REMOVAL EFFICIENCY OF PARTICLES BY COMBINING NEGATIVE IONS WITH MECHANICAL FILTERS

Transcription

1 REMOVAL EFFICIENCY OF PARTICLES BY COMBINING NEGATIVE IONS WITH MECHANICAL FILTERS Shinhao Yang 1,2, Hsiao-Lin Huang 3, Yi-Chin Huang 4, and Hsiu-Chin Chang 1 1 Department of Leisure and Recreation Management, Toko University, Chia Yi, Taiwan, R.O.C 2 Department of Environmental and Occupational Health, Toko University, Chia Yi, Taiwan, R.O.C 3 Department of Occupational Safety and Health, Chia Nan University of Pharmacy & Science, Tainan, Taiwan, R.O.C 4 Taiwan Research Institute, Taipei, Taiwan, R.O.C ABSTRACT The purpose of this work is to combine negative air ions and mechanical filters to remove indoor suspended particulates. Various factors, including the particle size, the face velocity, the species of aerosol, relative humidity, and the concentration of negative ions were considered to evaluate their effects on the aerosol collection characteristics. Experimental results show that the aerosol penetration through the mechanical filter without negative air ions (NAI) is higher than that through the mechanical filter with 2 4, 1 5, and 2 5 NAI/cm 3. This finding implies that the removal efficiency of the aerosol by combining negative ions with mechanical filter would be increased. The results also indicate that the variation of the aerosol penetration through the mechanical filter with NAI increased with the aerosol size. That is due to that the aerosol was easier to be charged when aerosol size was getting larger. Moreover, the aerosol penetration decreased with the NAI concentration increasing. The aerosol penetration through the mechanical filter with NAI increased with the face velocity and relative humidity. The filters with NAI performed better against solid aerosol than against liquid aerosol. KEYWORDS Negative air ions, Mechanical filter, Removal efficiency INTRODUCTION On average, people spend as much as 87.2% of their time in indoor environment (Lance, 1996). Hence indoor air quality has become an increasing important problem. Indoor suspended particulates play an important role in indoor air quality because it causes many respiratory diseases. Therefore more and more air-cleaning technologies have been used to remove the indoor particulates. filtration is an effective technology for removing aerosols from a gas stream. The traditionally mechanical filters are often used in indoors or Heating, Ventilating -conditioning (HVAC) system for removal indoor particulates. However, the removal efficiency of the mechanical filters was too lower. It is due to that the major collecting mechanism between particles and mechanical filter is mechanical force. Recently, negative air ionizers are also typically applied to clean air indoors. Daniels (2) reported that negative air ions (NAIs) reduce aerosol particles, airborne microbes, odors and volatile organic compounds (VOCs) in indoor air. The removal of aerosol particles using NAIs is efficient (Grabarczyk 1, Wu and Lee 3). The mechanisms of particle removal by NAI include particle charging by emitted ions and electromigration (Lee et al. 4a,b). The charged aerosol particles and the electric field produced by the electrical discharge increase the migration velocity towards the indoor surfaces (Lee et al. 4a,b, Mayya et al. 4). Particles are finally deposited on indoor surfaces, such as wall Corresponding Author: Tel: ext.684, Fax: address: shinhaoyang@ntu.edu.tw

2 surfaces. Although, this technology could charge the particles and cause them collected on the wall surface. However, the collected particles would pollute the wall face. This work tried to solve the lower removal efficiency of mechanical filters and the negative air ionizers would cause the wall surface dirty by collected particles. Therefore, the purpose of this work is to combine negative ions and mechanical filters to remove indoor suspended particulates. If these two cleaning technologies were combined to remove indoor suspended particulates, the negative ions would first contract with the particles and cause the particles charged, than the particles would be collected easier by the mechanical filter. Furthermore, the effects of the face velocity, relative humidity, and the concentration of negative ions on the aerosol collection characteristics were also considered. EXPERIMENTAL MATERIALS AND METHODS Tested Filters Polypropylene fibrous filters were employed in this study. The characteristics of the polypropylene fibrous filters were as follows: packing fraction.6; filter weight 56. g/m2; filter thickness 1. mm; mean fiber diameter 15.8 μm The weight of filters were measured by the electronic scale; the fiber diameter of the untreated and surfactant pretreated filters was measured by the scanning electron micrograph (SEM) experiments; the filter thickness was measured by ruler; the packing fraction were calculated by the empirical model of Davies (1973), νmuf ( α ) p = 2 r [1] f ( α ) = 64α (1 + 56α ) [2] where p is the pressure drop across the filter; ν is the air viscosity; m is the filter thickness; U is the face velocity; r is the fiber radius, and α is the packing fraction. Aerosol Generation System Figure 1 depicts the complete experimental setup. Sodium chloride (NaCl) and corn oil were employed as the test aerosols for solid and liquid aerosols. These test aerosols were generated in a polydisperse state using a Collison atomizer (model 76, TSI Inc.). Then, the dried and neutralized polydisperse aerosol was electrically classified using a Differential Mobility Analyzer (DMA, model 71, TSI Inc.). to obtain monodisperse singly charged aerosols in the submicron-sized range from.5 to. µm. The singly charged particles from the DMA passed through a Kr-85 radioactive source (model 77, TSI Inc.), which neutralized them to the Boltzmann charge equilibrium. An aerosol electrometer (model 68, TSI Inc.) was used to monitor the neutralization of the charge of the aerosol. Relative Humidity Control System This study considered the effect of relative humidity (RH) on the filtration characteristics. Two RHs were used in this work. The RH of the aerosol-flow stream was modified by changing the ratio of the flow rate of the dry gas stream to that of the humidified gas stream generated by the water vapor saturator. The final RH of the aerosol-flow stream was measured using a hygrometer (Hygromer A2, Rotronic Inc.). Two relative humidity conditions for experiments were % and % that stood for dry and humid condition respectively in this study. Aerosol Removal Unit The aerosol removal unit comprises a test column, a negative ion generator, a filter holder and the tested filter. The test column is made by stainless steel. It was connected with a ground wire to avoid the negative ion coated on the wall surface of the column. An NAI generator with a negative electric discharge was placed into the test chamber. The electrode of the generator was a cluster of copper needles; the discharge voltage was controlled with a power supply (Model SL, SPELLMAN, USA)

3 located outside the chamber. The NAI concentrations of 2 4, 1 5, and 2 5 NAI/cm 3 were chosen in the work. Measuring Penetration of Aerosol The aerosol penetration through the removal unit was measured using a condensation particle counter (CPC, model 22, TSI Inc.), which measured the aerosol concentrations upstream and downstream of the filter holder. Each aerosol penetration test was performed in triplicate for each filter. The pressure drop across the tested filter was measured using a pressure gauge (Model, Dwyer Instruments Inc). The face velocity through the tested filter was controlled using a flow meter and a pump. The testing face velocities ranged from to cm/sec and were applied to study the effects of the face velocity. Tests were performed at least in triplicate for each type of filter, face velocity, RH and species of aerosol. Figure 1 schematically depicts the complete experimental set-up. HEPA High-Volt Power Supply (-) Supply System Kr 85 HEPA HEPA Flow Meter Diffusion Dryer Supply System Collison Atomizer DMA Excess Saturator Flow Meter Negative Ions Generator Aerosol Electrometer CPC KR 85 Pressure Gage PC Filter Holder Exhausted Gas Removal Unit (Negative Ions and Mechanical Filter) Figure 1. Schematic diagram of the experimental system RESULTS AND DISCUSSION Aerosol Penetration through the Filter with Negative Ions Figure 2 shows the aerosol penetration through the mechanical filter without and with NAI. In each test, the face velocity was maintained at cm/s. The aerosol penetration of.3 µm aerosol through the filter without NAI was about 92%. The.3-µm aerosol penetrations through the filter with 2 4, 1 5, and 2 5 NAI/cm 3 were around 79%, 52%, and 39%. The penetrations through the filters with NAI were lower than that through filter without NAI. It indicates that combing mechanical filter with NAI

4 could decrease the aerosol penetration during the filtration process. It is due to that the NAI would first contract with the particles and cause the particles charged. Than the charged particles would be collected easier by the mechanical filter, that is working by image electrostatic force (image attracting force is between charged particles and uncharged filter). The results also indicate that the variation of the aerosol penetration through the mechanical filter with NAI increased with the aerosol size. That is due to that the aerosol was easier to be charged when aerosol size was getting larger. Figure 2 also plots the aerosol penetration through the filters with NAI was decreasing when NAI concentrations was increasing. It is because of that the image electrostatic force raised with the aerosols charges increasing. During the filtration process, when the NAI concentration was increasing, the aerosols have more chances to collide with NAI. Thus, testing with higher NAI concentration would cause the charges on the aerosol increased. Aerosol Penetration (%) Without NAI 2 * 4 NAI/cm 3 1 * 5 NAI/cm 3 2 * 5 NAI/cm Figure 2. Aerosol Penetration through the filters with negative air ions Effect of Face Velocity on Aerosol Penetration Figure 3 depicts aerosol penetration against aerosol size for the NaCl aerosol at various face velocities ( and cm/s), through the filter with 2 5 NAI/cm 3. The result reveals an increase in the penetration of.3 µm-aerosol through the filter with 2 5 NAI/cm 3 from approximately 39% to 49% as the face velocity increases from to cm/s. This result follows from the fact that the principal mechanisms between the submicron charged aerosols and uncharged filter is image electrostatic attraction and diffusion. A higher face velocity leads to a shorter residence time associated with aerosol deposition by electrostatic attraction and diffusion. Effect of Relative Humidity on Aerosol Penetration Figure 4 plots the aerosol penetration through the filter with 2 5 NAI/cm 3 versus aerosol size at two values of RH (% and %). The result reveals an increase in the penetration of.3 µm-aerosol through the filter with 2 5 NAI/cm 3 from approximately 39% to 42% as the RH increases from % to %.The experimental results indicate that the penetrations through the filter with NAI are getting a slight increase when test RH was raising. It is probably because of the NAI concentration was reducing in the higher RH. The water molecules maybe decrease the number of the NAI.

5 Aerosol Penetration (%) With 2* 5 NAI/cm 3 cm/s cm/s -2-1 Figure 3. Aerosol Penetration through the filters with NAI at different face velocities With 2* 5 NAI/cm 3 Aerosol Penetration (%) RH% RH% -2-1 Figure 4. Aerosol Penetration through the filters with NAI at different RHs Effect of Aerosol Type on Aerosol Penetration Figure 5 plots aerosol penetration versus aerosol size for filters with 2 5 NAI/cm 3 at a face velocity of cm/s. The penetrations of the.3 µm NaCl and corn oil aerosol were 39% and %. The experimental results reveal that the penetration through the filter with NAI against solid aerosol was lower than that against liquid aerosol. It is probably due to that the solid aerosol was easier charged by

6 NAI than liquid aerosol. The charged on the solid aerosol maybe was higher than liquid aerosol. Aerosol Penetration (%) With 2* 5 NAI/cm 3 NaCl Solid Aerosol Corn Oil Liquid Aerosol -2-1 Figure 5. Aerosol Penetration through the filters with NAI against NaCl and corn oil aerosols CONCLUSIONS The aerosol penetration of.3 µm aerosol through the filter without NAI was about 92%. The.3-µm aerosol penetrations through the filter with 2 4, 1 5, and 2 5 NAI/cm 3 were around 79%, 52%, and 39%. It indicates that combing mechanical filter with NAI could decrease the aerosol penetration during the filtration process. The results also indicate that the variation of the aerosol penetration through the mechanical filter with NAI increased with the aerosol size. That is due to that the aerosol was easier to be charged when aerosol size was getting larger. The aerosol penetration through the filters with NAI was decreasing when NAI concentrations was increasing. It is because of that the image electrostatic force raised with the aerosols charges increasing. The result reveals an increase in the penetration of.3 µm-aerosol through the filter with 2 5 NAI/cm 3 from approximately 39% to 49% as the face velocity increases from to cm/s. Moreover, result implies an increase in the penetration of.3 µm-aerosol through the filter with 2 5 NAI/cm 3 from approximately 39% to 42% as the RH increases from % to %. The experimental results indicate that the penetrations through the filter with NAI are getting a slight increase when test RH was raising. The aerosol penetration versus aerosol size for filters with 2 5 NAI/cm 3 at a face velocity of cm/s against.3 µm NaCl and corn oil aerosol were 39% and %.The experimental results reveal that the penetration through the filter with NAI against solid aerosol was lower than that against liquid aerosol. ACKNOWLEDGEMENTS The authors would like to thank the National Science Council of Republic of China for financially supporting this research under Contract No. NSC E CC3. REFERENCES 1. W. Lance (1996) Indoor Particles: A Review, Journal of the & Waste Management Association, 46,

7 2. S. L. Daniels (2) On the ionization of air for removal of noxious effluvia, IEEE Transactions on Plasma Science,, Z. Grabarczyk (1) Effectiveness of indoor air cleaning with corona ionizers, Journal of Electrostatics, 51 52, C. C. Wu and G. W. M. Lee (3). The temporal aerosol size distribution in indoor environment with negative electric discharge, Journal of Aerosol Science, 34, S999 S. 5. B. U. Lee et al. (4a) Removal of fine and ultrafine particles from indoor air environments by the unipolar ion emission, Atmospheric Environment, 38, B. U. Lee et al. (4b) Unipolar ion emission enhances respiratory protection against fine and ultrafine particles. Journal of Aerosol Science, 35, Y. S. Mayya (4) Aerosol removal by unipolar ionization in indoor environments, Journal of Aerosol Science, 35, C. N. Davies (1973) Filtration, Academic Press: London.