AIR PURIFICATION FILTER AND A METHOD FOR MANUFACTURING SUCH FILTERS Prior Art From WO 95/00248 a device for air purification is previ- ously known, said device including both a prefilter and a finefilter. More precisely the device includes a duct through which air may flow from an inlet towards an outlet, said air initially passing through a prefilter and subsequently a fine- filter consisting of a set of elongated, flexible or flimsy elements of electrostatically chargeable material. When the air passes these elements they are subjected to a motion, while generating static electricity that guarantees that also microscopically small particles in the air are attracted to the elements and adhere thereto.

A basic advantage of finefilters of the type defined above is that they are able to separate extremely small parti- cles from the air. In practice said finefilters require that coarse constituents are initially removed from the air. In the previously known device removal is effected by means of a separate, pre-separating filter or prefilter that is located closer to the inlet than the finefilter. Another prerequisite for having the finefilter to function effectively during operation is that the flimsy finefiltering elements, which may consist of extremely thin threads or strips, are kept together by an air permeable supporting element that completely cuts off the cross section of the duct in order to evenly distri- bute the air flow through the set of elements. This sets aside the risk of having voids in the set of elements through which air may pass without getting into contact with particle att- racting elements.

Although the device, previously known from WO 95/00248, has a very good performance in regard to the ability of puri- fying air, said device is associated with a number of incon- veniences. One inconvenience is that the particle-attracting, flimsy elements must be attached to or assembled with a parti- cular supporting element. In practice the work of assembling the elements and a rigid, still air permeable, supporting ele- ment is complicated and time-consuming. At the same time the

structure calls for a separate, preseparating filter that on one hand is expensive and on the other hand requires space in the air purification device in its entirety. It should also be pointed out that the structure of the finefilter is tricky inasmuch as it is difficult to guarantee, in real production, that the distribution of the particle attracting elements becomes even and dense in the set of elements traversing the duct.

Objects and Features of the Invention The present invention aims at removing the inconveni- ences mentioned above in connection with prior art and provide conditions not only for a more simple and cheaper air purify- ing function, but also for a more efficient filter manufactur- ing. According to a primary aspect the invention thus aims at providing a structurally simple filter that in itself combines a prefiltering function and a finefiltering function. A further object is to provide a filter which by its structural design makes it possible to overcome, in a simple way, the problem to establish an even and dense distribution of the particle attracting, flimsy elements in the set of such ele- ments.

According to the invention at least the primary object is attained by means of a filter having the features stated in claim 1. Preferred embodiments of the filter according to the invention are defined in the dependent claims 2 and 3.

According to a second aspect the invention also relates to a method for manufacturing of air purification filters according to the invention. Characteristic features of this method are defined in claim 4. Furthermore, preferred embodi- ments of the method according to the invention are defined in the dependent claims 5-li.

Further Illumination of Prior Art From WO 84/04467 it is previously known to use elonga- ted, flexible or flimsy strips in connection with air purifi- cation filters in the shape of cylindrical tubes, wherein said flimsy strips may include electrostatically chargeable fibres for the purpose of attracting small particles from the air.

However, in this case the strips have an annular location in the area inside a cylindrical wall of folded paper that forms a tube through which the air passes in a direction from inside and outwards. More precisely, at one of their ends the strips are attache to a common ring, the air passing through the layer of strips and the filter wall, the primary function of said strips being to remove particles, by mechanical action, from the inner side or the upstream side of the filter wall in order to increase the lifetime of the filter wall itself.

Brief Description of the Drawings In the drawings : Figure 1 is a schematic picture showing on one hand a wave shaped filter wall, and on the other hand a set of flexible finefiltering elements which are distanced from the wall and hang down from supports, the pic- ture illustrating a first step in connection with the manufacturing of a filter according to the pre- sent invention, Figure 2 is an analogue picture completed with a heating device, Figure 3 is a likewise schematic picture illustrating a second manufacturing step, where the filter wall and the finefiltering element are assembled in order to be connected with each other, Figure 4 is a picture showing a third and possibly final manufacturing step, where the supports for the fine- filtering elements have been removed, and Figure 5 is a picture illustrating a preferred, fourth step, in which the filter wall is compressed in its own plane.

Detailed Description of Preferred Embodiments of the Invention In figure 1 reference numeral 1 generally designates a filter wall for prefiltering purposes, while reference numeral 2 generally designates a set of flexible or flimsy elements that together form a means for finefiltering of through-pass- ing air. The filter wall 1 generally has a wave-like cross- sectional shape. Although the shape of the waves may vary con-

siderably, e. g. from smoothly rounded, sinusoidal configura- tions to sharp-edged U-shape, the example discloses a wall having a folded shape. More precisely, the wave formations-are V-shaped in cross section, each individual wave formation being defined by two planar wall portions 3, 3'that extend from common tips or crests 4 ; 4'. It is axiomatic that the wall has a certain arbitrary extension in the direction per- pendicular to the plane of the drawing. An upstream side of the filter wall is designated 5, while reference numeral 6 designates a downstream side. In other words, the air that is about to be purified passes in the drawing in a direction from above and downwards. The tips that are located at the upstream side are designated 4, while the tips at the downstream side are designated 4'.

In practice the filter wall may consist of paper having a fibrous structure and consequently air may pass through the wall but not constituents that are coarse, solid or in a liquid state, said constituents being adhered to the filter wall. Instead of using exactly fibrous paper also other mate- rials having a porous, air-permeable, though particle-filtrat- ing structure, may be of interest. It should also be mentioned that the wall may consist of two or more sublayers that toget- her form an air permeable wall having the desired, particle separating characteristics.

Individual flexible elements that are included in the set of elements 2 are designated 7 and are shown only schema- tically in figure 1. In a known way, the elements may be in the shape of threads, strips, fibres, fibre-like means or com- binations thereof. The material of the elements preferably consists of a plastic of meltable nature. Preferably, the ele- ments may consist of or include polypropylene that is an elec- trostatically chargeable material. It is essential when choos- ing the material that the material is electrostatically chargeable in order to bring the elements to generate static electricity when they are set in motion by the passing air, this giving the elements the ability to attract also microsco- pically small particles in the air and thus make the particles to adhere to the elements.

So far described the filter wall 1 and the set of ele- ments 2 respectively are in all essentials previously known.

According to the present invention the wave shaped fil- ter wall 1 is combined with the flexible, particle attracting elements 7, said combination providing a single, integrated filter unit. This can be effected in the following way : In a first step the elements 7 are applied in bundles along elonga- ted supports 8 that are shown in cross section in figure 1, said supports 8 having their axial extension perpendicular to the plane of the drawing. In practice the supports 8 should have at least the same length as the filter wall 1. When app- lying the elements 7 upon the supports 8 said elements are arranged in such a way that they straddle the supports. In other words each individual element 7 that suspends from a support mainly obtains a U-shape that includes two shank-like portions 10, 10'suspending from an intermediate portion 9.

Most preferably the elements are arranged in such a way that the shank portions 10, 10'are of substantially equal length.

In practice the elements 7 are applied in bundles and distri- buted essentially even along the entire length of the indivi- dual supports 8.

Elongated bars may be used as supports, said bars having an arbitrary cross section. However, in practice bars are pre- ferred that at least along a longitudinal side have a cross- sectional shape that corresponds to the shape of the cross section of a portion of the individual wave formation in the filter wall, said portion being intended to receive the bar, i. e. in the area of the wave crests or tips 4 at the upstream side. In the disclosed example, where the individual wave for- mation constitutes an essentially right-angled, V-shaped fold, the upper portion of the individual bar 8 is essentially V- shaped with two diverging, planar surfaces oriented at right angle relative each other.

In figure 2 reference numeral 11 designates a heating device that in the example constitutes a block, e. g. of metal, whose lower side is provided with grooves 12 that are V-shaped in cross section, the dimensions of said grooves essentially corresponding to the dimensions of the folds in the filter wall 1. At least in the area of its lower side this block may

be heated, in a suitable way, to a temperature where the plas- tic material of the elements 7 melts.

Figure 3 illustrates a second method step where the wave shaped filter wall 1 is assembled with the supports 8 and the finefiltering elements 7 suspending from said supports 8. At the same time the filter wall is shown pressed against the lower side of the heating block 11. Since the block 11 is able to emit heat to the intermediate portion 9, said portion 9 resting against the bars 8 simultaneously as the bars together with the filter wall is pressed against the block, the plastic material in the elements 7 is adhered, by melting, to the downstream side of the filter wall.

In a third step, illustrated in figure 4, the bars 8 are removed and the elements 7 will remain adhered to the filter wall, more precisely by their relatively short intermediate portions 9. The shanks 10, 10'of the elements, said shanks 10, 10'being longer than the intermediate portion 9, pro- trude, freely movable, from the downstream side of the filter wall.

In a fourth preferred method step the width of the indi- vidual wave formations is reduced, i. e. the distance between adjacent wave crests 4, 4'decreases. This is effected by com- pressing the filter wall 1 in its own plane and then steps are taken to maintain said compressed state. For instance, this can be effected by having the filter wall mounted in a sur- rounding rigid frame, although other methods are also feasi- ble.

By compressing the folded filter wall in the way described above the distances between adjacent filter elements in the set of elements 2 will be reduced proportionally. In other words the set of elements is condensed which means that the ability of the set of elements to separate microscopic particles from the passing air, at a given cross-sectional area of a duct, is increased proportionally to the condensa- tion. Furthermore, the risk is counteracted that empty voids are established within the set of elements..

Although it is feasible to supply heat via the bars 8 the method illustrated in figures 2 and 3 is preferred, i. e. to use a block or another device that emits heat to the ele-

ments 7 from the upstream side, i. e. via the filter wall. Rods of wood are preferably used as supporting bars, said rods easily getting loose from the plastic material in the elements 7 when said rods are removed.

The Function and Advantages of the Invention The finished, integrated filter according to figures 4 or 5 combines a prefiltering function with a finefiltering function. Thus coarse, usually visible, particles in the air to be purified may be separated by the filter wall 1 while microscopically small, usually invisible particles that accom- panies the air through the filter wall 1 are separated due to the ability of the elements 7 to generate static electricity that attracts such particles to the elements. In this connec- tion it should be emphazised that the illustrations in the drawings are schematic, especially as regards the closeness between the elements 7. Although the elements in the drawings are illustrated in the shape of simple, linear lines, the ele- ments are in reality in an essentially greater number and thus assembled or densely compacted relative to each other, espe- cially in case they have been compressed in the way that has been described in connection with figure 5. Further, the ele- ments are, in reality, not genuinely linear but rather crimp or wave shaped.

By the method described above the filter is manufactured in a simple and also cheap way. Few components are used during manufacturing.

Feasible Modifications of the Invention The invention is not restricted to the embodiments described above and schematically disclosed in the drawings.

Instead of having the finefilter elements adhered by melting to the filter wall it is thus feasible to secure them in other ways, e. g. by the aid of glue or other adhesives. Although the length of the finefiltering elements in the drawings is shown as being essentially larger than the depth of the wave forma- tions in the filter wall also other dimensional relations may occur. For instance, the wave formations may be so deep that they include the elements, i. e. the elements do not suspend

below the plane in which the wave crests 4'of the downstream side are located. The filter according to the invention may also be manufactured in another way than by means of supports that are removed from the finefiltering elements. Thus it is feasible to melt together or in another way connect ends of the finefilter elements in a ridge member that is common for all elements, said ridge member being inserted in the indivi- dual wave formation from the downstream side and being connec- ted with the wave crest portion at the upstream side. It should also be emphazised that the pre-separating, wave-shaped filter wall 1 may be manufactured in another way than in the shape of a continuous, folded paper web. Thus it is feasible to assemble individual, cross-section-wise V-shaped cartridges of air permeable material side by side and interconnect said cartridges via longitudinal edges. In doing so, the cartridges may be equipped with finefiltering elements of the defined type that at one end are connected with exactly the longitu- dinal edges of the cartridges that are internally interconnec- ted while creating a continuous, wave shaped filter wall. It should also be pointed out that the filter wall does not necessarily need to have a planar (although folded or wave shaped) basic shape. Thus the filter wall may also be given a curved or arched basic shape.