Abstract:
A spiral wound filter element is disclosed which includes alternating layers of filter medium and diffusion medium wrapped about a cylindrical core defining a central axis of the filter element. The alternating layers extend from a radially innermost layer of the filter element to a radially outermost layer of the filter element, the diffusion medium is defined by a continuous lengthwise sheet of mesh material, and the filter medium is defined by at least one sheet of filter material arranged along the length of the continuous sheet of mesh material. The alternating layers of filter medium and diffusion medium define three distinct radially disposed layered filtering sections surrounding the cylindrical core, and include a first filtering section having radially outer prequalifying layers, a second filtering section having middle prequalifying layers and a third filtering section having radially inner qualifying layers. The radially outer prequalifying layers and the middle prequalifying layers define about two-thirds of the radial distance from the radially outermost layer of the filter element to the radially innermost layer of the filter element.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to U.S. provisional patent application Ser. No. 60/103,233, filed Oct. 5, 1998, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE DISCLOSURE 
     1. Technical Field 
     The present disclosure relates, in general, to a filter, a method of making a filter and a method of filtering a fluid. 
     More particularly, the present disclosure relates to a filter having alternating layers of non-filtering diffusion medium and filter medium. Some of the layers of the filter medium are provided with bypass apertures, while the remaining layers do not include bypass apertures so that they act as qualifying layers for the filter. Filters according to the present disclosure have been found to provide improved fluid distribution over the filter medium, reduced pressure drop and increased filter life, without a reduction in filter rating. 
     2. Discussion of Background Disclosures 
     In general, a filter assembly is used for removing contaminants from fluids, i.e., liquids or gases. Such filter assemblies, for example, are used in chemical and hydrocarbon applications such as polyethylene manufacturing, food and beverage applications, electronic applications such as circuit board construction, coating applications such as high quality spray painting, and industrial applications such as paper manufacturing. Many filter assemblies include a tubular filter cartridge contained in a filter housing. The filter housing includes a sump, wherein the filter cartridge sits, and a head sealing the filter cartridge within the sump such that the housing acts as a fluid-tight pressure vessel. The filter head includes an inlet between the sump and the filter cartridge, and an outlet aligned with the tubular filter cartridge. Contaminated fluid is pumped into the filter housing through the inlet, and radially inwardly through the filter cartridge to produce filtered fluid, which then exits the filter housing through the outlet. 
     Normally, such a filter cartridge includes an elongated, tubular, perforated core wrapped with layers of depth filter medium. A typical depth filter medium is a non-woven, porous, melt-blown sheet or sheets of polypropylene micro fibers. The depth filter medium can have a uniform pore structure or a graded or tapered pore structure, whereby the pore size of the depth filter medium decreases in the direction of fluid flow, i.e. from an outer to an inner diameter of the filter. The depth filter medium can also be provided with fibers of varying diameter. 
     Even with a tapered pore structure and/or varying fiber diameters, however, it has been found that many depth filters actually act as “low area” surface filters, since only one or two of the multiple layers of filter medium within the depth are heavily loaded and plugged with contaminants after use, while the remaining layers are relatively clean (it should be noted that these are general observations, as the performance of a particular filter can depend on the particle size and distribution of contaminants in a fluid to be filtered). When a depth filter cartridge mimics a surface filter and collects contaminants in primarily one layer, the results are an inefficient distribution of fluid over the filter medium, a higher pressure drop for fluid passing through the filter and a lower flow rate capability for the filter. Such filters also tend to have a shorter useful life, and thus must be replaced more often. 
     A variety of depth filter cartridge configurations have been proposed and/or utilized over the years in efforts to provide improved performance. For example, U.S. Pat. No. 4,863,602 to Johnson shows a filter element that includes a plurality of layers of flexible, fluid permeable filtering material, at least one layer of which includes an opening through which fluid may pass, a layer of flexible, fluid-permeable, substantially nonfiltering transport material, and a layer of flexible prefiltering material positioned upstream of the plurality of layers of filtering material to “filter out from the fluid substantially all particles that could otherwise become lodged in the transport material but not to filter out significantly smaller particles.” 
     U.S. Pat. Nos. 5,174,895 and 5,015,379 to Drori disclose filter elements featuring at least one coiled filter strip defining first and second butt ends. The Drori filter elements fail to optimally enhance fluid flow while providing extended filter service life. Additional filter designs of background interest are disclosed in U.S. Pat. No. 4,877,526 to Johnson; U.S. Pat. No. 4,882,056 to Degen et al.; U.S. Pat. No. 5,468,382 to Cook et al.; and U.S. Pat. No. 5,591,335 to Barboza et al. 
     Despite the various configurations known in the art, however, there remains a need for a filter cartridge providing improved distribution of fluid over the filter medium therein, a lower pressure drop and long useful life, without reducing the filter rating and that is economical to manufacture and utilize. 
     SUMMARY OF THE DISCLOSURE 
     A filter for filtering contaminated fluid is disclosed herein. A preferred filter includes alternating layers of a filter medium and a diffusion medium, with at least a portion of the layers of the filter medium having bypass apertures and acting as pre-qualifying filter medium layer(s). 
     The diffusion medium includes a first plane of spaced-apart, substantially parallel strands defining first longitudinal passages. The longitudinal passages have a height dimension and a width dimension. The diffusion medium further includes a second plane of spaced-apart, substantially parallel strands defining second longitudinal passages. The second longitudinal passages also define a height dimension and a width dimension. The diffusion medium&#39;s second plane of strands are oriented in a non-parallel manner with respect to the strands of the first plane such that the first and the second planes define lateral openings. Those lateral openings define side dimensions. The first and the second longitudinal passages are sized such that at least one dimension is smaller than any of the side dimensions of the lateral openings. 
     Filters of the type disclosed herein demonstrate superior fluid distribution over the filter medium contained therein, and an optimum use of the filter medium. Filters according to the present disclosure, therefore, have an increased life and a lower pressure drop without a reduction in filter rating, and provide more cost effective filtering. 
     The filters of the present disclosure may be used in methods to filter contaminated fluids in a wide range of commercial applications. Such filters and filtration methods are described in greater detail hereinbelow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     To provide those of ordinary skill in the art to which the present disclosure pertains with an understanding as to how to construct a filter as disclosed and claimed herein, filters according to the present disclosure are described in detail below with reference to the attached drawings wherein: 
     FIG. 1 shows an isometric side/end view of a filter cartridge of the type disclosed herein; 
     FIG. 2 shows an enlarged sectional view of the filter cartridge taken along line  2 — 2  of FIG. 1; 
     FIG. 3 shows an enlarged isometric view of a portion of a non-filtering diffusion layer and a non-qualifying filter layer of the filter cartridge of FIG. 1; 
     FIG. 4 shows a sectional view of the diffusion layer and the non-qualifying filter layer of the filter cartridge of FIG. 1 taken along line  4 — 4  of FIG. 3; 
     FIG. 5 shows a sectional view of the diffusion layer and the non-qualifying filter layer of the filter cartridge of FIG. 1 taken along line  5 — 5  of FIG. 3; 
     FIG. 6 shows a sectional view, similar to FIG. 4, of a diffusion layer and an alternative filter layer for use with the filter cartridge of FIG. 1; 
     FIG. 7 shows a top plan view of a continuous sheet of non-filtering diffusion medium and sheets of filter medium being wound onto a core to form a filter cartridge of the type shown in FIG. 1; 
     FIG. 8 shows an isometric, exploded view of a continuous length wise sheet of non-filtering diffusion medium and sheets of filter medium prior to being wound onto a core to form an alternative filter cartridge according to the present disclosure; 
     FIG. 9 shows a sectional view, similar to FIG. 2, of a further filter cartridge according to the present disclosure; and 
     FIG. 10 shows an isometric view of a continuous sheet of non-filtering diffusion medium, a sheet of filter medium, and spaced-apart strips of filter medium being wound onto a core to form an alternative filter cartridge according to the present disclosure. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIGS. 1 through 5, a filter cartridge  10  according to the present disclosure includes an elongated, porous, rigid core  12  having a multiplicity of openings  36 , and an elongated, hollow filter  14  coaxially mounted on the core  12 . Annular end caps  38  are bonded to the ends of the filter to prevent contaminated fluid from by-passing the filter  14   
     The filter  14  includes at least one sheet of filter medium  16 , with at least a portion of the filter medium  16  including bypass apertures  18 , and a sheet of non-filtering diffusion medium  20 . The sheets of the filter medium  16  and the diffusion medium  20  are wrapped, or coiled, to form alternating layers of filter medium and diffusion medium extending from an innermost layer  22  to an outermost layer  24  of the filter  14 . In a preferred embodiment, the diffusion medium  20  is bonded to itself at the outermost layer  24  to prevent the filter  14  from unwinding or unwrapping during shipping, handling and use. As such, the diffusion medium  14  defines the outermost layer  24  of the filter  14 . 
     1. The Diffusion Medium 
     The diffusion medium  20  includes a first plane of spaced-apart parallel strands  26  forming longitudinal passages  28 , and a second plane of spaced-apart parallel strands  30  forming longitudinal passages  32 , as illustrated by arrows f 1 , f 2  in FIG.  3 . The strands  30  of the second plane are oriented such that they are not parallel with the strands  26  of the first plane, such that the first and the second planes form lateral openings  34 . In a preferred embodiment, strands  26  are substantially perpendicular to strands  30 . The longitudinal passages  28 ,  32  are preferably smaller in at least one dimension as compared to the smallest dimension of the lateral openings  34 . In particular, a height h of the longitudinal passages  28 ,  32 , as best shown in FIG. 4, is preferably smaller than the length or width of the lateral openings  34 . 
     The longitudinal passages  28 ,  32  of the diffusion medium  20  distribute the fluid to be filtered through flow channels f 1 , f 2 , such that the diffusion medium  20  allows for, and assists in, the longitudinal, or circumferential and/or axial, flow of the contaminated fluid within the filter  14  between the innermost layer of the filter medium  16  and the core  12 , and/or between adjacent layers of the filter medium. Such longitudinal flow assists in minimizing the pressure drop across the filter cartridge  10  and in dispersing the filtration function. The diffusion medium  20  is preferably positioned between the core  12  and the innermost layer of the filter medium  16  to facilitate the passage of fluid through the openings  36  in the core  12 . In a preferred embodiment, the core  12  is surrounded by a plurality of diffusion medium  20  layers to provide a collection area for the flow prior to exiting through openings in the core  12 . Positioning of the diffusion medium  20  between adjacent layers of the filter medium  16  similarly maximizes the use of the filter medium surface area within each layer for contaminant loading, thereby reducing pressure drop and optimizing filter medium usage to extend filter life. 
     In preferred embodiments, the dimensions of the lateral openings  34  and the longitudinal passages  28 ,  32  of the diffusion medium  20  are purposely selected to be substantially larger than any contaminant to be filtered from the contaminated fluid. As a result, the diffusion medium  20  does not act as a filter. Since the diffusion medium  20  does not act, and is not used, as a filter to trap contaminants, the diffusion medium does not substantially contribute to the pressure drop across the filter  14 , and in fact minimizes the pressure drop by providing unobstructed flow channels f 1 , f 2  for contaminated fluid. In addition, the diffusion medium  20  provides structural rigidity and protects the filter medium  16  from damage. The filter  14  is advantageously provided with an extra outer layer of the diffusion medium  20  to add support and protection to the filter  14 . 
     The diffusion medium  20  is made from a suitable material that is temperature and fluid compatible with the filtering application to be carried out. Preferably, the diffusion medium  20  is made of a suitable thermoplastic. For example, for lower temperature filtering applications (i.e., below 180° F.), the thermoplastic can comprise polypropylene, while for higher temperature applications (i.e., above 180° F.) or chemical compatibility with different fluids, the thermoplastic can comprise nylon, polyester, or melt-processible fluoropolymer. 
     The diffusion medium  20  preferably comprises thirty thousandths of an inch (30 mils) thickness, bi-planar strand orientation (17 mil strand size), twelve strands per inch, polypropylene extruded netting/mesh. Such netting is available, for example, under the trademark Plastinet®, manufactured by Applied Extrusion Technologies, Inc. of Middleton, Del., or Naltex®, manufactured by Nalle Plastic, Inc. of Austin, Tex. The strands  26  of the first plane may be transversely oriented with respect to the strands  30  of the second plane such that the two planes form generally square or diamond-shaped lateral openings  34  having side dimension of about 0.066 inches. Thus, a preferred diffusion medium  20  exhibits a ratio between lateral opening  34  side dimensions to lateral passage  28 ,  32  height (hereinafter “Side-to-Height Ratio”) of approximately 66:17 or 3.9:1. In addition, the sheet of the diffusion medium  20  is oriented so that the square lateral openings  34  form diamonds between ends  40 ,  42  of the cartridge  10  to advantageously distribute flow over the tubular filter. 
     Alternative netting dimensions may be utilized according to the present disclosure. In preferred embodiments, however, to ensure that the diffusion medium  20  does not function as a filter, the Side-to-Height Ratio should be greater than about 1.5:1 and preferably greater than 3:1. As noted hereinabove, a preferred diffusion medium  20  according to the present disclosure exhibits a Side-to-Height Ratio of about 4:1. 
     2. The Filter Medium 
     According to preferred embodiments of the present disclosure, the filter medium  16  is preferably of the depth filter type, wherein contaminants are trapped within the medium, as opposed to on an outer surface of the medium. A preferred depth filter medium  16  is comprised of one or more sheets of non woven thermoplastic micro fibers. The non woven thermoplastic micro fibers may be melt blown, spunbond, carded, or hydroentangled, for example. For lower temperature filtering applications (i.e., below 180° F.), the thermoplastic can comprise polypropylene, for example, while for higher temperature applications (i.e., above 180° F.) or chemical compatibility with other fluids, the thermoplastic can comprise nylon, polyester or melt-processible fluoropolymer, for example. 
     Furthermore, filter medium suitable for use in accordance with the present disclosure includes porous membrane, such as a cast nylon porous membrane available as Zetapore® from CUNO, Incorporated of Meriden, Conn. Other filter medium suitable for use in accordance with the present disclosure includes wet laid paper made with such raw materials as glass or cellulose. An example of a suitable wet laid filter medium is TSM®, available from CUNO, Incorporated of Meriden, Conn. Woven material can also be incorporated as the filter medium in accordance with the present disclosure. 
     The filter medium  16  is preferably provided in discrete sheet form, as opposed to being melt blown directly onto the diffusion medium, for example, such that the sheets can be inspected prior to being incorporated into the filter  14 . The use of discrete sheets of depth filter medium  16  has been found to simplify quality control inspection of the filter medium and make the physical properties of each filter cartridge  10  more consistent. The ability to control the consistency of the physical properties of the depth filter medium  16  provides a unique ability to achieve sharp, well-defined, and optimized control over the removal efficiency and dirt capacity of the resulting filter cartridge  10 . It should be understood, however, that a filter in accordance with the present disclosure could be provided with a single continuous sheet of filter medium. 
     According to preferred embodiments of the present disclosure, the porosity of the filter medium  16  may be constant between the inner and the outermost layers  22 ,  24  of the filter  14 . Alternatively, a filter medium  16  can be provided having a porosity that varies between the outermost layer  24  and the innermost layer  22  of the filter, e.g., a filter having a tapered or graded pore structure. If, as preferred, the filter medium  16  comprises melt-blown, non woven polypropylene micro fibers, the pore size and/or fiber diameter geometries can be constant or varied between the outermost layer  24  and the innermost layer  22  of the filter. A depth filter medium  16  having a relatively uniform pore size and fiber geometry is shown in FIGS. 4 and 5, while a filter medium  17  having a decreasing pore size is shown in FIG.  6 . The sheets of depth filter medium can also be processed, e.g., calendared or compressed, to change its porosity in instances where it is desired to utilize filter medium porosity to achieve desired filtration results. 
     3. The Bypass Apertures 
     According to preferred embodiments of the present disclosure, a portion of the depth filter medium  16  includes a multiplicity of spaced-apart bypass apertures  18 . Preferably, the bypass apertures  18  extend from the outermost layer  24  of the depth filter medium  16  for a distance equal to between fifty and eighty-five percent (50%-85%) of the overall radial distance from the outermost layer  24  to the innermost layer  22  of the filter  14 . Most preferably, the bypass apertures  18  are extend to about sixty-six percent (66%), i.e. two-thirds, of the radial distance from the outermost layer  24  to the innermost layer  22  of the filter  14 . 
     According to the preferred embodiments, the filter medium layers  16  closest to the core  12  do not include bypass apertures such that all of the fluid must pass through the inner layers. In this way, the innermost layers of the filter medium  16  act as qualifying layers for the filter  14 , thereby permitting the filter  14  to be rated based upon the particle retention of the qualifying layers. In like manner, the outer layers of filter medium  16  having the bypass apertures  18  act as pre-qualifying layers. 
     It should be noted, however, that if the filter cartridge  10  is to be used within a filter assembly wherein contaminated fluid is forced to flow radially outwardly therethrough, i.e., the orientation of the fluid flow through the filter cartridge  10  is to be reversed relative to the embodiments described heretofore, then the bypass apertures  18  may be advantageously provided to extend from the innermost layer of the depth filter medium  16  to a radial distance of about fifty to about eighty-five percent (50%-85%) of the overall radial distance between the innermost layer  22  and the outermost layer  24  of the filter  14 . When so oriented, the inner layers of the filter medium  16  will act as pre-qualifying layers, while the outer layers act as the qualifying layers. 
     It should also be noted that a filter according to the present disclosure is not limited to the coiled designs shown in the attached figures. The unique elements of the present disclosure, i.e., alternating layers of filter and diffusion mediums as disclosed and claimed herein, can be utilized in other filter structures, such as a pleated filter cartridge or a filter bag. 
     The bypass apertures  18  may be uniformly spaced-apart in predetermined patterns, and provided as generally circular openings. The geometry and relative sizes of the apertures  18 , however, may be advantageously varied, e.g., circular holes and elongated slots of varying sizes are contemplated, and combinations thereof. The apertures  18  may also be provided as slits, cuts or perforations in the filter medium  16 , and such slits, cuts or perforations may be designed such that they do not fully open until a predetermined pressure differential is created across the filter cartridge  10 . In addition, the multiplicity of bypass apertures may be provided in a number of different patterns, e.g., linearly aligned, diagonally aligned, or random, and the pattern(s) may vary from layer to layer of the filter medium  16 . 
     During operation with a filter cartridge  10  in which the fluid flow is radially inward, contaminated liquid or gas passes laterally (i.e., radially) inwardly through the lateral openings  34  in the outermost layer(s) of the diffusion medium  20 . The contaminated liquid or gas then contacts an outermost layer of the filter medium  16 . Contaminated liquid or gas that does not immediately pass through the outermost layer of the filter medium  16  or the bypass apertures  18  in the filter medium may be directed longitudinally, or substantially parallel with respect to the outermost layer of the filter medium  16 , through the longitudinal passages  28 ,  32  of the diffusion medium  20 , depending on the relative resistance to flow. 
     For each of the non-qualifying layers of filter medium  16 , the bypass apertures  18  allow a portion of the fluid to pass therethrough instead of passing through the filter medium of that particular layer. After passing through one of the non-qualifying layers of filter medium, the fluid passing through the bypass apertures  18  and the fluid passing through the filter medium  16  are re-mixed and diffused in the diffusion medium  20  before being filtered by the next layer of filter medium  16 . The bypass apertures  18 , accordingly, help utilize all available filter medium  16  and help to reduce the pressure drop through the filter  14 . Preferably, the bypass apertures  18  provide uniform contamination loading of the non-qualifying layers of filter medium  16 . 
     4. Performance 
     The combination of the filter medium  16 , the diffusion medium  20  and the bypass apertures  18  in the manner described hereinabove has been found to have the synergistic effect of simultaneously increasing filtration capacity and minimizing pressure drop across the filter cartridge  10 , without reducing the filter rating. This synergistic effect is demonstrated by the following test results: 
     A filter cartridge (“Test Cartridge  1 ”) utilizing non-filtering diffusion medium along with filter medium, but without bypass apertures, exhibits a filter life about two times greater than a “control” filter cartridge having neither non-filtering diffusion medium nor bypass apertures. 
     A filter cartridge (“Test Cartridge  2 ”) utilizing bypass apertures along with filter medium, but without diffusion medium as described herein, does not exhibit a greater filter life than the control filter cartridge. 
     A filter cartridge  10  (“Test Cartridge  3 ”) according to the present disclosure utilizing non-filtering diffusion medium  20  having a Side-to-Height Ratio of about 4:1 and relatively uniform bypass apertures  18  extending about two-thirds of the radial distance from the outermost layer to the innermost layer, exhibited three to four times the filter life of the control filter cartridge. 
     A filter cartridge  10  (“Test Cartridge  4 ”) according to the present disclosure utilizing both the non-filtering diffusion medium  20  and bypass apertures  18  as described for Test Cartridge  3 , and wherein the number of bypass apertures  18  increases towards the outer diameter of the filter  14 , exhibits from four to five times the filter life of a control filter cartridge. 
     Test Cartridge  4  exhibits from two and a half to three times the filter life of a filter cartridge utilizing both non-filtering diffusion medium and bypass apertures, wherein the number of bypass apertures increases towards the outer diameter of the filter, and wherein the diffusion medium comprises a polyolefin spunbond web available as POWERLOFT® media from Kimberly-Clark Corporation of Roswell, Ga. 
     The advantageous performance described above for Test Cartridges  3  and  4  is confirmed by visual inspection. Upon dissection of the Test Cartridge  3  after testing, the filter medium  16  displayed contaminant loading to a radial depth from the outermost layer  24  of about fifty percent (50%) of the filter  14 . In comparison, only the outermost layer of filter medium displayed contaminant loading in Test Cartridge  1 . Thus, the combination of the diffusion medium  20  and the bypass apertures  18  as described for Test Cartridges  3  and provides a synergetic effect that was not to be expected based upon the performances of Test Cartridges  1  and  2  possessing either non-filtering diffusion medium or bypass apertures, respectively, but not both. 
     The testing procedure included a single pass test at a flow rate of three gallons per minute of water containing between about 0.39 to about 1.0 grams per gallon of contaminant. Two standard contaminants were used: 0-30 micron contaminant (ISO COARSE, A.T.D. 12103-1, A4, available from Powder Technologies, Inc. of Burnsville, Minn.) and 0-10 micron contaminant (A.T.D. nominal 0-10 microns, also available from Powder Technologies, Inc). All of the filter cartridges tested had an outer diameter of about 2.5 inches and were about 10 inches long. The life of a filter for purposes of the tests is defined as the amount of contaminant challenged for the pressure drop across the filter to increase by 20 psid due to contaminant loading in the tested filter. 
     EXAMPLES 
     Additional exemplary filters made in accordance with the present disclosure are described hereinbelow. However, these exemplary filters are merely illustrative of filters that may be made according to the present teachings, and are not intended to be limiting thereof. 
     Example I 
     Referring to FIG. 7, an exemplary filter  48  according to the present disclosure is shown. The filter  48  includes a single continuous sheet of diffusion medium  20  comprising thirty thousandths of an inch (30 mils), bi-planar strand orientation (17 mil strand size), twelve strands per inch, polypropylene extruded netting. The Side-to-Height Ratio of such diffusion medium is approximately 4:1. The filter material, which comprises melt-blown, non woven polypropylene micro fibers, is provided in a plurality of discrete sheets  16   a ,  16   b ,  16   c . The plurality of sheets of filter medium  16   a ,  16   b ,  16   c  exhibit substantially equal and consistent pore size and fiber geometries. As shown, the ends of the sheets  16   a ,  16   b ,  16   c  are overlapped. The overlapping ends of the sheets  16   a ,  16   b ,  16   c , however, are not sealed or bonded since the tightly wound sheet of the diffusion material  20  provides an adequate seal between the overlapping ends of filter medium. 
     Inner (with respect to the core  12 ) sheets  16   a  of the depth filter material do not have bypass apertures, while outer sheets  16   b ,  16   c  of the filter material have bypass apertures  18  (it should be noted that only the ends of the non-perforated qualifying layers  16   a  need to be overlapped). The outermost sheets  16   c  of filter material are preferably provided with more numerous bypass apertures  18  than the intermediate sheets  16   b.    
     The bypass apertures  18  in sheets  16   b ,  16   c  are formed by perforating the sheets  16   b ,  16   c  prior to winding or coiling the sheets of diffusion medium  20  and filter medium  16   a ,  16   b ,  16   c . In particular, sheets  16   b  are provided with circular perforations having diameters of about {fraction (5/32)} inches, which are arranged in straight rows at intervals of about 1.2 inches, and wherein the rows are aligned and spaced at intervals of about 1.2 inch. Sheets  16   c  are provided with circular perforations having diameters of about {fraction (5/32)} inches, which are arranged in straight rows at intervals of about 1.2 inches, and wherein the rows are staggered and spaced at intervals of about 0.6 inches. In sum, sheets  16   c  contain almost twice as many bypass apertures  18  as do sheets  16   b . In general, it has been found that for a 2 to 2.5 inch outer diameter filter, rated between about 2 and about 70 microns, the apertures  18  should consume about 2.5 percent of the area of each of sheets  16   c , and should consume about 1.25 percent of the area of each of sheets  16   b.    
     A first end of the sheet of the diffusion medium  20  is secured to the core  12 , using heat bonding for example, and the sheet is wound about the core to create a first or innermost layer of the diffusion medium. The sheet of diffusion medium  20  and the sheets of filter medium  16   a ,  16   b ,  16   c  are then coiled together about the innermost layer. The sheet of the diffusion medium  20  is longer than the sheets of the filter medium  16   a ,  16   b ,  16   c  such that the sheet of diffusion medium will form an outermost layer around the filter medium. The outermost layer of the diffusion medium  20  is then secured to the adjacent layer of diffusion medium, using heat bonding for example, such that the filter is tightly and securely wound. Surprisingly, it has been found that winding the layers tightly does not affect either the removal efficiency or the dirt capacity of the filter  48 . 
     Example II 
     Referring to FIG. 8, a second example of a filter  50  according to the present disclosure is shown. The filter  50  includes a single continuous sheet of diffusion medium  20  comprising thirty thousandths of an inch (30 mils), bi-planar strand orientation (17 mil strand size), twelve strands per inch, polypropylene extruded netting. The Side-to-Height Ratio of the diffusion medium  20  is approximately 4:1. The filter material, which comprises melt-blown, non woven polypropylene micro fibers, is provided in a plurality of discrete sheets  16   a ,  16   b ,  16   c ,  16   d.    
     The sheets of filter medium  16   a ,  16   b ,  16   c  exhibit substantially equal and consistent pore size and fiber geormetry. Sheet  16   a  does not have bypass apertures, while outer sheets  16   b ,  16   c  have bypass apertures  18 . The outermost sheet  16   c  of filter material is preferably provided with more numerous bypass apertures  18  than the intermediate sheets  16   b . Most preferably, the sheets  16   b ,  16   c  are perforated in a manner substantially similar to the corresponding sheets of FIG.  7 . 
     Sheets  16   d  comprise melt-blown, non woven polypropylene micro fibers that are calendared, i.e., compressed between two rollers. Prior to being calendared, sheets  16   d  have an substantially identical fiber geometries to the fiber geometries of sheets  16   a ,  16   b ,  16   c . In the calendering process, to the extent the dimensions of the fibers are affected, the fibers assume a greater dimension in the plane of the sheet  16   d . As a result, after being calendared, the sheets  16   d  have a reduced pore diameter as compared to sheets  16   a ,  16   b ,  16   c.    
     As shown, prior to the filter  50  being coiled, sheet  16   a  is placed under sheet  16   d  adjacent sheet  16   b . After being coiled, the filter  50  includes: 1) inner layers of filter medium (innermost sheet  16   d ) having a reduced pore size, 2) intermediate layers of filter medium (laid over sheets  16   a  and  16   d ) that have a pore size that alternates between a relatively smaller and larger size, and 3) outer layers of filter medium (sheets  16   b  and  16   c ) that have a relatively larger pore size. 
     Example III 
     Referring to FIG. 9, another filter cartridge  70  according to the present disclosure is shown. The filter cartridge  70  is similar to the filter cartridge  10  of FIG. 7, and elements that are the same have the same reference numerals. The filter cartridge  70  includes a filter  72  having alternating layers of filter medium  74  and diffusion medium  76 . 
     The filter medium  74  has bypass apertures provided therein and aligned with one another so as to form radial from bores  78  extending from an outermost layer  80  towards an innermost layer  82  of the filter. The radial bores  78  each extend to a uniform depth within the filter  72 . Preferably, the bores  78  extend radially to between about fifty and eighty-five percent (50%-85%) of the radial distance from the outermost layer  80  to the innermost layer  82  of the filter  72 . More preferably, each of the bypass bores  78  extends radially to about sixty-six percent (66%) of such radial distance. It should be noted that the filter medium of the filter cartridge  70  can be provided with bypass apertures formed by bores radially extending from an outermost layer  80  towards an innermost layer  82  of the filter, but to non-uniform depths within the filter  72 . 
     A method for manufacturing the cartridge  70  generally includes winding or coiling the sheet of the diffusion medium  76  and the sheet(s) of the filter medium  74  into alternating layers extending between the innermost and the outermost layers  82 ,  80 , and piercing the layers from the outermost layer towards the innermost layer to produce the multiplicity of bypass bores  78  in the filter. The bypass bores may be created by piercing the outermost layer  80  of the filter  70  with one or more elongated, narrow, sharp instruments, such as steel pins. A multiplicity of parallel steel pins, for example, are mounted on a flat base, and the filter cartridge  70  is simply pushed onto the spikes and pierced to create the bypass bores. 
     Example IV 
     Referring to FIG. 10, a further filter cartridge  110  according to the present disclosure is shown (filter cartridge  110  is not shown with end caps; as will be readily apparent to persons of skill in the art). The filter cartridge  110  is similar to the filter cartridge  70  of FIG. 7, and elements that are the same have the same reference numerals. The filter cartridge  110  includes a filter having a single continuous sheet of diffusion medium  20  and at least one sheet of filter medium  16   a  wound around a core  12 . The filter  110  also includes strips of filter medium  114  wound within the sheet of diffusion medium  20  between the sheet of filter medium  14  and the outer diameter of the filter. The strips  114  are spaced apart to create gaps that comprise bypass apertures  116 . 
     As shown, the strips of filter medium  114  are arranged longitudinally with respect to the core  12 , but the strips can be oriented in other directions, such as diagonally with respect to the core. The strips  114  are equally spaced apart from one another such that the resulting bypass gaps  116  are of substantially equal size. Alternatively, the strips can be spaced such that the resulting bypass gaps become larger towards the outer diameter of the respective filters, for example. It has been found that a filter cartridge  110  of the type disclosed in FIG. 10 provides about the same improved performance as provided by the filter cartridge  10  disclosed in FIG.  7 . 
     The presently disclosed filter cartridges can be used in a variety of end uses, including, but not limited to, chemical and hydrocarbon applications such as polyethylene manufacturing, food amid beverage applications, electronic applications such as circuit board construction, coating applications such as high quality spray painting, and industrial applications such as paper manufacturing. It should be noted that while the examples of filters disclosed herein are elongated tubes with cylindrical cross-sections, filters in accordance with the present disclosure can be provided in other suitable configurations, such as elongated tubes with a square, elliptical, or oval cross-sections. 
     The filters and methods according to the present disclosure have been described in detail in the foregoing specification, with specific examples provided. Filters and methods in accordance with the present disclosure, however, are not to be construed as limited to the particular examples shown, as these examples are regarded as illustrious rather than restrictive. Moreover, variations and changes may be made to the exemplary filters by those skilled in the art without departing from the spirit of the present disclosure as set forth by the following claims.