Synthetic monofilament open mesh filtration fabric with asymmetric configuration for liquid / solid filtration with low load loss and ability to promote regeneration

Abstract

A synthetic monofilament open mesh type filter fabric having an asymmetric structure has a linear density / cm of warp threads that differs from a linear density / cm of weft threads, and wherein a diameter of the warp threads differs from a diameter of the weft threads. The synthetic monofilament open grid type filter fabric provided by the invention has improved solid particle blocking capability under the condition of ensuring equal liquid flow; the filter fabric has a higher flow rate (i.e., a lower pressure drop) while having the same ability to prevent the passage of particles, or, optionally, compared to the prior art, while having the same ability to prevent the passage of particles.

Description

Background of the Invention

[0002] The present invention relates to an open-mesh filter fabric with an asymmetric structure for technical or industrial use, which can be used to produce filters for removing various types of polluting particulate matter from liquids, in particular water.

[0003] Water is the most abundant compound on earth and a major component of the human body. The uses of water are innumerable. Consider, for example, its use in the domestic environment, in the industrial sector, in agriculture and more generally in the primary sector (agriculture, fisheries, aquaculture, forestry, mining).

[0004] In the complex water cycle, water undergoes multiple changes of state and important displacements (contact with other fluids, with inorganic or organic solid particles, with mineral and microbiological substances) that lead to its contamination, which in most cases makes it unsuitable for the specific uses within the industries listed above.

[0005] Therefore, several applications require different levels of water purification. Consider, for example, drinking water, softened water, water used in cooling circuits or recirculated water used in closed circuits, such as, for example, water used in breeding farms or swimming pools. Therefore, knowing the required purity of the water for a specific use and knowing the initial contamination level of the water (wastewater, black water, grey water, white water, rainwater), it is necessary to provide adequate and often complex treatment systems.

[0006] Specifically, filtration is the most commonly used technique for removing solid matter suspended in water. In its most general sense, filtration involves passing a contaminated liquid or gas through a purification medium that is sufficient to remove the contaminating compounds. There are a variety of water filtration systems, and they are often used in combination to obtain adequate quality.

[0007] For example, consider:

[0008] - Gravity separation, for the rough removal of material from a suspension by decantation;

[0009] - filtration using granular media (e.g. sand filters), to which chemical products having coagulation and flocculation capabilities may be added;

[0010] - Physical filtration through a barrier and without the use of chemicals;

[0011] -Membrane separation treatment.

[0012] Mechanical filtration allows the removal of particulate matter from 1 μm and above, microfiltration concentrates particulate matter in the range of 0.1 μm-1 μm, ultrafiltration allows the removal of particulate matter in the range of 0.01 μm-0.1 μm, reverse osmosis and nanofiltration allow the removal of particulate matter in the range of 0.001 μm-0.01 μm.

[0013] In this invention, we will focus on mechanical filtration (>1 μm), which is almost always an essential filtration step and is present in almost all water purification processes. In particular, the management of wastewater, drinking water and recirculation water in farms and aquaculture is considered. Regarding mechanical filtration, the most commonly used processes are of course:

[0014] -Drum filtration

[0015] -Disc filtration

[0016] -Sand filtration

[0017] -Bag filtration

[0018] - Cartridge filtration.

[0019] Different process types are selected according to the required level of filtration, according to the flows involved and the pressures at play, in order to have a high filtration efficiency and suffer as little as possible from any clogging problems. For this purpose, the process is usually divided into at least two stages: separation of coarse particles by sedimentation, followed by finer filtration.

[0020] In drum filtration, the liquid to be filtered passes through a rotating drum. The drum rotates slowly (several revolutions per minute) and the filter medium is held tight with the help of a metal frame or a frame made of polymer material to form a drum filter. Depending on the type of frame, different processes can be used to produce the filter. This ranges from simple bonding to compression molding or even injection molding.

[0021] During the rotational movement, the filter removes the captured impurities from the water, conveying the water thus filtered to the drainage channel. Depending on its application, the filter can be appropriately sized to filter particles from 5 μm and above, from 10 μm and above, and from 10 μm and above. 3 / h up to about 4500m 3 Typically, during its use, the filter is cleaned with a counter-current of water and there is an automatic control of the head to ensure its consistency during use.

[0022] In a disc filter, the water is transported inside a drum, inside which there is also a filter, which usually has a trapezoidal shape, connected to form a rotating disc in the water. During this movement, the water passes through the filter, removing the pollutant particles. Even in this case, the filter medium is held tight with the help of a metal frame or a frame made of polymer material to form a drum filter.

[0023] As far as sand filters are concerned, there is a wide variety of filter media, tailored for different flow rates and in order to ensure different filtration efficiencies. Typically, they consist of woven or nonwoven fabrics, filled with silica (typical particle sizes measure from 0.5 mm to 1 mm), which actually performs the filtering action. In addition to the filter layer, there is a support layer consisting of larger sized particulate matter. Sand filters can be in both vertical and horizontal configurations. The size and design of sand filters depends on many parameters, such as, for example:

[0024] - The filtration rate or water flow rate involved; for example, in aquaculture, a few tens of m 3 / m 2 h specific flow;

[0025] - The height of the filter bed, which is essential to ensure the desired efficiency level;

[0026] - The characteristic dimensions of the filter medium, which are equally important to ensure the desired efficiency level without generating excessive pressure drops;

[0027] - Arrangement of filtered water header;

[0028] - Washing system of the filter which, in case of clogging, generates an excessive pressure drop.

[0029] Bag filters are particularly suitable for applications where there are large amounts of contaminants, large flow rates, and relatively low pressure drops when a fairly economical and easy-to-use filtration solution is required. In this case, the fluid circulates through a filter sleeve mounted on a frame. There are many types of filter sleeves and frames, depending on the application.

[0030] Cartridge filtration consists of circulating a fluid through a container that includes a filter cartridge. The container usually has a frame of metal and plastic that has the function of giving mechanical strength to the filter medium, which can be made of fabric, non-woven fabric, wound thread, membranes made of polymeric materials and / or metallic materials, depending on the application. The fluid passes through the cartridge that retains the contaminants, and depending on the type of filter medium used, the cartridge can be removable and cleanable or disposable.

[0031] On a purely conceptual and theoretical level, all of the processes mentioned are united by the same type of desired properties, namely filtration efficiency on the one hand and permeability to liquids on the other hand.

[0032] In terms of filtration efficiency, filtration efficiency is defined as the ratio of particulate matter blocked by the filter to the total number of impinging particles on the filter medium. It is intuitively understood that, given a certain particle size, it is usually desirable to have a maximum filtration efficiency. It is also intuitively understood that in order to maximize filtration efficiency, the typical characteristic size of the filter medium (the size of the pores or filter mesh) must be smaller than the particulate matter to be blocked. In addition, the smaller the filter mesh, the greater the filtration efficiency.

[0033] It should be noted that the above considerations are fully valid only for filter fabrics made of monofilament yarns. In particular, the term "monofilament" or "monofilament yarn" means a monolithic yarn that is not composed of an interweaving of threads or strands of smaller dimensions, but, on the contrary, is a monolithic element of constant circular cross-section.

[0034] In this case, the geometry of the filter fabric is perfectly defined and the distance between the next two lines of each mesh or hole can be uniquely measured, defining a value for the mesh opening or hole size which determines the size of particulate matter that can be blocked by the filter mesh.

[0035] Vice versa, another required property relates to the flow rate of liquid that the filter medium manages to ensure at the initial time and during use. Even in this case, it can be intuitively understood that, with a defined maximum operating pressure drop, a flow rate as high as possible is sought. Vice versa, with a defined operating flow rate, a medium is sought that minimizes the pressure drop that causes energy consumption at the pump or electromechanical system that provides the fluid movement through the cartridge.

[0036] A further requirement arises from the desire to maximize the service life of the filter before it becomes clogged. Starting with a new filter medium that provides a very low pressure drop (ΔP) means ensuring a greater safety margin before reaching the ultimate pressure at clogging and thus a longer service life of the filter element before replacement or regeneration of the filter element.

[0037] Furthermore, in the case of regeneration, the open mesh monofilament fabric will also facilitate countercurrent washing, ensuring a high and free flow of wash water. Furthermore, the monofilament yarn will minimize the risk of contaminant particles being permanently trapped inside the yarn itself.

[0038] In general, minimizing pressure drop and / or optimizing flow leads to the use of particularly permeable filter media.

[0039] It is therefore clear that in the case of choosing a filter medium made of a monofilament fabric, the monofilament fabric must have an "open mesh" configuration, i.e., have appropriately spaced threads or yarns and have a diameter to the extent that a plurality of visible mesh openings are defined on the surface of the fabric in plan view and can be traversed by a fluid flow moving in a direction perpendicular to the plane of the fabric itself.

[0040] Under these conditions, the open mesh monofilament fabric of the invention allows defining the following values:

[0041] - "grid opening" or "aperture", which is measured as the distance between lines in plan view (see Figure 3 );and

[0042] - "Open Area %", which is calculated as the open grid area measured in the plane (see Figure 4a ) and the total area of ​​competence of a single grid (see Figure 4 ), the total effective area being measured relative to the center line of the line defining it.

[0043] It should be noted that the configuration of the open-mesh monofilament fabrics suitable for the purposes of the present invention differs from that of the closed-mesh monofilament fabrics, since the latter have a thread density or mesh count (number of threads / cm) and a thread diameter in one of the two directions of weft and warp, so as to reach a so-called "saturation", in which the parallel threads touch one another and no longer form any visible grid in plan view (Tressen fabrics, Reps fabrics, Dutch Weave fabrics, Double Dutch Weave fabrics). This involves obtaining very small channel sections that only allow cross-flow in the diagonal direction relative to the fabric plane. Particles of pollutants that are much smaller than the size of the threads are filtered in this way, however, the resulting pressure drop is significantly higher than that achieved with the open-mesh monofilament fabrics of the present invention.

[0044] In fact, the well-known closed-grid fabrics are often used in process filtration, where high pressure separation of large quantities of pollutants with medium-fine fineness (e.g. sludge) is preferred, to the detriment of the pressure drop, which for these fabrics reaches very large values.

[0045] In contrast, filters of the present invention made with open mesh fabrics can be used in all applications of solid / liquid filtration where flow must be maximized and pressure drop minimized.

[0046] Thus, the goal of minimizing pressure drop and / or optimizing flow leads to the use of a particularly permeable filter media and can be achieved using an open mesh monofilament fabric suitable for the application of the present invention by:

[0047] - by using media with particularly large pore / mesh openings, which however will impair the filtration performance;

[0048] - Maintaining small pore size in order to maximize filtration efficiency by maximizing the percentage of open area or volume of the media, defined as the % of open area of ​​the filter media as a percentage of the total area and the % of open volume of the media as a percentage of the total volume of the media.

[0049] It is obvious from the above description that these two properties will require materials characterized by widely different physical-geometric properties and therefore require a design and optimization phase in each case. When it comes to reusable and therefore washable filters ranging from 5 μm and above, and in particular to precision filters, the ideal solution in terms of filter media is represented by precision fabrics made of synthetic and metallic monofilaments. The filter media weight plays an important role in many applications (see drum filters).

[0050] Therefore, in order to save energy, the reduction of mass makes the use of synthetic monofilaments advantageous over metallic counterparts, while, on the contrary, the use of multifilament yarns consisting of several filaments twisted together will not guarantee obtaining precise mesh openings of the desired size, which will affect the filtration efficiency.

[0051] All of the above require that the fabric has a high open area in order to minimize the resistance of the material to the passage of liquids. At the same time, the same fabric must also ensure sufficient filtration efficiency and therefore have appropriately narrow mesh openings.

[0052] In summary, there is a need to reduce the mesh opening size while maintaining the void / full ratio, ie the open area of ​​the fabric. For prior art square mesh fabrics, these two conflicting requirements are only partially met, as we will now see.

[0053] In order to reduce the mesh size, there are two possible ways. The first way offers the possibility of inserting an increasing number of threads of the fabric, the diameter being fixed. Alternatively, the second way offers the use of the same number of threads, but with a higher thread diameter. It is obvious that in both cases the empty / full ratio decreases and the water penetration performance will deteriorate. Therefore, in order to obtain a mesh of smaller size while keeping the empty / full ratio constant, it is intuitively considered that the only feasible way consists in using an increasing number of threads, but at the same time with increasingly smaller thread diameters. However, this third way, which is obviously ideal, shows two limitations:

[0054] - technical limitations in the processing of the yarn, whereby below a certain diameter it is neither possible to extrude nor even to weave the monofilaments,

[0055] - Technical limitations in the weaving process, whereby it is not possible to increase the thread density (mesh / cm) of threads per cm above a certain threshold. In some cases where the application places high protection requirements and a mesh with particularly small dimensions is needed, the use of threads of smaller diameter may result in a structure with an impossible number of threads. This is particularly true for the warp threads, i.e. the warp threads arranged according to the roll length of the material produced, where there are major critical problems. These are related to the need to pass all the warp threads through the opening of the weaving reed, the thin blades of which impose restrictions on the spacing of the threads. In addition, there are mechanical limitations, since this group of warp threads is subject to great mechanical stresses due to the stresses exerted on the frame.

[0056] It is therefore clear that the conventional option mentioned, namely that of reducing the mesh size, necessarily involves the disadvantage of also reducing the open area of ​​the mesh itself, since there is no prior art solution that achieves both benefits simultaneously.

[0057] From all of the above, we infer that the choice of the best square mesh fabric for water filtration in the prior art is always a compromise: a very closed fabric is good for blocking but performs poorly from an energy point of view; conversely, a more open fabric provides acceptable permeability for the passage of fluids (with minimal pressure drop) but shows mesh openings that are too large to effectively block all particles of contaminants. SUMMARY OF THE INVENTION

[0059] The main object of the present invention is to provide an open mesh synthetic monofilament fabric for liquid / solid filtration, and in particular water filtration, preferably for the production of drum filters or cartridge filters, which, unlike the fabrics of the prior art, has a better solid particle blocking capability while ensuring the same liquid flow rate; or alternatively, compared to the prior art, the synthetic monofilament fabric shows a higher flow rate (i.e., a lower pressure drop) with the same ability to prevent the passage of particles.

[0060] These and other objects are achieved by the fabric of claim 1. Some preferred embodiments of the invention follow from the remaining claims.

[0061] The fabric of the invention offers the advantage of presenting a greater resistance to the passage of polluting particles relative to prior art fabrics with comparable air flow and water passage properties. Furthermore, if compared with prior art fabrics and similar fabrics with regard to the resistance to the passage of solid pollutants of interest, the fabric of the invention has better water flow properties, which increases the energy yield of the filtration system and also ensures a longer life of the filter element before clogging, as well as the possibility of easier regeneration by backwashing (where provided).

[0062] Of course, for each application of liquid / solid filtration, it is possible to choose the structure of the new fabric that best suits the purpose and to achieve partial improvements both in the first area (better protection) and in the second area (better flow), in any case to a total degree greater than what the prior art allowed.

[0063] The fabric of the present invention must be produced by weaving synthetic monofilament yarns, which are more effective than multifilament yarns in intercepting pollutant particles. The material from which the starting monofilaments are prepared can be a synthetic technical polymer belonging to the family consisting of polyesters, polyamides, polyaryletherketones, polyphenylene sulfides, polypropylenes, perfluorocarbons, polyurethanes or polyvinyl chlorides. Alternatively, the material from which the monofilaments of the fabric of the present invention are made can be a man-made polymer belonging to the family consisting of cellulose or viscose fibers.

[0064] The monofilaments from which the fabric of the invention is made may have diameters ranging from 10 μm up to 90 μm in both the warp and weft directions.The fabric of the invention may be produced with a textile structure requiring a number of threads ranging from 23 up to 450 per cm.

[0065] The fabric can be made with different open mesh textile structures having the common feature of being asymmetric in both directions of the weft and warp, in particular with respect to the thread density per centimeter and the thread diameter. Thus, the numerical density of the weft will be different from that of the warp and the weft will be different from the warp in terms of thread diameter or yarn properties.

[0066] Thus, depending on the choice of construction parameters of thread density, thread diameter and their mutual balancing in an asymmetric configuration, it is possible to produce fabrics according to the invention with a square grid as well as with a rectangular grid.

[0067] The mesh openings of the fabric of the invention may have values ​​in the range from 5 μm to 150 μm, which are perfectly defined in a plan view of the open-mesh fabric.

[0068] Further examples given in the following figures will better illustrate the textile structure of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0070] These and other objects, advantages and features result from the following description of some preferred embodiments of the fabric of the invention provided by way of non-limiting examples in the figures of the accompanying drawings.

[0071] In the attached picture:

[0072] - Figure 1 An example of a filter cartridge on which the fabric of the present invention may be used is shown;

[0073] - Figure 2 shows a cross section of a classic disc filter on which the filter fabric of the present invention can be used;

[0074] - Figure 3 , Figure 4 and Figure 4a A square grid of a prior art fabric is illustrated;

[0075] - Figure 4b Diagram showing the Figure 3 Grid;

[0076] - Figure 5a , Figure 6a shows a portion of a prior art open mesh monofilament filter fabric which serves as a basis for comparison with a corresponding embodiment of the fabric of the present invention;

[0077] - Figure 5b , Figure 6b Two different embodiments of the open mesh monofilament fabric of the present invention are shown, which are regarded as examples and are respectively Figure 5a, Figure 6a The prior art fabrics are compared;

[0078] - Figure 7 A prior art "Dutch weave" type closed grid monofilament fabric is illustrated;

[0079] - Figure 8 and Fig. 9 The results of the measurements of comparative data relating to pressure drop and filtration efficiency, respectively, during experimental filtration tests of fabrics of the invention compared to prior art fabrics are reported.

[0080] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0081] In the filter operation, such as Figure 1-Figure 2 As shown, a water flow (arrow F1 ) is conveyed in the direction of the fabric 2 so as to obtain a downstream water flow F2 which filters out pollutants 3 present in the upstream flow F1 .

[0082] exist Figure 3 and Figure 4 In the prior art shown, the mesh 4 of the fabric 2 is square and consists of threads 5 forming respective sides 6 of the square mesh 4 .

[0083] exist Figure 4 The open area of ​​the grid 4 of the prior art itself is calculated as a smaller square 7 ( Figure 4a ) and the larger square 8( Figure 4 ), the smaller squares 7 being included between the outlines or inner edges of the wires 5 forming the sides 6 of the grid 4, the larger squares 8 being measured up to the center line of the thickness of the wires 5 themselves.

[0084] When the prior art grid 4 receives particles of solid contaminants 3 having a diameter comparable to the length of the side 6 of the square grid itself from the water flow F1 to be filtered, the openings of the square grid are blocked, leaving only a small part 10 of the surface of the smaller squares 7 of the grid 4 for the water flow to pass through ( Figure 4b ).

[0085] As a result of the phenomena described, an increase in fluid pressure may occur in the upstream portion of the fabric 2 ( Figure 2 This enables the particles 3 of the pollutants to be deformed (arrow F1 in FIG. 4 ) until the particles 3 pass through the mesh 4. In this way, the filtration efficiency of the fabric is reduced relative to the filter design value.

[0086] Furthermore, the clogging of the square meshes 4 of the described prior art increases the pressure drop across the filter fabric, resulting in an increased energy requirement to maintain the same flow rate of water to be filtered.

[0087] In order to overcome these drawbacks, a fabric according to the invention is proposed which features a fabric structure characterized by a different number of threads per cm in both weft and warp directions, with the weft threads differing from the warp threads in diameter and possibly properties.

[0088] In particular, a ratio between the numerical density of the threads in the warp direction and the numerical density of the threads in the weft direction of between 0.4:1 and 2.5:1 is considered, as well as a ratio between the diameter of the warp threads and the diameter of the weft threads of between 0.5:1 and 2:1.

[0089] Figure 5b , Figure 6b Two possible and different embodiments of the fabric of the present invention are illustrated and compared with corresponding fabrics of the prior art to demonstrate the actual performance benefits of the invention itself.

[0090] In all the examples reported herein, the fabrics of the invention consist of synthetic monofilaments that offer optimal performances at the level of precision of the mesh openings and surface finishing, which prevents pollutants from being trapped inside the yarn itself. The polymeric nature of the fabrics also guarantees additional advantages in terms of lightness and at an environmental level (recyclability).

[0091] Some embodiments of the solution of the invention are now described in detail in comparison with equivalent textile structures of the prior art.

[0092] Figure 5a / Figure 5b

[0093] Figure 5a A portion of a prior art fabric 4 is shown, characterized in that the thread density per cm (N1) is the same for both the warp threads (vertical lines in the drawing) and the weft threads (horizontal lines in the drawing). Furthermore, the prior art fabric has the same thread diameter (d1) for both the warp threads and the weft threads. Thus, Figure 5a The opening grid 7 will be square and will be characterized in that the openings 6 of the grid 7 are of the same size in both the weft and warp directions.

[0094] exist Figure 5b In the present invention, a possible embodiment of the fabric 25 of the invention is now presented, in which the density of the threads per unit width in both directions and the corresponding diameter of the threads differ from each other. In this particular case:

[0095] - Number of warp threads per cm (N1, Figure 5b The vertical line in Figure 5a The number of warp threads of the prior art fabric is the same.

[0096] - Alternatively, the number of weft threads per cm (N2, Figure 5bThe horizontal lines in the fabric) are higher than the number of weft and warp threads per cm of the prior art fabric, and therefore also higher than the number of threads per cm of the prior art fabric (for both directions, N1, Figure 5a ). Note that this choice is usually possible in normal weaving processes, since the density of warp threads per cm is regulated by the presence of the reed, whereas for the weft threads, there is no such constraint, which can therefore be thicker. Therefore, an asymmetric configuration is possible and, as will be seen later, is advantageous.

[0097] - the diameter of the warp thread of the present invention (d1, Figure 5b The vertical line in Figure 5a The diameters of the warp threads of the prior art fabrics are the same.

[0098] - Alternatively, the diameter of the weft thread of the invention (d2, Figure 5b The horizontal line in the figure) is lower than the diameter of the weft and warp threads of the prior art fabrics and therefore also lower than the diameter of the threads (d1, for both directions, Figure 5a ). Note that this option is usually possible in normal weaving processes, because the weft threads are less stressed and smaller diameters can be used without compromising the quality of the fabric. Therefore, an asymmetric configuration is possible and, as will be seen later, is advantageous.

[0099] Furthermore, for the present example, the number of weft threads per cm (N2) is chosen such that, in combination with the value of the diameter d2 of the weft threads, it determines a mesh 7 having a perfect square shape with identical mesh openings 6 in both directions, just like the fabric 4 of the prior art ( Figure 5a ). In this way, the single mesh 7 intended to block the pollutant particles will be identical to that of the prior art, thereby determining the same filtration efficiency of both fabrics. At the same time, the fabric 25 of the invention will result in improvements with respect to: a reduction in the pressure drop across the filter, as well as an extension of the service life before clogging and the possibility of subsequent regeneration with backwashing. Due to the difference with the prior art fabric 4 ( Figure 5a ) is compared with the equivalent open area of ​​the fabric 25 ( Figure 5b ) has a larger open area, all of the above are achieved.

[0100] To prove this, in both cases the sum of the open surfaces at grid 7 must be considered compared to the relative reference area, which in this case is the area at Figure 5a and Figure 5b The portion of the fabric depicted in FIG. It can be seen that the fabric of the prior art ( Figure 5a ) has 4 open square grids in the area under consideration. Instead, the fabric of the present invention ( Figure 5b) has 4 open square grids in the equivalent reference area. Since the grid openings are the same in both cases for the above choices, it turns out that the open area of ​​the invention will be equal to 5 / 4 of the open area of ​​the prior art, which has a clear advantage in terms of lower pressure drop and no disadvantage in terms of filtration, since the value of the grid openings is the same.

[0101] Furthermore, given the greater number of square grids per unit surface presented by the new fabrics, it must be taken into account that in order to block them all, a greater number of pollutant particles would be required than in the prior art, thus extending the useful life of the filter before clogging and generally also increasing the possibilities of backwashing for regenerating the filter fabric.

[0102] Thus, in this first example, although both fabrics have the same square grid with equal openings, the present invention is a substantial improvement over the prior art.

[0103] Figure 6a / Figure 6b

[0104] For the previous Figure 5a , Figure 6a Also illustrated is a portion of a prior art fabric 4 having the same thread density per cm (N1) for both the warp threads (vertical in the figure) and the weft threads (horizontal in the figure). Furthermore, the prior art fabric 4 has the same thread diameter (d1) for both the warp threads and the weft threads. The mesh 7 will have a square shape and the mesh openings 6 will be the same size on the weft threads and the warp threads ( Figure 6a ).

[0105] With respect to the previous example, a second possible embodiment of the fabric 25 of the present invention is now presented ( Figure 6b ), where the density of threads per cm and the corresponding diameter of the threads are different in both directions. Similar to the previous case, the results show that:

[0106] - Number of warp threads per cm (N1, Figure 6b The vertical line in Figure 6a The number of warps in the prior art is the same.

[0107] - the number of weft threads per cm of the present invention (N2, Figure 6b The horizontal lines in the invention) are higher than the number of weft lines (N2, Figure 5b ) and the number of lines per cm of the prior art (for both directions, N1, Figure 6a ). As for the previous case, the compatibility of this option with the weaving process is confirmed, since the density of weft threads per cm is not regulated by the presence of the reed, which only has a binding force on the warp threads. Therefore, a greater thickening of the weft threads is possible and beneficial.

[0108] - the diameter of the warp thread of the present invention (d1, Figure 6b The vertical line in Figure 6a The diameter of the warp threads in the prior art is the same.

[0109] - Alternatively, the diameter of the weft thread of the invention (d2, Figure 6b The horizontal line in the figure) is lower than the diameter of the wires of the prior art fabric (for both directions, d1, Figure 5a ). This option is also compatible with normal weaving processes, since the weft threads are less stressed than the warp threads and smaller weft thread diameters can be used. Thus, asymmetric configurations are possible.

[0110] For this particular instance, choose Figure 5b The case of a higher number of weft threads per cm (N2): In this structure, the weft threads are thicker and have a corresponding opening ( Figure 6a Compared with the vertical direction, it is determined in the longitudinal direction ( Figure 6b The value of the mesh openings (in this case, the mesh openings are symmetrical in two directions and are therefore square) on the filter (vertical in the middle) is reduced. This will lead to an improvement in the filtration efficiency, resulting in the filter being able to block smaller particles, especially in the case of a pseudo-spherical shape.

[0111] However, it should be noted that this improvement is not achieved at the expense of ease of fluid passage (pressure drop, filter life, wash / regeneration capability) compared to the prior art. In fact, considering that the previous configuration of the present invention ( Figure 5a ) is an improvement in open area, a portion of such improvement may be given up (but never below the value of the open area of ​​the prior art) to convert it into an improvement in filtration efficiency. Figure 6b , indeed note that the displayed grid has now become 4 indeed note that, even if it is slightly smaller in size compared to the fabric of the prior art, it is much more numerous. By appropriately choosing the density N2, this construction solution of the invention can also be made advantageous compared to the prior art, even with regard to the open area.

[0112] In summary, the embodiments of the invention described herein demonstrate substantial improvements over the prior art in terms of filtration efficiency, while also maintaining the potential for advantages in terms of open area and low pressure drop.

[0113] The synthetic monofilament fabric of the present invention has an asymmetric structure with respect to the number of threads per cm and the diameter of the threads in both directions of the weft or warp.

[0114] Compared with the fabrics of the prior art, Figure 5bThe example ensures a clear increase in the value of the open area, which increase is beneficial in terms of improving the liquid flow and the resistance to clogging, the service life of the filter and, in some applications, also the possibility of regeneration and backwashing.

[0115] Instead, Figure 6b The examples described in show the improved capabilities of the invention in terms of lower mesh openings, which requires better filtration efficiency compared to the prior art, while maintaining values ​​of open area equal to or even slightly better than those of the prior art, thus without compromising the fluid dynamics and without introducing a higher pressure drop compared to the prior art (and in some cases even improving this feature).

[0116] Furthermore, the synthetic monofilament structure is ideal for minimizing weight and preventing pollutant particles from being trapped inside the yarn, which is a critical aspect in the case of multifilament yarns composed of several filaments. Finally, the smooth surface of the monofilaments minimizes the possibility of adhesion of pollutant particles and facilitates their sliding and removal during backwashing for regeneration of the filter.

[0117] In order to maximize the above benefits, the synthetic monofilament fabric with an asymmetric structure of the present invention must be characterized by a ratio between the numerical density of the threads in the warp direction and the numerical density of the threads in the weft direction in the range between 0.4:1 and 2.5:1, and a ratio between the diameter of the warp threads and the diameter of the weft threads in the range between 0.5:1 and 2:1.

[0118] In the filter fabric of the present invention, yarns of various types or different sizes may be arranged in combination in the same direction or in two different directions of weft and warp.

[0119] As previously explained, the object of the present invention can be classified as an "open mesh fabric" and therefore differs substantially from the closed mesh and asymmetric textile configurations of the prior art, such as the so-called "Tressen", "Reps" or "Dutch weave", in which the ratio between the thread density of the threads per centimeter in both directions is equal to 4:1 or higher, while for the present invention this ratio is at most equal to 2.5:1. In fact, the present invention aims to maximize the cross section of the liquid flow orthogonal through the filter, minimizing the pressure drop of the liquid flow; on the contrary, for the above-mentioned asymmetric fabrics, there is only the need to minimize the opening of the pores through which the fluid passes, in particular for filtration applications in which the pressure drop through the filter is not a problem. Therefore, in the asymmetric fabrics of the prior art defined as "Tressen", "Reps" or "Dutch weave", the threads in one of the two directions are adjacent to each other, reaching the so-called "saturation", leaving only the minimum openings for passage, which is suitable for ensuring advanced filtration, while generating a pressure drop significantly higher than that of the fabric of the present invention.

[0120] Just as an example, Figure 7 A typical "Dutch weave" structure of the prior art is shown, where the density of the weft threads (N4) is at least four times the density of the warp threads (N3) and reaches saturation, so that the weft threads are in contact with each other.

[0121] In particular, the "saturation" of a fabric in the weft or warp direction is defined as the ratio between the product of the thread density per centimeter multiplied by the diameter of the thread in the corresponding weft or warp direction and divided by the reference length used to calculate the thread density.

[0122] For example, defining the density of the wire N1 / cm and defining the corresponding wire diameter d1 measured in μm, we obtain:

[0123] Saturation = N1: ratio between the diameters of the wires.

[0124] Usually expressed as a percentage.

[0125] For open mesh filter fabrics, the saturation is less than or equal to 70%. Thus, adjacent threads do not touch each other in any case and a cross flow perpendicular to the plane of the fabric is always ensured.

[0126] In contrast, closed mesh filter fabrics are intentionally made to have a saturation close to 100% so that adjacent threads are in contact with each other. In this way, the cross-section of the liquid phase is reduced to Figure 7 The small cross-section 7 is penetrated by the oblique water flow, which does not allow the open area to be defined.

[0127] As can be noted, the grid openings are minimized and correspond to Figure 7 The small area 7 marked in the figure is able to block small particles as indicated by the reference (3) herein. However, a fabric so closed and having such a different purpose would generate a pressure drop significantly higher than that obtainable with the fabric of the present invention, and therefore it is not at all comparable to the asymmetric configuration of the prior art reported herein.

[0128] It can be seen from the present description that by choosing appropriately unbalanced and asymmetric configurations for the two characteristics of the number of threads per cm and the diameter of the threads for both the weft and warp threads, a larger open area than fabrics of the prior art and a smaller "effective" mesh opening (i.e. in the direction in which it is smallest) and even a higher open area than a symmetrical fabric can be obtained.

[0129] As explained, filtration efficiency is inversely proportional to mesh size and directly proportional to open area. It can be seen that the asymmetric textile structure of the present invention, which aims to maximize the open area with the same mesh size, can optimize the filtration efficiency of the filter.

[0130] The tests of filtration efficiency and flow rate relative to ΔP were performed according to ISO19438 / ISO16889 (filtration efficiency) and ISO4548-1:1997 (flow rate) standards, respectively.

[0131] In the filtration efficiency test according to the ISO19438 / ISO16889 standards, the filter fabric is subjected to a controlled flow of contaminated liquid according to the type and specific concentration of contaminants defined by the above standards. During the verification test, a gradual clogging of the filter occurs, which is monitored by acquiring the value of the differential pressure (pressure drop) across the filter fabric. Similarly, the sensors of the test equipment acquire the size and number of particles upstream and downstream of the filter. From these values, the filtration efficiency value ( Fig. 9 ).

[0132] During the liquid permeability hydrodynamic test according to ISO 4548-1:1997 standard, the fabric sample is subjected to a gradually increasing liquid flow. Depending on the value of the applied flow rate, the pressure sensor acquires the corresponding pressure drop value across the fabric, generating a value such as Figure 8 A graph of ΔP versus specific flow rate is shown in FIG.

[0133] exist Figure 8 and Fig. 9 The figures reported in represent the results of these tests, which were carried out on a prior art fabric 4 having a symmetrical structure and on a fabric 25 according to the invention having an asymmetrical structure.

[0134] Specifically:

[0135] - Prior art fabric 4 has a symmetrical structure with N1=215 threads / cm in both directions (both weft and warp) and thread diameter d1=31 μm; it has a mesh opening 6 equal to 12 μm and an open area of ​​7% ( Figure 6a );

[0136] - the fabric 25 of the invention has an asymmetric structure with N1 = 200 threads / cm for the warp and N2 = 245 threads / cm for the weft, resulting in an effective mesh opening equal to 10 μm (in the direction where it is smallest) and an open area equal to 8.5% ( Figure 6b ).

[0137] So, based on the tests performed, the results are:

[0138] - Using a test pollutant dust of type Arizona Test Dust A3 (medium), both fabrics had comparable filtration efficiencies.

[0139] - For the same guaranteed filtration efficiency and therefore the same particle removal in water filtration applications, the flow rate ensured by the fabric with an asymmetric structure is at least 20% higher at the same pressure drop.

Claims

1. A synthetic monofilament open mesh filter fabric having an asymmetric structure, characterized in that: The filter fabric has a linear density / cm of warp threads that is different from the linear density / cm of weft threads and is characterized in that the diameter of the warp threads is different from the diameter of the weft threads.

2. The filter fabric according to claim 1, characterized in that The ratio between the linear density / cm of the weft and the warp in their respective directions is in the range between 0.4:1 and 2.5:

1.

3. The filter fabric according to claim 1, characterized in that A ratio between the diameter of the warp threads and the diameter of the weft threads is in the range between 0.5:1 and 2:

1.

4. The filter fabric according to one or more of the preceding claims, characterized in that The openings of each grid range between 5 μm and 150 μm.

5. The filter fabric according to claim 1, characterized in that The synthetic monofilaments are monofilaments made of synthetic technical polymers belonging to the family consisting of polyesters, polyamides, polyaryletherketones, poly(p-phenylene sulfide), polypropylenes, perfluorocarbons, polyurethanes or polyvinyl chlorides.

6. The filter fabric according to claim 1, characterized in that The synthetic monofilaments are monofilaments made from artificial polymers belonging to the family consisting of cellulose or viscose fibers.

7. The filter fabric according to claim 1, characterized in that Yarns of various types or sizes are arranged in combination in the same direction or in two different directions in the weft and warp.

8. Filter fabric according to one or more of the preceding claims, for use in a cartridge filter, drum filter or disc filter intended for water filtration, capable of ensuring a sufficient water flow to minimize the pressure drop while optimizing the filtration efficiency and therefore the removal of pollutant particles contained in the liquid, wherein the filter fabric is of the open mesh type made of synthetic monofilaments with an asymmetric structure, with a linear density / cm of the warp threads that is different from the linear density / cm of the weft threads, and wherein the diameter of the warp threads is different from the diameter of the weft threads.

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