Patent Publication Number: US-9415436-B2

Title: Method and apparatus for bending a micro-channel heat exchanger

Description:
The invention relates to a bent micro-channel heat exchanger and a method to manufacture the same. Priority is claimed to U.S. provisional application 61/188,439, filed Aug. 8, 2008. 
    
    
     TECHNICAL FIELD OF INVENTION 
     Background of Invention 
     Brazed aluminum heat exchangers of the type having spaced header tanks (or manifolds), flat elongated tubes corrugated air fins or centers have been a commonplace in automotive applications, where they are of a relatively small face area and installed flat, such as air conditioning condensers. It is known to bend such automotive heat exchangers into a V or U shape, as shown in U.S. Pat. No. 4,876,778, but this is a relatively simple and straighforward bend in which the tubes and fins (core face) themselves are bent, perpendicular to the tubes, not the heavier manifolds themselves, which remain straight 
     That same U or V shaped bend of the core face can be applied to stationary air conditioning applications as well (residential heat pump, for example), but such applications often require a more difficult bending operation in which the tubes are left unbent, straight, and vertical, while the manifolds are bent into a rectangular perimeter. The vertical tubes drain condensation better, but the manifolds are heavier and more difficult to bend. Several different bending apparatuses and methods are known. A typical apparatus consists of a cylindrical solid mandrel that engages the core face, between the manifolds, and opposed flat clamps engaging the outer core face and/or manifolds, one of which is held stationary and the other of which is swung in to bend the core around the cylindrical mandrel. Another issue is the behavior of the tubes and fins at the “corners” where the manifolds are bent. These can buckle and deform, presenting at least an aesthetic objection, if not a dimunition in performance. Fins may also pull away from the tubes in the bend area, decreasing performance. This limits how tight or small a bend radius can be achieved. 
     Published Japanese application JP-2005090806 shows the basic bend configuration described above, and discloses some prior approaches to the bending problem. The most basic approach is to simply remove (leave out) the tubes and fins at the corners, and to cover the resulting open windows with a screen of some sort in the final installation. This has the obvious drawback of removing a considerable amount of heat exchange area out of the core face, besides necessitating the addition of some sort of screen at the corners to “fill in” the missing area and avoid disturbance of the forced air flow at the paths of least resistance. Alternate approaches proposed by JP-200509086 include removing only the fins at the bend corners, and placing the tubes more closely together in that area, and also brazing the corrugated fins to only one side of the tubes in the areas of the bend. All of these represent major changes to the way in which the basic core is stacked and brazed, and are therefore very undesirable in terms of cost and productivity. 
     Other methods shown in published US patent applications assigned to the assignee of the present application may be seen US2007227695 and US2008202733. The former discloses an air center of greater height that is located at the center of the bend, and which is more accommodating of the crush that occurs. The latter shows a dedicated bend spacer located in the same spot, which is designed only to act as a crush accommodater, and not as an air fin. While both systems improve the bend by accommodating or absorbing the crush, neither serves to actively control the behavior and alignment of the tubes in the area of the bend. 
     It is desirable to have an improved design of a heat exchanger and a method of manufacturing the improved heat exchanger that does actively control the tube alignment in the area of the bend, and so allows for easier, tighter bending without damage, and without significant change to the structure and manufacture of the basic core. 
     SUMMARY OF THE INVENTION 
     The heat exchanger design and the method of manufacturing and apparatus disclosed control and minimize the crushing of the air centers and buckling of the refrigerant tubes when the core is bent. A portion of the tube edges on the inside of the core bend is exposed by narrowing or offsetting the corrugated fins in the bend area. The tube edge offset provides room for a corresponding set of grooved vertical braces, one for each tube edge, to engage the clear portions of the tube edges. The vertical braces are fixed in the proper orientation by a flexible backing that allows them to bend from an initial flat shape on the core face into a bend radius, matching the typical cylindrical mandrel that controls the inner radius of the bend. The improved design allows a tighter radius than a conventional bend through the refrigerant tubes and centers. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       This invention will be further described with reference to the accompanying drawings in which: 
         FIG. 1  is the cross sectional view, taken through the tubes and between the manifolds, of a pre bent core. 
         FIG. 2  is a view like  FIG. 1  post bend. 
         FIG. 3  is a portion of the cross sectioned core in the section to be bent, in the process of being bent, with the novel apparatus of the invention in place, 
         FIG. 4  is a schematic view of the core being placed in the bend tooling, 
         FIG. 5  is a view of the tooling in operation. 
     
    
    
     DESCRIPTION OF INVENTION 
       FIG. 1  shows the flat, unbent core  10 , which consists of flat, parallel, regularly spaced tubes  12  extending between parallel upper and lower cylindrical manifolds  14 . Only a section thereof is shown, in the area where a bend would occur, and the remainder of the core would be identical. These basic parts of the core are conventional as to size, shape and material, typically a brazable aluminum alloy. The basic core differs only as to the particular corrugated air centers or fins that are installed between the tubes  12  encompassed by and within those areas intended to be bent. Outside of the bend areas, the fins  16  are also conventional as to size, shape and installation orientation. Most significantly, those fins  16  have a width substantially equal to the depth of the tubes  12 . The remaining centers  18 , those installed between those tubes encompassed by the bend areas, are significantly narrower and, in this embodiment, installed centrally between the tubes  12  so as to create a tube edge to fin clearance on both faces of the core  10 . This represents some change to the assembly process, requiring that the narrower centers  18  be installed just in select areas, and pushed into place with a spacer block or the like to set and maintain the tube edge clearance. However, the basic tube pitch and spacing remains the same, as would the stacker apparatus. 
       FIG. 2  shows core after bending, and shows that bend is distributed only over those narrower centers  18  and the associated tubes  12 , in a relatively tight radius of approximately 5 inches or less, with substantially no crush or deformation. This is accomplished by the apparatus and method described next. 
     Referring next to  FIG. 3 , the additional and novel apparatus used in the method of the invention is indicated generally at  24 . A series of solid metal braces  26 , one for each tube  12  in the bend area, each consists of an elongated rail with a width approximately equal to the spacing or pitch between tubes  12 , a thickness approximately equal or slightly more than clearance between the offset centers  18  and the edges of the tubes  12 , and a length substantially equal to the cross manifold length of the tubes  12 . Each brace  26  has a central groove  28  that closely receives the edge of a tube  12 , and a rounded edge  30  that engages the rounded edge  30  of an adjacent brace  26 . The braces  26  can be held together as a unitary apparatus in the edge to edge, parallel formation shown by a flexible backing  32 , which could be urethane or a similar material. The set of braces  26  can then be installed as one unit onto and over the edges of the tubes  12  in the bend area, and the back face of the apparatus rests nearly flush the inner face of the rest of the core  10 . 
     Referring next to  FIGS. 4 and 5 , the modified core  10 , and the novel braces  26  accommodated thereby, cooperate with a conventional bending apparatus to create the improved bends. A typical bending apparatus includes a cylindrical bending mandrel  34 , which has a clamp back up plate  36  fixed to one side. The core  10 , with braces  26  in place on the inside of the intended bend area, is placed between and clamped closely between the mandel back up plate  36  (inner core face) and an opposed clamp  38  (outer core face) with the braces  26  oriented over the 12 to 9 o&#39;clock quadrant of the cylindrical mandrel  34 . A swinging contact plate  40  is designed to engage the outer core face on the opposite side of the bend area and to swing forcefully down, about the pivot point P shown, to approximately a vertical position. As the bending occurs, the pattern of braces  26  bends around the mandrel  34  along with the core  10 , serving to actively keep the inner core face edges of the tubes  12  in the bend area aligned and undeformed. The adjacent cylindrical or rounded edges  32  roll around each other as the backing  34  flexes, allowing the grooves  28  to fan out and keep the inner edges of the tubes  12  in proper alignment. Concurrently, the outer core face edges of the same tubes  12  will fan out more evenly, by virtue of the inner edges having been maintained in alignment. While the bending apparatus and method steps (at least after the placement of the braces  26 ) are not significantly changed, the end result of the bent core is significantly improved, both as to the symmetry and lack of deformation and as to the tightness of the bend radius, which may be approximately 5 inches or less 
     Alternate embodiments of the core disclosed could be used, so long as edge clearance to accommodate the braces  26  was provided. will work, since all provide the clearance for the placement of the braces  26 . A narrower fin could be placed offset from the inner core face all the way to the core back face, rather than centered as show. Or, a conventional width fin could be offset from the inner core face and left to overhang the outer core face to an extent. The narrower, centered fin  18  shown may be best adapted in as well as providing a core  10  with no preferred orientation as to which face will accept the braces  26 , has no fin to tube attachment near the outer edges of those tubes  12  in the bend area. Consequently, as those outer tube edges fan out, there will be less tendency of the tube to fin braze joints to tear. This also enables tighter bend radii. 
     While this invention has been described in terms of the preferred embodiments thereof, it is not intended to be so limited, but rather only to the extent set forth in the claims that follow. Variations possible would include the complete absence of air centers in the bend area, which can be conceptualized as the air centers becoming vanishingly narrow, in effect, providing the ultimate tube edge clearance in the bend area to accommodate braces. In that event, with the extra and in fact complete clearance, the braces could support more than just the inner core face edges of the those tubes encompassed by the bend area, and could consist potentially of something like full width rubber blocks under compression that would support the entire profile of those tubes during bending, going into further compression on the inside of the neutral axis of the bend and going into less compression on the outside of the neutral axis. Fins of some sort in the bend area are preferred, however, as they add performance and prevent path of least resistance air flow out of the corners in operation. The braces  26  could, conceivably, be placed one at a time on the tube edges, especially if the core lay horizontal the inner face up, and the mandrel could keep them in place throughout the bend. It is advantageous to hold the braces together as a unit by some sort of flexible medium, however.