Electrostatic Enhancement of the Collection Efficiency of Stainless Steel Fiber Filters

This study investigated the effect of using a stainless steel fibrous filter as the ground electrode of a point-to-plate electrostatic precipitator on particle penetration. The effect of the electrical field on particle penetration was investigated at 4 different filter face velocities (25, 50, 100, and 125 cm/s) for monodisperse PSL and silica particles (size range 0.05-1 m) as well as polydis perse ammonium sulfate, ammonium nitrate, and ultrafine indoor air particles. Particle penetration was greatly reduced by the application of the electrical field. By comparison, the performance of electrically active fibrous filters has been shown to rapidly degrade as particle loading exceeds 2-3 g/m². The effect of particle loading on particle penetration was also investigated at a filter face velocity of 50 cm/s. Particle penetration seemed to slightly decrease with particle loading and was independent of particle size. These results indicated that the accumulation of nonconductive particles up to 15 cm³/ m² does not create ''back corona,'' which would substantially decrease the collection efficiency of the grounded filter. In conclusion, our experiments suggest that using metal filters as the collector electrodes may be an attractive alternative design for electrostatic precipitators.     

Penetration of Sub-50 nm Nanoparticles Through Electret HVAC Filters Used in Residence

Pleated electret HVAC filters are often used in residence to mitigate the particles that originate both indoors and outdoors. These filters are usually tested with particles larger than 300 nm. However, residential particles can contain a significant amount of nanoparticles with size below 50 nm due to cooking, smoking, cleaning, wood burning, and outdoor infiltration. In order to characterize the nanoparticle removal by electret HVAC filters, penetrations of 3–50 nm silver nanoparticles through five different flat sheet electret media used in commercial residential HVAC filters were tested with face velocities of 0.05, 0.5, and 1.0 m s^–1. Experimental results showed that all media had significantly high penetrations with 0.35–0.8 at the most penetrating particle sizes (MPPSs) for all three velocities, which were in the sizes of 10–30 nm. A model based on single fiber theory for particle penetration predictions was used and compared with the experimental data. Results showed that the model predicted the nanoparticle penetrations very well for all media and all face velocities tested. According to the model, for enhancing the nanoparticle efficiency of the current commercial HVAC filters, the fiber diameter should be reduced or the number of pleats should be increased. However, by doing these, pressure drop and cost may be largely increased. On the other hand, this study found the existing commercial mechanical HVAC filters were much capable for sub–50 nm nanoparticle removal when their minimum efficiency reporting values (MERVs) were larger than 13 and it is concluded mechanical HVAC filters can do a better job than electret ones. However, the quality factor analysis showed electret filters could be regarded as the best filter media for removing particles smaller than 300 nm.

Copyright 2015 American Association for Aerosol Research     

Effects of an external electric field on the collection efficiency of air filters: Filtration mechanisms with an external e-field

An external electric field was applied on the filter to improve its collection efficiency, and the collection efficiencies of the different filters under various conditions were evaluated. Dominant electrical filtration mechanisms for each condition were investigated using experimental and theoretical approaches. Four types of air filters were used as test filters: a charged fiber filter, a low-grade filter with 50% collection efficiency in the most penetration particle size (MPPS) zone, and two high-grade filters with more than 95% collection efficiency in the MPPS zone. Three different particle charge states—neutralized, single-charged and uncharged—were considered. For neutralized particles, the external electric field led to a 14.5%p. and 2.5%p. increase in the collection efficiencies of the low-grade filter and charged fiber filter, respectively. With the electric field, the collection efficiency of the low-grade filter increased by 30%p. for single-charged particles. The electric field also affected the collection efficiencies of the charged filter and high-grade filters, but the effect was not significant. For uncharged particles, the electric field did not lead to a remarkable increase in the collection efficiencies of any of the filters. Through experimental and theoretical analysis, it was found that the polarization force imposed on the charged fiber was the dominant factor for the charged fiber filter regardless of application of the external electric field. The Coulombic force imposed on the electric field was the dominant factor for the low-grade filter, while both the Coulombic and the polarization forces affected the collection efficiency of the high-grade filter.

Copyright © 2017 American Association for Aerosol Research     

Inactivation of Airborne Influenza Virus by Tea Tree and Eucalyptus Oils

Our previous studies demonstrated that precoating of filter fibers with biologically active tea tree oil (TTO) enhances physical collection efficiency of conventional heating, ventilation, and air conditioning (HVAC) filters, and provides cost effective and rapid inactivation of captured bacterial and fungal particles on the filter surface. The main aim of this study was to investigate the antiviral activity of two natural disinfectants, i.e., TTO and eucalyptus oil (EUO), against the influenza virus captured onto the filter surface. It was found that both tested oils possess strong antiviral properties when used as fiber coating materials, capable of inactivating captured microorganisms within 5–10 min of contact on the fiber surface. The antiviral activity of TTO was also successfully challenged in aerosol form by mixing viable airborne viral particles with oil droplets in the rotational aerosol chamber. The results look very promising for further development of virus inactivating procedures and technologies for air quality applications.

Copyright 2012 American Association for Aerosol Research     

Surface-collection efficiency of Nuclepore filters for nanoparticles

The flat surface of Nuclepore filters is suitable for observing collected particles with a scanning electron microscope (SEM). However, experimental data on surface-collection efficiency are limited because surface-collection efficiencies cannot be measured directly using aerosol measuring instruments. In this study, the surface-collection efficiencies of Nuclepore filters were determined by establishing the ratio of the number of particles deposited on the surface of the filter visually counted with an SEM to the number of inflow particles counted by a condensation particle counter, using monodispersed polystyrene latex particles (30–800 nm) and silver particles (15–30 nm). Because Nuclepore filters with smaller pore sizes would be expected to produce higher minimum surface-collection efficiency and a higher pressure-drop, 0.08 and 0.2 µm Nuclepore filters were chosen as the test filters in view of both collection efficiency and pressure drop. The results showed that the minimum surface-collection efficiencies of the 0.08 µm pores at face velocities of 1.9 and 8.4 cm·s^?1 were approximately 0.6 and 0.7, respectively, and those of the 0.2 µm pores at face velocities of 1.5 and 8.6 cm·s^?1 were approximately 0.8 and 0.6, respectively. Because the pressure drop of the 0.2 µm pore filter was lower than that of the 0.08 µm pore filter under the same flow-rate conditions, the 0.2 µm pore filter would be more suitable considering the pressure drop and collection efficiency. The obtained surface collection efficiencies were quantitatively inconsistent with theoretical surface-collection efficiencies calculated using conventional theoretical models developed to determine the collection efficiency of filters with larger pores.

© 2016 American Association for Aerosol Research     

Comparison of Test Methods for Determining the Particle Removal Efficiency of Filters in Residential and Light-Commercial Central HVAC Systems

Central heating, ventilating, and air-conditioning (HVAC) filters are often the dominant mechanism for particle removal in buildings. However, little is known about filter performance in real environments, particularly in residential and light-commercial buildings where particle concentrations and compositions can be very different from laboratory test conditions. This article explores differences in HVAC filter test protocols and refines a whole-house method for in situ testing of filters for size-resolved particle removal efficiencies. Results from the in situ method are compared with those from a simple upstream–downstream method for three types of commercially available filters in an unoccupied test house. Results from both field methods are compared with standardized laboratory test results as measured by an independent laboratory and as reported by the manufacturer. In general, comparisons between filter efficiency as measured by the refined whole-house method and as measured by the upstream–downstream method resulted in similar values of particle removal efficiency for many particle sizes and compared well with standardized lab tests, although experimental uncertainties were generally greatest for the whole-house method. However, the refined whole-house method has the added benefit of allowing an investigation of more particle interactions in an indoor environment, including deposition to ductwork and other HVAC system components, exfiltration through duct leakage, and bypass airflow around filters. Both field methods can be used to investigate the effects of HVAC system characteristics and dust loading on filter efficiency in real environments.

Copyright 2012 American Association for Aerosol Research     

Effects of Electrohydrodynamic Flow and Turbulent Diffusion on Collection Efficiency of an Electrostatic Precipitator with Cavity Walls

The effects of electrohydrodynamic (EHD) flow and turbulent diffusion on the collection efficiency of particles in a model ESP composed of the plates with a cavity were studied through numerical computation. Electric field and ion space charge density in the ESP were calculated by the Poisson equation of electric potential and the current continuity equation of ion space charge. The EHD flow field was solved by the continuity and momentum equations of gas phase, including the electrical body force induced by the movement of ions under the electric field. RNG k - l model was utilized to analyze turbulent flow. Particle concentration distribution was calculated from the convective diffusion equation of particle phase. As the ion space charge increased, the collection efficiency of charged particles increased because the electric potential increased over the entire domain in the ESP. The collection efficiency decreased as the EHD flow became stronger when the electrical migration velocity of charged particles was high. However, the collection efficiency could increase for the stronger EHD flow when the electrical migration velocity of charged particles was relatively lower. Also, the collection efficiency decreased as the turbulent diffusion of particles increased when the electrical migration velocity of particles was high. However, the collection efficiency could increase with the turbulent diffusion when the electrical migration velocity of particles was relatively lower.     

Centrifugal Filter for Aerosol Collection

Air filters collect particles by the mechanical collection mechanisms, namely, inertia, interception, gravitational settling, and Brownian diffusion. There exists the most penetrating particle size (MPPS) in submicron size range for which none of the collection mechanisms work effectively. In this study, we propose a new type of filter named as “centrifugal filter,” which collects aerosol particles by centrifugal force together with the conventional mechanical collection mechanisms. The centrifugal filter proposed in the present work may be rotated by a motor or compressed air. Air passes through the filter in the axial direction of filter rotation. The filter rotates so does the air embedded in the filter, and therefore centrifugal force exerts on particles. In addition to the mechanical collection mechanisms, small migration of particles due to the centrifugal force enhanced the collection efficiency of submicron particles significantly without increasing the pressure drop. The performance tests of centrifugal filter were conducted by changing the fiber diameter, the air flow velocity and the rotation speed. We found that the collection efficiency of filter is enhanced significantly by rotating the filter without increasing the pressure drop and that the filter efficiency is well predicted by the conventional filtration theory accounting for the centrifugal force.

Copyright 2015 American Association for Aerosol Research     

Optimizing the Design of Room Air Filters for the Removal of Submicrometer Particles

Room air filters, which usually closely resemble high-efficiency particulate air (HEPA) filters, should be designed to maximize the clean air delivery rate (CADR) rather than operate at the very high collection efficiencies and relatively high pressure drops associated with HEPA filters. That is, for fixed electrical energy consumption, filters should remove the most particles possible. This can be accomplished by designing room filters that operate at lower collection efficiencies and higher airflow rates than HEPA filters. Based on filtration theory, the CADR of a fibrous filter is maximized with respect to filter thickness and air velocity at fixed energy consumption and filtration area. For very small particles for which diffusional deposition is the predominant collection mechanism, it is shown that a filter thickness resulting in a collection efficiency of 82% is optimal. For somewhat larger particles having diameters close to the filter's most penetrating size, direct interception is included in the analysis. The importance of inertial impaction and electrostatic deposition is also considered. This article supports the belief that room air filters used for enhancing indoor air quality can be improved significantly and suggests a methodology to accomplish this improvement.     

High-Volume Aerosol Filtration and Mitigation of Inertial Particle Rebound

The performance of electrostatically charged blown microfiber filter media was characterized for high-volume sampling applications. Pressure drop and aerosol collection efficiency were measured at air pressures of 55.2 and 88.7 kilopascals (kPa) and filter face velocities ranging from 2.5 to 11.25 meters per second (m/s). Particle penetration was significant for particles above 0.5 micrometers (μm) in aerodynamic diameter where the onset of particle rebound was observed as low as 200 nanometers (nm). Particle retention was enhanced by treating filters in an aqueous solution of glycerol. Adding this retention agent eliminated electrostatic capture mechanisms but mitigated inertial rebound. Untreated filters had higher nanoparticle collection efficiencies at lower filter face velocities where electrostatic capture was still significant. At higher filter face velocities, nanoparticle collection efficiencies were higher for treated filters where inertial capture was dominant and particle rebound was mitigated. Significant improvements to microparticle collection efficiency were observed for treated filters at all air flow conditions. At high air pressure, filter efficiency was greater than 95% for particles less than 5 μm. At low air pressure, performance enhancements were not as significant since air velocities were significantly higher through the fiber mat. Measured single fiber efficiencies were normalized by the theoretical single fiber efficiency to calculate adhesion probability. The small fiber diameter (1.77 μm) of this particular filter gave large Stokes numbers and interception parameters forcing the single fiber efficiency to its maximum theoretical value. The adhesion probability was plotted as a function of the ratio of Stokes and interception parameter similar to the works of others. Single fiber efficiencies for inertial nanoparticle collection were compared to existing theories and correlations.

Copyright 2014 American Association for Aerosol Research     

Initial Collection Performance of Resin Wool Filters and Estimation of Charge Density

Abstract

A new type of resin wool filter (RWF) that persists the load with oil droplets was developed by Kimura and colleagues. In the present work, the initial collection performances of RWF (A and C) are measured for various particle sizes (0.03, 0.05, 0.1, 0.15, 0.2, 0.25, and 0.3 μm) with different charging states at various filtration velocities (0.05, 0.1, 0.15, and 0.3 m/s). As a result, it is shown that the present RWF impregnated with PTBP resin can attain high collection efficiency (99.999% at filtration velocity of 0.05 m/s) with a pressure drop of less than 30 Pa. The charge density is estimated by applying prediction equations of single-fiber collection efficiencies of electret filters with a dipolar charge distribution because no other prediction equation for RWF are available at present. The experimental single-fiber efficiencies for uncharged particles are successfully predicted by assigning a single value of charge density in the prediction equations for dipolar fibers. The estimated charge density on RWF fibers is 2.1 × 10^{? 4} C/m², which is much higher than those of conventional electret filter media. Therefore, RWF studied in the present work is suitable for the application to respirators as well as room air cleaners.     

Effect of particle loading on the collection performance of an electret cabin air filter for submicron aerosols

Electret filters are composed of permanently charged electret fibers and are widely used in applications requiring high collection efficiency and low-pressure drop. We tested electret filter media used in manufacturing cabin air filters by applying two different charging states to the test particles. These charging states were achieved by spray electrification through the atomization process and by bipolar ionization with an aerosol neutralizer, respectively. Polydisperse solid NaCl particles with 0.1%, and 1% solutions or liquid dicotyl sebacate (DOS) particles were generated from an atomizer, and they were loaded on the filter media. The amount of charge, the mean particle size, and the particle material significantly affected the collection performance of the electret filter media for submicron particles. The collection efficiency of the electret filter media degraded as more particles were loaded, and showed minimum efficiency at steady state. The electret filter media captured the highly charged particles more efficiently during the transient state. At steady state, the filter media loaded with smaller NaCl particles showed lower collection efficiency. The filter media loaded with liquid DOS particles showed collection efficiency much lower than those loaded with solid NaCl particles.     

Performance Evaluation of Electrostatically Augmented Air Filters Coupled with a Corona Precharger

An experimental study of electrostatically augmented air (EAA) filters coupled with a corona precharger has been conducted using Arizona road dusts and tobacco smoke. The measurements of filter efficiency and pressure drop across the EAA filter have been made using an ASHRAE 52.1-1992 filter test system and an opacity meter to measure the particle concentration upstream and downstream of the test filter. The two-stage EAA filter unit consists of the positive corona precharger upstream of a filter, to precharge particles with the electrical strength of 4.7 kV/cm, and an electrified filter collector, which has folded media with meshy metal separators, in the upstream and downstream side gaps. DC voltage of +1,000 V (1.4 kV/cm) is applied between the upstream and downstream separators to produce an electric field between the separators and media as well as across the media in a polarity so that most of the precharged particulates are collected on the upstream filter collector. A conventional filter was measured and had 70.0% efficiency with dusts of 1.96 w m in mass median diameter and 2.5 m/s face velocity, while the EAA filter had 92.9% efficiency. An electrical effect on the EAA filter was evaluated to both improve the filter efficiency and reduce the pressure drop across the filter. Also, the performance evaluation of the EAA filter using an air handling chamber system in occupied space was investigated with tobacco smoke particles.     

Performance of fibrous filters during nanoparticle cake formation

Fibrous filters are highly efficient in removing micrometer particles, but their performance in the nanometer particle range is still little known. The aim of this study was to evaluate pressure drop and collection efficiency during nanoparticles cake formation using commercial fibrous filters. The filter media used were High Efficiency Particulate Air (HEPA) and polyester filters. The aerosols were generated by a commercial inhaler using a 5 g/L solution of NaCl and the particles produced were in the size range from 6 to 800 nm, with a peak at around 40 nm. A superficial velocity (v_s) of 0.06 m/s was employed. During the filtration, the maximum pressure drop established was ?P ₌ ?P_f +980Pa, where ?P_f is the initial pressure drop of the filter. The collection efficiency was determined for a clean filter and for intermediate pressure drops. The filtration curves obtained showed that the HEPA filter provided greater surface filtration, compared to the polyester filter. Comparison of the collection efficiencies for clean filters revealed that the HEPA filter was highly efficient, even in the absence of cake, while the polyester filter showed initial collection efficiencies of between 20 and 40% for particles in the size range from 100 nm to 1000 nm. However, after formation of the filter cake, the collection efficiencies of both filters were almost 100% during the final stage of filtration. This shows that the fibrous filter can be applied in several industrial processes with highly efficient nanoparticle separation, after the formation of a thin layer cake filtration.     

A Simulation Study on the Collection of Submicron Particles in a Unipolar Charged Fiber

In this study, we developed a simulation method to predict the initial collection efficiency of a unipolar charged fiber and the particle deposition morphology in the electret filter composed of unipolar charged fibers. The particle sizes considered in this study were in the submicron range, and in the simulation method, Brownian motion of particles was also taken into consideration along with electrostatic forces acting on the particles. The simulation results were compared with other investigator's initial collection efficiency data, and it was found that simulation results are in good agreement with the experimental data. Based on this, we analyzed the effect of operating variables on the particle deposition morphology, which in turn affects the collection efficiency and pressure drop of the filter. In view of the simulation results on particle deposition morphology, it is clear that in the case of electret filters, particle deposition tends to take place onto the entire perimeter of fibers relatively uniformly, which may reduce the increase of pressure drop with time or extent of particle deposition compared to the conventional fibrous filter.     

Experimental Study of Electrostatic Aerosol Filtration at Moderate Filter Face Velocity

Aerosol collection efficiency was studied for electrostatically charged fibrous filters (3M Filtrete?, BMF-20F). In this study, collection efficiencies at moderate filter face velocities (0.5–2.5 m/s) representative of some high volume sampling applications was characterized. Experimental data and analytical theories of filter performance are less common in this flow regime since the viscous flow field assumption may not be representative of actual flow through the filter mat. Additionally, electrostatic fiber charge density is difficult to quantify, and measurements of aerosol collection efficiency are often used to calculate this fundamental parameter. The purpose of this study was to assess the relative influence of diffusion, inertial impaction, interception, and electrostatic filtration on overall filter performance. The effects of fiber charge density were quantified by comparing efficiency data for charged and uncharged filter media, where an isopropanol bath was used to eliminate electrostatic charge. The effects of particle charge were also quantified by test aerosols brought into the equilibrium Boltzmann charge distribution, and then using an electrostatic precipitator to separate out only those test particles with a charge of zero. Electrostatically charged filter media had collection efficiencies as high as 70–85% at 30 nm. Filter performance was reduced significantly (40–50% collection efficiency) when the electrostatic filtration component was eliminated. Experiments performed with zero charged NaCl particles showed that a significant increase in filter performance is attributable to an induction effect, where electrostatic fiber charge polarizes aerosol particles without charge. As filter face velocity increased the electrostatic filtration efficiency decreased since aerosol particles had less time to drift toward electrostatically charged fibers. Finally, experimental data at 0.5 m/s were compared to theoretical predictions and good agreement was found for both electrostatic and nonelectrostatic effects.

© 2013 American Association for Aerosol Research     

Determination of the Fractional Efficiencies of Fibrous Filter Media By Optical In Situ Measurements

ABSTRACT

An experimental setup is introduced to determine the fractional efficiency of fibrous filters. The device includes two optical particle counters for the in situ measurement of the particle flux and size upstream and downstream of a test filter. The simultaneous measurement in the raw and clean gas by a counting method allows a rapid determination of the fractional efficiencies of various filter media within a few minutes. In the first experiments performed with different aerosols (latex spheres, bacterial aerosol, limestone dust) the apparatus proved to be a reliable instrument for the investigation of the collection behaviour of fibrous filters influenced by various parameters.

The fractional efficiency yields more information about the filter performance, i.e., respirable fraction, collection minimum, particle bouncing off etc., than the total collection efficiency. Thus it is possible to compare different filter media of the same or different filter class and determine their optimum operational conditions.     

Analytic Solutions to Diffusional Deposition of Polydisperse Aerosols in Fibrous Filters

Deposition of polydisperse aerosols by Brownian diffusion was studied analytically using the penetration efficiency of monodisperse aerosols combined with the correlations among the moments of lognormal distribution functions. The analytic solutions, so obtained were validated using the exact solutions, which were applied to recalculate the filtration efficiencies of the existing experimental data for various filtration conditions. It was found that the collection efficiency of a fibrous filter should be corrected with respect to the position in the filter, if the particles are polydisperse. By considering the effect of the polydispersity of particle size, the analytic solutions showed good agreement with existing experimental data. It is believed that the present work makes it possible to determine the filtration efficiency of polydisperse aerosols in fibrous filters and to estimate errors associated with the degree of polydispersity of the particles quickly and accurately for the diffusion dominant regime.     

Water condensation-based nanoparticle charging system: Physical and chemical characterization

Abstract

A water condensation-based ion charging system has been developed to enhance both the charging efficiency and the concentration of sub-20?nm particles. This NanoCharger consists of a bipolar ion source followed by a parallel plate water-based condensation system, an embedded ion scavenger, and an aerodynamic focusing stage. Sufficient numbers of ions are transported through the system to attach to the formed droplets. An ion scavenger removes the ions immediately after the droplet formation to minimize multiple charging. A subsequent cold-walled condensation stage removes most of the water vapor, lowering the dew point to below 16?°C, while a set of focusing nozzles concentrates the droplets into ～10% of the flow. The flow is then slightly heated to evaporate the droplets. The physical enhancement of electrical charging was evaluated in the laboratory using mobility-selected particles, and found to provide ～40-fold enhancement over bipolar charging for 6–15?nm particles. Chemical artifacts were evaluated through thermal desorption chemical ionization mass spectrometry. Data comparing ion spectra for flow that passed through the NanoCharger to that obtained without it showed nearly equivalent ion spectra, indicating that no significant artifacts were introduced from the condensation–evaporation process.

Copyright © 2018 American Association for Aerosol Research     

The Filtration of Fibrous Aerosols

Fibrous particles constitute an important class of aerosols that are potential human health hazards. Filters can remove aerosols from the air. The capture of spherical and fibrous aerosols by fibrous filters was investigated in this study. The governing equations of motions for translation and rotation of fibrous particles are derived for airflow over a cylindrical object. Only impaction and interception losses were considered in this study. Transport and deposition of fibrous particles were found to depend on Stokes number, fibrous particle aspect ratio, and ratio of the fibrous particle diameter to the diameter of fibrous filters. Using the Kuwabara flow field, transport and single-fibrous filter capturing efficiency of spherical and fibrous particles were calculated numerically, and these calculations were compared with available data in the literature. The calculated results compared favorably with the results of Yeh and Liu (1974) for spherical particles. Good agreement for losses by interception for both spherical and fibrous particles was observed between our results and those of Lee and Liu (1982). Further experimental data are needed to verify the predicted losses of fibrous aerosols by impaction.