AU2005264525B2

AU2005264525B2 - Electric dust collecting apparatus and electric dust collecting system

Info

Publication number: AU2005264525B2
Application number: AU2005264525A
Authority: AU
Inventors: Hiroshi Hosono; Atsushi Katatani; Hikaru Murata
Original assignee: Panasonic Corp
Current assignee: Panasonic Corp
Priority date: 2004-07-23
Filing date: 2005-07-21
Publication date: 2010-06-24
Anticipated expiration: 2025-07-21
Also published as: ES2338837A1; WO2006009187A1; CN101014414A; CN101014414B; ES2338837B1; AU2005264525A1; JP4875983B2; JPWO2006009187A1

Description

[Name of Document] SPECIFICATION [Title of the Invention] ELECTRIC DUST COLLECTING APPARATUS AND ELECTRIC DUST COLLECTING SYSTEM [Technical Field] [0001] The present invention relates to an electric dust collecting apparatus and an electric dust collecting system for collecting generated dust. [Background Technique] [0002] At present, as a dust collecting apparatus for ventilating a tunnel, there is employed an electric dust collecting apparatus which gives electric charge to dust to collect dust generated in the tunnel. In the electric dust collecting apparatus having such uses, a discharge wire is mainly used as a discharge electrode of an electrically charging portion. If the discharge wire is used for a long term, the discharge wire is snapped in some cases. Therefore, minus polarity (discharge electrode is lower than grounding polarity in potential) having a long lifetime of the discharge wire is employed as voltage polarity applied to the discharge wire in many cases. In such a situation, even if the minus polarity is employed as the discharge wire, there still remains a possibility of snapping of the wire, there is a tendency to use a discharge electrode plate provided at its end surface with a plurality of projections which are not snapped, and there are proposed the discharge electrode plate provided at its end surface with the plurality of projections, and a dust collecting apparatus utilizing such a discharge electrode plate (e.g., patent documents 1 to 6). There is an electric dust collecting system (patent document 7) in which a plurality of dust collecting units each having an electrically charging portion and a dust collecting section are disposed to form a duct collecting block, a plurality of duct collecting blocks are provided, the duct collecting blocks are divided into plus discharge blocks and minus discharge blocks such that a difference between a quantity of processed air of the plus discharge block and a quantity of processed air of the minus discharge block becomes small and with this, dusts which were not collected by the dust collecting section are neutralized without deteriorating the dust collecting function, it is possible to prevent charged particles from adhering to a wall surface and contaminating the wall surface. [Patent document 1] Japanese Utility Model Application Laid-open No. S61-200146 [Patent document 2] Japanese Utility Model Application Laid-open No.H6-41849 [Patent document 3] Japanese Patent Application Laid-open No . H3-232554 [Patent document 4] Japanese Patent Application Laid-open No. H9-323048 [Patent document 5] Japanese Patent Application Laid-open No. H10 -28897 [Patent document 6] Japanese Patent Application Laid-open No.2000-126647 [Patent document 7] Japanese Patent Application Laid-open No.H2003-260383 [Disclosure of the Invention] [Problem to be Solved by the Invention] [0003] The electric dust collecting apparatus utilizes corona discharge in an electrically charging portion, and harmful ozone is generated as by-product. Since nitric oxides exist in volume in a tunnel, there is a problem that nitric oxide is oxidized and harmful nitrogen dioxides are increased. In recent years, concerns are rising that social citizens are adversely influenced by ozone or nitrogen dioxide discharged out from a tunnel, and an electric dust collecting apparatus which suppresses the increase of ozone or nitrogen dioxide is desired. Generally, more ozone is generated as the corona discharge current is increased. The generated ozone is consumed for oxidization of nitric oxide included in the air, and the ozone is gradually reduced with time. On the other hand nitrogen dioxide is increased with time. Therefore, the ratio of ozone and nitrogen dioxide is changed with time, but the sum thereof is substantially constant. That is, the sum of the increase amount of ozone and nitrogen dioxide which are increased under the lee of the electric dust collecting apparatus can be the as being a by-productive ozone amount produced by the electric dust collecting apparatus accessorily. When the electric dust collecting apparatuses have the same shapes and the consumed electricity values are also the same, the dust collecting efficiencies are substantially the same even in plus discharge and minus discharge, but the by-productive ozone amount is varied depending upon the discharge polarity. When a discharge wire is used as the discharge electrode, the by-productive ozone amount of the minus polarity is as many as about five to ten times of the by-productive ozone amount of the plus polarity per a unit consumed electricity. The by-productive ozone amount per a unit quantity of air and per a unit consumed electricity when 8 kV voltage is applied as plus polarity is 0.010 ppm, and the by-productive ozone amount peraunit quantityair andper aunit consumed electricity when 8 kV voltage is applied as minus polarity is 0.049 ppm. The by-productive ozone amount per a unit quantity of air and per a unit consumed electricity when 9.5 kV voltage is applied as plus polarity is 0.005 ppm, and the by-productive ozone amount per a unit quantity of air and per a unit consumed electricity when 9.5 kV of voltage is applied as the minus polarity is 0. 043 ppm. The by-productive ozone amount per a unit quantity of air and per a unit consumed electricity when 11 kV of voltage is applied as plus polarity is 0.006 ppm, and the by-productive ozone amount per a unit quantity of air and per a unit consumed electricity when 11 kV of voltage is applied as minus polarity is 0.037 ppm. The by-productive 5 ozone amount per a unit quantity of air and per a unit consumed electricity when 12.5 kV of voltage is applied as plus polarity is 0.004 ppm, and the by-productive ozone amount per a unit quantity of air and per a unit consumed electricity when 12.5 kV of voltage is applied as minus polarity is 0.034 ppm. If the lifetime of the discharge wire is taken into account, the minus 10 polarity is superior, but if the by-productive ozone amount is taken into account, plus polarity is superior. When a discharge electrode plate provided at its end surface with a plurality of projections is used, difference of by-productive ozone amount caused by voltage polarity or shape is not clearly confirmed. is [0004] Hence, the aspects of the present invention aim to provide an electric dust collecting apparatus and a discharge electrode plate for the electric dust collecting apparatus having a high dust collecting ability and in which the by-productive ozone amount is suppressed, using a discharge electrode plate which is not 20 snapped. The aspects of the invention also aim to provide an electric dust collecting system having a high dust collecting ability and in which the by-productive ozone amount is suppressed, when the electric dust collecting system in which dust which was not collected by a dust collecting section is neutralized, adhesion and 25 contamination of charged particles on a wall surface is prevented. [0005] 30 35 -4- A first aspect of the invention provides an electric dust collecting apparatus comprising a discharge electrode plate provided at its end surface with a plurality of projections each having a sharp tip end, a grounding plate disposed in parallel to the discharge electrode plate, and a high voltage power supply which applies corona discharge voltage between the discharge electrode plate and the grounding plate, in which intensity of an electric field formed between the discharge electrode plate and the grounding plate is set to about 0.67kV/mmtoO.8kV/mm, atipendangleof theprojectionof thedischarge electrode plate is set to about 100 to 400, voltage of about 8 kV to 12 kV is applied from the high voltage power supply such that potential of the discharge electrode plate becomes higher than potential of the grounding plate, and particle material in the air is charged, thereby adhering the particle material to a dust collecting section, wherein under the same voltage, a projection distance between the tip ends of adjacent projections is set such that a by-productive ozone amount becomes lower when electricity is supplied from the high voltage power supply such that potential of the discharge electrode plate becomes higher than potential of the grounding plate as compared with a case in which electricity is supplied from the high voltage power supply such that potential of the discharge electrode plate becomes lower than potential of the grounding plate. According to a second aspect, in the first aspect, the tip end angle of the projection of the discharge electrode plate is set to about 200 to 400, and the projection distance between the tip ends of adjacent projections is set to 12 mm or more. According to a third aspect, in the first aspect, the tip end angle of the projection of the discharge electrode plate is set to about 200, voltage of the high voltage power supply is set to about 9 kV to 12 kV, and the projection distance between the tip ends of adjacent projections is set to 8 mm or more. According to a fourth aspect, in the second aspect, the tip end angle of the projection of the discharge electrode plate is set to about 20* to 300 , and voltage of the high voltage power supply is set to about 9 kV to 12 kV. A fifth aspect of the invention provides an electric dust collecting system having a plurality of electric dust collecting apparatuses each comprising a discharge electrode plate provided at its end surface with a plurality of projections each having a sharp tip end, a grounding plate disposed in parallel to the discharge electrode plate, and a high voltage power supply which applies corona discharge voltage between the discharge electrode plate and the grounding plate, in which intensity of an electric field formed between the discharge electrode plate and the grounding plate is set to about 0 .67 kV/mm to 0.8 kV/mm, a tip end angle of the projection of the discharge electrode plate is set to about 100 to 400 , voltage of about 8 kV to 12 kV is applied from the high voltage power supply, and particle material in the air is charged, thereby adhering the particle material to a dust collecting section, wherein voltage is applied, such that potential of the discharge electrode plate becomes higher than potential of the grounding plate, to one or more of the electric dust collecting apparatuses in which under the same voltage, a projection distance between the tip ends of adjacent projections is set such that a by-productive ozone amount becomes lower when electricity is supplied from the high voltage power supply such that potential of the discharge electrode plate becomes higher than potential of the grounding plate as compared with a case in which electricity is supplied from the high voltage power supply such that potential of the discharge electrode plate becomes lower than potential of the grounding plate, and voltage is applied, such that potential of the discharge electrode plate becomes lower than potential of the grounding plate, to one or more of the electric dust collecting apparatuses in which under the same voltage, a projection distance between the tip ends of adjacent projections is set such that a by-productive ozone amount becomes lower when electricity is supplied from the high voltage power supply such that potential of the discharge electrode plate becomes lower than potential of the grounding plate as compared with a case in which electricity is supplied from the high voltage power supply such that potential of the discharge electrode plate becomes higher than potential of the grounding plate. A sixth aspect of the invention provides an electric dust collecting system having a plurality of electric dust collecting apparatuses each comprising a discharge electrode plate provided at its end surface with a plurality of projections each having a sharp tip end, a grounding plate disposed in parallel to the discharge electrode plate, and a high voltage power supply which applies corona discharge voltage between the discharge electrode plate and the grounding plate, in which intensity of an electric field formed between the discharge electrode plate and the grounding plate is set to about 0.67 kV/mmto O.8 kV/m, a tip end angle of the projection of the discharge electrode plate is set to about 100 to 400 , a projection distance between tip ends of adjacent projections is set to 4 mm or more, voltage of about 8 kV to 12 kV is applied from the high voltage power supply, and particle material in the air is charged, thereby adhering the particle material to a dust collecting section, wherein voltage is applied, such that potential of the discharge electrode plate becomes higher than potential of the grounding plate, to one or more of the electric dust collecting apparatuses in which under the same voltage, a tip end angle of the projection is set such that a by-productive ozone amount becomes lower when electricity is supplied from the high voltage power supply such that potential of the discharge electrode plate becomes higher than potential of the grounding plate as compared with a case in which electricity is supplied from the high voltage power supply such that potential of the discharge electrode plate becomes lower than potential of the grounding plate, and voltage is applied, such that potential of the discharge electrode plate becomes lower than potential of the grounding plate, to one or more of the electric dust collecting apparatuses in which under the same voltage, a tip end angle of the projection is set such that a by-productive ozone amount becomes lower when electricity is supplied from the high voltage power supply such that potential of the discharge electrode plate becomes lower than potential of the grounding plate as compared with a case in which electricity is supplied from the high voltage power supply such that potential of the discharge electrode plate becomes higher than potential of the grounding plate. A seventh aspect of the invention provides an electric dust collecting system having a plurality of electric dust collecting apparatuses each comprising a discharge electrode plate provided at its end surface with a plurality of projections each having a sharp tip end, a grounding plate disposed in parallel to the discharge electrode plate, and a high voltage power supply which applies corona discharge voltage between the discharge electrode plate and the grounding plate, in which intensity of an electric field formed between the discharge electrode plate and the grounding plate is set to about 0 .67 kV/mm to 0.8 kV/mm, a tip end angle of the projection of the discharge electrode plate is set to about 10' to 400 , a projection distance between tip ends of adjacent projections is set to 4 mm or more, voltage of about 8 kV to 12 kV is applied from the high voltage power supply, and particle material in the air is charged, thereby adhering the particle material to a dust collecting section, wherein voltage is applied, such that potential of the discharge electrode plate becomes higher than potential of the grounding plate, to one or more of the electric dust collecting apparatuses in which under the same voltage, a by-productive ozone amount is set lower when electricity is supplied from the high voltage power 5 supply such that potential of the discharge electrode plate becomes higher than potential of the grounding plate as compared with a case in which electricity is supplied from the high voltage power supply such that potential of the discharge electrode plate becomes lower than potential of the grounding plate, and voltage is applied, such that potential of the discharge electrode plate becomes 10 lower than potential of the grounding plate, to one or more of the electric dust collecting apparatuses in which under the same voltage, a by-productive ozone amount is set lower when electricity is supplied from the high voltage power supply such that potential of the discharge electrode plate becomes lower than potential of the grounding plate as compared with a case in which electricity is is supplied from the high voltage power supply such that potential of the discharge electrode plate becomes higher than potential of the grounding plate. [Effect of the Invention] [0006] According to aspects of the present invention, it is possible to provide an 20 electric dust collecting apparatus and a discharge electrode plate for the electric dust collecting apparatus in which an amount of generated ozone and an increase amount of nitrogen dioxide are small, and high dust collecting ability can be exhibited. According to aspects of the invention, when a discharge electrode plate for 25 an electric dust collecting apparatus in which a tip end angle of a projection is set to about 100 to 400 and a projection distance between tip ends of adjacent projections is set to 4 mm or more is 30 -9used, it is possible to appropriately set whether plus discharge or minus discharge such that the by-productive ozone amount is small, and to supply electricity. [Brief Description of the Drawings] [0007] Fig. 1 is a perspective view showing an electric dust collecting apparatus of an embodiment; Fig. 2 is a plan view showing a structure of an electrically charging portion of the electric dust collecting apparatus of the embodiment; Fig. 3 is a side view showing a structure of a discharge electrode plate of the electric dust collecting apparatus of the embodiment; Fig. 4 is a graph showing a relation between a distance between projections of the electric dust collecting apparatus of the embodiment and a by-productive ozone amount; Fig. 5 is a graph showing the relation between the distance between projections of the electric dust collecting apparatus of the embodiment and a by-productive ozone amount; Fig. 6 is a graph showing the relation between the distance between projections of the electric dust collecting apparatus of the embodiment and a by-productive ozone amount; Fig. 7 is a graph showing the relation between the distance between projections of the electric dust collecting apparatus of the embodiment and a by-productive ozone amount; Fig. 8 is a graph showing the relation between the distance between projections of the electric dust collecting apparatus of the embodiment and a by-productive ozone amount; Fig. 9 is a graph showing the relation between the distance between projections of the electric dust collecting apparatus of the embodiment and a by-productive ozone amount; Fig. 10 is a graph showing the relation between the distance between projections of the electric dust collecting apparatus of the embodiment and a by-productive ozone amount; Fig. 11 is a graph showing the relation between the distance between projections of the electric dust collecting apparatus of the embodiment and a by-productive ozone amount; Fig. 12 is a graph showing the relation between the distance between projections of the electric dust collecting apparatus of the embodiment and a by-productive ozone amount; Fig. 13 is a graph showing the relation between the distance between projections of the electric dust collecting apparatus of the embodiment and a by-productive ozone amount; Fig. 14 is a graph showing the relation between the distance between projections of the electric dust collecting apparatus of the embodiment and a by-productive ozone amount; Fig. 15 is a graph showing the relation between the distance between projections of the electric dust collecting apparatus of the embodiment and a by-productive ozone amount; Fig. 16 is a graph showing the relation between the distance between projections of the electric dust collecting apparatus of the embodiment and a by-productive ozone amount; Fig. 17 is a graph showing the relation between the distance between projections of the electric dust collecting apparatus of the embodiment and a by-productive ozone amount; and Fig. 18 is a graph showing the relation between the distance between projections of the electric dust collecting apparatus of the embodiment and a by-productive ozone amount. [Explanation of Symbols] [0008] 10 projection 50 electric dust collecting apparatus 51A, 51B high voltage power supply 52 electrically charging portion 52A discharge electrode plate 52B grounding plate 53 dust collecting section 53A charged plate 53B dust collecting plate [Best Mode for Carrying Out the Invention] [0009] According to an electric dust collecting apparatus of a first aspect of the present invention, the intensity of an electric field formed between a discharge electrode plate and a grounding plate is set to a value in a range of about 0.67 kV/mm to 0.8 kV/mm, a tip end angle of each of projections of the discharge electrode plate is set to a value in a range of about 100 to 400, voltage of about 8 kV to 12 kV is applied from a high voltage power supply such that the discharge electrode plate has higher potential than the grounding plate, and a distance between the projections at tip ends of adjacent projections is set such that a by-productive ozone amount becomes lower when electricity is supplied from the high voltage power supply so that the discharge electrode plate has higher potential than that of the grounding plate as compared with a case in which electricity is supplied from the high voltage power supply so that the discharge electrode plate has lower potential than that of the grounding plate. With this embodiment, if the distance between the projections at the tip ends of the projections is set to a value which is equal to or greater than a predetermined size, the generation of ozone can be reduced. According to the second aspect, in the first aspect, the tip end angle of the projection of the discharge electrode plate is set to about 40' , and the projection distance between the tip ends of adjacent projections is set to 12 mm or more. With this aspect, the by-productive ozone amount can be much smaller than minus polarity of the discharge wire, and high dust collecting ability can be obtained. According to the third aspect, in the first aspect, the tip end angle of the projection of the discharge electrode plate is set to about 20' , voltage of the high voltage power supply is set to about 9 kV to 12 kV, and the projection distance between the tip ends of adjacent projections is set to 8 mm or more. With this aspect, the by-productive ozone amount can be much smaller than minus polarity of the discharge wire, and high dust collecting ability can be obtained. According to the fourth aspect, in the second aspect, the tip end angle of the projection of the discharge electrode plate is set to about 20* to 300 , and voltage of the high voltage power supply is set to about 9 kV to 12 kV. With this aspect, the by-productive ozone amount can be the same level as that of the plus polarity of the discharge wire, and high dust collecting ability can be obtained. The fifth aspect of the invention provides an electric dust collecting system wherein voltage is applied, such that potential of the discharge electrode plate becomes higher than potential of the grounding plate, to one or more of the electric dust collecting apparatuses in which under the same voltage, a projection distance between the tip ends of adjacent projections is set such that a by-productive ozone amount becomes lower when electricity is supplied from the high voltage power supply such that potential of the discharge electrode plate becomes higher than potential of the grounding plate as compared with a case in which electricity is supplied from the high voltage power supply such that potential of the discharge electrode plate becomes lower than potential of the grounding plate, and voltage is applied, such that potential of the discharge electrode plate becomes lower than potential of the grounding plate, to one or more of the electric dust collecting apparatuses in which under the same voltage, a projection distance between the tip ends of adjacent projections is set such that a by-productive ozone amount becomes lower when electricity is supplied from the high voltage power supply such that potential of the discharge electrode plate becomes lower than potential of the grounding plate as compared with a case in which electricity is supplied from the high voltage power supply such that potential of the discharge electrode plate becomes higher than potential of the grounding plate. With this aspect, voltage is applied such that potential of the discharge electrode plate is higher than potential of the grounding plate when the projection distance between the tip ends of the projections is equal to or greater than a predetermined size, and voltage is applied such that potential of the discharge electrode plate becomes lower than potential of the grounding plate when the projection distance between the tip ends of the projections is equal to or smaller than the predetermined size. With this, the generation of ozone of any of the electric dust collecting apparatuses can be reduced, charged particle materials which were not collected in the electric dust collecting apparatuses can be neutralized, and it is possible to prevent particle materials from adhering to a tunnel wall surface. The sixth aspect of the invention provides an electric dust collecting system wherein voltage is applied, such that potential of the discharge electrode plate becomes higher than potential of the grounding plate, to one or more of the electric dust collecting apparatuses in which under the same voltage, a tip end angle of the projection is set such that a by-productive ozone amount becomes lower when electricity is supplied from the high voltage power supply such that potential of the discharge electrode plate becomes higher than potential of the grounding plate as compared with a case in which electricity is supplied from the high voltage power supply such that potential of the discharge electrode plate becomes lower than potential of the grounding plate, and voltage is applied, such that potential of the discharge electrode plate becomes lower than potential of the grounding plate, to one or more of the electric dust collecting apparatuses in which under the same voltage, a tip end angle of the projection is set such that a by-productive ozone amount becomes lower when electricity is supplied from the high voltage power supply such that potential of the discharge electrode plate becomes lower than potential of the grounding plate as compared with a case in which electricity is supplied from the high voltage power supply such that potential of the discharge electrode plate becomes higher than potential of the grounding plate. With this aspect, it is determined whether the discharge is plus discharge or minus voltage in accordance with the tip end angle of the projection, the generation of ozone of any of the electric dust collecting apparatuses can be reduced, charged particle materials which were not collected in the electric dust collecting apparatuses can be neutralized, and it is possible to prevent particle materials from adhering to a tunnel wall surface. The seventh aspect of the invention provides an electric dust collecting system wherein voltage is applied, such that potential of the discharge electrode plate becomes higher than potential of the grounding plate, to one or more of the electric dust collecting apparatuses in which under the same voltage, a by-productive ozone amount is set lower when electricity is supplied from the high voltage power supply such that potential of the discharge electrode plate becomes higher than potential of the grounding plate as compared with a case in which electricity is supplied from the high voltage power supply such that potential of the discharge electrode plate becomes lower than potential of the grounding plate, and voltage is applied, such that potential of the discharge electrode plate becomes lower than potential of the grounding plate, to one or more of the electric dust collecting apparatuses in which under the same voltage, a by-productive ozone amount is set lower when electricity is supplied from the high voltage power supply such that potential of the discharge electrode plate becomes lower than potential of the grounding plate as compared with a case in which electricity is supplied from the high voltage power supply such that potential of the discharge electrode plate becomes higher than potential of the grounding plate. With this aspect, it is determined whether discharge is plus discharge or minus voltage in accordance with the projection distance between the tip ends of the projections, the tip end angle of the projection or the difference in potential. With this aspect, it is determined whether the discharge is plus discharge or minus voltage in accordance with the tip end angle of the projection, the generation of ozone of any of the electric dust collecting apparatuses can be reduced, charged particle materials which were not collected in the electric dust collecting apparatuses can be neutralized, and it is possible to prevent particle materials from adhering to a tunnel wall surface. [Embodiment] [0010] An embodiment of the present invention will be explained below. Fig. 1 is a perspective view showing an electric dust collecting apparatus of the embodiment. In the electric dust collecting apparatus 50, an electrically charging portion 52 is disposed upstream in the air flow, and a dust collecting section 53 is disposed downstream in the air flow. The average electric field intensity of the electrically charging portion 52 is in a range of about 0.67 kV/mm to O.867 kV/mm. The average electric field intensity of the dust collecting section 53 is in a range of about 900 V/mm. Here, the electric field intensity is a ratio V/D of impressed voltage V to a distance D between the discharge electrode plate and the grounding plate. The electric dust collecting apparatus 50 is provided at its side surface with a high voltage power supply 51A which supplies electricity to the dust collecting section 53, and a high voltage power supply 51B which supplies electricity to the electrically charging portion 52. The electrically charging portion 52 has a plurality of grounding plates 52B arranged at predetermined distances from one another. Discharge electrode plates 52A are respectively disposed between the grounding plates 52B. In the dust collecting section 53, charged plates 53A and dust collecting plates 53B are alternately arranged at predetermined distances from one another. The electrically charging portion 52 applies high voltage to the discharge electrode plate 52A or the grounding plate 52B, and applies charge to dust by corona discharge generated between the discharge electrode plate 52A and the grounding plate 52B to charge the dust. The dust collecting section 53 applies voltage to the charged plate 53A to form an electric field between the dust collecting plate 53B and the dust collecting section 53, and collects the charged dust in the dust collecting plate 53B by the Coulomb force. In this embodiment, the dust collecting section 53 is provided in addition to the electrically charging portion 52, but in the electric dust collecting apparatus, the grounding plate 52B may constitute the dust collecting section. [0011] Fig. 2 is a plan view showing a structure of the electrically charging portion of the electric dust collecting apparatus of the embodiment. In the electrically charging portion 52, the plurality of grounding plates 52B are arranged in parallel to each other at predetermined distances from one another, and the discharge electrode plates 52A are respectively disposed between the grounding plates 52B. A plate distance D between a surface of the discharge electrode plate 52A and a surface of the grounding plate 52B is about 12 mm to 15 mm. [0012] Fig. 3 is a side view showing a structure of the discharge electrode plate of the electric dust collecting apparatus of the embodiment. The discharge electrode plate 52A includes a plurality of projections 10 each provided at its end surface with a sharp tip end. At that time, it is preferable that the projections 10 are provided at equal distances from one another, but projection distances H at the tip ends of the projections 10 need not be constant. A tip end angle A of the projection 10 of the discharge electrode plate 52A is in a range of about 100 to 400 . The tip end angle A may be smaller than 10' , but if the angle is less than 100 , it is difficult to machine the projection. Even if the tip end angle A exceeds 400 , if the end is enough sharp for the corona discharge, the same effect can be obtained. The plurality of projections 10 are provided on the upwind-side end surface and the downwind-side end surface of the discharge electrode plates 52A. Only the upwind-side end surface of the discharge electrode plate 52A may be provided with the plurality of projections 10, but if the downwind-side end surface is also provided with the projections 10, the dust collecting performance can be enhanced. A width between the upwind-side end surface and the downwind-side end surface of the discharge electrode plate 52A is about 30 mm to 150 mm. Notches may be provided between the upwind-side end surface and the downwind-side end surface of the discharge electrode plate 52A to provide the plurality of projections 10. When the plurality of steps of projections 10 are provided, it is preferable that the width between the upwind-side end surface and the downwind-side end surface of the discharge electrode plate 52A is about 150 mm to 200 mm. The heights of the projections 10 are set depending upon the tip end angle A and the projection distance H, but the heights are about 4 mm to 10 mm. The projection distance H of the projection 10 is preferably in a range of 4 mm to 12 mm as will be explained below. If the projection distance H of the projection 10 exceeds 12 mm, the dust collecting performance is deteriorated, but the by-productive ozone amount in the plus discharge is low. Therefore, by providing the plurality of steps of projections 10 to make the projection distance H greater than 12 mm, the amount of nitrogen dioxide generated can be reduced and the dust collecting ability of the electric dust collecting apparatus can be enhanced. In this embodiment, the thickness of the discharge electrode plate is 0.5 mm. To prevent the tip end of the projection 10 from being deformed by the corona discharge, the tip end of the projection 10 is provided with R of 0.3 mm. By providing the tip end of the projection 10 with R of 0.3 mm, the discharge current when the voltage is the same is slightly reduced, but it is less than 10%, and the characteristics of the by-productive ozone amount are not varied. [0013] Figs. 4 to 18 are graphs showing the relation between the by-productive ozone amount and the projection distance of teh electric dust collecting apparatus of the embodiment. Since the producing amount of ozone is received influence of the temperature and moisture, all measurements were carried out at the temperature of 20*C and moisture of 65%. The high voltage power supply rises commercial AC power supply and rectifies to produce DC high voltage and thus, AC component (ripple) remains in the high voltage power supply waveform. If the ripple is excessively large, local short-circuit (spark) frequently occurs between the discharge electrode plate and the grounding plate. Therefore, in this embodiment, a high voltage power supply having a ripple of 5% (effective value) or less was used. Fig. 4 shows a relation between a projection distance H, unit consumed electricity and the by-productive ozone amount per unit quantity of air at a tip end of the projection 10 under the condition that the tip end angle A of the projection 10 is 20' , voltage supplied from the high voltage power supply 51B is 10 kV, and the electric field intensity formed between the discharge electrode plate 52A and the grounding plate 52B is 0.67 kV/mm. The plate distance D between the discharge electrode plate 52A and the grounding plate 52B is 15 mm. When electricity was supplied from the high voltage power supply 51B (plus polarity) such that potential of the discharge electrode plate 52A became higher than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.071 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.009 ppm when the projection distance H was 8 mm, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0. 005 ppmwhen the projection distance H was 12 mm. When electricity was supplied from the high voltage power supply 51B (minus polarity) such that potential of the discharge electrode plate 52A became lower than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of airwas 0. 027 ppmwhen the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity andperunit quantity of airwas 0.031 ppmwhen the projection distance H was 8 nm, and the by-productive ozone amount per unit consumed electricity and per unit quantity of airwas 0. 031 ppm when the projection distance H was 12 mm. [0014] Fig. 5 shows a relation between a projection distance H, unit consumed electricity and the by-productive ozone amount per unit quantity of air at a tip end of the projection 10 under the condition that the tip end angle a of the projection 10 is 300, voltage supplied from the high voltage power supply 51B is 10 kV, and the electric field intensity formed between the discharge electrode plate 52A and the grounding plate 52B is 0.67 kV/mm. The plate distance D between the discharge electrode plate 52A and the grounding plate 52B is 15 mm. When electricity was supplied from the high voltage power supply 51B (plus polarity) such that potential of the discharge electrode plate 52A became higher than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.075 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.040 ppm when the projection distance H was 8 m, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was less than 0.001 ppm when the projection distance H was 12 mn. When electricity was supplied from the high voltage power supply 51B (minus polarity) such that potential of the discharge electrode plate 52A became lower than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.029 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.034 ppm when the projection distance H was 8 mm, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.035 ppm when the projection distance H was 12 mm. [0015] Fig. 6 shows a relation between a projection distance H, unit consumed electricity and the by-productive ozone amount per unit quantity of air at a tip end of the projection 10 under the condition that the tip end angle a of the projection 10 is 400, voltage supplied from the high voltage power supply 51B is 10 kV, and the electric field intensity formed between the discharge electrode plate 52A and the grounding plate 52B is 0.67 kV/mm. The plate distance D between the discharge electrode plate 52A and the grounding plate 52B is 15 mm. When electricity was supplied from the high voltage power supply 51B (plus polarity) such that potential of the discharge electrode plate 52A became higher than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.059 ppm when the projection distance H was 2.5 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.045 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.029 ppm when the projection distance H was 8 m, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was less than 0.015 ppm when the projection distance H was 12 mm. When electricity was supplied from the high voltage power supply 51B (minus polarity) such that potential of the discharge electrode plate 52A became lower than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.023 ppm when the projection distance H was 2.5 nun, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.027 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.026 ppm when the projection distance H was 8 mm, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.035 ppm when the projection distance H was 12 mm. [0016] Fig. 7 shows a relation between a projection distance H, unit consumed electricity and the by-productive ozone amount per unit quantity of air at a tip end of the projection 10 under the condition that the tip end angle a of the projection 10 is 200, voltage supplied from the high voltage power supply 51B is 12 W, and the electric field intensity formed between the discharge electrode plate 52A and the grounding plate 52B is 0.8 kV/mm. The plate distance D between the discharge electrode plate 52A and the grounding plate 52B is 15 mm. When electricity was supplied from the high voltage power supply 51B (plus polarity) such that potential of the discharge electrode plate 52A became higher than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.066 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.014 ppm when the projection distance H was 8 mm, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was less than 0.008 ppm when the projection distance H was 12 mm. When electricity was supplied from the high voltage power supply 51B (minus polarity) such that potential of the discharge electrode plate 52A became lower than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.022 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0. 023 ppm when the projection distance H was 8 mn, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.020 ppm when the projection distance H was 12 mm. [0017] Fig. 8 shows a relation between a projection distance H, unit consumed electricity and the by-productive ozone amount per unit quantity of air at a tip end of the projection 10 under the condition that the tip end angle a of the projection 10 is 300, voltage supplied from the high voltage power supply 51B is 12 kV, and the electric field intensity formed between the discharge electrode plate 52A and the grounding plate 52B is 0.8 kV/mm. The plate distance D between the discharge electrode plate 52A and the grounding plate 52B is 15 mm. When electricity was supplied from the high voltage power supply 51B (plus polarity) such that potential of the discharge electrode plate 52A became higher than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.074 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.028 ppm when the projection distance H was 8 mm, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was less than 0.006 ppm when the projection distance H was 12 mm. When electricity was supplied from the high voltage power supply 51B (minus polarity) such that potential of the discharge electrode plate 52A became lower than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.024 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0. 026 ppm when the projection distance H was 8 mm, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.025 ppm when the projection distance H was 12 mm. [0018] Fig. 9 shows a relation between a projection distance H, unit consumed electricity and the by-productive ozone amount per unit quantity of air at a tip end of the projection 10 under the condition that the tip end angle a of the projection 10 is 400, voltage supplied from the high voltage power supply 51B is 12 kV, and the electric field intensity formed between the discharge electrode plate 52A and the grounding plate 52B is 0.8 kV/mm. The plate distance D between the discharge electrode plate 52A and the grounding plate 52B is 15 mm. When electricity was supplied from the high voltage power supply 51B (plus polarity) such that potential of the discharge electrode plate 52A became higher than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.070 ppm when the projection distance H was 2.5 mn, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.072 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.019 ppm when the projection distance H was 8 mm, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was less than 0.011 ppm when the projection distance H was 12 mm. When electricity was supplied from the high voltage power supply 51B (minus polarity) such that potential of the discharge electrode plate 52A became lower than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.019 ppm when the projection distance H was 2.5 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.023 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.022 ppm when the projection distance H was 8 mm, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.026 ppm when the projection distance H was 12 mm. [0019] Fig. 10 shows a relation between a projection distance H, unit consumed electricity and the by-productive ozone amount per unit quantity of air at a tip end of the projection 10 under the condition that the tip end angle a of the projection 10 is 200, voltage supplied from the high voltage power supply 51B is 9 kV, and the electric field intensity formed between the discharge electrode plate 52A and the grounding plate 52B is 0.75 kV/mm. The plate distance D between the discharge electrode plate 52A and the grounding plate 52B is 12 mm. When electricity was supplied from the high voltage power supply 51B (plus polarity) such that potential of the discharge electrode plate 52A became higher than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.047 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.012 ppm when the projection distance H was 8 mm, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was less than 0.005 ppm when the projection distance H was 12 mm. When electricity was supplied from the high voltage power supply 51B (minus polarity) such that potential of the discharge electrode plate 52A became lower than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.021 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0. 025 ppm when the projection distance H was 8 mm, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.025 ppm when the projection distance H was 12 mm. [0020] Fig. 11 shows a relation between a projection distance H, unit consumed electricity and the by-productive ozone amount per unit quantity of air at a tip end of the projection 10 under the condition that the tip end angle cc of the projection 10 is 300, voltage supplied from the high voltage power supply 51B is 9 kV, and the electric field intensity formed between the discharge electrode plate 52A and the grounding plate 52B is 0.75 kV/mm. The plate distance D between the discharge electrode plate 52A and the grounding plate 52B is 12 mm. When electricity was supplied from the high voltage power supply 51B (plus polarity) such that potential of the discharge electrode plate 52A became higher than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.110 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.031 ppm when the projection distance H was 8 mm, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was less than 0.003 ppm when the projection distance H was 12 mm. When electricity was supplied from the high voltage power supply 51B (minus polarity) such that potential of the discharge electrode plate 52A became lower than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.038 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.036 ppm when the projection distance H was 8 mm, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.036 ppm when the projection distance H was 12 mm. [0021] Fig. 12 shows a relation between a projection distance H, unit consumed electricity and the by-productive ozone amount per unit quantity of air at a tip end of the projection 10 under the condition that the tip end angle a of the projection 10 is 40*, voltage supplied from the high voltage power supply 51B is 9 kV, and the electric field intensity formed between the discharge electrode plate 52A and the grounding plate 52B is 0.75 kV/mm. The plate distance D between the discharge electrode plate 52A and the grounding plate 52B is 12 mm. When electricity was supplied from the high voltage power supply 51B (plus polarity) such that potential of the discharge electrode plate 52A became higher than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.100 ppm when the projection distance H was 2.5 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.093 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.072 ppm when the projection distance H was 8 mm, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was less than 0.012 ppm when the projection distance H was 12 mm. When electricity was supplied from the high voltage power supply 51B (minus polarity) such that potential of the discharge electrode plate 52A became lower than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.030 ppm when the projection distance H was 2.5 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.031 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.034 ppm when the projection distance H was 8 mm, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.026 ppm when the projection distance H was 12 MM. [0022] Fig. 13 shows a relation between a projection distance H, unit consumed electricity and the by-productive ozone amount per unit quantity of air at a tip end of the projection 10 under the condition that the tip end angle A of the projection 10 is 200 , voltage supplied from the high voltage power supply 51B is 8 kVv, and the electric field intensity formed between the discharge electrode plate 52A and the grounding plate 52B is 0.67 kV/mm. The plate distance D between the discharge electrode plate 52A and the grounding plate 52B is 12 mm. When electricity was supplied from the high voltage power supply 51B (plus polarity) such that potential of the discharge electrode plate 52A became higher than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.042 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.022 ppm when the projection distance H was 8 mm, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was less than 0.013 ppm when the projection distance H was 12 mm. When electricity was supplied from the high voltage power supply 51B (minus polarity) such that potential of the discharge electrode plate 52A became lower than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.022 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.030 ppm when the projection distance H was 8 mm, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.034 ppm when the projection distance H was 12 mm. [0023] Fig. 14 shows a relation between a projection distance H, unit consumed electricity and the by-productive ozone amount per unit quantity of air at a tip end of the projection 10 under the condition that the tip end angle A of the projection 10 is 300 , voltage supplied from the high voltage power supply 51B is 8 kVv, and the electric field intensity formed between the discharge electrode plate 52A and the grounding plate 52B is 0.67 kV/mm. The plate distance D between the discharge electrode plate 52A and the grounding plate 52B is 12 mm. When electricity was supplied from the high voltage power supply 51B (plus polarity) such that potential of the discharge electrode plate 52A became higher than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.110 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.063 ppm when the projection distance H was 8 mm, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was less than 0.012 ppm when the projection distance H was 12 mm. When electricity was supplied from the high voltage power supply 51B (minus polarity) such that potential of the discharge electrode plate 52A became lower than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.049 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0. 042 ppm when the projection distance H was 8 mm, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.044 ppm when the projection distance H was 12 mm. [0024] Fig. 15 shows a relation between a projection distance H, unit consumed electricity and the by-productive ozone amount per unit quantity of air at a tip end of the projection 10 under the condition that the tip end angle A of the projection 10 is 40* , voltage supplied from the high voltage power supply 51B is 8 kVv, and the electric field intensity formed between the discharge electrode plate 52A and the grounding plate 52B is 0.67 kV/mm. The plate distance D between the discharge electrode plate 52A and the grounding plate 52B is 12 mm. When electricity was supplied from the high voltage power supply 51B (plus polarity) such that potential of the discharge electrode plate 52A became higher than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.107 ppm when the projection distance H was 2.5 mn, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.098 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.093 ppm when the projection distance H was 8 m, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was less than 0.027 ppm when the projection distance H was 12 m. When electricity was supplied from the high voltage power supply 51B (minus polarity) such that potential of the discharge electrode plate 52A became lower than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.034 ppm when the projection distance H was 2.5 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.040 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.040 ppm when the projection distance H was 8 mm, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.033 ppm when the projection distance H was 12 mm. [0025] Fig. 16 shows a relation between a projection distance H, unit consumed electricity and the by-productive ozone amount per unit quantity of air at a tip end of the projection 10 under the condition that the tip end angle a of the projection 10 is 100, voltage supplied from the high voltage power supply 51B is 10 kV, and the electric field intensity formed between the discharge electrode plate 52A and the grounding plate 52B is 0.67 kV/mm. The plate distance D between the discharge electrode plate 52A and the grounding plate 52B is 15 mm. When electricity was supplied from the high voltage power supply 51B (plus polarity) such that potential of the discharge electrode plate 52A became higher than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.076 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.029 ppm when the projection distance H was 8 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.022 ppm when the projection distance H was 12 mm, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was less than 0.016 ppm when the projection distance H was 20 mm. When electricity was supplied from the high voltage power supply 51B (minus polarity) such that potential of the discharge electrode plate 52A became lower than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.027 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.029 ppm when the projection distance H was 8 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.031 ppm when the projection distance H was 12 mm, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.023 ppm when the projection distance H was 20 m2. [0026] Fig. 17 shows a relation between a projection distance H, unit consumed electricity and the by-productive ozone amount per unit quantity of air at a tip end of the projection 10 under the condition that the tip end angle A of the projection 10 is 100 , voltage supplied from the high voltage power supply 51B is 8 kVv, and the electric field intensity formed between the discharge electrode plate 52A and the grounding plate 52B is 0.67 kV/mm. The plate distance D between the discharge electrode plate 52A and the grounding plate 52B is 12 mm. When electricity was supplied from the high voltage power supply 51B (plus polarity) such that potential of the discharge electrode plate 52A became higher than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.076 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.059 ppm when the projection distance H was 8 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.026 ppm when the projection distance H was 12 mm, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was less than 0.021 ppm when the projection distance H was 20 mm. When electricity was supplied from the high voltage power supply 51B (minus polarity) such that potential of the discharge electrode plate 52A became lower than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.029 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.027 ppm when the projection distance H was 8 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.024 ppm when the projection distance H was 12 mm, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.023 ppm when the projection distance H was 20 M. [0027] Fig. 18 shows a relation between a projection distance H, unit consumed electricity and the by-productive ozone amount per unit quantity of air at a tip end of the projection 10 under the condition that the tip end angle A of the projection 10 is 100 , voltage supplied from the high voltage power supply 51B is 9 kVv, and the electric field intensity formed between the discharge electrode plate 52A and the grounding plate 52B is 0.75 kV/mm. The plate distance D between the discharge electrode plate 52A and the grounding plate 52B is 12 mm. When electricity was supplied from the high voltage power supply 51B (plus polarity) such that potential of the discharge electrode plate 52A became higher than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.070 ppm when the projection distance H was 4 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.040 ppm when the projection distance H was 8 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.015 ppm when the projection distance H was 12 mm, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was less than 0.014 ppm when the projection distance H was 20 mm. When electricity was supplied from the high voltage power supply 51B (minus polarity) such that potential of the discharge electrode plate 52A became lower than potential of the grounding plate 52B, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.028 ppm when the projection distance H was 4 mn, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.026 ppm when the projection distance H was 8 mm, the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.022 ppm when the projection distance H was 12 mm, and the by-productive ozone amount per unit consumed electricity and per unit quantity of air was 0.021 ppm when the projection distance H was 20 mm. [0028] According to the embodiment as described above, under the same voltage, when the intensity of the electric field formed between the discharge electrode plate 52A and the grounding plate 52B is in a range of about 0.67 kV/mm to 0.8 kV/mn and the tip end angle of the projection 10 of the discharge electrode plate 52A is in a range of about 100 to 40* nd voltage of about 8 kV to 12 kV is applied from the high voltage power supply 51B so that potential of the discharge electrode plate 52A becomes higher than potential of the grounding plate 52B, the projection distance H is set such that the by-productive ozone amount becomes lower when electricity is supplied from the high voltage power supply 51B such that potential of the discharge electrode plate 52A becomes higher than potential of the grounding plate 52B, as compared with a case in which electricity is supplied from the high voltage power supply 51B such that potential of the discharge electrode plate 52A becomes lower than potential of the grounding plate 52B. With this, the by-productive ozone amount can be reduced, and high dust collecting ability can be obtained. According to the embodiment, when the plate distance D between the discharge electrode plate 52A and the grounding plate 52B is about 12 mm to 15 mm and a tip end angle of the projection 10 of the discharge electrode plate 52A is set to about 100 to 400 and voltage of about 8 kV to 12 kV is applied from the high voltage power supply 51B so that potential of the discharge electrode plate 52A becomes higher than potential of the grounding plate 52B, if the projection distance H is set in the same manner as that described above, the by-productive ozone amount can be reduced, and high dust collecting ability can be obtained. According to the embodiment, the intensity of the electric field formed between the discharge electrode plate 52A and the grounding plate 52B is in a range of about 0.67 kV/mm to 0.8 kV/mm, the tip end angle A of the projection 10 of the discharge electrode plate 52A is in a range of about 20* to 40* , voltage of about 8 kV to 12 kV is applied from the high voltage power supply 51B such that potential of the discharge electrode plate 52A becomes higher than potential of the grounding plate 52B, and the projection distance Hat tip ends of adjacent projections 10 is 12 mm or more. With this arrangement, the by-productive ozone amount can be much smaller than minus polarity of the discharge wire. The tip end angle A of the projection 10 of the discharge electrode plate 52A is set to about 200 to 300 , and the voltage of the high voltage power supply 51A is set to about 9 kV to 12 kV. With this, the by-productive ozone amount can be the same as plus polarity of the discharge wire, and high dust collecting ability can be obtained. According to the embodiment, the plate distance D between the discharge electrode plate 52A and the grounding plate 52B is about 12 mm to 15 mm, the tip end angle A of the projection 10 of the discharge electrode plate 52A is about 200 to 400 , voltage of about 8 kV to 12 kV is applied from the high voltage power supply 51B such that potential of the discharge electrode plate 52A becomes higher than potential of the grounding plate 52B, and the projection distance H at the tip ends of adjacent projections 10 is 12 mm or more. With this, the by-productive ozone amount can be mush much smaller than minus polarity of the discharge wire. The tip end angle A of the projection 10 of the discharge electrode plate 52A is set to about 200 to 300 , and the voltage of the high voltage power supply 51B is set to about 9 kV to 12 kV. With this, the by-productive ozone amount can be same as the plus polarity of the discharge wire. According to the embodiment, the intensity of the electric field formed between the discharge electrode plate 52A and the grounding plate 52B is set to about 0.67 kV/mm to 0.8 kV/mm, the tip end angle A of the projection 10 of the discharge electrode plate 52A is set to about 20' , the projection distance H at the tip ends of the adjacent projections 10 is set to 8 mm or more, and voltage of about 9 kV to 12 kV is applied from the high voltage power supply 51B such that potential of the discharge electrode plate 52A becomes higher than potential of the grounding plate 52B. With this, the by-productive ozone amount can be mush much smaller than minus polarity of the discharge wire. According to the embodiment, the plate distance D between the discharge electrode plate 52A and the grounding plate 52B is set to about 12 mm to 15 mn, the tip end angle aof the projection 10 of the discharge electrode plate 52Ais set toabout 200, the projection distance H at tip ends of the adjacent projections 10 is set to 8 mm or more, and voltage of about 9 kV to 12 kV is applied from the high voltage power supply 51B such that potential of the discharge electrode plate 52A becomes higher than potential of the grounding plate 52B. With this, the by-productive ozone amount can be much smaller than minus polarity of the discharge wire. [0029] According to the embodiment, in the discharge electrode plate 52A, the plurality of projections 10 having the tip end angle a of about 100 to 40* are provided such that the projection distance H becomes 4 mm or more, and using this discharge electrode plate 52A in which a plurality of projections 10 each having a tip end angle a of about 100 to 400 are provided such that the projection distance H becomes 4 rmn or more, the intensity of the electric field formed between the discharge electrode plate 52A and the grounding plate 52B is set to about 0.67 kV/mm to 0.8 kV/mm, and when voltage of about 8 kV to 12 kVis applied, it is possible to select plus discharge andminus discharge such that the ozone to be generated is reduced from the plus discharge, the tip end angle A, the plate distance D and voltage value to be impressed. As one embodiment, when the plurality of electric dust collecting apparatuses are disposed as an electric dust collecting system, and when it is desired to use an electric dust collecting apparatus using minus discharge and an electric dust collecting apparatus using plus discharge together, if a discharge electrode plate 52A in which only the projection distance H is changed is used, it is possible to realize a system having a small by-productive ozone amount although other conditions are the same. That is, when the voltage is set to 9 kV, the grounding plate is set to 12 mm and the tip end angle of the projection 10 is set to 300, an electric dust collecting apparatus in which the projection distance H is set to 4 mm is minus discharge, and an electric dust collecting apparatus in which the projection distance H is set to 10 mm is plus discharge. With this, the electric dust collecting apparatus which is the plus discharge and the electric dust collecting apparatus which is the minus discharge can reduce the by-productive ozone amount (see Fig. 13). As another embodiment, when the plurality of electric dust collecting apparatuses are disposed as the electric dust collecting system, and when it is desired to use an electric dust collecting apparatus using minus discharge and an electric dust collecting apparatus using plus discharge together, it is possible to realize a system having a small by-productive ozone amount although other conditions are same only by changing voltage. That is, when the plateus discharge is set to 15 mm and the tip end angle of the projection 10 is set to 40* and the projection distance H is set to 8 mm, and when an electric dust collecting apparatus in which voltage of 10 kV is impressed is minus discharge and an electric dust collecting apparatus in which voltage of 12 kV is impressed is plus discharge., With this, the electric dust collecting apparatus which is the plus discharge and the electric dust collecting apparatus which is the minus discharge can reduce the by-productive ozone amount (see Figs. 6 and 9, or 11 and 14). As another embodiment, when the plurality of electric dust collecting apparatuses are disposed as the electric dust collecting system, and when it is desired to use an electric dust collecting apparatus using minus discharge and an electric dust collecting apparatus using plus discharge together, it is possible to realize a system having a small by-productive ozone amount although other conditions are the same only by changing the tip end angle a. That is, when voltage is set to 12 kV and the plate distance is set to 15 mm and the projection distance H is set to 8 un, an electric dust collecting apparatus in which the tip end angle of the projection 10 is set to 300 is minus discharge, and an electric dust collecting apparatus in which the tip end angle of the projection 10 is set to 20*. With this, the electric dust collecting apparatus which is the plus discharge and the electric dust collecting apparatus which is the minus discharge can reduce the by-productive ozone amount (see Figs. 7 and 8). It is possible to realize a system (not shown) having a small by-productive ozone amount by changing the plate distance D. [0030] In an electric dust collecting system, using a discharge electrode plate 52A in which a plurality of projections 10 each having a tip end angle a of about 100 to 40 are provided such that the projection distance H becomes 4 mm or more, the intensity of an electric field formed between the discharge electrode plate 52A and the grounding plate 52B is set to about 0.67 kV/mm to 0.8 kV/mm and a plurality of electric dust collecting apparatuses each applying voltage of about 8 kV to 12 kV are disposed, when an electric dust collecting apparatus which is plus discharge and an electric dust collecting apparatus which is minus discharge are used together in the same system, if at least one of parameters, i.e., the projection distance H, the tip end angle A, the plate distance D and the voltage value to be impressed is changed, the plus discharge and minus discharge can be selected such that the generation of ozone is reduced. The electric dust collecting apparatus which is plus discharge and the electric dust collecting apparatus which is minus discharge are provided together in the same system. With this, charged particle materials which were not collected in the electric dust collecting apparatuses can be neutralized, and it is possible to prevent particle materials from adhering to a tunnel wall surface. In this electric dust collecting system, it is preferable to determine whether plus discharge or minus discharge such that the generation of ozone is reduced for all of the electric dust collecting apparatuses, but in some of electric dust collecting apparatuses which are arranged such that the by-productive ozone amount is not largely varied depending upon the plus discharge or minus discharge, even when an electric dust collecting apparatus in which ozone generation is increased by measuring is included, if t influence on environment is negligible, there is no problem. The above embodiment has been explained based on the by-productive ozone amount, but in the case of an electric dust collecting apparatus used in a tunnel, chemical reaction occurs between ozone and nitric oxide existing in volume in a tunnel, and nitrogen dioxide is generated in volume. Therefore, if the by-productive ozone amount is reduced, the generation of the nitrogen dioxide is also reduced. [Industrial Applicability] [0031] The present invention provides an electric dust collecting apparatus which applies electric charge to dust by corona discharge to charge the dust, and the charged dust is collected by Coulomb force, and this is suitable for a route dust collecting apparatus and a tunnel dust collecting apparatus which generates nitric oxide.

Claims

1. An electric dust collecting apparatus comprising a discharge electrode plate provided at its end surface with a plurality of projections each 5 having a sharp tip end, a grounding plate disposed in parallel to said discharge electrode plate, and a high voltage power supply which applies corona discharge voltage between said discharge electrode plate and said grounding plate, in which intensity of an electric field formed between said discharge electrode plate and said grounding plate is set to about 0.67 kV/mm to 0.8 kV/mm, a tip end angle of said 10 projection of said discharge electrode plate is set to about 100 to 400, voltage of about 8 kV to 12 kV is applied from said high voltage power supply such that potential of said discharge electrode plate becomes higher than potential of said grounding plate, and particle material in the air is charged, thereby the particle material adhering the particle material to a dust collecting section, wherein is under the same voltage, a projection distance between the tip ends of adjacent projections is set such that a by-productive ozone amount becomes lower when electricity is supplied from said high voltage power supply such that potential of said discharge electrode plate becomes higher than potential of said grounding plate as compared with a case in which electricity is supplied from said 20 high voltage power supply such that potential of said discharge electrode plate becomes lower than potential of said grounding plate.

2. The electric dust collecting apparatus according to claim 1, wherein the tip end angle of said projection of said discharge electrode plate is set 25 to about 200 to 40*, and the projection distance between the tip ends of adjacent projections is set to 12 mm or more.

3. The electric dust collecting apparatus according to claim 1, wherein the tip end angle of said projection of said discharge electrode plate is set 30 to about 200, voltage of said high voltage power supply is set to about 9 kV to 12 kV, and the projection distance between the tip ends of adjacent projections is set to 8 mm or more. - 42 -

4. The electric dust collecting apparatus according to claim 2, wherein the tip end angle of said projection of said discharge electrode plate is set to about 200 to 300, and voltage of said high voltage power supply is set to about 9 kV to 12 kV. 5

5. An electric dust collecting system having a plurality of electric dust collecting apparatuses each comprising a discharge electrode plate provided at its end surface with a plurality of projections each having a sharp tip end, a grounding plate disposed in parallel to said discharge electrode plate, and a high 10 voltage power supply which applies corona discharge voltage between said discharge electrode plate and said grounding plate, in which intensity of an electric field formed between said discharge electrode plate and said grounding plate is set to about 0.67 kV/mm to 0.8 kV/mm, a tip end angle of said projection of said discharge electrode plate is set to about 100 to 400, voltage of about 8 kV to 12 kV is is applied from said high voltage power supply, and particle material in the air is charged, thereby adhering the particle material to a dust collecting section, wherein voltage is applied, such that potential of said discharge electrode plate becomes higher than potential of said grounding plate, to one or more of said electric dust collecting apparatuses in which under the same voltage, a projection 20 distance between the tip ends of adjacent projections is set such that a by productive ozone amount becomes lower when electricity is supplied from said high voltage power supply such that potential of said discharge electrode plate becomes higher than potential of said grounding plate as compared with a case in which electricity is supplied from said high voltage power supply such that 25 potential of said discharge electrode plate becomes lower than potential of said grounding plate, and voltage is applied, such that potential of said discharge electrode plate becomes lower than potential of said grounding plate, to one or more of said electric dust collecting apparatuses in which under the same voltage, a projection 30 distance between the tip ends of adjacent projections is set such that a by productive ozone amount becomes lower when electricity is supplied from said high voltage power supply such that potential of said discharge electrode plate becomes lower than potential of said grounding plate as compared with a case in which electricity is supplied from said high voltage power supply such that -43- potential of said discharge electrode plate becomes higher than potential of said grounding plate.

6. An electric dust collecting system having a plurality of electric 5 dust collecting apparatuses each comprising a discharge electrode plate provided at its end surface with a plurality of projections each having a sharp tip end, a grounding plate disposed in parallel to said discharge electrode plate, and a high voltage power supply which applies corona discharge voltage between said discharge electrode plate and said grounding plate, in which intensity of an electric io field formed between said discharge electrode plate and said grounding plate is set to about 0.67 kV/mm to 0.8 kV/mm, a tip end angle of said projection of said discharge electrode plate is set to about 100 to 40", a projection distance between tip ends of adjacent projections is set to 4 mm or more, voltage of about 8 kV to 12 kV is applied from said high voltage power supply, and particle material in the is air is charged, thereby adhering the particle material to a dust collecting section, wherein voltage is applied, such that potential of said discharge electrode plate becomes higher than potential of said grounding plate, to one or more of said electric dust collecting apparatuses in which under the same voltage, a tip end 20 angle of said projection is set such that a by-productive ozone amount becomes lower when electricity is supplied from said high voltage power supply such that potential of said discharge electrode plate becomes higher than potential of said grounding plate as compared with a case in which electricity is supplied from said high voltage power supply such that potential of said discharge electrode plate 25 becomes lower than potential of said grounding plate, and voltage is applied, such that potential of said discharge electrode plate becomes lower than potential of said grounding plate, to one or more of said electric dust collecting apparatuses in which under the same voltage, a tip end angle of said projection is set such that a by-productive ozone amount becomes 30 lower when electricity is supplied from said high voltage power supply such that potential of said discharge electrode plate becomes lower than potential of said grounding plate as compared with a case in which electricity is supplied from said high voltage power supply such that potential of said discharge electrode plate becomes higher than potential of said grounding plate. - 44 -

7. An electric dust collecting system having a plurality of electric dust collecting apparatuses each comprising a discharge electrode plate provided at its end surface with a plurality of projections each having a sharp tip end, a grounding plate disposed in parallel to said discharge electrode plate, and a high 5 voltage power supply which applies corona discharge voltage between said discharge electrode plate and said grounding plate, in which intensity of an electric field formed between said discharge electrode plate and said grounding plate is set to about 0.67 kV/mm to 0.8 kV/mm, a tip end angle of said projection of said discharge electrode plate is set to about 100 to 400, a projection distance between 10 tip ends of adjacent projections is set to 4 mm or more, voltage of about 8 kV to 12 kV is applied from said high voltage power supply, and particle material in the air is charged, thereby adhering the particle material to a dust collecting section, wherein voltage is applied, such that potential of said discharge electrode plate is becomes higher than potential of said grounding plate, to one or more of said electric dust collecting apparatuses in which under the same voltage, a by productive ozone amount is set lower when electricity is supplied from said high voltage power supply such that potential of said discharge electrode plate becomes higher than potential of said grounding plate as compared with a case in which 20 electricity is supplied from said high voltage power supply such that potential of said discharge electrode plate becomes lower than potential of said grounding plate, and voltage is applied, such that potential of said discharge electrode plate becomes lower than potential of said grounding plate, to one or more of said 25 electric dust collecting apparatuses in which under the same voltage, a by productive ozone amount is set lower when electricity is supplied from said high voltage power supply such that potential of said discharge electrode plate becomes lower than potential of said grounding plate as compared with a case in which electricity is supplied from said high voltage power supply such that potential of 30 said discharge electrode plate becomes higher than potential of said grounding plate. -45 -

8. An electric dust collecting apparatus, said apparatus being substantially as herein disclosed with reference to any one or more of Figs. 1-18 of the accompanying drawings. 5

9. An electric dust collecting system, said system being substantially as herein disclosed with reference to any one or more of Figs. 1-18 of the accompanying drawings. DATED this Twenty-Eighth Day of May, 2010

10 Panasonic Corporation Patent Attorneys for the Applicant SPRUSON & FERGUSON 15 - 46 -