A FILTER ASSEMBLY HAVING A FILTER ELEMENT AND A SEALING DEVICE

Abstract: The present invention provides a filter assembly in which a ceramic tube filter (9) may be preassembled with first (2) and second (13) sealing elements prior to installation. The preassembled filter assembly has an advantage in that the seal between the ceramic filter and the metal sealing device may be tested in the manufacturing stage prior to installation in the field. Additionally, a tube sheet including filter assemblies according to the present invention may locate the filter assemblies closer together, thus reducing the pitch between the filter elements.

Description: A FILTER ASSEMBLY HAVING A FILTER ELEMENT AND A SEALING DEVICE This application is a continuation-in-part of U.S. Application 08/478,618, filed June 7, 1995, which is a continuation-in-part of U.S. Application 08/277,869, filed July 20, 1994, which is a continuation-in-part of U.S. Application 07/988,642, filed December 11, 1992, and now U.S. Patent 5,401,406, herein incorporated by reference. Technical Field The present invention relates to a ceramic tube-type filter arrangement which may be used, for example, in industrial processes for purifying fluids in either the gaseous or liquid phase. Conventional applications include, but are not limited to, coal gasifiers, fluidized bed combustors, smelters, and catalytic crackers. Background Art A typical arrangement generally comprises a tank or a pressure vessel divided into an inlet portion and an outlet portion by a relatively rigid support member called a tube sheet. A plurality of tube-type filters are typically coupled to apertures in the tube sheet. A fuse may also be coupled to each tube-type filter to prevent particulate bypass in the event that the tube-type filter is damaged. A conventional tube-type filter element is made of a porous ceramic material and is commonly referred to as a candle filter. Candle filters are particularly effective in removing particulates from high pressure, high temperature gases. A candle filter typically comprises a hollow, cylindrical tube disposed between a closed end and an open end with a flange disposed about the open end. A conventional fuse is similar in shape to a conventional filter element. For example, a conventional fuse may comprise a hollow, cylindrical tube disposed between a closed end and an open end with a flange located at the open end. In a conventional fuse, back pressure created by the fuse may be minimized by using a tubular shaped fuse to maximize the surface area of the fuse. However, the tubular shape of a conventional fuse is difficult and expensive to manufacture. Further, mounting a tubular shaped fuse to a tube sheet and a filter element is difficult and problematic. In conventional tube-type filter assembly, it is difficult to seal the fuse and/or the filter to the tube sheet. The seal between a tubular element and a tube sheet has long been a cause of failure in the filtration of hot gasses. The seal may fail directly or induce failure in the fuse or filter element as a result of stress at the flanges. For example, the filter elements may crack in an area immediately adjacent to the flange. In conventional tubular ceramic filter assemblies, the compressive seal between the filter elements and the tube sheet must be assembled in the field, i.e., in an operational unit. Improper clamping and/or over or under compression of the filter elements is a common cause of failure of tube-type filters and/or fuse assemblies. In typical installations, a number of parallel connected filter elements are disposed within a single tank. The parallel connection and operational environment make it difficult or impossible to determine if a particular filter element has been clamped correctly, and to isolate a failed clamping arrangement from among several parallel connected filter elements. Further, the compression of the gasket material disposed between the sealing element and the tube sheet should be adjusted within precise limits in order to provide an optimum seal and not over or under compress the gasket. In embodiments where space is constrained and filtration area is to be maximized, conventional filter assemblies are disadvantageous in that the filter tubes are disposed through an aperture of sufficient size to accommodate the filter tube. The filter tubes may have large external diameters of several inches or more. Additionally, in some embodiments a clamping mechanism is provided between the filter elements on the surface of the tube sheet. Often, sufficient clearance must be maintained for installation and welding of the sealing element which typically includes an annular ring disposed around the outside diameter of the filter element. Thus, the clamping mechanism becomes the limiting factor determining the spacing or pitch of the filter elements. The large apertures through the tube sheet may also require a predetermined spacing before another large aperture may be inserted in the tube sheet due to thermal and structural support considerations. Accordingly, in conventional embodiments, the spacing or pitch of the filter elements within the tube sheet is limited by the outside diameter of the filter elements and by a minimum separation necessary for ensuring structural integrity and proper clamping. Accordingly, there is a need for an improved seal between the filter element and the tube sheet and for an improved fuse to increase the reliability of the filter assembly and decrease manufacturing costs. Disclosure of the Invention: A principal object of the present invention is to provide a filter assembly which overcomes the above problems by providing a simple and reliable sealing mechanism for sealing the filter element to a tube sheet. The filter assembly may also provide an improved fuse which enhances the reliability of the filter assembly while reducing manufacturing costs. Another principal object of the present invention is to provide a preassembled filter assembly in which the seal between the ceramic filter element and the ceramic tube filter is defined and tested under controlled manufacturing conditions. Additional objects of the present invention include sealing the filter element throughout the entire range of operating temperatures of the filter assembly; providing multiple sealing surfaces to enhance reliability; providing a sealing mechanism which is less susceptible to installation errors; providing a sealing mechanism and filter assembly in which the seal between the ceramic filter and the metal sealing device can be tested at the factory as a single unit with or without temperature variations; providing a filter assembly including a sealing element for coupling to the tube sheet, where a cross- sectional area of the sealing element is less than that of an outside of the filter element; decreasing the pitch between adjacent filter elements; and providing an improved fuse. The present invention provides a filter assembly having a ceramic tube filter which may be preassembled with first and second sealing elements prior to installation. The preassembled filter assembly has an advantage in that the seal between the ceramic filter and the sealing elements may be tested in the manufacturing stage prior to installation in the field. Further, the filter and the sealing elements may be assembled under controlled manufacturing conditions to ensure consistent quality in the seal, thus providing a significant advantage over conventional arrangements. After the filter assembly is assembled according to the present invention, the metal sealing device is simply attached to the tube sheet in the field without the need for the application of a pre-compressive force or concern with the seal between the ceramic filter element and the metal sealing device. Thus, the installation of the tube filter is substantially reduced in complexity and the potential for improper installation is likewise reduced. Further, a tube sheet including filter assemblies according to the present invention may locate the filter assemblies closer together, thus reducing the pitch between the filter elements and reducing the size of the tank. Accordingly, the present invention provides a filter assembly connectable to a tube sheet comprising a ceramic tube filter having a first opening and a compressive assembly separate from and connectable to the mbe sheet. The compressive assembly includes first and second metal elements coupled together to compress at least a portion of the ceramic tube filter therebetween, and a second opening in fluid communication with the first opening. The present invention may also provide a ceramic fuse having a substantially planar shape for use with a ceramic tube filter. The ceramic fuse may have either a monolithic or a multi-layer structure. Additionally, the present invention provides a method for attaching ceramic elements to a mbe sheet including preassembling a filter assembly. The filter assembly is preassembled by compressing a ceramic element between first and second sealing elements to form a fluid tight seal between the first and second sealing elements and a ceramic element. Then, the filter assembly is attached to a tube sheet. Substantial research has been expended in deterπώύng preferred embodiments for overcomming the aformentioned problems and achieving the above mentioned objectives. Accordingly, the present invention may also include a filter assembly having one or more of the elements described herein and/or shown in Figures 1-8, in any combination or subcombination. Brief Description of Drawings Figure 1 is a sectional view of a first embodiment of a filter assembly according to the present invention; Figure 2 is a sectional view of a second embodiment of a filter assembly according to the present invention; Figure 3 is a sectional view of a trjird embodiment of a filter assembly according to the present invention; Figure 4 is a sectional view of a fourth embodiment of a filter assembly according to the present invention; Figure 5 is a sectional view of a fifth embodiment of a filter assembly according to the present invention; Figure 6 is a sectional view of a sixth embodiment of a filter assembly according to the present invention; Figure 7 is a sectional view of a seventh embodiment of a filter device according to the present invention; Figure 8 is a sectional view of an eighth embodiment of a filter assembly according to the present invention; Figure 9 is a sectional view of a ninth embodiment of a filter assembly according to the present invention; Figure 10 is a sectional view of a tenth embodiment of a filter assembly according to the present invention. Best Modes For Carrying Out The Invention Referring to Figure 1, a first exemplary filter assembly 1 embodying the present invention generally comprises a compressive assembly 19 coupled to and sealing a filter element 9. In general, the compressive assembly 19 includes first and second elements which cooperate to compress the filter element 9, a fuse 8, and/or compressive material 12 therebetween. The first and second elements may be variously configured to include first and second sealing elements, a ring 18, or other suitable member. In accordance with the embodiment of the invention shown in Figure 1, the compressive assembly includes a first sealing element 2 and a ring 18 compressing the fuse 8 therebetween. The compressive assembly 19 may also include a second sealing element 13 cooperating with the ring 18 and/or first sealing element 2 to compress and seal the filter element 9. The first sealing element 2 may be variously configured. For example, the first sealing element 2 may be adapted for securing the filter assembly 1 to a tube sheet 7. The first sealing element 2 may be fabricated as a single unitary piece or from two or more separate sections by, for example, machining, forming, stamping, or casting. If it is made from two or more separate sections, the sections of the first sealing element 2 may be joined together by various methods such as a welded, threaded, or press fit connection. Further, the first sealing element 2 may be variously configured without departing from the scope of the invention. For example, the illustrated first sealing element 2, as shown in Figure 1, may include a tubular portion 3 and a neck portion 4. The tubular portion 3 and neck portion 4 may be concentrically arranged, with the tubular portion 3 having a larger, smaller, or the same cross-sectional area as the neck portion 4. In a preferred embodiment, it may be desirable for the tubular portion 3 to have a cross- sectional area larger than the neck portion 4. In this manner, the size of an aperture 22 in the mbe sheet 7 may be reduced, thereby allowing the filter assemblies 1 to be located closer together when installed in the tube sheet 7. In preferred embodiments, the neck portion 4 may have an outer diameter less than an outer diameter of a tubular portion of the filter element 9. This configuration allows for closer spacing of the filter elements in the mbe sheet 7 since the spacing is not limited by the large apertures normally required to accommodate the outer diameter of the filter elements. In some embodiments, the neck portion 4 may have an outer diameter less than an inner diameter of an opening in the filter elements 9 and may act as a venturi or backflow nozzle. For example, the diameter of the neck portion 4 is preferably between about 1 cm and 15 cm, and more preferably between about 2 cm and 10 cm, and even more preferably between about 3 cm and 5 cm, and most preferably about 4 cm. The first sealing element 2 may include an internal passage 23 passing through the element from one open end to another. In the embodiment illustrated in Figure 1, the internal passage 23 extends axially through the tubular portion 3 and through the neck portion 4 of the first sealing element 2. The configuration of the internal passage 23 may or may not be arranged in such a manner as to act as a venturi or nozzle. If the internal passage 23 is configured to act as a venmri or nozzle, it is preferred to locate the venmri in the neck portion 4. The nozzle and/or an alternative capillary (jet) configuration can be used to increase the velocity of fluid flowing in a back flow di¬ rection and, thus, to enhance the blowback effectiveness when the filter elements 9 are cleaned. In an alternative embodiment, it may be preferable for the internal passage 23 to be configured as a plurality of internal passages. In some embodiments, at least one of these plurality of internal passages may be formed into a nozzle, capillary, or jet configuration to provide a means for increasing back flow efficiency. In yet another embodiment, the internal passage may simply be a cylindrical bore. In the embodiment illustrated in Figure 1, the venmri is formed by a nozzle portion having an inner diameter tapered such that the inner radius increases in the direction toward the filter 9. The neck portion 4 may include any suitable connecting mechanism including a welded, threaded, or press fit connection for connecting the filter assembly 1 to a mbe sheet 7. In the embodiment illustrated in Figure 1, the neck portion 4 includes an outer flange 6 and a tapered portion 5. The outer flange 6 is preferably configured for coupling to a mbe sheet 7. In a preferred embodiment, the filter assembly 1 is adapted to be coupled to the mbe sheet only along the periphery of the outer flange 6 at an upper portion of the mbe sheet 7. In this embodiment, the aperture 20 through the mbe sheet 7 is preferably slightly larger than the outer flange 6 to facilitate coupling the outer flange 6 to the mbe sheet 7. The mbe sheet 7 preferably forms a fluid tight seal between the inlet portion and the outlet portion of a tank (not shown) and typically has a series of holes or apertures 22 spaced from one another. The mbe sheet 7 may be variously configured to include a sheet, mbe, conduit, or other configuration suitable for accommodating a plurality of filter assemblies and for dividing the tank into an inlet portion and an outlet portion. The filter assemblies 1 are typically coupled to the mbe sheet such that an outlet of each filter assembly is in fluid communication with one of the apertures in the mbe sheet. The tubular portion 3 of the first sealing element 2 may be variously configured. In the embodiment illustrated in Figure 1, the first sealing element 2 is adapted to accommodate a fuse 8 within the tubular portion 3. The fuse 8 typically comprises a coarse ceramic or metallic material which provides a redundant mechanism for blocking particulate flow should the filter element 9 and/or seal about the filter element become damaged. In some embodiments, the fuse may be designed to clog/foul with particulate matter in order to prevent or impede particulate flow when the filter element 9 is damaged. The fuse 8 may be narrow and elongated. In preferred embodiments, the fuse 8 includes a closed or blind end, an open end, and a porous side wall extending therebetween and defining an internal cavity that opens at the open end. The fuse 8 may be constructed of a metallic material or a porous ceramic composition, such as Si 3 N 4 , mullite, cordierite (MgO, Al 2 O 3 , SiO 2 ), fireclay, aluminosilicate fibers, alumina, mullite, alumina/mullite, and silicon carbide-based materials. In some embodiments, the fuse 8 may be fabricated using an aluminosilicate binder. Further, the fuse may be sintered. Additionally, the fuse may be of a reticulated foam construction. In the most preferred embodiments, both the fuse and the ceramic filter is formed from a silicon carbide material. The fuse may be variously configured to have an axial length of between 50 and 1000 mm, and preferably between 100 and 500 mm, and more preferably between 150 and 400 mm, and even more preferably between 200 and 300 mm, and most preferably about 260 mm in length. The fuse may have an inner diameter of between 10 and 400 mm, and preferably between 20 and 200 mm, and more preferably between 30 and 100 mm, and most preferably about 40 mm. The fuse may have an outer of between 1 and 100 mm larger than the inner diameter, and preferably between 2 and 50 mm, and more preferably between 5 and 30 mm, and even more preferably between 8 and 20 mm, and most preferably about 10 mm larger than the inner diameter. The fuse may be constructed as a single layer or a multi-layer design as discussed below with respect to planar fuses. If a single layer design is utilized, it is preferably to form the layer from a ceramic powder having a monolithic layer of between 20 and 400 grit and preferably between 30 and 300 grit, and more preferably between 40 and 200 grit and even more preferably between 50 and 100 grit and most preferably about 60 grit. The resulting pore size may be between 10 and 200 microns and preferably between 30 and 150 microns, and even more preferably between 40 and 120 microns and most preferably about 80 microns. In preferred embodiments, the pore size/structure of the fuse 8 is proportional to the pore size/ structure of the filter element 9. For example, where the fuse is of a tubular configuration, the fuse 8 may range from the same permeability as the filter element 9 up to about 200 times as permeable as the filter element 9, and more preferably range between 2 and 200 times more permeable than the filter element 9, and even more preferably range between 5 and 50 times more permeable than the filter element, and most preferably about 10-15 times more permeable than me filter element. For a substantially planar fuse configuration, the fuse 8 may range from the same permeability as the filter element 9 up to about 400 times as permeable as the filter element 9, and more preferably range between 20 and 300 times more permeable than the filter element 9, and even more preferably range between 40 and 200 times more permeable than the filter element, and most preferably about 80-120 times more permeable than the filter element 9. In the embodiment shown in Figure 1, the fuse 8 is slidably inserted through the first sealing element 2 through a hole or aperture 21 in the tubular portion 3 and divides the internal passage 23 into an inlet side and an outlet side. The open end of the fuse 8 may include a flange 10 which prevents the fuse 8 from passing completely through the aperture 21. The flange 10 may be at any suitable angle to an axis 29 of the fuse 8. If the flange 10 is at a right angle, the flange may engage the top of the first sealing element 2 or a lip formed in the wall defining the aperture 21. In the some embodiments, the outer surface of the flange 10 may slope upwardly at an acute angle and may include a concave portion. The flange 10 preferrably cooperates with a corre- sponding locating profile or ring portion 11, e.g., an annular bevel and/or inwardly extending flange in the aperture wall. In preferred embodiments, a compressible material 12 is disposed between the annular ring portion 11 and the flange 10. The first sealing element 2 may optionally include an extended neck portion 17 extending axially from the annular ring portion 11 and surrounding the flange 10. Referring to Figure 1, the compressive assembly 19 preferably includes a second sealing element 13 in addition to the first sealing element 2. In accordance with one aspect of the invention, the second sealing element 13 is preferably utilized to couple the filter element 9 to the first sealing element 2. The second sealing element 13 may be fabricated as a single unitary piece or from two or more separate sections by, for example, machining, forming, stamping, or casting. If it is made from two or more separate sections, the sections of the second sealing element 13 may be joined together by various methods such as a welded, threaded, or press fit connection. Further, the second sealing element 13 may be variously configured without departing from the scope of the invention. The illustrated second sealing element 13 in Figure 1 includes an annular or disk section portion 24 and a hollow cylindrical or neck portion 16 which may be unitarily formed. The second sealing element 13 may include an internal passage 23 passing through the element from one open end to another. For example, in the illustrated second sealing element 13, the passage 23 extends axially through the hollow annular portion 24 and through the neck portion 16. The second sealing element 13 may be variously configured. In preferred embodiments, the second sealing element 13 includes an aperture for accommodating the filter element 9. For example, the filter element 9 may be disposed within the internal passage 23 within the second sealing element 13 and divide the internal passage 23 into an inlet portion and an outlet portion. Further, the internal passage 23 through the second sealing element 13 may be arranged to act as a positioning device for filter element 9. For example, the second sealing element 13 may include an annular ring, internal flange, and/or bevelled portion 14. The bevelled portion 14 is preferably configured to mate with a flange 15 of filter element 9. In preferred embodiments, a compressible material 12 is disposed between the bevelled portion 14 and the flange 15. The neck portion 16 may optionally extend axially from the flange 15 and surround the filter element 9. In yet another embodiment, the internal passage 23 through the second sealing element 13 need not be tapered or bevelled. For example, the passage 23 through the second sealing element 13 may simply be a cylindrical bore. The outer shape of the first and second sealing elements 2, 13 may be variously configured to include a circular, oval, square, star, rectangle or other multi-sided or irregularly shaped configuration. In preferred embodiments, the outer shapes of the first and second sealing elements 2, 13 are configured so as to allow the filter assemblies 1 to be closely spaced within the mbe sheet 7. Accordingly, it is desirable for the first and second sealing elements 2, 13 to minimize the thickness at the portion of the first and second sealing elements 2, 13 attached to the mbe sheet 7 and surrounding the flange 15 of the filter element 9. Each filter element 9 is preferably narrow and elongated. It may include a closed or blind end, an open end, and a porous side wall. When a closed end is present, the side wall extends from the closed end to the open end and has an internal surface which defines a cavity that opens at the open end. When the filter includes first and second open ends, the side wall extends from the first open end to the second open end and has an internal surface which defines a cavity. The filter element 9 is preferably constructed of a porous ceramic material. The ceramic may be made of such compositions as Si 3 N 4 , mullite, cordierite (MgO, Al 2 O 3 , SiO 2 ), fireclay, aluminosilicate fibers, alumina, alumi- na/mullite, and silicon carbide-based materials. In one preferred embodiment, a hollow, porous, ceramic candle filter is used as the filter element 9. The first and second sealing elements 2, 13 are preferably disposed opposed to each other and coupled together at a coupling location 28 either directly or through a ring 18. If a ring 18 is utilized, the ring 18 is preferably disposed within the first and second sealing elements 2, 13 and between the fuse 8, (if any), and the filter element 9. In preferred embodiments, a compressible material 12 is disposed between the ring 18 and the filter element 9 and between the ring 18 and the fuse 8. In the embodiment shown in Figure 1, the filter element 9 is slidably inserted through the aperture 20 in the second sealing element 13. The flange 15 preferably prevents the filter element 9 from passing completely through the aperture 20. The flange 15 may extend at any suitable angle to the axis 29. If the flange extends at a right angle, the flange may engage the top of the second sealing element 13 or a lip formed in the wall defining the aperture 20. In the first embodiment, the flange 15 slopes upwardly at an acute angle and cooperates with a corresponding locating profile 14, e.g. , an annular bevel, in the aperture wall. The ring 18 may be fabricated as a single unitary piece or from two or more separate sections by, for example, machining, forming, stamping, or casting. If it is made from two or more separate sections, the sections of the ring 18 may be joined together by various methods such as a welded, threaded, or press fit connection. Further, the ring 18 may be variously configured without departing from the scope of the invention. For example, the ring 18, may include a flat planar or disk section 25 and one or more hollow cylindrical sections 26, 27 which may be unitarily formed. In a preferred embodiment, it may be desirable for the disk section 25 to have a larger outer diameter than the first and second cylindrical sections 26, 27 and to extend radially out beyond an outer surface of the cylindrical sections 26, 27, forming first and second annular surfaces of ring 18. The ring 18 may include an annular groove 36. In embodiments where the first and second sealing elements 2, 13 are welded together, the annular groove may be fully or partially filled with a weld. In this manner the ring 18 may serve as a backup element for the weld. The ring 18 preferably includes an internal passage 23 passing through the ring 18 from one open end to another. For example, in the embodiment of Figure 1, the internal passage 23 extends axially through the first and second hollow cylindrical sections 26, 27 and through the disk section 25. The configuration of the internal passage 23 may or may not be arranged in such a manner as to act as a venmri or nozzle. In accordance with one aspect of the invention, the ring 18 and the filter element 9 preferably have different rates of thermal expansion, and these different rates of thermal expansion effect a seal between the filter element 9 and the ring 18 throughout the entire range of system temperatures. At certain times, such as during start up or shutdown, the filter assembly is at a relatively low temperature and pressure, e.g. atmospheric pressure and room temperature. A seal is then preferably maintained between the top of the filter element 9 and the disc section 25 of the ring 18 by compression of the compressible material 12 due to a compressive preload force in the filter assembly 1. On the other hand, the operating temperature and pressure may be much higher. For example, the filter assembly 1 may be operated at a temperature of around 300°C to 1000°C or more and at a pressure of around 2 to 10 atmospheres. Embodiments of the present invention preferably utilize a ring 18 and a filter element 9 having different coefficients of thermal expansion and take advantage of the different rates of thermal expansion to further seal the filter element 9 to the ring 18 at these elevated temperatures. For example, ceramics typically have a much lower coefficient of thermal expansion than metals. Thus, as the temperature of the filter assembly 1 is increased during operation, the metal ring 18 may expand at a faster rate than does the ceramic filter element 9 due to the different coefficients of thermal expansion of metals and ceramics. The increase in temperature may result in a gradual decrease in the axially directed preload force between the sealing elements 2, 13, ring 18, and filter element 9 due to differing coefficients of expansion between the metal sealing elements and the ceramic filter element 9. However, although the sealing efficiency may be reduced between the top of the filter element 9 and the bottom of the disc section 25 or ring 18, the overall sealing efficiency is retained. This result occurs because the increase in temperature also causes the neck portions 26, 27 of the ring 18 to expand radially at a faster rate than the filter element 9 and/or fuse. This has the effect of reducing the clearance between the cylindrical sections 26, 27 and the internal surface of the filter element 9 and/or fuse, thus compressing the compressible material 12 and sealing the cy¬ lindrical section 26, 27 of the ring 18 against the internal surface of the filter element 9 and/or fuse even tighter. In this way an effective seal is maintained throughout the entire range of system temperatures. A similar sealing mechanism may also be employed between ring 18 and fuse 8. Chemical corrosion may occur as a result of chemicals in the fluids processed through the filter assembly 1. As a result, the first and second sealing elements 2, 13 and the ring 18 are preferably manufactured from a material which resists corrosion, such as a high temperature, corrosion resistant, metal alloy such as RA333, available from Rolled Alloys of Temperance, MI. Other high temperature metal alloys, such as stainless steels and nickel alloys may instead be utilized. The compressible material 12 may be a high temperature gasket material, most preferably a ceramic fiber material. In some embodiments a suitable gasket material may be, for example, a gasket material available from 3M Corporation under the trade designation Interam and/or more preferably Nextel. The compressible material may provide a cushioning, sealing, and/or gasket mechanism between opposed surfaces of the first and second sealing elements 2,13, the ring 18, the fuse 8, and/or the filter element 9. The compressible material 12 may also act to prevent uneven stresses from being generated in the filter element 9 and/or fuse 8 due to non-uniformities on the interacting surfaces of the filter element 9, the fuse 8, the first and second sealing devices 2, 13, and/or the ring 18. The use of the compressible material 14 is particularly useful when the filter element 9 and/or the fuse 8 include a ceramic material. In embodiments of the invention, the filter assembly 1 may be preassembled at the factory and then shipped to the field as an assembled unit. The preassembly operation allows the seal between filter element 9 and/or fuse 8 to be formed and tested in the factory under controlled conditions. For example, each filter assembly 1 may be assembled using a precise compressive force. Further, each filter and ceramic/metal seal may be factory tested individually over a variety of temperature ranges. A preload force may be used to compress the first and second sealing elements 2, 13 together to form a compressive assembly 19 and seal the filter element 9. If the ring 18, fuse 8, and/or compressible material 12 are utilized, the preload force may also compress these components between the first and second sealing elements 2, 13 to form the compressive assembly 19. A preload force may be applied by any suitable mechanism including a hydraulic, pneumatic, mechanical, or electromagnetic press or other pressure exerting arrangement. In preferred embodiments, the preload force may be of a magnitude sufficient to compress the compressible material 14 to prevent fluid flow around the filter element 9 and/or the fuse 8, while not over compressing the filter elements and/or compressible material 12. In exemplary embodiments, the preload force may range from 2 to 200 psi, preferably from 5 to 100 psi, more preferably from 20 to 80 psi, even more preferably from 30 to 60 psi and most preferably about 40 psi. The preferred preload force is determined based on the structural considerations such as characteristics of the filter element, compressible material, and/or sealing elements. In exemplary methods according to the present invention, a preload force is applied between the first and second sealing elements 2, 13 using any suitable technique. For example, the compressive assembly 19 may be placed in a press and compressed to a predetermined pre-load force. Thereafter, the preload force may be maintained by, for example, securing the first sealing element 2 to the second sealing element 13 in a fixed relationship. The fixed relationship may be maintained by any suitable mechanism as, for example, a welded, bolted, threaded, latched, locked, and/or a pin-and-hole connec¬ tion. The method of connection should be capable of withstanding vibrations and thermal shock resulting from, for example, pulse cleaning of the filter elements. After the filter assembly is completed by securing the first and second sealing elements 2, 13 in a fixed relationship, the filter assembly may 1 be shipped from the factory for installation in the field. In the field, the first sealing element 2 may simply be attached to the mbe sheet 7. There is no need for a complicated sealing arrangement to the tube sheet 7 since the first sealing element 2 is preferably made of a material, e.g., a metal or metal alloy, that closely matches the thermal expansion characteristics of the tube sheet 7. Accordingly, the filter assembly may be coupled to the mbe sheet by, for example, welding, threading, and/or clamping the first and/or second sealing elements 2, 13 to the mbe sheet. In a preferred embodiment, the neck portion 4 of the first sealing element 2 is welded within an aperture of the mbe sheet 7. The filter assembly, according to embodiments of the present invention, may be assembled by, for example, one or more of the following steps: a) placing the compressible material 12 on the first sealing element 2 and/or the fuse 8; b) inserting the fuse 8 into the first sealing element 2; c) placing the compressible material 12 on the second sealing element 13 and/or the filter 9; d) inserting the filter element 9 into the second sealing element 13; d) disposing the compressible material 12 about opposed first and second annular surfaces of the ring 18; e) configuring the filter assembly 3 by bringing the first sealing element 2, the second sealing elements 13, the fuse 8, the filter 9, the ring 18 and/or the compressible material 12 together; d) compressing components of the filter assembly 1; and e) fixing the first and second sealing elements 2, 13 relative to each other and/or the ring 18 in order to maintain the compression. In a preferred embodiment, the filter assembly according to Figure 1 is placed in a press where a predetermined pre-load force is applied. The preload force may be maintained by, for example, forming a girth weld along coupling location 28 to form a welded joint between the first and second sealing elements 2, 13 and the ring 18. In alternative embodiments, a mechanical stop machined into one or more components of the filter assembly 1, may be utilized in conjunction with known manufacturing dimensions of the filter assembly elements to determine the compressive force on the elements within the filter assembly 1. In embodiments utilizing a mechanical stop, the filter assembly 1 is compressed until the stop prevents further compression. The first sealing element 2, second sealing element 13, and/or the ring 18 are then coupled together to maintain the force. For some applications, this embodiment is not preferred since machining tolerances may be more costly and may not permit precise control of the pre-compressive force. The filter assembly according to the present invention may be variously configured to include more or less elements than those illustrated in Figure 1. For example, in Figure 2, first and second filter assemblies 1A, IB are included for coupling first and second filter elements 9A, 9B together. (Many of the components of the second and subsequent embodiments are similar to the components of the first embodiment and are identified by similar reference numerals.) For example, the first and second filter assemblies 1A, IB are similar to the filter assembly 1 shown in Figure 1. Similiarly, the first and second filter compressive assemblies 19A, 19B are similar to the compressive assembly 19 shown in Figure 1. However, in the second filter compressive assembly 19B of Figure 2, first and second filter elements 9A, 9B, are coupled together using two second sealing elements 13B, 13C. In order to reduce fabrication costs, the second sealing elements 13A, 13B, 13C may be identical. In some embodiments, it may be preferable to dispose a ring 18B and one or more pieces of compressible gasket materials 12 between the first and second filter elements 9A, 9B. It may also be desirable for the two second sealing elements 13B, 13C to each be made from two or more separate sections and joined together by various methods such as a welded, threaded, or press fit connection. In this manner, the second sealing elements 13A, 13B can be assembled over a filter 9A having a flange located at both first and second ends. The elements of the first and second filter assemblies 1 A, IB may be assembled together, loaded (compressed) with a preload force, and fixed in relative position in the same manner as discussed above for the first embodiment. In the embodiment shown in Figure 3, the filter assembly is configured to include less elements than those illustrated in Figure 1. For example, in Figure 3, the function of the ring is incorporated directly into the first sealing element 2A, with the fuse being omitted entirely. In this embodiment, the first and second elements of the compressive assembly 19C are respectively the first and second sealing elements 2A, 13. The first and second sealing elements 2 A, 13 may overlap each other in one or more locations. The filter 9 may be disposed in the second sealing element 13 with or without the compressible material 12. The first and second sealing elements 2 A, 13 and the filter 9 are then assembled together, loaded (compressed) with a preload force, and fixed in relative position in the same manner as discussed above for the first embodiment. Figure 4 illustrates an alternative embodiment of the filter assembly 1. In this embodiment, the ring and fuse are omitted entirely such that the first sealing element 2B is coupled directly to the second sealing element 13D to form compressive assembly 19D. It may be preferable for the first and second sealing elements 2B, 13D to overlap each other in one or more locations. The filter 9C may be disposed in the second sealing element 13D with or without the compressible material 12. Further, a mechamcal stop may optionally be included to regulate the pressure and/or prevent the compressible material 12 from being over compressed. The first and second sealing elements 2B, 13D and the filter 9C are then assembled together, loaded (compressed) with a preload force, and fixed in relative position to each other in the same manner as discussed above for the first embodiment. Regulation of the compression by mechanical stops formed and/or machined into the components of the filter assemblies may be less preferred in some embodiments. For example, in some embodiments, mechanical stops necessitate that each of the components of the assembly and the gaskets be machined to very rigid tolerances. These close tolerances may be difficult to achieve and add substantially to the cost of manufacturing the components of some filter assemblies. However, mechanical stops may provide a useful mechanism to prevent over or under compression of the compressible material - particularly where the filter assembly is assembled in an operational environment. The embodiment of Figure 4 illustrates an alternative mechanism for attaching the first sealing element 2B to the mbe sheet 7 which may be employed in each of the embodiments of the filter assembly 1. For example, the first sealing element 2B may include a threaded or press-fit connection 33 and/or one or more locking arrangements such as pins and/or a lock nut 32. Figure 5 illustrates another embodiment of the filter assembly 1 according to the present invention. In this embodiment, the filter element 9D does not include a flange. Accordingly, the first and second sealing elements 2C, 13E are disposed on opposite sides of the filter 9D and coupled together to form compressive assembly 19E. In some embodiments, it may be desirable to include a bolt 34, and nut and/or spring assembly 35 to couple the first and second sealing elements 2C, 13E to the filter 9D. The filter 9D may be disposed between the first and second sealing elements 2C, 13E with or without the compressible material 12. The first and second sealing elements 2C, 13E and the filter 9D are assembled together, loaded (compressed) with a preload force, and fixed in relative position using a bolt and nut, pin, spring assembly and/or other clamping arrangement. This embodiment is less preferred because of the long distances between the points of compression on the ceramic filter 9D. Accordingly, gasket material 12 must be able to absorb the differential of expansion over the entire length of the mbe filter. This can have adverse affects on the reliability of the filter assembly 1. Each of the aforementioned embodiments shown in Figures 1-5 include either a conventional fuse or omit the fuse entirely. Figures 6-8 illustrate a second major aspect of the present invention in which the filter assembly 1 includes a fuse 8A having a substantially planar configuration. In these embodiments, elements of the compressive assembly including the ring, first sealing element, fuse, and second sealing element have been modified from previous configurations. A planar fuse may be configured to have high reliability and low manufacturing costs due to the simple geometry. A substantially planar fuse has excellent reliability because the center of mass of the fuse is substantially in-line with the points of attachment between the fuse and the first and second sealing elements and/or ring. When a fuse has a center of mass which is cantilevered and/or substantially out of line with the point of attachment/compression of the fuse, the reliability of the fuse may be decreased and the fuse may tend to crack at a location adjacent to the point of attachment. To attempt to increase the reliability of the fuse, the point of attachment compression is often made thicker than the remaining portions of the fuse. For example, it may be desirable to construct a fuse such that the flange is thicker than other portions of the fuse. I some situations, the flange may be an inch or more in thickness. Although increasing the flange thickness helps prevent the flange from cracking, it often makes sealing the fuse more difficult. For example, when the fuse is made of a ceramic material while the remainder of the sealing device is made of a metal, differing coefficients of thermal expansion between the fuse and the metal sealing elements cause the sealing device to expand at a greater rate than the flange of the fuse. Where large temperature variations are present, the seal between the sealing elements and the fuse may be reduced, particularly in an axial direction. The differential expansion problems become increasingly severe with increasing thickness of the flange. Accordingly, it is desirable to minimize the thickness of the fuse in an axial direction in the area where the fuse is held by the sealing elements. Embodiments, according to the present invention, minimize the thickness of the portion of the fuse coupled to the sealing devices by utilizing a generally planar fuse which has a cross- sectional thickness which may be less than that of a flange because the center of mass of a generally planar fuse is proximate to the point of attachment of the fuse. For example, the thickness of a planar fuse in the area clamped by the first sealing element may be about half an inch or less in thickness. Thus, a substantial reduction in thickness (e.g., about 50 percent) may reduce the problems caused by the differing coefficients of thermal expansion, including reducing stress on the compressible material 12 and fuse 8 while improving overall reliability of the filter assembly. Further, a substantially planar fuse is less complex to manufacmre and therefore can be made more reliably and at less expense than other fuse configurations. Referring to Figure 6, for example, the filter assembly 1 may include a filter 9 and a compressive assembly 19F. In Figure 6, the compressive assembly 19F may include a generally planar fuse 8 A, a ring 18C, and first and second sealing elements 2D, 13F. As with the other embodiments of the present invention, the first and second sealing elements 2D, 13F, ring 18C and/or fuse 8A may be disposed above, below, through, and/or be integral with the mbe sheet 7. For example, where the tube sheet 7 is relatively thick (e.g., one or more inches in thickness), the second sealing element 13F may be formed in the mbe sheet using any suitable technique such as machining, stamping, forming, and/or molding. In the illustrated embodiment, the ring 18C, the filter 9, the fuse 8A, and the second sealing element 13F are disposed through and generally below the mbe sheet 7, while the first sealing element 2D is disposed through and generally above the mbe sheet 7. The first sealing element 2D, the second sealing element 13F, and the ring 18C may abut, partially overlap, and/or completely overlap each other in any suitable combination. For example, where the ring 18C overlaps or extends axially over the first and/or second sealing elements 2D, 13F, the ring 18C may be disposed about the interior, the exterior, and/or between the first and/or second sealing elements 2D, 13F. Where the first sealing element 2D overlaps or axially extends over the ring 18C and/or the second sealing elements 13F, the first sealing element 2D may be disposed about the interior, the exterior, and/or between the ring 18C and/or the second sealing element 13F. Similarly, where the second sealing element 13F overlaps or axially extends over the ring 18C and/or the first sealing elements 2D, the second sealing element 13F may be disposed about the interior, the exterior, and/or between the ring 18C and/or the first sealing element 2D. As with the other embodiments of the present invention, the aperture 22 through the mbe sheet 7 may be variously configured to be larger, smaller, or about the same size as the filter mbe 9. For example, where the ring 18C, the first sealing element 2D, and/or the second sealing element 13F are disposed within the aperture 22, the aperture is preferably sized to accommodate the portions of the filter assembly 1 passing through the tube sheet. If the second sealing element 13F is disposed above the tube sheet, the aperture through the mbe sheet 7 may approximate the size of the filter element 9 or be slightly larger than the filter element 9. If the first sealing element 2D is located below the mbe sheet, the aperture 22 through the mbe sheet 7 may be sized to accommodate the neck portion 4 or the tubular portion 3 of the first sealing element 2D. As discussed above, in some embodiments where minimizing the pitch of the filter assemblies 1 is important, it may be desirable to minimize the size of aperture 22 and to only couple the neck portion 4 to the mbe sheet 7 either within or adjacent to the aperture 22. However, in other embodiments, it may be advantageous to insert or replace the filter assembly 1 from the outlet side of the mbe sheet 7. In these embodiments, the aperture 22 in the tube sheet is preferably sized at least large enough to accommodate the filter element 9. Of course, the aperture may be larger than the filter element 9 in order to accommodate the first sealing element 2D, the second sealing element 13F, and/or the ring 18C. The filter assembly 1 according to embodiments shown in Figures 6-8 of the present invention may be assembled using the methods discussed above or other suitable methods. For example, embodiments of the filter assembly 1 may be assembled by one or more of the following steps (performed in any suitable order): a) placing the compressible material 12 on the filter element 9 and/or on the second sealing element; b) coupling the filter element to the second sealing element; c) placing the compressible material on a first substantially planar surface of the ring and/or on the filter element adjacent to the open end; d) positioning the ring within and/or adjacent to the filter element; e) disposing the compressible material 12 about a second planar surface of the ring and/or about the fuse; f) disposing a first planar surface of the fuse adjacent to the second planar surface of the ring with the compressible material optionally disposed therebetween; g) disposing the compressible material adjacent to a second planar surface of the fuse and/or about a first planar surface of a first sealing element; h) disposing the first sealing element adjacent to the second planar surface of the fuse with the compressible material optionally disposed therebetween; i) bringing the second sealing element and the ring together; j) bringing the first sealing element and the ring together; k) compressing the filter assembly; 1) fixing the first sealing element relative to the ring, the second sealing element and/or the mbe sheet; and/or m) fixing the second sealing element relative to the ring, the first sealing element 2D and or the mbe sheet. As discussed above, it may be desirable to place the filter assembly 1 in a press where a predetermined pre-load force is applied. The pre-load force may be measured directly to determine the amount of force applied. Alternatively, and/or additionally, one or more optional mechamcal stops 31 may be machined, formed, molded, and/or stamped into one or more components of the filter assembly 1. If a mechamcal stop 31 is utilized, the stop may prevent over compression of the compressible material and/or determine the amount of compressive force applied to the elements within the filter assembly 1. The compressive force may be maintained by a) securing the first sealing element to the ring, b) securing the second sealing element to the ring, c) securing the second sealing element to the first sealing element, d) securing the second sealing element and the ring to the mbe sheet, e) securing the first sealing element and the ring to the mbe sheet, and/or f) securing the first and second sealing elements to the mbe sheet in any suitable order and/or combination. In embodiments of the present invention, it may be desirable construct the filter assembly 1 in two or more consecutive steps. For example, in a first step, it may be desirable to seal the filter element by fixing the second sealing element to the ring to compress the filter therebetween. Thereafter, in a second step, it may be desirable seal the fuse by fixing the ring to the second sealing element in a separate operation than the sealing of the filter element. A separate operation allows independent control over the compression of the gaskets disposed about the filter and the gaskets disposed about the fuse, having the ring fixed to both the first and second sealing elements ensures that expansion and contraction of the filter assembly is not entirely absorbed by only one gasket, but rather equally shared by gaskets surrounding the fuse and filter element. As with previous embodiments, the fixed relationship between the components of the filter assembly 1 may be maintained by any suitable mechanism including a welded, bolted, threaded, latched, locked, press fit and/or pin-and-hole connection. In the embodiment illustrated in Figure 6, the fixed relationship is maintained by welding the ring 18C to the first sealing element 2D and to the second sealing element 13F, either simultaneously or in two sperate operations. If a weld is utilized, the weld may be either a spot weld or a girth weld. Alternatively, the first sealing element 2D may be fixed to the second sealing element 13F and the ring may be allowed to float therebetween, i.e., the ring need not be secured to either the first or second sealing element 2D, 13F. The filter assembly 1 may be mounted to the mbe sheet 7 in any suitable manner. Where the second sealing element 13F is disposed below the mbe sheet, the second sealing element 13F may be coupled to the mbe sheet 7 at an upper end surface or spaced from the mbe sheet 7. If the second sealing element 13F is spaced from the mbe sheet 7, then the mbe sheet is preferably coupled and/or integral with the ring 18C or the first sealing element 2D. If the tube sheet is coupled to the first sealing element 2D, the mbe sheet 7 may be coupled to either the tubular portion 3 or the neck portion 4 as discussed above. In embodiments of the invention, the filter assembly may be coupled to the mbe sheet using any suitable mechamcal mechanism including a welded, threaded, latched, locked, and/or pin-and-hole connection. Referring to Figure 6, the filter assembly 1 may be either pre-assembled prior to coupling to the mbe sheet or assembled after one or more components of the filter assembly 1 are coupled to the mbe sheet. For example, in some embodiments, the entire filter assembly may be preassembled (e.g., using a press and girth weld 39) and then coupled to mbe sheet 7 using, for example, a girth weld 40 disposed at a first planar surface, a second planar surface and/or at a location within the aperture 22 in the mbe sheet 7. Alternatively, only some of the components of the filter assembly 1 may be preassembled. For example, the second sealing element 13F, the filter element 9, the ring 18C, and the compressible material 12 may be preassembled. Thereafter, the second sealing element 13F may be coupled to the mbe sheet 7 as discussed above. If a fuse is to be included, the fuse 8 A, compressible material 12, and first sealing element 2D may thereafter be compressed and coupled to the mbe sheet 7, and/or second sealing element 13F. In a further alternate arrangement, the ring 18C, first sealing element 2D and/or second sealing element 13F may be initially coupled directly to the mbe sheet, with the remaining components of the filter assembly 1 positioned thereafter. For example, in Figure 6, the second sealing element 13F may be coupled to the tube sheet 7 prior the positioning of the other components of the filter assembly 1. Thereafter, the filter element 9, compressible material 12, ring 18C, fuse 8A, and first sealing element 2D are assembled, compressed, and fixed in place using any suitable mechanism as discussed above. In the embodiment shown in Figure 6, the components of the filter assembly 1 are fixed in place using a girth weld 39. Referring to Figure 7, the second sealing element 13G may be modified to include an outer flange 41 arranged to cooperate with a surface of mbe sheet 7 to effect a seal between the second sealing element 13G and the mbe sheet 7. In this arrangement, the filter assembly 1 may be pre-assembled prior to shipment from the factory. Thereafter, the filter assembly 1 may be easily replaced by securing the filter assembly 1 within the aperture 22 in mbe sheet 7 using any suitable mechanism such as clamping arrangement 37. In Figure 7, the compressive assembly 19G is similar to the compressive assembly 19F. However, the mechanism for securing the compressive assembly 19G to the mbe sheet is different. In Figure 7, clamping arrangement 37 includes a bolt 42, a nut 44, a clamping member 46 and one or more hollow risers 45. In preferred embodiments the bolt 42 is preferrably a tee bolt inserted through a hole 47 in one or more risers 45. The risers may be L-shaped or retangular. In preferred embodiments, the risers are retangular with at least one, and preferrably two open sides through which the bolt 42 may be inserted in hole 47. The clamping member 46 may be any suitable shape such as a ring, a rectangle, or a U-shaped member. In preferred embodiments, the clamping member engages the top of the first sealing element 2D. Conventionally, ceramic candle filters are welded or held using a pin-in-hole arrangement. However, a problem arises in that after extended use the pins are difficult to remove due to corrosion. Further, where a sealing element is welded directly to the mbe sheet, the sealing element may need to be cut from the mbe sheet prior to replacement, possibly resulting in damage to the mbe sheet. However, these problems may be overcome by employing clamping arrangement 37. In operation, the clamping arrangement 37 is disposed for pressing the filter assembly 1 tightly against the mbe sheet 7, and thereby effecting a seal. The seal between the filter assembly 1 and the mbe sheet 7 may be facilitated by angling an outer surface of the flange 41 and the inner surface of the aperture 22. The coupling location between the mbe sheet 7 and the filter assembly may optionally include a compressible material (not shown) to even more effectively seal the tube sheet 7 to the filter assembly 1. To remove the clamping arrangement 37, the nut 44 may be cut off of the bolt 42 so that the bolt 42 may be removed from the hole in the hollow risers 45. In one preferred embodiment, the bolt 42 is designed to shear when over-torqued such that the bolt may be easily removed by over-tightening the bolt. Referring to Figure 8, the second sealing element 13F may be coupled directly to the mbe sheet 7 using, for example, weld 40. Subsequently, the components of the filter assembly may be positioned and clamped to the mbe sheet using clamping arrangement 37. The clamping arrangement 37 provides a compressive force to compress the compressible material 12 and to clamp the ring 18C, fuse 8A, filter element 9, and first sealing element 2D to the second sealing element 13F. In the embodiment shown in Figure 8, the optional mechanical stops 31 may be desirable to regulate the amount of compression of the compressible material 12. In this embodiment, the compressive assembly 19H may be configured to include a portion of the mbe sheet 7. Referring to the embodiment of the invention shown in Figure 9, the second sealing element 13F may be coupled directly to the mbe sheet 7 using any suitable mechanism as discussed above. As shown in Figures 6-9, the second sealing element 13F may be substantially extended to provide a top mountable filter assembly 1 which has a low profile above the mbe sheet. In some applications, it is desirable to have both a low profile and a top mountable filter assembly 1. As shown in Figures 6-9, this may be accomplished by extending the second sealing element 13F. The components of the filter assembly 1 may be positioned and clamped to the mbe sheet 7 using clamp 37. The clamp 37 provides a compressive force to compress the compressible material 12, ring 18D, fuse 8, filter element 9, and first sealing element 2E to the second sealing element 13F. Mechanical stops 31 may be desirable to regulate the amount of compression of the compressible material 12 when forming the compressive assembly 191. In Figure 9, a tubular fuse is utilized inside of a extended hold-down vessel or first sealing element 2E. The second sealing element preferrably is arranged such that a back-pulse nozzel (not shown) is disposed directly above the first sealing element 2E facing the internal passage 23 and axially aligned with axis 29. The back-pulse nozzel directs a pulse of back flow gas into the first sealing element 2E. The first sealing element 2E is preferrably configured to cooperate with the tubular fuse 8 such that the back pressure gas is channeled evenly around the fuse 8 so as to minimize resistance to back pressure cleaning of filter 9. The channeling function provided by the extended hold-down vessel or first sealing element 2E substantially improves the back pulse cleaning efficiency of the filter assembly sown in Figure 9. The arrangement shown in Figure 9 is also advantageous in that the mbe sheet may be constructed to have a relatively thin cross sectional area since the structural support for supporting filter element 9 is provided by the second sealing element 13F. Referring to the embodiment of the invention shown in Figure 10, the second sealing element 13G is formed directly in the tube sheet 7. In Figure 10, the first sealing element includes two separate pieces 2Fa and 2Fb. This configuration facilitates the substimtion of different fuse dimensions to accommodate different applications. Subsequently, the components of the filter assembly 1 may be positioned and clamped to the mbe sheet 7 using clamp 37. The clamp 37 provides a compressive force to compress the compressible material 12, ring 18D, fuse 8, filter element 9, and first sealing elements 2Fa, 2Fb to the mbe sheet 7. Mechanical stops 31 may be desirable to regulate the amount of compression of the compressible material 12 when forming the compressive assembly 19J. In Figure 10, the mbe sheet 7 is made thicker than other embodiments such that the mbe sheet may also serve as the second sealing element 13G. In alternate embodiments, the first sealing element, second sealing element and/or the ring may be fixed to or be part of the mbe sheet 7 with one or more components of the filter assembly 1 subsequently assembled either above, below, or within the mbe sheet 7. For example, if the ring is first coupled directly to or is part of the mbe sheet 7, the fuse, the compressible material 12, and the first sealing element may thereafter be compressed and coupled to the ring and/or the mbe sheet 7 in any suitable manner as discussed above. Further, where the ring is pre-coupled to the mbe sheet, the filter element 9, compressible material 12 and second sealing element may thereafter be compressed and coupled to the ring and/or mbe sheet 7 in any suitable manner as discussed above. Similarly, if the first sealing element is pre-coupled to the mbe sheet 7, one or more of the remaining components of the filter assembly 1 may thereafter be positioned and coupled to the tube sheet 7 and or first sealing element. In a third major aspect of the present invention, an improved fuse may include a modified construction including an improved shape and composition. For example, the fuse may include one or more layers formed from different materials or the same material. The layers may have an identical pore structure or a different pore strucmre. Further, the pore strucmre within each layer may be a uniform or a graded pore strucmre. Additionally, the fuse may be formed into a tubular shape, a substantially planar shape, or other shape which facilitates incorporation into the process fluid stream. As discussed above, a substantially planar fuse may be preferable in some applications over mbe shaped fuses. However, heretofore when a fuse with a relatively small surface area is utilized, it has been difficult to obtain adequate solids blockage/removal in the event of a failure of the filter element 9 while at the same time achieving an acceptably low pressure drop across the fuse during normal operation of the filter element 9. In some cases, a fuse having a single layer with a umform pore strucmre will produce a pressure drop that is larger than an optimal value and have a solids blockage/removal capacity that is less than an optimal value. Further, because a large pore size and large porous area may be used to reduce the pressure drop across the fuse, the time required for sufficient cake to build-up prior to effective solids blockage/ removal is often excessive. When an attempt is made to reduce the porosity of the fuse to increase the solids blockage/removal ability while minimizing cake build- up time, an unacceptable pressure drop often occurs across the fuse. Accordingly, conventional, single layer, uniform pore strucmre fuse designs have traditionally been less than satisfactory. It has been found that this problem may be over come by modifying the strucmre of the fuse to include a multi-layer pore strucmre and more particularly to include a porous membrane layer having a finer pore strucmre than a supporting layer. For example, a membrane fuse may be constructed from a substrate layer of a relatively coarse grade material (e.g., a ceramic) and a membrane layer of a relatively fine grade material (e.g., a fine ceramic powder). In typical embodiments, the coarse material comprises about 50-99% of the thickness, and preferably about 70-99%, and more preferably about 80-99%, and even more preferably about 90-99% of the total thickness of the fuse. The coarse substrate layer preferably provides support while having sufficiently large pores so as to minimize the pressure drop. In exemplary embodiments, the substrate layer may be as coarse, and preferably coarser than that of conventional fuse elements. On top of the substrate layer is preferably disposed a thin membrane layer of a fine medium. The fine medium actually provides the majority of the filtration properties which are desirable for the fuse. However, because resistance to air flow is a function of both porosity and thickness, the thin membrane layer provides excellent filtration capabilities while minimizing pressure drop. In other words, a layered fuse allows a low initial pressure drop while providing the same dirt trapping performance. A multilayered fuse may be constructed by disposing a thin layer of a fine grit (e.g., a fine ceramic powder) or other membrane layer on a substrate layer which may be a relatively thick layer formed from a coarse grit. Since air flow permeability is dependent on the porosity (coarseness) and on the thickness of the material, a very thin fine layer provides the same or a decreased pressure drop as opposed to a thick layer of coarser material. Further, the thin membrane layer substantially improves the fouling and filtration characteristics of the fuse. Planar fuses may be constructed using a single monolithic composition or a plurality of layers having different compositions. Further, planar fuses may be constructed using a plurality of layers having the same or different compositions and/or the same or different pore structures. Where different compositions are utilized, the compositions should have compatible thermal expansion properties and chemical compositions. In preferred embodiments, the different layers are formed from different granularity powders from the same base material. In exemplary embodiments, the fuse may be constructed from one or more of the following materials: Si 3 N 4 , mullite, cordierite (MgO, Al 2 O 3 , SiO 2 ), fireclay, aluminosilicate fibers, alumina, alumina/mullite, and silicon carbide-based materials and/or other ceramic or metallic compositions. Ceramic compositions are preferred for many processes including high temperature and corrosive environments. Where the fuse is to operate in a highly corrosive environment, a mullite may be preferably. However, in environments where extreme thermal shock occurs, silicon carbide may be preferred. In some embodiments it may be desirable for the planar fuse 8A to include a substrate layer formed from a coarse ceramic material. The substrate layer may have a thickness of about 4mm to 25mm, and preferably between 8 and 15 mm, and more preferably about 10-12 mm. If a membrane layer is used, the membrane layer may be about between 5mm and 0.005mm, and preferably between 1mm and 0.01mm and even more preferably between 0.5 mm and 0.05mm and most preferably about 0.1mm in thickness. The membrane may be either on the upstream or downstream surface depending on the application. The planar fuse may be variously configured to have a diameter of between 30 and 200 mm and more preferably between 40 and 100 mm and even more preferably between 50 and 75 mm and most preferably about 65mm. In the monolithic construction, it is preferred to have an average pore size of between 10 and 600 microns and preferably between 100 and 500 microns and more preferably between 200 and 400 and even more preferably between 250 and 350 microns and most preferably about 300 microns. Where a multi-layered fuse is utilized, the membrane layer may be disposed either on the upstream or the down stream surface of the substrate layer. In preferred embodiments, the membrane layer is disposed on the upstream surface of the fuse so that particulate matter impinges directly on the membrane and a particulate cake builds-up quickly. The particulate cake acts in conjunction with the membrane layer to substantially improve the ability of the fuse to prevent particulate flow through the fuse. In the multi-layer planar fuse construction, the fuse preferably has a coarse substrate having an average pore size of between 250 and 1000 microns and preferably between 350 and 900 microns and even more preferably between 500 to 750 microns and a thin membrane layer having an average pore size of from 10 to 500 microns and more preferably between 50 to 400 microns and most preferably between 100 to 300 microns. If one or more of the layers has a graded pore strucmre, the pores may be arranged such that either the smaller or larger pores are disposed near the upstream surface. In many applications, it may be preferred to arrange the pore strucmre such that the smaller pores are disposed near the upstream surface with the larger pores disposed near the downstream surface to facilitate the build-up of particulate cake. At the present time, the most preferred embodiments which provide the best mode for carrying out the invention are illustrated in Figures 9 and 10. Figures 8 provides the best mode for carrying out a substantially planar fuse design which, at the present time, is less preferred than the ceramic tubular fuse designs illustrated in Figures 9 and 10. At the present time, the most preferred embodiment which provide the best mode for the bular fuse element is a 260 mm long candle fuse with one closed end and one open end surrounded by a flange. A flange portion of the candle fuse at the flange has an inner diameter of 40 mm and an outer diameter of 70 mm. A mbular portion of the candle fuse has an inner diameter of 40 mm and an outer diameter of 50 mm. The fuse is preferably constructed of a single monolithic layer of 60 grit (approximately 250 micron) silicon carbide particles bonded together using an aluminosilicate binder forming a ceramic having an average open pore size of approximately 80 microns. At the present time, the most preferred embodiment which provides the best mode of the disk fuse element is an element having a diameter of about 65mm and a thickness of approximately 10-12 mm. The disk is preferably formed from either aluminosilicate bonded particles or a reticulated foam. The disc fuse may be either a monolithic construction or a two layer composite. In the monolithic construction, it is preferred to have an average pore size of about 300 microns. In the multi-layer disk construction, the disk preferably has a coarse substrate having an average pore size of 500 to 750 microns and a thin membrane layer having an average pore size of 100 to 300 microns. The membrane layer preferably comprises between 1 to 10% of the total thickness of the planar fuse. The membrane may be either on the upstream or downstream surface depending on the application. While several exemplary filter assemblies embodying the present invention have been shown, it will be understood, of course, that the invention is not limited to these embodiments. Modifications may be made by those skilled in the art, particularly in light of the foregoing teachings. For example, each of the embodiments shown may utilized either a mbular fuse, a substantially planar or disc shaped fuse, or omit the fuse entirely. Further, elements from the various embodiments may be combined or substimted for corresponding elements of another embodiment and embodiments may include more or less components than those shown. For example, a compressible material may be disposed between adjacent portions. Further, the substantially planar fuse 8A of Figures 6-8 may optionally be interchanged with the fuse 8 shown in Figures 1-5 and 9-10. Additionally, the ring 18C may be omitted such that the first and second sealing devices compress the filter and/or fuse directly with or without the compressible material. Further, the fuse may be formed directly on or within the open end of the filter such that the filter and fuse become an integral component - eliminating one of the aforementioned seals. It is, therefore, intended that the appended claims cover any such modifications which incorporate the features of this invention or encompass the true spirit and scope of the invention.

Claims: CLAIMS 1. A filter assembly connectable to a mbe sheet comprising: a first ceramic mbe filter having a first opening and a first flange disposed about the first opening; a second ceramic mbe filter having a second opening and a second flange disposed about the second opening; and a compressive assembly including a compressible material disposed between the first and second ceramic mbe filters, a first substantially annular sealing element disposed around the first ceramic mbe filter abutting the first flange, and a second substantially annular sealing element disposed around the second mbe filter abutting the second flange, the first and second substantially annular sealing elements being coupled together to compress the flanges and compressible material therebetween. 2. A filter assembly connectable to a mbe sheet for filtering high temperature gases, the filter assembly comprising: a ceramic candle filter having a first opening and a flange disposed about the opening; a compressive assembly separate from and connectable to the mbe sheet, the compressive assembly including first and second metal elements coupled together to compress the flange therebetween, a second opening in fluid communication with the first opening, and a compressive material disposed between the ceramic candle filter and the first and second metal sealing devices. 3. A filter assembly for filtering high temperature gases, the filter assembly comprising: first and second mbular ceramic elements, each having an opening and a flange disposed around the opening; and a metal sealing ring including first, second, and third annular portions, the first and third annular portions having a diameter less than a diameter of the second annular portion, the first and third annular portions being disposed within the openings of the first and second mbular ceramic elements, respectively. 4. A filter assembly comprising: a mbe sheet having an aperture of a first diameter; a ceramic mbe filter having and outer diameter and a first opening; and a compressive assembly including first and second metal sealing elements coupled together to compress at least a portion of the ceramic mbe filter therebetween, and a second opening in fluid communication with the first opening wherein the first metal sealing element includes a tubular portion and a neck portion, the neck portion being coupled to the mbe sheet wherein the first diameter is substantially less than the outer diameter of the filter element. 5. A filter assembly comprising: a fuse; a ceramic mbe filter a sealing ring disposed between the fuse and the ceramic mbe filter; a compressive assembly including first and second metal sealing elements coupled together to compress at least a portion of the ceramic mbe filter, the fuse, and the sealing ring therebetween, wherein the fuse is disposed within the first metal sealing element. 6. A filter assembly comprising: a substantially planar fuse having first and second major surfaces; a ceramic mbe filter having an opening disposed adjacent to the first major surface of the substantially planar fuse; a compressive assembly disposed about the fuse and the ceramic mbe filter for compressing and sealing the ceramic tube filter. 7. A filter assembly connectable to a mbe sheet comprising: a ceramic mbe filter having a first opening; and a compressive assembly including first and second metal elements coupled together to compress at least a portion of the ceramic mbe filter therebetween, and a second opening in fluid communication with the first opening. 8. A fuse for use with a ceramic mbe filter comprising a substantially planar ceramic fuse. 9. A fuse for use with a ceramic tube filter comprising a fuse having a pore strucmre which varies in the direction of fluid flow. 10. A method of attaching ceramic elements to a mbe sheet comprising: preassembling a filter assembly by compressing a mbular ceramic element between first and second metal sealing elements to form a fluid tight seal between the first and second sealing elements and the ceramic element; and then attaching the filter assembly to a mbe sheet. 11. The filter assembly of any of claims 1-2 and 4-7 wherein the compressive assembly is separate from and connectable to the mbe sheet. 12. The filter assembly of any of claims 1, 2, 4, and 7 wherein the first sealing element is welded to the second sealing element at an overlapping portion. 13. The filter assembly of any of claims 1, 2, 4, 6 and 7 including a metal ring disposed between the first and second sealing elements and within the ceramic mbe filter. 14. The filter assembly of any of claims 3, 5, and 13 wherein the first and second sealing elements are welded to each other and the metal ring. 15. The filter assembly of any of claims 1, 2, 4, 5, and 7 including a fuse disposed within the first sealing element. 16. The filter assembly of claim 15 wherein the fuse has a mbular shape and is disposed in a mbular portion of the first metal sealing element. 17. The filter assembly of any of claims 1 to 7 including a clamping mechanism having a bolt, a nut, a clamping member, and a riser, the riser being fixed to the mbe sheet and the bolt and nut being joined to fix the clamping member to the riser. 18. The filter assembly of any of claims 1, 2, 4, 5, 6, and 7 wherein the first metal element includes first and second annular surfaces and the ceramic candle filter element includes first and second annular surfaces respectively disposed about the first surface of the first metal element and adjacent to the second surface of the first metal element wherein the ceramic candle filter element has a coefficient of thermal expansion that dif¬ fers from a coefficient of thermal expansion of the first metal element and wherein the first metal element, the ceramic candle filter and the compressible material comprise sealing means responsive to the differing coefficients of thermal expansion and the positioning of the first metal element, the ceramic candle filter, and the compressible material, the sealing means for compressing the compressible material between the first and second surfaces at a first temperamre, the compression between the first surfaces increasing responsive to an increase in temperamre above the first temperamre. 19. The filter assembly of any of claims 1-2 and 4-7 including disposing a compressible material between the elements of the compressive assembly. 20. The method of claim 9 wherein preassembling the filter assembly includes disposing a compressible material between the first and second sealing elements and the ceramic element. 21. The method of claim 20 wherein preassembling the filter assembly includes: placing a ceramic fuse in the first sealing element; placing a gasket between the fuse and the first sealing element; and fixing the first sealing element relative to the second sealing element. 22. The filter assembly of claim 6 wherein the compressive assembly includes a metal ring disposed between the substantially planar fuse and the ceramic mbe filter. 23. The fuse of any of claims 6, 8, and 9 wherein the fuse includes a plurality of layers, each layer differing in porosity. 24. The fuse of any of claims 6, 8, and 9 wherein the fuse includes a membrane layer. 25. The fuse of any of claims 6, 8, and 9 wherein a pore structure of the fuse includes first and second layers differing in porosity. 26. The fuse of claim 25 wherein the first layer includes a ceramic membrane layer. 27. The fuse of claim 26 wherein the membrane layer is substantially thinner than the second layer.

Inventors: JOHNSON MARK W, SUTHERLAND DANIEL W, JOHNSON, MARK, W., SUTHERLAND, DANIEL, W.
Applicants: JOHNSON MARK W, PALL CORP, SUTHERLAND DANIEL W, JOHNSON, MARK, W., PALL CORPORATION, SUTHERLAND, DANIEL, W.
Application year: 1995
Main classification: B01D46
IPC codes: B01D46, C10K1