S&F Ref: 644458 AUSTRALIA PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT Name and Fuji Electric Co., Ltd. Address 1-1, Tanabeshinden, Kawasaki-ku, Kawasaki-shi of Applicants: Kanagawa 210-0856 Japan Tairo Ito 2154-41, Naruse Machida-shi Tokyo Japan Actual Akinori Zukeran, Koji Yasumoto, Yoshihiro Kono, Tairo Inventor(s): Ito Address for Spruson & Ferguson Service: St Martins Tower Level 35 31 Market Street Sydney NSW 2000 (CCN 3710000177) Invention Title: Electric Dust Collecting Apparatus The following statement is a full description of this invention, including the best method of performing it known to me/us:- ELECTRIC DUST COLLECTING APPARATUS BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an electric dust collecting apparatus using alternating-current high voltage of a rectangular waveform, preferable for cleaning air in a tunnel. Description of the Related Art As has been known conventionally, air in tunnel of an automobile road is contaminated by harmful gasses and soot in exhaust gas exhausted from an automobile and by suspended particulate matter of a submicron order of dust or the like brought about by wearing tire or road asphalt produced in accordance with running of an automobile. Hence, in order to remove those soot and particles in the contaminated air, an air cleaning facility using a two-stage type electric dust collecting apparatus including a charging portion and a dust collecting portion is put into practice. Fig. 20 shows a structure of a generally known two-stage type electric dust collecting apparatus. The electric dust collecting apparatus 100 shown in Fig. 20 includes a charging portion 1 and a dust collecting portion 2. The charging portion 1 is provided with a structure of a "line (4) versus flat plate electrodes (3a, 3b)". By applying direct-current high voltage between the electrodes, corona discharge is generated. The dust collecting portion 2 is provided with - 1 a structure of parallel flat plate electrodes (5a, 5b and 6). By applying high direct-current voltage between the parallel flat plate electrodes, an electrostatic field is formed. In the conventional two-stage type electric dust collecting apparatus having the above structure, particles are charged in a single polarity at the charging portion 1 and are caught and collected onto the dust collecting electrodes 5a and 5b by the electrostatic field of the dust collecting portion 2. The two-stage type electric dust collecting apparatus is provided with a high dust collecting rate also with respect to nanometer (submicron) particles and suitable for processing a large flow rate. However, when carbon or the like having low electric resistance is included as a major component of suspended particulate matter as in the tunnel of the automobile road, there is a case in which particles 5 caught and collected onto the dust collecting electrodes are scattered again and exhausted from the electric dust collecting apparatus along with a gas flow. The phenomenon is referred to as a reentrainment phenomena. The reentrainment phenomena poses a significant problem to be improved since a dust collecting rate of particles having a large o size is significantly lowered when the reentrainment phenomena occurs. Fig. 21 illustrates explanatory views showing a mechanism of the above-described reentrainment phenomena. Here, at the charging portion, particles are charged in a single polarity of negative polarity. In this case, the mechanism of the reentrainment phenomena 5 is as follows. -2- First, as shown in Fig. 21(A), a particle 9 charged in a negative polarity in the charging portion is caught and collected onto a dust collecting portion grounding electrode 8. The carbon particle 9 collected onto the grounding electrode plate 8 immediately loses electric charge and is provided with a polarity the same as that of the grounding electrode. Therefore, an electric field is intensified at a vicinity of the collected particle on the grounding electrode 8. Further, as shown in Fig. 21(B), when the particle charged in a negative polarity in the gas flow is collected onto the grounding electrode 8, the particle is flocculated with the particle on the grounding electrode 8 and forms moniliformed particles in a rosary-like shape in a direction of an electrode having a negative polarity by Coulomb' s force by the electric field. As the moniliformed particles in the rosary-like shape on the grounding electrode 8 are flocculated .to be bulky (see Fig. 21(C)), exfoliating force of hydrodynamic resistant force and of Coulomb's force is intensified and the particles become rescattered when the forces become larger than adhering force between the grounding electrode and moniliformed particles. As a method of preventing the reentrainment phenomena extremely effectively, an electric dust collecting apparatus using alternating-current high voltage of a rectangular waveform has been proposed. Fig. 22 shows an outline structure of an electric dust collecting apparatus using alternating-current voltage of a rectangular waveform. The apparatus includes a charging portion 40 and a dust collecting - 3 portion 50. The charging portion 40 is structured in the line versus flat plate electrodes structure and includes grounding electrodes 21 and 22 as a pair of flat plates and a high voltage electrode 23 formed in a line-like shape. Direct-current high voltage is applied between the grounding electrodes 21 and 22 and the high voltage electrode 23 from a high voltage power source 20 and the corona discharge is generated at the charging portion 40. The polarity of the direct-current high voltage may be either of positive or negative and the direct-current high voltage may be a pulse voltage. The dust collecting portion 50 is structured as a parallel.flat plate electrodes structure and includes grounding electrodes 31 and 32 as a pair of flat plates and a high. voltage electrode 33 as one sheet of a flat plate. Alternating-current high voltage of a rectangular waveform is applied between the grounding electrodes 31 and 32 and the high voltage electrode 33 from an alternating-current high voltage power source 30. Instead of the alternating-current high voltage power source 30, an alternating-current high voltage power source that generates alternating-current high voltage of a sine waveform may be used. A voltage range of the alternating-current high voltage power source of this kind is pertinently equal to or lower than 3kV per 1mm of an interval between the electrodes and is generally about 0.9kV per 1mm. Further, a frequency of the applied voltage falls in a range of several Hz through several kHz. However, there poses a problem that the higher the frequency, the larger the power source capacity needs -4 - -5 to be. Conversely, when the frequency is set to be low, there poses a problem that rescattering is generated and a dust collecting rate of particles having a large particle size is lowered. 5 SUMMARY The present invention has been made to solve the above problems, and therefore an object of the present invention is to provide an electric collecting apparatus capable of maintaining a high dust collecting rate even when a frequency of alternating-current high voltage applied to a dust collecting portion is lowered, that is, even when a power source 10 capacity is reduced. Further, it is another object of the invention to provide an electric dust collecting apparatus capable of achieving a high dust collecting rate while having a necessary minimum voltage change rate of dV/dt value (that is, an inclination of the rectangular waveform in rising and falling) when alternating-current high voltage of the rectangular is waveform is used and capable of simplifying a power source apparatus and cables. Further, it is another object of the invention to provide an electric dust collecting apparatus capable of realizing a high dust collecting rate at low cost by effectively preventing rescattering. In order to achieve the objects, according to a first aspect of the invention, there 20 is provided an electric dust collecting apparatus comprising a charging portion of a corona discharge type configured to charge a particle wafting in air, the particle including carbon having low electrical impedance as a major component and a dust collecting portion arranged downstream from the charging portion and configured to collect the charged particle, wherein the corona discharge type charging portion comprises a direct-current 25 high voltage generating portion configured to apply a direct-current high voltage between electrodes thereof for charging the particle, and wherein the dust collecting portion comprises an alternating-current high voltage generating portion configured to apply an alternating-current high voltage of a rectangular waveform at a frequency of 0.1Hz through 2Hz between electrodes thereof for changing the particle flocculated at the 30 electrodes in a rosary-like shape into a flocculated particle in a spherical shape to collect the particle.
-6 According to a second aspect of the invention, there is provided an electric dust collecting apparatus comprising a charging portion of a corona discharge type configured to charge a particle wafting in air, the particle including carbon having low electrical impedance as a major component and a dust collecting portion arranged downstream from 5 the charging portion and configured to collect the charged particle, wherein the charging portion comprises a direct-current high voltage generating portion configured to apply a direct-current high voltage between electrodes thereof for charging the particle, and wherein the dust collecting portion comprises an alternating-current high voltage generating portion configured to apply an alternating-current high voltage of a rectangular 10 waveform having a voltage change rate dV/dt within a range of from 50V/msec to 2000V/msec between electrodes thereof for changing the particle flocculated at the electrodes in a rosary-like shape into a flocculated particle in a spherical shape to collect the particle. According to a third aspect of the invention, there is provided an electric dust 15 collecting apparatus comprising a charging portion of a corona discharge type configured to charge a particle wafting in air and a dust collecting portion arranged downstream from the charging portion and configured to collect the charged particle, wherein the charging portion comprises an alternating-current high voltage generating portion configured to applying an alternating-current high voltage between electrodes thereof for charging the 20 particle, and wherein the dust collecting portion comprises an alternating-current high voltage generating portion configured to apply an alternating-current high voltage between electrodes thereof for collecting the particle. BRIEF DESCRIPTION OF THE DRAWINGS 25 The above and other objects and advantages of the present invention will become more fully apparent from the following detailed description taken with the accompanying drawings, in which: Fig. I is a sectional constitution view of an electric dust collecting apparatus according to a first embodiment of the invention. 30 -7 Fig. 2 illustrates explanatory views showing a model of catching and collecting charged particles and preventing rescattering thereof in a dust collecting portion shown in Fig. 1. Fig. 3 is a diagram showing a waveform of a high voltage applied to the dust s collecting portion. Fig. 4 illustrates diagrams showing a frequency characteristic (in applying direct current (DC), in applying frequency of 0.001Hz through 1Hz) of a dust collecting rate provided by an experiment. Fig. 5 illustrates diagrams showing a frequency characteristic 10 (in applying direct current (DC), in applying frequency of 0.01Hz through 1Hz) of a dust collecting rate provided by the experiment. Fig. 6 is a diagram showing a frequency characteristic (in frequency of 0.lHz through 10Hz) of a dust collecting rate provided by another experiment. Fig. 7 illustrates diagrams showing a frequency characteristic of a dust collecting rate when alternating-current high voltage of a sine waveform is applied to the dust collecting portion. Fig. 8 is an explanatory view showing a particle oscillation model at respective frequencies. Fig. 9 is .a sectional structure view of an electric collecting apparatus according to a second embodiment of the invention. Fig. 10 is a diagram showing a waveform of voltage applied to the dust collecting portion shown in Fig. 9. Fig. 11 is a diagram showing a relationship between an allowable difference Dh of a dust collecting rate and dV/dt in a direct-current electric dust collecting apparatus and an alternating-current electric dust collecting apparatus of a rectangular waveform. Fig. 12 is a block diagram of an apparatus used in an experiment. Fig. 13 illustrates views showing an electrode constitution of the electric dust collecting apparatus according to the second embodiment. Fig. 14 is a view showing an electrode arrangement in a duct. Fig. 15 is a diagram showing an influence of dV/dt on a particle size characteristic of a dust collecting rate. - 8- Fig. 16 is a sectional structure view showing an electric dust collecting according to a third embodiment of the invention. Fig. 17 is a sectional structure view showing an electric dust collecting apparatus according to a fourth embodiment of the invention. Fig. 18 illustrates views exemplifying various electrode constitutions of a charging portion. Fig. 19 illustrates views exemplifying general various electrode shapes. Fig. 20 is a view showing a generally known structure of a two-stage type electric dust collecting apparatus. Fig. 21 illustrates explanatory views showing a mechanism of a reentrainment phenomena. Fig. 22 is view showing an outline structure of an electric dust collecting apparatus using alternating-current voltage of a rectangular waveform. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the. accompanying drawings, a description will be given in detail of preferred embodiments of the invention. (First embodiment of the invention) Fig. 1 is a sectional structure view of an electric dust collecting apparatus according to a first embodiment of the invention. The electric dust collecting apparatus includes a charging portion 40 and a dust collecting portion 50. The charging portion 40 has a structure of line versus flat plate electrodes and includes - 9 grounding electrodes 21 and 22 as a pair of flat plates and a high voltage electrode 23 in a line-like shape. Direct-current high voltage is applied between the grounding electrodes 21 and 22 and the high voltage electrode 23 from a high voltage power source 20 so that a corona discharge is generated at the charging portion 40. The polarity of the direct-current high voltage may be either of a positive or negative polarity and the voltage may be a pulse voltage. The dust collecting portion 50 has a structure of parallel flat plate electrodes and includes grounding electrodes 31 and 32 as a pair of flat plates and a high voltage electrode 33 as one sheet of.a flat plate. Alternating-current high voltage (frequency of 0.1 Hz through 2 Hz) is applied between the grounding electrodes 31 and 32 and the high voltage electrode 33 from an alternating-current high voltage power source 60 for generating alternating-current high voltage of a rectangular waveform. By applying the high voltage, at the dust collecting portion 50, an electrostatic field is generated. Agas flow including suspended particulate matter is charged by passing through the charging portion 50 and the suspended particulate matter are caught and collected onto the dust collecting electrode by the electrostatic field of the dust collecting portion 50. Next, an explanation will be given of a rescattering preventing mechanism when the alternating-current high voltage is applied to the dust collecting portion 50 in reference to Fig. 2 and Fig. 3. Fig. 2 shows a model of collecting charged particles and preventing rescattering of the charged particles at the dust collecting portion - 10 - 50. Here, the charging portion 40 (refer to Fig. 1) is applied with negative direct-current high voltage andparticles are chargedinminus. Fig. 3 shows a waveform of alternating-current high voltage applied to the dust collecting portion 50. In Fig. 3, the voltage applied to the dust collecting portion 50 is divided into three sections. Section A is a region in which positive high voltage is applied to the dust collecting portion 50. Section B is a transient region in which the voltage applied to the dust collecting portion 50 is changed from positive to negative (in several milliseconds) . . Section C is a region in which negative high voltage is applied. to the dust collecting portion 50. At section A, particles negatively charged at the charging portion 40 are caught and collected onto the high voltage dust collecting electrode plate having a positive polarity (refer to Fig. 2(a)). The collected particles are immediately charged to positive to form moniliformed particles in a rosary-like shape. Thereafter, at section B, the polarity of the voltage is rapidly changed from positive to negative (refer to Fig. 2(b)). Since the polarity of the dust collecting electrode plate is rapidly changed from positive to negative, the particles flocculated at the electrode plate in the rosary-like shape are changed to flocculated particles in a spherical shape by being exerted with a force in a direction of the dust collecting electrode plate by static electricity. In this way, by changing into the flocculated particles in the spherical shape, wind force or electrostatic force operated as exfoliating force is reduced and - 11 rescattering is not brought about (refer to Fig. 2(C)). (First experiment) Fig. 4 and Fig. 5 show a frequency characteristic of a dust collecting rate provided by a first experiment (in applying direct current (DC), in applying the voltage of a rectangular waveform in frequency of 0.001Hz through 1Hz). As experimental conditions, wind speed is set to 5m/s, a length of the dust collecting portion 50 is set to 206mm, charging voltage is set to 11kV, the dust collecting voltage is set to a rectangular wave of ±5kV and a distance between the electrodes of the dust collecting portion 50 is set to 6mm. The result of the first experiment wherein the particle size is set to 0.3mm through 0.5mm is shown in Fig. 4 (A) . The result of the first experiment wherein the particle size is set to 0.5mm through 1mm is shown in Fig. 4 (B) . The result of the first experiment wherein the particle size is set to 1mm through 2mm is shown in Fig. 5 (A) . The result of the first experiment wherein the particle size is set to 2mm through 5mm is shown in Fig. 5(B). As a result of the experiment, the higher the frequency, the more increased is the dust collecting rate in any of particle sizes, particularly, the highest dust collecting rate is shown at frequency of 0.1 Hz through 1 Hz. (Second experiment) Fig. 6 shows a frequency characteristic of a dust collecting rate provided by a second experiment (when rectangular wave frequency is 0.1 Hz through 10 Hz) . As experimental conditions, wind speed is - 12 set to 7m/s, temperature is set to 13'C, humidity is set to 25%, atmospheric pressure is set to 1031hPa, the charging portion is constituted by 1 unit, the dust collecting portion is constituted by 206mm'2units, the charging voltage is set to 11kV, the dust collecting voltage is set to a rectangular wave of ±7.5kV, the distance between the electrodes of the dust collecting portion 50 is set to 9mm and a time period of operating the dust collecting portion is set to 30 minutes. As a result of the experiment, the dust collecting rate tends to be maximized at the.particle size of 0.5mm through 2mm in any of the frequencies. Further, a high dust collecting rate is constituted at the frequency of 0.1Hz and 1Hz than 4Hz and 10Hz. As described above, it can be said that an optimum frequencies in the electric dust collecting apparatus would be within a range of 0.1Hz through 2Hz. (Reference experiment) Further, as a reference, Fig. 7 shows a frequency characteristic of a dust collecting rate when alternating-current high voltage of sine waveform is applied to the dust collecting portion 50. In Fig. 7, Fig. 7 (A) shows a case of a particle size of 0.3mm through 0.5mm and Fig. 7 (B) shows a case of a particle size of 2 through 5mm. As experimental conditions, wind speed is set to 5m/s, the length of the dust collecting portion 50 is set to 206mm, charging voltage is set to llkV in direct current, dust collecting voltage is set to 5kVrms in alternating-current having sine waveform and frequency is changed - 13 in a range of 25Hz through 100Hz. The distance between the electrodes of the dust collecting portion is set to 6mm. As a result of the experiment, the higher the frequency, the lower the dust collecting rate. The reason is that as shown in Fig. 8 (particle oscillationmodel in respective frequencies), since the frequency is high, charged particles flowing into the dust collecting portion 50 are trapped at a space between the electrodes and discharged without being caught and collected onto the dust collecting electrode. As has been explained above, according to the electric dust collecting apparatus of the first embodiment, a high dust collecting rate can be achieved by effectively preventing rescattering with a small power source capacity. In other words, an optimum frequency for maintaining a high dust collecting rate at a low frequency (small power source capacity) can be selected. (Second embodiment of the invention) Here, it is presumable that in a voltage waveform having a waveform in a rectangular shape, the larger the voltage change rate dV/dt, which is an inclination of changing voltage in correspondence with section B shown in Fig. 3, the more restrained is the rescattering ) and the higher the dust collecting rate can be maintained. However, there poses a problem that the higher the voltage change rate dV/dt, an induction current value is more increased, and therefore, a current resistance value of a part at inside of a high voltage power source apparatus needs to be increased. Similarly, there also poses a problem that a cable size is thickened from such reason. It becomes an - 14 important problem in practice to select an optimum value of the voltage change rate dV/dt. In this connection, a description will be given hereinafter of an electric dust collecting apparatus according to a second embodiment of the invention capable of achieving a high dust collecting rate while having a necessary minimum voltage change rate of dV/dt value when alternating-current high voltage of the rectangular waveform is used and capable of simplifying a power source apparatus and cables. Fig-. 9 is a sectional structure view of the electric dust collecting apparatus according to -the second embodiment of the invention. The electric dust collecting apparatus of the second embodiment includes a charging portion 40 and a dust collecting portion 50. The charging portion 40 has a structure of line versus flat plate electrodes and includes grounding electrodes 21 and 22 as a pair of flat plates. and a high voltage electrode 23 in a line-like shape. Direct-current high voltage is applied between the grounding electrodes 21 and 22 and the high voltage electrode 23 from a high voltage power source 20 and corona discharge is generated at the charging portion 40. The polarity of the direct-current high voltage may be either of a positive or a negative polarity and the voltage may be pulse voltage. The dust collecting portion 50 has a structure of parallel flat plate electrodes and includes grounding electrodes 31 and 32 as a pair of flat plates and a high voltage electrode 33 in a form of one sheet of a flat plate. Alternating-current high voltage (high voltage in - 15 a trapezoidal waveform) is applied between the grounding electrodes 31 and 32 and the high voltage electrode 33 from an alternating-current high voltage power source 70 for generating alternating high voltage of a rectangular waveform. A frequency of the alternating-current high voltage (high voltage in trapezoidal waveform) is set to be equal to or lower than several kHz and a change rate dV/dt in a rising edge and a falling edge falls in a range of 50V/msec through 2000 V/msec. A gas flow including suspended particulate matter is charged by passing through the charging portion 40 and the suspended particulate matter are caught and collected onto the grounding electrodes 31 and 32 (dust collecting electrode) of the dust collecting portion 50. (Theoretical study) Hereinafter, an explanation will be given of a theoretical dust collecting rate of the electric dust collecting apparatus according to the second embodiment. A so-called "Deutsch equation", which is a calculating equation of a dust collecting rate in the electric dust collecting apparatus of the embodiment, is shown below in Equation (1). 77, =(l-exp(-aV))x100 [%] Equation (1) Here, the parameter "VD" designates direct-current voltage [V] applied to the dust collecting portion and the parameter "a" designates a proportional factor. When a density of particles at an inlet of the electric dust collecting apparatus is designated by parameter "Ni" [particles/M 3 ], a density of particles at an outlet thereof "No," is - 16 as shown below in Equation (2). No = N, exp(- aVD ) [particles / n 3 ] Equation (2) When a waveform of alternating-current voltage applied to the dust collecting portion is defined as shown in Fig. 10, a density of particles "Nou 1 " on an outlet side of the electric dust collecting apparatus at time tl is as shown below in Equation (3). No = N exp(- aV) [particles /n 3 ] Equation (3) Meanwhile, a density of particles "Not 2 " on the outlet side of the electric dust collecting apparatus at time t2 is as shown below in Equation (4) No 1 =N exp -a- [particles/ni 3 ] Equation (4) 2 Therefore, an average density of particles "Not" on the outlet side of the electric dust collecting apparatus at time t is as shown below in Equation (5). N, = N, {-I exp(- aV)+ exp - a V} [particles/rm 3 ] Equation (5) it (_ 2 Therefore, a dust collecting rate "A" of the electric dust collecting apparatus is as shown below in Equation (6). q7 = 1-i exp(-aV)--Lexp -a- x 100 [%] Equation (6) tt 2 Therefore, a difference "Air" between a dust collecting rate "ID" when direct current is applied and a dust collecting rate "nA" when alternating-current high voltage of a rectangular waveform is applied is as shown below in Equation (7). - 17 - AqyD qA t 2{exp(-aV)±exp akVJ .100 Equation (7) In the above, it is assumed that direct-current voltage "VD" is equal to an absolute value [VI of the alternating-current high voltage of the rectangular waveform. The following relationship is established between dV/dt and t2/t as shown below in Equation (8). dV - 2V- Equation (8) dt t t 2 From the Equations (7) and (8) , the following equation (Equation (9)) is derived. dV _2V1V - - V exp(-aV)+exp -a-Jx100 [%] Equation (9) dt t A77 2 By the Equation (9) , there can theoretically be calculated dV/dt necessary for an allowable difference "Ar" between the dust collecting rate "riD" of the direct-current electric dust collecting apparatus and the dust collecting rate "riA" of the alternating-current electric dust collecting apparatus. A relationship between the allowable difference "Arl" calculated from Equation (9) and dV/dt is shown in Fig. 11. From a result of the calculation, dV/dt necessary for making the allowable difference "Ari" fall within a range of 1% through 15% is as shown in Table 1 and generally becomes about 50V/msec through 2000V/msec. - 18 - Table 1 Dust collecting rate in DC (%) dV/dt necessary in AC of rectangular wave (V/msec) 70 167 80 129 90 83 95 55 A value of dV/dt necessary when allowable difference Ar < 15% Fig. 12 shows a block diagram of an experimental apparatus. In the experiment, as particles to be sampled, there are used particles in emission gas of a diesel engine (DEP) whose major component is carbon. A portion of emission gas is taken in by a brancher 71 and mixed with the atmosphere to dilute at a mixing tank 72. The diluted gas is delivered into a duct by a booster fan 73 and is diluted again with the atmosphere taken an intake port 2. The gas is processed by passing an ESP (Electrostatic Precipitator) 74 by a suction fan 75 and is discharged to outside of the duct. Flow speed at inside of the duct is set to 7m/s by a rotational number of the suction fan 75. The electric dust collecting apparatus of the second embodiment is disposed within the duct between the booster fan 73 and the ESP 74. At this occasion, the flow rate between the electrodes of the dust collecting portion becomes about 9m/s. Fig. 13 shows outlines of electrode structures of the charging portion and the dust collecting portion used in the above-described - 19 experiment. The charging portion (as shown in Fig. 13(A)) has a grounded flat plate electrode made of aluminum and a wire electrode made of tungsten having a diameter of 0.2 6mm for applying high voltage. In the charging portion, charging spaces configured by a pair of electrodes are aligned in 5 stages in parallel with the gas flow and 3 stages in series. Therefore, a total of 15 charging spaces are provided. As the dust collecting portion (as shown in Fig. 13 (B) ), flat plate electrodes for grounding and the flat plate electrodes for applying high voltage made of aluminum having the same size are alternately aligned at intervals of 9mm. Fig. 14 shows an arrangement of the respective electrodes in the duct. As shown in the drawing, the charging portion 40 and the dust collecting portion 50 are arranged in series in the direction of the gas flow and the dust collecting portion 50 is installed 5 downstream from the charging portion 40. Applied voltage at the charging portion is respectively set to 11kV having a direct current in negative polarity and applied voltage at the dust collecting portion is se to +7.5kV in alternating-current of a rectangular waveform. A frequency of the alternating-current high voltage of the rectangular 0 waveform (in trapezoidal shaped waveform) is set to 1Hz and dV/dt falls in a range of 46V/msec through 646V/msec. That is, particles charged in a negative polarity by negative corona discharge at the charging portion 40 are collected onto the grounding electrodes and the high voltage flat plate electrodes by an alternating-current electric field 5 of the rectangular waveform of the dust collecting portion 50. - 20 - Fig. 15 shows dependency of dV/dt on a particle size characteristic of the dust collecting rate. As experimental conditions, wind speed is set to 7m/s, temperature is set to 15.5 0 C, humidity is set to 35%, atmospheric pressure is set to 1032hPa, the charging portion is constituted by 1 unit, the dust collecting portion is being structured by 4 units, each unit having a length of 208mm, charging voltage is set to 11kV, dust collecting voltage is set to a rectangular waveform of ±7.5kV, the distance between electrodes of the dust collecting portion 50 is set to 9mm and the time period of > operating the dust collecting portion is set to 20 minutes. The drawing shows also a result of a case of applying-direct-current high voltage to the dust collecting portion (direct-current voltage of -7.5kV is applied, the dust collecting rate is about 70%) for comparison. With regard to the dust collecting rate in the range of the particle of 0.3mm through 1mm, the larger the dV/dt, the more increased is the dust collecting rate and when dV/dt=433V/msec or higher, the dust collecting rate substantially equivalent that in applying direct current is achieved. Meanwhile, at the particle size of 1mm or larger, the dust collecting rate shows a high value in any of dV/dt in comparison with the case of applying direct current. The result of the experiment of Fig. 15 does not coincide with the result of calculation shown in Fig. 11. This is because in any of particle sizes, the dust collecting rate in applying direct current is lowered by the reentrainment phenomena. From the above-described result, dV/dt of the waveform of the - 21 alternating-current high voltage in the electric dust collecting apparatus can be said to be equal to or higher than 50V/msec. Further, when dV/dt is equal to or higher then about 400V/msec, the dust collecting rate equivalent to or higher than that in applying direct current can be achieved. Therefore, a sufficient high dust collecting rate is achieved when dV/dt of the alternating-current high voltage of the rectangular waveform falls in a range of 50V/msec through 2000V/msec. As has been explained above, according to the embodiment, high function is achieved by providing the necessary minimum value of the voltage change rate dV/dt, which represents an inclination of rectangular wave in rising and falling. Further, according to the embodiment, the power source apparatus can be simplified and a cable thickness or, the like can be simplified in view of the operation. (Third Embodiment) Fig. 16 shows an electric dust collecting apparatus according to a third embodiment of the invention. Electrode constitutions of the charging portion 40 and the dust collecting portion 50 are similar to those in Fig. 1 and therefore, a detailed explanation thereof will be omitted. Fig. 16 illustrates a case in which voltage applied to the charging portion 40 is constituted by alternating-current high voltage (that is, alternating-current high voltage of sine waveform or alternating-current high voltage of rectangular waveform). Alternating-current high voltage is applied between the line - 22 electrode 23 and the flat plate electrodes 21 and 22 of the charging portion 40. As the alternating-current high voltage, not only alternating-current high voltage of a sine waveform but also alternating-current high voltage of a rectangular waveform can be used. In this way, according to the embodiment, an output waveform of an alternating-current high voltage power source 80 can arbitrarily be selected. Similarly, also with regard to an output waveform of an alternating-current high voltage power source 90 at the dust collecting portion 50, not only alternating-current high voltage of a sine 0 waveform but also alternating-current high voltage of a rectangular waveform can arbitrarily be selected. (Fourth Embodiment) Fig. 17 shows an electric dust collecting apparatus according to a fourth embodiment of the invention. According to the fourth 5 embodiment, the alternating-current high voltage power source 80 is commonly used for the charging portion 40 and the dust collecting portion 50 and a simpler system can be constructed by applying the same alternating-current high voltage. Also the output waveform of the alternating-current high voltage 0 power source 80 shown in Fig. 17 can arbitrarily be selected. That is, as the alternating-current high voltage, an alternating-current high voltage of a rectangular wave, a sine wave or the like can be used. According to the embodiments shown in Fig. 16 and Fig. 17, the 5 charging portion 40 is applied with alternating-current high voltage - 23 and therefore, positive and negative corner discharges are alternately generated. Therefore, by the alternating-current corona discharge, also particles passing through the charging portion 40 are alternately charged in positive and negative polarities and are efficiently corrected at the dust collecting portion 50. Particularly, by adopting the electric dust collecting system shown in Fig. 17, rescattering at the charging portion 40 is restrained, high voltage generating apparatus respective needed for the charging portion 40 and the dust collecting portion 50 are dispensed with and dust can be collected by a single high voltage generating apparatus. Fig. 18 enumerates to exemplify various electrode constitutions of a charging portion (cited from "Theory and practice of removing dust and collecting dust" by Chotaro Ono, Ohm-sha, 1978) . As shown by the drawing, any o.f the electrode constitutions may be used and is not restricted. Further, Fig. 19 shows general various discharge electrode shapes (cited from "Applied Electrostatic Precipitation" by K.R. Parker, Blackie Academic and Professional) . Any of the discharge electrode shapes may be used and is not restricted. As has been explained above, according to the third and the fourth ) embodiments of the invention, rescattering can effectively be prevented and a high dust collecting rate can be realized at low cost. Particularly, according to the fourth embodiment, by applying the same alternating-current high voltage to the charging portion and the dust collecting portion, the system can be simplified and 5 rescattering can be prevented. - 24 - The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and-with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents. - 25 -