Patent Application
20070240571
The present invention relates to a device for cleaning of an air stream from electrically charged particles (aerosols), said air stream passing through the device, said device comprising at least two electrode elements that are arranged in planes parallel to each other and at a gap distance between adjacent electrode elements, that the electrode elements are connected to a respective terminal of a high voltage source, and that spacers are provided between adjacent electrode elements.
Two techniques are used today to clean air from particles and aerosols (very small particles):
Mechanical filters constituting thin fibres of glass, synthetics or cellulose material: In order to catch small particles very fine fibres are necessary. Via the Manufacturing process the synthetic fibres may be given a certain electrostatic charge that especially when the filter is new improves the filtering of small particles.
Electrostatic filters, so-called two-step electro filters, comprise an ionizing chamber in which the particles carry an electric charge and a so called precipitator. The charged particles pass the precipitator in the air stream and are deposited on the electrodes of the precipitator.
Mechanical filtering is the number one filtering method in the world. Its primary drawback is that proper filtering demands fine filters that create high air resistance (so called pressure drop). High pressure drops bring about high energy costs and also result in a high sound level from the fans.
The use of higher filter classes often brings about that such a filter must be protected from heavy dust deposit by means of a filter with a lower filter class. Naturally, the pressure drop increases to a corresponding degree.
The other way around electrostatic filters have a more cost efficient filtering compared to the mechanical filters, since they work at considerably lower air resistance. Recent development of the electro filter technique, described in Swedish patent no. 0103684-7, has to a substantial degree improved the traditional electro filter technique. However, certain drawbacks are still present.
In connection with higher air velocity through the filter (1-4 m/s), e.g. in ventilation ducts, heavy dust has a tendency to migrate through the filter and follow the air stream.
A further disadvantage in connection with higher air velocities is that the electrodes of the precipitator must be arranged at a gap distance of some millimeters between adjacent electrode elements in order to achieve the desired separation result. This brings about that heavy dust easily surrounds the inlet area of the precipitator and the operating parameters are negatively affected to a considerable degree.
In order to separate particles from combustion engines ceramic filters are normally used. These filters have extremely large air resistance which means increased wear of the engines and increased fuel consumption.
From WO 87/02274 A1 an electrically stimulated filter is previously known. This filter comprises electrode elements that are perforated. The air stream direction is generally transverse to the planes that the electrode elements are located in. Between the electrode elements a filter medium is provided, said filter medium normally constituting glass fibre. The filter also comprises means for charging of the air that will pass the filter.
From EP 0 808 660 A1 an electrostatic dust separator is previously known, said separator comprising a unit for corona discharge and a unit that creates an electric field. The last mentioned unit comprises electrode elements that are arranged at a certain mutual distance. In one embodiment (working example 6; FIG. 10) of the dust separator according to EP 0 808 660 A1 a non-conductive, dielectric element is provided between the electrode elements, said dielectric element may be designed as a net of polypropylene that is received in a V-shaped house, likewise of polypropylene. In this connection it should be pointed out that the non-conductive, dielectric element does not constitute a spacer between the electrode elements. By spacer it should be understood an element that keeps the electrode elements at a certain mutual distance.
A primary object of the present invention is to present a new type of particle filter that is characterized by a relatively seen very low and almost constant pressure drop in connection with very high separation ability of both large and micro sized particles as well as a relatively seen substantial increase of the dust collection ability.
A further object of the present invention is to present an embodiment of a filter for cleaning of particles in combustion exhaust gases from motor driven vehicles.
At least the primary object is realized by means of a particle filter that has been given the features of the appending independent claim 1. Preferred embodiments of the invention are defined in the dependent claims.
Below preferred embodiments of the present invention will be described, reference being made to the accompanying drawings, where:
a shows a perspective view of a portion of an air stream duct, in which a particle filter according to the present invention is provided;
b shows a perspective view of an electrode element that is part of the particle filter in
As is evident from
The spacers 11, shown in
Preferably, the spacers 11 are of the same material that is used in mechanical filters (glass fibre, synthetic fibre or plastic fibre), foamed plastic or ceramic structures with high-temperature resistance. Regardless of the material that is used, said material must be permeable to an air stream 40 and electrically insulating.
As is evident from
In the shown embodiment the electrode elements 20 and 21 respectively are made from a cellulose based material, preferably cardboard and coated with a micrometer thin plastic film in order to protect the material from moisture. Also the edge sections of the electrode elements are encased, e.g. by hot melt adhesive or tape, in order to prevent moisture penetration.
In connection with ventilation or corresponding areas of use it is a clear advantage that the electrode elements 20, 21 constitute material with very high resistivity and of disposable type as for instance cellulose material. In the latter case disc shaped electrodes from plastic coated paper are especially suitable.
The electrode elements 20 and 21 respectively are in a previously known way connected to a respective terminal of a high voltage source (not shown in the figures), whereby an electrostatic field is present in the gap between the adjacent electrodes 20, 21.
As is shown in
By the substantial velocity drop of the air stream 40 the separation of larger and micro sized particles on the surfaces of the electrode elements 20 and 21 respectively increases several times, also at a relatively seen very large gap distance “d” between the adjacent electrode elements 20, 21. As regards the gap distance “d” it should preferably be in the interval 3-30 millimeter. However, both larger and smaller gap distances “d” are possible within the scope of the invention.
A separation ability of more than 85% was achieved in lab tests with spacers 11 manufactured from coarse filter media, corresponding to filter class G3, gap distance “d” between the electrodes 20, 21 of about 15 mm and air velocity corresponding to 2.4 m/s in the air stream duct 30.
The spacers 11 may of course constitute the same material that is used in higher classes of mechanical filters, e.g. fibre filters. Regardless which material that is used it should in itself be electrically insulating. For instance glass filter material or synthetic filter material fulfills these demands. Also ceramic filters are of electrically insulating material.
Swedish patent no. 0103684 presents a description of particles separators designed from paper, the energizing of the electrode elements is arranged by means of semi-conductive coating printed on paper in the shape of thin current lines. Such energizing is also a preferred embodiment in connection with the present invention in case the electrode elements 20, 21 constitute cellulose material. In this connection it should be pointed out that the device according to the present invention may be part of both a precipitator and a particle separator according to Swedish patent no. 0103648-7.
The charging of particles and aerosols, before they in the air stream pass between the electrode elements 20, 21, is effected upstream, seen in the direction of the air stream through the device, in a previously known way. Normally, this is effected by having an ionizing unit installed in the air stream duct upstream the device according to the present invention.
The air stream through the device may be initiated by means of fans or natural draught or in a different way, e.g. by having the device mounted in the exhaust duct of a combustion engine.
In the latter case the electrodes 20, 21 and the spacers 11 should be made from a material that resists high temperature. It is preferred that the spacers constitute ceramic (glass) material and that the electrode elements constitute sheet metal.
The particle filter according to the present invention has excellent separation parameters also at a relatively seen very large gap distance between the electrodes 20, 21. This brings about that considerable amounts of dust may be gathered on the electrode elements 20, 21 without the risk of flash over between the electrode elements 20, 21. When used in extreme environments, that usually demand guarantee that no particles may pass the filter, the spacers 11 may constitute very high classes of known mechanical filters or known filters for cleaning exhaust gases from particles. The initially high pressure drop through the device may be kept at a constant level instead of increasing when the precipitated amount of dust increases.
It should especially be pointed out that the spacers 11 included in the device functions on one hand as mechanical filters and on the other hand as spacers. It should also be pointed out that the spacers 11 preferably have a certain transverse extension relative to the air stream direction. Within the scope of the present invention the lateral extension of the spacers 11 may be relatively seen small, which means that the spacers 11, and thus also the electrode elements 20, 21, may have a length of several meters.
In the embodiment shown in
As regards the spacers 11 included in the device according to the present invention they should belong to all established filter classes, i.e. rough filters, fine filters, micro filters and HEPA filters.
The spacers of the present invention may also constitute relatively thin strips that are oriented standing and equipped with a fine perforation, i.e. the openings that create the perforation are proportionately small.
In the embodiment described above the air passes through a spacer 11 and then continues in the air stream duct 30. Within the scope of the present invention it is also feasible that there are several sets of spacers arranged one after the other in the direction of the air stream. This means that the air must bypass two or more spacers that is favorable as regards the separation on the electrode elements since the velocity between two consecutive spacers is extremely low.
In connection with the embodiment described above it is stated that the spacers 11 may be glued to the lower electrode element 20, 21. Within the scope of the present invention it is also feasible with alternative ways to connect the spacers 11 with the electrode elements 20, 21. In exemplifying and not restricting purpose it may be mentioned that the spacer on one side may be equipped with a two sided adhesive tape.
In the embodiment described above all spacers 11 included in the V-formation are permeable to air. However, within the scope of the present invention it is also feasible that only every second spacer is permeable to air. In such a case the spacers that are not permeable to air may have an extension straight in the direction of the air stream while the permeable spacers have a certain lateral longitudinal direction relative to the air stream.
It may be suitable that the different groups of electrode elements 20 and 21 respectively are displaced somewhat relative to each other in the direction of the air stream. Hence, the projecting portions of one group of electrode elements may easily be electrically connected to each other.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0302691-1 | Oct 2003 | SE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/SE04/01469 | 10/13/2004 | WO | 2/22/2007 |
