Abstract:
An electrode comprises a shaft extending from an electrode head. A cooling passage extends from an open end disposed at an attachment end of the shaft to a closed end, which is disposed within the electrode head. The electrode head is formed to have approximately a teardrop shape, which may be formed according to a radial profile rotated about a centerline of the shaft, where the radial profile has a center disposed within the electrode head and on the centerline. The radial profile may exhibit a single maximum within a middle portion of the radial profile. Alternatively, or in addition, the middle portion of the radial profile may be negatively curved.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The field of the present invention is liquid cooled electrodes, such as those used in glass melting furnaces. 
         [0003]    2. Background 
         [0004]    Glass is typically processed by heating and refining in batch within a melting furnace. Glass batches are typically heated from both flames from burners, which serve as the primary heat source, and from glass melt electrodes embedded in the wall of the melting furnace. The number of electrodes depends upon the size of the melting furnace and the characteristics of the glass being processed. These glass melt electrodes introduce additional thermal energy into the furnace by passing a current through the glass melt. 
         [0005]    Current state-of-the-art glass melt electrodes use a two piece assembly, having a head, which is typically constructed from a refractory metal (such as molybdenum), affixed to a shaft cooled by an internal passage through which cooling water is passed. This shaft is often constructed from a variety of materials, such as stainless steel, a nickel based alloy, or even molybdenum. Such electrodes are disclosed in U.S. Pat. No. 3,983,309, U.S. Pat. No. 4,965,812, and U.S. patent application publication No. 20070064763. The disclosures of these documents are incorporated herein by reference in their entirety. Due to the high temperatures existing near the electrodes in the glass melt, a water-tight joint is difficult to achieve between the head and the shaft. Thus, the cooling passage is contained only within the shaft and cannot be extended into the head without compromising the durability of electrode under operating conditions. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention is directed toward an electrode which is usable in glass furnaces and the like. The electrode includes a shaft extending from an electrode head. A cooling passage extends from an open end disposed at the attachment end of the shaft to a closed end, which is disposed within the electrode head. The electrode head is formed to have approximately a teardrop shape. 
         [0007]    The teardrop shape of the electrode head may be formed according to a radial profile rotated about a centerline of the shaft, with the radial profile having a center disposed within at the closed end of the cooling passage and on the centerline. The middle portion of the radial profile may exhibit a single maximum. Alternatively, or in addition, the middle portion of the radial profile may be negatively curved. 
         [0008]    Additional options for the electrode may also be incorporated, either alone or in combination. As one option, the closed end of the cooling passage may be formed to have a double “U” shape in cross-section. As another option, the shaft and electrode head may be constructed from a refractory metal, such as molybdenum or a molybdenum alloy. 
         [0009]    Accordingly, an improved electrode is disclosed. Advantages of the improvements will appear from the drawings and the description of the preferred embodiment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    In the drawings, wherein like reference numerals refer to similar components: 
           [0011]      FIG. 1  illustrates a liquid cooled electrode found in the prior art; 
           [0012]      FIG. 2  illustrates a one-piece liquid cooled electrode; 
           [0013]      FIG. 3  graphically illustrates an electrode head radial profile; and 
           [0014]      FIGS. 4A &amp; 4B  illustrate a drill head for forming the closed end of the cooling passage. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0015]    Turning in detail to the drawings,  FIG. 1  illustrates a liquid cooled electrode  11  as is known in the prior art. The electrode  11  includes a liquid-cooled shaft  13  which is attached to the electrode head  15  via a threaded tip  17  inserted into a complimentary threaded receptacle  19 . The shaft  13  has a cooling passage  21  extending substantially along its entire length. The cooling passage  21  includes an inlet  23  and an outlet  25  at the attachment end  27  of the shaft  13 . Since the cooling passage  21  is normally machined into the shaft  13 , the closed end  29  of the cooling passage  21 , takes on the form of the tip of the drill bit used to drill the passage  21 . During use, a coolant delivery tube  31  is inserted into the passage  21 , through the inlet  23 , so that it extends nearly to the closed end  29 . Liquid coolant, typically water, is delivered into the closed end  23  of the passage  21  via the tube  31 . The tube  31  has a smaller overall diameter than the passage  21 , thereby allowing the coolant to return down the passage  21  and exit through the outlet  25 . 
         [0016]    A single piece electrode  51  is illustrated in  FIG. 2 . Here, the shaft  53  and the electrode head  55  are constructed out of one piece of material. The entire electrode  51  is symmetrical about the centerline  57 . A cooling passage  59  is formed down the length of the shaft  53  and extends into the electrode head  55 . The cooling passage  59  includes an inlet  61  and an outlet  63  at the attachment end  65  of the shaft  53 . As with the electrodes of the prior art, a coolant delivery tube  67  is inserted into the passage  57  during use for the delivery of a liquid coolant through the inlet  61 . Here too, the tube  67  has a smaller overall diameter than the passage  59  so that the coolant can return down the passage  59 , between the outer wall of the tube  67  and the inner wall of the passage  59 , and exit through the outlet  63 . 
         [0017]    The electrode head  55  is roughly “teardrop” shaped, and the passage  59  extends about halfway into the electrode head  55 , although the passage may extend as deeply into the electrode head based upon desired design specifications. With the closed end  69  of the passage  59  placed in this manner, the cooling of the entire electrode head is improved over the two-piece electrodes of the prior art. Elimination of the joint between the shaft and the electrode head also improves heat transfer from the electrode head into the shaft, thereby increasing the efficiency of overall heat dissipation for the electrode. Elimination of this joint also serves to remove a potential point of mechanical failure. 
         [0018]    The teardrop shape of the electrode head  55  serves to add longevity to the life of the electrode. This is thought to be the result of better control of localized thermal gradients within the electrode head. By reducing significant localized thermal gradients in the electrode head, longer life spans have been observed in these electrodes before cracks begin appearing on the outside of the electrode head. Whereas some prior art electrodes are known to have a lifespan of about 45 minutes under certain use conditions, electrodes having a teardrop shaped head have been observed to have a lifespan on the order of 1-3 weeks, under the same use conditions, before cracks appeared in the electrode head. As those skilled in the art will recognize, the use conditions of the electrode play a significant role in the lifespan of the electrode. These conditions may include the type of glass or ceramic mixture being melted and the temperature at which the melt is maintained, among other things. 
         [0019]    The teardrop shape of the electrode head  55  is represented by the radial profile  81  shown in  FIG. 3 . For purposes of this description and to highlight certain features, the radial profile  81  is shown on a Cartesian coordinate system, with θ=0° being perpendicular to the centerline  57  of the electrode. The radial profile  81  takes the shape of the electrode head shown in  FIG. 2  when the radial profile is shown in a polar coordinate system. This radial profile  81  representation of the electrode head has it&#39;s radial center located along the centerline  57  of the electrode  51  at the closed end  69  of the cooling passage  59 . In this radial profile  81  representation, the vertical axis is the length of the radius, r, and the horizontal axis is the radial position, θ. The overall shape of the electrode head  55  can be represented by rotating this radial profile  81  (based on polar coordinates) about the centerline  57  of the electrode  51 . The features of this radial profile that are believed to contribute to the longevity of the electrode, as compared to electrodes of the prior art, are the reduced variation in the distance of the exterior of the electrode head from the cooling shaft, the smooth transition in the radial profile, i.e., the lack of cusps that would be caused by corners or hard edges formed in the electrode head, and the presence of a single maximum  83  in the radial profile  81 . As shown in  FIG. 3 , this single maximum is located at about θ=15°, although it is anticipated that this maximum could be located nearly anywhere within a middle range of the radial profile, from about −30° to 30°. This maximum also means that this portion of the radial profile is negatively curved, unlike the radial profile of the electrode head depicted in  FIG. 1 . Preferably, the entire middle portion of the radial profile is negatively curved, i.e., the radial profile curves downward. The ends of the radial profile, located closer to θ=90° and θ=−75°, may be slightly positively curved. As shown in  FIG. 3 , the radial profile remains negatively curved at θ=90°, and transitions to a slight positive curve around θ=−75°. This transition to a slight positive curve at around θ=−75° results from the transition between the electrode head  55  and the shaft  53 . Experimentally, the electrode head depicted in  FIG. 2 , for which the radial profile  81  is a representation, has exhibited the greatest longevity. 
         [0020]    The one piece electrode may be manufactured using a controlled partial extrusion process, which is well known to those of skill in the art. While any refractory metal may be used, for glass or ceramics melting applications, molybdenum or a molybdenum alloy is preferred. This process results in a near net shape part which then undergoes rotary forging to properly size the electrode and straighten the shaft. The electrode is then subjected to final machining to form the connection end and the cooling passage. 
         [0021]    The closed end of the cooling passage is formed using the specially developed flat drill head  91  shown in  FIGS. 4A &amp; 4B .  FIG. 4A  shows a side view of the drill head  91 , showing the dual blades  93  that are appropriately rounded to provide the double “U” shape to the closed end  69  of the cooling passage  59 . The dual cutting edges  93  have fairly large radii and converge in the center of the drill head at an inward-facing 120° angle.  FIG. 4B  shows a top planar view of the drill head  91 , showing the formation of the dual cutting edges  93  and flutes. The curvature of the cutting edges  93  and the angle at which they converge may be made according to desired design specifications. Those of skill in the art will recognize that the angle and curvature of the cutting edges may depend upon such things as operating temperature of the electrode, the type of cooling liquid used, and the rate of flow of the cooling liquid, among other things. By making the closed end in this double “U” configuration, even fluid flow is promoted at the closed end of the cooling passage, thereby reducing, and possibly eliminating, hot spots which may lead to localized boiling of the coolant. Such hot spots can lead to cracking and degradation, resulting in erosion of the electrode from the inside out. The double “U” configuration of the closed end therefore aids in reducing mechanical stresses to which the electrode is subjected and in increasing the overall longevity of the electrode. 
         [0022]    Through the manufacturing processes described above, the electrode head and connection end may be constructed so that the electrode can serve as an appropriate replacement part in nearly any furnace. 
         [0023]    Thus, an electrode is disclosed. While embodiments of this invention have been shown and described, it will be apparent to those skilled in the art that many more modifications are possible without departing from the inventive concepts herein. The invention, therefore, is not to be restricted except in the spirit of the following claims.