Abstract:
A system and method for joining a tube to a header is provided. The system includes a header, tube and internal ferrule, wherein the ferrule is fixed inside the tube. This design reduces manufacturing cost and time, as well as reducing and more evenly distributing stress along the tube-header assembly.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention generally relates to a system and method of joining a tube to a header, more particularly, an assembly of a fluid transporting tube and a header joined using a ferrule. 
       BACKGROUND OF THE INVENTION 
       [0002]    Much development has been made in the area of joints formed between ends of fluid transporting tubes and a header plate receiving those tubes. For example, various methods of joining fluid tubes, such as coolant tubes, to a header has been developed to extend duration and durability of heat exchangers. 
         [0003]    One of the well known methods of joining a tube to a header involves a ferrule. Often, a ferrule is designed to gradually transfer stress from the tube to the ferrule, thereby increasing allowable loading and life span of the joint. Without the ferrule, a header-tube joint is subjected to a maximum stress of the system, and thus, often becomes a point of failure. In some applications, the ferrule is integrally formed with the header, wherein the tube is received and seated. Such a joining method using an integral ferrule-header is commonly practiced in manufacturing of afterburner spray bars for some jet engines. 
         [0004]    There are several limitations with the integral ferrule-header designs. First, the integral ferrule-header systems can be relatively costly to manufacture. This is because manufacturing of the integral ferrule-header can involve complex engineering and equipment, which can be a lengthy and expensive process. Second, a design of the integral ferrule-header is limited by current manufacturing capabilities, which often does not allow for production of a desired geometry of a ferrule-header. Further, in such a ferrule-header system a stress concentration can be induced in the tube proximate a ferrule. 
         [0005]    In view of these limitations, there is a need in the art for improved system and method of joining fluid transporting tubes with a header. The present invention pertains to such improvements to the state of the art of a tube-header assembly using an internal ferrule. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    In view of the above, embodiments of the present invention provide a new and improved system and method of joining tubes to a header that overcomes one or more of the limitations existing in the art. More particularly, embodiments of the present invention provide a new and improved tube-header assembly incorporating an internal ferrule. Such embodiments significantly improve the performance of the tube-header assembly by more evenly distributing external stress. Such embodiments are relatively simple to engineer and easy to manufacture, and thus reduce overall cost of the tube-header assembly. 
         [0007]    In one aspect, the invention provides a tube-header joint for a fluid transport system including a tube, an internal ferrule and a header, wherein the internal ferrule is attached within the tube. Further, the header includes an aperture, wherein the tube is fixed. 
         [0008]    In another aspect, the invention provides a method of assembling a tube-header joint for a fluid transport system including forming a tube, forming a flared end in at least one end of the tube, making an internal ferrule, fitting the internal ferrule in the flared end of the tube, forming a header including an aperture, and attaching the flared end of the tube in the aperture 
         [0009]    In yet another aspect, the invention provides a method of reducing an external stress concentration on a tube-header joint for a fluid transport system including forming a tube-header joint comprising a tube, a header and a ferrule, wherein the ferrule is positioned inside of the tube. The method of reducing an external stress concentration on a tube-header joint also excludes an external ferrule from the tube-header joint. 
         [0010]    Other aspects, objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings: 
           [0012]      FIG. 1  is a cross sectional view of a tube-header joint comprising a tube, a header and an internal ferrule, according to an embodiment of the present invention; 
           [0013]      FIG. 2  is a cross sectional view of the internal ferrule of  FIG. 1 ; 
           [0014]      FIG. 3  is a cross sectional view of a conventional tube-header joint; and 
           [0015]      FIG. 4  is a schematic illustration of stress distribution on the tube-header joint of  FIG. 1  compared to the conventional tube-header joint of  FIG. 3 . 
       
    
    
       [0016]    While the invention will be described in connection with certain preferred embodiments, there is no intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the spirit and scope of the invention as defined by the appended claims. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0017]      FIG. 1  shows a cross sectional view of a tube-header joint  10  according to an embodiment of the present invention. The tube-header joint  10  comprises a tube  12 , a header  14 , and an internal ferrule  16 . The tube  12  is generally a cylindrical in its shape and swaged at one end, wherein a flared end  18  is formed. The header  14  includes an aperture  22 , which extends from an upper surface  28  of the header  14 , and down through the thickness of the header  14 . The header  14  and the tube  12  are configured such that the flared end  18  of the tube  12  fits tightly into the aperture  22  of the header  14 . The internal ferrule  16  is a separate piece, configured to fit into the flared end  18  of the tube  12 . 
         [0018]    The tube  12  of this embodiment is adapted to transport fluid, for example, fuel in an afterburner system in some jet engines, through a flow path  30  defined within the tube  12 . The cylindrical tube  12  has a circular cross section having an inner diameter  32 , which gradually increases from point  34  to point  36 , thereby forming the flared end  18 . The flared end  18  has a sloped portion  20 , defined by a portion of the tube  12  between the point  34  and the point  36 , and a tube insert  40  having a constant inner diameter  38 . 
         [0019]    The aperture  22  of the header  14  has a circular cross section to fit the tube insert  40 . Thus, an inner diameter  42  of the aperture  22  is almost equal to an outer diameter  44  of the tube insert  40  with just enough clearance to tightly fit them together. In other embodiments, the tube insert  40  may be sloped, either flaring or constricting, as such the aperture  22  of such embodiments is configured accordingly to fit such shapes. 
         [0020]      FIG. 2  shows the internal ferrule  16  of  FIG. 1  before it is fitted into the flared end  18  of the tube  12 . The internal ferrule  16  is generally cylindrical in its shape and includes an inlet end  46  and an outlet end  48 . The shape of an outer surface  52  of the internal ferrule  16  mirrors the inner surface of the flared end  18 . That is, the internal ferrule  16  comprises a body  54  corresponding to the tube insert  40  and a tip  56  corresponding to a part of the sloped portion  20  of the flared end  18 . As shown, the internal ferrule  16  is shorter in length than the flared end  18  of the tube  12 , as such the tip  56  of the internal ferrule  16  does not extend into the entire sloped portion  20  of the flared end  18 . However, in other embodiments, the internal ferrule  16  and the flared end  18  of the tube  12  may be designed to have a same length, such that the internal ferrule  16  extends through the entire flared end  18  to the point  34 . 
         [0021]    Now that each component of the tube-header joint  10  is described, a method of assembling the tube-header joint  10  according to the present invention will be explained. 
         [0022]    In one embodiment, the tube  12  is swaged at one end in a swaging machine using swaging dies designed to form the flared end  18 . For example, the tube  12  can be fed into a swaging die opening, wherein the swaging die rotates to widen an end of the tube  12  into a desired shape by centrifugal force. Once the tube  12  is swaged to form the flared end  18  the internal ferrule  16  is inserted, with the tip  56  first, into the flared end  18  through the tube insert  40 . In other embodiments, the tube  12  may be preformed to include a flared end  18 . 
         [0023]    In the tube  12 , the internal ferrule  16  is joined with the flared end  18  of the tube  12  by a suitable manufacturing process such as brazing, welding, press-fit, shrink-fit, hold-down spring, etc. For example, in a brazing process, an inner surface  24  of the flared end  18  and/or an outer surface  52  of the internal ferrule  16  maybe applied with a filler material, such as silver, tin, zinc, copper, etc., and heated to the melting temperature of the filler material such that the filler material melts and metallurgically bonds the tube  12  and the internal ferrule  16  together. In such an embodiment, the tube  12  and the internal ferrule  16  are formed of a metallic material having a higher melting temperature than the brazing filler material. The tube  12  and the internal ferrule  16  may be formed of a same or different metallic materials. The tube  12  joined with the internal ferrule  16  is fitted into the aperture  22  of the header  14 , wherein the tube insert  40  is joined with the header  14  by a suitable manufacturing process similar to the process used for joining the internal ferrule  16  and the flared end  18  of the tube  12 . 
         [0024]    In a different embodiment, the tube  12  may be first joined with the header  14  by attaching an outer surface of tube insert  40  with an inner surface of the header aperture  22  by a suitable manufacturing process. The internal ferrule  16  may then be inserted into the flared end  18  of the tube  12  such that the outlet end  48  of the internal ferrule  16  is flushed with an end of the tube insert  40 . The internal ferrule  16  may be attached with the flared end  18  via suitable process. For example, the internal ferrule  16  can be press-fit into the flared end  18 . 
         [0025]    The internal ferrule  16  is advantageous over a conventional system, wherein a ferrule is integrally formed with a header plate. An example of the conventional tube-header joint is illustrated in  FIG. 3 . As shown, a conventional tube-header joint  60  comprises a tube  62 , a header  64  and a ferrule  66 , wherein the ferrule  66  is integrally formed with the header  64 . Further, the ferrule  66  is external to the tube  62 . Production of such integrated ferrule  66 -header  64  piece can often involve complex engineering and manufacturing processes. On the other hand, the internal ferrule  16  of the present invention is relatively simple to design and manufacture since it is a separate piece from the header, and thus, can reduce manufacturing costs compared to the conventional integrated ferrule-header piece. 
         [0026]    Further, the placement of a ferrule inside of a tube also improves stress distribution on a tube-header joint.  FIG. 4  is a schematic illustration of stress distribution around the tube-header joints  60 ,  10 . As shown, the conventional tube-header joint  60  of  FIG. 3  comprising the external ferrule  66  can drive the stress that it is under to a focused region  68  on the tube  62 , immediately above the external ferrule  66 . On the other hand, the tube-header joint  10  comprising the internal ferrule  16  allows for the loading to be spread more uniformly throughout the tube-header joint  10 , thereby decreasing a maximum stress at any one particular point. This is because the placement of the ferrule inside of the tube eliminates a built-in stress concentration in the conventional tube-header joint  60  due to a step from a header  64  to tube  62  from a difference in their outer diameters. By distributing an external stress applied to the tube-header joint  10  more uniformly over a larger area, it reduces the maximum stress at a focused area, and thus, increases the amount of stress that the tube-header joint  10  can endure before failure. Therefore, the tube-header joint  10  of the present invention can have increased product life span with respect to high cycle fatigue. 
         [0027]    Test results confirmed reduced von Mises stress at an intersection of the ferrule and tube for the tube-header joint  10  of the present invention when compared with the conventional tube-header joint  60 . The test results showed that the tube-header joint  10  had von Mises (Max) stress of 164.3 Ksi at a tube-ferrule intersection  26 . ( FIG. 1 ). The conventional tube-header joint  60  had a von Mises (Max) stress of 178.9 Ksi at an tube-ferrule intersection  70 . ( FIG. 3 ). In other words, the tube-header joint  10  comprising the internal ferrule  16  of the present invention had about 9% lower von Mises maximum stress than the conventional tube-header joint  60  using the external ferrule  66 . 
         [0028]    A tube-header joint comprising an internal ferrule according to the present invention provides many benefits. The internal ferrule substantially decreases manufacturing costs, because it is a separate piece from a header, and has a relatively simple structure. As such, the internal ferrule allows for a decrease in complexity of engineering and manufacturing the header and the ferrule, resulting in savings in manufacturing costs. Further, the internal ferrule improves overall performance of the tube-header joint, by eliminating a stress concentration point. 
         [0029]    All references, including publications, patent applications, and patents cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein. 
         [0030]    The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
         [0031]    Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.