Filters Comprising Anisometric Compressed And Bonded Multilayer Knitted Wire Mesh Composites

Abstract

Filters and filter units are provided which comprise as the filter sheet anisometric compressed and bonded knitted wire mesh composites composed of a plurality of sheets of knitted wire mesh, superimposed at random orientation with respect to each other, compressed or densified to a voids volume within the range from about 10 to about 90 percent, and bonded together. The sheets are taken in sufficient number, usually at least five and preferably ten or more, and as much as 1,000 or more, to form a self-supporting relatively non-resilient composite of high tensile strength and high breaking strength having an average pore diameter of less than 200 microns, and preferably less than 100 microns, that is relatively uniform in any unit area of the surface, and having an anisometric porosity, the through pores extending crosswise of the sheet greatly exceeding in number the through pores extending laterally of the sheet, which latter pores can be reduced virtually to zero in a highly compressed composite. The composite is formed by superimposing a plurality of knitted wire mesh sheets, annealing the composite to avoid wire breakage during later processing, compressing the composite to the desired density and anisometricity by application of pressure in a direction approximately perpendicular to the plane of the layers of the composite, and bonding the sheet layers and wire filaments of the sheets together at their points of contact and/or crossing. The bonding holds the composite at the selected density, prevents relative movement of the wires in the composite, and in conjunction with the multilayer structure imparts the self-supporting nonresilient characteristic, together with high tensile strength and high breaking strength.

Parent Case Text

This application is a division of Ser. No. 732,443, filed May 27, 1968, now U.S. Pat. No. 3,690,606, patented Sept. 12, 1972.

Description

Woven wire mesh have been in use for some years as filter materials. They have the advantages of being readily available, permitting close control of uniformity in the number, size and shape of the pores, and in tensile strength, as well as being adapted for fabrication and being relatively low in cost. Various forms of such materials have been provided, ranging from the woven wire mesh as commercially available, to wire mesh specially treated so as to better suit them for filter uses.

U.S. Pat. No. 2,423,547 to Behlen, dated July 8, 1947, suggests rolling a wire mesh to form a flat sheet, and thereby produce a filter or screen material having a reasonably smooth surface, analogous to a perforated sheet material prepared by drilling holes in a metallic sheet in the desired pattern. However, such screens have the disadvantage that the dirt capacity is very greatly reduced, as compared to the woven wire mesh starting material.

The amount of dirt that can be taken up by a filter before it is effectively clogged is referred to as the dirt capacity of the filter, and this can be measured in various ways. For reference purposes, it is usually expressed in terms of grams of standardized dirt per unit surface area of the filter, as determined by a standardized test procedure.

U.S. Pat. Nos. 2,925,650 and 3,049,796 to Pall describe and claim woven wire sheet material specially treated by sinter-bonding, with a slight or great deformation of the wires at their points of crossing, which possess several advantages over the Behlen material. Not only are the wires held against a relative shift in position during treatment, because of the sintering operation, but the material also retains much of the nature of the starting wire mesh, and therefore much if not all of the original dirt capacity.

Nonetheless, one of the difficulties in using woven wire mesh-type filters is their relatively low dirt capacity, as compared to other filter materials.

Filter media can generally be classified as being one of two types, depth filters and surface filters. A depth filter removes suspended material from the fluid passed through the filter by collecting it not only on the surface of the element but also within the pores. A depth filter has a considerable thickness, and has a plurality of pores of distinct length. The longer the pores, the higher the dirt capacity of the filter, because there is more room for dirt along the pores. Most depth filters are made of masses of fibers, or other particulate material, held together by mechanical means or by bonding. One or several layers of such materials can be employed, and these layers can vary in porosity. In most cases, however, the greater percentage of contaminants unable to pass through the filter is trapped at the surface of the filter.

A surface filter removes suspended material from the fluid passed through the filter by collecting such material on its surface, and the material thus removed forms a filter cake or bed upon the filter. This material naturally obstructs the openings in the surface of the filter, because the fluid must flow through this material, which thus effectively reduces the diameter of the filter openings to the size of the pores in the filter cake. This reduction in effective diameter of pore openings in the filter increases the pressure differential required to maintain flow through the filter.

Woven wire mesh filters of the square weave type fall in the category of surface filters, because the depth of the pores through the sheet is substantially no greater than the diameter of the filaments making up the weave. Consequently, these filters have a rather limited dirt capacity, as compared to depth filters. U.S. Pat. No. 3,327,866 to Pall et al. describes woven wire mesh which, by an appropriate selection of wire size and wire count, in both warp and shute, is formulated to specified pore size Dutch twill weaves of extraordinarily high dirt capacity, as compared to Dutch twill weave wire mesh woven of wires of other sizes and/or counts.

Knitted wire mesh filter elements have been known for many years. However, the physical properties of knitted mesh made of fine diameter wires are such as to defy any modification previously attempted to render them suitable as filters for anything other than coarse filtration of liquids and gases, such as air, since they have lacked a reliable pore size uniformity and their maximum pore size has been rather high, well in excess of 200 microns.

A number of U.S. Pats. have described air filters made in whole or in part of knitted wire mesh, among them, No. 1,676,191 to Jordahl, No. 1,905,160 to De Angelis, No. 1,829,401 to Kamrath, No. 2,274,684 to Goodloe, No. 2,327,184 to Goodloe, No. 2,334,263 to Hartwell, No. 2,439,424 to Goodloe et al., No. 2,462,316 to Goodloe, No. 2,672,214 to Goodloe, No. 2,792,075 to McBride et al., No. 2,929,464 to Sprouse, and No. 3,085,381 to Sobeck. Virtually all of the filter elements thus proposed comprise a plurality of layers of knitted wire mesh. However, the problem presented by knitted mesh is best summarized by Goodloe in No. 2,327,184:

"Although it is highly desirable from the standpoint of efficiency to employ layers of fine mesh, such fine mesh, especially when of knitted character, is generally of flimsy character, is not self-sustaining, and consequently a filter body composed of layers thereof is easily subject to compression by the force of the air or gas stream moving therethrough, whereby tendency to crowd the layers together is increased, so that the undesirable conditions above referred to are further enhanced."

Goodloe and the other workers in the field have resolved these problems as best they could, using reinforcing spacers (as in U.S. Pat. No. No. 2,327,184), by crushing or compacting the plural layers endwise or crosswise (as in U.S. Pat. Nos. 2,439,424, 2,462,316 and 2,672,214) or by supporting them within a filter unit frame (as in U.S. Pat. Nos. 1,676,191, 1,829,401, 2,792,075, 2,929,464 and 3,085,381). Such expedients are acceptable for gas filters, but they are not capable of overcoming the flimsy, nonself-supporting nature of knitted wire mesh to render them suitable for liquid filtration, where the fluid pressures are considerably higher, and where high strength combined with low or minimal flow resistance are indispensable prerequisites.

One of the outstanding characteristics of a knitted material, as opposed to a woven material, is its resiliency or low modulus of elasticity (Young's Modulus). A knitted wire mesh can be stretched over 100 percent in any direction, despite the nonresiliency of the wire filaments of which it is made up. In an air filter, this inherent resiliency (which is due to the looping of the filaments in the knit weave) is an advantage, as Jordahl pointed out in U.S. Pat. No. 1,670,191, and it also makes it possible to fold and crush the material in any direction, as in U.S. Pat. Nos. 2,439,424 and 2,462,316. In a liquid filter, however, this resiliency is a disadvantage, since it means, in effect, that pore size varies with pressure drop across the filter, the pores of the material closing as the pressure drop increases.

It is equally evident that if the filaments be locked in position, by bonding or other means, the resiliency is not diminished appreciably, because the looped condition of the filaments is unaffected by the bonding. Indeed, Jordahl pointed out that even when a plurality of layers of knitted mesh are superimposed, and compressed to any desired density, a relatively great density "may be obtained without danger of losing the characteristic indicia of the structure above described, since the uneven or irregularly roughed surfaces of the fabric, due to the multiplicity of interlocked strand loops distributed thereover, will always tend to produce sufficient separation of the component folds, sheets or layers as to assure the requisite low air pressure resistance of the structure." It was because of this that Goodloe in U.S. Pat. No. 2,274,684 used comparatively stiff wire, "whereby the interengaged loops forming the fabric tend to resist relative displacement and consequently tend to retain and maintain the initial shapes and uniform distribution of the loop defined openings throughout the area of the fabric, as well as a considerable degree of self-supporting stability due to inherently greater resistance to both contraction and elongative stretch of the fabric;". Because of these difficulties, which are not found in woven wire mesh, and because knitted wire mesh presents no apparent advantage over woven wire mesh, whether in one or in several layers, knitted wire mesh has not been preferred to woven wire mesh in liquid filters.

In accordance with the invention, knitted wire mesh is formed into a self-supporting relatively rigid anisometric multilayer structure that is not only eminently suited for use in liquid filtration, as well as other uses, such as in gas filtration, but also has unexpected and advantageous properties as compared to similar materials made of woven mesh. The knitted mesh material of the invention has lower flow resistance than comparable woven mesh material of the same pore size, and it is also stronger, is more uniform in permeability, has a higher modulus of elasticity (Young's Modulus), usually at least 3.3% of the modulus of solid sheet of the same material, and frequently much higher, and when materials of equal particle removal are compared, has a higher dirt capacity. For stainless steel this value is at least 1 .times. 10.sup.6 psi. Why this is so is not at present understood, and no explanation thereof can be offered, but the superiority is clearly to be seen in the data that has been collected, of which a representative selection is given in the examples.

The knitted wire mesh used as the filter sheet in the filter elements and filter units of the invention comprises a plurality of sheets of knitted wire mesh, superimposed at random orientation with respect to each other, compressed or densified to a voids volume within the range from about 10 to about 90 percent, and bonded together. The sheets are taken in sufficient number, usually at least five and preferably ten or more, and as many as 50 to 1,000 or more, to form a self-supporting relatively nonresilient anisometric composite sheet of high tensile strength, high specific strength and a high modulus of elasticity, having an average pore diameter of less than 200 microns, and preferably less than 100 microns, that is relatively uniform in any unit area surface of the sheet. The porosity of the composite is anisometric, the number of through pores extending across the sheet exceeding the number of through pores extending laterally of the sheet. The number of lateral through pores can be reduced virtually to zero, if the degree of compression or densification is great enough, and this can be an advantage in many uses. The thickness need not be great, provided the sheet is self-supporting, and preferably is within the range from about 0.001 to about 0.5 inch.

The composite is formed, by superimposing a plurality of knitted wire mesh sheets, preferably (but optionally) annealing the composite to avoid wire breakage during later processing, compressing the composite to the desired density by application of pressure in a direction approximately perpendicular to the plane of the layers of the composite, and bonding the sheet layers and the wire filaments of the sheets together at their points of contact and/or crossing.

The annealing softens the wires which are work hardened as a result of the knitting operation, and permits them to bend or deform during compression without breaking. While it is preferable to anneal the composite to reduce annealing cost, annealing of the knit mesh sheets before they are superimposed into a composite is an equivalent step, serving the same purpose.

If the composition of the wire is such that very little work hardening occurs during knitting, the annealing step may be omitted.

The bonding holds the composite at the selected density, prevents relative movement of the wires in the composite, and in conjunction with the multilayer structure imparts the high modulus of elasticity, usually at least 1.1 .times. 10.sup.6 psi., together with high tensile strength and high specific strength.

In a preferred embodiment of the invention, the filaments of the knitted wire mesh are sintered to integrate them, at the bonding stage of the process. The filaments can be integrated by sintering prior to compressing, so that they no longer are able to shift their relative positions. The sintering process also anneals the filaments. However, since in the stack of knitted mesh sheets the wires are sufficiently stable against relative movement during compressing, being held in position by the interlocked loops of wires of adjacent sheets, they may be compressed, preferably by rolling, before sintering.

The rolling and sintering can be repeated as many times as desired to meet any desired porosity and density requirement. In certain cases, the effect of a rolling operation can be imparted to the work by the application of pressure during sintering.

This method makes it practical to prepare composite sheet products from knitted mesh sheets formed of very fine wires, appreciably finer than 10 mils in diameter. In fact, wires 2 to 4 mils and smaller in diameter can be utilized to provide strong self-supporting relatively nonresilient anisometric knitted very thin multilayer composite sheet products having a relatively large number of pores in a uniform pattern, and a uniform porosity across but not laterally of the sheet.

The anisometric knitted wire mesh composites are also susceptible of being made with a relatively low voids volume, using knitted mesh of relatively large wires and small pores, and a relatively high thickness, so that they are especially suited for use in transpiration cooling, when they can serve as porous walls or wall liners for passage of cooling gases to efficiently remove heat from the chamber walls, as in jet engine combustion chambers, rocket engine fuel injection systems, turbo-jet blades, and the surface skin of hypersonic aircraft, reentry and aerospace craft, and other space vehicles and projectiles.

By use of the knitted mesh composites, it is possible to make porous sheets from wire materials which cannot be woven into suitable wire mesh. If a wire mesh is not to be too sleazy (i.e., if the wires are not to be free to slide around), the wires must be elongated and deformed to small radius during weaving. The most common woven meshes (e.g. 325 .times. 325 .times. 0.0014, 200 .times. 200 .times. 0.0021) require wire with an elongation of at least 25 percent, and very few weaves can be made from wires having elongations of less than 15 percent. By contrast, wire materials with very low elongations can be knitted, and then used to make the composites of the invention.

Several knitted wire mesh composites, and filter elements containing the same, all in accordance with the invention, are described in detail below, having reference to the embodiments shown in the accompanying drawings, in which:

FIG. 1 is a plan view of a rigid warp-knitted mesh composite, showing in two parts the starting mesh and the composite, respectively;

FIG. 2 is a view in transverse section taken along the line 2--2 of FIG. 1, and looking in the direction of the arrows;

FIG. 3 is a photographic top view enlarged six times of a loose-weft-knitted mesh stack, ready for compressing and bonding to form a composite of the invention;

FIG. 3A is a photographic top view enlarged six times of a weft-knitted mesh composite of the invention, made from the mesh stack of FIG. 3.

FIG. 3B is a photographic end view of the composite of FIG. 3A;

FIG. 4A is a photographic top view enlarged six times of another knitted mesh composite of the invention;

FIG. 4B is a photographic end view of the composite of FIG. 4;

FIG. 5 is a plan view (with the top layer partly broken away) of a tubular loose-weft-knitted mesh composite made of mesh of different needle ends, showing the starting mesh in one part and the composite in another part;

FIG. 6 is a view in transverse section, taken on the line 6--6 of FIG. 5, looking in the direction of the arrows;

FIG. 7 is a plan view of a flat close-weft-knitted mesh composite, showing the starting mesh in one part and the composite in another part;

FIG. 8 represents a longitudinal sectional view of a filter unit and filter element incorporating a knitted mesh composite of FIG. 1;

FIG. 9 represents a cross-section taken along the lines 9--9 of FIG. 8.

A knitted mesh is composed of rows of loops, each caught into the previous row, and depending for its support on both the row above and the row below. There are two types of knitting, weft and warp. In weft-knit mesh the loops run crosswise of the fabric, and each loop is linked into the loop on the preceding row. In warp-knit mesh, parallel yarns are united in a chain stitch, first one yarn and then the other zig-zagging to tie the yarns together; and the loops interlock both weftwise and warpwise in the fabric. Warp-knitted mesh has about four times as many stitches to the inch as weft-knit mesh, and is of a stronger and closer construction.

When knitted mesh layers are superimposed, at random, the pores of adjacent mesh layers do not necessarily line up, because the mesh have an uneven surface, with projecting loops, resulting in relative displacement of adjacent mesh layers. Upon compression of the composite in a direction perpendicular to the plane of the mesh, this displacement may be increased. Thus, because of the random orientation of the mesh layers of the composite, the through pores follow an extremely tortuous path. At the same time, the anisometric characteristic is impressed on the sheet, because of the relative lateral displacement of the fibers of adjacent mesh layers, to selectively block laterally extending through pores.

This relative displacement is consequently an advantage, since it has the effect of reducing the size of the through pores in the composite. Loops of adjacent mesh layers project into and partially obstruct the pores of the next adjacent layers, and upon compression this effect can be repeated many times, with loop wires from layers as far as three or four layers away joining in this obstruction. Thus, in a composite of five to ten layers, through crosswise pores can be reduced to as low as an average of 50 microns or less, using mesh having an initial 500 to 10,000 micron pore size.

At the same time, these projecting loops selectively block the laterally-extending through pores to a greater extent than the cross-wise pores, and this effect is increased as the number of layers and the degree of compression or densification is increased.

The result is an accentuation of the anisometricity of the starting knitted mesh, to the point where the through pores that extend laterally can be blocked together, and thereby extinguished.

The tortuousness of the through pores in these composites is in contrast to the pores through a woven wire mesh, such as a square weave mesh, which are of the straight-through variety, or of a Dutch twill weave material, which are angled.

The knitted wire mesh composites can be made up of warp-knitted or weft-knitted wire mesh in any combination of mesh, wires, pore sizes, and knit types and stitches, such as plain stitch or purl stitch, flat stitch or rib stitch, open work stitch or tuck stitch, weft-knit mesh; and single-bar tricot, double bar tricot and Milanese warp-knit mesh. Flat knit and circular knit mesh can be used. Circular knit mesh can be cut open, or used double.

The knitted mesh composites can be knitted of wires of any metal. For filter uses, metals which are inert to and non-corroded by the fluid being filtered or the gas in contact therewith are of course preferred. Stainless steel is a very suitable material. Aluminum, brass and bronze wires can also be used. Other wires that are useful include copper, iron, steel, Monel metal, molybdenum, tantalum, colombium, titanium, tungsten, nickel-chromium alloys, cobalt-based alloys, chromium-plated wires of all types, zinc-plated wires of all types, and cadmium-plated wires of all types. All of these wires give knitted mesh composites of high modulus of elasticity, usually at least 3.3 percent of the modulus of solid sheet of the same material, because of the construction of the composite.

These can be knitted using conventional textile knitting machinery to mesh of the required needle ends, or loops per inch, wire diameters, and pore sizes. In general, the mesh should not have more than about 30 needle ends per inch, but there is no lower limit. If the knitted mesh is rather open, i.e., if the needle ends are only 2 per inch, or less, more layers may be needed to reduce pore size to the desired maximum than if closer knitted mesh is used, but if large initial thickness of the composite is not a factor, this is not a disadvantage.

The wires are usually monofilaments. Wires less than 10 mils in diameter, and preferably from 1 to 5 mils in diameter, are preferred for filter uses. The wires can be of any cross-sectional configuration, such as round, square, flat, polygonal, elliptical and rectangular. Stranded multifilament wire can be used.

It is sometimes advantageous for some filter uses to use magnetic wires, or to interleave mesh of magnetic wires with mesh of non-magnetic wires, in the mesh composites. In some cases, it may be useful to alternate mesh of magnetic wires with mesh of non-magnetic wires.

The composite is prepared by superimposing a selected number of knitted wire mesh sheets, one above the other. The orientation is random, preferably, since this best enables each sheet to remedy any nonuniformity in the next sheet, and produce a composite that is uniform throughout, but an orderly or a patterned orientation, such as laying alternate sheets at right angles, or other specific orientation, to the one below may have advantages in some instances.

If the knitted wire mesh have not previously been annealed, the composite is preferably annealed first, to soften the wire filaments. This is especially desirable when the wire filaments are 4 mils or less in diameter. Annealing is at a temperature and for a time appropriate for the metal of which the filaments are made, and is usually at from about 150.degree. to about 1,125.degree.C. for from 10 minutes to 48 hours. The following are exemplary:

Temperature Time Metal (.degree.C.) (Minutes) Stainless steel 1000-1125 10-30 Copper 260-650 10-30 Steel 810-875 10-30 Monel 875-1000 10-30 Aluminum 350-415 10-180

After annealing, the composite is compressed. The compression can be in a single step, or in several steps. If several steps are used, the composite is preferably bonded after the first compression, and then rebonded after each succeeding compression step. The degree of compression at the final step then is determined by the desired density or voids volume, and pore size.

The compression is applied perpendicularly to the plane of the layers of the composite, or approximately perpendicularly, but not more than 10.degree. from the perpendicular, since the displacement component may be excessive at such larger angles. The compression can be applied with restraint, as in a mold, but it is preferably without restraint, as by platens, or by pressure rollers. Rolling is preferred,

The composite should be subjected to a pressure of the order of 100 to 200,000 lbs. per square inch, the pressure applied depending upon the ductility of the metal, and applied normal to the metal surface, as by rolling or coining. If the pressure is less than the deforming pressure for the metal of the wires, usually about 50,000 lbs. per square inch, it merely results in densification of the composite, by forcing the layers and the wires closer together.

If the pressure applied is sufficiently great, a coining action can be obtained, in which the composite is compressed to as little as about 10 percent of the starting thickness. Reductions of as little as 30% in the starting thickness can be sufficient, however, and preferably the reduction is to from about 30 percent to about 65 percent of the starting thickness.

After compression, the layers and the filaments are set in their new relative positions by bonding them together at their points of contact. The layers and filaments can if desired be bonded by welding, brazing, soldering or sintering, or by use of resinous bonding agents, applied as solutions, dispersions, or from a fluidized bed of the resin. They will, of course, be mechanically interlinked or interleaved or interlocked, as a result of the compression, so that a very strong structure results.

Brazing, soldering, resin bonding and welding, while fully satisfactory, may reduce porosity and pore size to an undesirable extent. Consequently, it is frequently preferred to integrate the filaments at their points of contact by sintering.

The composite can be sintered by passing it through a furnace in a non-oxidizing atmosphere, such as, for example, in a reducing atmosphere of hydrogen or carbon monoxide, or mixtures thereof; or in an inert atmosphere such as nitrogen, argon, helium, or combinations thereof; or in a vacuum. The mesh is heated to a temperature not exceeding approximately 20.degree. below the melting point of the metal of which the filaments are formed. Generally, the temperature will be in excess of 1,000.degree.F. The result is a sintered integration of the metal at the points of crossing.

After bonding has been effected, the composite can be compressed again, such as by rolling, and then bonded again, such as by sintering, and these steps can be repeated as many times as necessary to give a composite having the desired characteristics, for filtration, for acoustic insulation, or for other uses. The final composite generally will have been reduced to between about 10 percent and about 95 percent of the starting thickness, and the pore anisometricity will be such that the permeability for flow through the pores extending laterally will be less than about 75 percent of that for flow through the pores extending across the sheet, and preferably less than 60 percent, and this can be reduced to zero. The permeability is defined as the volume of flow of any fluid at unit differential pressure through a unit cube.

As one or several of the juxtaposed layers there can also be used woven wire mesh, or metal plates or sheets, which can be perforated or imperforate, and which can be at the surfaces or in the interior, and which can be bonded thereto by any of the procedures indicated above. The combination of the knitted wire mesh composite with a perforated material is particularly useful, as it permits the manufacture of light weight high strength materials, useful in sound absorption in airborne applications. A layer of metal powder can be dusted into the knitted wire mesh composite, or superposed on one or both surfaces thereof, and bonded thereto, for example in accordance with U.S. Pat. No. 3,017,917, dated Nov. 6, 1962.

If desired, the knitted mesh wire composites can also be laminated to other materials, such as woven wire mesh, and metal plates and sheets, perforated, if desired.

The knitted wire mesh composite can also be impregnated and/or coated with fibrous material such as inorganic, metallic or organic fibers, as disclosed for instance in U.S. Pat. Nos. 3,158,532; 3,238,056; 3,246,767 and 3,353,682.

After bonding has been effected, the composite can be formed into filter elements of the invention of any desired form or configuration, with or without a support. The composite can be set in the desired configuration by corrugating, folding, or other shaping techniques, following which it can be put into the form of a filter element, as, for example, by folding a corrugated sheet into a cylindrical form, lapping over the free ends of the sheet, and bonding them together to complete the cylinder. This cylinder can be end-capped, if desired. The result is a rigid structure in which the knitted wire mesh composite serves as the filtering sheet. The knitted wire mesh composite is quite resistant to deformation or distortion, under rather high liquid pressures. The preceding is given merely as an example. It will be apparent that any desired configuration can be adopted.

FIGS. 1 and 2 show an anisometric knitted wire mesh composite, made up of ten layers 1 of a warp-type single-bar tricot knitted wire mesh, 12 needle ends per inch, of 4 mil stainless steel wire 2, rolled and sintered to a voids volume of 85 percent and a thickness of 0.04 inch.

This composite can be cut into disks, and it can also be corrugated into a sheet having a plurality of corrugation folds. Such a sheet can be made into a cylindrical filter element, as shown in FIGS. 8 and 9, for example.

FIG. 3 shows photographically, enlarged six times, a stack of sixteen layers of loose-weft-knitted mesh, of 0.004 inch diameter wire, ready for rolling and sintering. The looped wires of the mesh retain their knitted identity quite clearly.

FIG. 3A shows photographically, enlarged six times, the same stack after rolling and sintering. The knitted pattern is still evident, but the consolidation of the mesh has resulted in a considerable reduction in the size of the mesh openings. The end view 3B shows that the consolidation has in fact resulted in a sheet that is plate-like in character, with a smooth surface. The porosity is anisometric, with many through pores extending across the sheet, and few through pores extending laterally.

FIG. 4A shows photographically, enlarged six times, another sintered and rolled composite, made of 10 layers of the loose-weft-knitted mesh of FIG. 3. The composite is more open, and the pore size and voids volume are greater, partly because the number of layers is less, and partly because of a lower pressure and less percent reduction in thickness during the rolling and sintering operation. Nonetheless, as FIG. 4B shows, the sheet is platelike in character, and anisometric in porosity, the porosity laterally being lower than that across the sheet.

FIGS. 5 and 6 show another anisometric knitted wire mesh composite, made up of thirty layers of knitted mesh sheet. The first 15 are of a loose-knit weft type stainless steel knitted mesh 4, 12 needle ends per inch, and the second 15 are of a loose-knit-weft type stainless steel mesh 5, 18 needle ends per inch. Both knitted mesh are made of 2 mil wire 3. The difference in needle ends of the two mesh produces a composite having coarse 50 microns average pores on the upper side (shown in FIG. 6) and fine 10 microns average pores on the lower side.

FIG. 7 shows an anisometric knitted wire mesh composite made of five layers 6 of close-knit weft type stainless steel knitted mesh, made of 10 mil wire 7.

The voids volume of the anisometric knitted wire mesh composite is determined by measuring apparent volume and true volume. The apparent volume of the material is determined by measurement of its area and thickness. The true volume is determined by fluid displacement techniques, using a fluid capable of wetting the product. The voids volume is then determined by the following equation:

Voids volume = 100 .times. 1 - [true volume of composite/apparent volume of composite]

Calculated by this method, the knitted wire mesh composites preferably have voids volumes of at least 50 percent and in some instances 80 percent and even higher.

The pore size or diameter of the knitted wire mesh composites is evaluated by the following test, which is substantially in accordance with the procedure of U.S. Pat. No. 3,007,334.

A disk of the material to be tested is wetted with a fluid, preferably ethyl alcohol, capable of wetting the porous material, and clamped between rubber gaskets. The volume above the disk is filled with the fluid. Air pressure is increased in the chamber below the disk until a stream of air bubbles is observed emerging from one point of the test piece. The effective pore diameter is then calculated by the well-known formula:

pore diameter (microns) = K/pressure (inches of water)

This formula is discussed in WADC Technical Report 56-249, dated May, 1956, entitled "Development of Filters for 400.degree.F. and 600.degree.F. Aircraft Hydraulic Systems" by David B. Pall, and available from the ASTIA Document Service Center, Knott Building, Dayton 2, Ohio. A detailed description of the bubble point test and determination of pore size from the maximum particle passed will be found in Appendix I of this report. See also U.S. Pat. No. 3,007,334, dated Nov. 7, 1961, to David B. Pall.

K is determined by measuring the maximum spherical glass bead or carbonyl iron particle which passes through the element, in accordance with WADC Technical Report 56-249 and MIL-F-8815 B Paragraph 4.6.2.5 (Aug. 10, 1967).

The pore diameter obtained by this method is the maximum pore diameter. By continuing to increase air pressure until the whole surface of the filter vacuum is bubbling (known as the "open bubble point"), the same constant can be used to compute an average diameter characteristic of most of the pores. Tests have shown that if air is passed at a velocity of 70 to 170 cm/min, the pressure necessary to achieve the open bubble point taken together with the K value given above gives a value for the pore opening approximately the true average value. The ratio between the maximum pore size and the average pore size of the microporous media of this invention generally ranges from about 2:1 to about 4:1, a relatively small difference which greatly increases the safety and reliability of the product.

The following examples in the opinion of the inventors represent preferred embodiments of their invention.

EXAMPLE 1

Four anisometric knitted wire mesh compositions were prepared, made of 0.0011 inch diameter AISI 347 stainless steel wire, using a weft knit mesh that had from 12 to 18 needle ends per inch. Sixteen layers of this mesh were stacked at random orientation to make a composite, and sintered at 1,200.degree.C. The composite was cut into four pieces which were rolled to thicknesses of 0.007 inch, 0.0045 inch, 0.003 inch and 0.002 inch, respectively. The four layers were stacked in that order, and resintered to make the final anisometric composite.

The dirt capacity of the final composite was determined in accordance with the following test procedure, which represents a modification of the procedure of Military Specification MIL-F-8815B. The composite described in the preceding paragraph was clamped in a flow jig fitted with gaskets 31/2 inches OD and 3.06 inches ID, and connected to a pressure build-up and collapse pressure apparatus, as defined in Section 4.6.2.7 of MIL-F-8815B, Aug. 20, 1967. Hydraulic fluid conforming to Specification MIL-H-5606 was run through the mesh at a flow of 40 gpm/ft.sup.2. Direction of flow was such that the upstream face of the test piece was the 0.007 inch (highest voids volume) face. Standardized fine air cleaner (A-C fine) test dust in a slurry was added through the dust valve in 0.2 gram increments at four-minute intervals. The clean-up filter was not by-passed during this test. Two minutes after each test dust addition, the pressure differential at rated flow through the apparatus was recorded. The initial pressure drop was 0.2 psid., and the weight of contaminant added in the same manner to develop a differential pressure across the mesh of 15, 40 and 90 psid. was, respectively, 85, 91 and 97 grams/sq. ft. After cleaning the composites, a suspension of glass beads in oil was passed through them. The maximum bead passed was 62 microns. This is the maximum particle rating.

These data represent a very high dirt capacity, considerably higher than a woven wire mesh of equivalent maximum particle rating. For a 325 .times. 325 .times. 0.0014 stainless steel square weave wire mesh, of nominal opening 43 microns, and maximum particle rating 51 microns, the weights of contaminant (AC Test Dust) were 20, 24, and 26 g./sq.ft. for differential pressures of 15, 40, and 90 psig., respectively for a 200 .times. 200 .times. 0.0021 stainless steel square weave wire mesh of nominal opening 74 microns and maximum particle rating 38 microns, the weights of contaminant were 56, 63, 68 g./sq.ft. to 15, 40 and 90 psig., respectively. These two woven meshes are industry standards for removal ratings in the range of 43 to 83 microns. Thus, the knitted wire mesh composite of the invention has a dirt capacity greater than that of woven wire mesh of comparable or somewhat larger pore size.

EXAMPLE 2

An anisometric knitted wire mesh composite was prepared, made of layers of 0.002 inch A151347 stainless steel wire weft knit mesh having from 12 to 18 needle ends per inch. Ten layers of this were stacked, annealed, rolled and sintered at 1,200.degree.C. to a composite 0.006 inch thick. Twenty layers were stacked, annealed, rolled and sintered at 1,200.degree.C. to a composite 0.008 inch thick. Ten layers were stacked, annealed, rolled and sintered at 1,200.degree.C. to a composite 0.0023 inch thick. The three composites were then stacked in that order, and resintered to make the final anisometric composite.

The dirt capacity and maximum particle rating of this wire mesh composite were determined in accordance with the test procedure of Example 1. The weights of contaminant to 15, 40 and 90 psig. were 56, 60 and 78 grams/sq.ft., respectively.

The maximum particle rating was 71 microns.

EXAMPLE 3

An anisometric knitted wire mesh composite was prepared made from 64 layers of 0.0011 inch AISI 347 stainless steel wire weft knit mesh having 12 to 18 needle ends per inch, stacked, annealed, rolled and sintered at 1,200.degree.C. to a composite 0.028 inch thick.

The dirt capacity and maximum particle rating of this composite were determined in accordance with Example 1, except that the flow was 50 g.p.m./sq. ft. The weights of contaminant to 15 40 and 90 psig. were 91, 107 and 120 g./sq.ft., respectively.

The maximum particle rating was 60 microns.

EXAMPLE 4

The procedure of Example 3 was repeated except that the final composite was rolled and sintered to a composite 0.018 inch thick.

The dirt capacity and maximum particle rating of this composite were determined in accordance with Example 3. The weights of contaminant to 15, 40 and 90 psig. were 64, 68 and 80 grams/ft.sup.2, respectively.

The maximum particle rating was 31 microns.

EXAMPLE 5

The composites of Examples 3 and 4 were stacked and sintered at 1,200.degree.C. The resultant composite was clamped in a flow jig as in Example 1 with the 0.028 inch thick portion upstream.

The dirt capacity and maximum particle rating of this composite was determined as in Example 3. The weights of contaminant to 15, 40 and 90 psig. were 81, 90 and 97 grams, respectively.

The maximum particle rating was 30 microns.

EXAMPLES 6 to 19

A number of anisometric knitted wire mesh composites were prepared, from weft knitted wire mesh made of AISI 347 stainless steel wire, 0.002, 0.003 or 0.004 inch in diameter, as noted in Table I. These were stacked, using the number of layers noted in the Table, sintered at 1,150.degree. - 1,400.degree.C., rolled to the thickness noted in the Table, and then resintered at 1,100.degree. - 1,250.degree.C. The bubble points and air flow at the pressure differential noted and the Rayl numbers were determined, and are listed. The Rayl number is a measure of flow resistance, and is discussed below. ##SPC1##

The tensile breaking strength, specific strength (ratio of breaking strength to weight per unit area) and Young's Modulus were determined for several of the Examples given in Table I. This data is given in Table II:

TABLE II

Tensile Specific Example Breaking Strength Young's Modulus No. Strength ft.* psi. lb./ft. 6 3468 7332 p.6 7 3432 7150 p.6 8 3744 7800 up.6 17 3564 6854 up.6 18 7140 7933 up.6 19 8820 8820 up.6 *lb./ft./lb./sq./ft.

For comparison with Examples 1 to 19, similar data is given in Table III for a number of sintered woven wire mesh sheets of comparable weights and pore size, made of the same stainless steel, AISI 347, as these examples. ##SPC2##

It will be noted that all the above data is for material which has been resintered, thus at least partially annealing the mesh, and the variation in specific strength is not very large. This indicates that, for any alloy, the breaking strength is about proportional to the weight of material used, with the amount of compression performed on the material having at most a small effect.

These anisometric materials were satisfactory as filters for air and for liquids.

EXAMPLES 20 TO 32

A number of anisometric knitted wire mesh composites were prepared, from weft knitted wire mesh made of AISI 347 stainless steel wire, 0.002, 0.003, and 0.004 inch in diameter, as noted in Table IV. These were stacked, using the number of layers noted in the table, sintered at 1,150.degree. - 1,400.degree.C., rolled to the thickness noted in the table (0.01 to 0.05 inch), resintered at 1,100.degree. - 1,250.degree.C., and the Rayl number and tensile strength determined.

TABLE IV

Wire Actual Ex. Diameter Weight No. of Rayl Thickness No. (inch) (lb./ Layers No. (inch) sq.ft.) 20 0.004 0.47 24 10 0.0125 21 0.003 0.75 112 59 0.031 22 0.003 0.58 94 43 0.0245 23 0.004 1.0 82 32 0.0485 24 0.004 0.85 66 50 0.0325 25 0.004 0.72 52 45 0.0235 26 0.004 0.61 44 44 0.0215 27 0.004 0.53 40 35 0.0175 28 0.004 0.63 50 32 0.0245 29 0.004 0.75 60 38 0.029 30 0.002 0.65 212 32 0.0425 31 0.002 0.65 212 50 0.036 32 0.002 0.80 242 32 0.048

EXAMPLES 33 TO 35 published number

For applications requiring high strength, the resintering step can be omitted, so that the knitted mesh composite remains in a work-hardened condition, or heat treatable materials can be used. Example 33 is an instance of the former approach, Examples 34 and 35 the latter. These composites were prepared from weft-knitted wire mesh made of the steel alloy noted in Table IV, stacked using the number of sheets noted in the table, sintered at 1,150.degree.-1,400.degree.C., and rolled to the thickness noted in the Table. Examples 34 and 35 were resintered at 1,100.degree.-1,250.degree.C., and heat treated for maximum strength following manufacturer's published recommendations. The Rayl number, breaking strength and specific strength were then determined for all the examples. ##SPC3##

These materials were quite satisfactory as filters.

EXAMPLES 36 TO 39

It is also remarkable that the knitted mesh composites provide more uniform permeability over the face of a composite than woven wire mesh of comparable nominal permeability. Four knit mesh composites and four woven wire mesh sheets were checked for Rayl number, 16 places each on sheets 18 .times. 48 inches in size. Results are given in Table VI.

TABLE VI

Example Average No. Type Rayl Variation % Variation No. 36 Knitted 10 .+-.0.3 3.0 Control E Woven 13 .+-.1 7.7 37 Knitted 20 .+-.1 5.0 Control F Woven 27.5 .+-.2.5 9.1 38 Knitted 31 .+-.2 6.5 Control G Woven 34 +6 +17.6 -3 -8.8 39 Knitted 50 .+-.3 6 Control H Woven 50 .+-.8 16

In all cases, the percentage variation for the knitted wire mesh composites was lower than for the woven mesh, ranging from 3 to 6.5 percent for the knitted mesh composites as against 7.7 to 17.6 percent for the woven mesh.

EXAMPLE 40

An anisometric knitted wire mesh composite was prepared made from 122 layers of 0.002 inch AISI 347 stainless steel wire weft knit mesh having from 12 to 18 needle ends per inch. These layers were stacked on top of a perforated AISI 304 stainless sheet having 0.028 diameter holes on 0.063 inch centers in an equilateral triangular pattern, sintered at 1,200.degree.C., rolled, and sintered again. The final composite had a permeability of 50 Rayls, a weight of 1.1 lbs/ft..sup.2, a Young's modulus of 13 .times. 10.sup.6 psi., a breaking strength of 12,240 lbs/ft. and a specific strength of 11.12 ft.

By using perforated sheet made of high strength alloys, for example, the precipitation hardening alloys such as 17-7 PH, 17-4 PH, AM 350, AM 355, etc., bonded to lower or higher yield strength knitted wire mesh composites, even better mechanical properties can be obtained.

Some high strength alloys are not readily available, or are very expensive, when made as fine wire. By combining perforated material made of very high strength alloys with knitted mesh composites made of readily available wire, a high strength product is obtained at low cost.

The anisometric wire mesh composites of the invention are especially suited for use as liquid filter elements because of their disproportionately high flow crosswise pf the sheet, their unusually high dirt capacity, low flow resistance, and particle removal rating, and their high strength. A typical filter unit including an anisometric knitted wire mesh composite of the invention as the filter element is shown in FIGS. 8 and 9.

The filter unit of FIGS. 8 and 9 comprises a filter housing or lead 40 having an inlet passage 41 and an outlet passage 42, opening into a filter bowl 43 which is threadably attached to a dependent portion 44 of the head. Disposed in the bowl 43 in a manner to intercept liquid flow from the inlet 41 to the outlet 42 through the bowl 43 is a filter element 45 composed of a corrugated cylinder of an anisometric knitted stainless steel wire mesh composite 46 of the invention, and an internal supporting core 47 held between top and bottom end caps 49 and 50, respectively. The top end cap 49 with the biasing action of Belleville spring 48 at the bottom of bowl 43, engages the dependent wall 51 of outlet 42 in a leakproof seal, so that all liquid entering the bowl 43 from inlet 41 can leave the bowl only by passing through the filter element 45.

A by-pass line 60 is provided, with a relief valve 61 arranged to open at a predetermined pressure differential between inlet and outlet passages 41 and 42, to ensure continued liquid flow in the event of clogging of the filter element.

A pressure indicator 62 is provided, also responsive to a predetermined pressure differential between the inlet and outlet passages to indicate a clogged condition of the filter.

Thus, liquid in normal flow enters the head 40 via inlet 41, passes into bowl 43 outside the filter mesh 46, passes through the mesh and core 47 into the open space 63 enclosed thereby, and emerges as filtered flow via outlet 42.

As the filter mesh 46 becomes clogged by the suspended contaminants removed thereby, the pressure differential thereacross rises, and eventually reaches the predetermined value at which the pressure indicator 62 is actuated to show the clogged condition, and the by-pass valve 51 is opened to ensure a continuing supply of liquid to the outlet 42. The filter unit can then be taken out of service, the bowl removed and the filter element replaced.

The filter element as shown is cylindrical, but any closed form can be used, as well as flat sheets. It can be supplied with any type of fitting, to secure it in the housing of the filter unit in a manner to ensure that all liquid flow passes through the filter. It is usually preferable to corrugate or fold the composite filter sheet, to provide maximum surface area in a small space.

Other variations will be apparent to those skilled in the filter art.

Claims

I claim:

1. A filter element comprising, in combination, a porous fluid-permeable anisometric knitted wire mesh composite of integrated multilayer structure capable of removing small particles from fluids comprising a plurality of layers of knitted wire mesh, compressed substantially throughout its surface area to a maximum pore diameter below about 200 microns, and having the wires lying almost entirely in planes approximately parallel to the plane of the composite, having an anisometric porosity, and a voids volume of at least 10 percent, the wires at the interface of the interior layers being intermingled and interlocked with each other substantially throughout by such compression, and the wires and the layers being bonded together at their points of contact, formed in a cylinder having two open ends and an end cap closing off each of the open ends.

2. A filter element as in claim 1 having an anisometric metallic filter sheet material whose thickness is within the range from about 0.001 to about 0.5 inch.

3. A filter element in claim 1, wherein the wires within each layer and at the interface of the layers of the filter sheet material are sinterbonded together.

4. A filter element as in claim 1, wherein said layers of the filter sheet material are of weft-type knitted mesh.

5. A filter element as in claim 1, wherein said layers of the filter sheet material are of warp-type knitted mesh.

6. A filter element as in claim 1, wherein the layers are of knitted mesh having less than thirty needle ends per inch.

7. A filter element as in claim 1, made of stainless steel wire.

8. A filter element as in claim 1, in which there are at least five layers of knitted wire mesh in one composite.

9. A filter element as in claim 1, wherein the wires of the filter sheet material are deformed at their points of crossing, so as to have a lesser height and a greater width at those points.

10. A filter element as in claim 1, having the wires of the filter sheet material sinter-bonded at their points of crossing.

11. A corrugated filter element comprising a porous fluid-permeable anisometric knitted wire mesh composite of integrated multilayer structure capable of removing small particles from fluids comprising a plurality of layers of knitted wire mesh, compressed substantially throughout its surface area to a maximum pore diameter below about 200 microns, and having the wires lying almost entirely in planes approximately parallel to the plane of the composite, having an anisometric porosity and a voids volume of at least 10 percent, the wires at the interface of the interior layers being intermingled and interlocked with each other substantially throughout by such compression, and the wires and the layers being bonded together at their points of contact, in corrugated form.

12. A filter unit comprising, in combination, a housing, a fluid inlet and a fluid outlet therein, and, disposed across the line of flow between the inlet and the outlet in a manner to intercept fluid flowing therebetween, a porous fluid-permeable anisometric knitted wire mesh composite of integrated multilayer structure capable of removing samll particles from fluids comprising a plurality of layers of knitted wire mesh, compressed substantially throughout its surface area to a maximum pore diameter below about 200 microns, and having the wires lying almost entirely in planes approximately parallel to the plane of the composite, having an anisometric porosity and a voids volume of at least 10 percent, the wires at the interface of the interior layers being intermingled and interlocked with each other substantially throughout by such compression, and the wires and the layers being bonded together at their points of contact.

13. A filter unit in accordance with claim 12, wherein the wires of the metallic filter sheet are sinter-bonded at their points of contact.

14. A filter element as in claim 12 having an anisometric metallic filter sheet material whose thickness is within the range from about 0.001 to about 0.5 inch.

15. A filter element as in claim 12, wherein the wires within each layer and at the interface of the layers of the filter sheet material are sinter-bonded together.

16. A filter element as in claim 12, wherein said layers of the filter sheet material are of weft-type knitted mesh.

17. A filter element as in claim 12, wherein said layers of the filter sheet material are of warp-type knitted mesh.

18. A filter element as in claim 12, wherein the layers are of knitted mesh having less than thirty needle ends per inch.

19. A filter element as in claim 12, made of stainless steel wire.

20. A filter element as in claim 12, in which there are at least five layers of knitted wire mesh in one composite.

21. A filter element as in claim 12, wherein the wires of the filter sheet material are deformed at their points of crossing, so as to have a lesser height and a greater width at those points.