Filter cartridge with end densification ring

Abstract: A filter cartridge comprising a pleated, cylin­ drical filter element is provided with an end densifi­ cation ring (5) to cause the ends of the pleats to pack together more tightly. In this configuration they can be more easily and securely sealed in an end cap (6) closing off an end of the filter cartridge.

Description: This invention relates to pleated cylindrical filter elements and specifically the means by which such filter elements are sealed into filter devices containing them. Cylindrical filters are widely used to filter fluids and comprise a cylindrical housing containing a replaceable filter cartridge, which cartridge comprises a porous cylindrical core member and a pleated filter element surrounding the core. The axes of the pleats are generally parallel to the axis of the core. In normal operation an inlet port communicates with the interior of the housing and an outlet port communicates with the porous axial core member such that the fluid to be filtered is constrained to pass from inlet to outlet through the cylindrical pleated filter element. To ensure that no fluid passes around the ends of the filter element, these are usually sealed into end caps at each end of the cartridge. The end cap through which the outlet port communicates with the axial core member is called the open end cap and the other, which seals off the other end of the core, is called the blind end cap. In some constructions, the axial core member has exit ports at both ends as, for example, when a plurality of cartridges are joined together to provide a longer filter device. DE-C-3739297 discloses a tubular filter element including a pleated web contained within a perforated supporting tube. A spring ring or clip presses each end of the web radially outwards into contact with the supporting tube. A flanged cap is provided at each end of the pleated web. The sealing of the ends of the filter element in the end caps is frequently done by placing in the cap a potting compound, which in this context means a material that is liquid under filter cartridge-sealing conditions but solid under normal operating conditions. Typically this will be a molten polymer, a plastisol, a two-part epoxy resin, a wax, a liquid polymer that can quickly be cured to a rigid state, or some such similar material. In one preferred embodiment, the potting compound is in fact the material of the end cap itself. In this embodiment, the surface of the end cap to which the pleats are to be bonded is melted and the pleats are pressed into the molten surface. The end cap is then allowed to cool. In order for this sealing process to be effective, it is necessary that the end of each pleat be firmly anchored in the potting compound and in practice this means ensuring that all the ends of the pleats penetrate into the compound to approximately equal depth and stay there until the compound solidifies. In some cases this proves a difficult objective. The viscosity of the potting material may be so high or the rigidity of the filter element so low that the pleats may actually buckle before adequate penetration can be achieved. This problem is exacerbated where the pleats are relatively widely spaced so as to accommodate dirt removed from a filtered fluid. Filter elements embodying the present invention are better able to resist buckling when the end is inserted into a potting compound to seal it in place in an end cap of a filtration cartridge. In addition, the extrusion of potting material between the individual pleats is more uniform and controllable. Thus, the uniformity of performance of filtration cartridges containing a cylindrical pleated filter element is improved by insuring a reduced rate of filter failure as a result of filtration fluid bypassing the filter element through inadequate end seals. The present invention provides a filter cartridge comprising a cylindrical pleated filter element including a first end and axially extending pleats and an end cap attached to the first end of the pleated filter element characterized in that an end densification ring is located near said first end of the pleated filter element, the end densification ring displacing ends of the pleats clockwise or counterclockwise to one side at said first end and forming a portion of compacted pleats. The present invention also provides for a filter cartridge comprising a porous cylindrical core member, end caps at opposed ends of the cartridge, and, disposed around the cylindrical core member and retained in the cartridge by the end caps, a cylindrical pleated filter element with the axes of the pleats extending generally parallel to the axis of the cylindrical core member, and, fitted over an end of the core member, an end densification ring having a generally wedge-shaped cross-section with a wedge angle of from 18° to 23°, said end densification ring acting to restrict the local radial dimension of the space occupied by the pleated filter element such that the pleats, in the area in contact with the ring, are reshaped and compacted in close proximity to one another. The present invention also provides for a filter cartridge comprising a porous cylindrical core member, end caps at opposed ends of the cartridge, and, disposed around the core member and retained in the cartridge by the end caps, a cylindrical pleated filter element with the axes of the pleats extending generally parallel to the axis of the core member, and an end densification ring located in the area adjacent at least one end cap so as to restrict the local radial dimension of the space occupied by the pleated filter element and cause the pleated filter element to have a densification factor of from 0.7 to 0.9. The present invention also provides for a filter cartridge comprising a cylindrical pleated filter element including a first end and axially extending pleats, an end densification ring located near the first end of the pleated filter element so as to restrict the local radial dimension of the pleated filter element, and an end cap attached to the first end of the pleated filter element. The present invention also provides for a method for attaching a pleated filter element to an end cap comprising the steps of compacting a first end of the filter element by displacing ends of pleats clockwise or counterclockwise to one side at the first end of the element with an end densification ring; and joining the end cap to the first end of the pleated filter element. The end densification ring may be a separate component or it may be an integral part of the core member. The ring, which may be of a generally wedge-shaped cross-section, displaces the ends of the pleats of the filter element to one side, (clockwise or counterclockwise), so as to realign them at an angle to the radial direction. In practice this realignment results in the pleats being forced in the area of the densification ring to be packed in close proximity to one another. In this configuration they have a much greater ability to resist buckling of the individual pleats or bending over of the ends of the pleats when the ends are forced into the potting compound and the extrusion of potting compound between the pleats is more uniform and controllable. The end densification ring may be located adjacent to at least one end of the filter element. In preferred embodiments, end densification rings are provided at both ends of the filter element. The end densification ring acts by restricting the local radial dimensions of the space occupied by the pleated filter element such that the pleats are forced by contact with the ring to be reshaped and compacted in close proximity to one another. It will be appreciated that this may be most conveniently done by attaching the densification ring to the core member. It will however be understood that a similar result may be obtained by restricting the radial dimensions of the space by applying the densification ring to the inner surface of a cage member surrounding the cylindrical pleated filter element and sealed with it into the end caps. The end densification ring may have a generally wedge-shaped cross-section. In some embodiments, however, it is preferred that the end densification ring be fitted onto a rabbeted end of the axial core member. In this event the wedge-shaped cross-section of the end densification ring will conveniently be truncated with the extent of the truncation corresponding to the depth of the rabbet. In general, therefore, it is preferred that the densification ring act to increase the effective local diameter of the core member at its end. This increase is preferably done gradually with the maximum restriction of the space occupied by the filter medium to be found at the axial end of the core member. The wedge angle, that is to say, the acute angle subtended by the outside surface of the ring and the hypothetically continued surface of the core member without the ring, is preferably from 15° to 30°, such as from 18° to 23°. The densification factor (as this term is hereinafter defined) achieved by the use of the end densification ring, is preferably from 0.5 to 1.1 and most preferably from 0.70 to 0.90. While the densification ring has been described in terms of a wedge-shaped cross-section, it is to be understood that this includes configurations that approximate the wedge shape such as those in which the external surface of the ring, when seen in cross-section, has a concave configuration. The end densification rings can be made of a variety of materials that will retain their structural integrity at the temperatures at which the end caps are applied. In general polymeric material is satisfactory, such as polyethylene, polypropylene, polytetrafluoroethylene, and polyfluorinated alkyl vinyl ether polymers and copolymers. In one preferred embodiment the rings are provided with slots to allow for the escape of gases which might otherwise be trapped in the pleat ends by application thereto of a solid ring. This is particularly important when the filter is oriented vertically in a system and is used to filter carefully metered volumes of liquid. The pleated filter element used in the invention can be made of any suitable medium such as polymeric fibers, glass fibers, cellulosic or mixed fibers, or metal fibers. It may also be in the form of a porous membrane. The pleated filter element may also comprise support material such as non-woven or woven fabrics including those made from metal or polymeric materials and/or extruded webbings. The invention is particularly useful in the context of a cylindrical pleated depth filter medium formed of polymeric micro-fibers which are physically entangled to define a tortuous filtration path for the fluid passing therethrough. As indicated above, the potting compound into which the ends of the pleats are sealed can be any compound that is liquid under sealing conditions but which under use conditions is a solid. Thus, it can be a plastisol, a two-part epoxy resin, a wax, or some other such material. A preferred expedient is to melt a surface of the end cap and, while the surface is molten, press the pleats into it until the ends are completely and uniformly covered. The porous core member may be made of a molded plastic such as a polyolefin, a fluoropolymer, a nylon, or a polyester. Alternatively, it may be made of a metal mesh, a perforated sheet of metal such as steel or aluminum, or a porous ceramic or any one of a number of porous rigid materials adapted to form a support for the cylindrical pleated filter element. As explained above, the end densification ring can be integral with the core member or the cage or it can be applied separately, preferably with a locking fit, to the core member or the cage. Whatever the embodiment it is preferred that, when the pleated filter element is first forced into contact with the end densification ring, the movement is accompanied by a slight rotational movement about the filter axis, perhaps of up to 45°, and preferably from 20° to 30°, so as to assist the pleats to be reshaped and compacted in a uniform manner. The invention is further described with specific reference to the attached drawings in which: Figure 1 is an exploded cross-section of part of a filtration cartridge according to the invention. Figure 2a is a cross-section of an end densification ring according to the invention and Figure 2b is a plan view of the end densification ring illustrated in Figure 2a. Figure 3 is a perspective view of a filter cartridge of the invention with a cutaway portion in cross-section showing the pleats of a cylindrical pleated filter element as they appear in the bulk of the filter element, (i.e., nondensified), and also in the portion adjacent the end densification ring in which the pleats are laid over on one another and compacted. Also shown is a prior art cartridge without the densification ring. Figure 4 is a schematic cross-section of an end of the filter cartridge according to the invention and is provided to identify the various dimensions referred to in the specification. Figure 5 is a schematic cross-section of the pleated filter element before it is formed into the cylindrical shape required in the cartridge. It is also provided to identify the various dimensions used in the specification. In the embodiment illustrated in Figure 1, which incorporates an end densification ring as shown in Figures 2a, 2b and 4, a cylindrical core member 1, provided with apertures 2, is surrounded by a pleated filter element (not shown) located between the core member and a cage member 3. The function of the cage member is to contain and protect the pleated filter element against sudden pressure fluctuations. The core member is provided with a rabbeted portion 4 adjacent its end, and an end densification ring 5 having a truncated wedge cross-section such that the truncated portion of the wedge abuts the rabbet on the core member. In use, an end cap 6 is heated to melt one major surface 7 thereof and this is then forced against the body of the filter device such that the ends of the pleats of the filter medium and the end densification ring and the cage member become sealed into the end cap. Figures 2a and 2b illustrate in greater detail the end densification ring appearing in Figure 1. The acute angle Θ is the hypothetical angle between the external sloping surface of the wedge-shaped ring and the hypothetical continuation of the surface of the core member. Clearly, the size of the angle ϑ and the axial length L of the ring control the amount of densification that occurs at the pleat end. The larger the values of ϑ and L the greater the densification that will occur. This densification concept is expressed mathematically as a densification factor or D F . This is derived by the following calculation. Reference is made to Figures 4 and 5 for the significance of the various terms used. The cross-sectional area, A₁, of the filter end actually occupied by the filter member is calculated by measuring the dimensions of the filter pack in a flat and also in a fully dense state as shown in Figure 5. The area, A₁, is then given by A₁ = bh. This is in the "flat" form before it is formed into a cylinder. In the flat configuration there are no void areas such as are formed when the pleated element is formed into a cylinder thus separating the apices of the pleats. The area, A₂, occupied by the filter end in the cylindrical pack form is determined by A₂ = π(R I ² - R O ²). The difference in areas in the fully dense state and in the final filter shape is the void area, A V , of the filter end: A V = A₂ - A₁ A V = π(R I ² - R O ²) - bh. The final cross-sectional area, A F , occupied by the filter when densified by the pack end ring is given by A F = π(R I ² - R R ²) where R R = R O + X or A F = π[R I ² - (R O + X)²]. The final area of voids, A VF , in the cross-sectional area with the pack densification installed is given by A VF = A F - A₁ or A VF = π[R I ² - (R O + X)²] - bh. The required dimension, X, which is a measure of the maximum reduction, in the radial direction, of the space occupied by the filter end in its densified state, may then be determined by X = [R I ² - A VF + bh π ] ½ - R O . If we define a desired densification factor, D F , such that D F = A V - A VF A V then A VF = (1 - D F )A V . It should be noted that, if the filter element is somewhat loosely consolidated, the end densification ring could compress the material of the pleats as well as compact the pleats together such that A F could be less than A₁. Thus, A VF could be a negative figure and therefore D F could be greater than 1. Then, having once defined a derived densification factor, D F , it is possible to deduce the necessary end densification ring dimensions to achieve that degree of densification: X = [D F ( R I ² - bh/π) + (1 - D F )R O ²] ½ - R O and b₂ = b₁ + X. Generally, the axial length of the ring L is chosen such that the angle ϑ is from 15° to 30° and preferably from 18° to 23°. EXAMPLE One embodiment of an end densification ring was incorporated into a particular filter configuration to illustrate the attainment of the objectives of the invention. The filter chosen was rated for absolute removal of 0.2 micrometer size particles. The filter member consisted of a 0.08 mm. (0.003 inch) thick polytetrafluoroethylene expanded membrane with a 0.30 mm. (0.012 inch) thickness of non-woven polypropylene on the upstream side and a 0.46 mm. (0.018 inch) thickness of the same material on the downstream side. The filter member was pleated on a conventional reciprocating pleater with a pleat height of 10.29 mm. (0.405 inch). The filter had cage, core, and end cap members molded from polyproplyene. The dimensions of the filter, using the parameters discussed above with respect to Figures 4 and 5, were: End densification rings were constructed so as to provide densification factors, D F , of 0.71 and 0.94. The dimensions of the larger height of the ring cross-sections, b₂, were determined to be 2.79 mm. (0.110 inch) and 3.43 mm. (0.135 inch), respectively. A single value for the length of the ring, L, was chosen at 6.35 mm. (0.25 inch). This resulted in values for the wedge angle, ϑ, of 18° and 23°. Table 1 summarizes the results attained by incorporating these end densification rings into the above filters. The data in the third column represent the results of a standard production acceptance test. The test measures the rate of air flow through the filter in the forward direction after the filter has been wetted with a solution of tertiary butyl alcohol and water. In this series of tests a differential pressure of 8578 kg/square meter (12.2 psi) was applied to the filter and the resulting flow was measured in cc/min. When the calculated diffusional flow (0.9 cc/min in this embodiment) is subtracted from the total, the remaining leakage flow is a measure of flow through "holes" in the filter, side seals, and end seals. The average value of this leakage flow for each case is shown in column 3. The results clearly show that embodiments with the above densification factors attain the objects of the invention.

Claims: A filter cartridge comprising a cylindrical pleated filter element including a first end and axially extending pleats and an end cap (6) attached to the first end of the pleated filter element characterized in that an end densification ring (5) is located near said first end of the pleated filter element, the end densification ring displacing ends of the pleats clockwise or counterclockwise to one side at said first end and forming a portion of compacted pleats. A filter cartridge according to claim 1 further comprising a porous cylindrical core member (1), the filter element being disposed around the core member (1). A filter cartridge according to claim 2 in which the end densification ring (5) is applied to an end of the core member (1). A filter cartridge according to claim 2 or 3 in which the core member (1) is rabbeted adjacent an end and the end densification ring (5) has a truncated wedge cross-section such that the truncated portion of the wedge is accommodated in the rabbet (4) of the core member (1). A filter cartridge according to claim 1, 2, 3, or 4 in which the end densification ring (5) and the core member (1) are provided with means for insuring a locking fit when the end densification ring (5) is located on the core member (1). A filter cartridge according to claim 2 or 3 in which the end densification ring (5) is an integral part of the core member (1). A filter cartridge according to any preceding claim further comprising a cylindrical cage (3) surrounding the pleated filter element. A filter cartridge according to claim 7 when dependent on claim 1 or claim 2 in which the end densification ring (5) is applied to an end of the cage member (3). A filter cartridge according to any preceding claim in which the ends of the pleats of the filter element penetrate into the body of the end cap (6). A filter cartridge according to any preceding claim in which the end densification ring (5) is provided with slots to allow the escape of gases. A filter cartridge according to any preceding claim wherein the filter element includes first and second ends and wherein end densification rings (5) are provided at both ends of the filter element. A filter cartridge according to any preceding claim wherein the end densification ring (5) has a generally wedge-shaped cross-section. A filter cartridge according to claim 12 in which the wedge has a wedge angle of from 18° to 23°. A filter cartridge according to any preceding claim in which the portions of the pleats in contact with the end densification ring (5) have a densification factor (D F ) of from 0.5 to 1.1, where D F = A v -A vF A v where A v = the void area of the filter end A vF = the final area of voids in the cross-sectional area when densified A filter cartridge according to claim 14 in which the densification factor (D F ) is from 0.7 to 0.9. A filter cartridge according to any preceding claim in which the end densification ring (5) is made from a polymer that has a melting point above the temperature at which the ends of the pleats are sealed in the end cap (6). A filter cartridge according to any preceding claim in which the end densification ring (5) is made from a polymer of a fluorinated or unfluorinated olefin. A filter cartridge according to any preceding claim in which the end densification ring (5) and the end cap (6) are both made from olefinic polymer. A method for attaching a pleated filter element to an end cap (6) comprising the steps of: compacting a first end of the filter element by displacing ends of pleats clockwise or counterclockwise to one side at the first end of the element with an end densification ring (5); and joining the end cap (6) to the first end of the pleated filter element A method according to claim 19 wherein the step of compacting the first end of the pleated filter element includes restricting a local radial dimension of the space occupied by the first end of the pleated filter element such that the pleats in the vicinity of the first end are reshaped and compacted in close proximity to one another. A method according to claim 20 wherein restricting a local radial dimension includes fitting an end densification ring (5) to the first end of the pleated filter element.

Inventors: FEINT STEPHEN D, FEINT, STEPHEN D.
Applicants: PALL CORP, PALL CORPORATION
Application year: 1990
Main classification: B01D29
IPC codes: B01D29, B01D36, B01D46