Patent Publication Number: US-6662615-B2

Title: Method to reduce air center middle margin turnaround for folded tube applications

Description:
TECHNICAL FIELD 
     The present invention relates to heat exchangers and more particularly to the formation of the folded fins in a heat exchanger. 
     BACKGROUND OF THE INVENTION 
     Heat exchangers for a wide variety of applications, such as radiators, heater cores, condensers, and evaporators are well known in the art and are generally of a similar basic construction. The basic construction of such a heat exchanger typically includes two headers for the input and output of a heat exchanging liquid. A plurality of regularly spaced tubes extend between the two headers to permit the heat exchanging liquid to flow therebetween. A plurality of heat conductive fins, also known as an air center, occupy spaces between each pair of adjacent tubes and are oriented to permit a flow of air therethrough. Most typically, the fins are constructed as a convoluted folded fin, which when viewed from one edge are folded in an accordion-like pattern. The folded fins are bonded, most typically metallurgically bonded, to the sides of the tubes to enhance the heat transfer from the liquid flowing in the tubes to the folded fins so that the excess heat can be convectively transferred to an air stream flowing through the fins. Features of a typical prior art heat exchanger core  10  are illustrated in FIGS. 1 and 2. 
     Prior art heat exchanger core  10  typically comprises a plurality of tubes  12  having an elliptical shape wherein the width of tube ends  16  are generally smaller than the width of a central section  14  of tube  12  such that central section  14  forms a compressible crown. A plurality of air centers  18  formed as accordion-like folded fins are placed between adjacent ones of tubes  12 . Each air center  18  is generally comprised of a plurality of adjacent convolution legs  24 , wherein each convolution leg  24  is connected to a previous leg  24  by top tip radius  20  and to a subsequent convolution leg  24  by bottom tip radius  22 . Each convolution leg  24  can further have a plurality of louvers  26  formed therein for improved heat transfer properties. Prior art core  10  is assembled by abutting a plurality of tubes  12  and air centers  18  in an alternating fashion such that the tip radii  20 ,  22  of an air center  18  are abutted to facing sides of adjacent tubes  12 . Since tubes  12  have center sections  14  that are generally wider than tube ends  16 , the arranged tubes  12  and air centers  18  can then be compressed to form core  10  to a desired dimension. By compressing the stacked tubes  12  and air centers  18 , tip radii  20  and  22  can be caused to substantially contact the facing sides of tubes  12 . The compressed core  10  is then processed to bond tip radii  20  and  22  to tubes  12 , most typically by a metallurgical bonding process. 
     Each air center  18  in prior art core  10  is generally formed by passing a strip of heat conductive metal through a pair of intermeshing star-shaped form rolls similar to the rolls illustrated in FIG.  6 . The intermeshing of the star-shaped form rolls form the generally flat metallic strip into an accordion-like folded fin. A partial cross-section of a prior art form roll  30  is illustrated in FIG. 3 wherein FIG. 3 corresponds to the cross-section along the lines  7 — 7  of FIG.  6 . Form roll  30  is generally comprised of a plurality of discs  32 - 36  wherein each disc forms a specific portion of convolution legs  24  and tip radii  20  and  22 . Star-shaped discs  32 - 36  have a plurality of teeth about the circumference wherein each tooth has a top edge  38  and a valley  39  is defined at the bottom convergence of adjacent teeth. The teeth of outer discs  32  and  36  have a plain face  40  to form ends  25  of leg  24 . Discs  33  and  35  are positioned interiorly of discs  32  and  36 , and their teeth have faces  42  for forming louvers  26  in leg  24 . Stripper disc  34  is positioned between discs  33  and  35 . Stripper disc  34  has no teeth and has a diameter substantially less than the diameter circumscribed by valleys  39 . Stripper disc  34  in combination with adjacent discs  33  and  35  define a gap  44  between discs  33  and  35  to permit a stripper finger  46  to be at least partially contained therein. Stripper finger  46  facilitates the removal of formed air center  18  from form roll  30 , and thus remains below valleys  39  to permit the proper intermeshing of teeth from the discs of the two intermeshing form rolls. 
     The intermeshing form rolls produce three different specific characteristics of the air center  18 ; the angle of louvers  26 , the height of air center  18 , and the size of tip radii  20  and  22 . The form rolls operate under minimal clearance to produce the desired effect onto the blank heat conductive strip. The placement of stripper disc  34  corresponds to middle turnaround  28  of air center  18 . Stripper disc  34  does not come in contact with air center  18  but allows for the clearance of stripper finger  46  to enter form roll  30  without creating an interference with the heat conductive strip being formed thereon. The heat conductive strip is trapped by the mating top and bottom form rolls  30  and is in turn drawn over the corresponding edges  38  of the discs to form the top and bottom tip radii  20  and  22 . As the clearance between form rolls is reduced, the bend radii  20  and  22  of the heat conductive strip are also reduced, thereby resulting in sharper tip radii  20  and  22 . As tip radii  20  and  22  become sharper, the height of air center  18  correspondingly becomes higher. The gap  44  created by stripper disc  34  allows that portion of the heat conductive strip in the region between discs  33  and  35  to be pushed toward gap  44  rather than forming a clean bend at the radius. This interaction results in middle turnaround  28  to be at a higher height than the rest of each individual top and bottom tip radii  20  and  22 . Thus, as the mating top and bottom form rolls are setting the corresponding tip radii between them, the material at gap  44  is formed at a sharper radius, resulting in a higher center height of middle margin turnaround  28  of air center  18 . 
     During assembly, core  10  is compressed to meet a predetermined core package dimension prior to placing a header on the ends of tubes  12 . The height of air centers  18  should be substantially constant from convolution to convolution since a center of excessive height will cause air center  18  to collapse. Similarly, a center height that is too low will cause the air centers  18  to drop out of the core block  10  prior to bonding air centers  18  to tubes  12 . The higher middle margin turnaround  28  could thus interfere with the proper assembly of core  10 . However, as previously discussed, tubes  12  have a compressible crown  14  to permit some compression of tube  12  during assembly. This compressibility allows the increased height of air center middle margin turnaround  28  to be absorbed by the compression of tubes  12 . 
     In the past, tubes  12  have been fabricated of either welded or extruded construction. However, a folded tube  50  as shown in FIG. 4 has now been introduced into the construction of heat exchanger cores. Folded tube  50  is designed and formed to have legs  52  and  54  in middle section  56  of tube  50 . Legs  52  and  54  are non-compressible thereby removing the flexibility exhibited by tube  12  having a compressible crown center section  14 . The non-compressibility of center section  56  results from legs  52  and  54  bottoming out on an opposite side of the folded tube  50 . The tube sections between middle portion  56  and ends  58  do however, retain a degree of compressibility. Since legs  52  and  54  align directly with the high middle margin turnaround  28  of air center  18 , the excess height of middle margin turnaround  28  cannot be compensated for since center portion  56  is no longer compressible. Therefore, assembling a heat exchanger core from tubes  50  in combination with air centers  18  having an increased height middle margin turnaround  28  provide additional difficulties in insuring contact between the tip radii  20  and  22  with the sides of tubes  50  during assembly of the core  10 . Further, the combination of the middle margin turnaround  28  height with the non-compressible middle portion  56  height of tube  50  can cause collapsed air centers or a poor bond therebetween if the center height is too low. 
     Thus, there is a need for a method of forming an air center wherein its middle margin turnaround is at or below the height of the remainder of the tip radii. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention is an improved air center form roll for use in combination with a like form roll to produce an accordion-like folded fin for a heat exchanger core. The form roll being of the type comprising a plurality of form discs abutted one to the other and having a plurality of star-like teeth thereabout. Points of the star-like teeth define a major diameter of the form roll and valleys between adjacent ones of the star-like teeth define a minor diameter of the form roll. The form roll further includes at least one stripper disc having no teeth therearound and interposed between at least two of the plurality of form discs wherein the stripper disc has a diameter less than the minor diameter of the form roll. The improvement to the form roll comprises a modified disc having a plurality of modified star-like teeth thereabout. The modified disc is interposed at each abutment between the stripper disc and one of the abutting form discs. The modified disc further having a major diameter defined by points of the modified star-like teeth, the major diameter of the modified disc being smaller than the major diameter of the plurality of form discs. 
     Another aspect of the present invention is a method of improving a form roll utilized to form a folded fin for use in a heat exchanger core. The form roll being of the type comprising a plurality of form discs abutted one to the other and having a plurality of star-like teeth thereabout. Points of the star-like teeth define a major diameter of the form roll and valleys between adjacent ones of the star-like teeth define a minor diameter of the form roll. The form roll further includes at least one stripper disc having no teeth therearound and interposed between at least two of the plurality of form discs to abut with the two form discs. The stripper disc has a diameter less than the minor diameter of the form roll. The method includes the steps of fabricating a plurality of modified discs, each modified disc having a plurality of modified star-like teeth thereabout and further having a major diameter defined by points of the star-like teeth, the major diameter of the modified disc being smaller than the major diameter of the plurality of form discs. One modified disc is interposed between each abutted form disc and stripper disc, and the star-like teeth of the modified disc are aligned with the star-like teeth of the form discs. 
     Yet another aspect of the present invention is a method of making a folded fin air center having a reduced height middle margin turnaround for use in a heat exchanger core. The method includes the steps of providing a pair of form rolls, each form roll being of the type comprising a plurality of form discs abutted one to the other and having a plurality of star-like teeth thereabout. Points of the star-like teeth define a major diameter of the form roll and valleys between adjacent ones of the star-like teeth define a minor diameter of the form roll. The form roll further includes at least one stripper disc having no teeth therearound and interposed between at least two of the plurality of form discs. The stripper disc has a diameter less than the minor diameter of the form roll. The form discs abutting the stripper disc are modified form discs having modified star-like teeth therearound. Points of the modified teeth define a major diameter of the modified disc to be less than the major diameter of the form roll. The form rolls are rotated in opposite directions in a manner to cause the star-like teeth of one form roll to intermesh with the star-like teeth of the other form roll. A blank strip of heat conductive material is fed between the rotating form rolls. The teeth of each form roll are allowed to engage the blank strip. The blank strip is then formed over the point of an opposing tooth to form a tip radius adjoining two legs of a folded fin. The strip is caused to be further drawn over the point of the opposing tooth to form a leg having a middle margin turnaround in the region over the modified discs and the stripper disc wherein the height in this region is less than the height of the remainder of the leg over the region of the plurality of form discs. 
    
    
     These and other advantages of the invention will be further understood and appreciated by those skilled in the art by reference to the following written specification, claims and appended drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a segment of a prior art heat exchanger assembly of air center convoluted folded fins alternating with welded coolant tubes. 
     FIG. 2 is an elevation front view of a prior art air center fin. 
     FIG. 3 is a partial cross-sectional view of a disk stack utilized to form the prior art air center fins. 
     FIG. 4 is a cross sectional view of a folded coolant tube. 
     FIG. 5 is a is an elevation front view of an air-center fin formed according to a preferred embodiment of the present invention. 
     FIG. 6 is a is a side elevation view of the intermeshing disk stack utilized to form the air center fins with a reduced center margin. 
     FIG. 7 is a partial cross-sectional view of the disk stack of FIG. 6 taken along the Line  7 — 7 . 
     FIG. 8 is a view of a segment of a heat exchanger assembly of air center convoluted folded fins with a reduced middle margin alternating with folded coolant tubes. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For purposes of description herein, the terms “upper”, “lower”, “left”, “rear”, “right”, “front”, “vertical”, “horizontal”, and derivatives thereof shall relate to the invention as oriented in FIG.  7 . However, it is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification, are simply exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific dimensions and other physical characteristics relating to the embodiments disclosed herein are not to be considered as limiting, unless the claims expressly state otherwise. 
     Turning to the drawings, FIG. 5 shows an air center  60  having a leg  62  with middle margins  68  of reduced height formed by form rolls  70  which include one of the preferred embodiments of the present invention and is illustrative of its various components. 
     Turning to FIGS. 6-7, interneshing form rolls  70  and  71  according to an embodiment of the present invention are shown rotating in their respective rotational directions  74  and  76 . When blank heat conductive strip  78  is fed between form rolls  70  and  71  in direction  77 , teeth  72  of form rolls  70  and  71  engage strip  78  to create folded strip  80 . Folded strip  80  is later cut to various lengths to form the air center  60  as illustrated in FIG.  5 . 
     Air center  60  is formed in an accordion-like manner and includes a plurality of legs  62  wherein leg  62  is adjoined to a previous leg  62  by top tip radius  64  and is also connected to a subsequent leg  62  by bottom tip radius  66 . Each tip radii  64  and  66  has a middle margin turnaround  68  wherein the height of leg  62  from top tip radius  64  to bottom tip radius  66  is smaller at middle margin turnaround  68  than at the remaining portions of leg  62 . 
     To form air center  60  with a reduced height middle margin turnaround  68 , a form roll disc stack that is modified from the disc stack described with respect to FIG. 3, above is utilized. Such a modified form roll  70  is illustrated in FIGS. 6-7. Form roll  70  (form roll  71  being substantially identical thereto) is comprised of a stack of form discs  82 - 84  and  86 - 88  and at least one stripper disc  85 . Form roll  70  has a plurality of star-shaped teeth  72  wherein each tooth  72  has a top point  93 , the tops points  93  defining a major diameter of the form roll  70 . Each adjacent pair of teeth  72  adjoin at their respective bases to define valleys  94 , the valleys  94  defining a minor diameter of the form roll  70 . End discs  82  and  88  are spaced one from another and include plain faces  90  on each side thereof to form ends  65  of convolution leg  62 . Louver forming discs  83  and  87  are abutted to and positioned interiorly of end discs  82  and  88  respectively. Louver forming discs  83  and  87  include on each side of their respective teeth  72 , faces  91  configured to form louvers  63  in convolution leg  62 . Discs  82 ,  83 ,  87 , and  88  combine to define top point  93  of form roll  70 . 
     Modified discs  84  and  86  are positioned such that disc  84  is positioned interiorly of disc  83  and disc  86  interiorly of disc  87 . Each of discs  84  and  86  have a louver forming face  92  substantially coplanar with the louver forming faces  91  of discs  83  and  87 . Discs  82 - 84  and  86 - 88  have their respective teeth aligned to form teeth  72  with top points  93  and valleys  94  of form roll  70 . A stripper disc  85  is positioned interiorly of discs  84  and  86  and is the central disc of the disc stack. Stripper disc  85  has a diameter that is smaller than the diameter circumscribed by valleys  94  and thus in combination with discs  84  and  86  defines a gap  96  therebetween. A stripper finger  73  is at least partially received within gap  96  while remaining below valleys  94  so as not to interfere with the intermeshing teeth  72  and the formation of folded strip  80 . Discs  82 - 84  and  86 - 88  are constructed such that valley  94  is colinear thereacross. However, uppermost point  93  of tooth  72  is defined by the top edge of the teeth  72  of discs  82 ,  83 ,  87 , and  88 . Discs  84  and  86  have modified teeth  89  with a reduced pitch top edge  95  that circumscribes a diameter smaller than the diameter circumscribed by top points  93  of teeth  72 . Since faces  92  of discs  84  and  86  are substantially coplanar with faces  91  of discs  83  and  87 , top edge  95  of discs  84  and  86  have a larger tip radius than the top edge corresponding to top point  93  of discs  82 ,  83 ,  87 , and  88 . 
     In operation, as form rolls  70  and  71  rotate in their respective directions  74  and  76 , blank strip  78  is fed therebetween along direction  77  such that teeth  72  of the respective form rolls  70 ,  71  engage blank strip  78  and form folded strip  80  therefrom. Form rolls  70 ,  71  intermeshingly engage with respect to each other in a manner to produce the desired height of air center  60  and the corresponding tip radii  64 ,  66 . Top edge  95  of discs  84  and  86  being lower than the top edge  93  of the remaining discs results in an additional clearance above top edge  95 . The additional clearance above top edge  95  in combination with the larger radius thereof result in the portion of the top and bottom tip radii at middle margin turnaround  68  formed over the width of discs  84  and  86  to then form with a larger tip radius. The top edges  95  of discs  84  and  86  being of reduced height no longer push a portion of the strip  78  toward the region above stripper disc  85 . Discs  82 ,  83 ,  87  and  88  set the top tip and bottom tip radii  64  and  66  over ends  65  and louver banks  63 , and a smooth transition occurs at the tip radius above discs  84  and  86 . Since discs  84  and  86  do not push material into gap  96 , the tip radii formed in this area results in a middle margin turnaround  68  that has a height smaller than the remainder of leg  62 . As folded strip  80  exits from between form rolls  70  and  71 , stripper finger  73  being partially received in gap  96  causes folded strip  80  to disengage from the teeth  72  of form roll  70 . Folded strip  80  can then be cut to a desired length to form a desired air center  60 . 
     FIG. 8 illustrates a portion of a heat exchanger core  100  illustrating the mating of a folded tube  50  with adjoining air centers  60 . Tubes  50 , having a central portion  56  that is essentially non-compressible as a result of folded legs  52  and  54  is abutted to a top tip radius  64  of a first air center  60  and on an opposite side thereof to the bottom tip radius  66  of a second air center  60 . Air centers  60  are positioned along tube  50  such that central portion  56  is substantially aligned with the reduced height of middle margin turnaround  68 . Since the portion of tube  50  between center portion  56  and ends  58  remains compressible, a stack of a plurality of tubes  50  and air centers  60  can be compressed to conform to the overall required width of core  100 . In this manner top tip radii  64  and bottom tip radii  66  are substantially abutted against the respectively adjacent tubes  50  without middle margin turnaround  68  interfering with the non-compressible central portion  56  of tube  50 . After compression of the stacked core  100 , the tip radii  64  and  66  are sufficiently abutted to the sides of tube  50  to permit the metallurgical bonding therebetween to form the completed core  100 . 
     In the foregoing description those skilled in the art will readily appreciate that modifications may be made to the invention without departing from the concepts disclosed herein. Such modifications are to be considered as included in the following claims, unless these claims expressly state otherwise.