CA1204391A

CA1204391A - Air cleaning apparatus

Info

Publication number: CA1204391A
Application number: CA000430372A
Authority: CA
Inventors: Ryozo Kawashima
Original assignee: NICHIELE Corp
Current assignee: NICHIELE Corp
Priority date: 1982-12-30
Filing date: 1983-06-14
Publication date: 1986-05-13
Also published as: DE3367337D1; KR850000260A; EP0114178B1; US4516991A; EP0114178A1

Abstract

ABSTRACT OF THE DISCLOSURE
An air cleaning apparatus produces by a corona discharge an air stream without mechanical section and removes impurity particles or contaminated particles contained in the air in the flowing course. The apparatus has a plurality of duct collecting panel electrodes alternately arranged oppositely to each other at a predetermined interval to become air flow passages, corresponding panel electrode group, and a number of ionizing wires extending at the position isolated from the ends of the electrodes. A value of voltage applied between the corresponding panel electrodes and the duct collecting panel electrodes is set to substantially one-second of the value of voltage applied between the ionizing wires and the dust collecting panel electrodes, and the length of the intervals between the correspondig panel electrodes and the dust collecting panel electrodes is determined to become a predetermined potential gradient in response to the value of the voltage applied between both the panel electrodes, thereby generating a corona discharge between the dust collecting panel and the ionizing wire to improve the dust collecting efficiency. The dust collecting panel electrodes and the corresponding panel electrodes are treated with an ozone decomposition accelerating noble metal plating layer. Activated charcoal ozne decomposing filter is arranged in the air stream outlet, thereby reducing the ozone flow rate. Further, this apparatus has another second ionizing wires installed at a predetermined distance from the first ionizing wires substantially on extension lines from the respective dust collecting panel electrodes at the further outside position from the position of arranging the first ionizing wires. Corona discharge is also produced between the first ionizing wires and the second ionizing wires, thereby increasing the treating air flow rate to improve the air cleaning efficiency in the room.

Description

439~

This invention relates to an air cleaning apparatus.
Prior art and the present invention will be described hereinafter with reference to the accompanying-drawings, wherein:
Fig. 1 is a plan view schematically showing a conventional air cleaner;
Fig. 2 is a partially fragmentary front view of a preferred embodiment of an air cleaning apparatus according to the present invention;
Fig. 3 is a partially fragmentary plan view of the apparatus in Fig. 2;
Fig. 4 is a partially fragmentary side view of the apparatus in Fig. 2;
Fig. 5 is a partially fragmentary back view of the apparatus in Fig. 2;
Fig. 6 is a circuit diagram showing the connecting relation between ionizing wires, dust collecting panel electrodes and a power source;
Fig. 7 is a plan view of the essential part of the embodiment.
Fig. 8 is a circuit diagram showing the connecting relation between the first, second ionizing wires and the panel electrodes, the power source in the apparatus in Fig.
7; and Fig. 9 is a circuit diagram showing the connecting relation between the panel electrodes and the power source in a modified embodiment of the panel electrode wiring state.

lza43sl As a conventional air cleaning apparatus lC
disclosed an air cleaner which is, for example, disclosed in Japanese Patent No. 996,051 is known. More particularly, a conventional air cleaner has, as shown in Fig. 1, a plurality of dust collecting electrodes 1 formed of aluminum, corresponding electrodes 2 alternately arranged oppositely to each other at an interval such as approx. 10 mm to become an air flow passage, and ionizing wires 3 installed at the outside isolated at a distance r from the line for connecting the ends of the respective electrodes 1 on an extension line from the electrodes 2. The distance r is defined to approx. 20 mm. The wires 3 and the electrodes are commonly connected as positive polarity, the electrodes 1 are as negative polarity, and a voltage of approx. 15 kV is applied from a power source 4 therebetween. A
corona discharge is produced between the wire 3 and the electrode 1 upon application of the voltage therebetween, thereby charging kinetic energy to neutral gas molecules to generate an air stream directed from the wires 3 toward the interval when numerous ions are moved to the side of the electrodes 1. Further, when 3~1 the air stream is produced, fine particles in the air are charged in one ion, is thus collected to the d~st collecting elec~rodes 1. Moreover, remaining fine particles which are not completely col~ected are collected to the electrodes 1 by an electric field formed between the electrodes 2 and 1 in the course of flowing in the interval. In case that the voltage supplied from the power source 4 is constant value, the force for producing an air stream in parallel with the panel surface of the electrodes 1 in the interval is, as shown in Fig. 1, given by the component force Fcos ~ of the force F directed from the wire 3 to the electrode 1, where ~ is an opening angle from the wire 3 as an origin between the end of the electrode 1 and-the end of the electrode 2. When ~he distance r is approached to O in this case, the angle ~ approaches 90 and accordingly the force Fcos ~ approaches 0. Thus, the force for producing the air stream is almost vanished. When the distance r is, on the other hand, increased, a magnetic field between the end of the wire 3 and the end of the electrode 1 decreases proportionally to 1/r , thereby remarkably weakening the corona discharge electric field. Thus, similarly to the above, air stream is almost vanished. Accordingly, when the power source voltage is constant, an adequate value exists in the set ~0439~

range of the distance 4, thereby defining the velocity of the air stream to be produced substantially ~o a predetermined value. - - ~
However, in such a conventional air cleaner, the duct collecting efficiency is disadvantageously insufficient in a practical use due to the long interval of approx. 10 mm of the interval between the dust collecting electrode and the corresponding electrode.
In addition, since no remedy means is provided against zone generated in case of corona discharge, the ozone flow rate out of the air cleaner excessively becomes approx. 200 ppb, which may affect the influence to human body.
The volume and the cleaning efficiency of a cleaning chamber to be cleaned by an air cleaner are different depending upon the purpose of using the cleaning chamber. Thus, the air cleaner requires the corresponding performance. However, in the conventional air cleaner, the velocity of the air stream to be produced is defined substantially to a predetermined value when the voltage of the power source is defined to a constant value. Therefore, the conventional air cleaner has drawbacks so as not to sufficiently respond to the above-described desires.

4 _ ~204391 It is an object of thi~ invention to provid~ an air cleaning apparatus which is capable of improving dust collecting efficiency and reducing ozone flow rate.
It is another object of thls invention to provide an air cleaning apparatus which has large processing capacity.
In order to achieve the above first object, there is provided according to this invention a cleaning apparatus which comprises a plurality of dust collecting panel electrodes and co~responding panel electrodes arranged oppositely to each other via a predetermined interval to become air flow passages in a casing having air flow inlet and outlet, and a number of ionizing wires installed at a predetermined distance from the end of the dust collecting panel electrodes substantially on extension lines from the respective corresponding electrodes at the position out of the interval.
Further, the dust collecting panel electrodes, and the corresponding panel electrodes and the 1onizing wires are provided at a narrow interval so that the corresponding panel electrodes and the ionizing wires are equal polarity to the dust collecting panel electrodes, the voltage applied between the dust collecting panel electrodes and the corresponding panel electrodes is set to substantially one-second of that lZ0~391 applied between the dust co~lecting panel electrodes and the ionizing wires, and the length of the interval between the dust collecting panel electrodes and the corresponding panel electrodes is at a predetermined potential gradient in response to the applied voltage value. In addition, ozone decomposition accelerating noble metal plating layer is coated on each of the dust collecting panel electrodes and the corresponding panel electrodes, and ozone decomposing filter formed of activated coal is arranged at the air flow outlet. This air cleaning apparatus of the invention can thus improve the dust collecting efficiency to sufficient degree in practical use and can reduce the ozone flow rate.
In order to further achieve the second object, there is provided acco~ding to the invention an air cleaning apparatus which further comprises another serond ionizing wires installed at a predetermined distance from the first ionizing wires substantially on extension lines of the respective dust collecting panel electrodes at the position further outer than the arranging position of the first ionizing wires, thereby generating a corona discharge between the first and the second ionizing wires to accelerate the produced air streamO
With this structure, the velocity of the air stream can be accelerated (air flow rate per unit time), thereby remarkably improving the air cleaning efficiency.
The above and other objects and features of the present invention will beGomes apparent from a reading of the followiny description with reference to the accompanying drawings.
Figs. 2 to 6 show a typical embodiment of an air cleaning apparatus according to the present invention. In Figs.

2 to 4, reference numeral 5 designates a casing, in which inlet and outlet side mask frames 6 and 7 respectively having mask nets 6a and 7a are detachably mounted to become air flow inlet and outlet at left and right sides. A stand 8 is mounted at the lower portion of the casing S, and a handle 9 is mounted on the top of the casing 5.
Said casing 5 has therein units which respective have ionizing function, dust collecting function and ozone decomposing function, and a containing frame 10 for holding the units. More particularly, as shown in Fig. 3, a unit containing frame lO is fixedly secured substantially to the center of the casing 5, and a dust collecting unit frame 13 is detachably mounted on the frame lO. The frame 13 has a plurality of dust lZO~91 collecting panel electrodes 11 and corresponding panel electrodes 12 alternately arranged oppositely to each other at a predetermined interval 14. The interval or air gaps 14 form air flow passages, which axe maintained, for example, at approx. 5 mm. The electrodes ll and 12 are formed of substrates such as metal plates made of brass or copper, and are treated with ozone decomposition accelerating silver plating layers. The metal for accelerating the ozone decomposition may include, for example, not only the silver, but noble metals such as gold, or platinum.
Each of the electrodes 12 is formed ~arrower in width and shorter in length than the electrode 11, and is, as shown in Fig. 3, disposed at the edge lla inside the interval by a predetermined distance from the line for connecting the edges lla of the electrodes 11. A pair of terminal boards 15 and 16 are bonded, as shown in Fig. 4, on the inner surfaces in the vicinities of the rear edge of the frame 13 (In the description, the front and the rear define the air flow inlet side and outlet side, respectively.), the electrodes ll are commonly connected to the upper board 15, and the electrodes 12 are commonly connected to the lower board 16. 16a designates a plug socket, and 16b designates a terminal receptacle, and the other board 15 also has similarly lZ~439~.

a plug socket and a terminal receptacle (not shown).
Thus, the electrodes 11 and 12 are respectively connected to a power source E1 through the plug socket and the terminal receptacle as will be described with reference to Fig. 6. The frame 13 is formed, as shown in Fig. 3, in tapered surfaces on the four outer peripheral surfaces. The inner surface of the frame 10 are also formed in the tapered surface corresponding to the tapered surfaces of the frame 13, which is detachably from the rear side to the frame 10. 17 and 17 depict night latches, and the frame 13 is anchored by the latches 17 at the inserted position. The electrodes 11 and 12 are detachably provided integrally to the casing 5 by the insertion or removal of the frame 13.
The frame 10 is slightly expanded at the front side. An ionizing unit frame 18 formed of metal is engaged with the expanded portion (Figs. 3 and 4).
Reference numeral 19 illustrates an ionizing unit retaining frame, and the frame 18 is secured fixedly by the frame 19 at the engaged position. Ionizing wires 20 are installed between the upper and lower beams at the frame 18. The wires 20 are formed of tungsten wires having approx. 1 mil of thickness, and are treated with noble metal plating layer of gold similarly to the above. Each wire 20 has a eoil spring 21 elastically 1~043gl extended at the lower portion thereof.` The lower end of each spring 21 is engaged with a hole 18a perforated at the frame 18, and the upper end of each wire 20 is engaged fixedly by a screw 22 with the frame 18. The wire 20 is defined at the position isolated at a predetermined distance such as, for example, approx. 20 mm from a line for connecting the front edges lla of the electrodes 11 on the front extension line of the respective electrodes 12. The po~ition of the wire can be readily defined by elastically engaging the spring 21. Each wire 20 is connected to the power source E
via a lead wire (not shown) led from the frame 18.
Shielding plates 23a and 23b formed of plastic for preventing ozone are stood in a predetermined height between the vicinity of the installing ends of the wires 20, the electrodes 11 and 12.
On the other hand, filter frame mounts 24 are extended from four rear corners o-f the frame 10, and an ozone decomposing filter 25 is enaged with the mounts 24. The filter 25 is formed of activated coal, which is pulverized in mesh of approx. 12 cells/square inch, thereby enhancing the ozone decomposing function.
Fig. 6 shows the connecting state of the electrodes 11 and 12 and the power source El, and the electrodes 11 are connected to a negative terminal 26 in negative 12~439~L

polarity. The wires 20 and the electrodes 12 are connected in positive polarity, and the wires 20 are connected through a discharge current regulating resistor R to the positive terminal 26a, and the electrodes 12 are connected to an intermediate terminal 26b of 1/2 voltage point. The voltage value of the terminal 26a is, for example, 15 kV. In this connection state, +15 kV is applied to the wires 20 with respect to the electrodes 11, and +? .5 kV of 1/2 voltage is applied to the electrode 12. The length of the interval between the electrodes 12 and 11 is defined to approx. 5 mm to maintain a predetermined potential gradient, approx. 1.5 kV/mm corresponding to 1/2 of the applied voltage value.
In Figs. 2 and 4, reference numeral 27 designates a power switch, 28 a power cord, PL a pilot lamp, 29 and 30 safety limit switches, and 29a and 30a limit switch mounting brackets. The switches 29 and 30 are composed of normally closed contacts connected in series with the switch 27 and switched to OFF when the inlet or outlet side mask 6 or 7 is removed, thereby preventing the high voltage from contacting a hand.
The operation of the above embodiment of the air cleaning apparatus will be described.
The air cleaning appara~us is installed at a predetermined position in a room. When the switch 27 is ~Q~313i closed ON, with the electrodes 11 in negative polarity 15 kV is applied between the electrodes 11 and the wires 20, and 7.5 kV of 1/2 of 15 kV is applied between the electrodes 11 and 12. Corona discharges are produced by the 15 kV applied between the wires 20 and the electrodes 11. When the numerous ions moved by the corona discharges to the electrodes 11 side, their kinetic energy is applied ~o the neutral gas molecules, a type of air stream is generated in this manner, thereby producing an air stream flowing toward the interval 14 at a predetermined velocity such as approx.
60 m/min. Simultaneously, impurity particles in the air are charged in one ions and are collected to the electrodes 11. On the other hand, since 7.5 kV is also applied through the interval 14 between the electrodes 11 and 12, the remaining particles which are not collected by the previous corona discharge of the impurity particles in the air are attracted to the electrodes 11 and are collected. In the present invention, the length of the interval 14 is narrow such as 5 mm. Accordingly, the impurity particle collecting probability in the course of passing the interval 14 is increased, thereby effectively performing the dust collection. The measured example of the efficiency is shown as below:

i2~43C31 Impurity particles (~) Dust collecting èfficiency~) 0.3 98.70 0~5 9~.59 ..
1.0 99.99 . _ , ~ .
The dust collecting efficiency of the conventional air cleaner of electrostatic type is normally approx.
50% .
A large quantity of ozone i5 produced when the above corona discharge with the high electric field.
However, the ozone is contacted with the silver plating layer coated on the electrodes 11 and 12 in the course of flowing the interval 14 and is decomposed to oxygen molecules. Since the electric field is concentrated in the vicinity of the ends of the wires 20, the quantity of produced ozone in this part tends to increase as compared with the other part. Since the plates 23a and 23b are however stood on this part, the corona discharge is disturbed by the plates, thereby suppressing the production of the ozone in this part.
The quantity of the produced zone can be reduced to approx. 20 ppb of approx. 1/10 of the conventional one even by the ozone decomposition of the silver plating lZ0~3gl layer and the ozone production preventing operation of the plates 23a and 23b. The ozone thus reduced is further decomposed in contact with the ozone decomposing filter 25 of activated coal in the course of flowing out from the outlet side. Since the filter 25 is formed in the predetermined mesh of 12 cells/square inch, the flowing ozone can be progressively decomposed effectively in contact with the surface of the activated coal, and can be further reduced. The degree of decomposing the ozone of the filter 25 depends upon the quantity of the ozone flowed to the f~lter, but decomposes 25 to 40% of the ozone is decomposed by the filter. Since the filter 25 is inactivated as it is used, it is necessary to suitably exchange the filter, but since the filter 2~ in this invention is formed of activated ~oal, its lifetime is maintained over one year.
As described above, the noble metal plating layers coated on the plates 23a and 23b, and the electrodes 11 and 12 as well as the filter 15 cooperate to suppress the production of ozone or to effectively decompose the ozone so as to remar~ably reduce the ozone less than the stipulated quantity so as not to produce a defect on human body.
As the progressive use, impurity particles in the lZ~3gl air are adhered to the wires 20, resulting in extension of thè particles in stylus state to the electrode 11 side. The variation in the electric field occurs between the wires 20 and the electrodes 11 due to the adherence of the particles in the stylus state to the wires 20, and a trend of generating a self-exciting vibration noise takes place at the wires 20. Since the spring 21 is installed elastically at the wires 20, it can alleviate the vibration, thereby reducing the production of the noise.
Further, the impurity particles in the air are accumulated on the electrodes 11 due to the above described effectively dust collecting operation.
Accordingly, it is necessary to clean the electrodes 11.
At this time, the electrodes 11 and 12 are removed from the casing 5 together with the frame 13 and are cleaned.
Then, in Figs. 7 to 9, another preferred embodiment of the air cleaning apparatus according to the invention is shown. In Figs. 7 to 9, the members or those equal or equivalent to those members are designated by the same reference numerals in Figs. 2 to 8 and will not accordingly be described but will be omitted.
In this embodiment, as shown in Figs. 7 and 8, another second ionizing wires 20b are installed at a predetermined distance from the first ionizing wires 20a 12~4391 substantially on extension lines from the respective electrodes 11 at the position outer than the arranging positions of the wires 20a, and corona discharges are also produced even between the wires 20a and the wires 20b.
This arrangement will be further described in more detail. First and second ionizing unit frames 31a and 31b formed of metal are, for example, engaged fixedly at a predetermined interval such as approx. 13 mm at the inlet side expanded part of the unit c~ntaining frame lOo The first ionizing wires 2da are installed between the upper and the lower beams in the frame 31a, and the second ionizing wires 20b are installed between the upper and the lower beams in the second ionizing unit frame 31b. Both the wires 20a and 20b-are constructed similarly to those in the first embodiment at the points that the ozone decomposition accelerating noble metal plating layers are coated and that the coil spring is mounted at the lower parts. With this installing state, the first ionizing wires 20a are defined at the position isolated at a predetermined distance such as, for example, 13 mm from the line for connecting the edges lla of the respective wires 11 on the front extension lines from the respective electrodes 12. Further, the second ionizing wires 20b are installed at the position ~Z~3g~

isolated at a predetermined distance such as, for example, 13 mm from the line for conn~ecting the respective wires 20a on the extension lines from the respective electrodes 11 at the position outer from the arranging positions of the wires 20a. In the invention, the first and second wires 20a and 20b are arranged in two stages. When the wires 20a and 20b are elastically installed by the coil spring, the wires can be readily defined at the position to be installed. The wires 20a and 20b are respectively connected to the terminals of a power source E2, which will be described later via lead wires (not shown) led from the frames 31a and 31b.
Fig. 8 shows the connecting states of the electrodes 11 and 12, the wires 20a and 20b and the power source E2. The wires 20a are connected to a 0 volt terminal 32a, the wires 20b are connected to positive V terminal 32b, the electrodes 11 are connected to negative V terminal 32c, and the electrodes 12 are connected to negative 1/2V terminal 32d. The voltage value V is defined as an example, to 12.5kV.
Accordingly, with the electrodes 11 as reference, the wires 20a are applied with positive 12.5 kV, the electrodes 12 are applied with positive 6.25 kV of 1/2 of the 12.5 kV, and the wires 20b are applied with positive 25 kV. A predetermined discharge voltage of ~Z0~3~1 12.S kV is applied between the electrodes 11 and the wires 20a and between the wires 20a and the wires 20b.
On the other hand, the length of the interval between the electrodes 11 and 12 is defined to slightly longer than 4 mm corresponding to the applied voltage value of the 1/2 so as to set a predetermined potential gradient of approx. 1.5 kV/mm.
The operation of this embodiment will be described.
When the power switch tnot shown) is closed ON, 12.5 kV
is applied between the electrodes 11 and the wires 20a and between the wires 20a and the wires 20b, thereby producing corona discharges therebetween. When numerous ions move with the corona discharges toward the wires 20a and the electrodes 11 sides, their kinetic energy is applied to the neutral gas molecules, thereby producing a type of yas stream. Thus, an air stream is produced at a predetermined velocity from the wires 20a and 20b toward the interval side. In this embodiment, the discharge sections are formed in two stages. Then, the initial flow produced by the corona discharge of the first stage between the wires 20a and 20b is accelerated by the corona discharge of second stage between the wires 20a and the electrodes 11, performing the velocity of the air stream to reach approx. 85 m/min.
This velocity is accelerated by approx. 40~ as compared lZ()43~

with that in the first embodiment. As this air stream is produced, impurity particles in the air are charged to one ions and are collected to the electrodes 11. On the other hand, a voltage of 6.25 kV is applied through the interval between the electrodes 11 and 12.
Accordingly, the remaining particles not collected by the corona discharge of the particles in the air are attracted onto the electrodes 11 by the electric field produced in this manner and are collected. This particle collecting operation is formed in a narrow width such as, for example, approx. 4 mm in the length of the interval. Even if the velocity is accelerated, this operation can be remarkably effectively performed~
In Fig. 9. a modified example of the panel electrode arranging state in the above second embodiment is shown. In this modified example, the arrangement of the corresponding panel electrodes is omitted as compared with that in Figs. 7 and 8. According to this modified example, since no arrangement of the corresponding panel electrodes exists, the velocity of the air stream flowing in the interval, and hence the point of air flow rate, can be further accelerated.
While there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that various i~O4391 modifications may be made therein, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An air cleaning apparatus comprising a plurality of dust collecting panel electrodes and corresponding panel electrodes alternately arranged oppositely to each other at a predetermined interval to becomes air low passages in a casing having air flow inlet and outlet, a number of ionizing wires installed at a predetermined distance from the edges of said dust collecting panel electrodes substantially on extension lines from said respective corresponding electrodes at the position outside the interval so that said corresponding panel electrodes and said ionizing wires are in the same polarity as said dust collecting panel electrodes and a voltage of a predetermined value is applied between said dust collecting panel electrodes and said corresponding electrodes, said ionizing wires, corona discharges are produced between said ionizing wires and said dust collecting panel electrodes, an air stream is produced at said interval by the corona discharges and a predetermined potential gradient is produced between said respective dust collecting panel electrodes and said corresponding panel electrodes, characterized in that the voltage value applied between said corresponding panel electrodes and said dust collecting panel electrodes is set to substantially 1/2 of the voltage value applied between said ionizing wires and said dust collecting panel electrodes, the length of the respective intervals between said corresponding panel electrodes and said dust collecting panel electrodes are set to those becoming a predetermined potential gradient in response to the applied voltage value, and further ozone decomposition accelerating noble metal plating layer is coated on said dust collecting panel electrodes and said corresponding panel electrodes, and activated coal ozone decompositing filter is arranged in said air flow outlet.

2. The air cleaning apparatus as claimed in claim 1, wherein ozone production preventing shielding plates are stool between the vicinity of the installing ends of said ionizing wires, said dust collecting panel electrodes and said corresponding panel electrodes.

3. The air cleaning apparatus as claimed in claim 1, wherein a spring is interposed between each said ionizing wire and said casing side, and each said ionizing wire is elastically installed.

4. The air cleaning apparatus as claimed in claim 1, wherein said dust collecting panel electrodes and said corresponding panel electrodes are mounted on a dust collecting unit frame detachably provided on said casing.

5. An air cleaning apparatus comprising a plurality of dust collecting panel electrodes provided at least as panel electrodes in parallel with each other at a predetermined interval to become air flow passages, first ionizing wires respectively installed at predetermined distance from the ends of said dust collecting panel electrodes at the positions substantially corresponding to the intermediate positions for dividing said interval into two segments at the position outside said intervals, second ionizing wires respectively installed at predetermined distance from said first ionizing wires substantially on the extension lies from said respective dust collecting panel electrodes at the position outside said first ionizing wires so that the voltages of the same polarity are applied to said first and second ionizing wires with respect to said dust collecting panel electrodes, the voltage value between said dust collecting panel electrodes and said second ionizing wires is higher by a predetermined value than the voltage value between said dust collecting panel electrodes and said first ionizing wires, thereby producing corona discharges between said dust collecting panel electrodes and said first ionizing wires and between said first ionizing wires and said second ionizing wires and producing an air stream in said interval by the corona discharges.

6. The air cleaning apparatus as claimed in claim 5, wherein said panel electrodes comprises a plurality of dust collecting panel electrodes and corresponding panel electrodes, said dust collecting panel electrodes and said corresponding panel electrodes are alternately arranged oppositely to each other at predetermined interval to become air flow passages, said first ionizing wires are installed substantially on an extension liens from said respective corresponding electrodes, the voltage as the same polarity as said first ionizing wires is applied to said corresponding panel electrodes, the voltage value applied between said dust collecting panel electrodes and said corresponding panel electrodes is substantially 1/2 of the voltage value applied between said dust collecting panel electrodes and said first ionizing wires, and the length of said each interval between said dust collecting panel electrode and said corresponding panel electrodes is set to that becoming a predetermined potential gradient in response to the applied voltage value.