ELECTROSTATIC FILTER

Abstract: An air filter comprises a first (21) and second (22) layer of woven electrostatic material (100). The woven electrostatic material (100) comprises a warp (120) of positively or negatively chargeable material and a weft (110) of oppositely chargeable material. The material (100) has the combined electrostatic characteristics of both positively and negatively charged material. The layers of material are separated in a filter by an air permeable material (22) comprising a block of woven polypropylene, open-cell polyurethane or polyester. The woven material (100) and/or the air permeable material (22) is impregnated with a microbiological growth control agent. The filter is reversible and may be placed in a bidirectional air flow.

Description: ELECTROSTATIC FILTER The present invention relates to woven electrostatic material and in particular to an air filter comprising such woven electrostatic material. It is well know that dust particles in a stream of air may be attracted to a charged surface by electrostatic interaction. Such surfaces may be made from fibrous material which may be woven. This woven material may be provided with an electrostatic charge which may be sustained over a long period. Filters which use such woven sheets of material have been shown to provide superior filtration. In a known air filter, the filter comprises two pairs of woven electrostatic material layers, typically polypropylene. Each pair of layers comprise a first layer of material that will develop a positive charge and a second layer of material that will develop a negative charge in response to an air flow therethrough. The air flow entry side of such a filter comprises a first pair of layers where the positively chargeable layer of woven material is outwardly disposed. The exit side comprises a second pair of layers where the negatively charged layer of said material is outwardly disposed. Between said electrostatic filter layer pairs there is provided a foam material, for example, a polyurethane material which also has electrostatic properties. This type of filter is exemplified in the disclosure of United States Patent No. 4,902,306. Prior art electrostatic air filters are known to have certain disadvantages. Firstly, due to the electrostatic charge, dust particles adhere to the woven material blocking its pores and thus restrict air flow. Furthermore, due to the build-up of dust particles on said woven material, the electrostatic charge tends to decrease within a relatively short period of time. Additionally, due to the restricted air flow and the build-up of dust particles in the filter, a breeding ground for bacteria, fungi, yeasts and other micro-organisms may be provided. With most electrostatic filters air flow direction must be determined before installation of the filter. The filter must be installed to present a predetermined face to the air flow for maximum efficiency. If the filter is incorrectly installed or the air flow direction changes the filter operates below its optimum efficiency. It is an object of the present to seek to alleviate the above disadvantages and to provide an improved electrostatic filter. Accordingly, the present invention provides a woven electrostatic material characterised in that the material comprises a warp of negatively or positively chargeable material and a weft of oppositely chargeable material, which warp and weft are interwoven to form a material having the combined electrostatic characteristics of both a positively and negatively charged material. Preferably, the woven material is impregnated with at least one microbiological growth control agent. Most preferably, the growth control agent or agents is or are selected from the group comprising fungicides, fungistats, biocides, biostats, bactericides and bacteriostats. Conveniently, the warp and the weft each comprise polypropylene. The present invention further provides an air filter comprising first and second layers of a woven electrostatic material characterised in that said electrostatic material comprises an interwoven negatively or positively chargeable warp and an oppositely chargeable weft, said layers of woven electrostatic material being separated by an intermediate air permeable material. Advantageously, the or each layer of woven material and/or the intermediate material is or are impregnated with a microbiological growth control agent. Conveniently, the woven material comprises an anti-clogging means for filtering dust particles from the air flow path, and the air filter further comprises a collecting tray for collecting dust particles repelled from the electrostatic material. Preferably, the intermediate air permeable material comprises an electrostatic medium. Advantageously, the intermediate material comprises woven polypropylene, open-cell polyurethane or polyester. Optionally, the layers of woven material and the intermediate air permeable material are housed in a retaining frame, whereby the filter may be installed in a bi-directional air flow system or without consideration to air flow direction in a uni-directional system so as to provide a reversible filter. The present invention will now be more particularly described with reference to the accompanying drawings which show, by way of example only, one embodiment of filter according to the invention. In the drawings: Figure 1 is a partially cut-away perspective view of a filter; and Figure 2 is a diagrammatic front elevation of a portion of a layer of electrostatic woven material. Referring to the drawings, Figure 1 shows a perspective view of a filter housed in a metal frame 10. The filter elements are held within the frame 10 by a wire mesh 12. The first filter element 21 comprises a layer of interwoven positively and negatively chargeable materials comprising a warp of negatively or positively chargeable material and a weft of oppositely chargeable material. The warp and weft are interwoven to form a material which displays the combined electrostatic characteristics of a pair of separate positively and negatively chargeable layers. A second filter element 22 comprises a layer of polyester material which may optionally be substituted by an open-cell polyurethane block or a polypropylene block. The third filter element 23 comprises an interwoven layer similar to that of the first layer. The first layer 21 and third layer 23 comprise warps and wefts of polypropylene fibres which have a microbiological growth control agent impregnated therein. The microbiological growth control agent is selected to have high efficiency against fungi, yeasts or bacteria, particularly gram-positive and/or gram-negative bacteria. The microbiological growth control agent is selected for its action and is one or more of the following: fungicide, fungistat, biocide, biostat, bactericide bacteriostat and similar agents. Suitable control agents are well known to those skilled in the microbiological growth control art. Quantities used for required control depend on the selected agent. Such agents may include Microban (Microban Products Company), Micro-Chek (Ferro Corporation), Biosperse (Ashland Chemical Company), Biomate (Dearborn Chemicals Ltd.) and Fungi ex (Ciba Geigy PLC) . The preferred microbiological growth control agent comprises Microban (Microban Products Company, 9801 Kincey Avenue, Suite 100, Huntersville, NC 28078, United States of America). The filter of the present invention may be used as a general purpose air filter for use in office or home environments. Tests conducted indicate an efficiency of 96.5% in extraction of dust particles from the air stream. The elimination of gram-positive and gram-negative bacteria and various fungi and yeasts from the air reduces the risk of development of a number of potentially hazardous conditions. So-called "sick building" syndrome has been attributed to airborne bacteria, in association with several other biological factors such as fungi and yeasts. The present filter aims to eliminate one or more of those biological factors. Figure 2 illustrates an interwoven material 100 of the present invention. The weft 110 is represented by a thick line (which does not represent the thickness of the individual fibres or filaments) and the warp 120 is represented by a thin line for clarity. Either the weft 110 or the warp 120 is selected to develop a positive electrostatic charge and the other fibres or filaments are selected to develop an opposite charge. The material 100 combines the electrostatic characteristics of a positively charged and negatively charged pair of layers in one layer. The extraction of dust particles from the air stream occurs in the same way as with separately charged sheets but clogging does not tend to occur. A high electrostatic field gradient between the weft 110 and warp 120 of the material 100 dislodges dust particles which aggregate on the surface and then fall under gravity into a collection area within the filter frame 10. This anti-clogging mechanism allows the weft 110 and warp 120 to retain their electrostatic charges for a relatively long period of time. This arrangement provides a filter which is still electrostatically active after a period in which there is no air flow. In a preferred embodiment, each electrostatic element, the weft 110, the warp 120 and the intermediate air permeable material block 22, comprise self-charging electrostatic media. Thus, a depressed electrostatic charge is revitalised when charge is developed by an air flow through the filter. The installation of these filters is facilitated by the bi-directional characteristic of the material 100. Installation instructions with reference to air flow direction are not required as the filter may be installed in a bi-directional air flow system or without consideration to air flow direction in a uni-directional system. Thus, a reversible filter is provided. It will of course be understood that the invention is not limited to the specific details described herein, which are given by way of example only, and that various modifications and alterations are possible within the scope of the appended claims.

Claims: CLAIMS : 1. Woven electrostatic material (100) characterised in that the material (100) comprises a warp (120) of negatively or positively chargeable material and a weft (110) of oppositely chargeable material, which warp and weft are interwoven to form a material having the combined electrostatic characteristics of both a positively and negatively charged material. 2. Woven electrostatic material as claimed in claim 1 , characterised in that the woven material is impregnated with at least one microbiological growth control agent. 3. Woven electrostatic material as claimed in claim 2, characterised in that the growth control agent or agents is or are selected from the group comprising fungicides, fungistats, biocides, biostats, bactericides and bacteriostats. 4. Woven material as claimed in any one of claims 1 to 3 characterised in that the warp and the weft each comprise polypropylene. 5. An air filter comprising first (21) and second (23) layers of a woven electrostatic material characterised in that said electrostatic material comprises an interwoven negatively or positively chargeable warp and an oppositely chargeable weft, said layers of woven electrostatic material being separated by an intermediate air permeable material (23) . 6. An air filter as claimed in Claim 5 characterised in that the or each layer of woven material and/or the intermediate material is or are impregnated with a microbiological growth control agent. 7. An air filter as claimed in Claim 5 or Claim 6 characterised in that the woven material comprises an anti-clogging means for filtering dust particles from the air flow path, and in that the air filter further comprises a collecting tray for collecting dust particles repelled from the electrostatic material. 8. An air filter as claimed in any one of Claims 5 to 7, characterised in that the intermediate air permeable material comprises an electrostatic medium. 9. An air filter as claimed in any one of Claims 5 to 8, characterised in that the intermediate material comprises woven polypropylene, open-cell polyurethane or polyester. 10. An air filter as claimed in any one of claims 5 to 9 characterised in that the layers of woven material and the intermediate air permeable material are housed in a retaining frame (10), whereby the filter may be installed in a bi-directional air flow system or without consideration to air flow direction in a uni-directional system so as to provide a reversible filter.

Inventors: FORDE GEORGE, FORDE, GEORGE
Applicants: E V E HOLDINGS LIMITED, FORDE GEORGE, E.V.E. HOLDINGS LIMITED
Application year: 1993
Main classification: B03C3
IPC codes: B01D39, B03C3