Method and device for cleaning of a gaseous fluid

[0002] This application is a National Stage of International Application No. PCT/SE98/00818 filed May 5, 1998.

[0003] The present invention refers to a method for cleaning of a gaseous fluid from particles being present in said fluid, said gaseous fluid being ionised, i.e. the particles are subjected to a charging, and that said ionised gaseous fluid is brought to pass through a precipitator unit including a filter medium. The invention also refers to a device for cleaning of a gaseous fluid.

[0004] From SE-A-9604817-8 a device is previously known in connection with an active electret filter of the type defined above. Thereby, an electric field is directed towards the precipitator unit that consists of medium being electrically non-conductive and whose molecules are easily polarised or oriented by an electric field directed towards the precipitator unit. By this arrangement an electric field is created inside the fibre material that the precipitator unit preferably consists of. The particles that pass through the filter medium, said particles being charged by the ionization unit, are attracted and repelled respectively by the filter fibres depending on the charging and polarity of the fibres and the particles respectively.

[0005] The object of the present invention is to define a method and a device of the type defined above, the present invention being essentially more simplified compared to the device that has been described under the headline “Prior Art”. The objects of the invention is realised by a method and a device that have been given the features defined in the appending claims.

[0006] In the drawings:

[0007] FIG. 1 is a schematic view of a device according to the invention;

[0008] FIG. 2ais a schematic view of a molecule before and after polarisation, said molecule being included in the fibre material of the precipitator unit;

[0009] FIG. 2bis a schematic view showing how the molecules of the filter medium are oriented;

[0010] FIG. 3ais a schematic view showing how an electric field is created between the polarised fibres that are included in the filter medium; and

[0011] FIG. 3bshows schematically how the charged particles that pass the filter are attracted and repelled respectively by the fibres included in the filter medium.

[0012] The device shown in FIG. 1 comprises a ionization unit 1 that in the shown embodiment includes a corona tip 3. However, within the scope of the invention also other types of ion sources are feasible, e.g. a coal fibre brush or a corona wire. The device also includes a precipitator unit 2 that below will be described more in detail.

[0013] The ionization unit 1 comprises in a known way a high voltage source 4 that is connected to the corona tip 3. The ionization unit 1 does not need to be located adjacent to the precipitator unit 2 but said ionization unit 1 may in principle be located anywhere in the space that contains gaseous fluid, said fluid being cleaned by means of the method and the device according to the present invention.

[0014] The precipitator unit 2 consists of a filter of an electrically non-conductive medium, preferably a fibre filter of polypropylene. It is also advantageous if the filter medium is a so called fine filter medium that in the present application is defined as a filter medium having a fibre diameter down to 1 μm and a spacing between the fibres of the magnitude 10 μm. The air velocity through such a fine filter medium is usually 2-10 cm/s.

[0015] It is preferred that the fibres, included in the filter medium, are not pre-charged but charged by the particles charged in the ionization unit, i.e. said particles emit their charging to the fibres 7 (see FIGS. 3aand 3b) in the filter medium when said particles adhere to said filter medium. This can also be expressed in such a way that the present invention preferably does not constitute an electret filter. It is also advantageous if the fibres 7 in other respects are untreated.

[0016] The device according to the invention functions in the following way. In the space where the ionization unit 1 is located the gaseous fluid, normally air, contains particles that are charged by the ions that the ionization unit 1 emits. In this connection it should be repeated that the ionization unit 1 may in principle be located anywhere in the space that contains the gaseous fluid/air that is to be cleaned.

[0017] The particles charged by the ionization unit 1 are in a suitable way, e.g. by means of a fan (not shown), brought to pass through the precipitator unit 2, this being indicated by the arrows 5. When these particles hit the fibres of the filter medium a polarisation of the molecules of said fibres takes place. If it is assumed that a particle having a positive charging, i.e. positive corona is used, hits the upper molecule 6 in FIG. 2asaid particle will be polarised in the way shown at the bottom of FIG. 2a.In a corresponding way a fibre 7 that thus contains a lot of such polarised molecules 6, see FIG. 2b,will have a negative side and a positive side. Charged particles that pass through the filter, after chargings have been induced upon the surfaces of the fibres 7, will adhere to said fibres 7, thus a particle having positive charging will adhere at the left side of the fibre 7 shown in FIG. 2b.

[0018] In FIG. 3ait is schematically shown how a fibre 7 upon whose surface chargings have been induced by the polarised molecules may induce chargings upon the surface of an adjacent fibre 7.

[0019] In FIG. 3bis schematically shown how charged particles 8 that pass through the precipitator unit 2 are attracted and repelled respectively by the fibres 7 depending on the charging and polarity of the fibres 7 and particles 8 respectively.

[0020] The invention is in no way restricted to a positive corona but negative corona is preferred from certain aspects, a negative corona for instance generates less ozone than a positive corona.