Assessment of surface and depth filters by filter quality

This paper reviews the concept of filter quality (q_F) for dust filtration media composed of different structures: metal fiber beds (MFB), fabric filters (BF), and fly ash filters (FAF). Filter quality is a useful index of the filtration performance, which incorporates both pressure drop and filtration efficiency. Major parameters affecting the filter quality are filtration velocity in the range of 0.06-0.19 m/s, dust loading, porosity of the medium in the range of 75-93%, and internal structure of the medium. The experimental observation showed that filter quality decreased with increasing filtration velocity or dust concentration. A unique increase in filter quality during the initial stage of filtration appeared with the FAF as a result of the predominately surface filtration with less pore clogging. Nevertheless, the filter quality cannot be taken as an absolute indicator of filter performance, but rather it should be used just as a reference parameter depending on operating conditions. The results of this work show that fly ash filters are capable of providing more stable performance, particularly during the initial stage of filtration, and thereafter of a certain time filter quality initiates to decline as other filters.     

Clogging of fibrous filters by liquid aerosol particles: Experimental and phenomenological modelling study

Fibrous filters are the most common means used to separate liquid aerosol particles from an industrial gas stream. The pressure drop and penetration (=1-efficiency) are the most important performance criteria of the filter. In this study, experimental and modelling results describing the pressure drop and penetration evolution of a glass microfibre HEPA filter are presented. For the experimental part, the pressure drop and penetration evolutions of a HEPA filter are described as well as the influence of the filtration velocity on those evolutions. For the modelling part, the physical collection mechanisms taken into account and their mathematical expressions which are the basis of the phenomenological model are described in a first step. After that the experimental values are compared to their modelled counterparts. Different efficiency models from the literature have been tested in order to determine the one closest to the experimental values. The influence of the filtration velocity on the model is studied in the last part. The model presented here is capable of describing the pressure drop and penetration evolution of a HEPA filter over the whole filtration period.     

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

Nanofibers 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(PM_1.0) 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 PM_1.0. 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.     

Experimental Study on Charge Decay of Electret Filter Due to Organic Solvent Exposure

The electret filter is a potential component to remove airborne particles due to its high collection efficiency and low pressure drop. However, its filtration performance is gradually decreased by exposure to organic solvents, which limits the application of electret filters. The effect of ethanol exposure on the filtration performance of polypropylene electret filters was investigated experimentally to clarify the charge decay phenomenon in this study. Experimental results revealed that filter performance is strongly dependent upon the challenged mass and existing state of an ethanol solvent. The filter performance was drastically degraded by exposure to ethanol droplets generated from a solution with ethanol concentrations above 30%; however, it was maintained during exposure to ethanol vapors. This tendency was also seen in the surface potentials of the exposed filter media. In addition, we found that the critical challenging amount of ethanol droplets was in the vicinity of 0.045 g/cm² to neutralize a tested electret filter in this study.

Copyright 2015 American Association for Aerosol Research     

并列和错列纤维过滤器稳态除尘过程的 格子Boltzmann模拟

大部分纤维捕集效率和压降的理论模型认为纤维性能仅取决于来流速度、颗粒粒径、纤维体积分数、过滤层厚度、纤维直径等因素。实际上,布袋除尘器的性能还与纤维配置方式直接相关。利用LB(lattice Boltzmann)两相流模型对多层纤维捕集颗粒物过程进行了数值模拟,研究了不同纤维配置方式下系统压降与捕集效率的变化。结果表明,错列纤维的性能参数优于并列纤维;纤维排列间距增大,压降增幅大于捕集效率,导致性能参数下降。通过比较不同位置纤维的捕集能力发现,在布朗扩散和拦截捕集机制主导下,前方纤维捕集能力略强于后方纤维;而在惯性碰撞捕集机制主导时,对捕集贡献最大的主要是前两排纤维,后方纤维对捕集效率的贡献非常小,可以忽略。这些研究结果可以对布袋除尘器的多层纤维配置方式的优化提供理论依据和工程建议。     

Figure of Merit of Composite Filters with Micrometer and Nanometer Fibers

We investigate the filtration performance of composite filters composed of micrometer and nanometer fibers. The filter quality is evaluated using the figure of merit, also known as the quality factor. We use analytical expressions for the pressure drop and filtration efficiency to compute the figure of merit. The effects on the figure of merit by fiber diameter, solidity, and thickness of nanometer and micrometer fibers and face velocity are investigated. Experimental data obtained using conventional filter media and nanofiber composite filters are then used to verify the calculated results. We find that for large particles (approximately 0.1 μm and above), nanofibers can improve the figure of merit compared to conventional filters. Smaller fiber size, larger solidity, and thickness of the nanofiber layer lead to better filtration performance in this size range. For small particles (approximately below 0.1 μm), nanofibers do not improve the figure of merit compared to conventional filter media. Larger fiber size, smaller solidity, and thickness of the nanofiber layer are preferred in this size range. We demonstrate that our procedure using analytical expression is a fast and effective tool for filter media design.     

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.     

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     

Filtration and loading characteristics of granular bed filters

The purpose of this study was to investigate the filtration and loading characteristics of granular bed filters. Stainless steel holders (diameter 71.6 mm, height 70 mm) were fabricated to accommodate 500 g of zirconium oxide (ZrO₂) beads, as the packed media of granular bed. Monodisperse ZrO₂ granules (0.3, 0.8, 2 and 4 mm in diameter) were used to demonstrate the effect of the granule size and packing geometry on both pressure drop and aerosol penetration. From the filter quality perspective, the selection of the ‘best” filter is complicated. Assuming a low face velocity (e.g., 0.58 cm/s), large granule size is more cost-effective because of the higher filter quality factor. The phenomenon implies that the gain in filtration efficiency due to larger surface area (of small granules in the filter) did not compensate for the increase in air resistance. After the cake formation point, the dust cake on glass fiber filter became compressed. This dust cake compaction caused the pressure to drop precipitously and intermittently. In contrast, the rate of increase in pressure drop of the dust cake formed on the granular bed filters decreased with time probably due to the pinhole channels in the increasing mass load. The size and density of the pinholes are determined by the granule size, the face velocity and the size of the challenge aerosols.     

Development of high efficiency particulate absorbing filter materials

We have developed new high efficiency particulate absorbing filter materials by bonding the fiber web with the help of high pressure water jets emerging from micron sized nozzles and subsequently coating the filters with a chemical binder. Two different types of nonwoven filters are produced by varying the water jet pressure during the bonding process. The performance characteristics of the filter materials are evaluated in terms of filtration parameters, such as filtration efficiency, dust holding capacity, and pressure drop. Filtration efficiency depends on the pore characteristics, namely pore size and their distribution in the filters. The developed filter materials have shown promising performance characteristics by capturing higher amount of dust particles with a relatively low pressure drop during use. These filter materials can be used for a wide range of industrial applications, where high filtration efficiency is required at low energy consumption. A fluid flow simulation is carried out by computational fluid dynamics (CFD) to understand flow pattern during the bonding process. The CFD is also used to predict the pressure drop in the nonwoven filter materials during filtration process. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

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.     

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     

孔径梯度分布对亲油型滤材气液过滤性能的影响

在天然气净化、大型旋转机械曲轴箱通风和压缩空气过滤等领域,气液聚结过滤器具有广泛的应用。利用滤材过滤性能实验装置,分析了气液过滤过程中不同孔径梯度分布的亲油型滤材的压降、穿透率和饱和度变化,比较了其过滤性能、内部液体分布特性以及对液滴二次夹带现象的影响。结果表明：在气液过滤过程“通道压降”阶段,孔径递增滤材压降和0.8 μm以上液滴穿透率的变化曲线具有明显的分层特征。不同孔径梯度分布滤材的稳态过滤性能存在明显差异,主要原因是滤材内部存在液体运移通道的传递现象。通过与孔径递减和孔径均匀分布滤材的稳态过滤性能对比,发现孔径递增滤材在保证较低压降的同时具有最高的品质因子,有利于减少液滴二次夹带现象的发生,且对0.8 μm以上不同粒径液滴均具有最高的过滤效率,即孔径递增滤材在气液聚结过滤器设计中更具优势。     

Aligned carbon nanotube sheet high efficiency particulate air filters

Aerosol filters, made with conventional micro-fiber fabrics, are designed to efficiently capture small particles from the air. Filters constructed of nano-fiber fabric structures provide even greater filtration efficiency than conventional micro-fiber fabrics due to their higher surface area and smaller pore size. Carbon nanotubes (CNTs) are very small diameter fibers that have the potential to be integrated into filters to further increase particle capture efficiency. In this study, CNT sheets, drawn from millimeter tall CNT arrays, were integrated between traditional micro-fiber fabrics to produce aerosol filters. The filtration performance of the novel filters showed that when the number of CNTs layers increased, the filtration efficiency increased dramatically, while the pressure drop also increased. In order to meet high efficiency particulate air (HEPA) filter requirements with a reasonable pressure drop, CNTs were laid in a cross-plied structure within the filter. The results demonstrated that the three layer cross-ply structure provided 99.98% filtration efficiency at 0.3 μm particle size at a 10 cm/s face velocity, making it a viable method for producing low basis weight HEPA filters utilizing CNTs as the main filtration component.     

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.     

驻极体过滤器过滤特性的数值研究

利用计算流体动力学(Computational Fluid Dynamics,CFD)对驻极体过滤器内的气-固两相流动特性进行数值研究,计算了过滤器内部电场的分布及带电颗粒经过驻极体过滤器时的压力损失和渗透率。结果表明:对于颗粒粒径dp≤0.1μm的颗粒,静电力在其捕集过程中占主导作用;随着粒径的增加,静电力的影响逐渐减弱。对于粒径范围在0.05～0.5μm的颗粒,驻极体过滤器的渗透率随迎面风速的增大而增大。另外,从驻极体过滤器内颗粒运动轨迹可以看出,在驻极体过滤器的背风面有带电颗粒的沉积,这种颗粒沉积可能是驻极体过滤器低渗透率,低阻力的主要原因之一。     

折叠滤芯气液过滤性能测试及优化

采用聚结型滤芯气液过滤性能实验装置,研究了油雾加载率和表观过滤速度对折叠滤芯过滤性能的影响及涂覆粘合剂对折叠滤芯过滤性能的优化作用。结果表明,涂覆粘合剂后,滤材抗张力强度明显增大,滤材孔径减小。随油雾加载率增大,滤芯过滤层液体运移通道数增加,通道压降升高。初始压降随表观过滤速度增加而升高。粘合剂主要凝固在渗透性低的区域,压降变化较小。表观过滤速度增加抑制了二次夹带,折叠滤芯过滤效率升高,而由于粘合剂脱落,涂覆粘合剂的滤芯过滤效率下降。表观过滤速度为0.10 m/s时,随油雾加载率增大,聚结在滤材表面的粘合剂抑制夹带,滤芯过滤效率升高。     

Preparation and Properties of PVDF/Fe3O4 Nanofibers with Magnetic and Electret Effects and their Application in Air Filtration

Electrospun electret filter material is widely studied because of its excellent removal effect on particulate matter from air streams. Here, a kind of high efficiency and low pressure drop nanofiber (NF) membrane with both electret effect and magnetic effect is developed, and compounded it with glass fiber mesh and polyester (polyethylene terephthalate) mesh to prepare a sandwich structure of NF anti‐haze window screen. Under the synergistic effect of magnetic particles, when the density is 2.06 g m^?2, the filtration efficiency of NF anti‐haze window screen can reach 99.95% for the fine particles below 0.3 µm, while the filtration pressure drop is only 58.5 Pa, with good light transmittance and excellent breaking strength (6.32 MPa). Therefore, polyvinylidene fluoride/Fe₃O₄ composite NF membrane has a potential application prospect in the field of air filtration.     

Characteristics of Nylon 6 nanofilter for removing ultra fine particles

Electrospinning is a fabrication process that uses an electric field to make polymer nanofibers. Nanofibers have a large specific surface area and a small pore size; these are good properties for filtration applications. In this paper, the filtration characteristics of a Nylon 6 nanofilter made by electrospun nanofibers are tested as a function of the fiber diameter. Nanofilter media with diameters in the range of 100–730 nm can be produced in optimized conditions. The pressure drop of a Nylon 6 nanofilter linearly increases with the increasing face velocity. An electrospun Nylon 6 filter (mean fiber diameter: 100 nm) shows a much lower pressure drop performance relative to the commercial HEPA filter media when the filtration efficiency of the Nylon 6 nanofilter and the HEPA filter are over 99.98% with test particles of 0.02–1.0 μm in diameter. The pressure drop at 5 cm/s of the face velocity is measured as 27 mmAq for the Nylon 6 nanofilter media, and 37.1 mmAq for the HEPA filter media. The particle size with minimum efficiency decreases with the decreasing fiber diameter. And the minimum efficiency becomes greater as the fiber diameter is decreased.     

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.