US 3798879 A
A filter, for purifying air at least in part by electrostatic collection of particulates contained therein, has at least one charged gas-permeable filter surface extending transversely to the gas stream and designed mechanically to retain the particles by interposing a surface in the path of the gas. The electrostatic field is applied in the direction of movement of the gas stream. The particles may be charged by an ionizing device at the upstream side of the apparatus.
Description (OCR text may contain errors)
United States Patent [191 Schmidt-Burbach et a1.
[ AIR FILTER WITH ELECTROSTATIC PARTICLE COLLECTION  Inventors: Gerhard Schmidt-Burbach, Wetzlar;
Anton L. Jung, Herborn; Helmut Hubert, Erda; Manfred Lapczyna, Wetzlar; Erhard Ledwon, Guntersdorf; Wilfried Rock, Ganderkesee; Heinz Strauss, Lohnberg, all of Germany  Assignee: Buderussche Eisenwerke, Wetzlar,
Germany  Filed: Nov. 29, 1971  Appl. No.: 202,747
 Foreign Application Priority Data Nov. 28, 1970 Germany 2058685 May 21, 1971 Germany 2125234  U.S. Cl 55/102, 55/120, 55/131, 55/138, 55/146, 55/151, 55/155, 55/302,
 Int. Cl. B03c 3/38  Field of Search 55/102, 131, 132, 154, 55/155,136,137,138,120,146,151, 302,
 References Cited UNITED STATES PATENTS 1,381,660 6/1921 Rathbun 55/138 X Mar. 26, 1974 1,931,436 10/1933 Deutsch 55/131 2,067,822 1/1937 Biederman 128/146 2,579,441 12/1951 Palmer 55/131 2,593,377 4/1952 Wintermute.. 55/131 X 2,593,869 4/1952 Fruth 55/102 2,616,165 11/1952 Brennan. 29/195 UX 2,908,347 10/1959 Roos 317/261 X 2,973,054 2/1961 Kurtz 55/131 3,114,877 12/1963 Dunham 73/28 3,128,378 4/1964 Allen et al. 250/43 3,403,252 9/1968 Nagy 21/74 X FOREIGN PATENTS OR APPLICATIONS 931,625 7/1963 Great Britain 55/139 44-13477 6/1969 Japan 55/131 Primary ExaminerDennis E. Talbert, Jr. Attorney, Agent, or Firml(arl F. Ross; Herbert Dubno 5 7] ABSTRACT A filter, for purifying air at least in part by electrostaticcollection of particulates contained therein, has at least one charged gas-permeable filter surface extending transversely to the gas stream and designed mechanically to retain the particles by interposing a surface in the path of the gas. The electrostatic field is applied in the direction of movement of the gas stream. The particles may be charged by an ionizing device at the upstream side of the apparatus.
4 Claims, 6 Drawing Figures AIR FILTER WITH ELECTROSTATIC PARTICLE COLLECTION FIELD OF THE INVENTION Our present invention relates to an apparatus for purifying air by the filtration of particulates therefrom and, more particularly, to a gas-purifying filter operating at least in part by electrostatic collection of particulates.
BACKGROUND OF THE INVENTION While numerous techniques for removing particulates from fluid streams have been proposed heretofore, considerable research is expended continuously on devising more economical, efficient and simple means for removing particles from gas streams and especially particulates from air. Filters have been made in the past in a variety of configurations and designs, operating by mechanical or electrical entrapment of particles contained in the air stream. For example, a nonwoven or woven fibrous or filamentary web may be provided across the air flow for traversal thereby to trap dust particles and particles in the greater-thanmicron range.
As is well known, such filters operate by mechanical interception of the particles, i.e., having a pore size which governs the size of the particles which can be intercepted by a filter. Smaller particles traverse the filter and attempts to reduce the pore size lead only to high pressure drops and increasing impediment to gas flow.
Electrostatic precipitators have been proposed to alleviate this problem, the gas stream being passed through channels defined by oppositely charged plates, wires or screens, the latter representing collector electrodes which lie in arrays parallel to the direction of flow of the gas. The gas stream is subjected to a charging field, e.g., a corona discharge, and charged particles are accumulated by absorption or dipole association with the dust particles in the gas stream. The electrically charged dust particles then transfer charge to the collector electrode to which they adhere.
While electrostatic precipitators provide substantially less of an obstruction to the gas stream and hence operate with less pressure drop than a throughflow filter of the same dust-removal capacity, some difficulties are encountered. Firstly, the particles collected on the electrode are held only by mechanical forces and frequently are dislodged or entrained by the gases passing along the electrode parallel thereto. As a result, particulates which originally were precipitated electrostatically may eventually reach the outlet. Furthermore, the collector electrodes become contaminated readily by grease particles or the like, which cannot be readily removed and which prevent rappers or the like from dislodging the particles collected on the electrodes. Also electrostatic precipitators, in which the electrode arrays run parallel to the direction of flow of the gas stream, frequently require extremely high potentials or potential gradients to effectively collect the contaminants contained in the gas stream.
OBJECTS OF THE INVENTION It is the principal object of the present invention to provide an improved air filter adapted to avoid the aforementioned disadvantages and drawbacks.
Another object of the invention is to provide a compact high-frequency filter requiring lower electrostatic potential gradients than conventional electrostatic precipitators while having an increased efficiency and reduced tendency to pass small particles.
SUMMARY OF THE INVENTION These objects and other which will become apparent hereinafter, are attained in accordance with the present invention which provides in the path of a particulatecarrying gas stream, usually air, at least one web, mat or layer of relatively tightly-packed conductive fibers forming a collector electrode for charged particles carried by the gas stream and means for applying in the region of this layer an electrostatic potential gradient in the direction of fluid flow. By contrast with prior art systems in which the collector electrodes were constitued by arrays of rods, wires or bars parallel to the direction of fluid flow, therefore, the present invention provides a mechanical entrapment filter directly across and at right angles to the path of the gases, i.e., a layer which is perpendicularly traversed by the gases and which is capable of mechanically trapping particles contained therein. In addition, the filter layer is made up of electrically conductive filaments or at least filaments, fibers or strands of such conductivity that the electrostatic particles collecting potential can be applied across this electrode-filter element and a counterelectrode spaced therefrom in the direction of flow of the gas. It will be apparent that this arrangement prevents particles which are gathered in the layer from being entrained by gas even after the particle has lost its charge.
The electrically conductive filter mats, according to the present invention, are conductively tied to the electrostatic potential source and may include two spacedapart but parallel mats forming the two electrodes of the system. The, or each, mat can be made simply and inexpensively from filamentary materials or synthetic resin or glass fibers to which sufficient metal particles or carbon have been added to provide a suitable electrical conductivity, or with which metal fibers have been intercollated so that the mat as a whole may be metallized by galvanic or chemical metal plating. The filter mat may be of the nonwoven or random-fiber type or may be woven or knitted, e.g., from steel wire in a mesh or interstitial-opening size sufficiently small as to prevent particles with a diameter of 1 micron from passing through. When such mats are electrostatically charged, they are found to be able to collect particles with a size substantially smaller than 1 micron.
According to another feature of the invention, upstream of the, or both conductive filter mats, there is provided a device for ionizing the air, preferably a B-ray generator, or a corona-discharge and/or ultraviolet ionization device. Best results are obtained with a U.V. lamp having corona-discharge wires extending therefrom and across which a high potential is applied.
Among the advantages of the present system are the following:
Firstly, the filter material, mat or web is relatively inexpensive and can constitute a reusable or disposable mechanical filter of high efficiency. Since either metal fibers or metallized nonconductive fibers may be used, a wide variety of materials can be employed and the costs can be minimized. The filter mats can be formed by conventional procedures at a small cost above that of fabric filter layers, a vast improvement in filter efficiency and a considerable reduction in the particle size of the substances traversing the filter may be obtained.
Secondly, because of the increased efficiency of the electrostaticallycharged filter mat as a particle collector over the particle-collecting ability of a nonconductive or uncharged filter mat, the pressure drop of the air traversing the filter can be reduced so that the filter may have less resistance. A simpler blower or fan may thus be used in conjunction with the device.
Thirdly, the electrostatic potential gradient is applied in the direction of gas flow and does not have to resist or interfere with the inertia or kinetic energy of the moving particles and thus can be reduced. Lowpotential sources such as low-capacity generators, distribution circuits and the like may be employed.
Fourthly, the air filter has been found to be particularly suitable for the removing of particles in the physiologically interesting range of one micron and therebelow. In the event of failure of the electrostatic source, the conductive mats remain nevertheless effective to a limited extent.
In addition, cleaning of the filter by backflow of air is possible and there is little danger that, in the event of an accident, particles (e.g., micro-organisms) or the like will be carried through the filter.
DESCRIPTION OF THE DRAWING The above and other objects, features and advantages of the present invention will become more readily apparent from the following description, reference being made to the accompanying drawing, in which:
FIG. 1 is an axial cross-sectional view of an air filter embodying the present invention;
FIG. 2 is a view similar to FIG. 1 illustrating a modification;
FIG. 3 is another axial cross-sectional view showing a feature of the present invention;
FIG. 4 is an enlarged diagrammatic cross-sectional view through a portion of a fiber mat according to the invention;
FIG. 5 is an elevational view of a portion of the mat;
FIG. 6 is a view similar to FIG. 4, illustrating another embodiment of the mat.
SPECIFIC DESCRIPTION From FIG. 1, it will be apparent that an air filter-according to the present invention comprises a duct 1 having an inlet la and an outlet lb, at least one of which may be provided with a fan, blower or other conventional mechanism for displacing the air through the system. The housing 1 may be flanged at 10 to enable removal of a filter cartridge generally represented at 2 and comprising a pair of conductive fiber mats 3 of steel wool pressed into disk configuration and received in frames 3a and 3b. As is also apparent from FIG. 1, an electrostatic potential source having a voltage of, say, 100 300 volts do. is provided at 10 and has its positive and negative terminal connected to the mats 3 through insulators l1 and 12. The frames 3a and 3b are spaced apart by an insulating ring 4. It will be apparent that charged particles traversing the filter 2,. 3, 3a, 3b and 4, will be mechanically trapped on the mats 3 and electrostatically trapped if negatively and positively charged in accordance with principles of electrostatic precipitators. Upstream of the cartridge 2, which may be disposed of when fully contaminated, an ionization source may be provided as described in connection with FIG. 3.
In FIG. 2, we show an arrangement wherein the housing 13 is formed at its upstream side with an ultraviolet lamp 5 whose positive and negative terminals extend from the duct and which is provided with coronadischarge wires 6 connected to a high negative potential. The positive terminal of the electrostatic field generator is provided in the woven steel wire mat 14 in its insulating frame 15.
FIG. 3 illustrates yet another modification of the invention therein the upstream perforated disk 20 is constituted as a B-particle emitter by the incorporation of radioactive material therein. The downstream electrode 21 is composed of a sintered metal-wire mat in which the filaments are fused together at their contact points to produce a rigid body. As is also apparent from FIG. 3, the electrostatic potential source is shown to be a pair of terminals 23 connected across the normal alternating current supply line 110, 220, 440, etc., volts a.c. in circuit with a switch 24 and a full-wave rectifier bridge 25. A simple propeller-type fan 26 is mounted in the downstream side of the housing and is reversible to induce a flow of air through the system. For normal operation, the butterfly flap 27 of the inlet 28 of housing 29 is opened and flap 3% is closed so that air flows from the inlet 28 to the outlet 31 with normal filtration. When switch 24 is opened, motor 26 reversed, flap 30 opened and flap 27 closed, a cleaning flow of air in the opposite sense carries particles collected on the electrode 21 through the branch 32 and into a collecting vessel.
In FIG. 4, we have shown a metallized web which may be used according to the present invention and which is woven from glass fibers 40 provided with nickel coatings 41. As illustrated in FIG. 5, the resulting web has the appearance of a tightly woven fabric layer 42.
In FIG. 6, it can be seen that the randomly packed fiber mat 50 is composed of a mixture of filaments 51 of synthetic resin coated with metal and filaments 52 of the uncoated synthetic resin. It is thus possible to control the conductivity of the mat as a whole.
1. A filter comprising conduit means for conducting a stream of air from an inlet side to an outlet side; at least one electrically conductive filter of metallic fibers mat spanning said path and perpendicular thereto while traversed by the gas stream; a counterelectrode in said conduit means upstream of said metallic fiber mat; means for generating a potential difference parallel to said gas stream between said electrodes in the region of said mat to cause particles in said gas stream to be drawn electrostatically to said mat; and an ultraviolet source in said conduit means upstream of said counterelectrode for generating ions in said gas stream, said counterelectrode including wires extending generally parallel to the direction of flow of said gas stream and reaching to said ultraviolet source.
2. The filter defined in claim 1 wherein said mat consists at least in part of electrically nonconductive fibers and electrically conductive fibers mixed therewith.
3. The filter defined in claim 1 wherein said mat is composed of electrically nonconductive fibers provided with a conductive metal coating.
4. The filter defined in claim 1 wherein said mat is composed of a steel-wire woven fabric.
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