected folds of the folded fins  320  such that when seated on the support surface provided by the spacer tabs, the trailing edges  110  of the forward heat exchange tube segments  106  are spaced the desired spacing, G, from the leading edges  208  of the aft heat exchange tube segments  206 . 
     In the assembly of the heat exchanger  10 , it is desirable to limit the depth of insertion of the respective ends of the heat exchange tube segments  106 ,  206  into the manifolds  102 ,  104  and  202 ,  204 , respectively. During manufacture of the manifolds  102 ,  104 ,  202 ,  204 , slots  162  are cut, punched or otherwise machined into the manifolds at appropriate locations for receiving the ends of the tube segments  106 ,  206 . The receiving slots  162  are sized to receive an end of a respective one of the heat exchange tube segments  106 ,  206  in a snug interference fit. If the neighboring manifolds  104  and  204  or  102  and  202  are formed as a single piece extrusion or formed separately but welded or otherwise connected together, the slots  162  may be simultaneously punched in both manifolds of the pair. If the neighboring manifolds are separate bodies, an integral one-piece end cap covering each manifold end and maintaining a desired separation between the manifolds may be inserted simultaneously into the ends of the manifolds at each end of the paired manifolds to control manifold spacing during the simultaneous punching of slots  162  in the paired manifolds and during assembly of the heat exchange tube segments  106 ,  206  into the slots  162 . 
     Referring now to  FIGS. 9-11 , in accordance with an aspect of the method disclosed herein for fabrication of a multiple bank heat exchanger, an insertion depth control rod  160  is inserted into each manifold  102 ,  104 ,  202 ,  204  prior to assembly the manifolds to the respective ends of the heat exchange tube segments  106 ,  206 . Each insertion depth control rod  160  is positioned within the interior chamber of its respective manifold opposite the side of the manifold into which are formed the slots  162  into which the tube ends are to be inserted. During the assembly process, each tube end is inserted into a respective receiving slot  162  in a respective one of the manifolds  102 ,  104 ,  202 ,  204  until the end of the heat exchange tube segment strikes the insertion depth control rod  160  positioned in the manifold. The diameter of the insertion depth control rod  160  is sized relative to the interior dimension in the direction of insertion of the respective manifold in which the control rod is positioned to limit depth of insertion to a desired depth, thereby preventing over insertion of the tube ends into the interior chamber of the manifold. 
     In the embodiment depicted in  FIG. 9 , the insertion depth control rods  160  are of a uniform diameter along their longitudinal length and are positioned against the inside wall of the manifold opposite the slots  162 . In the embodiment depicted in  FIG. 10 , the insertion depth controls rods  160  are positioned away from the inside wall of the manifold, while still being positioned to extend longitudinally along the interior chamber of the manifold to limit the depth of insertion of the ends of the tube segments extending through the receiving slots  162 . In this embodiment, the insertion depth control rod  160  can include a stepped portion  164 , as illustrated in  FIG. 12 , which is sized to establish an interference fit with the inside wall of the manifold so as to hold the insertion depth control rod  160  in a desired positioned during the assembly process of inserting the ends of the tube segments into the receiving slots. 
     In the embodiment depicted in  FIG. 9 , the manifolds  104  and  204  in are connected in direct fluid flow communication through a flow passage defined by a central bore  242  in a block insert  240  positioned between the manifolds  104  and  204  as illustrated in  FIG. 9 . The block insert  240  is positioned such that the central bore  242  aligns with holes  244  and  246  formed through the respective walls of the manifolds  104  and  204 , respectively. So aligned a continuous flow passage is established through which refrigerant may pass from the interior of the second manifold  204  of the second tube bank  200  through the hole  246 , thence through the central bore  242  of the block insert  240 , and thence through the hole  244  into the interior of the second manifold  104  of the first tube bank  100 . The side faces of the block insert  240  are contoured to match and mate with the contour of the abutting external surface of the respective manifolds  104 ,  204 . The block insert  240  is metallurgically bonded, for example by brazing or welding, to each of the second manifolds  104  and  204 . 
     In the embodiments depicted in  FIGS. 10 and 11 , the neighboring manifolds  104  and  204  are connected in fluid flow communication through at least one external conduit  224  opening at a first end  226  into the interior chamber of the manifold  204  of the second tube bank  200  and opening at a second end  228  into the interior chamber of the manifold  104  of the first tube bank  100 . In fabrication of the heat exchange unit  10 , after assembly of the second manifolds  104  and  204  to the first and second tube banks  100 ,  200 , respectively, the first end  226  of the conduit  224  is inserted into a mating hole extending through the wall of the second manifold  204  of the second tube bank  200  and the second end  228  of the conduit  24  is inserted into a mating hole extending through the wall of the second manifold  104  of the second tube bank  100 . More than one conduit  224  may be provided to establish fluid flow communication between the second manifold  104  and the second manifold  204 . For example, a plurality of external conduit  224  may be provided at spaced longitudinal intervals. 
     In an embodiment of the method disposed herein, each conduit  224  is installed before the insert depth control rods  160  are removed from the manifolds  104 ,  204 . Thus, as illustrated in  FIG. 10 , the depth insertion control rods  160 , which are disposed along the inside wall of the manifold opposite the receiving holes  162 , limit the depth of insertion of the ends  226  and  228  into the manifolds  204 ,  104 , respectively, thereby preventing over insertion of the ends  226 ,  228  into the manifolds. 
     In another embodiment of the method disclosed herein, the depth insertion control rods  160  are removed from the manifolds  104 ,  204  and end caps secured to the respective ends of the manifolds before the external conduit  224 . To guard against an excessive depth of insertion of the first and second ends  226 ,  228  of the conduit  224  into the manifolds  104 ,  204 , respectively, a block or rod  230  may be temporarily positioned, as depicted in  FIG. 11 , between the conduit  224  and the external surface of the manifolds  104 ,  204  to restrict the depth of insertion of the first and second ends  226 ,  228  of the conduit  230  into the respective mating holes of the first manifold  104  and the second manifold  204 . After the first and second ends  226 ,  228  of the conduit  224  are metallurgically bonded, for example by brazing or welding, to the second manifolds  104  and  204 , respectively, the block  230  may be removed. 
     While the present invention has been particularly shown and described with reference to the exemplary embodiments as illustrated in the drawing, it will be recognized by those skilled in the art that various modifications may be made without departing from the spirit and scope of the invention. For example, it is to be understood that the multiple bank flattened tube finned heat exchanger  10  disclosed herein may include more than two tube banks. It is also to be understood that the tube banks  100 ,  200  could include serpentine tubes with the heat exchange tube segments  106 ,  206  being parallel linear tube segments connected by U-bends or hairpin turns to form a serpentine tube connected at its respective ends between the first manifold and the second manifold of the heat exchanger slab. Further, although the multiple tube bank heat exchanger disclosed herein is depicted having flattened tube segments, various aspects of the invention may be applied to multiple bank heat exchangers having round tubes or other forms of non-round tubes. Therefore, it is intended that the present disclosure not be limited to the particular embodiment(s) disclosed as, but that the disclosure will include all embodiments falling within the scope of the appended claims.