Novel Wire-on-Plate Electrostatic Precipitator (WOP-EP) for

Jun 26, 2015 - A new wire-on-plate electrostatic precipitator (WOP-EP), where discharge wires are attached directly on the surface of a dielectric pla...
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Novel Wire-on-Plate Electrostatic Precipitator (WOP-EP) for Controlling Fine Particle and Nanoparticle Pollution Ziyi Li,† Yingshu Liu,† Yi Xing,‡ Thi-Minh-Phuong Tran,§ Thi-Cuc Le,§ and Chuen-Jinn Tsai*,§ †

School of Mechanical Engineering, and ‡School of Civil and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China § Institute of Environmental Engineering, National Chiao Tung University, University Road, Hsinchu 30010, Taiwan S Supporting Information *

ABSTRACT: A new wire-on-plate electrostatic precipitator (WOPEP), where discharge wires are attached directly on the surface of a dielectric plate, was developed to ease the installation of the wires, minimize particle deposition on the wires, and lower ozone emission while maintaining a high particle collection efficiency. For a lab-scale WOP-EP (width, 50 mm; height, 20 mm; length, 180 mm) tested at the applied voltage of 18 kV, experimental total particle collection efficiencies were found as high as 90.9−99.7 and 98.8−99.9% in the particle size range of 30−1870 nm at the average air velocities of 0.50 m/s (flow rate, 30 L/min; residence time, 0.36 s) and 0.25 m/s (flow rate, 15 L/min; residence time, 0.72 s), respectively. Particle collection efficiencies calculated by numerical models agreed well with the experimental results. The comparison to the traditional wire-in-plate EP showed that, at the same applied voltage, the current WOP-EP emitted 1−2 orders of magnitude lower ozone concentration, had cleaner discharge wires after heavy particle loading in the EP, and recovered high particle collection efficiency after the grounded collection plate was cleaned. It is expected that the current WOP-EP can be scaled up as an efficient air-cleaning device to control fine particle and nanoparticle pollution.

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magnetic devices to remove the deposited dust cake.9 However, rapping is not feasible for small-scale or indoor air cleaners. Additional problems caused by the contamination of electrodes include back-corona discharge,19 particle re-entrainment,20 ozone generation,5 and other undesirable operations.21,22 In traditional wire-in-plate EPs (WIP-EPs), discharge wires are installed between two parallel collection electrodes, as shown in Figure 1a. Particle contamination on the discharge wires occurs readily because they are exposed to the particleladen flow and frequent cleaning of the wires and the collection electrodes is inevitable to keep the particle collection efficiency high. The collection electrodes can be washed off by water film or water spray in a wet EP for a long-term operation.23,24 However, cleaning the wires is much more difficult when the high-voltage power supply has to be interrupted during water spraying to avoid short circuit between the wires and the plates.24 In this paper, a modified EP configuration, called wireon-plate EP (WOP-EP), is presented with its schematic diagram shown in Figure 1b. A dielectric plate is used to replace the original upper collection plate shown in Figure 1a. Discharge wires are positioned at regular intervals on the inner

INTRODUCTION In recent years, pollution of ambient fine particles (PM2.5) has drawn worldwide attention because of their mortality and morbidity effects on humans.1,2 Exposure to nanoparticles has also been shown to pose adverse health effects,3,4 and the nanoparticle concentration can be elevated in the presence of indoor sources5 or during the handling of nanomaterials.6,7 Therefore, there is an increasing demand to control the pollution and emission of PM2.5 and nanoparticles. Among all particulate control devices, electrostatic precipitators (EPs) have been shown to remove fine and nanoparticles efficiently at a low pressure drop.8−12 When introduced into an EP, particles are charged typically by corona wires based on diffusion and field-charging mechanisms and then migrate to and collected by the collection electrodes by electrostatic force. The particle collection efficiency of a well-designed EP can reach as high as 99%.13 Generally, the minimum particle collection efficiency is observed in the 100−1000 nm diameter range,14−16 and the efficiency drops with decreasing diameter for nanoparticles smaller than 30 nm, attributing to partial charging effects.8,17 In practical use of an EP, efficiencies for sub-micrometer and nanoparticles were also found to drop off quickly with increasing operation time as particles deposit on the collection electrodes and discharge wires, leading to the reduction in the electric field.17,18 Both collection electrodes and wires are rapped periodically by mechanical or electro© 2015 American Chemical Society

Received: Revised: Accepted: Published: 8683

April 12, 2015 June 16, 2015 June 26, 2015 June 26, 2015 DOI: 10.1021/acs.est.5b01844 Environ. Sci. Technol. 2015, 49, 8683−8690

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Environmental Science & Technology

advantage of the current WOP-EP, the particle loading effect on the collection efficiency was then examined and compared to that of the traditional WIP-EP.

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MATERIALS AND METHODS Lab-Scale WOP-EP. Figure 2 shows a lab-scale one-channel WOP-EP with the channel dimension of 20 mm in height, 50 mm in width, and 180 mm in length. Four parallel stainlesssteel (SS) wires of 0.35 mm in diameter attached on the inner surface of a dielectric polypropylene (PP) plate surface were used as the discharge electrodes. The distances of the discharge wires to the collection plate are 20 mm, and the wires are spaced at 40 mm. To ensure a uniform particle distribution at the inlet of the EP, a porous metal plate (thickness, 5 mm; pore diameter, 100 μm; Series 1100, Mott Corp., Farmington, CT) was installed between the inlet diverging section and the buffer section before the EP. Positive corona discharge was chosen to lower ozone production.25 For the comparison of particle collection performance, a traditional WIP-EP was made with the same plate−plate (20 mm) and wire−wire spacing (40 mm) as those of the WOP-EP (see Figure S1 of the Supporting Information), to ensure that the residence time at the same test aerosol flow rate is equal to that of the WOP-EP. This makes the wire−plate spacing of the WIP-EP equal to one half of that of the WOP-EP, which could result in a much higher ozone generation of the former. Therefore, for the comparison of ozone emission with the WOP-EP, an additional WIP-EP with the wire−plate spacing of 20 mm, which is the same as that of the WOP-EP, was made for effluent ozone concentration measurements. Particle Collection Efficiency Test. The experimental setup for the particle collection efficiency is shown in Figure S2 of the Supporting Information. The discharge wires of the EPs were connected to a high-voltage direct current (DC) power supply (model SL150, Spellman High Voltage Electronics Corp., Hauppauge, NY), while the collection plate was grounded. The voltage was monitored on the DC power supply, while the corona current was measured by an electrometer (model 6514, Keithley Instruments, Inc., Cleveland, OH). At the downstream of the EP, the O3 concentration was measured using an ozone monitor with the resolution of 0.1 ppb and the accuracy of 2 ppb (model 106-L, 2B Technologies, Inc., Boulder, CO). Sodium chloride (NaCl) and aluminum oxide (Al2O3) were used as the test particles with the electrical mobility diameter (dp) of 16.5−600 and 600−1870 nm, respectively. The collision atomizer (model 3076, TSI, Inc.) was used to atomize NaCl solution, and the generated NaCl particles were dried in a diffusion dryer. The small-scale powder disperser (SSPD,

Figure 1. Schematic of the (a) traditional WIP-EP, (b) proposed WOP-EP, and (c) concept of the scale-up device for the WOP-EP.

surface of the dielectric plate. In comparison to the WIP-EP, wires in the WOP-EP are not contaminated easily because electrostatic force is pushing particles away from the wire surface, where the air flow velocity is almost zero. Figure 1c shows the scale-up version of the WOP-EP, in which several modules of the configuration in Figure 1b are arranged in parallel. In this work, a lab-scale one-channel WOP-EP was designed and tested for the current−voltage characteristics, ozone generation, and particle collection efficiency. Numerical methods were applied to predict particle collection efficiencies and validated by the experimental results. To show the

Figure 2. Schematic diagram of the lab-scale one-channel WOP-EP. 8684

DOI: 10.1021/acs.est.5b01844 Environ. Sci. Technol. 2015, 49, 8683−8690

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Environmental Science & Technology

collection efficiencies following the models developed previously.11,26 Laminar flow field was assumed because the maximum flow Reynolds number (Re) for the experimental conditions was calculated to be 1203.91, which is smaller than 2000.14 Two-dimensional (2D) Navier−Stokes equations were solved using the semi-implicit method for pressure-linked equation (SIMPLER method). The electric field and ion concentration distribution were then solved on the basis of Poisson and convection−diffusion equations, respectively. Finally, the charged particle concentration field can be obtained for the calculation of particle collection efficiency. For particles with dp ≤ 100 nm, the Eulerian method was adopted,30 in which the particle charging model of Fuchs27 or Marlow and Brock28 was used to calculate the charge of particles with 30 ≤ dp ≤ 100 nm and dp ≤ 30 nm, respectively. For particles with 100 ≤ dp ≤ 650 nm, the Lagrangian method11 was adopted and the particle motion equation, particle charging rate, and particle charge were solved using the fourth-order Runge−Kutta method based on the combined charging model of Lawless.29 Details of the numerical gird, governing equations for the flow field, electric field, ion concentration field, and charging models as well as the calculation procedure are provided in the Supporting Information.

model 3433, TSI, Inc.) was used to generate Al2O3 particles from Al2O3 powder (QF-Al-8000, Sipernat, Japan). The characteristics of the test particles can be seen in Table S1 of the Supporting Information. After charge equilibrium, the test aerosol was then drawn through the Kr-85 radioactive source (model 3077, TSI, Inc.), where particles were chargeneutralized. The scanning mobility particle sizer (SMPS, model 3936, TSI, Inc.) and the aerodynamic particle sizer (APS, model 3320, TSI, Inc.) were used to measure the particle size distributions upstream and downstream of the EP for NaCl and Al2O3 particles, respectively. The aerodynamic diameter of Al2O3 particles measured by the APS was converted into the electrical mobility diameter by assuming the particle density of 3.5 g/cm3, which is slightly smaller than the density of alumina particles of 3.95−4.1 g/cm3, to extend the collection efficiency curve smoothly from the sub-micrometer to micrometer size. Particle collection efficiency tests were conducted at two air flow velocities of 0.25 and 0.50 m/s at the corresponding flow rates of 15 and 30 L/min, respectively, at three different applied voltages of 14, 16, and 18 kV. The collection efficiency for particles with the diameter dp considering the electrostatic precipitation effect, ηelec(dp), is calculated by the following equation: ηelec(d p) (%) = =

Cout,OFF(d p) − Cout,ON(d p) Cout,OFF(d p)

ηtotal (d p) (%) − ηpm(d p) (%) 100% − ηpm(d p) (%)

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× 100%

RESULTS AND DISCUSSION Electrical Characteristics and Ozone Concentration. Figure 3a shows the experimental results of the corona current−voltage characteristics with clean air flow and aerosol flow at both clean conditions and after 170.85 mg of loaded mass in the WOP-EP when the flow rate is 15 L/min. General agreement is seen between numerical and experimental values at the initially clean condition assuming the positive ion mobility value of 1.15 × 10−4 m2 V−1 s−1. For ions, there is a range of ion mobility values, which will influence the predicted corona current.26 In the presence of 15 L/min NaCl aerosol flow, the corona-starting voltage increased and the corona current decreased in comparison to the initial clean condition because of ion consumption by particle charging. The difference increased when the WOP-EP was loaded with 170.85 mg of nano-TiO2 particles because the electric field strength was reduced further in the presence of the dust layer on the collection plate. At the same applied voltage and the clean air flow rate of 15 L/min, a much higher positive corona current of the WIP-EP than that of the WOP-EP is observed in Figure 3b, where netgenerated O3 concentrations are also compared. The spark-over voltage is much lower for the WIP-EP compared to the WOPEP, which is about 11.2 kV (WIP-EP, wire-to-plate spacing d = 10 mm), 15 kV (WIP-EP, d = 20 mm), and 19 kV (WOP-EP, d = 20 mm), respectively. For all EPs, the net ozone concentration increased with an increasing voltage. It is from 37.6 ± 2.1 ppb (7.3 kV) to 3056.0 ± 119 ppb (11.0 kV) for the WIP-EP (d = 10 mm), from 17.7 ± 3.0 ppb (8.6 kV) to 2277.3 ± 41.1 ppb (14.8 kV) for the WIP-EP (d = 20 mm), and from 14.0 ± 1.1 ppb (10.9 kV) to 280.3 ± 4.9 ppb (18.2 kV) for the WOP-EP. At the same applied voltage, the ozone concentration generated by the WOP-EP is 1−2 orders of magnitude lower than that of the WIP-EP. For example, at the applied voltage of 11 kV, it is 1/168 times that of the WIP-EP (d = 10 mm) (18.2 versus 3056 ppb) and 1/29.2 times that of the WIP-EP (d = 20 mm) (18.2 versus 532 ppb), and at the applied voltage of 14.5 kV, it is 1/17.5 times that of the WIP-EP (d = 20 mm) (129.7

× 100% (1)

where Cout,ON and Cout,OFF are the outlet particle number concentrations (number/cm3) of the WOP-EP with and without applying a high voltage, respectively, ηtotal is the total collection efficiency, and ηpm is the efficiency as a result of the porous metal and other mechanisms. ηtotal and ηpm can be calculated by the following equations with the additional measurement of the inlet particle concentration, Cin, as ηtotal (%) = ηpm (%) =

C in − Cout,ON C in C in − Cout,OFF C in

× 100%

× 100%

(2)

(3)

Particle Loading Test. To study the particle loading effect on the collection efficiency, nano-TiO2 powder (AERODISP P25, Degussa, Germany) was dispersed by the SSPD at the flow rate (Qa) of 15 L/min to achieve a heavy particle loading condition. After the WOP-EP was loaded with a certain nanoTiO2 mass, NaCl particles were then used to test the collection efficiency. The loaded nano-TiO2 mass, Wloaded (g), can be obtained by the following equation: Wloaded =

Qa Qs

(Winlet − Woutlet) − Wp

(4)

where Winlet and Woutlet are nano-TiO2 mass upstream and downstream of EPs, respectively, measured by the filter cassettes at the sampling flow rate (Qs) of 1.82 L/min, and Wp is the deposited particle mass on the porous metal, which was measured by weighing the mass differences of the porous metal before and after each particle loading test. Numerical Simulation. Numerical models based on Eulerian and Lagrangian methods were used to predict particle 8685

DOI: 10.1021/acs.est.5b01844 Environ. Sci. Technol. 2015, 49, 8683−8690

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Environmental Science & Technology

Figure 3. (a) Corona current versus applied voltage for the WOP-EP with clean air flow and aerosol flow (clean and particle-loaded WOPEP) at 15 L/min (0.25 m/s). (b) Comparison of the net ozone concentration versus applied voltage between the WOP-EP and the WIP-EP with clean air flow at 15 L/min (0.25 m/s) (net ozone concentration is the total measured ozone concentration minus the background value of 27.8 ppb).

versus 2111 ppb). Even at the same corona current, the ozone concentration is also much lower for the WOP-EP than that of the WIP-EP, as shown in Figure S3 of the Supporting Information. For example, at the corona current of 0.1 mA, it is 1/2.7 (230.1 versus 629.0 ppb) to 1/3.6 (230.1 versus 825.0 ppb) times that of the WIP-EP (d = 10 mm) and the WIP-EP (d = 20 mm), respectively. Even at the worse case of 280.3 ppb when the applied voltage is 18.2 kV, the O3 emission rate of the WOP-EP is only 0.5 mg/ h, which is 1−2 orders of magnitude lower than the reported values of 21.8−60.4 mg/h for commercial EP indoor cleaners.5 Therefore, the current WOP-EP has a very good potential to be used as an indoor air-cleaning device to meet the 8 h average O3 standard of 75 ppbv of the United States Environmental Protection Agency (U.S. EPA)5 or the ozone concentration limit of 50 ppb for 24 h of operation of ESP air cleaners set as the Underwriters Laboratories, Inc. (UL) safety standard for electrostatic air cleaners, UL 867.30 Particle Collection Efficiency. Figure 4a shows the total collection efficiency of the WOP-EP as a function of the electrical mobility particle diameter (16.5−1870 nm) at three different applied voltages (14, 16, and 18 kV) under the initially

Figure 4. Total particle collection efficiency of the WOP-EP at the flow velocity of (a) 0.50 m/s and (b) 0.25 m/s.

clean condition when the flow velocity is 0.50 m/s. The collection efficiency at no applied voltage with and without the porous metal as the flow straightener is also shown. The efficiency decreased with decreasing the applied voltage because of the reduction in the electric field strength and the corona current, as shown in Figure 3. When the applied voltage was decreased from 18 to 14 kV, the total efficiency decreased from 90.9−99.7% at 18 kV to 87.2−99.5% at 16 kV and further decreased to 76.2−99.6% at 14 kV for particles in the entire size range. For particles from 100 to 1000 nm, U-shape efficiency curves were observed, with the minimum efficiency for 14, 16, and 18 kV applied voltages occurring at the electrical mobility diameter of 108, 171, and 171 nm, respectively, which are similar to the single-stage EP reported in the literature.8 In the left-hand side of the U-shape curve, where the diffusion charging mechanism dominates, it can be explained by the fact that the particle charge decreases with decreasing the particle diameter, while the mechanical mobility increases more rapidly 8686

DOI: 10.1021/acs.est.5b01844 Environ. Sci. Technol. 2015, 49, 8683−8690

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Environmental Science & Technology with decreasing the particle diameter.13 For particles below 30 nm, the efficiency decreased slightly with decreasing the particle diameter because of the partial charging effect.11 This is also consistent with the observation and suggestion by Zhuang et al.20 and Huang et al.,8 where a fraction of small nanoparticles was found not to be charged. Figure 4b shows the experimental results for the total particle efficiency at the flow velocity of 0.25 m/s (residence time of 0.72 s). At the applied voltages of 14, 16, and 18 kV, the total collection efficiency increased to 91.1−98.6, 94.9−99.3, and 98.8−99.9%, respectively, in the particle diameter range of 30− 1870 nm. Decreasing the flow velocity from 0.50 to 0.25 m/s had a substantial effect on the total collection efficiency with a less apparent U-shape curve, particularly for particles below 1000 nm. On the basis of these experimental results, the WOPEP is able to remove particles with the minimum efficiency of >90 and >98% at the air flow velocity of 0.50 and 0.25 m/s, respectively. The low corona current of the WOP-EP shown in Figure 3b can still achieve high particle collection efficiency. The collection efficiency is enhanced by the ionic wind effect, which is defined as a secondary flow caused by the ions that flow from the discharge wire to the grounded plate.31 It was reported that the ionic wind increased as the air flow velocity became smaller.8 In the present WOP-EP, the velocity near the surface of the dielectric plate is low because of the non-slip boundary condition. Thus, a strong ionic flow around discharge wires can force particles to drift toward the collection plate, which increases the particle collection efficiency. The function of the porous metal to make the aerosol flow uniform across the channel is of great importance because the total collection efficiency (16 kV, 0.25 m/s) was found to be only 35.9−58.8% for the electrical mobility diameter of 16.5− 1870 nm if the porous metal was not used. To obtain the net WOP-EP electrostatic precipitation efficiency, eq 1 was used to decouple the collection efficiency of the porous metal from the total efficiency. The convection−diffusion deposition is seen to play an important role for very small nanoparticles, especially for those below 50 nm, while the impaction effect dominates the deposition of particles larger than 300 and 800 nm at the flow velocity of 0.50 and 0.25 m/s, respectively. As the flow velocity is reduced from 0.50 to 0.25 m/s, the convection− diffusion deposition with or without the porous metal increases slightly for particles below 100 nm, while the decrease in the impaction efficiency for particles above 300 nm is very obvious. This can be attributed to the fact that diffusion is important for nanparticles when the collection efficiency is inversely proportional to U02/3 (where U is the filtration velocity), while the impaction predominates for particles larger than 300 nm with the corresponding efficiency proportional to U for a glass fiber filter media, which resembles that of the porous metal.14 Comparison of Experimental and Numerical Results. After the flow field was obtained, the electric potential and ion density fields were solved to obtain the charged particle concentration field, which were then used to predict the particle collection efficiency. Only the electrostatic precipitation mechanism was considered in the numerical simulation. An example of the numerical results for the electric potential and the ion concentration fields in the WOP-EP at the applied voltage of 16 kV and the flow velocity of 0.50 m/s (15 L/min) is shown in Figure S4 of the Supporting Information. Panels a and b of Figure 5 show the comparison of the numerical and experimental results of the electrostatic precipitation efficiency, ηelec(dp), at the flow velocity of 0.50 and 0.25 m/s, respectively.

Figure 5. Comparison of the electrostatic precipitation efficiency between numerical and experimental results at the flow velocity of (a) 0.50 m/s and (b) 0.25 m/s.

As seen from the figures, overall good agreement between numerical and experimental results is obtained, with a relatively larger deviation observed for particles smaller than 30 nm. In Eulerian method, particle charges predicted using Fuchs model for particles in the size range of 30−100 nm appeared to be accurate because the electrostatic precipitation efficiency was predicted very well. However, partial charging that occurred in the smaller size range of 70 nm, the partial charging effect on the collection efficiency became less insignificant. A similar trend of the discrepancy versus particle size can be seen in the comparison between the numerical results and experimental data of Huang et al.8 reported in our previous study.26 The agreement is very good for particles with the diameter range of 70−400 nm at the applied voltage of 16 and 18 kV (0.50 m/s), with the deviation of less than 5% (0.75−4.97 and 0.81−4.89%, respectively). At the applied voltage of 14 kV, a similar good agreement was found within the diameter range of 80−150 nm. Outside of this range, the numerical method overestimated the electrostatic precipitation efficiency with the deviation of 5.23−18.71% for smaller nanoparticles and 5.19− 8687

DOI: 10.1021/acs.est.5b01844 Environ. Sci. Technol. 2015, 49, 8683−8690

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Environmental Science & Technology 14.43% for sub-micrometer and micrometer particles, respectively. The region of the dielectric plate surface nearby the wires, which cannot be modeled easily, is believed to somehow influence the ion generation and dispersion in the WOP-EP, which leads to errors in predicting the collection efficiency. This effect becomes more influential as the electrical field strength and ion concentration are decreased with decreasing the applied voltage. More investigation is needed to improve the numerical model in this aspect. Numerical results obtained by Lagrangian method coupled with Lawless charging model are also in fairly good agreement with the experimental values in the particle size range of 100− 1000 nm, with the deviation of less than 10%. These could be attributed to the fact that the Lawless combined charging model predicted particle charges well in the continuum charging regime (Kn ≪ 1).29 When the electric field strength is very strong, diffusion charging can be neglected for particles larger than 1 μm but cannot be neglected for sub-micrometer particles. In the simulation, a slightly larger discrepancy existed for microsized particles (dp > 1 μm) with the deviation, for example, of 7.38−14.43% at the applied voltage of 14 kV and the flow velocity of 0.25 m/s. One of the reasons could be the ion-quenching effect because of particle charging, which plays a significant role as the field charging becomes the predominant charging mechanism for microsized particles. The use of the dielectric plate on which ions might be accumulated around the discharge wires is also believed to have some effect on particle charging and electrostatic precipitation. Particle Loading Test. The degradation of the collection efficiency of the WOP-EP at different loaded nano-TiO2 particle masses was compared to the traditional WIP-EP. Only the electrostatic efficiency was compared to avoid the diffusion and impaction effects of the porous metal. Panels a and b of Figure 6 show the particle loading test results during the operation time of 10 h at the flow velocity of 0.25 m/s. For the WOP-EP, the applied voltage was set at 18 kV, and for the WIP-EP (d = 10 mm), the applied voltage was set at the maximum applied voltage from 10.7 to 9.5 kV that can be applied at each loaded mass before spark-over occurred. Filter samples at the inlet and outlet of the EPs determined the loaded nano-TiO2 for the WIP-EP to be 20.94, 40.77, 62.09, and 83.24 mg (corresponding to 1.16, 2.27, 3.44, 4.62 g/m2 on the collection plate), and 38.22, 97.02, 139.36, and 170.85 mg (corresponding to 4.24, 10.78, 15.48, 18.98 g/m2 on the collection plate) for the WOP-EP, respectively. After each loaded mass, the electrostatic precipitation efficiency for NaCl particles from 16.5 to 615 nm in the electrical mobility diameter was tested at least 5 times. Initial electrostatic precipitation efficiency of the WIP-EP and the WOP-EP in the range of 71.3−97.7 and 89.0−99.3%, respectively, can also be seen in the figure. The WOP-EP exhibited a much lower degradation rate in the electrostatic collection efficiency with increasing the loaded particle mass compared to the WIP-EP, especially for larger (>200 nm) or smaller (