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     

Size-Resolved Penetration Through High-Efficiency Filter Media Typically Used for Aerosol Sampling

We have developed a new, fully controlled filter testing device and have used it to measure size-resolved penetration through a typically used filtration media for (but not only) atmospheric aerosol sampling. Twenty membrane and fiber filter pieces (mixed cellulose ester filters, polytetrafluoroethylene filters, quartz fiber filters, glass fiber filters, and polycarbonate filters) of various manufacturers and filter codes were examined. High variability in the penetration curve shapes, most penetrating particle size (MPPS) (from 20 nm to 90 nm) and penetration maxima (from 0.001% to almost 100%) has been found. The dependence of pressure drop on face velocity generally agrees with theory, the comparison of penetration at various face velocities proved the theoretical equations being able to determine MPPS only partially correctly. Although the variability within an individual filter of the same code is not negligible, it is small compared to the differences between the various filter types. The results not only differed from the information provided by the manufacturers, but in many cases also provided information otherwise unavailable, although affecting the sampling and also the ability of comparison with theory. To have enough information for the proper choice of the filter for a given purpose, it would be necessary to have not only total penetration given from the DOP standard measurement, but the MPPS, penetration maximum value and pressure drop as well.

Copyright 2015 American Association for Aerosol Research     

Penetration of Monodisperse,Singly Charged Nanoparticles through Polydisperse Fibrous Filters

The article presents experimental results and theoretical analysis of aerosol nanoparticle penetration through fibrous filters with a broad fiber diameter distribution. Four fibrous filters were produced using the melt-blown technique. The analysis of the filters’ SEM images indicated that they had log-normal fiber diameter distribution. Five kinds of proteins and two types of silica particles were generated by electrospraying and were then classified using a Parallel Differential Mobility Analyzer to obtain well-defined, monodisperse, singly charged challenge aerosols with diameters ranging from 6.3 to 27.2 nm. Particle penetration through the filters was determined using a water-based CPC. Experimental results were compared first with predictions derived from the classical theory of aerosol filtration. It is demonstrated that it is inappropriate to apply it to the arithmetic mean fiber diameter, as this results in turn in a huge underestimation of nanoparticle penetration. A better, but still unsatisfactory agreement is observed when that theory was used together with the pressure drop equivalent fiber diameter or when the Kirsch model of nonuniform fibrous media was applied. We show that the classical theory applied to any fixed fiber diameter predicts a stronger dependence of nanoparticle penetration on the Peclet number as compared to experimental data. All these observations were successfully explained by using our original partially segregated flow model that accounts for the filter fiber diameter distribution. It was found that the parameter of aerosol segregation intensity inside inhomogeneous filters increases with the increase in particle size, when the convective transport becomes more pronounced in comparison to the diffusive one.     

3-D simulation of particle filtration in electrospun nanofibrous filters

Virtual 3-D geometries resembling the internal microstructure of electrospun fibrous materials are generated in this work to simulate the pressure drop and collection efficiency of nanofibrous media when challenged with aerosol particles in the size range of 25 to 1000 nm. In particular, we solved the air flow field in the void space between the fibers in a series of 3-D fibrous geometries with a fiber diameter in the range of 100 to 1000 nm and a Solid Volume Fraction (SVF) in the range of 2.5 to 7.5%, using the Fluent CFD code, and simulated the flow of large and fine particles through these media using Lagrangian and Eulerian methods, respectively. Particle collection due to interception and Brownian diffusion, as well as the slip effect at the surface of nanofibers, has been incorporated in the CFD calculations by developing customized C++ subroutines that run in the Fluent environment. Particle collection efficiency and pressure drop of the above fibrous media are calculated and compared with analytical/empirical results from the literature. The numerical simulations presented here are believed to be the most complete and realistic filter modeling published to date. Our simulation technique, unlike previous studies based on oversimplified 2-D geometries, does not need any empirical correction factors, and can be used to directly simulate pressure drop and efficiency of any fibrous media.     

Experimental and modelled efficiencies during the filtration of a liquid aerosol with a fibrous medium

The aim of this study was to determine and model efficiency during the filtration of a liquid aerosol through a fibrous filter. A series of experiments demonstrated that liquid particle filtration is different from solid particle filtration in that a drainage state appears, characterized by a constant pressure drop at the end of filter clogging. Moreover, during filter clogging, the number efficiency presents a minimum level for particles close to 100 nm in diameter (the most penetrating particle size). The results also reveal that during filter clogging there is a decrease in the medium's performance for particles smaller than 100 nm and an increase in efficiency for particles with a diameter >200 nm. Both effects are induced by the amount of liquid collected in the medium. Finally, a model is proposed to describe filter efficiency during clogging with a liquid aerosol.     

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.     

Filtration Efficiency and Pressure Drop of Miniature Diesel Particulate Filters

A method was developed to evaluate miniature diesel particulate filters (DPFs). To validate the performance of the instrumentation and test apparatus, measurements were made using silicon carbide (SiC) and cordierite miniature filters with representative microstructures. Filtration efficiency (FE), the most penetrating particle size (MPPS), and pressure drop were measured for catalyzed and uncatalyzed advanced ceramic material (ACM) acicular mullite and representative commercial filters to determine the impact of substrate morphology, the formation of a soot cake, and the presence of a catalyst coating on filtration properties. FE measurements demonstrated that filter geometry and microstructure significantly influence initial filtration performance. ACM filters had high initial FE and the MPPS near ～200 nm. Reduction of the ACM pore size in the absence of a reduction in porosity increased initial FE even more, but its influence on MPPS was not resolvable. The presence of a catalyst and washcoat on the ACM increased the pressure drop but increased initial FE and reduced MPPS to <100 nm. The addition of a washcoat allowed the rapid buildup of a soot cake, which resulted in a more rapid rate of increase in FE compared to uncatalyzed ACM. The similarity in the ACM and cordierite soot cakes after a long loading time is consistent with theory that suggests the formation of the soot cake depends primarily on the Péclet (Pe) number, which is influenced only by macroscopic filter geometry and prevailing test conditions.

Copyright 2013 American Association for Aerosol Research     

Filtration efficiency analysis of fibrous filters: Experimental and theoretical study on the sampling of agglomerate particles emitted from a GDI engine

Fibrous filters are commonly used for aerosol purification and sampling. The filtration efficiency has been extensively studied using standard aerosol generators, yet the literature on experimental data and theoretical study concerning the filtration of agglomerates from real engines remains scarce. A filtration efficiency test system was developed to determine the filtration efficiency of two types of filters (uncoated and fluorocarbon coated) loaded by particulate matter (PM) emissions from a gasoline direct injection (GDI) engine. The experimental results showed that the filtration efficiency in terms of PM mass and number increased over time for both types of filters. The fractional efficiency (penetration efficiency) curves for the test fibrous filters rendered a U-shaped curve for particle sizes from 70 to 500 nm, and the most penetrating particulate size (MPPS) decreased over time. A small fraction of accumulation mode particles with the size between 70 nm to 500 nm penetrated the filters while almost all nucleation mode particles with the size below 50 nm were captured by the filters. The filtration efficiency derived from an empirical model based on classical single-fiber theory for laden filters generally agreed with the experimental data for the first 500 s, but suffered a significant deviation by approximately one order of magnitude at 948 s. A better estimate of the filtration efficiency trend with the maximum deviation of about 20% (except for large particles at the high end of the measurement spectra) was obtained by using a revised model which incorporates the effects of the increase in filter solidity, local velocity, dynamic shape factor and effective total length of fibers during the filtration process.

© 2017 American Association for Aerosol Research     

Intermediate Air Filters for General Ventilation Applications: An Experimental Evaluation of Various Filtration Efficiency Expressions

Neither the European standard nor the US standard for classification of intermediate class filters comprises testing of filter performance with respect to ultrafine particles (UFPs) or particles of the most penetrating size (MPPS). This could turn out to be a major lack in classification standards since UFPs have been pointed out as a serious health hazard. In this study, fractional efficiencies of eight new full-scale bag filters and twenty-three new filter medium samples were determined. The influence of air velocity and aerosol type was investigated, and correlations between efficiencies for UFPs (EF_UFPs), MPPS-sized particles (EF_MPPS) and 0.4 μm-sized particles (EF_0.4μm) were established. The tested bag filters were challenged by four aerosol types: a neutralized atomized oil aerosol, the same oil aerosol but non-neutralized, a non-neutralized thermally generated oil smoke, and a “natural” indoor aerosol. The tests were carried out at different air velocities through the filter medium, ranging between 0.08 m/s and 0.22 m/s. The relationships that were observed between EF_UFPs, EF_MPPS, and EF_0.4μm appeared to be linear within the observed filtration efficiency ranges. These relationships were similar regardless of the test aerosol type used, but somewhat different for glass fiber filters than for charged synthetic filters. Generally, EF_MPPS was 10–20% lower than EF_0.4μm. The influence of air velocity variations on the size resolved efficiency was determined. The glass fiber filters showed practically the same fractional efficiencies regardless of whether the test aerosol was neutralized or not. However, the charged synthetic filters showed substantially lower efficiencies when tested with the non-neutralized aerosol compared to the case when the aerosol was neutralized.

Copyright 2013 American Association for Aerosol Research     

Experimental Study on the Loading Characteristics of Needlefelt Filters with Micrometer-Sized Monodisperse Aerosols

In this work, three types of needlefelt filters, made of Polyester (PE), Ryton Sulfar (RS), and Polyaramid (PA), were tested to in- vestigate the aerosol loading characteristics of fabric filters when challenged with micrometer-sized monodisperse potassium sodium tartrate (PST) particles. A fibrous filter with packing density of 9%, thickness of 0.38 mm, and fiber diameter of 5.1 θ m was included for comparison. A vibrating orifice monodisperse aerosol generator was used to produce three different sizes (5, 10, and 20 θ m) of PST particles for aerosol loading experiment. An ultrasonic atomizing nozzle and a TSI constant output nebulizer were used to generate polydisperse PST particles for the aerosol penetration test. The aerosol penetration of submicrometer-sized particles through the filters was measured by using a Scanning Mobility Particle Sizer. An Aerodynamic Particle Sizer was used to measure the penetration fraction of aerosol particles larger than 0.8 θ m. The pressure drop across the filter was monitored by using pressure transducers, which were calibrated against an inclined manometer. Airflows of 5, 10 , 20, and 30 cm/s were used to study the flow dependency. The aerosol penetration results showed that the particles larger than 3 θ m did not penetrate the clean fabric filters tested in the present study. The loading curves (plots of pressure drop against sampling time) displayed three regions: an initial region of fast increase, a transition region, and a final linear region after the dust formation point. After the formation point of the dust cake, both fabric and fibrous filters shared the same slope (of the loading curves). The slope of different regions of the loading curves was determined by many factors, such as size of challenge aerosol, face found to be critical to the performance of the fabric filters. In order lower porosity, which caused an extra rise in pressure drop across velocity, surface treatment, and the compressibility of the dust cake forming on the filter. The method of final surface treatment was to avoid the unnecessary rise in air resistance, the melting clumps formed during final surface treatment should be as thin and narrow as possible, just enough to support the filter bag cleaning. From the standpoint of filter quality and energy consumption, the low filtration velocity has to be adopted whenever possible, because high filtration velocity not only led to lower filter quality (in particular for submicrometer-sized particles) but also created dust cake of lower porosity, which caused an extra rise in pressure drop across thet dust cake.     

Comparison of the relative performance efficiencies of melt-blown and glass fiber filter media for managing fine particles

The purpose of this study was to compare the performance efficiency of melt-blown and currently used glass fiber filter media under the same environmental conditions. To evaluate filter efficiency, filter class was determined according to ISO and European standards (EN 1822-1:2009) using an automated filter tester (0.3 μm size), taking into account particle filtration, fractional efficiency for negative pressure devices, and consumption of electrical power. The average fractional efficiency, quality factor (QF), fractional efficiency by dust loading amount, pressure by dust loading amount, and QF by dust loading amount were higher in the case of melt-blown media than in the case of glass fiber filters. The fractional efficiency of hydrocharged melt-blown filters was higher than that of uncharged media. Based on performance efficiency, melt-blown filters are more effective high efficiency particulate air filters than glass fiber media.

Copyright © 2018 American Association for Aerosol Research     

Experimental Study on the Effect of Fiber Orientation on Filter Quality

Most filtration models assume that the air stream runs perpendicularly to the orientation of the filter fibers. However, cigarette filters remove aerosol particles apparently by a different filter configuration in that the fiber orientation almost parallels the air streamlines. To focus on the effect of fiber orientation, cellulose acetate filters were used in this work to facilitate the filter performance comparison. A piece of original round cigarette filter was molded to form a cube. The same piece of filter was used for both perpendicular and parallel orientations, to avoid the variability caused by the non-uniform filter media distribution. DOP aerosol particles used in the tests were generated by either a constant output aerosol nebulizer or an ultrasonic atomizing nozzle. A Po-210 radiation source was used to neutralize the challenge aerosols to the Boltzmann charge equilibrium. A scanning mobility particle sizer (for < 0.8 μm) and an aerodynamic particle sizer (for > 0.8 μm) were used to measure aerosol number concentrations and size distributions upstream and downstream of the cigarette filters. The results showed that parallel and orthogonal filters behave similarly. However, the pressure drop across parallel filter was lower than for the perpendicular filter, indicating that the airflow is more laminar passing through the parallel filters. Possibly for the same reason, aerosol penetration through parallel filter was higher than the orthogonal filter, although the difference may not be statistically significant. When a comparison of the fiber orientation is based on filter quality, orthogonal filter performs better than parallel filter, if face velocity is lower than 60 cm/s. Parallel filter performs better only when the particles are smaller than the most penetrating size and under high face velocity.     

Filtration of liquid aerosols with a horizontal fibrous filter

The aim of this study was to determine the filtration efficiency and pressure drop in liquid aerosol capture mechanisms using a fibrous coalescent filter. The experimental set-up was designed to measure filtration efficiency and pressure drop through the filter. To avoid the gravimetric effect, the filter was placed horizontally. A series of experiments highlighted the importance of operating conditions such as filtration velocity and aerosol concentration. The results demonstrated the advantage of working at high velocities to minimise the resistance of the medium and to enhance the mass efficiency of the coalescent filter. In parallel, no effect of aerosol concentrations in the pressure drop increase was noted.     

Characterization of An Open-Pored Nickel Foam with Respect to Aerosol Filtration Efficiency by Means of Measurement and Simulation

The deposition of micron and submicron particles in metallic, ceramic, or synthetic open-pored foams is a special field of aerosol filtration in porous media. This is due to the more complicated pore structure than, for example, fibrous filter media. Therefore, the measurement as well as the simulation of aerosol filtration in open-pored foams involves certain custom-built techniques.

The filter efficiency for micron and submicron particles can be measured by differential electrical mobility analyser systems (DEMAS) or optical particle counters (OPC). Empirical formulas are available in literature for open-pored polyurethane foams to determine their aerosol filtration efficiency and pressure drop. An additional method for characterization is direct numerical simulation (DNS) by means of a three-dimensional (3D) model of the pore structure. These models can be obtained either by tomography or by mathematical generation.

In this work, the filter efficiency of an open-pored nickel foam with a cell diameter of 450 μm is determined by the methods previously mentioned. The experimental results are in good agreement with the results of the 3D simulation and a semi-empirical approach for polyurethane foams is adapted for a nickel foam.

Copyright 2015 American Association for Aerosol Research     

Enhancing the performance of fibrous filters by means of acoustic waves

The influence of low-frequency acoustic waves on the operating time of fibrous filters was investigated experimentally. It was found that the application of acoustic waves can dramatically extend the operating time of fibrous filters. The experiments indicate that at frequencies of 50–1000 Hz and sound pressure levels of 110–130 dB the operating time of fibrous filters increases 2–10 times, as compared to filtration without acoustics. The effect is more pronounced at lower frequencies and higher sound intensities. A simple theoretical model shows that, because of secondary acoustic streaming, the aerosol particles moving toward the filter surface are deflected toward the deposited particles, resulting in increased voids and a more porous structure of the filter cake, which in turn leads to a lower pressure drop across the filter. This conjecture is supported by pictures of the filter cake surface.     

基于微观结构的褶式滤芯拟态化模型及其过滤性能的数值模拟

基于随机多层纤维过滤介质算法建立褶式滤芯三维拟态化结构模型,对褶式滤芯内部气-固两相流动进行数值模拟,计算不同运行参数及结构参数下滤芯的压力损失及过滤效率,并与文献计算值进行比较. 结果表明,压力损失随过滤风速增大呈线性增加;随褶尖角增大,压力损失呈先减小后增加,压力损失计算值与文献计算值吻合较好. 褶尖角和过滤风速一定时,过滤效率随粒径增加先减小后增大,在给出的颗粒直径范围内存在最易穿透颗粒直径(MPPS). 不同过滤风速下,当颗粒粒径小于0.5 mm时,扩散作用使过滤效率随过滤风速增加而减小;大于0.5 mm时,惯性作用使其随过滤风速增加而增加;MPPS随风速增加而减小;本计算值与文献计算值趋势一致. 不同褶尖角下,当颗粒粒径小于1 mm时,扩散作用使过滤效率随褶尖角增大而减小;大于1 mm时,惯性作用使其随褶尖角增大而增加.     

Effect of the Packing Density of the Fibrous Bed on the Flow Resistance and Dimensions of an Aerosol Filter

The theory of aerosol filtration is used to demonstrate the feasibility of reducing the dimensions and flow resistance of high-performance fibrous aerosol filters, and the lower limits of these values are determined. Increasing the filter packing density by a factor of 3 to 4 and reducing the scale of spatial configuring of the bed by a factor of 5 to 6 will reduce the filter volume by a factor of 2 to 3 and the filter resistance by more than one order of magnitude, without affecting the capturing performance of the filter with respect to the most easily penetrating particles.     

Retracted: Effects of the Operating Conditions and Geometry Parameter on the Filtration Performance of the Fibrous Filter

The gas‐solid two‐phase flows in fibrous filters were simulated by computational fluid dynamics (CFD) technology. The pressure drops and filter efficiencies with different operating conditions and geometry parameter, including face velocity, particle size, and solid volume fraction (SVF) were calculated. The effects of the operating conditions and geometry parameter on the filter performance of the fibrous filter were obtained. The results indicate that the pressure drop increases linearly with the face velocity and the predicted values of the pressure drops are in excellent agreement with the experimental correlation. Filtration efficiency decreases with the face velocity for submicrometer particles (0.1 μm) and, for larger particles (1 μm) the tendency is just the opposite. The filtration mechanism is different for different particle sizes. For the filter in this paper, when the particle size is smaller than 0.2 μm, Brownian diffusion plays a significant role in the filtration process. When the particle size is greater than 0.5 μm, inertial impaction becomes an important capture mechanism. For particle sizes in the range of 0.2–0.5 μm, the Brownian diffusion and inertial impaction are both relatively weak and, therefore, the filtration efficiency has the least value in this range. Additionally, the SVF distribution is an important geometry parameter in the filter. The filtration efficiency of the filter with a decreased SVF (geometry B) along the thickness of the filter is higher than that of the filter with the even SVF (geometry A), while maintaining a low pressure drop.     

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.     

Particle Bounce During Filtration of Particles on Wet and Dry Filters

This paper experimentally examines the bounce and immediate re-entrainment of liquid and solid monodisperse aerosols under a stable filtration regime (precake formation) by wet and dry fibrous filters. PSL and DEHS were the solid and liquid aerosols, respectively, used in four monodisperse sizes of 0.52, 0.83, 1.50, and 3.00 w m. Three different fibrous filters were used to filter the aerosol streams, and the efficiency of the filtration process for each aerosol type under dry and wet regimes was measured. It was found that the solid particles generally exhibited a lower fractional filtration efficiency than liquid particles, although this difference decreased in the smaller size fractions. The difference between solid and liquid efficiencies was found to be greatest in the 1.5 w m size range. As particle sizes of liquid/solid aerosols and filtration parameters were similar, this difference is most likely to be due to the effect of particle bounce and or immediate re-entrainment occurring inside the filter, with the greater efficiency of filtration of the liquid particles being due to their greater capacity to plastically/elastically deform in order to absorb the impact forces. However, for the wet filtration regime (each fibre of the filter was coated by a film of water), no significant difference in filtration efficiency was detectable between solid and liquid aerosols. Therefore, the conclusion can be drawn that the either the bounce effect of the particles is inhibited by the liquid film, or the filtration conditions in the wet filter are so different that the aerosol properties are less significant with respect to capture.