Electret Filter Research Articles

Evaluation of electrostatic properties of electret filters for aerosol deposition

Electret filter media are nonwovens with electrostatically charged fibers and are commonly applied for separation of fine particles from gases. However, the distribution of the electrostatic charge on the fiber surfaces – which dependens on the applied charging method – could not be completely clarified so far. The aim of this work is to determine the amount of electrostatic charge and its distribution in the porous structure of the nonwoven by comparison of experiments and 3D DirectNumerical Simulations coupling the gas flow and electrostatic field to the particle movement.As example, corona and triboelectrically charged filter media were investigated. The charge distributions on the fibrous structure were estimated by comparison of the numerically and experimentally obtained fractional deposition efficiencies. Furthermore, the simulations showed that different charge distributions led to significant differences in the fractional deposition efficiency up to 15%, even if the amount of charge and the area ratio of positive and negative charges were constant.

Effects of relative humidity and particle hygroscopicity on the initial efficiency and aging characteristics of electret HVAC filter media

Because of the low pressure drop, electret filter media are widely used in indoor environments to remove particles for air quality control. The effects of water soaking, air relative humidity (RH), and particle hygroscopicity on the filtration performances of fresh and aged media, i.e., size-fractioned efficiency, however remains unclear. Electret media from a commercial high-efficiency particulate air (HEPA) filter, a commercial Heating, Ventilating, and Air Conditioning (HVAC) filter with minimum efficiency reporting value of 13 (MERV13) and a homemade filter (CM) were investigated in this work. In the initial test, the performance of above three media were evaluated at 10, 60 and 90% RHs. In the aged test, the media performances were characterized at 30, 60 and 80%, and using three different sample particles, i.e., hygroscopic NaCl, non-hygroscopic SiC, and NaCl + SiC mixture). In addition to clean filters, the collection efficiency and pressure drop of used MERV13 electret media, which had been utilized in a house for few months, were evaluated. SEM analysis was conducted to examine particle loading characteristics. The major finding of this study is that operating the filtration systems having electret media at ~60% RH can greatly enhance the particle holding capacity and minimize the efficiency decline. The result of this study provides the insights of the RH effects on the performance of electret media.

In situ efficiency of filters in residential central HVAC systems.

High-efficiency filtration in residential forced-air heating, ventilation, and air conditioning (HVAC) systems protects equipment and can reduce exposure to particulate matter. Laboratory tests provide a measure of the nominal efficiency, but they may not accurately reflect the in situ efficiency of the filters because of variations in system conditions and changes in filter performance over time. The primary focus of this paper is to evaluate the effective filtration efficiency, which is inclusive of any loading and system impacts, in 21 occupied residential homes through in-duct concentration measurements. We considered the role of filter media by testing both electret and non-electret media, as well as the role of loading by considering new and used filters. The results show that filters with higher nominal efficiency generally had higher effective filtration efficiency in the same home. In terms of performance change, there is no significant difference in efficiency between initial and 3-month non-electret filters, but the efficiency of electret filters generally decreased over time. However, both nominal efficiency and performance change were vastly overshadowed by the wide variety in loading and system conditions across homes, making it hard to predict filter efficiency in a given home without in situmeasurements.

Effectiveness of N95 Facepiece Respirators in Filtering Aerosol Following Storage and Sterilization

ABSTRACT The use of electret N95 filtering facepiece respirators (N95FFRs) after prolonged storage or treatment can reduce the expense of buying non-expired N95 and thus enable developing and developed countries to optimize their use of limited resources to against airborne particles and diseases, such as coronavirus disease 2019 (COVID-19). The filtration performance of five N95FFR models following long-term storage, removal of charge using isopropanol alcohol (IPA), autoclaving, or treatment with gamma-radiation was assessed using a TSI 8130 automated filter tester. Statistically significant differences were found in the penetration (P), pressure drop (Δp) and quality factor (qf) between non-expired and expired N95 models 3M-8210 and 3M-8511. A statistically significant linear correlation was also obtained between the N95 penetration ratio (PR) and the difference between year of manufacture and time of test (DYM). The PR of N95 was more strongly influenced by eliminating the charge (for extremely out-of-date respirators) on the electret filter than by the DYM. Sterilization by gamma irradiation increased the P into non-expired and expired N95FFR models (p < 0.05), reducing their qf. The qf of all N95FFR models, except UVEX-3200, was strongly affected by gamma irradiation, the removal of charge using IPA, autoclaving, and storage in that order. All expired models maintained acceptable filtration performance and still could be used to collect aerosol effectively, even though models 3M-8511 and 3M-1860 had been manufactured up to 13 years previously. As the COVID-19 outbreaks in 2019 and is getting worse in 2020, these data are useful in developing a global stockpiling strategy to maximize the longevity of N95FFRs for public health and healthcare workers. However, the aging of the straps and seal materials (rubbers, plastics) of the N95FFRs may affect their fit factor and effectiveness.

In-situ effectiveness of residential HVAC filters.

In this study, we explore different filter and contextual characteristics that influence effectiveness of high-efficiency filters in 21 residences in Toronto, Canada. The in situ effectiveness was assessed with decay tests at the beginning andthe end of filter life with four different filters (MERV 8-14 from ASHRAE Standard 52.2) installed in operational HVAC systems, compared with either the system off or with no filter installed. There was considerable difference between median PM2.5 effectiveness of the non-electret filters when compared to electret filters (16% vs. 36%) of the same nominal efficiency (MERV 8). However, median PM2.5 effectiveness of electret filters only slightly improved (between 5% and 9% absolute increase) as MERV increased from 8 to 14. There was more variation in filter effectiveness between the same filter in different homes than there was between different filters in the same home. Variations in filter performance arose because home-specific particle loss rates (eg, ventilation rate) vary greatly in different buildings. The higher the loss rates due to non-filter factors, the lower the effectiveness of a filter. Given the relatively large variation in effectiveness for a given filter over time and in different homes, increasing system runtime may be a productive way to improve filter performance in many homes.

Filtration performance and charge degradation during particle loading and reusability of charged PTFE needle felt filters

Electret filter materials have attracted wide attention because of their low pressure drop and high filtration efficiency. Few studies have measured the filtration performance and the charging status of the charged filters during soot particle loading. In this study, the polytetrafluoroethylene (PTFE) needle felt filters were charged by a corona discharge method. The filtration efficiency and surface potential were investigated during the soot particle loading process. The effects of NaCl particles, soot particles and A2 dust on surface potential and filtration efficiency were investigated. The reusability of charged filters and the effect of temperature on surface potential were studied. The results indicated that the initial filtration efficiency of charged PTFE needle felt filters (PNFFs) was greatly improved compared with that of uncharged PNFFs. The attenuation rate of the surface potential at 100 °C was faster than that at room temperature. The surface potential decreased with the start of particle deposition on the filters, correspondingly, the filtration efficiency of the charged PNFFs decreased. The efficiency of charged PNFFs loaded by soot particle decreased first and then increased, with the transition point corresponding to 20 Pa of pressure drop growth. With the same increment of pressure drop, soot particles with higher conductivity caused more efficiency degradation than NaCl particles. The reusability of the charged filters was affected by the effectiveness of dust cake removal and the particle conductivity.

Electrostatically assisted air coarse filtration for energy efficient ambient particles removal: Long-term performance in real environment and influencing factors

Electrostatically assisted air (EAA) coarse filter can improve the particle filtration performance of low efficiency filters without increasing their pressure drop. In this study, we investigated the long-term single pass filtration efficiency for 0.3–10 μm ambient particles, pressure drop, PM2.5 collecting amount, and energy consumption of an EAA coarse filter during a 51-day operation period. We found that the efficiency of EAA coarse filter varied with average values of 87.6%–97.1% for 0.3–10 μm particles, respectively, while the pressure drop increased from 14.6 Pa to 63.1 Pa at face air velocity of 1 m/s. Using K-means clustering and principal components analysis methods, we discovered that before 33 days, the significant influencing factors of the efficiency for 0.3–0.5 μm particles included air temperature, air moisture content, 3–10 μm particle number concentration, and PM2.5 collecting amount. While after 33 days, only PM2.5 mass collecting amount has a significant negative correlation with the efficiency for 0.3–0.5 μm particles. The efficiency became relatively unsteady and the PM2.5 collecting amount reached 131 g/m2 after 33 days. The total energy consumption of EAA coarse filter rose more slowly as PM2.5 collecting amount increased when compared with that of a conventional electret filter (KAFPO78A4, Daikin Inc., Japan). We suggest the high efficiency EAA coarse filtration as an energy efficient method for ambient particle removal in the view of its long-term performance.

Electrostatic Transparent Air Filter Membranes Composed of Metallized Microfibers for Particulate Removal.

Particulate matter (PM) from ever-increasing industrialization poses a great public health risk. Although fiber-based filters are used effectively to block PM, filters with high packing densities suffer from excessive pressure drops. Electret filters bypass intermediate- or large-sized particles and thus capture only small particles, the motion of which can be influenced by weak electrostatic fields. In this study, we demonstrate the fabrication of metallized fibers that produce intense electric fields, thereby enabling capture of PMs of a variety of sizes produced by burning incense. The filter consisting of these metallized fibers effectively removes moving particles from air. An electricity-driven filter is relatively thin and has a low packing density, making it light, portable, transparent, and inexpensive. The sizes of the pores between the metallized fibers are readily controlled by manipulating the electrospinning and electroplating times. Sufficiently large pores permit efficient airflow and thus increase permeability without risking an excessive pressure drop. The metallized fiber filter is washable and thus reusable. In this study, a PM removal rate of >97% was recorded using a filter designed under optimal conditions.

Characterization of filter media prepared from aligned nanofibers for fine dust screen

ABSTRACTNanofibers for fine dust filters of four structures (random, aligned, orthogonal, and nanofiber net) were prepared by electrospinning method using polymers such as PAN and PA6. While conventional electret filters experienced deterioration problems in fine dust(PM1.0) capture as its surface charge decayed, the electrospun nanofibers prepared contributed to the removal capacity. The filters from aligned fibers showed high quality factors ( q F: filter performance indicator) and filtration efficiency from 22 to 50% depending on particle size than simple electret media at a face velocity of 15.92 cm/s. The fiber structure of nanofiber net (NFN) presented almost absolute collection efficiency, particularly on dust particles smaller than 300 nm. Furthermore, the composite filters which are composed both of a commercial electret mask filters and nanofiber nets effectively enhanced the overall filtration efficiency by 59.46%, resulting in more than 99% for PM1.0. Consequently, electrospun polymer nanofibers offer a promising plausible mask filter material with air permeability. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 48166.

Filter media performance under the oscillating flow condition

This work investigated the performance of two mechanical glass-fiber and one electret filter media under the oscillating flow conditions (i.e., cyclic flows in this study). DMA-classified particles of 30, 50, 100 and 150 nm, and oscillating flow conditions with the Average Suction Flow Rates (ASFRs) of 6.4, 25.6 L/min and oscillating frequency of 6, 24, 40 min−1 were chosen for the experiments. It is found in general the particle penetration of test media increases with the increase of oscillating frequency. The above-observed penetration enhancement is found to be more pronounced for small particles (i.e., 30 and 50 nm) as compared to that for large particles (i.e., 100 and 150 nm). At the flow oscillating frequency of 24 and 40 min−1, the particle penetration through filter media is also higher than it under the corresponding constant flow conditions. The increase of ASFR increases the particle penetration of filter media for all the selected particle sizes. The apparent single fiber efficiency for media A and B under cyclic flow conditions was lastly found. The obtained efficiency enables the single fiber theory to calculate the average penetration of diffusive particles through mechanical filter media under the cyclic flow conditions.

Evaluation the effect of fiber alignment on particle collection performance of mechanical/electret filters based on Voronoi tessellations

The effect of fiber alignment on particle collection performance of mechanical/electret filters are investigated numerically. Four sets of fiber networks, namely, nearly isotropic, aligned, highly aligned and layered orientations are constructed using Voronoi tessellations. The particle transport and deposition process in these fiber networks under the coupling of flow field and electrostatic field are modeled using hybrid lattice Boltzmann Lagrangian method. The particle collection efficiencies and pressure drops of the mechanical/electret filters are calculated and compared. Results indicate that the mechanical filter with layered networks has the best filtration performance for large particles, and then the aligned networks has the next best performance. However, the sequence reverses when filtering the nanoparticles. In contrast to a mechanical filter, the electret filter with isotropic networks exhibits the best performance for full range of particles size, indicating that the dielectric attraction is less impacted by the 3D fiber orientations. Increasing fibers orientations in the airflow direction and constructing 3D pores of high sphericity are recommended to improve the performance of an electret filter.

Operation of the electret filter in an external electric field

Experiments were performed to investigate the filtration efficiency of electret filters in external electric field. The electret filter layer was located between two conductive nonwovens (electrodes). The top conductive fibrous layer was applied to a DC voltage ranging from 0 to 6.5 kV while the bottom one was grounded. At the DC voltage, a strong external electric field was generated between the two conductive electrodes, and the electret filter layer can be charged more effectively. The filtration efficiency and the quality factor (QF) were obviously increased with an increase of the applied voltage or electric field in each filter.

Charged PVDF multi-layer filters with enhanced filtration performance for filtering nano-aerosols

Filtration of neutrally charged nano- and sub-micron aerosols (50–500 nm) using PVDF electret nanofiber filters based on dielectrophoretic effect was investigated experimentally for the first time. Dielectrophoretic effect is refers to inducing dipole charges on neutrally charged aerosols when they are in close proximity to the charged nanofibers; subsequently, the polarized aerosols are captured electrostatically by the charged fibers. Given the nanofiber diameter is less than 500 nm, the electrostatic attraction force in the proximity of the fiber was very strong, resulting in good aerosol capture without incurring higher pressure drop across the filter. We were able to charge a PVDF nanofiber filter using corona discharge and the performance of the filter remained relatively unchanged within 90 days of repeated testing with quality factor well reaching 0.14 Pa−1 or higher. Optimization of charging parameters was carried out and charge density on filters indicated by surface potential (SP) was found to increase with increasing charging voltage and decreasing charging distance. Correspondingly, filtration efficiency increased due to the enhanced electrostatic attraction between charged fibers and polarized aerosols. The influence of filter basis weight (W) on the filtration performance of electret filters was studied, which showed that the improvement in filtration performance was insignificant with limited increase in filtration efficiency and higher pressure drop. Although higher fiber amount led to higher surface potential, it also caused unsatisfactory charge density increment and higher electrical interference between adjacent fibers due to higher packing density. To enhance the performance of PVDF electret filters, a ‘multi-layering’ approach was proposed for the first time on electret nanofiber filter whereby the total charged fibers were distributed in thinner stack-up layers, with each layer being isolated by a porous permeable substrate, to reduce the electrostatic interference among fibers of adjacent layers. The latter has been the key culprit in reducing the performance when charged fibers were all stacked into a single layer, resulting in overlapping of intense electrostatic fields from different nanofibers. Moreover, through distributing fibers to multilayers, the filter porosity was increased, leading to lower air flow resistance and pressure drop. Therefore, compared with a filter with only one layer, a multi-layer filter with similar total basis weight of fibers had much better performance, which was further verified by halving the basis weight of each individual layer (WL) while maintaining the overall value. The single-fiber efficiency based on dielectrophoretic effect confirms larger aerosols got better charge induction resulting in stronger dipoles and better capture. The face velocity dictates the retention time and increasing retention time proves beneficial for dielectrophoretic capture. Multilayering could reduce electrostatic interference, thereby facilitating increasing basis weight of fibers to be used in a filter to achieve high capture efficiency. Moreover, filter durability/stability was investigated, which indicates the multi-layer PVDF electret filters could maintain high performance and are promising for long-term use. It was concluded that multilayer PVDF electret filters have a great performance in aerosol removal and a potential filtration enhancement mechanism was elaborated.

Enhancement of filtration performance of polyester (PET) filters by compositing with schorl powder

Improving the filtration performance of prefilters to protect high efficiency filter media is of great significance for air conditioning systems. Based on this, a schorl/polyester composite (SPET) filter was produced by embedding schorl powder into a polyester (PET) filter surface via a thermoplastic polyurethane (TPU) membrane. Its filtration performance parameters, including pressure drop, filtration efficiency, and quality factor (Q), were studied experimentally and compared with those of PET. The filtration efficiencies of the SPET filter for dust particles with diameters between 0.3 and 10 μm were improved, with a more significant enhancement effect for smaller dust particles. Additionally, the quality factor was improved by the addition of schorl powder to make SPET. The enhancement mechanism of SPET has been studied. The electret filters model made good predictions of the filtration efficiencies of SPET by ignoring the piezoelectricity of schorl. This study indicates that the spontaneous polarization of schorl is the main reason for filtration efficiency enhancement.

Evaluation of different discharging methods on HVAC electret filter media

Electret filters with electrostatic charges added on fibers are widely used in HVAC system of residential and commercial buildings to mitigate PM2.5 exposure in indoor environments. Filtration applications require the knowledge of minimum filtration efficiency of electret filters under worst condition. To determine the minimum efficiency, different electret discharging methods are used in test standards to eliminate electrostatic effects. This study evaluated the isopropanol (IPA) liquid discharging method in EN 779 and the IPA saturated vapor treatment in ISO 16890. It was found that although both IPA liquid immersion and saturated vapor treatment can significantly remove fiber charge, saturated vapor treatment showed better discharging performance. The charge density of media discharged by IPA liquid immersion and saturated vapor treatment were 2 μC/m2 and 0 μC/m2, respectively. It demonstrated that IPA saturated vapor treatment can completely remove fiber charge, while undesirable fiber charge occurred after being treated by IPA liquid due to interface electrification. The test results of unsaturated vapor treatment indicated that IPA in the phase of vapor was not capable of removing fiber charges. The fact that saturated vapor treatment was more effective than liquid immersion was due to the different discharging process. For saturated vapor treatment, vapor condensed on fiber surface and formed a liquid layer with thickness of 15 nm. This ultra-thin layer of IPA liquid can discharge the fiber charge completely without inducing extra charge after liquid evaporation. This study concluded that discharging method in ISO 16890 should be used in electret filter test.

Numerical and experimental study of submicron aerosol deposition in electret microfiber nonwovens

Highly efficient microfiber electret nonwovens, in which the fibers are electrostatically charged to complement the mechanical deposition mechanisms by electrostatic effects, are nowadays widely used in numerous applications for example to improve the indoor air quality or to reduce emissions. Simulations of the filtration efficiency of electret filters are typically based on a single fiber approach and thus imply strong simplifications. Here, a novel simulation method based on direct numerical solution of the Navier-Stokes equations and the Euler-Lagrange approach for particle tracking taking into account the real three-dimensional microscopic structure of the electret media is presented.Besides the conventional mechanical deposition mechanisms, the model also considers the Coulomb effect as well as dielectrophoresis. It furthermore allows to allocate different surface charge amounts and distributions of both polarities to the microfibers and thus to investigate their influence on the filtration efficiency.To validate the simulations, the results were compared to an empirical approach and measurements for an exemplary electret medium. In the investigated case, the results from the simulations agree well with the measurements for a bipolar charge distribution on the microfibers. The simulation results also point out that by the used empirical approach, for a given surface charge amount, the filtration efficiency is overestimated.

PM collection performance of electret filters electrospun with different dielectric materials-a numerical modeling and experimental study

Electrospun electret filter has been regarded as a promising medium to remove particulate matter (PM) from air stream because it could fulfill the requirements for both high filtration efficiencies and low pressure drops. The physico-chemical characteristics of the polymer material have significant effects on the PM collection performance of the electret filter. In this study, polyvinyl chloride (PVC), polyacrylonitrile (PAN), polycarbonate (PC) and polyethyleneimine (PEI) electret filters with similar structures are electrospun using polymers of different permittivity and their collection efficiencies for neutral particles are evaluated experimentally. Then the lattice Boltzmann coupled with discrete element method (DEM) is applied to simulate the particle transport and deposition through the virtual 3-D electret filters, to give further insight into the particle capture mechanisms due to the PM-fiber interaction and the dielectrophoretic attraction. The simulation results compare favorably with the experimental data. Results indicate that the PM collection efficiency of PVC electret filter is the highest, and followed by the PAN, PC and PEI electret filters. It is revealed that the dielectric property of the polymer material is an influential factor shaping the interaction of particles and fiber surfaces, as well as the charge storage abilities of the electrospun electret filters. A higher polymer permittivity would lead to a larger PM-fiber adhesion energy, a larger fiber charge density and then a better PM collection performance.

Factors Affecting Particle Depositions on Electret Filters Used in Residential HVAC Systems and Indoor Air Cleaners

Filters made from electret media with quasi-permanent electrical charges have been widely applied to control particulate matter (PM) pollution. However, studies using parametric analysis to examine the effects of the operating face velocity, charge density, fiber diameter, porosity, and thickness on the energy efficiency of the filtration are lacking. A reliable parametric analysis requires an accurate filtration model. Without adding any empirical parameters, a modified model developed earlier by the authors was the first to accurately predict the efficiency of electret filters at different face velocities and with different filter charge densities for neutralized particles. To further verify the applicability of this model, we conducted filtration experiments in which singly charged, neutral, and neutralized monodisperse particles with 3–500 nm through two different electret filters, one with a charge density of 0.075 and the other with a charge density of 0.025 mC m–2, as well as through discharged electret filters. The results of the modified model agreed well with the experimental data. The validated model was then used to conduct the parametric analysis to clarify the effects of the aforementioned parameters on filter performance. It was found that the increase in efficiency due to the fibers’ charge states varied largely with the face velocity and the charge density of the electret. Furthermore, when the pressure drop was held constant, using thicker filters with less solidity reduced particle penetration. The results in this work can be applied to the design and operation of future electret filters.

Filtration efficiency and loading characteristics of PM2.5 through commercial electret filter media

Abstract Atmospheric PM2.5 pollution has been attracting much public attention for decades because of its adverse health effects. China and other Asian countries are frequently exposed to high PM2.5 concentrations, and filtration techniques through HVAC systems and indoor air cleaners are often used to mitigate PM2.5 in indoor environments, e.g. commercial and residential buildings, schools, and hospitals. Electret filter media provide lower pressure resistance and are frequently used in air purification systems. However, existing test standards, e.g. ASHRAE 52.2-2012, only challenge filters with particles larger than 300 nm which makes it impossible to assess filter performance at the most penetrating particle size (MPPS) since the inherent MPPS of electret media is smaller than 100 nm. In this study for investigating the performance of the electret filter media against PM2.5, the filter media were first challenged with monodisperse particles of 3–500 nm (the majority of PM2.5 number concentration resides in this range) at face velocities of 0.05, 0.25 and 0.5 m s−1 to obtain initial efficiency curves. Then the time-dependent efficiencies during the loading process were investigated using polydisperse NaCl particles which mimicked the size distribution of typical atmospheric PM2.5. Additionally, the effects of fiber charge and particle charge on initial efficiency and loading characteristics were evaluated by intentionally removing the fiber charges and charging the particles bipolarly (neutralized) or unipolarly. Results showed that the effects of fiber and particle charges on initial efficiency enhancement depended on particle sizes. The polarization force enhanced the collection of particles larger than ∼20 nm particles, the image force enhanced the collection of particles smaller than ∼100 nm, and the Columbic force enhanced the collection of particles smaller than ∼300 nm. Charging the particles unipolarly can slow down the reduction of electrical efficiency during loading and enhance the minimum efficiency by 10–30% for 50–500 nm particles, compared to that of bipolarly charged particles. A model using an electric degradation parameter and mechanical collection efficiency raising factor was applied to explain the performance of electret media in loading processes.

Enhancement of filtration efficiency by electrical charges on nebulized particles

There have been few investigations of effects of electrical charge, carried by lab-generated particles, on filtration efficiency testing. Here, we measured the elementary charge on particles and the fraction of particles carrying that charge with a combined electrometer, differential mobility analyzer, and scanning mobility particle sizer. A typical solid NaCl aerosol and liquid diethylhexyl sebacate (DEHS) aerosol were generated with Collison and Laskin nebulizers, respectively. Our experimental results showed that NaCl aerosols carried more charge after aerosol generation. The average net elementary charge per particle was approximately 0.07. The NaCl aerosol was overall positively charged but contained a mixture of neutral and charged particles. Individual particles could carry at most four elementary charges. According to constant theorem, we speculated that original NaCl aerosol contained 17% neutral, 45% positive-, and 38% negative-charged particles in the diameter range from 30 to 300nm. A Kr-85 neutralizer was used to decrease the charge on the NaCl particles. Our results indicated that the DEHS aerosol was electrically neutral. The effects of electric charge on particle collection by electret and electroneutral high efficiency particulate air (HEPA) filters were analyzed. Theoretical calculations suggested that charges on original NaCl aerosol particles enhanced the filtration efficiency of HEPA filters.