Abstract:
A method of establishing a leak tight and structural connection between a core tube and an accommodating header plate in a tubular heat exchanger, including the steps of positioning the core tube into bores of the header plate and a braze foil, installing a ferrule inside the core tube end, radially expanding the ferrule in the core tube end, thus expanding the core tube end into intimate contact with the header plate in which it is received, deforming a ferrule into pinching contact with the braze foil plate, directing the flow of the braze material towards the contact surface area, and brazing the tubular heat exchanger in order to form a seal at the intimate contact area. A leak tight connection and a tubular heat exchanger having a leak tight connection produced via the noted method are also set forth.

Description:
CROSS-REFERENCE TO RELATED CASES 
     The present application claims the benefit of the filing date of U.S. Provisional Application Ser. No. 60/347,085; filed Jan. 4, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     In the art of fabricating tube and shell heat exchangers, it is well known that a proper seal and support structure is required at each header plate-core tube interface. Even minor leaks at the tube joints will impair the function of the heat exchanger. Prior inventions have taken two approaches to establishing such a connection at this interface. The first approach was via a purely mechanical connection. The second approach has been via a metallurgical bonding, particularly brazing. 
     The first noted approach to be discussed is the method of mechanically sealing this interface. Prior art, such as U.S. Pat. No. 4,152,818 to Mort et al., sets forth an example of such a technique. The mechanical sealing process first involves inserting an end of the core tube into a hole in the header plate. A rivet is then inserted into the core tube end and expansion of the rivet subsequently creates a high load friction connection end of the core tube into a hole in the header plate. A rivet is then inserted into the core tube end and expansion of the rivet subsequently creates a high load friction connection between the core tube and the header plate. This resulting connection serves as the joint for the interface. Several methods can be used to expand the rivet, but a complete expansion requires all contact areas between the core tube and header plate to be a maximum of 0.001 inch to intimate. In order to provide this complete seal over 100% of the interface, several process steps may be required. Even with a 100% complete seal, varying load forces can damage this mechanical seal. For example, vibrations and pressure fluctuations may cause one of the header plate and core tube to move relative to the other. In order to assure completion of the seal, an added step, as shown in prior art U.S. Pat. No. 4,482,415 to Mort et al., of using a sealant material at each header plate-core tube interface can be used. In this type of process, the joint is codependent on the mechanical process and the sealant process. 
     Another approach for sealing this joint is via a braze joint construction. Prior art, such as U.S. Pat. No. 4,207,662 to Takenaka, sets forth an example of using clad braze materials for this process. In such a process, the clad braze material is located on the exterior surface of one of the objects to be joined. For example, a core tube is inserted into a hole in a header plate having braze material located on at least one of its sides. Upon brazing, the clad material melts and forms the joint. Alternatively, the core tube could be clad with braze material. However, clad materials are typically produced as flat stock and the products shaped therefrom, for example the header plate or core tube, is preferably also flat in order to satisfactorily retain the clad material. During brazing, the clad braze material melts and, like any liquid, will flow and take the path of least resistance. With a flat surface, it is therefore difficult to direct the flow of the melted braze material. In order to overcome the difficulties in directing the flow of melted clad material, the previously mentioned prior art patent sets forth an example of using a flat, inclined surface to direct the flow of the melted material towards the intended area of joining. 
     Another prior art braze joint construction approach involves the use of diffusion bonded braze material. Typically such a manufacturing process first includes the initial diffusion bonding of a braze foil alloy to the header plate in order to bond the braze material in place. The core tubes are then inserted into holes in the header plate, followed by a mechanical staking operation of the tube ends in order to form a clearance controlled or intimate bond at the header plate-core tube surface. Subsequent vacuum brazing is then employed to bond the tube end to the header plate. An intimate bond is critical to any brazing operation. It is the intimate contact between the header plate and core tube that promotes the wetting of the joint surfaces with the braze alloy. A mechanically staked core tube, though, exhibits overall distortion due to the biaxial (radial and axial) stressing of the tubes that occurs during the noted staking operation. The core tube staking process is generally performed manually, and in addition to it being labor intensive, is largely uncontrolled thus introducing excessive process variations and large compressive stresses in the core tubes. This process often creates product rejections ranging from braze joint leaks to unacceptable dimensional distortions. In addition, the braze alloy diffusion bonding process is dependent on a complex vacuum process and often produces unacceptably low yields. The diffusion bonding process also produces changes in the aluminum header plate material, via diffusing out the silicon, which has a negative effect on the brazing process as well as the header plate material. 
     Various other methods have been used in order to create the critical intimate contact between the bonding surfaces. Prior art, such as U.S. Pat. No. 3,496,629 to Martucci et al., teaches welding the core tube to the header plate in order to produce the intimate contact area. 
     Another example of a prior art brazing technique is set forth in U.S. Pat. No. 5,464,145 to Park et al. This technique does not address the need for an intimate contact area between the bonding surfaces. Other prior art brazing techniques are in U.S. Pat. No. 2,267,315 to Stikeleather and U.S. Pat. No. 5,507,338 to Schornhorst et al. These two references set forth a process of joining the tubes to each other, but not to the header plate. A further reference, U.S. Pat. No. 6,170,738 to Otsuka et al. sets forth the use of a specific material for brazing low-melting point aluminum material parts. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the prior art&#39;s inability of producing quality braze joints by providing a method of achieving the necessary contact pressure between core piece parts prior to brazing. The invention also provides properly sealed tubular heat exchanger core tube-header plate joints and tubular heat exchangers having properly sealed core tube-header plate joints. A proper sealing joint will ensure that the two distinct fluids that flow through the heat exchanger are not intermixed. 
     It is an advantage of the present invention to provide a leak tight connection between a core tube and a header plate using a braze process for aluminum alloy tubular heat exchangers. This is accomplished by first aligning all of the necessary heat exchanger components: header plates, braze foil, and core tubes. The core tubes are placed inside its mating header plate apertures and a braze foil plate is positioned on top of the header plate. A ferrule is then inserted into each core tube along with an expanding mechanism. A feature of an illustrated embodiment of the present invention includes radially expanding the ferrule in order to provide a desired intimate contact area between the header plate and an adjacent core tube. This expansion of the ferrule provides another feature, namely pinching or compressing the braze foil, thereby providing an impetus for the direction of the flow of the braze foil material during the brazing process. During the brazing process, the braze foil melts and flows in its intended path or direction, namely into the header plate-core tube junction contact area. The molten braze material enters the microscopic pores at the header plate-core tube junction contact area via capillary action and bonds the core tube to the header plate. The brazing process, via vacuum brazing or via brazing in an inert atmospheric furnace, ensures that the joint will withstand any detrimental forces and elements, such as corrosion, vibration and pressure variations, encountered by the connection. 
     Another advantage of an illustrated embodiment of the present invention includes limiting the core tube stresses, during the ferrule expansion, solely to radial stresses. The present invention overcomes difficulties encountered in the prior art due to the biaxial stresses resulting from the mechanical staking operation. The noted staking operation also causes axial, or longitudinal, stresses which result in undesired longitudinal displacement, as well as subsequent weakening, distortion and/or elongation, of the core tube material. 
     Another advantage of the present invention is that it is relatively simple to alter the length of the braze joint by varying the dimensions of the ferrule. The length of the ferrule shank portion helps to determine the length of the braze joint. With predetermined changes in the thickness of the braze foil and tolerances of the involved parts, the braze joint length can readily be changed by altering the length of the ferrule shank. The braze contact area between the tube end and the header plate is largely determined by the length of the ferrule shank. Only this junction area can provide the contact pressure needed for a proper braze joint. Likewise, the annular area of the ferrule head bottom surface determines the amount of braze material that can be used to form the joint. By varying the surface area of the ferrule head, the amount of the pinched braze material is also varied. 
     A further embodiment of the present invention pertains to the deformation of an annular sleeve in order to provide the required peripheral contact area between the header plate aperture and its inserted, adjacent core tube end. Deformation of the sleeve not only determines the noted contact area between the header plate and core tube, but also the size of the bottom surface area of the sleeve that pinches the braze foil for subsequent material flow into the contact area. 
    
    
     
       As previously described, the advantages of the present invention are fulfilled via a simplified process of preparing a header plate-core tube joint prior to the braze process for an aluminum alloy tubular heat exchanger and the heat exchangers produced with this process. Further features and advantages of the present invention will become apparent to those skilled in the art upon review of the following specification in connection with the accompanying drawings. 
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic, simplified side elevation of a tubular heat exchanger, comprised of opposed header plates joined by tubes mounted therein, with a portion of one of the header plates being broken away to show a tube joint; 
         FIG. 2  is an enlarged cross-sectional detailed view of the broken away portion of one embodiment of the tube joint of  FIG. 1 , showing a portion of a header plate, braze foil plate, and tube, with a rivet inserted into the tube prior to the rivet being expanded; 
         FIG. 3  is a longitudinal, cross-sectional view of an expansion tool inserted into the unexpanded rivet in the tube joint; 
         FIG. 4  is a detail view, similar to that of  FIG. 2 , but showing the rivet now expanded in place; 
         FIG. 5  is a schematic, exploded view of the assembly of the heat exchanger tubes with the header plate and the braze foil plate; 
         FIG. 6  is an enlarged detail view of the broken away portion of another embodiment of a tube joint of this invention, showing a portion of the header plate, braze foil plate and tube with a deformable ferrule inserted into the tube, prior to the ferrule being expanded; and 
         FIG. 7  is a detail view, similar to that of  FIG. 6 , but showing the ferrule swaged and expanded in place. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings and particularly to  FIGS. 1 and 2 , a first preferred embodiment is shown generally at  20  in the form of a tubular heat exchanger. This invention has a specific, but not limited, utility in the field of tubular heat exchangers, particularly when manufactured of aluminum alloys. The main components include two opposed header plates  25  and  26 , joined via a plurality of core tubes  35  (only a few of which are illustrated in the interest of simplicity), rivets or ferrules  45 , and a braze foil  55 . 
     The plurality of core tubes  35  are positioned between and joins the two header plates  25  and  26 , and, for assembly purposes, core tubes  35  are initially affixed to the header plates  25 ,  26  with ferrules  45 . Core tubes  35  are made of a rigid, thin, metallic material, such as from aluminum or an aluminum alloy for example, for good heat transfer properties. Core tubes  35  are parallel to each other and perpendicular to header plates  25  and  26  and, when finally affixed thereto, present fluid-tight joints therebetween. As is well known to those skilled in the heat exchanger art, when provided with the necessary header tanks (not shown), a first fluid flows through bores  38  in the hollow core tubes  35  at core tube ends  36  and is discharged at their opposite ends. A second fluid flows over and in between core tubes  35 . The two fluids do not intermix, but a transfer of heat from one fluid to the other takes place. Proper sealing joints, where the core tubes  35  are affixed to the header plates  25  and  26 , ensure that the two fluids are kept separate. 
     The configurations of header plates  25  and  26  are substantially similar so that a description of one will be applicable for both. Header plates  25  and  26  are made of a metallic material, such as from aluminum, or an aluminum alloy, for example. Also, the method of affixing core tubes  35  to header plates  25 ,  26  is the same for all core tubes  35  so a description of one will be applicable to all. Referring to  FIGS. 2 and 5 , core tubes  35  are readily deformable so that the portions thereof that adjoin apertures or through bores  27  in header plate  25  may be readily radially expanded into close contact with the header plate aperture wall that defines through bore  27 . The outside diameter of core tubes  35  is approximately equal to the diameter of header plate through bores  27 . 
     Once an end  36  of core tube  35  has been inserted through header plate aperture  27  so that it axially extends slightly therefrom, a braze foil  55 , having through bores  57  that are substantially coincident in size and spacing with bores  27  of header plate  25 , is placed on top of header plate  25  with core tube ends  36  extending through at least a portion of the axial extent of braze foil bores  57 . Extending core tube ends  36  thus also function as locators and retainers for braze foil  55 . The annular end surfaces  37  of extending core tube ends  36  are preferably coplanar with the foil upper surface  58  within a reasonable tolerance. 
     Following the noted assembly of core tubes  35  into header plate  25  and the placement of braze foil  55  around core tube ends  36 , a rivet or ferrule  45  is placed into each tube end opening  36 . As best seen in  FIG. 2 , ferrule  45 , which may, for example, take the form of a hollow rivet, includes a head portion  46  and an annular shank portion  47  as well as a central, longitudinal through bore  48 . Ferrule head  46  may be of any predetermined size or shape, such as, for example, having a domed head as shown in  FIGS. 2 and 4  or a flat head, as shown in FIG.  3 . Ferrule head  46  extends, at a minimum, both axially and radially beyond the annular end surface  37  of tube end  36  and over a portion of the top surface  58  of braze foil  55 , preferably for a distance greater than the wall thickness of core tube  35 . The outside diameter of shank portion  47  is approximately equal to the inside diameter of core tube  35  so as to permit a slip fit thereof into core tube bore  38 . Ferrule or rivet  45  is made of a rigid malleable, non-ferrous metallic material, such as of an aluminum alloy, for example. 
     Referring to  FIGS. 2 and 3 , the ferrule  45  is radially expanded by means of an expansion mandrel tool  65 . This tool  65  includes a connector  66 , such as a wire or rod, having a teardrop shaped, or ball shaped, expander portion  67  affixed at one end. Expander portion  67  has an outside diameter slightly greater than the diameter of ferrule bore  48 . Tool  65  also includes a collet  68  having an axial through bore  69  extending therethrough. Connector  66  is adapted to slidably move within collet  68  through bore  69 . In order to radially expand ferrule  45 , expander portion  67  is placed inside core tube  35  with connector  66  extending outwardly from the header plate bore  27 . Ferrule  45  is placed around connector  66  and inserted into core tube bore  38 . The collet  68  is placed in contact with ferrule head  46 . Expander  67  is then pulled upwardly through ferrule bore  48 , in the direction of arrow  71 , while the collet  68  pushes ferrule head  46  into intimate contact with tube end  36  and braze foil  55 . As expander  67  is pulled through ferrule bore  48 , ferrule  45  will tend to move in the same direction as expander  67 . Collet  68  counteracts this motion, thus keeping ferrule head  46  in intimate contact with tube end  36  and braze foil  55 . Referring now to  FIGS. 3 and 4 , as expander  67  is pulled through ferrule bore  48 , ferrule shank  47  radially expands into core tube  35 , thus in turn radially expanding core tube  35  into a physical contact area  80  with the header plate aperture wall that defines bore  27 . At the same time, the noted radial expansion also causes ferrule head  46  into intimate pinching contact with tube end  36  and braze foil  55 . Expansion of ferrule shank  47  also ensures that fluid is not restricted when flowing through the core tube  35  at the ferrule shank inserted portion. 
     As can best be seen in a comparison of  FIGS. 2 and 4 , not only is ferrule  45  radially expanded into contact with its adjacent parts, namely tube  35  and braze foil  55 , but these parts, in turn, are subsequently moved into intimate contact with their adjacent part, namely header  25 . For example, as can be seen in  FIG. 2 , prior to the noted radial expansion an annular gap  75  exists between the ferrule shank  47  and core tube  35 . Another gap  76  exists between the lower surface of ferrule head  46  and braze foil surface  58 . A further annular gap  77  also exists between core tube  35  and the header plate aperture wall that defines bore  27 . Referring to  FIGS. 2 and 4 , gaps  75 ,  76 , and  77  are substantially eliminated as a result of the radial expansion of ferrule  45 . Ferrule shank  47  is thus radially expanded into intimate contact with core tube  35 , thus eliminating gap  75 . Due to its rigid, but malleable metallic material construction, core tube  35  is also radially expanded into intimate contact with header plate  25 , thus eliminating gap  77 . Ferrule head  46  is expanded or deformed into contact with core tube end  36  and braze foil  55 . Due to the inherent malleable property of braze foil  55 , ferrule head  46  is able to pinch or compress the foil  55  between head  46  and plate  25  thus providing the impetus for the braze alloy to follow a specific, desired directional path upon brazing, in a manner to be described hereinafter. 
     With the noted expansion at the header/tube surface providing a provisional connection, the mechanically assembled heat exchanger is then subjected to a braze and preferably to a vacuum braze operation. The heat exchanger is placed in a vacuum furnace and heated to a predetermined elevated temperature and for a length of time sufficient to melt the braze foil  55 , while not significantly altering any of the metallic properties of core tube  35 , header plate  25 , or ferrule  45 . The time and temperature needed to perform this process is known to those skilled in the art. 
     Referring to  FIG. 4 , the plastically deformed ferrule head  46 , by pinching braze foil  55  in the vicinity of core tube-header plate contact area  80 , directs the braze foil alloy  55  to flow towards core tube  35 . Although core tube  35  and header plate  25  are in close intimate contact, the melted braze foil alloy flows by capillary attraction into the microscopic pores of the metal joint at core tube-header plate contact area  80 , thus forming a fillet. During the brazing process, in addition to the capillary flow, some of the constituents, including silicon, of braze alloy  55  also diffuse into the core tube  35  and header plate  25  on a molecular level. Thus, upon melting, silicon molecules from braze foil alloy  55  diffuse into core tube  35  and header plate  25  thereby providing a braze joint fillet. Due to the position or location of expanded ferrule  45 , the braze material does not deviate from its directed path between core tube  35  and header plate  25 . Specifically the braze material does not flow into contact with ferrule shank  47 . The formation of eutectic silicon at the joint fillet with diffusion at least partially into the parent metal thicknesses is required for achieving adequate braze joint strength. As a general rule, for maximum structural life based on shear loading, it is known that the braze joint fillet length should be four times the thickness of the thinnest material to be joined. Therefore the length of ferrule shank  47 , the thickness of braze foil alloy  55 , the tube thickness, as well as the header bore and foil bore tolerances are all predicated on this ratio and can easily be determined by those of ordinary skill in the art. Further, the size of ferrule head  46  can determine the amount of braze foil alloy  55  that is pinched during the expansion process, thus defining the amount of braze material available for formation of the joint fillet upon heating. 
     As an alternative to vacuum brazing, such brazing could be accomplished in an inert environment furnace up to a predetermined temperature at which the braze material is melted, while the other metallic components remain unmelted. This inert environment may include both a partial vacuum and an oxide-reducing gas atmosphere. 
     A second embodiment  120  of the present invention, which differs from first embodiment  20  only as to ferrule  145 , is illustrated in FIG.  6 . Since all parts, except for ferrule  145 , are substantially similar to the first embodiment  20  the detailed description thereof will not be repeated. After the positioning of header plate  25 , core tube  35 , and braze foil  55 , ferrule  145  is positioned inside core tube  35 . Ferrule  145  preferably takes the form of an annular sleeve having a central longitudinal through bore  148 . The outer diameter of ferrule  145  is selected so as to permit a slip fit thereof into core tube bore  38  and minimizing gap  75 . Ferrule  145  is made of a malleable non-ferrous metallic material such as an aluminum alloy for example and includes a top portion  146  and a bottom portion  147 . Top portion  146  axially extends, at a minimum, past the top surface  58  of braze foil  55 , preferably for a distance greater than the wall thickness of core tube  35 . Ferrule bottom portion  147  extends longitudinally into core tube  35  beyond the inner end surface of header plate  25  (located on the outside of core tube  35 ). 
     Referring now to  FIG. 7 , in order to provide the desired intimate contact between ferrule  145 , core tube  35 , and header plate  25 , as well as between ferrule  145  and braze foil  55 , ferrule  145  is radially deformed, such as via swaging. During the swaging process the inside and outside diameters of ferrule  145  are enlarged so that the ferrule&#39;s cylindrical wall is radially pressed against core tube  35 , thus eliminating any remaining portion of gap  75 . Core tube  35  is thereby outwardly pressed against header plate  25  so that gap  77  is substantially eliminated. Referring to  FIG. 7 , ferrule top portion  146  is also deformed, such as via swaging, then mushrooming same outwardly over core tube end  36  so that the top cylindrical surface is laterally pressed into contact with braze foil  55 . As in first embodiment  20 , at least a part of ferrule top portion  146  is pinched into contact with braze foil  55  such that braze foil  55  later flows into the header plate-core tube interface  80  during the brazing process. 
     EXAMPLE 
     The following is a method exemplifying a successful vacuum braze process utilized with the first embodiment of the above-described invention. 
     Core tube  35  was constructed of a 6000 series, specifically 6951 aluminum alloy material. 
     Header plate  25  was also constructed of a 6000 series, specifically 6061 aluminum alloy material. 
     The composition of braze foil  55  was Aluminum Association Number 718 aluminum braze alloy. 
     The installation mandrel is obtainable from Textron Avdel Cherry Corporation as part number 7150-6003. 
     All core parts were cleaned and dried in a known manner in order to ensure that process oils, finger prints, and aluminum oxide formations were removed. As is well known in the art, this is accomplished by using a sequence of degreasers, deoxidizers, water rinses and drying cycles. 
     The braze cycle is performed to a recognized industry standard which entailed the following process. Thermal couples were positioned at varying spots in the core. Receptacles holding a predetermined amount of magnesium chips (for use as sacrificial getters) were placed at each end of the braze furnace. The furnace and specimen temperature were driven from room temperature to the final braze temperature using a series of temperature elevations and stabilization hold steps. It should be understood that the exposure and holding times are based on the mass of the specimen material. In this example, the final hold point prior to driving to the brazing temperature was maintained for 30 minutes insuring the degree of vacuum pressure was in the 0.00001 torr range. When the specimen reached 1090° F., the heat was turned off and the part was allowed to cool under vacuum to 1080° F. At 1080° F., a non-oxidizing atmospheric gas, such as an Argon backfill was introduced. The specimen was then gradually cooled to 650° F. The specimen was subsequently subjected to a solution heat treat process which involved heating the specimen to a range of approximately 950° F. to 1000° F. followed by rapidly quenching same in water. As is known in the industry, this process traps alloying elements in the grain boundary structure of the material to enable enhanced strength properties. 
     It should be noted that the present invention is not limited to the specified preferred embodiments and principles. Those skilled in the art to which this invention pertains may formulate modifications and alterations to the present invention. These changes which rely upon the teachings by which this disclosure has advanced are properly considered within the scope of this invention as defined by the appended claims.