WO1998046358A1

WO1998046358A1 - Device in connection with a filter

Info

Publication number: WO1998046358A1
Application number: PCT/SE1998/000646
Authority: WO
Inventors: Johan WENNERSTRÖM
Original assignee: Blue Air Ab
Priority date: 1997-04-16
Filing date: 1998-04-08
Publication date: 1998-10-22
Also published as: SE9701410D0; SE522210C2; SE9701410L; AU7091598A

Abstract

The present invention relates to a device in connection with a filter, especially an active electret filter for air cleaning, said filter comprising an ionisation portion (10; 10'), said portion (10; 10') having a high voltage source at a certain potential, and a precipitator portion (18; 18'), said portion (18; 18') having a downstream side facing away from the ionisation portion (10; 10'). In known electret filters the fibre material eventually loses its charge, alternatively is it necessary to use a fibre material that is easily polarised when an electrical field is applied. The aim of the present invention is to define a filter of the previously mentioned type that is characterised in that as filter material a number of different pre-filter media are useful and that at the downstream side of the filter a layer (22; 22') of electrically conductive material is applied, said material having an absolute potential that is lower than the potential of the high voltage source (12).

Description

DEVICE IN CONNECTION WITH A FILTER

Technical Field of the Invention

The present invention relates to a device in connection with a filter, especially an active electret filter for air cleaning, said filter comprising an ionisation portion and a precipitator portion.

Prior Art Electret filters for air cleaning are previously known, said filters comprising a filter material that consists of a filter material having relatively coarse fibres arranged in a relatively open pattern. The fibres in the filter material are subjected to a charging, said charging having the effect that although the filter is relatively open small particles may be separated. However, a prerequisite for this is that the particles have a weak charging. Since the filter is open the pressure drop across the filter is low. A disadvantage in connection with known electret filters is that the charging of the fibres and also the separating ability of the filter decline after a time.

From SE-A-9604817-8 is previously known a so called active electret filter of the type defined above. This filter consists of an electrically non-conductive material, preferably polypropylene having the characteristic that its fibres includes molecules that are easily polarised or oriented of a field directed towards the filter. This in its turn has the effect that charges are induced on the surfaces of the filter fibres and hence an electric field is created within the filter material. The particles passing through the filter, said particles being charged in the ionisation portion, are attracted and repelled respectively of the charged fibres, said particles normally adhere to a fibre having the opposite charging compared to the particles. A limitation with this filter is that the filter material must include fibres that in its turn includes molecules that are easily polarised from an applied electric field. Objects and Features of the Invention

The aim of the present invention is to define an active electret filter of the type defined above, said filter having' a freedom of choice as regards the filter material that is essentially improved compared to the device according to

SE-A-9604817-8. In connection with the device according to the present invention all sorts of pre-filter media may be used as filter material, this being a considerable advantage since said filter materials being extremely cheap. The aim of the present invention is realised by means of a device that has been given the characteristics of the appended claims.

Brief Description of the Drawings

Below the principle of the invention and embodiments of said invention will be described, reference being made to the enclosed drawings, where Figure 1 schemetically shows how a device according to the present invention is designed; Figure 2 shows in section an embodiment of a filter in accordance with the principle of the present invention; and Figure 3 shows a section along III-III in figure 2.

Detailed Description of the Principle of the Invention and Embodiments of said Invention

As is evident from figure 1 the device according to the invention includes an ionisation portion 10, said portion including a high voltage source 12 and a corona electrode 14 connected to said source 12, said corona electrode for instance being in the shape of a point, a brush or a wire.

As is indicated by the arrow 16 in figure 1 an air flow bypasses the corona electrode 14 in the direction of the arrow 16. When the air flow bypasses the corona electrode a charging takes place of the particles of the air flow that are to be separated. It is possible to use either negative or positive corona, in the present invention negative corona is preferred since it is advantageous as regards prevention of ozone generation. The device according to figure 1 includes also a precipitator portion 18, said portion 18 in its turn including a filter 20 that consists of a pre-filter medium and a layer ^■ ' 22 of an electrically conductive material, said layer 22 being located downstream of the filter 20, seen in the direction 16 of the air flow. In the disclosed embodiment the layer constitutes of a grid 22.

The pre-filter medium that constitutes the filter 20 may consist of a number of different materials. In exemplifying but not restricting purpose foamed materials having open pores should be mentioned, e.g. polyester or other synthetic materials. In the present application should by the expression pre-filter medium be understood a filter having a fibre diameter down to about 20 μm and a spacing between the fibres of 200-400 μm. A material named CM 355 and marketed by the company Camfill has turned out to be a suitable material. However, in this connection should be pointed out that it has turned out empirically that a large number of different filter materials may be used as filter material in the filter 20. In this connection it should be pointed out that the velocity of the air flow, indicated by the arrow 16, depends on the coarseness of the filter. In connection with a pre-filter an air velocity of 0.2 m/s is suitable while in connection with a fine filter an air velocity of about 0.5 m/s is possible. The principle is that the more fine the filter is the higher air velocity.

The grid 22 consists preferably of a metallic material. It has turned out to be important for the function of the device that the grid 22 closely abuts the filter 20. Within the scope of the invention it is also feasible that the grid consists of a non-metallic material that is electrically conductive, e.g. a plastic material. Nor is it necessary to have a grid but the corresponding function may for instance be achieved by an electrically conductive paint that is applied to the downstream side of the filter.

The primary characteristic feature of the present invention is that the layer/grid 22 has an absolute potential that is lower than the potential of the corona electrode. By absolute potential should in this connection be understood the magnitude of the potential regardless if it is negative or positive. The corona may for instance have a negative potential of 6-7 kV while the grid 22 has a positive potential- of 1 kV. According to a preferred embodiment the grid 22 is grounded, this being marked by a symbol in figure 1. In case the grid 22 is not grounded it must be connected to a voltage source .

The device schematically shown in figure 1 functions in the following way. When the air flow bypasses the corona electrode 14 a charging is effected of the particles in the air flow that will be separated, this being pointed out above. When these particles reach the precipitator portion 18 the filter 20 will be charged due to the fact that the charged particles emit their charges to the filter 20 when they collide with the fibres in the filter 20. The particles thus adhere to the fibres, i.e. the air is cleaned from said particles. A charging of the fibres of the filter also takes place due to the fact that ions generated at the corona electrode 14 are transported, via the electric field, to the filter where said ions adhere to the fibres of the filter. Since a continuous generation of negative ions take place at the corona electrode 14 charged particles and free ions will continuously be supplied to the filter 20. When a sufficiently high potential has been built up in the filter negative ions will be transferred to the grid 22 since this has a lower absolute potential than the corona electrode 14. Negative ions will thus continuously be deflected via the grid 22, i.e. it is important that the grid closely abuts the filter 20.

When the filter has been used such along time that its separating function no longer is sufficient the filter 20 is replaced by a new one. This is not a heavy expense since the preferred pre-filter medium is cheap.

In the description of the device according to figure 1 it has been stated that the ionisation source 10 includes a negative corona. However, it is fully feasible within the scope of the invention to use a positive corona, on condition that the problems of ozone generation are solved. In such a case positive ions will of course be generated at the corona electrode. However, the corresponding reasoning as regards charging of the filter 20 and separation of particles from the air are valid also in such a case.

In figure 2 and 3 an application of the invention in the shape of an air cleaner is shown. The device 1' according to figures 2 and 3 includes a casing 24, said casing 24 in its upper portion having an opening 26 where an axial fan 28 is mounted. The right part of the casing 24 in figure 2 and 3 has an ionisation portion 10', i.e. a corona electrode 14' in the shape of a wire is mounted in said part of the casing 24, said wire having an extension in the axial direction of the fan 28. As especially is evident from figure 3 the casing 24 has an essentially curved shape in the part constituting the precipitator portion 18', said filter 20' and said grid 22' constituting a part of the casing 24. The curved shape makes the distance between the corona electrode 14' and the filter 20' more uniform. The curved shape also facilitates e proper contact between the filter 20' and the grid 22'.

The device 1' according to figures 2 and 3 functions in principally the same way as has been described in connection with figure 1. However, especially for the device 1' the air, symbolised by the arrow 30, is supplied to the casing by means of the fan 28. The air thus supplied is ionised by the wire- shaped corona electrode 14', i.e. particles that are to be separated are charged and free ions are supplied to the air. The air thus ionised is forced, by the fan 28, out through the filter 20', said air transport being symbolised by the arrows 32.

In a corresponding way as in figure 1 the corona electrode 14' has a higher absolute potential than the grid 22', said grid 22' preferably being grounded. The separation process, i.e. the cleaning of the air, takes place in a corresponding way as has been described in connection with the device according to figure 1. Empirically, it has turned out that the device according to figures 2 and 3 give a purification of 90%.

A still further feasible application of the present invention is to use the principles of the invention in connection with so called supply air terminal device, i.e. where the ventilating duct emerges into spaces. Thus such a device may be used that principally corresponds to the device according to figures 2 and 3, the difference being that the fan 28 is deleted since the air flow emerging from the ventilating duct already has a certain, relatively low velocity.

The invention is in no way restricted to the embodiments/applications described above but can be varied freely within the scope of the appending claims.

Claims

1. Device in connection with a filter, especially an active electret filter for air cleaning, said filter comprising an ionisation portion (10; 10'), said portion (10;10') having high voltage source at a certain potential, and a precipitator portion (18; 18'), said portion (18; 18') having a downstream side facing away from the ionisation portion (10; 10'), c h a r a c t e r i z e d in that the precipitator portion (18; 18') includes a filter (20; 20') consisting of a filter medium of electrically non-conductive material, that upon the downstream side of the precipitator portion (18; 18') a layer (22; 22') of an electrically conductive material is applied, and that said layer has an absolute potential that is lower than the potential of the high voltage source (12) .

2. Device according to claim 1, c h a r a c t e r i z e d in that the high voltage source (12) has a negative potential and that the layer (22; 22') is grounded.

3. Device according to claims 1 or 2 , c h a r a c t e r i z e d in that the layer (22; 22') is of metallic material.

4. Device according to any of claims 1-3, c h a r a c t e r i z e d in that the layer (22; 22') constitutes of a mesh/grid.

5. Device according to claim 1, c h a r a c t e r i z e d in that the layer (22; 22') constitutes of an electrically conductive paint that is applied upon the filter (20; 20') .

6. Device according to any of claims 1-5, c h a r a c t e r i z e d in that the filter (20; 20') is a fibre filter medium.

7. Device according to any of claims 1-5, c h a r a c t e r i z e d in that the filter (20; 20') constitutes of a foamed material having open pores.

8. Device according to any of claims 1-7, c h a r a c t e r i z e d in that the filter (20_; 20'₎ constitutes of a pre-filter medium.