Abstract:
A method of finishing a length, particularly a leading end (66), of an aluminum heat exchanger core tube (30) that has a flattened, oblong cross section to facilitate lacing through an aluminum fin stack. A widthwise intermediate portion of the tube that, in the cross section, is intermediate opposite widthwise end portions is corrugated by squeezing the widthwise intermediate portion in the direction of the cross section thickness (FIGS. 5 and 6). While the corrugated widthwise intermediate portion is being held squeezed, the widthwise end portions of the tube cross section are reformed to size the tube end to a desired overall width and a desired overall thickness free of substantial springback when the corrugated widthwise intermediate portion ceases to be held squeezed (FIGS. 7, 8, 9).

Description:
REFERENCE TO A RELATED APPLICATION 
     Certain subject matter that is disclosed in the present application is also the subject of the commonly owned, co-pending patent application of the same inventor, METHOD OF MAKING A ROBUST GOSPER FIN HEAT EXCHANGER, Ser. No. 09/382,754, filed Aug. 25, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to heat exchange structures, and is particularly concerned with improvements in heat exchangers of the type disclosed in U.S. Pat. No. 5,501,270 issued Mar. 26, 1996 in the names of the present inventor and Barry W. Blumel. 
     2. Background Information 
     U.S. Pat. No. 5,501,270 shows a heat exchange structure that comprises a stack of metal fins laced together by parallel tubes. Consecutive fins are substantially uniformly spaced from each other throughout the stack. The fins comprise identical patterns of collared holes through which the tubes lace the stack. The tubes have oval transverse cross sections. In plan view, the fin holes have oval shapes just slightly larger then the oval cross sections of the tubes. The fins and tubes are brazed together around each hole through which a tube passes. 
     When the heat exchanger is in use, a first heat exchange fluid flows through the parallel tubes, and a second heat exchange fluid flows through the stack from a front face of the stack to a rear face of the stack. The second heat exchange fluid enters the front face from a direction that is generally perpendicular to the tubes. 
     Within the interior of the stack, a major portion of the area of each fin lies in a respective plane that is non-perpendicular to the direction from which the second fluid approaches it. In general, those interior portions of the fins are disposed in planes that are non-perpendicular to the direction from which the second fluid approaches the front face of the stack. 
     In addition to the collared holes, the interior portions of the fins contain rows of louvered slots arranged to cause the second fluid to flow through the slots as it passes through the stack. The novel core constructions disclosed in the referenced patent are efficient in respect of both heat exchange and fluid pressure drop. 
     SUMMARY OF THE INVENTION 
     In a general respect, the present invention relates to improvements in fabricating heat exchanger tubes, especially tubes having generally flat, oblong cross sections, that facilitate the lacing of tubes through fins stacks. 
     A general aspect of the within claimed invention relates to a method of finishing an end of a metal tube that comprises a nominally oval cross section having a width and a thickness, the method comprising: corrugating a widthwise intermediate portion of the tube end that, in the cross section, is intermediate opposite widthwise end portions by squeezing the widthwise intermediate portion in the direction of the cross section thickness; holding the corrugated widthwise intermediate portion squeezed, and while the tube end is being so held, reforming the widthwise end portions of the tube cross section to size the tube end to a desired overall width and a desired overall thickness free of substantial springback when the corrugated widthwise intermediate portion ceases to be held squeezed. 
     Another general aspect of the within claimed invention relates to a method of processing a length of metal tube that comprises a nominally oval cross section having a width and a thickness, the method comprising: corrugating a widthwise intermediate portion of the tube that, in the cross section, is intermediate opposite widthwise end portions by squeezing the widthwise intermediate portion in the direction of the cross section thickness; holding the corrugated widthwise intermediate portion squeezed, and while the tube end is being so held, reforming the widthwise end portions of the tube cross section to size the tube end to a desired overall width and a desired overall thickness free of substantial springback when the corrugated widthwise intermediate portion ceases to be held squeezed. 
     Other general and more specific aspects will been set forth in the ensuing description and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings that will now be briefly described are incorporated herein to illustrate a preferred embodiment of the invention and a best mode presently contemplated for carrying out the invention. 
     FIG. 1 is a plan view of a known tube used in certain heat exchangers. 
     FIG. 2 is a right side view of FIG. 1. 
     FIG. 3 is an enlarged and exaggerated view, in section, in circle 3 of FIG. 1. 
     FIG. 4 is an end view of a tube that is about to be processed in accordance with the present invention. 
     FIG. 5 is an end view of the tube of FIG. 4 during a step in the process. 
     FIG. 6 shows the shape that the tube end would tend to assume if it were not processed further. 
     FIG. 7 is an end view of the tube during another step in the process. 
     FIG. 8 is an end view of the tube during a further step in the process. 
     FIG. 9 is an end view of the tube after the step of FIG. 8, as taken in the direction of arrows 9--9 in FIG. 10. 
     FIG. 10 is a plan view of the tube after the step of FIG. 8. 
     FIGS. 11-18 is a sequence of related steps subsequent to the step of FIG. 10. 
     FIG. 19 is a cross section view in the direction of arrows 19--19 in FIG. 18. 
     FIG. 20 is an enlarged view of a portion of a tube within the area marked 20 in FIG. 19 
     FIG. 21 is a front elevation view of core structure of a heat exchanger that has been fabricated using steps shows in previous Figures. 
     FIG. 22 is an enlarged fragmentary cross section view related to the core structure. 
     FIG. 23 is a fragmentary perspective view related to the core structure. 
     FIGS. 24 and 25 are somewhat diagrammatic views useful in illustrating certain benefits of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1-3 illustrate an end of a tube 30 that has heretofore been used in the manufacture of core structures of heat exchangers like those shown in the referenced patent. Such core structures contains a number of such tubes that are have been laced through aligned holes in the fin stack and joined to the individual fins in the stack. The act of cutting a length of tube from tube stock may create a condition at the tube end which distorts the nominal cross section. The nominal cross section comprises an elongate oval having a width W and a thickness T. Generally parallel, longer sides 32 are joined at opposite ends by much shorter sides 34 which are essentially semi-circularly curved. The distortion is shown by the exaggerated view of FIG. 3 where it can be seen that the shorter sides 34 bulge outward to impart a somewhat bellmouthed shape to the tube end in the direction of the long dimension of the tube cross section. Small burrs 36 may also be present after the cutting. 
     Because of this distortion of the tube end, there may be a problem with lacing the tubes through the holes in the fin stack. If a hole is too small, a tube may not pass freely through the stack as it is being laced. The tube end may instead catch, snag, or hang up, on the edge of a fin hole before complete insertion through the stack. On the other hand, if the holes in the stack are sized sufficiently large to avoid that problem, it becomes more difficult to place the tubes in thermally conductive relationship with the fins because there is larger clearance between the nominal cross section of a tube and the fin holes through which it is laced. 
     FIGS. 5-10 disclose a series of steps for processing a tube end 30 like that in FIGS. 1-3 in order to avoid both of the aforementioned problems. FIG. 4 shows an initial shape for tube 30 like that described above. The distortion that has been described is not apparent in FIG. 4 due to the scale of the Figure, but it is present. In order to eliminate the effects of the bellmouth and of any burrs 36 on the lacing process, the end portion of tube 30 is squeezed in the direction of the cross section thickness between opposing metal dies 40, 42 in a suitable machine, such as a press. Only an intermediate portion of the tube cross section however is squeezed, leaving the shorter rounded ends of the cross section free. Confronting faces of dies 40, 42 that squeeze the tube comprise matching corrugations 44, 46 that act on the widthwise intermediate portion of the tube end to corrugate that portion. The corrugations may be considered to have a somewhat sinusoidal shape, as shown. The tube is squeezed to an extent that forces the opposite sides 32 against each other. If the dies were to be retracted, sides 32, although now corrugated, would exhibit some degree of springback that would separate them, as shown by FIG. 6. However, instead of being retracted, the dies continue to hold the sides 32 against each other as in FIGS. 7 and 8 while a further operation that reforms the widthwise end portions 34 of the tube cross section is performed. 
     That operation comprises forcing respective dies 48, 50 over the respective widthwise end portions of the tube cross section that protrude from the sides of the closed dies 40, 42. Each die 48, 50 comprises a respective cavity 52, 54 that engages the respective protruding widthwise portion of the tube, and that has a shape for reducing the extent to which the respective portion protrudes from the closed dies 40, 42 in the direction of the tube width W, and for coining any burrs 36 that may be present. Dies 40, 42 can coin any burrs that are in the corrugated portion. Moreover, the cavity shapes, and the extent to which the protrusions are shortened in the direction W, are chosen such that when dies 48, 50 are retracted, followed by retraction of dies 40, 42, the cross section of the tube end will have an overall width and thickness that do not exceed the nominal width W and nominal thickness T. It is especially desired that the final shape, as shown by FIGS. 9 and 10, have a width that is less than the nominal width and a thickness less than the nominal thickness. In other words, after all dies have been retracted, the tube end has been sized to a desired overall final width and a desired overall final thickness, free of substantial springback. Although the foregoing has described processing only an end portion of a tube, it should be understood that a tube may be processed up to as much as its full length in the manner described. 
     FIGS. 11-18 disclose a series of steps in fabricating a heat exchanger core utilizing tubes that have been processed in the manner of FIGS. 4-10. The finished heat exchanger core 56, and certain of its details, are shown in FIGS. 19-22. FIG. 11 shows a stack 58 of individual heat exchanger fins 60 sandwiched between header plates 62, 64. Fins 60 are identical, each having a matching hole pattern comprising individual collared holes each of which is adapted to be laced by a tube 30. When fins 60 are arranged in registration to form stack 58, each hole of an overlying or underlying fin assumes registration with a corresponding hole of an underlying or overlying fin. A uniform spacing distance between consecutive fins in the stack is maintained by abutment of one fin with the collars that surround each hole of a consecutive fin. 
     The leading end 66 of tube 30 has been processed according to FIGS. 4-10 and has clearance to the holes in the fins through which it passes when laced into stack 58. Clearance holes are also present in header plates 62, 64. FIG. 11 shows the relative positions of parts prior to lacing tube 30 through stack 58. As the lacing begins, tube 30 is inserted through a hole in header plate 62 into stack 58, leading end 66 first. FIG. 12 shows the lacing partially complete. FIG. 13 shows the completed lacing where tube 30 has passed completely through the stack, including passing through holes in header plates 62, 64. 
     Next a mandrel 68 is introduced into the interior of tube 30 at the end opposite the end that was processed in accordance with FIGS. 4-10. The mandrel is then advanced through the tube. FIG. 14 shows relative positions of parts prior to mandrel insertion, FIG. 15 shows relative positions at an intermediate stage where mandrel 68 has been inserted and partially advanced, and FIG. 16 shows relative positions after full advancement of mandrel 68. The distal end 70 of mandrel 68 has a cross section that is enlarged from that of the remainder that enters the tube. That enlarged distal end has a transverse cross sectional shape that passes freely through those portion of the tube of nominal oval cross section that have not been corrugated. However, wherever the tube has been corrugated according to the processing of FIGS. 4-10, the enlarged distal end of the mandrel engages the inner wall surface of tube 30 to expand the corrugations as it passes along them. Wherever corrugations in a tube pass through a collared hole in a fin, their expansion by distal end 70 of mandrel 68 forces them against the collar of the hole to thereby create a certain mechanical joining between tube and fin. That joining is sufficient to maintain the fin and tube in assembly relationship until they can be brazed together at a brazing operation that is subsequently performed. In its expanded corrugated cross sections, each tube has a flow area that is almost as large as those in its uncorrugated cross sections. FIGS. 17 and 18 illustrate withdrawal of mandrel 68 out of tube 30. 
     Wherever any tube 30 contains corrugations that have been created by the process of FIGS. 4-10 and those corrugations pass through a collared hole, the corrugations in the tube expand against the collared hole in the manner portrayed by FIGS. 19 and 20. Even if only the lower tube ends have been corrugated and they are expanded against the holes in only the lowermost fins in the stack, the resulting joints are sufficient to maintain all the fins and laced tubes in proper assembly relationship during handling of the core structure until the fins and tubes are brazed together at all collared holes through which the tubes pass as long as the core structure is maintained substantially upright. With the core structure upright, each higher fin in the stack continues to be supported on a lower one via the collars surrounding the holes in one of the two fins with the spacing distance established by the height of the collars. 
     If the tubes have also been corrugated where they pass through the uppermost fins in the stack, those corrugations too will be expanded against the holes in those uppermost fins, and intermediate fins that are between the uppermost and the lowermost fins will be captured in the stack regardless of the presence or absence of any corrugations in the tubes between the uppermost and lowermost fins to which the tubes have been staked. Of course, in the presence of such corrugations, the tubes will be staked to those intermediate fins, too. 
     While it is preferred that header plates 62, 64 be staked to the tubes as the fins are being staked, it should be appreciated that principles of the invention contemplate that one of both header plates can be assembled to a core structure in any suitable manner after the tubes have been staked to the fins in the manner described. 
     Although it was mentioned above that up to an entire length of a tube could be corrugated, it is preferred that the tube ends at which the mandrels enter not be corrugated. It is believed that leaving a short length of each tube free of corrugations at the end through which a mandrel enters facilitates mandrel entry into a tube by avoiding potential interference that might have an undesired effect on the outcome of the staking process. 
     FIG. 21 illustrates finished heat exchanger core structure 56, including header plates 62, 64. In the finished heat exchanger, tanks (not shown) are assembled to top and bottom of the core structure, with tubes 30 opening at one end to the interior of one tank and at the opposite end to the interior of the other tank. 
     In a representative use of a heat exchanger that comprises core 56, liquid flows from one tank through tubes 30 to the other tank while gas flows through stack 58 in the manner suggested by arrow 80 in FIGS. 18, 22, and 23. FIGS. 22 and 23 show a representative embodiment of gosper fins like one of those in the above-referenced patent. Each fin comprises identical spaced apart rows 82 of louvered slots 84. The inner rows 82 are between adjacent tubes 30 while the two outer rows are outboard of the two outboard tubes 30. Upstream and downstream margins 86, 88 of fins 58 are essentially parallel to the incident gas flow entering the core of the heat exchanger. The more expansive intermediate area of each fin between its margins 86, 88 is inclined to the incident flow, and it is in that area of each fin that the louvered slots 84 are disposed. As suggested by the arrows in FIG. 22, the gas can flow through the louver slots thus passing across surfaces of multiple fins as it wends its way through the core. FIG. 22 shows collars 90 forming the collared holes in the fins through which the tubes pass and which set the spacing distance between fins in the stack. 
     FIGS. 24 and 25 illustrate the benefit of fabricating a heat exchanger using tubes 30 processed by the process of FIGS. 4-10. Because the lead end of a tube has a smaller cross section, while an immediately following portion of the tube length has a larger one, the lacing of a tube through the stack is analogous to gun drilling, portrayed by FIG. 25. A tube 30 is kept straight as it passes through aligned holes in the stack, and does not experience a snow plow effect, as portrayed by FIG. 24, where a tube like the one in FIGS. 1-3 is not kept straight and is hence prone to snagging. 
     Aluminum is typically used for both fins and tubes, and it is a preferred material in the practice of the present invention. While the foregoing description has referred to the tubes and holes as having specific oval shapes, as in FIG. 4 for example, it is to be appreciated that reference to an oval shape means any generally oblong, flattened shape. A specific example of a tube that is suitable for use in the practice of the invention is 3003 or 3005 aluminum having an oval cross section like that in FIG. 4 with a width W of about 2.08 millimeters, a length of about 25.97 millimeters, and a nominal wall thickness of about 0.33 millimeters. 
     While a presently preferred embodiment has been illustrated and described, it is to be appreciated that the invention may be practiced in various forms within the scope of the following claims.