Prior Art Air cleaning filters of the type generally defined above are previously known from SU 1 125 05 and WO 95/00248. An important advantage of these known filters is that they have the ability of separating also microscopically small particles simultaneously as the particle catching, flexible elements are continuously kept electrostatically charged without the need of supplying external electric energy. A common drawback of both filter structures is, however, that the bunch or set of flexible elements sometimes fill up the air duct in an incom- plete way. The individual elements are kept together in a frame structure that in the first-mentioned case consists of a grating and in the last-mentioned case consists of a band arranged in loops, said band having a certain axial extension, said frame structure comprising pipe forming means having the task of distributing the air flow through the frame structure.

The portions of the flexible elements located downstream the frame structure are freely movable and in order to achieve an optimal filtering effect said elements should be distributed uniformly across the cross section of the duct. If portions of the space, housing the set of elements, are not filled up by the elements voids are created through which air can pass through without being in contact with and cleaned by said ele- ments. Since the flow resistance is minimal in such void por- tions of the duct area the air will of course find its way to and pass through such voids. In other words also relatively

small voids may bring about that a large portion of the air to be cleaned passes as an bypassing air flow or leakage flow that is subjected to a poor or non-existent filtration. Voids not filled up exist already in vertical or upright air ducts, but this phenomena will be especially articulated when the air duct is oriented horizontally. In the last-mentioned case the flexible elements are packed together by their own weight in direction towards the bottom area of the duct simultaneously as the upper portion of the duct will be totally void of such flexible elements, at least along a major portion of the length of the set of elements.

Objects and Features of the Invention The present invention aims at overcoming the problems mentioned above in connection with previously known filters and to create an improved filter. Thus, a primary object of the invention is to create a filter using flexible and elec- trostatically chargeable elements that offers a substantially uniform filtering effect in the entire duct area that is occu- pied by the set of elements. A further object is to create a filter that is structurally simple and thus inexpensive to manufacture.

The objects mentioned above are realized by the features that are defined in the characterizing portion of claim 1.

Brief Description of the Appended Drawings In the drawings: Figure 1 is a perspective exploded view illustrating the dif- ferent details included in the filter according to the invention, Figure 2 is a partial side view illustrating how a number of flexible, particle attracting elements are attached to a supporting disc, Figure 3 is an air cleaning assembly in a horizontal position including two filters according to the present inven- tion, and Figure 4-6 are end views showing air ducts having different cross sections.

Detailed Description of Preferred Embodiments of the Invention In the drawings reference numeral 1 generally designates a filter designed in accordance with the invention, said filter in the disclosed example being intended to be housed in a tube 2 that defines a duct 3, through which air may pass, more precisely in a direction from the top and downwards in the drawing. In the given example the tube 2 has a polygonal, and more precisely rectangular cross-sectional shape since it is defined by two relatively wide main walls 4, 4' and two nar- rower side walls 5, 5'.

The filter 1 includes a multitude of elongated, flexible or floppy elements 6 which at their ends are assembled or kept together in a common frame structure 7 of stiff nature that allows air to pass therethrough, said frame structure 7 extending transverse to the longitudinal axis of the tube 2 and intersecting or covering essentially the entire cross section of the tube 2. The elements 6 may be in the shape of threads, strips, fibres, fibre-like means or combinations thereof. At least certain of these elements, and preferably all of them, consist of a material that is capable of being charged electrostatically when the elements are put in motion relative to each other. The chargeable elements may be of the same material having only one polarity, but it is also possible to use two different types of material having different polarity, e.g. a first material having a strong positive charge and a second material having a strong negative charge. The first- mentioned material is for instance chosen from the group of wool, cotton or the like, while the second material is a plastic polymer, like polypropylene. It is also possible to use threads or elements of the electret fibre type and also metal fibres, e.g. copper fibres. It is characteristic for the active elements that they generate static electricity when they are put in motion, more precisely due to the fact that they on one hand are rubbed against each other and on the other hand that friction action is established between the elements and the passing air.

The assembling and supporting frame structure 7 may in practice be realized in several different ways. It is however essential that the frame structure, which totally intersects

the cross section of the tube, is able to distribute the incoming air uniformly in a plane transverse to the longitudi- nal extension of the duct. For this reason the frame structure should include a multitude of small, individual partial pas- sages through which the air may pass in smaller partial flows.

The frame structure and the partial passages should thus have at least a certain axial extension, e.g. within the range of 5- 50 mm, preferably 10-30 mm. In the given example the frame structure 7 is composed of a multitude of individual box units 8 including two longitudinal side walls 9 and several parti- tions 10 extending between said side walls 9, said partitions 10 defining between themselves pipe-like cells having a re- stricted cross section area, e.g. within the range of 1-16 mm2.

As far as the filter according to the present invention is described herein it is in all essentials previously known from SE 9302117-8.

New and characteristic for the filter according to the present invention is that it comprises several partitions or separating walls 11 that divide the set of flexible elements into several partial sets 6', 6", 6in ,etc. and thus the main duct 3 into several partial ducts 3', 3", 3in, etc (see also figure 3).

Reference is now made to figure 2 that illustrates how a number of flexible elements 6 are with their upper ends applied to opposite sides of a lower portion of a thin disc 12. In practice the elements may be connected with the disc by adhesive bonding. As is apparent from figure 2 they fall down in an abundantly flow of elements that initially has a width that is considerably larger than the thickness of the box units 8. Although the elements are shown as being attached to both sides of the disc 12 it is also possible to apply elements to one side thereof only. Each individual disc 12 is inserted between adjacent box units and connected with said units, preferably by adhesive bonding, the box units and the intermediate discs together forming the stiff frame structure 7. Since the discs 12 have a larger height than the box units 8 the discs will project downwards beyond the assembled frame structure 7. Since each air flow that passes through a box unit 8 by the partitions 10 is divided into utterly fine partial

flows, the air that passes the frame structure will be distributed essentially uniformly in the individual compart- ments 13 defined between adjacent discs 12.

The separating walls 11 are inserted at suitable rela- tive distances, e.g. five box units 8 between said walls. Also the separating walls 11 are preferably connected with adjacent box units by adhesive bonding. In practice the separating walls 11 should have such a length that their free ends are located in substantially the same cross section plane as the free ends of the elements 6. However, in practice there may exist a deviation of dimensions between said free ends. In practice it is preferred to manufacture the separating walls (likewise the tube 2) from a material being impervious to air, e.g. plastic.

It is, however, also conceivable to manufacture the separating walls pervious to air, e.g. in the shape of gratings, provided that they are given sufficient rigidity as to shape. Especially when the filters are adapted to be oriented horizontally in their active state there may be provided spacer elements, e.g. in the shape of a frame, between the free ends of the separating walls to guarantee parallelism between the separating walls even if the walls are somewhat flexible.

In figure 3 is shown how two filters 1 are mounted in a common, duct forming tube 2. When air is introduced via the left end of the duct, e.g. by means of a pressure fan, the air initially passes through the individual cells of the different box units in the frame structure 7 and then through the partial ducts 3', 3", 3"' that are separated by the separating walls 11.

Since these partial ducts have limited height or width it is guaranteed that the different partial sets of flexible elements to essentially completely fill up the partial duct, said flexible elements being included in each partial duct. Even if the individual partial set of elements initially has a tendency to collapse somewhat when the filter is inactive, the limited amount of elements will start to flutter when air is put to motion through the duct and thus fill up the restricted volume that is defined between the separating walls. Seen over the entire cross section of the duct it is thus guaranteed a complete filling-up that will result in that the different partial air flows will not have any untreated bypassing air

flows. In other words the air will in a uniform way, during its passage between the inlet and the outlet, rub along the particle separating elements along their entire length. In other words an effective cleaning of all air passing through the filter is guaranteed.

Figure 4 illustrates how first and second separating walls 11', 11" may be arranged at an angle relative to each other, preferably perpendicular to each other, to define seve- ral individual partial ducts in each of two planes oriented at an angle to each other. In the embodiment according to figure 4 the individual partial ducts have a rectangular cross section.

In figure 5 is shown how the partial ducts also may have square cross section. Finally figure 6 discloses how a tube 2 having circular cross section is divided into individual partial ducts. Also in this case the filter includes not only a number of parallel, spaced-apart separating walls but also a further separating wall oriented perpendicular to said first separating walls.

Feasible Modifications of the Invention The invention it not solely restricted to the embodi- ments described above and shown in the drawings. Although the flexible, particle catching elements in the embodiments are shown as attached only at one end to a supporting and assemb- ling frame structure it is thus possible to attach the opposite ends of the elements to two frame structures that are spaced apart. Although the filter according to the invention is described and defined in the claims as suitable in connection with the separation of particles from air, said filter is also useful for the same purpose in connection with other gases than air. It should also be pointed out that the duct, in which the filter is to be mounted, may be defined otherwise than by a tube. Instead of planar separating walls also walls having another arbitrary shape may be used. Thus it is e.g. conceivable to provide inside an outer tube or shell, having e.g. circular cross section, one or more internal separating walls having circular (or possibly polygonal) cross section, said internal separating walls defining between themselves partial ducts having annular cross sections. It is also possi- ble to divide said annular partial ducts into several smaller partial ducts by means of radial separating walls.