Abstract:
A fin collar having a shape that enhances flux application and brazing clad flow into the tube to fin joint to provide an improved thermal and structural bond.

Description:
PRIOR RELATED APPLICATION  
       [0001]     The present application is a continuation application based on U.S. patent application Ser. No. 10/761,448 filed Jan. 20, 2004. 
     
    
     FIELD OF INVENTION  
       [0002]     This invention is directed to heat exchanger fin collars, and more particularly to an improved fin collar for use in a brazed plate fin heat exchanger.  
       BACKGROUND OF THE INVENTION  
       [0003]     Plate fin and tube heat exchangers are used in a wide variety of applications including, but not limited to, air conditioning and refrigeration where it is desired to exchange heat between two fluids, usually a pure liquid or a liquid undergoing a phase change to or from a gas, flowing in the heat exchanger tubes and a gas, usually air, flowing around the heat exchanger plate fins and tube exteriors. In such a heat exchanger, a plurality of thin plate fins are arranged parallel to each other between two tube sheets. Heat exchanger tubes pass through holes in the tube sheets and plate fins. There is a firm fit between the tubes and the plate fins so that the effective surface area, and thus the heat transfer area, of the heat exchanger tubes is increased by the area of the plate fins. Because of this increase in surface area, a plate fin and tube heat exchanger offers improved heat transfer performance over a plain tube type heat exchanger of the same size.  
         [0004]     A common method of manufacturing this type of heat exchanger is to first assemble a plurality of plate fins between two tube sheets, then lace a plurality of hair pin tubes through selected holes in the plate fins and similar holes in each of the tube sheets. Next, bells are formed in the end of hairpin tubes, then the legs of the tubes are expanded to insure a tight mechanical fit between the tubes and plate fins.  
         [0005]     In order to improve the thermal and structural bond resulting from mechanical joining of the tubes and plate fins, there is a need for a brazed plate fin heat exchanger with an improved braze joint at the tube-to-fin joint.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention meets the above-described need by providing a fin collar having a shape that enhances flux application and brazing clad flow into the tube-to-fin joint to provide an improved thermal and structural bond. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The invention is illustrated in the drawings in which like reference characters designate the same or similar parts throughout the figures of which:  
         [0008]      FIG. 1  is a perspective view of a plate fin heat exchanger of the present invention;  
         [0009]      FIG. 2  is a perspective view of a tube of the present invention disposed through several plate fins;  
         [0010]      FIG. 3  is a perspective view of a fin collar of the present invention;  
         [0011]      FIG. 4  is a perspective view of an alternate embodiment of the fin collar of the present invention;  
         [0012]      FIG. 5  is a perspective view of another alternate embodiment of the fin collar of the present invention;  
         [0013]      FIG. 6  is a perspective view of the fin collar of the present invention shown with a representation of the air flow over the collar; and,  
         [0014]      FIG. 7  is a top plan view of the fin collar of  FIG. 6 . 
     
    
     DETAILED DESCRIPTION  
       [0015]      FIG. 1  depicts a plate fin and tube heat exchanger  10  containing plate fins  12  that embody the present invention. Each plate fin has a plurality of holes  16 . A common method of manufacturing heat exchanger  10  is to first assemble a plurality of plate fins  12  between two tube sheets  18 , then lace a plurality of hairpin tubes  20  through selected holes  16  in the plate fins  12  and similar holes  16  in each of tube sheets  18 . The heat exchanger assembly is completed by fitting up a plurality of return bends  22  to the ends of hairpin tubes  20  so as to form one or more closed fluid flow paths through the tubes of the heat exchanger.  
         [0016]     When installed and operating in a device such as an air conditioner, a first fluid, such as a refrigerant, flows through heat exchanger  10  via a fluid flow path or paths defined by interconnected hairpin tubes  20  and return bends  22 . A second fluid, such as air, flows over and around plate fins  12  and tubes  20 . If there is a temperature differential between the two fluids, then heat transfer from the warmer to the cooler of the two takes place through the tube walls and plate fins.  
         [0017]     Turning to  FIG. 2 , a single tube  20  is shown disposed through a plurality of plate fins  12 . Each plate fin  12  is provided with an upstanding fin collar  30  disposed around the openings  16 . As shown, the collar  30  may be curved so that a convex surface  31  faces the tube  20 . The number of plate fins  12  that can be placed around the tube  20  is determined by the height of the collar  30 .  
         [0018]     In order to manufacture the heat exchanger of the present invention, the tube  20  to fin  12  joint is brazed in a controlled atmosphere braze furnace. The brazing temperatures will range between 1070° F. and 1120° F. depending on the clad used.  
         [0019]     The tube  20  may be constructed of an aluminum alloy that is clad or unclad. The tube  20  may be roll formed with a welded seam or a lock seam. As an alternative, the tube  20  may be extruded. The tube  20  may have a wall thickness of 0.016″ to 0.05″ depending on the tube diameter and the working pressure. The tube  20  may have a cross-sectional shape that is round, circular, oval, or the like. The tube material is a long life, high strength, corrosion resistant alloy. For extruded tubes, a 3003 aluminum alloy may be used. For roll formed tube an Alcan X-1000 may be used. The clad alloys may be 4045 or 4343 aluminum alloys.  
         [0020]     The fins  12  and fin collar  30  may be constructed out of an aluminum alloy 3003 with a 4045 or 4343 alloy clad. If unclad, the fin may be constructed from an 1100 aluminum alloy. The fins may be constructed with a thickness of 0.003″ to 0.016″.  
         [0021]     In addition to the aluminum alloys described above, the present invention may be used for brazing a copper fin to copper tubing or brazing an aluminum fin to copper tubing, as will be evident to those of ordinary skill in the art.  
         [0022]     In  FIG. 3 , a first embodiment of the fin collar  30  of the present invention is shown. A plurality of slits  32  are disposed around the circumference of the fin collar  30 . The slits  32  may be formed by removing material from the collar and may be disposed equidistantly around the perimeter of the collar  30 . The slit may extend from the top  43  of the collar  30  and terminate at a point approximately 0.02″ from the underside of the fin. The slit  32  is defined by a pair of opposed walls  34  and  36 . The walls  34  and  36  may be angled such that the width  40  across the slit  32  gradually increases from the bottom  42  of the slit  32  to the top  43  of the collar  30 . The slit  32  may range from 0.015″ to 0.15″ in width depending on the collar height and the number of slits. The slits  32  improve the tube-to-fin joint both thermally and structurally.  
         [0023]     With regard to structural properties at the joint, the fin collar  30  of the present invention enhances the flux application and the brazing clad flow because the slits  32  allow the cladding to flow through on both sides of the collar  30 .  
         [0024]     With regard to heat transfer performance, the split fin collar  30  increases heat transfer between the air and tube surfaces. The slits  32  open access to a portion of the surface of the primary tube  20  for the air flow allowing direct heat transfer from air to the tube  20  without the resistance from secondary sources. Ordinarily these portions of the primary tube  20  would be covered by a solid fin collar.  
         [0025]     Turning to  FIG. 4 , an alternate embodiment of the fin collar of the present invention is shown. Fin collar  40  has a rectangular-shaped slit  42 . The slit  42  is defined by a bottom wall  44  and opposed side walls  46  and  48 . The bottom wall  44  may extend to a point approximately 0.02″ from the underside of the fin  12 . The collars  40  may have a curvature such that they have a convex shape on the side that faces the tubes  20 .  
         [0026]     In  FIG. 5 , another alternate embodiment of the fin collar of the present invention is shown. Fin collar  60  is elongated in the longitudinal (tube axis) direction. The collar  60  has a plurality of slits  62  defined therein. The slits  62  also have a rectangular shape and are defined by a bottom wall  64  and a pair of opposed side walls  66 ,  68 . The bottom wall  64  may extend to a point approximately 0.02″ from the underside of the fin  12 .  
         [0027]     In  FIGS. 6 and 7 , the fin collar  30  of the present invention is shown with arrows  70  representing air flow around the collar  30  during use. The shape of the fin collar  30  provides interruptions around the circumference of the fin collar  30  perpendicular to air flow. The interruptions will provide turbulence, which is indicated by curved lines  80 , in the boundary layer of air along the fin collar  30  which will increase the rate of heat transfer between the air and the tube  20 . The increased turbulence will also occur around the area near the base of the fin collar  30  in the area of highest fin efficiency, increasing heat transfer rates in that area.  
         [0028]     While the invention has been described in connection with certain embodiments, it is not intended to limit the scope of the invention to the particular forms set forth, but, on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.