Abstract:
A filter cartridge for a molten metal filtering system is provided with a modular construction in which the filter elements are adapted to be removably mounted to an end plate. The filter elements may be removed separately from the filter and replaced with new filter elements without removing the entire cartridge assembly. According to another aspect of the invention, the filter elements are provided with an elliptical cross-section.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to the field of molten metal casting and to the field of filtration. Specifically, the invention relates to a filtering device for molten metal. 
     Wrought aluminum castings are used to produce high-grade stock for use in cans, as foil, litho sheets, memory disc substrates and aircraft components, for example. The casting process typically involves providing metal in a molten state using a furnace. The molten metal is then ultimately delivered to a casting mold or a secondary receiver for holding the molten metal at an elevated temperature for later casting. 
     It is desirable to remove any impurities or inclusions that may have been introduced into the molten metal at some stage during the casting process. Although sedimentation and flotation methods, which capitalize on the density differences between the molten metal and impurities, have been used, it is well recognized that filtration techniques are much more efficient than those methods. 
     Molten metal filtration is usually accomplished using filter elements formed of a porous refractory material. The molten metal is directed through the filter elements such that impurities are removed. Typically, inclusions that are above 5 microns in dimension are filtered from the molten metal in high-grade applications. Filtration can be performed by flowing molten metal through a filter box, which houses a filter assembly in such a way that the assembly may be removed and replaced as it becomes contaminated with the removed impurities. Similarly, in a trough filtration system, a plate filter is positioned as a type of trap in the trough. Molten metal, therefore, passes through the plate as gravity flows it through the trough. 
     Much attention has been given to the design of filter assemblies used in casting processes. It is desirable to minimize the footprint “floor space,” and volume occupied by the filter assembly in order to provide sufficient casting space. On the other hand, it is desirable to maximize the filter surface area in order to minimize the velocity of molten metal through the filter itself. An optimized design must balance the factors of footprint, total flow and velocity through the filter. Because filters must be periodically replaced, their cost is a significant factor that affects the feasibility of using a particular filter construction. Of equal importance is the down-time of a casting line required when a filter must be replaced. Obviously, productivity can be increased by decreasing filter change time. There have thus been efforts to provide relatively low-cost filtering devices which achieve a requisite throughput while occupying as little space as possible. 
     It is known to provide a filter assembly in the form of a cartridge filter unit that is constructed of a plurality of tubular filter elements adhesively bonded between end plates. Such a cartridge is described in U.S. Pat. No. 5,126,047, the subject matter of which is incorporated herein by reference. The cartridge comprises a plurality of generally horizontally oriented cylindrical tubes that are connected at their ends to a pair of spaced, generally parallel, ceramic refractory end plates. One end plate is provided with holes that are in fluid communication with the hollow interiors of the tubes. The other end plate abuts the closed ends of the tubes. In use, the filter cartridge is disposed within a filter box into which molten metal flows. The molten metal is filtered as it travels from the exterior of the tubes to the interior of the tubes. The filtered molten metal then flows through the tube interior and through the holes in the end plate to the outlet of the filter box. An advantage of a cartridge filter over a plate filter is that a cartridge filter provides a greater surface area within the same size footprint as a two-dimensional plate filter. 
     While such a filter cartridge construction offers superior filtration capabilities as compared to other filter constructions, its high cost may make it uneconomical. One of the primary reasons for the high cost associated with such prior art filter cartridges relate to their unitary construction. Specifically, the filter cartridge must be replaced as a single unit because filter elements are permanently bonded to the end plates using a refractory cement. 
     More importantly, this construction also results in considerable down-time during filter changes, since the entire cartridge must be preheated before use, typically requiring up to 24 hours for a cartridge with a large number of filter tubes. Another disadvantage is that the filter cartridge is generally a bulky structure which is difficult to lift and which must be carefully installed, often requiring significant manpower. To this end, holes are usually provided in the end plate to permit lifting equipment to attach to the end plate. 
     SUMMARY OF THE INVENTION 
     The present invention solves the aforementioned and other problems in the prior art by providing a modular construction for a filter cartridge in which the filter elements are adapted to be removably mounted to an end plate which is provided in the filter box or trough. The term “modular” as used herein refers to the capability for the separate interchange of parts that comprise an assembly. The term “removably mounted” refers to the ability of one component to be connected to or engaged with another such that the components can be later separated while remaining intact thereafter. According to one aspect of the invention, the filter elements are adapted to be removably mounted to the end plate, which may itself be removably mounted within the filter well. According to an alternative aspect of the invention, the filter elements are formed as tubular elements having an elliptical cross-section as opposed to a circular cross-section. 
     The modular construction of the filter cartridge according to the present invention provides the advantage of reducing the overall cost of using the filter cartridge since the end plate need not be replaced each time the filter is replaced. This construction also provides the advantage of reducing pre-heating times, since the filter elements may be pre-heated separately and outside of the filter box before filter replacement occurs. Filter replacement is simple since the individual filter elements may be handled, removed, and installed separately. Because the end plate may remain in the filter box while the filter elements are removed, the production delay associated with pre-heating the end plate is eliminated. Furthermore, since each tube is individually seated and sealed, the complex task of assuring simultaneous seating of several tubes on the large surface area of an end plate is avoided. Similarly, it will be recognized that a tool or structural design of the base of the filter box can enable the proper register of multiple tube. 
     The elliptical construction of filter elements according to the present invention provides improved priming characteristics associated with the filter well because the elliptical construction provides more area in a given horizontal plane, thereby reducing the pressure differential in the vertical direction within the filter element. This results in increased flow through the filter element at a given elevation within the filter well as compared to a filter element having a circular cross-section. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take physical form in certain parts and arrangements of parts, a preferred embodiment of which will be described in detail in this specification and illustrated in the accompanying drawings which form a part hereof, wherein: 
     FIG. 1 is an isometric of a filter kit according to a preferred embodiment of the invention. 
     FIGS. 1A-1C illustrate preferred mounting configurations for a filter element according to the present invention. 
     FIG. 2 is a top view of a filter well according to the present invention. 
     FIG. 3 is a cross-sectional view taken along lines  3 — 3  of FIG.  2 . 
     FIGS. 4A-4D represent alternative mounting configurations for a filter element according to the invention. 
     FIG. 5 is a cross-sectional view of an alternative filter element according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings wherein the showings are for the purposes of illustrating the preferred embodiment of the invention only and not for purposes of limiting same, the FIGURES show a filter element, filter kit and filter box in accordance with the present invention. 
     FIG. 1 illustrates a filter kit  20  according to the present invention. It comprises an end plate  22  which may be constructed of a molten-aluminum resistant refractory material. End plate  22  is adapted to be mounted in a filter well and is provided with a number of openings  24  (two are illustrated) for conveying filtered molten metal to an outlet of a filter well, as will be described below. Two filter elements  26  are provided for removable engagement with the end plate  22 . While one row of two elements is described here, it will be recognized that more than a single tube row may be provided, i.e., two rows of two elements, two rows of four elements, etc. Each filter element  26  is provided as a hollow tubular element of particulate refractory material and having a closed end  28  and an open end  30  and an inner cavity  32  communicating therewith. In accordance with the invention, filter elements  26  are adapted to be removably mounted to end plate  22 . For example, as illustrated in FIG. 2, the open end  30  of each filter element  26  is provided with a flat surface for abutting engagement with the face of the end plate in the area immediately surrounding an associated opening  24 . To retain the removable filter elements in place, a wedge block  34  is provided for each filter element  26  (only one wedge block is shown) and cooperates therewith in a manner that will be described. 
     FIG. 1A illustrates a preferred construction for the mounting details of a filter element  26  according to the present invention. End plate  22  is provided with a tapered inlet opening  25  around an opening  24  in the end plate  22 . A gasket  27  is provided on the tube end for providing sealing engagement with the tapered face  25 . FIG. 1B illustrates a second preferred construction for the mounting configuration according to the present invention. Like the embodiment in FIG. 1A, the embodiment of FIG. 1B is provided with a tapered face  25  on the end plate  22 . The tapered face  25  provides a larger opening into which the filter element  26  may be initially introduced for providing ease of alignment of the filter element with the end plate. In the embodiment of FIG. 1B, the end plate is provided with a shoulder  29  for abutting one face of gasket  27 , the opposite face of gasket  27  engaging the end of filter element  26 . FIG. 1C shows another preferred construction of the mounting details. In this embodiment, no tapered face is provided on the end plate  22 . However, a shoulder  29  is provided for abutting engagement with the gasket  27 . In the embodiment in FIG. 1C, a gap is shown between the outer circumference of the filter element  26  and the recess  31  formed in the end plate  22  for receiving the end of the filter element. While the gap is illustrated at FIG. 1C, it is in exaggerated form as the clearances between the recess  31  and the outer circumference of filter element  26  will be only large enough so that installation of the filter element  26  into engagement with the end plate  22  may be easily facilitated. 
     FIGS. 2 and 3 represent a top and a side cross-sectional view of a filter box, generally referenced  40 , which is adapted according to the present invention. Filter box  40  generally comprises an inlet  42  for conveying molten metal to a filter well  44  and an outlet  46  for conveying filtered molten metal from the filter well  44 . End plate  22  is, at its sides, disposed in complementarily shaped recesses or slots  48  which permit the end plate  22  to be slid into and out of engagement with the filter well in the direction of arrow A in FIG.  3 . 
     According to the present invention, filter elements  26  are retained in abutting engagement with the face  23  of end plate  22  by wedge blocks  34 , which are constructed of a refractory material. The interior cavity  32  of the filter elements are in fluid communication with the openings  24  in end plate  22  such that filtered molten metal may be conveyed to outlet  46 . Wedge blocks  34  engage the closed end  28  of filter elements  26  as well as the filter wall  50  to provide a lateral force on the filter elements which retains them in abutting engagement with the end plate  22 . It will be recognized that filter elements  26  may be removed separately from engagement with the end plate  22  by first removing an associated wedge block  34  and then lifting the filter element  26  from the filter well. It will further be recognized that the force provided by wedge blocks  34  may be adjusted to provide a fluid tight connection between filter elements  26  and end plate  22 . 
     The configuration illustrated in FIGS. 1-3 is one preferred technique for adapting the filter elements to be removably mounted to the end plates. Other techniques have been contemplated as shown in the cross-sections of FIGS. 4A-4C which depict a single filter element/end plate connection. Specifically, FIG. 4A illustrates an end plate  122  which is provided with an enlarged opening  124  for receiving the outer diameter of filter element  126 . A shoulder  125  is provided for abutting the end  130  of the filter element  126 . A tubular projection  127  is provided for engaging the outer diameter of filter element  26  to provide additional support and sealing capability thereto. 
     FIG. 4B illustrates another embodiment of the present invention in which the filter element  226  is adapted to be removably mounted to the end plate  222  by incorporating an inner recess  228  which receives a tubular projection  227 . FIG. 4C illustrates another embodiment in which the filter element  326  is provided with a taper  328  which is received in a complementarily shaped tapered recess  330  on end plate  322  which also includes a shoulder  325  on the interior of opening  324 . FIG. 4D illustrates a filter element  426  that is provided with an internal taper  428  which sealingly engages a tapered projection  427  on end plate  422 . While FIGS. 1-3 depict filter elements  22  which are of a generally circular cross-section, it will be appreciated that the inventive principles demonstrated thereby are applicable to other filter element configurations. 
     In accordance with another aspect of the invention, the filter elements  22  may be provided with a generally elliptical cross section as shown in FIG.  6 . The elliptical cross-section provides an advantage related to the priming of the filter element. An elliptical cross-section provides a larger horizontal surface area as compared to a circular tube of the same outer surface area. As molten metal is introduced into the box, a head pressure is required for priming of the filter element. In turn, the smaller vertical cross-sectional height of the elliptical tube may allow complete priming of all elevations of the tube at a lower overall fill depth of the filter box. 
     The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon a reading and understanding of this specification without departing from the spirit and scope of the invention described herein. The invention is intended to include all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.