Synthetic monofilament open mesh filter fabric with asymmetric structure for liquid / solid filtration with low pressure drop and easy regeneration

Abstract

The present invention provides an open mesh filter fabric made of synthetic monofilament with an asymmetric structure, in which the linear density / cm of the warp threads is different from the linear density / cm of the weft threads, and the diameter of the warp threads is different from the diameter of the weft threads. The open mesh filter fabric made of synthetic monofilament of the present invention has an improved ability to block solid particles when an equal amount of liquid passes through. Alternatively, compared to the prior art, it can achieve a higher flow rate (i.e., lower pressure loss) when the particle capture capacity is equivalent.

Description

[Technical Field]

[0001] The present invention relates to an open mesh filter fabric with an asymmetric structure that can be used to manufacture filters for removing various pollutant particles from liquids, especially water, for technical or industrial applications. [Background technology]

[0002] Water is the most abundant chemical compound on Earth and a major component of the human body. It has countless uses, including in the domestic environment, in industry, in agriculture, and more generally in the primary industries (agriculture, fishing, livestock, forestry, and mining).

[0003] In the complex water cycle, water is subjected to multiple changes of state and important fluctuations that can cause contamination (contact with other fluids, inorganic or organic solid particles, minerals and microorganisms), which in most cases makes it unsuitable for certain applications in the above mentioned sectors.

[0004] Therefore, several applications require different levels of water purification: drinking water, demineralized water, water used in cooling circuits or circulating water used in closed circuits (e.g. livestock farms, swimming pools, etc.). Therefore, when the water purity required for a particular application and the initial contamination level of the water (wastewater, sewage, greywater, whitewater, stormwater) are known, appropriate and complex treatment systems must be provided.

[0005] Filtration is the most common technique for removing suspended solids from water. In its most general sense, filtration refers to passing a contaminated liquid or gas through a purifying medium that adequately removes the contaminants. There are several water filtration systems, often used in combination to achieve the desired quality.

[0006] For example, the following methods can be mentioned. - Gravity separation for coarse removal of suspended solids by decantation. - Filtration with granular media (e.g. sand filters) which can be supplemented with the addition of chemical products with flocculating and flocculating properties. -Physical filtration through a chemical-free barrier. -Membrane separation process.

[0007] Mechanical filtration can remove particulate matter larger than 1 μm, microfiltration in the 0.1-1 μm range, ultrafiltration in the 0.01-0.1 μm range, and reverse osmosis and nanofiltration in the 0.001-0.01 μm range.

[0008] In this invention, we focus on mechanical filtration (>1 μm). This filtration step is almost always essential and is present in almost all water purification processes, especially in the management of wastewater, drinking water, and recycled water used in livestock farms and aquaculture. With regard to mechanical filtration, the most frequently used processes are undoubtedly: -Drum filtration -Disc filter -Sand filtration -Bag filtration -Cartridge filtration

[0009] Depending on the required level of filtration, the flow rates involved, and the pressures involved, different processes are selected, which ensure high filtration efficiency and minimize clogging problems. To this end, the process is usually divided into at least two stages: separation of coarse particles by sedimentation, followed by fine filtration.

[0010] In drum filtration, the liquid to be filtered passes through a rotating drum. The drum rotates slowly (a few revolutions per minute) and the filtration media is held firmly in place by a metal or polymer frame, forming a drum filter. Depending on the type of frame, different processes are used to manufacture the filter. These can range from simple gluing to compression molding or even injection molding.

[0011] During the rotational movement, the filter removes trapped impurities from the water and directs the filtered water to the drain. The filter is appropriately sized for the application, capable of filtering 5 μm or more, and has a filter capacity of 10 m. 3 / h to typically approximately 4500m 3 / h. The filter is normally washed with backflow water during use and the head is automatically controlled to ensure stability during use.

[0012] In a disc filter, water is introduced into a drum, inside which a filter, usually trapezoidal in shape, is attached, forming a disk that rotates in the water. During this movement, the water passes through the filter, removing contaminant particles. Again, the filtration medium is held firmly in place by a metal or polymer frame, forming a drum filter.

[0013] When it comes to sand filters, there is a huge variety of filtration media, customized for different flow rates to ensure different filtration efficiencies. They are usually made up of woven or non-woven fabrics and filled with silica (particle size usually 0.5-1 mm), which is responsible for the actual filtration. In addition to the filtration layer, there is also a support layer made up of larger particles. Sand filters can be both vertical and horizontal. The size and design of a sand filter depend on many parameters, such as: - Filtration rate or associated water flow rate. For example, in aquaculture, it can be several tens of m 3 / m 2 A specific flow rate of the order of 1000 s is recommended. - the height of the filtration bed, essential to ensure the desired efficiency level, - The characteristic size of the filtration medium, which is equally important to ensure the desired efficiency level and at the same time does not create excessive pressure losses, - Placement of filtered water header, -Filter cleaning systems that can become clogged and cause excessive pressure loss.

[0014] Bag filters are particularly suited to applications with large amounts of contaminants, high flow rates, and relatively low pressure drop, and are ideal when an economical and easy-to-use filtration solution is required. In this case, the fluid is circulated through a filter sleeve attached to a frame. There are various types of filter sleeves and frames available depending on the application.

[0015] Cartridge filtration is a method of circulating a fluid through a vessel containing a filtration cartridge. This usually has a metal or plastic frame that provides mechanical strength to the filtration media, which may be made from woven, nonwoven, wound, polymeric, and / or metallic membranes, depending on the application. The fluid passes through the cartridge, which retains contaminants. Depending on the type of filtration media used, the cartridge may be removable and washable or disposable.

[0016] On a purely conceptual and theoretical level, all the aforementioned processes are linked by the same type of required performance: filtration efficiency on the one hand, and permeability to liquids on the other.

[0017] Filtration efficiency is defined as the proportion of particulate matter rejected by a filter relative to the total number of particles reaching the filtration medium. Once a specific particle size is defined, it is intuitively obvious that maximum filtration efficiency is typically sought. To maximize filtration efficiency, it is intuitively obvious that the typical characteristic size of the filtration medium (pore or filter mesh size) must be smaller than the particulate matter being rejected. Furthermore, the smaller the filter mesh, the higher the filtration efficiency.

[0018] It should be noted that the above considerations apply entirely only to filter fabrics made from monofilament yarns. In particular, the terms "monofilament" or "monofilament yarn" do not refer to an interwoven structure of smaller threads or strands, but rather to a monolithic element having a constant circular cross section, i.e., a monolithic yarn.

[0019] In this case, the geometry of the filter fabric is fully defined, and the distance between two adjacent threads in each mesh or pore can be uniquely measured, defining the mesh opening or pore size that determines the size of particulate matter that can be blocked by the filter mesh.

[0020] Conversely, the other performance requirement concerns the liquid flow the filtration media will guarantee, both initially and during use. Again, it is intuitive to understand that once a maximum operating pressure drop is defined, the highest possible flow rate is desired. Conversely, once an operating flow rate is defined, the media is sought that minimizes the pressure drop that results in energy consumption in the pump or electromechanical system responsible for moving the liquid through the cartridge.

[0021] Furthermore, the desire to maximize filter life before clogging creates a requirement: using filtration media that has a very low pressure drop (ΔP) when new allows for a greater headroom before the critical pressure for clogging is reached, thereby extending the life of the filter element before replacement or regeneration.

[0022] Additionally, for regeneration, the use of open-mesh monofilament fabrics facilitates backwashing, ensuring a high and free flow of wash water. Additionally, the use of monofilament yarns minimizes the risk of contaminating particles becoming permanently trapped in the yarn itself.

[0023] In summary, minimizing pressure drop and / or optimizing flow rate requires the use of particularly permeable filtration media.

[0024] It is therefore clear that when selecting a filtration medium consisting of a monofilament fabric, the latter must necessarily have an "open mesh" structure, i.e., threads or yarns that are optimally spaced, e.g., of a diameter such that in plan view there are multiple visible mesh openings on the surface of the fabric, and that the flow of fluid is permitted through the fabric in a direction perpendicular to the plane of the fabric.

[0025] Under these circumstances, the following values ​​are defined for the open mesh monofilament fabric of the present invention: - "Mesh opening" or "pore size" is measured as the distance between the threads in a plan view (see Figure 3). - "Open area %" is calculated as the ratio of the open mesh area measured in a plane (see Figure 4a) to the corresponding area of ​​a single mesh measured relative to the centre line of the yarn that defines it (see Figure 4).

[0026] It should be noted that the structure of an open-mesh monofilament fabric optimal for the purposes of this invention differs from that of a closed-mesh fabric. The latter has a linear density or mesh count (number of threads / cm) and diameter of the threads in one of the two directions, weft or warp, that reaches a so-called "saturation" state. For example, parallel threads touch each other and no longer form a visible mesh in plan view (Tressen, Reps, Dutch Weave, and Double Dutch Weave fabrics). This creates a very small cross-section of passages that allows flow only diagonally to the plane of the fabric. This method allows for the filtration of contaminant particles much smaller than the size of the threads, but generates a significantly higher pressure drop than with the open-mesh monofilament fabric of this invention.

[0027] In fact, well-known closed mesh fabrics are usually used in process filtration, where the priority is to separate large amounts of contaminants with a medium to fine level of coarseness (e.g. sludge) at high pressure, with the disadvantage that the pressure drop through the fabric is very high.

[0028] On the other hand, filters according to the present invention made from open mesh fabrics can be used in any solid-liquid / liquid filtration application where maximum flow rate and minimum pressure drop are required.

[0029] The goal of minimizing pressure drop and / or optimizing flow rate requires the use of a particularly permeable filtration medium, which can be achieved by using an open mesh monofilament fabric, which is ideally suited for the present invention. -By using media with particularly large pores / mesh openings, although filtration performance may be reduced. -Pore size is kept small to maximize filtration efficiency by maximizing the open area or volume fraction of the filtration medium (defined here as the ratio of open area to total area of ​​the filtration medium, or the ratio of open volume to total volume of the filtration medium).

[0030] From the above explanation it is clear that the two performance characteristics require materials with contrasting physical and geometrical properties, which is why a design and optimization step is necessary in every case. When it comes to reusable and therefore washable filters, in the range above 5 μm, especially in the case of precision filters, the ideal solution in terms of filtration media is constituted by precision fabrics made from synthetic and metallic monofilaments. The weight of the filtration media plays an important role in many applications (see drum filters).

[0031] Therefore, to save energy, synthetic monofilaments are preferred over metallic ones due to the reduced mass, while the use of multifilament yarns, which are made by twisting together multiple filaments, affects filtration efficiency, as it does not guarantee the achievement of precise mesh openings of the desired size.

[0032] All of the above require that the fabric have a high open area to minimize the material's resistance to the passage of liquid, while at the same time ensuring adequate filtration efficiency, which requires that the fabric have suitably narrow mesh openings.

[0033] Consequently, there is a need to reduce the mesh opening size while maintaining the void / fill ratio (open area of ​​the fabric). In prior art square mesh fabrics, these two conflicting requirements are only partially met, as will be discussed below.

[0034] There are two possible ways to reduce mesh size. The first offers the possibility of increasing the number of threads inserted into the fabric while keeping the diameter fixed. The second is to instead keep the number of threads the same and increase the thread diameter. In both cases, it is clear that the porosity / filling ratio decreases, resulting in a decrease in water permeability. Therefore, it is natural to assume that the only realistic way to reduce mesh size while keeping the porosity / filling ratio constant is to increase the number of threads and simultaneously decrease the thread diameter. This third approach, although seemingly ideal, has two limitations: -Technical limitations in the yarn process: monofilaments cannot be extruded or woven below a certain diameter. -Technical limitations in the weaving process. The linear density of threads per cm (mesh count / cm) cannot be increased above a certain threshold. For applications requiring a high level of protection, particularly where small mesh sizes are required, the use of thinner diameter threads can result in structures with an unattainable number of threads. This is particularly true for the warp threads, i.e., the threads that are laid out along the length of the roll of produced material, which poses significant problems. This is because all warp threads must pass through the openings in the reed of the loom, and the reed's thin blades impose limitations on the spacing of the threads. There are also mechanical limitations, as the setting of the warp threads places a high mechanical strain on the frame due to the stresses imposed on the frame.

[0035] It is therefore clear that the aforementioned conventional option, i.e., reducing the mesh size, comes with the inevitable drawback of also reducing the open area of ​​the mesh itself, as there is no prior art solution to achieve both benefits simultaneously.

[0036] From the above, it can be concluded that the selection of the best square mesh fabric for water filtration in the prior art always involves a compromise: very tightly woven fabrics provide good protection but perform poorly from an energy standpoint; opener fabrics provide acceptable transparency for the passage of fluid (minimal pressure loss), but the mesh openings are too large to effectively block all contaminant particles. Summary of the Invention [Problem to be solved by the invention]

[0037] The main object of the present invention is to provide an open mesh synthetic monofilament woven fabric for use in liquid / solid filtration, particularly water filtration, preferably for the manufacture of drum or cartridge filters, which, unlike prior art fabrics, has a better ability to capture solid particles while ensuring the same liquid flow rate, or exhibits a higher flow rate (i.e., lower pressure drop) while providing the same protection from particles compared to prior art fabrics. [Means for solving the problem]

[0038] These and other objects are achieved by the fabric of claim 1. Some preferred embodiments of the invention are derived from the remaining claims.

[0039] Compared to prior art fabrics with comparable breathability and water permeability properties, the fabrics of the present invention have the advantage of providing better protection from contaminant particles. Furthermore, compared to prior art and similar fabrics with respect to their protection from solid contaminants, the fabrics of the present invention have better water flow properties, which may improve the energy yield of the filtration system and may also extend the life of the filter elements before clogging and make regeneration easier if backwashing is provided.

[0040] Of course, for each liquid / solid filtration application, it is possible to select a new fabric structure that best suits the purpose and achieve partial improvements in both the first area (better protection) and the second area (better flow), and in either case the overall improvement will be greater than could be achieved with the prior art.

[0041] The fabric of the present invention must be produced by weaving synthetic monofilament yarns, which are more efficient than multifilament yarns in terms of capturing polluting particles. The starting monofilament may be made of synthetic technopolymers from the group consisting of polyester, polyamide, polyaryletherketone, polyparaphenylene sulfide, polypropylene, perfluorocarbon, polyurethane, or polyvinyl chloride. Alternatively, the monofilament from which the fabric of the present invention is made may be made of artificial polymers from the group consisting of cellulose or viscose.

[0042] The monofilaments used in the fabrics of the present invention may have diameters in both the warp and weft directions of from 10 μm to 90 μm. The fabrics of the present invention may be produced with fiber structures requiring thread counts of from 23 to 450 threads per cm.

[0043] Woven fabrics can be manufactured with different open-mesh fiber structures, but they share the common feature of being asymmetric in two directions, the warp and the weft, particularly in terms of linear density of threads per centimeter and diameter of the threads. Thus, the weft thread density differs from the warp thread density, and the weft thread differs from the warp thread in terms of diameter or yarn properties.

[0044] As a result, depending on the choice of structural parameters such as linear density, yarn diameter and mutual balance of the yarns in an asymmetric configuration, it is possible to produce the fabric of the invention with both a square mesh and a rectangular mesh.

[0045] The mesh openings of the fabric of the present invention are well defined in the plan view of the open mesh fabric and may have a value in the range of 5 to 150 μm.

[0046] Further examples shown in the following figures better illustrate the fiber structure according to the invention. [Brief explanation of the drawings]

[0047] These and other objects, advantages and properties will emerge from the following description of some preferred embodiments of the fabric according to the invention, given by way of non-limiting example in the figures of the accompanying drawings. In the diagram: [Figure 1] FIG. 1 shows an example of a filter cartridge in which the fabric of the present invention may be used. [Figure 2] FIG. 2 shows the cross section of a typical disc filter in which the filter fabric of the present invention can be used. [Figure 3-4a] Figures 3, 4 and 4a show a square mesh of prior art woven fabric. [Figure 4b] FIG. 4b shows the mesh of FIG. 3 when clogged with contaminant particles. [Figures 5a-6a] 5a and 6a represent a portion of a prior art open mesh monofilament filter fabric, which is used as a basis for comparison with corresponding embodiments of fabrics according to the invention. [Figures 5b-6b] Figures 5b and 6b compare two different embodiments of an exemplary open mesh monofilament woven fabric according to the invention compared to the prior art woven fabric of Figures 5a and 6a, respectively. [Figure 7] FIG. 7 shows a prior art "Dutch Weave" type closed mesh monofilament fabric. [Figure 8-9] 8 and 9 report comparative data measurements for pressure drop and filtration efficiency, respectively, during filtration laboratory tests of fabrics according to the present invention compared to prior art fabrics. DETAILED DESCRIPTION OF THE INVENTION

[0048] In the operation of the filter shown in Figure 1-2, a water flow (arrow F1) is discharged in the direction of the fabric 2, resulting in a downstream flow F2 from which contaminants 3 present in the upstream flow F1 have been filtered.

[0049] In the prior art shown in FIGS. 3 and 4, the mesh 4 of the fabric 2 is square and is made up of yarns 5 forming each side 6 of the square mesh 4.

[0050] The open area of ​​the prior art mesh 4 itself, shown in Figure 4, is calculated as the percentage ratio between the area of ​​the smaller square 7 (Figure 4a) enclosed by the outline or inner edge of the threads 5 forming the sides 6 of the mesh 4 and the area of ​​the larger square 8 (Figure 4) measured to the centerline of the thickness of the threads 5 themselves.

[0051] When the prior art mesh 4 receives from the water flow F1 to be filtered particles of solid contaminants 3 having a diameter comparable to the length of the side 6 of the square mesh itself, the openings of the latter become clogged and only a small portion 10 of the area of ​​the smaller square 7 of the mesh 4 allows the water flow to pass through (Figure 4b).

[0052] As a result of the above phenomenon, fluid pressure increases in the upstream part of the fabric 2 (arrow F1 in Figure 2) until it passes through the mesh 4, which may deform the contaminant particles 3. This reduces the filtration efficiency of the fabric below the filter design value.

[0053] Furthermore, clogging of the prior art square mesh 4 increases the pressure drop across the filter fabric, resulting in more energy being required to maintain the same velocity of the filtered water flow.

[0054] To overcome these drawbacks, the fabric of the present invention proposes a fiber structure characterized by a different number of threads per cm in the two directions, warp and weft, and by the weft having a different diameter and, in some cases, a different material from the warp.

[0055] In particular, it is conceivable that the ratio of warp to weft yarn densities is between 0.4:1 and 2.5:1, and that the ratio of warp diameter to weft diameter is between 0.5:1 and 2:1.

[0056] Figures 5b and 6b illustrate two possible different embodiments of the fabric of the present invention compared to corresponding fabrics of the prior art to demonstrate the actual performance advantages of the present invention itself.

[0057] In all examples reported here, the fabrics of the present invention are constructed from synthetic monofilaments, which provide optimal performance in terms of mesh opening precision and surface finish, and prevent trapping of contaminants in the yarn itself. The polymeric nature of the fabric also provides additional benefits in terms of lightness and environmental (recyclability) benefits.

[0058] Below we will explain in detail some embodiments of the solution according to the invention in comparison with equivalent fiber structures of the prior art. Figure 5a / 5b

[0059] Figure 5a shows a portion of a prior art woven fabric 4, characterized by the same thread density (N1) for both the warp (vertical threads in the figure) and weft (horizontal threads in the figure) per centimeter. The prior art woven fabric further has the same thread diameter (d1) for both the warp and weft. Thus, the open mesh 7 in Figure 5a is square, characterized by the fact that the size of the openings 6 in the mesh 7 is the same in both the weft and warp directions.

[0060] In Figure 5b a possible embodiment of a fabric 25 according to the invention is shown, in which the thread density per unit width in the two directions and the respective diameter of the threads differ from one another. In this particular case: - The number of warp threads per cm (N1, vertical threads in Figure 5b) is the same as in the prior art fabric of Figure 5a. The number of weft threads per cm (N2, horizontal threads in Figure 5b) is greater than the number of weft and warp threads, and therefore greater than the number of threads per cm of prior art fabrics (Figure 5a, N1 in both directions). While the density of the warp threads per cm is conditioned by the presence of the loom's reed, for example, the weft threads have no such constraints and can therefore be thicker. Note that this option is therefore generally possible in normal weaving processes. Asymmetric configurations are therefore possible and, as will be seen, advantageous. - The diameter of the warp threads (d1, vertical threads in Figure 5b) in the present invention is the same as that of the prior art fabric in Figure 5a. The diameter of the weft threads in the present invention (d2, horizontal threads in Figure 5b) is smaller than the diameters of the weft and warp threads, and therefore smaller than the diameter of the threads in prior art fabrics (d1 in Figure 5a, both directions). It should be noted that this choice is usually possible in the normal process of weaving, since the weft threads are subjected to less stress, and therefore smaller diameters can be used without compromising the quality of the fabric. Asymmetrical constructions are therefore possible and, as will be explained later, are advantageous.

[0061] Furthermore, in this embodiment, the number of weft threads per cm (N2), in combination with the value of the weft thread diameter d2, is selected to determine a mesh 7 with a perfectly square shape, with identical open meshes 6 in both directions, as in the prior art woven fabric 4 (FIG. 5a). Thus, each mesh 7 blocking contaminant particles is identical to that of the prior art, resulting in the same filtration efficiency for the two fabrics. At the same time, the inventive woven fabric 25 offers improvements in terms of reduced pressure loss during filtration, a longer service life before clogging, and improved regeneration possibilities through countercurrent washing. All of this is achieved thanks to the increased open area of ​​the inventive woven fabric 25 (FIG. 5b) compared to the open area of ​​the prior art woven fabric 4 (FIG. 5a).

[0062] To prove this, we must consider the total open area of ​​the mesh 7 in both cases compared to the relative reference area. The reference area in this case is the portion of the fabric shown in Figures 5a and 5b, respectively. The prior art fabric (Figure 5a) has 4 x 4 = 16 open square meshes in the area under consideration. The fabric of the present invention (Figure 5b) in the same reference area instead has 4 x 5 = 20 open square meshes. Since the mesh openings are the same in both cases, the open area of ​​the present invention turns out to be 5 / 4 of that of the prior art. This has the obvious advantage of reduced pressure loss, and there are no drawbacks in terms of filtration, since the mesh openings are the same.

[0063] Furthermore, given that the new fabric has a higher number of square meshes per unit area, it would take more contaminant particles to cover them than the prior art, thereby extending the filter's useful life before clogging and, in many cases, improving the possibility of backwashing for regeneration of the filter fabric.

[0064] In this first example, the present invention provides a significant improvement over the prior art, even though both fabrics have the same square mesh and the same openness. Figure 6a / 6b

[0065] Similar to Figure 5a, the portion of Figure 6a shows that the prior art fabric 4 has the same thread density per centimeter (N1) for both the warp (vertical in the figure) and weft (horizontal in the figure). The prior art fabric 4 has the same thread diameter (d1) for both the warp and weft. The mesh 7 has a square shape, and the size of the mesh openings 6 for the weft and warp are identical (Figure 6a).

[0066] As in the previous example, a second possible embodiment of a fabric 25 according to the invention is now proposed (Fig. 6b). In this case, both the density of threads per cm and the respective diameter of the threads are different in the two directions. As in the previous example, it was found that: - The number of warp threads per cm (N1, vertical threads in Figure 6b) is the same as that of the prior art in Figure 6a. - the number of weft threads per cm (N2, horizontal threads in Figure 6b) of the invention is greater than both the number of weft threads of the invention itself (N2, Figure 5b) and the number of threads per cm of the prior art (N1 in both directions, Figure 6a). As in the previous example, the compatibility of this choice with the weaving process can be confirmed by the fact that the density of weft threads per cm is not influenced by the presence of a weaving reed, which only binds the warp threads. Therefore, a larger weft thickness is both possible and advantageous. - The diameter of the warp threads (d1, vertical threads in Figure 6b) in the present invention is the same as that of the prior art in Figure 6a. - The diameter of the weft threads in the present invention (d2, horizontal threads in Figure 6b) is rather smaller than the diameter of the threads in prior art fabrics (d1 in both directions, Figure 5a). This choice is compatible with the normal weaving process, since there are fewer stresses in the weft threads than in the warp threads. It also allows for the use of smaller thread diameters in the weft threads. Asymmetric constructions are therefore possible.

[0067] In this particular example, the number of weft threads per cm (N2) is higher than in the previous case in Figure 5b. The current structure determines that the weft threads are thicker and the open mesh value is reduced (in this case, square, due to symmetry in two directions). The openings in the warp direction (vertical in Figure 6b) are smaller than those in the prior art (vertical in Figure 6a). This leads to improved filtration efficiency and, consequently, the filter's ability to capture smaller particles, especially when they have a pseudo-spherical shape.

[0068] Note, however, that this improvement is not achieved at the expense of ease of fluid passage (pressure drop, filter life, cleaning / regeneration ability) compared to the prior art. Indeed, considering that the pre-invention configuration of Figure 5a had an improved open area compared to the prior art, it is possible to forgo some of that improvement (but not below the open area value of the prior art) in exchange for improved filtration efficiency. Referring to Figure 6b, it can be seen that the mesh now displayed is 4 x 6 = 24, a significant increase in number, even though the size is slightly smaller compared to the prior art woven fabric. By selecting an appropriate density N2, this structural solution according to the present invention also offers advantages over the prior art, as does the open area.

[0069] In conclusion, the inventive embodiments described herein have been found to be a definite improvement over the prior art in terms of filtration efficiency, and may also maintain advantages in terms of open area and low pressure drop.

[0070] The synthetic monofilament woven fabric according to the present invention has an asymmetric structure with respect to the number of threads per cm and the diameter of the threads in the two directions, warp or weft.

[0071] The example in Figure 5b ensures a clear increase in the open area values ​​compared to prior art fabrics, which helps to improve the resistance to liquid flow and clogging, extend the service life of the filter and in some applications also increase the possibility of regeneration and backwash cleaning.

[0072] The example shown in Figure 6b rather demonstrates the ability of the present invention as an improvement over the prior art in terms of lower mesh openings to achieve better filtration efficiency compared to the prior art while maintaining comparable or even slightly higher open area values ​​compared to the prior art, thus without compromising fluid dynamics or resulting in higher pressure drops compared to the prior art (and in some cases even improving this feature).

[0073] The synthetic monofilament construction further minimizes weight and is ideal for preventing contamination particles from becoming trapped within the yarn, an important aspect in the case of multifilament yarns. Finally, the smooth surface of the monofilament minimizes the possibility of contamination particles becoming attached and allows them to slide off and be removed during backwashing for filter regeneration.

[0074] To maximize the above advantages, the asymmetric synthetic monofilament fabric of the present invention should have a yarn linear density ratio in the warp direction to the weft direction in the range of 0.4:1 to 2.5:1, and a warp to weft diameter ratio in the range of 0.5:1 to 2:1.

[0075] In the filter fabric according to the invention, yarns of different types or sizes may be provided combined either in the same direction or in two different directions, warp and weft.

[0076] As mentioned above, the object of the present invention is to create what can be classified as an "open mesh fabric" and therefore significantly differs from prior art closed mesh fabrics and asymmetric fiber structures, such as so-called "Tressen," "Reps," or "Dutch weave." In these fabrics, the ratio of linear thread density per centimeter in two directions is 4:1 or more, whereas in the present invention, it is at most 2.5:1. Indeed, the present invention aims to maximize the cross-sectional area of ​​the fluid flowing orthogonally through the filter, thereby minimizing pressure loss. In contrast, the asymmetric fabrics mentioned above only need to minimize the pore openings through which the fluid passes. This is particularly true for filtration applications where pressure loss through the filter is not an issue. Thus, in the asymmetrical fabrics of the prior art, defined as "Tressen", "Reps" or "Dutch Weave", the threads in one of the two directions become adjacent to each other, reaching a so-called "saturation", leaving only minimal openings for passage, optimal for ensuring a high degree of filtration, but resulting in significantly higher pressure losses than in the fabrics according to the invention.

[0077] As an example, Figure 7 shows a typical "Dutch weave" construction of the prior art, where the density of the weft yarns (N4) is at least four times the density of the warp yarns (N3), reaching saturation and the weft yarns touching each other.

[0078] In particular, the "saturation" of a fabric in the weft or warp direction is defined as the ratio of the product of the yarn linear density per cm and the yarn diameter in the corresponding weft or warp direction, divided by the reference length used to calculate the yarn linear density.

[0079] For example, if the density of yarns per cm is defined as N1 and the corresponding yarn diameter d1 in μm, we obtain: Saturation = N1×d1 / 10000 It is usually expressed as a percentage.

[0080] The open mesh filter fabric has a saturation level of no more than 70%, so adjacent threads never come into contact with each other, ensuring constant fluid passage perpendicular to the plane of the fabric.

[0081] On the other hand, closed mesh filter fabrics are manufactured with an intentionally high saturation level close to 100% so that adjacent threads are in contact with each other. In this way, the cross section of the liquid is reduced to a small cross section where the diagonal water flow intersects, such as small cross section 7 in Figure 7, and the open area cannot be defined.

[0082] It is worth noting that the area of ​​the open mesh is minimized, corresponding to the small area 7 identified in Figure 7, making it possible to stop small particles at the scale (3) shown here. Nevertheless, such a closed and differently-purposed fabric clearly results in a higher pressure drop than that obtained with the fabric according to the invention, and is therefore completely incomparable to the asymmetric configuration of the prior art reported here.

[0083] This can be seen from the description herein that by selecting an appropriately unbalanced and asymmetrical configuration of weft and warp yarns, both in terms of thread count per cm and yarn diameter characteristics, a larger open area than prior art fabrics can be obtained, and a smaller "effective" mesh opening (in the direction of smallest) can be obtained when compared to symmetrical fabrics, potentially resulting in even higher open areas.

[0084] As explained, filtration efficiency is inversely proportional to mesh size and directly proportional to open area. Therefore, an asymmetric woven fabric structure according to the present invention, which aims to maximize open area for an equal mesh size, can optimize the filtration efficiency of the filter.

[0085] The tests of filtration efficiency and flow rate vs. ΔP are carried out according to the standards ISO19438 / ISO16889 (filtration efficiency) and ISO4548-1:1997 (flow rate), respectively.

[0086] In filtration efficiency tests based on the ISO 19438 / ISO 16889 standards, filter fabrics are exposed to a controlled flow of contaminated liquid according to the type and concentration of specific contaminants defined in the standards. During the validation test, the filter clogging is gradually increased and monitored by obtaining values ​​of the differential pressure (pressure drop) across the filter fabric. Similarly, sensors in the test equipment obtain the size and number of particles upstream and downstream of the filter. From these values, it is possible to calculate the filtration efficiency value as a function of the contaminant particle size (graph in Figure 9).

[0087] In the liquid permeability hydrodynamic test according to the ISO 4548-1:1997 standard, an increasing flow of liquid is applied to the fabric sample. Depending on the applied flow value, a pressure sensor captures the pressure drop through the fabric, generating a graph of ΔP vs. specific flow rate, as shown in Figure 8.

[0088] The graphs shown in Figures 8 and 9 represent the results of tests carried out on a prior art fabric 4 having a symmetrical structure and a fabric 25 according to the invention having an asymmetrical structure. especially: - The prior art fabric 4 has a symmetrical structure in both directions (weft and warp) with a thread density N1 = 215 threads / cm and a thread diameter d1 = 31 μm, resulting in an open mesh 6 equivalent to 12 μm and an open area of ​​7% (Figure 6a). The fabric 25 according to the invention has an asymmetric structure with N1 = 200 threads / cm in the warp direction and N2 = 245 threads / cm in the weft direction, which results in an effective open mesh equal to 10 μm in the direction of minimum value and an open area equal to 8.5% (FIG. 6b).

[0089] As a result of the testing, the following conclusions are drawn: -When Arizona Test Dust A3 (medium) was used as the test pollutant dust, the filtration efficiency obtained from the two types of fabrics was comparable. If the same filtration efficiency is guaranteed and therefore the same particle removal is obtained in water filtration applications, the flow rate ensured by fabrics with an asymmetric structure is at least 20% higher, given the same pressure drop.

Claims

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

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

1.

3. 2. A filter fabric according to claim 1, characterized in that the 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. 10. A filter fabric according to one or more of the preceding claims, characterized in that the openings of each mesh range from 5 μm to 150 μm.

5. 2. The filtering fabric according to claim 1, characterized in that the synthetic monofilaments are monofilaments made of synthetic technopolymers belonging to the group consisting of polyester, polyamide, polyaryletherketone, polyparaphenylene sulfide, polypropylene, perfluorocarbon, polyurethane or polyvinyl chloride.

6. 2. A filter fabric according to claim 1, characterized in that the synthetic monofilaments are monofilaments made of artificial polymers belonging to the group consisting of cellulose or viscose.

7. 2. A filter fabric according to claim 1, characterized in that it is provided with a combination of yarns of different types or sizes, either in the same direction or in different weft and warp directions.

8. 10. A filter fabric according to one or more of the preceding claims for use in a cartridge, drum or disc filter for the filtration of water, It optimizes filtration efficiency and the removal rate of contaminant particles contained in the liquid while minimizing pressure loss and ensuring sufficient water flow. The filter fabric is an open mesh type of synthetic monofilament having an asymmetric structure, the linear density / cm of the warp threads being different from the linear density / cm of the weft threads, and the diameter of the warp threads being different from the diameter of the weft threads.

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