ELECTROSTATIC DUST COLLECTOR

Abstract: An electrostatic dust collector (2) comprises a strip (6) of pliable porous dielectric permitting gas passage and is provided with a polymer coated (24) electrode (7).

Description: The present invention relates to an electrostatic dust collector according to the preamble of claim 1 which is known from US-A-2 868 319. To eliminate microparticles floating in exhaust gases or the like, it is the commonest to use a filter device which permits the particles to pass through a porous member to collect them. This system, however, involves problems that if the bore of a particle collecting material is less than a predetermined value, it is hard to collect the particles, and that if the thickness of a collecting material is made larger, the resistance of fluids which pass therethrough increases to increase a pressure loss. There was a limit in dust collecting performance. On the other hand, for the purpose of eliminating air pollution by smoke or the like, an electrostatic dust collector has been used in which microparticles in gases to be eliminated are permitted to be charged in a corona discharge area, and gases are permitted pass through and between plates to which high voltage is applied to electrostatically adsorb the charged particles. This system has merits that microparticles having a diameter of approximately 0.1µ can be collected, and the pressure loss resulting from the dust collector is very small. However, this system is sufferred from disadvantages that a corona discharge section and a collecting section have to be provided resulting in complex construction; if the collecting performance is intended to be increased, high applied voltage has to be used or a voltage applied section has to be extended, in which case, however, concentration of electric fields on a raised portion of microparticles accumulated on the plates causes a discharge to again scatter the collected microparticles; and the device becomes large-scaled. There poses a further problem that if any of these devices are installed in an existing duct or the like, they have to be specially designed to meet the size and shape of the duct. Moreover, in such devices as described above, since the collecting plates are exposed, the durable period is extremely short if they are used in corrosive environments, thus failing to actually use them. Because of this, the devices have been difficult to be used not only in installations involving exhaust gases in boilers containing SO x but in hospitals, animal breeding farms and the like containing formalin. The present inventor has previously developed an electrostatic dust collector which has not been found in the past, wherein electrodes are disposed on opposite surfaces of a porous dielectric, an intense electric field is applied to the porous dielectric so that even particles having a particle size smaller than the bore of the porous dielectric may be collected, and a portion between electrodes is insulated by the dielectric to eliminate a danger of discharge resulting from accumulation of collected particles and the intense electric field can be applied. (Japanese Patent Laid-Open No. 19564/84). The present invention is an improvement over the aforesaid dust collector to make application thereof to various uses possible. The present invention provides an electrostatic dust collector including a gas inlet, a gas outlet, a filter element in a gas flow path between said gas inlet and gas outlet, said filter element comprising a narrow strip of a pliable porous dielectric having, on at least one surface thereof, a gas barrier high molecular layer, a metal strip electrode on said gas barrier layer, and a further gas barrier high molecular layer provided on said electrode, said filter element being arranged in said gas flow path so that gas to be treated passes through the porous dielectric of the filter in a direction substantially parallel to the electrode thereof, and means to apply a high voltage to said electrode. Fig. 1 is a sectional view of one embodiment of a dust collector according to the present invention; Figs. 2 and 3 are perspective views, respectively, showing a construction of a filter element; Fig. 4 illustrates a method for adhering a metal foil to a filter medium; Fig. 5 is a schematic view of a device for coating a paraffin to a metal foil; Fig. 6 is an overall view of a first embodiment of a dust collector for disorderly packing into a duct; Fig. 7 is a fragmentary perspective view of a filter element as claimed in EP-0 207 203; Fig. 8 is a fragmentary perspective view of another example of the filter element; Fig. 9 is a fragmentary perspective view of one example of a filter element used under the corrosive atmosphere; Fig. 10 is a fragmentary perspective view of one example of a filter element having a plurality of electrodes; and Fig. 11 illustrates the dust collecting state. The present invention will now be described in detail with reference to the accompanying drawings. Fig. 1 is a sectional view showing the conception of a dust collector in accordance with the present invention. A filter element 2 is disposed in the central portion of a casing 1. An intake flow A is drawn by a fan 3 and flows through an inlet 4. For reducing blinding of the filter element, a filter bag 5 is disposed to collect coarse dusts. The filter element 2 is manufactured by cylindrically winding, as shown in Fig. 3, a plurality of filter media formed with a metal film 7 such as Aℓ on one surface of a porous dielectric material 6 such as urethane foam as shown in Fig. 2, and a high voltage is applied between adjacent electrodes by a DC high voltage power source 8. A reference numeral 10 designates a support net for the filter element 2. In the arrangement as described above, microparticles such as dusts floating in the air stream A drawn through the inlet 4 are physically collected in narrow holes of the filter media and also electrostatically collected while being charged by slipping relative to the filter media which are dielectric. Therefore, since a relatively small thickness of the filter element will suffice, pressure loss can be minimized. Electrodes used to apply an electric field comprise the metal films 7 formed on the porous dielectric material 6, and the electrodes apart through the thickness of the dielectric material 6 are disposed merely by winding the metal film and therefore the spaced apart electrodes can be arranged very simply and held securely. Therefore, a uniform intense electric field can be formed within the filter element. A corona discharge section for charging dusts need not be provided, which has been necessary in conventional electrostatic dust collectors. The ratio between the length and diameter of the air stream passage is large and the charged particles are collected on passage walls by slight displacement of electrostatic attraction, and therefore the collecting efficiency is extremely good, and in addition, the portion between the electrodes is insulated by the dielectric material, and therefore no short-circuiting and discharge occur due to the accumulated dusts. Even if the short-circuiting and discharge should occur, microelectrode surfaces vaporise and the short-circuiting and discharge extinguish thus providing safety. Therefore, the thickness of the foamed dielectric material is made small and the spacing between electrodes is made small whereby an applying voltage can be reduced to about one-thirds of that of conventional electrostatic dust collectors. Preferably, this filter element is produced for example in the following procedure. In the embodiment shown in Fig. 4, an Aℓ foil 71 coated with paraffin is placed on one surface of a sheet 6 of dielectric foamed filter medium, heated by a heater 11 to a temperature at which the paraffin becomes molten, and lightly pressed to thereby bond them together. At that time, the width of the Aℓ foil is made slightly smaller than that of the filter medium sheet, leaving portions 61 to which electrode is not bonded on both sides. Thereafter, the filter medium 6 is cut along the center line thereof, and these are superposed each other and wound as shown in Fig. 2 to obtain the filter element 2. With this arrangement, end edges of electrodes adjacent to each other are mutually exposed to the reverse surface of the element 2, and therefore, it is convenient to provide a terminal 9 for aligning the side edges to apply a voltage to each of the electrodes. Fig. 5 illustrates one example of a device for coating paraffin on an Aℓ foil. The Aℓ foil drawn from an Aℓ foil supply roll 12 is preheated by a preheating fan 13 and thereafter comes into contact with a coating roll 16 which rotates within a paraffin bath 15 held at approximately 50°C by a heater 14. Then, the foil is entirely coated with a predetermined quantity of paraffin, cooled and solidified by a cooling fan 17 and wound onto a winding roll 18. Thereafter, the winding roll is adhered to the dielectric foamed filter medium as shown in Fig. 4. However, in the case where the manufacturing process is continuously carried out, the winding roll can be adhered to the filter medium immediately after paraffin has been coated and thereafter wound together with the filter medium. In this case, the cooling fan 17 and heater 9 can be omitted. Formation of electrodes on the dielectric foamed filter medium is not limited to the manner of the above-described embodiment but vacuum vaporisation can be employed. In this case, preferably the surface is treated to be smooth to facilitate vaporisation, and cellulose acetate or cellulose ethyl is coated by spraying or by a roll to the thickness of dozens of microns on the surface of the filter medium. After the coated film has been dried, Aℓ or Zn film is coated on the surface thereof by vaccum vaprisation. This dust collector has an extremely simple construction as described above and has realized a dust collector which has the merits obtained by a dust collector consisting of an electrostatic dust collector and a filter. Moreover, the filter media can be easily produced in volume as described above, and if the lowering in efficiency due to the blinding or the like should occurs, the filter medium may be exchanged simply to always maintain a high dust collecting efficiency. Because of low cost and low applied voltage, the present device can be used even in fields which have been impossible to apply the electrostatic dust collector in the past. For example, the device can be incorporated into an air heater for home use, a window fan and the like to collect pollen which causes asthma, dusts and the like to maintain the indoor clean, and besides, the device can be utilized as an air cleaner for home use which collects smoke of cigarettes. Fig. 6 shows one embodiment of an improved filter element. Reference numerals 6, 6' and 19 designate porous dielectric materials formed of urethane foam or the like, which are in the form of a narrow strip having a suitable width. To one surface of the materials 6, 6' is adhered Aℓ foils 7, 7' narrower than the dielectric material by paraffin, adhesives and the like as described above, which form electrodes. Three dielectric materials 6, 6' and 19 are adhered so that the electrodes may not be exposed outside to form a filter element. This lengthy filter element 2 is disorderly forced into, for example, an insulating bag 20 such as a nylon net and forced into a duct 21, as shown in Fig. 7, as claimed in EP-0 207 203. A reference numeral 22 designates a high voltage power source. Since the filter element 2 is pliable and has a moderate elasticity, as described above, the filter element is wholly spread inside the duct 21 to cover the entire section irrespective of the size and shape in section of the duct 21. Under the aforesaid condition, when a high voltage is applied by the high voltage power source 22 to the electrodes 7, 7′, the filter element 2 exhibits a great dust collecting performance with less pressure loss different from a mere filter. That is, the filter elements 2 disorderly forced into the net bag 20 are porous themselves and can form a flowpassage for exhaust gases and in addition, form disorderly bended clearances between the intertwined elements 2 to impart only a relatively small resistance to an exhaust stream flowing through the clearances, and thus pressure loss of exhaust is small. However, the dusts contained in the exhaust come into contact with the porous dielectric material forming walls of a narrow and bended passageway and are mechanically collected, charged by the adherence of ions created due to the slipping or a high voltage between electrodes, and collected and retained by the porous dielectric material by the electric field formed between electrodes. As is known, the electric field formed between electrodes is produced not only in portions where the electrodes are opposed each other but bulges towards both sides thereof and also greatly bulges externally of the porous dielectric material. Therefore, as shown in Fig. 8, the electrodes can be copper wires 23, 23′ instead of foils. With the above-described construction, this embodiment has the following characteristics: (1) In the present invention, the filter element 3 is merely readily forced into the bag and spread fully over the passage for gases to be dust-eliminated such as an exhaust duct by its own resilient force. Therefore, the dust collector can be easily installed irrespective of the size and section of the existing gas passage. (2) Since both sides of each of the electrodes are covered with the porous dielectric material, even if the filter element 3 is disorderly forced, there occurs no possible short-circuiting between the electrodes, and handling thereof is extremely easy. (3) Despite that the filter element is forced in a complicated shape, it is in the form of a narrow strip, and therefore, one terminal for application of voltage will suffice and thus the whole construction of the device is extremely simple. (4) Since air can pass through the disordery spaces of the element 3, the resistance is small and the pressure loss is extremely small. Fig. 9 shows an embodiment which is used for gases containing corrosive components. Electrodes 7, 7′ are provided on porous dielectric materials 6, 6′ in a manner similar to that as described in connection with Fig. 4. Paraffin coated on the electrodes 7, 7′ forms a protective layer to prevent the Aℓ foil of electrode from direct exposure to treated gases. However, if this is not sufficient, cellulose acetate or cellulose ethyl is applied by spraying or roll to surfaces 62, 62′ of the narrow strips of the porous dielectric materials 6, 6′ to further complete gas cut-off. As described above, polystyrene liquid is coated by spraying or roll on the narrow strips 6, 6′ formed with electrodes to form films 24, 24′ to provide a complete bag-like cover to thereby prevent the electrodes 7, 7′ from direct contact with the treated gas. The aforesaid narrow strips 6, 6′ are superposed to be wound into a disc-like configuration as shown in Fig. 3 or fully forced into the duct disorderly as shown in Fig. 7 and a high voltage is applied between the electrodes 3 and 4 whereby microparticles in gases passing through the element 2 can be collected in the porous dielectric. Also, terminal portions of lead electrodes can be molded by heating them at a low temperature by use of paraffin after lead wires have been fixed to easily interrupt contact thereof with exhaust gases. In this embodiment, a unique construction in which electrodes are provided on the dielectric can be utilized to easily form gas barrier covers on both surfaces of electrodes to completely prevent the lowering of a dust collecting performance due to the corrosion of electrodes. Furthermore, the filter element can be easily produced continuously from inexpensive materials such as urethane foam and can be of disposable type, and therefore, the filter element is suitable for eliminating gases containing corrosive components which are troublesome in treatment after collection. Therefore, the device according to the present invention is suitable for use as a dust collector in facilities such as hospitals, animal breeding farms and the like which were not able to find suitable devices despite the fact that the necessity of such provisions has been recognized. Fig. 10 shows an embodiment in which a plurality of electrodes are provided on narrow strips of porous dielectric to thereby enhance the collecting performance of microparticles. Aℓ foils 72, 73; 72′, 73′ having a width of approximately 10 mm are attached at intervals of approximately 10 mm to one surface of a urethane foam having a thickness of approximately 10 mm and a width of approximately 50 mm, a filter is wound thereon, said filter having narrow strips 6, 6′ superposed thereon formed with films 24, 24′ by spraying polystyrene liquid to form a disc-like filter element 2 as shown in Fig. 3, and lead electrodes 9, 9′ are connected to the electrodes 72, 73 and 72′, 73′ of the narrow strips 6, 6′, respectively. The electrodes 72, 73 and 72′, 73′ can be of the same polarity or opposite polarity, and if the same polarity is employed, the construction of the lead electrodes becomes simple. Gases containing microparticles such as smoke is permitted to flow in a vertical direction relative to the disc of the filter element and a DC voltage of a few KV is applied between the lead electrodes to measure the collection rate of microparticles in the gases. The results obtained therefrom is as follows: Particle Size µ Collection Rate 0.3 above 80 % (According to to the calculation method) It has been found from the section of the element that as shown in Fig. 11, the microparticles are most materially collected on the electrode end B on the gas inlet side and the high rate next thereto is obtained at the second electrode end C. Electrostatic adsorption requires the width of an electric field enough to receive an electrostatic force during the time the microparticles reach the collection surface and at the same time, needless to say, the intenser electric field, the higher the collection effect is obtained. According to the present invention, the electrodes 3, 4, 5 and 6 comprise foils which have a predetermined width, and lines of electric force are concentrated at the end edges of the electrodes by the edge effect as is well known, at which the high collection efficiency is exhibited. Since the end edges of the electrodes are present in both edges of the plurality of electrodes, portions where the collection performance is high appear through magnification of electrodes, and the collection performance as a whole seems to be increased.

Claims: An electrostatic dust collector including a gas inlet (4), a gas outlet, a filter element (2) in a gas flow path between said gas inlet and gas outlet, said filter element comprising a narrow strip of a pliable porous dielectric (6) and a metal strip electrode (7) thereon, and means (8) to apply a high voltage to said electrode, characterised in that said strip of a pliable porous dielectric (6) having on at least one surface thereof, a gas barrier high molecular layer, said metal strip electrode (7) on said gas barrier layer, and a further gas barrier high molecular layer provided on said electrode, said filter element (2) being arranged in said gas flow path so that gas to be treated passes through the porous dielectric (6) of the filter in a direction substantially parallel to the electrode thereof. An electrostatic dust collector according to Claim 1, wherein said electrode comprises a metal film, subjected to gas barrier coating, adhered onto the porous dielectric. An electrostatic dust collector according to Claim 1, wherein said electrode comprises a gas barrier film on the surface of the porous dielectric, a metal electrode vaporized onto said film, and a gas barrier film coated thereon. An electrostatic dust collector according to any one of Claims 1 - 3, wherein said narrow strip is wound into a disc-like configuration.

Inventors: YANAGAWA, MOTOO
Applicants: ISHIMORI & CO., LTD., RESEARCH DEVELOPMENT CORPORATION OF JAPAN
Application year: 1985
Main classification: B03C3
IPC codes: B03C3