AU2008207611B2

AU2008207611B2 - Electric dust collector

Info

Publication number: AU2008207611B2
Application number: AU2008207611A
Authority: AU
Inventors: Yoshiyasu Ehara; Yasuhiro Takagi; Takeo Takahashi; Koji Yasumoto; Akinori Zukeran
Original assignee: Fuji Electric Co Ltd
Current assignee: Fuji Electric Co Ltd
Priority date: 2007-08-31
Filing date: 2008-08-28
Publication date: 2013-01-17
Anticipated expiration: 2028-08-28
Also published as: KR101375468B1; AU2008205431A1; CN101412006A; KR20090023148A; CN101412006B; JP2009072772A; AU2008207611A1

Abstract

[Abstract] [Object] To provide an electric dust collector capable of efficiently collecting nanometer-class ultrafine particles such as diesel emission particles (DEPs) without the need for adding a special device. 5 [Means for solution] In an electric dust collector (10) which is configured in such a manner that a charging unit (20) and a collecting unit (30) are disposed as a front stage and a rear stage, respectively, the collecting unit (30) is formed by a parallel-plate-like high voltage electrode (35) and a parallel-plate-like ground electrode (32) that are arranged parallel with each other so as to be opposed to each other via spaces where 10 processing air flows, and a plurality of punching holes (35h) are formed in at least one of the two kinds of electrodes in a dispersed manner over the entire area. [Selected drawing] Fig. I

Description

- 1 AUSTRALIA PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT ORIGINAL Name of Applicant/s: Fuji Electric Systems Co., Ltd. and Yoshiyasu Ehara Actual Inventor/s: Koji Yasumoto and Akinori Zukeran and Yasuhiro Takagi and Yoshiyasu Ehara and Takeo Takahashi Address for Service is: SHELSTON IP 60 Margaret Street Telephone No: (02) 9777 1111 SYDNEY NSW 2000 Facsimile No. (02) 9241 4666 CCN: 3710000352 Attorney Code: SW Invention Title: ELECTRIC DUST COLLECTOR The following statement is a full description of this invention, including the best method of performing it known to me/us: File: 59253AUP01 -2 [Title of the Invention] Electric Dust Collector [Technical Field] The present invention relates to an electric dust collector for cleaning air or a gas 5 existing in a room, a tunnel, or the like and polluted with fine dust particles, various floating particles, etc. by removing such pollutants from the air or gas. [Background Art] Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common 10 general knowledge in the field. For example, the air in an automobile road tunnel is polluted with fine floating particles such as smoke produced being contained in exhaust gases emitted from passing vehicles and abrasion dust particles that are produced from tires and a road surface asphalt pavement material as vehicles run. A two-stage electric dust collector equipped 15 with a charging unit and a collecting unit is used for removing pollutants such as floating particles from such polluted air. A general two-stage electric dust collector is configured as shown in Fig. 9. The electric dust collector 10 is equipped with a charging unit 20 and a collecting unit 30. In the charging unit 20, a positive or negative corona discharge is generated by 20 applying a high DC voltage between a flat-plate-like or linear discharge electrode 21 and flat-plate-like ground electrodes 22 that are arranged parallel with each other, whereby the air flowing between the electrodes is ionized and floating particles contained therein are charged positively or negatively (i.e., to a single polarity). In the collecting unit 30, a static electric field is formed by applying a high DC voltage between a flat-plate-like 25 high voltage electrode 31 and ground electrodes 32 that are arranged parallel with each -3 other, whereby the high voltage electrode 31 collects floating particles that have been charged by the charging unit 20 by attracting them by Coulomb force. With this configuration, when polluted air containing floating particles is supplied through an inlet 11, located upstream of the charging unit 20, of the dust collector 10 by a fan or the like, 5 clean air that has been deprived of floating particles can be taken out from an outlet 12, located downstream of the collecting unit 30, of the dust collector 10. Having high collection efficiency even for fine particles of I jim or less and being suitable for processing of high-flow-rate air, such an electric dust collector is used as one for cleaning the air in an automobile road tunnel. 10 Incidentally, the air in an automobile road tunnel contains a large amount of diesel emission particles (DEPs) that are emitted from diesel vehicles etc. DEPs, which are ultrafine particles (diameter: 100 nm or less) and nanoparticles (diameter: 50 nm or less), are light and hence raise a problem that they cannot be collected efficiently even by an electric dust collector. Since ultrafine particles and nanoparticles such as DEPs 15 adversely affect human health, it is strongly desired to remove them by collecting them efficiently. One conventional means for increasing the collection efficiency is to form many punching holes in the collecting electrode plate in a dispersed manner (Refer to Japanese Patent No. 3427165). 20 In Japanese Patent No. 3427165, the punching holes are provided in such a manner that their total hole area accounts for 10 to 50% of the area of the collecting electrode plate. Japanese Patent No. 3427165 states as follows: The punching holes provide advantages that current from the discharge electrode plate is concentrated in portions between the punching holes, that dust stuck to the collecting electrode plate is 25 prevented from growing further because of the small area of the collecting electrode -4 plate, and that air flowing through the punching holes suppress excessive growth of dust. Therefore, dust lumps stuck to the collecting electrode plate fall and collected dust does not grow. As a result, the collection efficiency does not decrease even during long-term use. 5 [Disclosure of the Invention] [Problem to be solved by the invention] As described above, in the conventional electric dust collector, no consideration is given to increasing the collection efficiency for ultrafine particles and nanoparticles though it is improved in that the collection efficiency is increased by forming many 10 punching holes in the collecting electrode plate in a dispersed manner. An object of the present invention is to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative. In one aspect, the invention provides an electric dust collector which is configured in such a manner that a charging unit and a collecting unit are disposed as a 15 front stage and a rear stage, respectively, wherein the collecting unit is formed by a parallel-plate-like high voltage electrode and a parallel-plate-like ground electrode that are arranged parallel with each other so as to be opposed to each other via spaces where processing air flows; and wherein at least one of the two kinds of electrodes has a plurality of punching holes formed therein in a dispersed manner over an entire area of 20 the least one of the two kinds of electrodes and a diameter of the punching holes is in a range of diameter size, the range of diameter size being such that, in use, a region around a circumference of each of the punching holes where electric field strength is enhanced becomes wider than a region around the circumference of each of the punching holes where electric field strength is weakened.

- 4a In the aspect of the invention recited above, it is preferable that the diameter of the punching holes be 2 to 10 mm. Furthermore, it is further preferable that the punching holes be formed in the high voltage electrode.

-5 [Advantages of embodiments of the invention] The invention employs the simple structure that many punching holes are merely formed, in a dispersed manner over the entire area, in at least one of the two kinds of electrodes of the collecting unit for collecting floating particles contained in processing 5 air. Since the electric field is concentrated in peripheral regions around the punching holes of the electrode plate, the electric field between the electrodes of the collecting unit becomes non-uniform, that is, a non-uniform electric field develops that includes strong electric field regions and weak electric field regions. Nanometer-class ultrafine floating particles can be collected efficiently in the strong electric field regions, and the 10 collection (dust collection) efficiency for ultrafine particles, which are hard to collect ordinarily, can be increased. [Brief Description of the Drawings] A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: 15 Fig. I shows a basic configuration of an electric dust collector according to an embodiment of the invention; Fig. 2 is a front view of a high voltage electrode used in the invention; Fig. 3 shows the configuration of a dust collection experimental apparatus including the electric dust collector according to the invention; 20 Fig. 4(a) and 4(b) are perspective views showing the structures of examples of electrodes of a charging unit and a collecting unit of the electric dust collector according to the invention; Fig. 5 is a graph showing collection efficiency characteristics with respect to the diameter of collected fine particles of electric dust collectors that use a first group of 25 sample electrodes according to the invention as the high voltage electrode; -6 Fig. 6 is a graph showing collection efficiency characteristics with respect to the diameter of collected fine particles of electric dust collectors that use a second group of sample electrodes according to the invention as the high voltage electrode; Fig. 7 is a graph showing a collection efficiency characteristic with respect to the 5 total circumferential length of punching holes of the high voltage electrode of the electric dust collector according to the invention; Fig. 8 is a graph showing a relationship between the hole diameter and the electric field strength in peripheral regions around the punching holes of the high voltage electrode of the electric dust collector according to the invention; 10 Fig. 9 shows the configuration of a conventional electric dust collector; and Fig. 10 shows the configuration of an electric dust collector according to another embodiment of the invention. [Description of symbols] 10: Electric dust collector 15 20: Charging unit 22: Ground electrode 25: Discharge electrode 30: Collecting unit 32: Ground electrode 20 35: High voltage electrode 35h: Punching hole [Best Mode for Carrying Out the Invention] An illustrated embodiment according to the invention will be described below.

-7 Fig. I shows a basic configuration of an electric dust collector according to the embodiment of the invention. In Fig. 1, reference numeral 10 denotes an electric dust collector which is equipped with a charging unit 20 and a collecting unit 30. In the charging unit 20, a 5 stainless steel wire discharge electrode 25 and flat-plate-like ground electrodes 22 that are arranged parallel with each other are provided in a body 11. A corona discharge is generated by applying a high DC voltage between the discharge electrode 25 and the ground electrodes 22, whereby the air flowing between the two electrodes is ionized and floating particles contained therein are charged to a single polarity. In the collecting unit 10 30, a static electric field is formed by applying a high DC voltage between parallel-plate like ground electrodes 32 and a high voltage electrode 35 obtained by forming many punching holes 35h in a parallel-plate electrode (the electrodes 32 and 35 are arranged parallel with each other) and the ground electrodes 32 collect floating particles in the air that have been charged by the charging unit 20 by attracting them by Coulomb force. 15 With this configuration, when polluted air containing floating particles is supplied through an inlet 11, located upstream of the charging unit 20, of the dust collector 10 by a fan or the like, clean air that has been deprived of floating particles can be taken out from an outlet 12, located downstream of the collecting unit 30, of the dust collector 10. Basically, the electric dust collector 10 according to the invention is almost the 20 same as the conventional one. The electric dust collector 10 according to the invention is different from the conventional one in that as shown in Fig. 2 the high voltage electrode 35 of the collecting unit 30 is obtained by forming many punching holes 35h, in a dispersed manner over the entire area, in a parallel-plate-like electrode plate made of stainless steel or the like. It is preferable that the punching holes 35h be formed 25 uniformly.

-8 The inventors conducted various experiments and have found that when the many punching holes 35h are formed in the high voltage electrode 35 of the collecting unit 30, an electric field is concentrated in peripheral regions around the punching holes 35h of the high voltage electrode 35 and finer floating particles can be collected by the 5 concentration of the electric field, which concentration occurs in the peripheral regions around the punching holes 35h. The invention is based on the above knowledge. That is, the inventors have found that the simple structure that the many punching holes 35h are merely formed, in a dispersed manner over the entire area, in at least one of the two kinds of electrodes 32 10 and 35 of the collecting unit 30 can increase the collection efficiency for ultrafine particles and nanoparticles of the electric dust collector 10, and that the removal ratio of diesel emission particles (DEPs) can be increased when the invention is applied to processing of the air in a tunnel. Next, results of dust collection experiments that were conducted with the dust 15 collector 10 according to the invention will be described. Fig. 3 shows the configuration of a dust connection experimental apparatus including the electric dust collector 10 according to the invention. This experimental apparatus is to clean an exhaust gas that is emitted from a diesel engine 50 by collecting diesel emission particles contained in the exhaust gas. 20 An exhaust gas from the diesel engine 50 is thinned down being mixed with external air by an air mixer 51 and then sent to the electric dust collector 10 at a flow speed of 7 m/s by a fan 52. The electric dust collector 10 processes the exhaust gas supplied (i.e., separates and collects floating particles such as diesel emission particles contained therein) and outputs a cleaned exhaust gas. To measure the collection 25 efficiency of the electric dust collector 10, particle counters 61 and 62 capable of -9 counting the number of floating particles for each particle diameter are disposed on the exhaust gas input side and output side of (i.e., upstream and downstream of) the electric dust collector 10. The collection efficiency 'i (%) of the electric dust collector 10 is given by the 5 following Equation (1): i = (1 - Nd/Nu) x 100(%) (1) where Nu and Nd are the counts of the counters 61 and 62 disposed upstream and downstream of the electric dust collector 10, respectively. Fig. 4 shows specific electrode structures of the charging unit 20 and the 10 collecting unit 30 of the electric dust collector 10 that was used in the dust connection experiments. The charging unit 20 is configured in such a manner that ground electrodes 22 each of which is a flat-plate-like stainless steel electrode measuring 65 mm (width) x 70 mm (height) and a discharge electrode 25 which is a tungsten wire of 0.26 mm in 15 diameter are arranged at intervals of 9.5 mm. A negative corona discharge is caused by applying a high DC voltage of -9 kV to the discharge electrode 25 with respect to the potential of the ground electrodes 22. The collecting unit 30 is configured in such a manner that ground electrodes 32 each of which is a flat-plate-like stainless steel electrode measuring 160 mm (width) x 20 70 mm (height) and a flat-plate-like stainless steel high voltage electrode 35 having the same size as the ground electrodes 32 are arranged at intervals of 9 mm so as to be parallel with an air flow. A DC voltage of -7.5 kV is applied to the high voltage electrode 35 with respect to the potential of the ground electrodes 32. Although the embodiment shown in Fig. 1 shows the collecting unit 30 consisting of one high voltage 25 electrode 35 and two ground electrodes 32, the present invention is not limited to such a -10 configuration. It is desirable, when a high air flow is expected through the collecting unit 30, that the collecting unit 30 include several ground electrodes 32 and several high voltage electrodes 35, each of which is arranged at a predetermined interval, alternatively, as shown in Figure 10. 5 Tables 1 and 2 show particulars of sample electrodes which were used as the high voltage electrode 35 in the dust collection experiments of the invention. In Tables I and 2, a comparative electrode which is given a sample No. SO is a flat-plate-like electrode in which no punching holes are formed. 10 [Table 1] Sample Hole Number Total area Hole area Total circumferential No. diameter of holes of holes ratio length L (mm) of (mm) (mm 2 ) (%) holes so 0 0 0 0 0 SAl 2.5 21 103 1.1 165 SA2 5 21 412 4.3 330 SA3 10 21 1,649 17.2 659 SA4 13 21 2,786 29.0 857 The sample electrodes in Table I which are given sample Nos. SAI-SA4 are such that the number of punching holes 35h formed in a flat-plate-like electrode measuring 160 mm x 70 mm is fixed at 21 and the hole diameter is varied from 2.5 mm 15 to 13 mm. In these sample electrodes, since the number of punching holes is fixed at 21, as shown in Table 1 the total area of holes, the hole area ratio, and the total circumferential length L of holes increase as the hole diameter increases.

- 11 [Table 2] Sample Hole Number Total area Hole area Total circumferential No. diameter of holes of holes ratio length L (mm) of (mm) (mm2) (%) holes SO 0 0 0 0 0 SBI 2.5 336 1,649 17.2 2,638 SB2 5 84 1,649 17.2 1,319 SB3 10 21 1,649 17.2 659 The sample electrodes in Table 2 which are given sample Nos. SBI-SB3 are such 5 that the hole area ratio ((total area of punching holes)/(electrode area) x 100 (%)) of punching holes 35h formed in a flat-plate-like electrode having the same size as the sample electrodes in Table 1 is fixed at 17.2 % and the hole diameter is varied from 2.5 mm to 10 mm. In these sample electrodes, since the hole area ratio is fixed at 17.2%, as shown in Table 2 the number of holes and the total circumferential length L of holes 10 decrease and the total area of holes increases as the hole diameter increases. A dust collection experiment was conducted by using the above-described experimental apparatus (see Fig. 3) for each electric dust collector 10 in which each of the sample electrodes SAI-SA4 and SBI-SB3 shown in Tables 1 and 2 was used as the high voltage electrode 35 of the collecting unit 30. Collection efficiency -q (%) for 15 diesel emission particles (DEPs; classified as ultrafine particles and nanoparticles in terms of particle diameter) was measured for each particle diameter. Fig. 5 shows a collection efficiency measurement result of the electric dust collectors 10 in which the sample electrodes of the sample Nos. SAI-SA4 shown in - 12 Table I (the number of punching holes was fixed) were used as the high voltage electrode 35. Fig. 5 is a graph in which the horizontal axis represents the particle diameter d (nm) of collected fine particles and the vertical axis represents the collection efficiency 71 5 (%) and which hence shows collection efficiency characteristics with respect to the particle diameter of the respective high voltage electrodes (SO and SA I -SA4). The definition of the collection efficiency rj is the same as defined by Equation (1). As seen from Fig. 5, the collection efficiency for ultrafine particles of 100 nm or less in particle diameter and nanoparticles of 50 nm or less was higher when the sample 10 electrodes SAI-SA4 having the punching holes were used as the high voltage electrode 35 than when the comparative electrode SO without punching electrodes was used as the high voltage electrode 35. In the experimental result, when the sample electrode SAl (hole diameter: 2.5 mm) was used as the high voltage electrode 35, the collection efficiency for ultrafine particles of around 100 nm in particle diameter sometimes 15 became lower than when the comparative electrode SO was used as the high voltage electrode 35. However, this seems to be a measurement error. We think that the reason why the collection efficiency is increased by forming punching holes 35h is as follows. Where punching holes 35h are formed, the electric field strength is enhanced in a 20 peripheral region around each hole and weakened around the center of each hole. In the region where the electric field strength is enhanced, the movement speed of charged particles is increased and the collection efficiency is increased. The sizes of the electric field-strength-enhanced region and the electric-field-strength-weakened region vary depending on the hole diameter.

- 13 Fig. 8 is a graph showing a relationship between the hole diameter and the sizes of the electric-field-strength-enhanced region and the electric-field-strength-weakened region. The graph of Fig. 8 is a result of an experiment in which ground electrodes measuring 20 mm x 20 mm were disposed on both sides of a high voltage electrode 5 measuring 20 mm x 20 mm at intervals of 9 mm and a punching hole of 2.5 to 13 mm in diameter was formed at the center of the high voltage electrode. How the areas of the regions where the electric field strength was higher and lower, respectively, than a value 8.3 x 105 V/m of a flat-plate-like electrode without a punching hole varied with respect to the hole diameter was measured and analyzed. 10 In the graph of Fig. 8, symbol Rs represents the area ratio (Ss/S x 100 (%)) of the area Ss of the region where the electric field strength is higher than 8.3 x 105 V/m of the flat-plate-like electrode without a punching hole to the area S of each of the spaces between the high voltage electrode and the ground electrodes (i.e., the area of each of the spaces between the high voltage electrode 35 and the ground electrodes 32 as viewed 15 from above in Fig. 1) and symbol Rw represents the area ratio (Sw/S x 100 (%)) of the area Sw of the region where the electric field strength is lower than 8.3 x 105 V/m to the area S. Fig. 8 thus shows how the area ratios Rs and Rw (vertical axis) vary with respect to the hole diameter (horizontal axis). As seen from the graph of Fig. 8, the electric-field-strength-enhanced region is 20 wider than the electric-field-strength-weakened region when the hole diameter is small. However, when the hole diameter is larger than 11 mm, the electric-field-strength enhanced region is narrower than the electric-field-strength-weakened region. This is considered due to the fact that the hole diameter is sufficiently greater than the distance -14 between the high voltage electrode and the ground electrodes (in this experiment, 9 mm). It is understood from the above discussions that the collection efficiency can be increased by widening the electric-field-strength-enhanced region and thereby increasing 5 the movement speed of charged particles, that is, by increasing the punching hole diameter and thereby increasing the total circumferential length L of holes. It is seen that the upper limit of the hole diameter should be determined taking the distance between the high voltage electrode 35 and the ground electrodes 32 into consideration. As seen from Fig. 5, the electric dust collector 10 using the sample electrode SA4 10 in which the total circumferential length L of the punching holes 35h in the high voltage electrode 35 is equal to 857 mm (longest) can attain high dust collection performance that the collection efficiency for ultrafine particles and nanoparticles (diameter: 30 to 100 nm) is higher than 60%. However, increasing the punching hole diameter to increase the total 15 circumferential length L of holes is associated with problems of reduction in electrode area and in the strength of the electrode plate. Fig. 6 shows a collection efficiency measurement result of the electric dust collectors 10 in which the second group of sample electrodes of the sample Nos. SB 1 SB3 shown in Table 2 (the hole area ratio was fixed) were used as the high voltage 20 electrode 35. Like Fig. 5 which was referred to above, Fig. 6 is a graph showing collection efficiency characteristics with respect to the collected particle diameter d (nm) of the respective high voltage electrodes (SO and SB I -SB3). As seen from Fig. 6, when the sample electrodes SB 1 -SB3 in which the punching 25 holes 35h were formed according to the invention were used as the high voltage - 15 electrode 35, the collection efficiency rj for ultrafine particles and nanoparticles (diameter: 100 nm or less) was higher by more than 10% than when the comparative electrode SO without punching holes 35h was used as the high voltage electrode 35. In particular, the collection efficiency il was higher by more than 20% in the case of the 5 sample electrodes SB1 and SB2 whose hole diameters were 2.5 mm and 5 mm, respectively (i.e., the total circumferential lengths L of holes were long). From this fact, it would be understood that the collection efficiency can be increased more by increasing the total circumferential length L of holes by forming more punching holes 35h having a small hole diameter. 10 Fig. 7 shows a relationship between the collection efficiency 11 (%) and the total circumferential length L of the punching holes 35h formed in the flat-plate-like electrode in the case where the hole area ratio was fixed (at 17.2 %), which is based on the result of the above-described dust collection experiment. The term "collection efficiency" as used herein means collection efficiency for all fine particles (the diameter is irrelevant). 15 As seen from Fig. 7, the collection efficiency is 48% when the total circumferential length L is 0 mm (i.e., the flat-plate-like electrode without punching holes), increases as the total circumferential length L increases, and is saturated when the total circumferential length L is longer than 1,319 mm. The collection efficiency is equal to 74% when the total circumferential length L is 2,638 mm. The tendency that 20 the collection efficiency is saturated when the total circumferential length L exceeds the above value is considered due to the fact that the electric field becomes weaker in peripheral regions around the holes because the hole diameter decreases and the number of holes increases which mean reduction in the distance between the holes. It is understood from the above results that the collection efficiency can be 25 increased by decreasing the punching hole diameter, increasing the number of holes, and - 16 increasing the total circumferential length L of holes and the upper limit of the hole diameter for attaining collection efficiency 60% is 10 mm. The lower limit of the hole diameter is 2 mm because the collection efficiency cannot be increased even if the hole diameter is made smaller than 2.5 mm and consideration should be given to the time and 5 labor of machining work, strength reduction of the high voltage electrode plate, and other factors. In the invention, punching holes 35h may be formed in the ground electrodes 32 rather than the high voltage electrode 35 or in both of the high voltage electrode 35 and the ground electrodes 32. 10 As described above, according to the invention, the collection efficiency for ultrafine particles and nanoparticles can be increased without the need for adding any new energy or device by a simple configuration that many punching holes are merely formed, in a dispersed manner over the entire area, in least one kind of electrode(s) of the collecting unit, consisting of the high voltage electrode and the ground electrodes, of 15 the electric dust collector. Therefore, when applied to an electric dust collector to be used for air cleaning in an automobile road tunnel where ultrafine particles and nanoparticles such as diesel emission particles are prone to be occur, the invention provides advantages that the collection performance can be enhanced and the apparatus can be miniaturized. 20

Claims

1. An electric dust collector which is configured in such a manner that a charging 5 unit and a collecting unit are disposed as a front stage and a rear stage, respectively, wherein the collecting unit is formed by a parallel-plate-like high voltage electrode and a parallel-plate-like ground electrode that are arranged parallel with each other so as to be opposed to each other via spaces where processing air flows; and wherein at least one of the two kinds of electrodes has a plurality of punching 10 holes formed therein in a dispersed manner over an entire area of the at least one of the two kinds of electrodes, and a diameter of the punching holes is in a range of diameter size, the range of diameter size of the punching holes being such that, in use, a region around a circumference of each of the punching holes where electric field strength is enhanced becomes wider than a region around the circumference of each of the 15 punching holes where electric field strength is weakened.

2. The electric dust collector according to claim 1, wherein the diameter of the punching holes is 2 to 10 mm.

3. The electric dust collector according to claim I or claim 2, wherein the punching holes are formed in the high voltage electrode. 20

4. An electric dust collector substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings and/or examples.