Description

Technical Field

[0001 ] The present invention relates to a layered filtering structure which is adapted for micro-filtration purposes. The term "micro-filtration" refers to filtering structures, which are able to retain particles with a size in the range of a few micrometers.

Background Art

[0002] Presently available filter material for applications such as micro-filtration and for in situ cleanable filtration media conveniently comprise ceramic membrane layers fixed to the surface of porous sintered metal powder or metal fiber substrates. The high pressure drops across these filter laminates, however, is a considerable drawback since the filtering process requires additional energy due to the high pressure and robust mechanical supports for the filter layers. In addition, repeated backflushing is difficult and, after all, the ceramic layers are quite brittle, which adversely affects durability.

[0003] An improvement is to use wire mesh layer instead of ceramic membrane layers as support to the filter layers. As disclosed by US patent no. 6889852, a layered filtering structure comprises two sintered filter layers which has two different porosities. One wire net is fixed as support to each side of the filter layer. The layered filter structure combines the advantage of a small filter rating with a low pressure drop. However, repeated backflushing may adversely cause fatigue of the fiber layers and lead to a delamination of layered filter structure.

Disclosure of Invention

[0004] It is an object of the present invention to avoid the drawbacks of the prior art. [0005] It is also an object of the present invention to provide a filtering structure adapted for microfiltration without causing high pressure drops across the structure.

[0006] It is still another object of the present invention to provide a filtering structure which allows for repeated backflushing without delamination of the layered structure.

[0007] According to a first aspect of the present invention, there is provided a filtering structure having a filter inlet side and a filter outlet side, said layered filtering structure comprising at least

-a support metallic mesh, used to provide strength for said filtering structure,

-a filter metallic mesh made from metallic wires, used to determine filtering rate of said filtering structure,

-a metallic fiber layer, positioned in-between said support metallic mesh and said filter metallic mesh, wherein the average pore size of said metallic fiber layer is bigger than the width of apertures of said filter metallic mesh, and wherein said support metallic mesh, said metallic fiber layer and said filter metallic mesh are sintered together.

[0008] As an example, the filter metallic mesh is at the upstream side, so most close to the filter inlet side, is a fine mesh such that determine the filtering rate, i.e. , the size of the particles the majority of which still pass the filter. The support metallic wire mesh, at the downstream side, so closer to the filter outlet side, is a robust wire mesh such that support the filtering structure.

[0009] According to the present invention, the fine metallic filter wire mesh layer is fixed to the metallic support wire mesh via a metallic fiber layer positioned in-between said support metallic mesh and said filter metallic mesh. Herein, the wires of support wire mesh have bigger diameter compared with the wire of fine wire mesh. Also, the apertures of the support wire mesh is much bigger than the apertures of the fine filter wire mesh. In-between these two wire mesh, there is a metallic fiber layer. It cannot completely exclude that this metallic fiber layer can have certain filtering effect, however it does not determine the filtering rate of the filtering structure since its pore size is wider than the apertures of the filter metallic wire mesh. The main function of the metallic fiber layer is used as an adhesion layer to bond the fine wire mesh and support wire mesh together. This overcomes the delamination problem of the prior art filter where a fine filtration mesh is directly sintered to a coarse support mesh to connect them together. In the prior art, there are already spots that are delaminated after sintering fine filtration mesh directly on coarse support mesh. The delamination problem is tackled in the prior art by stacking several meshes to create gradient meshes from fine to coarse. According to the present invention, the metallic fiber layer is applied as a bonding layer between fine filtration mesh and coarse support mesh. The present filtering structure in a simple three-layer construction is absent of delamination after sintering. The fiber layer can be a nonwoven metal fiber media. In addition, since the fine wire mesh is sintered together with the fiber layer and the coarse support wire mesh, the mesh structure of the fine wire mesh is well maintained. This is beneficial to have a consistent filtering rate and keep low pressure drop. The sintered structure between the fine wire mesh and fiber layer is robust and not easy to laminate. The filtering structure of the invention has a significantly improved lifetime.

[0010] Generally, the pressure drop over a filter is about proportional to its thickness. In this case, the limited thickness of the filter wire mesh limits the resultant pressure drop to an acceptable degree. The degree of pressure drop over the fiber layer is considerably lower than the pressure drop over the conventional filter due to the thin thickness and the greater porosity of the invention fiber layer. So, the total pressure drop over the whole filtering structure is about equal to the limited pressure drop over the filter metallic layer. An incoming fluid is immediately able to expand in the fiber layer once it has passed the filter metallic wire mesh layer.

[0011 ] According to the present invention, the metallic wire of the filter metallic mesh can have a diameter in a range of 10 to 100 pm and preferably in a range of 20 to 50 pm and have apertures with a width in a range of 10 to 50 pm. The filter metallic mesh can have openings in a range of 300 to 1000 meshes per linear inch. Herein, the size of apertures of the filter metallic mesh determines the filtering rate of the layered filtering structure.

[0012] The metallic fiber in the fiber layer can have a diameter in the range of 8 to 60 pm. The metallic fiber media can be sintered together with the fine metallic filter wire mesh and the metallic support wire mesh. The sintered metallic fiber layer acts as bonding layer for the two wire mesh nets, so that its thickness can be limited to a range of between 0.05 mm to 0.15 mm.

[0013] The support metallic mesh has openings in a range of 30 to 100 meshes per linear inch. The support metallic mesh can have a weight ranging between 300 to 2000 g/m ²

[0014] The filtering structure may further comprise an inferior support metallic wire mesh in contact with the filter metallic mesh. The inferior support wire mesh is between the fine filter metallic mesh and a drum of filter candle. Next to the function of support, the inferior wire net still has another function and advantage. This inferior wire net generates some turbulence in the incoming flow which improves the anti-fouling behaviour.

[0015] The support metallic mesh, filter metallic mesh and metallic fiber layer can be made from austenitic & martensitic stainless steel, nickel chromium- based alloys, Inconel® or Hastalloy®. Preferably, the metallic support mesh, metallic filter mesh and metallic fiber layer are made from the same material, although they can be made by different material.

[0016] The filtering structure is at least 90 percent efficient at removing particles having diameter of 10 pm and larger, and at least 98 percent efficient at removing particles having diameter of 50 pm. In preferred example, the filtering structure is at least 98 percent efficient at removing particles having diameter of 10 pm and larger.

[0017] According to a second aspect of the present invention, it is provided a method of manufacturing a filtering structure, said method comprising:

(a) providing a support metallic mesh having 30 to 100 meshes per linear inch;

(b) bring a web of metal fibers having a diameter in the range of 10 to 50 pm, to form a metallic fiber layer in contact with said support metallic layer; (c) bring a filter metallic mesh made from metallic wires, in contact with said metallic fiber layer, to form a layered sandwich assembly, and

(d) sintering said layered sandwich assembly to form a coherent structure. [0018] Preferably, the web of metal fibers is a non-woven and non-sintered web.

The metallic wires have a diameter in a range of 10 to 100 pm and preferably in a range of 20 to 50 pm.

Brief Description of Figures in the Drawings

[0019] The invention will now be described into more detail with reference to the accompanying drawing, wherein

[0020] FIGURE 1 shows an illustrated cross-section view of a layered filtering structure of prior art.

[0021 ] FIGURE 2 shows an illustrated cross-section view of a layered filtering structure according to the invention.

[0022] FIGURE 3 (a)-(c) shows respectively filter efficiency of three types of invention layered filtering structures.

Mode(s) for Carrying Out the Invention

[0023] A layered filtering structure 10 in prior art is illustrated in Fig. 1 . The layered filtering structure 10 is sandwiched in-between an outer cage 11 and a drum 12. The layered filtering structure comprises two or three non-woven metallic filter layers 14,15,16, and two support wire meshes 17,18 respectively at each side of the metallic filter layers 14,16. The filtering rate are determined by the two or three non-woven metallic filter layers 14,15,16.

[0024] As shown by an illustrated cross-section view in Fig. 2, the layered filtering structure 20 is sandwiched in-between an outer cage 21 and a drum 22. A layered filtering structure 20 according to the invention comprises a support metallic mesh 27, used to provide strength for said filtering structure, a filter metallic mesh 26 made from metallic wires, used to determine filtering rate of said filtering structure, a metallic fiber layer 25, positioned in-between said support metallic mesh 27 and said filter metallic mesh 26, and wherein said support metallic mesh 27, said metallic fiber layer 25 and said filter metallic mesh 26 are sintered together.

[0025] According the first example of the invention, the support metallic mesh 27 can be a commercially available so-called K, J, S type of mesh. The characteristics of these standard mesh are illustrated in Table 1 below.

[0026] Table 1 Standard K, J, S type of mesh

[0027] As an example, type K is taken as a support mesh. The metallic fiber layer has a weight of 300 g/m2 and is made by fibers having an average equivalent diameter of 12 pm. For comparison, three type I, II, III of filter structure are made, wherein different fine meshes are applied as filter metallic mesh. These types of filter mesh are made by different diameter wires and have different apertures or different mesh count. The specification and characteristics of these three types of layered filter structure are shown and compared in table 2 below.

[0028] Referring to table 2 type I, a layered filtering structure according to the invention comprises support metallic mesh, a nonwoven fiber layer and a fine filter mesh. The support metallic mesh is a standard type of K mesh. The fibers in the nonwoven layer are stainless steel 316L and have a diameter of 12 pm. The weight of the fiber nonwoven layer is 300 g/m ² The thickness is about 0.062 mm. The fine filter mesh is made from stainless steel wires having a diameter of 39 pm. The mesh has 400 meshes per inch and the width of apertures is 25 pm. The overall thickness of this filter structure is 0.64 mm. Its weight is about 600 g/m ² .

[0029] Table 2 The specification and characteristics of three types of filter structure of the invention

[0030] As another example, another type of fiber medium is applied as bonding layer in-between filtration mesh and support mesh. The fibers in the nonwoven medium are stainless steel 316L and have a diameter of 22 pm. The weight of the fiber nonwoven layer is 300 g/m ² . The thickness is about 0.162 mm. The support metallic mesh is a standard type of K mesh. The fine filter mesh is made from stainless steel wires having a diameter of 39 pm. The filter mesh has 400 meshes per inch and the width of apertures is 25 pm. The overall thickness of this filter structure is about 0.74 mm. By using bigger diameter fiber medium as bonding layer, it improves air permeability of the filtering structure to about 1397 l/dm ² /min. In addition, the mean flow pore size of the filtering structure, which is about 46 pm, is much bigger compared to the aperture of the filter mesh. [0031 ] A filtering structure as shown in Fig. 2 can be made in the following way. Stainless steel fibers with a diameter of 12 pm are obtained by means of the technique of bundled drawing, such as described e.g., in US-A-3,379,000. A non-woven fiber web is then produced by means of an air-lay web former apparatus which is disclosed e.g., in GB 1 190 844. The non-woven fiber web is then put on a support type K wire mesh. A fine wire mesh as filter mesh is then put on the non-woven web. Herein, the support wire mesh, the fine wire mesh and the non-woven fiber web can be pre-rolled respectively. The thus obtained layered assembly is sintered together under a light pressure to obtain the layered filtering structure 20. An inferior support metallic mesh 28 can be set in contact with the filter metallic mesh 26.

[0032] As shown above in table 2, the three type of filter structure of the invention respectively has a filter metallic mesh with an aperture size of 25, 26, 30 pm, which are small than the pore size of the metallic non-woven fiber layer. Hence, the filter rate of the layered filtering structure is determined by the fine filter mesh.

[0033] Two types of conventional filter A & B and their corresponding characteristics are also listed in table 2 for comparison. The overall weight of type I, II, III filtering structure of the invention has thinner thickness and lower weight compared with conventional filter structures. This results from the thickness of non-woven fiber layer of the invention is much smaller than the conventional non-woven fiber filter layer. The bubble point pressure of the invention types is comparable with conventional type B and higher than that of conventional type A. The filter rate of the three types of invention filtering structures is all around 20 pm and comparable with conventional type B filter. Thanks to the thin thickness of the filter mesh layer, the air permeability of the invention filtering structure is much higher than that of the conventional filters.

[0034] Moreover, the three types of the invention filtering structure are subjected to a conventional filter efficiency evaluation. The results are presented in Fig. 3. As can be seen in Fig. 3, all the invention types of filtering structures have a filter efficiency rate of more than 98% for particles more than 30 pm. This is acceptable for most filtration applications.

[0035] The material used for the filtering structure according to the invention may be conventional compositions such as stainless steel 316®, Hastelloy®, Inconel® or Nichrome®. The latter composition can be applied for gas filtration at a high temperature.