GB2139131A - A method of welding a metallic filtering material - Google Patents

Abstract

Two portions of metallic filtering material (1) comprising filaments or fibres are joined together by resistance welding in a region (8) by being passed between roller electrodes (9) acting indirectly or, as shown, directly on the portions. The portions (1) are passed continuously through the electrodes (9) and subjected to compression, but welding takes place intermittently so as to provide an elongate joint with welded regions separated by unwelded, but compressed, regions. A strip of metallic material may be placed between or on one or both sides of the portions. <IMAGE>

Description

SPECIFICATION A method of welding a metallic filtering material This invention relates to the welding of a metallic filtering material, and in particular to the electrical resistance welding of such material. The welding may be to join the material to itself or to like filtering material, e.g. for forming a filtration bag.

The metallic filtering material with which the invention is concerned comprises metallic fibres or filaments and is flexible, compressible and capable of being rolled up like conventional textile materials.

In preferred embodiments, the material is inhomogeneous. By this is meant that the structure varies on a macroscopic scale through the material, whether in a random or a regular manner. Such variations are not concerned with the properties of the fibres themselves or their general orientation.

Thus the material could comprise non-woven fibres and a supporting fabric, or could be in the form of a woven fabric. In either case there will be macroscopicvariations, e.g. from an area where a warp passes over a weft to an area where it passes under a web.

On the other hand, a non-woven random web of fibres would not exhibit macroscopic variations in structure and for the purposes of this specification would be homogeneous.

In general, the filtering material may consist of metallic fibres or continuous filaments with a diameter lower than 50 microns, preferably lower than 30 microns. Such material is designed to permit the flow of air or other gas therethrough, while retaining any liquid or solid particles entrained therewith. The filtration characteristics of this material by means of these very thin, metallic fibres or filaments can be obtained in several ways.

The filtering material should have sufficient strength to withstand the rather severe working conditions of a filtration unit, e.g. to be able to withstand pulses for cleaning purposes, and it is for this reason that filtering materials may need to incorporate a strong basic or supporting structure to take up the necessary forces.

A first example of such a filtering material is a needle punched felt obtained by applying a web of metallic fibres along one or both sides of a fabric of continous metallic filament yarns and by needle punching the web or webs to the fabric or supporting structure. This first example is a combination of staple fibres (web) and fibres in the form of continuous filaments (yarns of the fabric). This special needle punched felt is sold by N.V. Bekaert S.A. of Zwevegem, Belgium, under the Trade Mark BE KIPOR -NP.

A second example of such a filtering material is a fabric of continuous metallic filament yarns, woven close enough to serve as a filter. This second example consists substantially only of fibres in the form of continuous filaments and is sold by N.V.

Bekaert S.A. under the Trade Mark BEKIPOR -FA.

These filtering materials can be composed of appropriate metals for high temperature filtration applications. Appropriate metals for use in such applications include stainless steel alloys, nickel and cobalt high-temperature alloys (e.g. Inconel, Hastelloy (Trade Marks) and so on ...), as well as other high-temperature resistant alloys. Filtration assemblies, such as filter bags of these materials are used for filtration of gases at high temperatures, e.g. up to 5500C or even more and offer the possibility of eliminating the cooling of the hot gases and recovering in this way energy with a high efficiency and in some cases recovery of some process products.

The construction of filter or filtration assemblies, e.g. bags or tubes, containing a layer of such metallic filtration material presents difficult problems when attempting to join such a layer to another layer of the same or similar material.

A number of techniques have been proposed and are used to join filtering materials composed of textile materials, glass fibres, and so on. However, these filtering materials are used at much lower temperatures, e.g. up to a maximum of 1 50 C to 2000C and therefore the problems are totally different for making joints for high temperature filtration assemblies, e.g. up to 550 C or more.

Afirsttechnique,which has been proposed to connect a layer of metallic filtering material to another layer of the same or similar material is to use special adhesives. However, this techinique of bonding is very time-consuming and gives brittle and stiff ridges at the joints. The joints often open under the severe working and cleaning conditions, e.g. pulses to clean the filtration housing during the use of the filtration assemblies. Furthermore the joined filtering material cannot easily be rolled up because of these stiff brittle joints. This problem does not arise with e.g. assemblies made from conventional textile materials in which the layers are stitched to each other with textile yarns to form the joints.Another technique, which has been proposed to connect a layer of the metallic filtration material to another layer of the same or similar material to form joints, is by sewing or stitching. It has therefore been necessary to develop a special, sewing yarn of a high temperature resistant metal or alloy. However, the problems for stitching or sewing with a metallic yarn through two layers of the material are very great or at present unsolvable.

It has therefore been proposed by the Applicant to connect a layer of such metallic filtration material to another layer of the same or similar material by welding techniques, particularly by means of resistance welding techniques. However, here again, unexpected problems have arisen, particularly when resistance welding is used for connecting such a layer to another layer to form elongate welded joints, e.g. longitudinal joints in filtration assemblies of cylindrical shape such as filtration bags. Such joints can be made using roller electrodes.

One particular problem in the case of inhomogeneous material is that it is very difficult to find the exact welding conditions, e.g. welding pressure, welding current, welding time, and so on between the two welding electrodes to obtaine a sufficiently strong, well sealed joint without the formation of openings or leaks by overheating or burning through due to overheating at some spots.

On the other hand, the welding conditions, i.e.

temperature and pressure, should be sufficiently high to obtain a strong joint but on the other hand as soon as these conditions are too high, the filtration material is burned through and can no longer be used for filtration purposes. It has been found that it is very difficult or impossible to weld continuously acceptable lengths, e.g. joints of 1 metre or more, without the formation of openings or leaks by burning through when welding such a metallic filtration material layer. The reason for this problem is thought to be due inter alia to the inhomogeneous structure of the metallic filtration material layer.

It has been thus been proposed by the Applicant that in the case of non-homogeneous filtering material a homogeneous metallic element should be placed between the filtering material and each welding electrode. In the embodiments tested continuous welded joints are formed using roller electrodes. A problem with this method, however, is that the formed longitudinal joint can be rather stiff and brittle with the joint being in the form of a ridge or rib. A first disadvantage is, that it is not possible to roll up the formed filtration assembly, e.g. the formed filtration bag, due to the presence of such stiff and brittle joints. This leads to additional costs for packaging and transporting the so formed filtration assemblies. Another disadvantage is that with certain arrangements, cracks or ruptures may be likely to form in the joints.The formation of such cracks, e.g. during packaging and transporting, when placing the filtration assembly, such as a filtration bag, around a supporting frame in a filtration house, or during the cleaning operation by means of applying pulses to the filtration bags, may lead to openings or leaks, which are not acceptable for an efficient filtering operation.

One object of this invention is therfore to provide a welding technique which whilst giving a strong and well sealed joint will still retain flexibility. Thus, preferably the welding process will not form a stiff ridge, rib or ripple, so that it is still possible to roll up e.g. a formed filtration bag as with known filtration bags made from conventional textile materials.

Viewed from one aspect the invention provides a method of electrical resistance welding together two portions offlexible metallic filtering material each comprising metallic fibres orfilaments wherein the portions are passed through roller electrodes and subject to continuous compression therebetween, whereas welding is effected intermittently.

The joint between the two portions will thus consist of a series of welded regions spaced apart by unwelded regions which are preferably relatively short and where, nevertheless, the two portions of material are compressed together. The unwelded regions retain full flexibility and act as "hinges" permitting the jointed portions to be rolled up. The unwelded regions do not however create leaks, since the portions are compressed together and held in this condition by the welded regions either side. It has also been found that, in contrast to completely continuous welding of non-homogeneous filtering material, the intermittent process permits extended joint length without the risk of openings or leaks caused by the welding.

The portions being welded together could be parts of the same piece of filtering material, or of two different pieces of filtering material. In the latter case, the material of each portion could be different.

Preferably, the welded portions are in the range of 3 to 7mm long, and the unwelded regions in the range 0.5 to 3mm long. The width of the welded regions could be between 2 and 6mm in preferred embodiments.

It is also possible to use a portion of e.g.

homogeneous metallic material in conjunction with inhomogeneous material in a technique in accordance with the invention. Thus a strip of a homogeneous material may be placed between the two portions of inhomogeneous material being welded together. The strip itself should be flexible so that the joint will not become too stiff and brittle.

Such a strip can therefore improve the effectiveness of welding and sealing in the welded regions without seriously detracting from the flexibility of the joint.

It will be noted that an importantfeature of the method according to the invention is that the welding electrodes can form a direct contact with inhomogenous metallic filtering material and do not have to be separated therefrom by other material, although in some embodiments it may be desirable to do so.

The invention also extends to a filtering assembly including a metallic filtering material welded in accordance with the invention.

Some embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a metallic filtering material for welding in accordance with the invention; Figure 2 is a cross-section through a metallic filament yarn used in the material; Figure 3 is a perspectice view of a filtration assembly, namely a filtration bag, including a layer of metallic filtering material of the kind shown in Figure 1.

Figure 4a is a cross-section through two roller electrodes, as well as through two superposed layers of metallic filtering material layers being welded in accordance with the invention; Figure 4b is a cross-section similar to Figure 4a, but showing a modified embodiment of the formed welded joint; and Figure 5 is a perspective view of a part of a longitudinal joint formed by the seam-welding method according to the invention.

Figure 1 shows a perspective view of a flexible and compressible metallicfiltering material 1. This material 1 comprises a basic or supporting structure 2 in the form of a fabric composed of continuous metallic filament yarns 3, woven close enough for the fabric to act as a filtering medium in its own right. The warp of this fabric 2 consists of 13 yarns per cm and the weft of this fabric consists of 10 yarns per cm, each yarn 3 comprising two bundles of 275 substantially continuous filaments with a diameter of 12 microns (12 Fm) and the step or pitch of the 2 bundles in the yarn 3 being, e.g. 5mm. The weight of this fabric 2 is substantially 1100 grams per square m. An important feature of this fabric 2 is that this fabric 2 can be rolled up as known textile fabrics.

This metallic filtering fabric 2 can e.g. be rolled up around a core or tube with a diameter of 5 cm or even less. This fabric is sold by N.V. Behaert S.A.

under the Trade Mark BEKIPOR-FA material. Whilst this fabric 2 could be used by itself, in the shown embodiment of Figure 1, there is provided a web 4 of metallic fibres, e.g. fibres with a diameter of 12 microns ( > m) and a web weight of 200 grams/m2 or more, along one or both sides of the supporting fabric structure 2. The web 4 can be obtained in many known ways, e.g. by means of known carding machines using short broken, e.g. 50 to 60 mm, metallic fibres. Each web 4 is anchored to the supporting fabric 2 by means of known needle punching machines and this known filtration material is sold by N.V. Behaert S.A. under the Trade Mark BEKIPOR-NP material. Here again, an important feature of this known filtration material 1, i.e.

BEKIPOR-NP material, is that it can be rolled up as known textile filtration fabrics, e.g. can be rolled up around a core or tube with a diameter of 5 cm or even less.

Figure 2 shows a cross-section through a metallic yarn 3 of two bundles of substantially 275 continuous filaments with a diameter of 12 microns, as used for weaving the supporting fabric 2 in the first described embodiment of the filtering material 1.

Figure 3 shows a perspective view of a filtration assembly 5 in the form of a filtration bag. This filtration bag 5 comprises a jacket 6 of filtering material, e.g. BEKIPOR-NP material and a metallic ring 7. This jacket 6 is closed at the bottom and the longitudinal side by welded joints 8 and the ring 7 is enclosed in the jacket 6 at the upper end by folding the upper edge of the jacket 6 around the metallic ring 7 and fixing the folded upper edge of the jacket to the jacket by a circular or circumferential welded joint 8. As can be seen all these welded joints 8 are made by welding a layer of filtration material 1 to another layer of filtration material 1.

The method of forming a welded joint 8 in accordance with the invention will be clearly explained with reference to Figures 4a, 4b and 5. In Figure 4a, two superimposed, edge regions of layers 1 for forming the welded joint 8 are guided between two roller electrodes 9 of an electrical resistance welding machine. In Figure 4b, the layers completely overlie each other. The basic feature of the method is that the two layers 1 are welded to each other by intermittently welding by means of these roller electrodes 9. These roller electrodes 9 continuously press the two layers 1 against each other, but the welding operation itself is interrupted at regular intervals so that the formed joint 8 is provided with interruptions 10 and fixed or welded parts 11 (see particularly Figure 5).The length of these interruptions 10 can very between 0.5 mm and 3mm and the length of the welded parts 11 between 3 and 7 mm.

The width of the joint 8 or the welded parts can vary between 2 and 6mm. However, this data is given for illustrative purposes and is not limiting.

An advantage of this welding method is that the welded joint 8 does not form a stiff and brittle ridge or rib. The formed joint 8 acts as a chain, which can be folded up, whereby the interruptions 10 act as hinges. These interruptions 10 do not cause leaks in the filtration assembly because these interruptions are completely filled with compressed filtering material of the two superposed layers 1. A great advantage is that it is now possible to roll up the formed joint 8 without forming cracks in the welded joint and in the filtering layers 1.

Figure 5 shows in more detail a portion of a welded joint 8 of a preferred embodiment. In this preferred embodiment a metallic strip 12 is interposed or situated between the two layers 1 of the filtering material. This strip 12 must be made of a type of flexible material, which can be rolled up and which does not lead to the formation of a stiff ridge when the welded joint 8 is formed. A preferred material for this strip 12 comprises a web of metallic fibres sintered to each other, e.g. stainless steel fibres with a diameter ranging between 4 and 35, preferably between 4 and 22 microns. The weight of this strip can range between 200 and 1200 grams per square meter, preferably between 300 and 500 gr/m2. The strip should be porous, having a density which is lower than 50% and preferably lower than 30%.Such strips of low density, e.g. ranging between 20% and 50% have the great advantage of being very trans formable or deformable under pressure. The width of the strip 12 is preferably somewhat greater than the width of the welded joint 8, e.g. a strip with a width ranging between 5 and 10mm. The thickness of the strip depends on its weight (gr/m2) and porosity, and preferably ranges between 0,1 mm and 0,3 mm. A suitable material composed of sintered metallic fibers is sold by N.V. Behaert S.A. under the Trade Mark BEKIPOR-ST (Al and B1-series) and is used inter-alia for fluid filtration. This sintered material should not be reinforced by a supporting structure, such as a welded mesh or the like, because this will lead to increased stiffness or to the formation of a stiff ridge at the formed joint 8.

It is clear, that this invention is not limited to the described embodiments and that many variants can be made. For example, instead of providing a strip 12 between the two layers 1, it is also possible to place a strip of such transformable and deformable material at both sides of the two superimposed layers 1, so that four layers are welded to each other.

It is also possible to place such a strip 12 at only one side.

Claims (7)

1. A method of electrical resistance welding together two portions of flexible and compressible metallic filtering material, each comprising metallic fibres or filaments wherein the portions are passed through roller electrodes and subject to continuous compression therebetween, whereas welding is effected intermittently.

2. A method as claimed in claim 1 wherein at least one of the portions is of an inhomogeneous material.

3. A method as claimed in claim 2 wherein a portion of flexible homogeneous metallic material is interposed between the two portions.

4. A method as claimed in claim 1, 2 or 3 wherein the welded joint consists of welded regions of a length in the range of 3 to 7mm separated by unwelded regions of a length in the range of 0.5 to 3mm.

5. A method of electrical resistance welding together two portions of a metallic material substantially as hereinbefore described with reference to the accompanying drawings.

6. A filtering assembly including two portions of metallic filtering material welded together by a method as claimed in any preceding claim.

7. Afiltering assembly as claimed in claim 6, substantially as hereinbefore described with reference to the accompanying drawings.