Optimization of Pleated Filter Designs Using a Finite-Element Numerical Model

A numerical model has been developed to optimize the design of pleated filter panels. In this model, the fluid flow is modeled by a steady laminar flow and the filter media resistance is governed by the Darcy-Lapwood-Brinkman equation. A finite element method with a nine-node Lagrangian element is used to solve the governing equations. For the rectangularly pleated filter panel, the numerical results agree well with the analytical model of Yu and Goulding (1992) and with his experimental data. The pressure drop increases at small pleat count due to increased media face velocity, and at large pleat count due to increased viscous drag in the pleat spacings. Therefore, an optimal pleat count for minimum pressure drop exists at a certain pleat height for each filter media type. The optimization of rectangular pleated filters, e.g., mini-pleated filter panels, has been performed for six commercial filter media. The optimal pleat count is shown to increase with decreasing media permeability of the filter media. A generalized correlation curve has been found for the six filter media by using a nondimensional parameter analysis. The results can be used to design pleated filter panels with minimum pressure drop.     

Air flows and pressure drop modelling for different pleated industrial filters

A dimensionless model was developed to determine the pressure drop across clean pleated filters, according to filter medium type, geometric characteristics of the pleating (distance between two pleats, pleat height, etc) and air flow parameters (filtration velocity, air density, etc). The model was derived from both experimental and numerical results obtained from nuclear and automotive filters — high efficiency particulate air (HEPA) and low efficiency particulate (LE), respectively. The major findings were that a more homogeneous air flow distribution occured over the surface of the pleated HEPA filter, while geometric characteristics had a greater influence on the initial pressure drop across the LE filter. The numerical model highlighted the fundamental importance of the filter medium's air flow resistance on air flow distribution.     

Experimental study of pleated fabric cartridges in a pulse-jet cleaned dust collector

Six pleated filter cartridges with different base media and geometrical dimensions were tested in a full-size dust collector periodically cleaned by a short burst of pulse-jet. The evaluation was performed under two different cleaning modes called clean-on-demand (to clean the filter once the pressure drop reaches a preset value) and clean-on-time (to clean the filter at a fixed time interval). The filter performance was evaluated by the effective residual pressure drop and downstream particle concentration. The results showed that the pleat ratio, defined as the ratio of pleat height to pleat pitch, had a great influence on the preferred operating mode for cleaning the filter. Clean-on-time mode demonstrated better performance for filters with a high pleat ratio (> 4.0), while clean-on-demand mode performed better for filters with a low pleat ratio (< 4.0). The test results also showed the tank pressure was critical for cleaning the pleated filter cartridges, whereas the pulse duration only had a small effect on the cleaning efficiency. With the same base media, cartridges with surface treatment such as fine fiber were superior to those without surface treatment. Pulse-jet cleaning could also promote particle penetration through filter media. The downstream particle concentration during cleaning was at least twice of that during filtration process for all pleated filter cartridges tested. Further, the downstream particle concentration was independent of the applied cleaning mode and the cleaning intensity in this study.     

Modeling instantaneous pressure drop of pleated thin filter media during dust loading

In this paper, we present a modeling methodology for studying the effects of dust loading on the pressure drop across pleated filters. Our simulations demonstrate that there exists an optimum pleat count for clean filters at which pressure drop reaches a minimum regardless of the in-plane or through-plane orientation of the fibers. With the particle deposition included in the analysis, our results indicated that the rate of increase in pressure drop decreases with increase in the pleat count. We demonstrated that a higher pleat count results in a higher flow velocity inside the pleat channels causing more non-uniformity in the dust deposition across the pleat. Especially when particles are sufficiently large, the dust cake tends to form deeper inside the pleated channel when the pleat count is high. This effect is observed to be less pronounced when the pleats have a triangular shape. We also showed that if the dust cake permeability is higher than that of the filters fibrous media, the rate of increase in pressure drop does not always decrease with increase in the pleat count. Finally, by comparing filters having 15 pleats per inch, we observed that rectangular pleats are preferred over the triangular pleats when the particles are highly inertial, i.e., filtering high-speed large particles. When particle's inertia is small, our results indicate that triangular pleats cause less pressure drop, and so are recommended.     

A semi-analytical model for gas flow in pleated filters

A semi-analytical model of gas flow in pleated fibrous filters is developed for large filtration velocities. This case presents two main new and distinguishing features compared to the low filtration velocity situations studied in previous works: the velocity profiles are not parabolic within the pleat channels and the filtration velocity is not uniform along the pleated filter element and this has a great impact on the filter loading. The model relies on similarity solutions to the Navier–Stokes equations in the channels formed by pleating the filter medium. After validation by comparison with direct CFD simulations and experimental data, the model is used to determine the optimal pleat density, i.e. the pleat density minimizing the overall pressure drop across the filter for given flow rate, pleat length and given filter medium properties. As illustrated in the paper, this model greatly facilitates the study of flow within the pleated filter compared to a standard CFD approach. It represents an excellent basis for the more involved problem of filter loading computation. In particular, no remeshing across the width of the pleated filter entrance channels is needed when a filtration cake forms at the channel walls.     

NUMERICAL ANALYSIS OF MEDIUM COMPRESSION AND LOSSES IN FILTRATION AREA IN PLEATED MEMBRANE CARTRIDGE FILTERS

A computer model has been developed to simulate the fluid flow in pleated filter cartridges. This model has been used to evaluate the performance and design of pleated cartridge membrane filters. The effects of medium compression, pleat deformation, and pleat crowding are analyzed. At higher flow rates due to the exerted fluid pressure the medium is deformed, which leads to a reduction in the material permeability. Further, due to pleating and bending there is a loss in effective filtration area. The combined effects of compression and reduction in filtration area cause deviations from Darcy's law. To interpret such deviations, permeability models based on the data obtained from the flat sheets of the filter material used in cartridge fabrication have been developed. The incorporation of the permeability model within the main hydrodynamic model determines the percentage loss in filtration area, percentage medium compression, and the pressure drop across the filters. Results of this study have been presented for fiberglass medium. The simulated results have been compared against experimental industrial data for purposes of model validation. The developed simulation tool offers a robust, cost-effective, and user-friendly design and analysis tool for pleated cartridge membrane filters, which can be easily used by engineers in industry.     

Development of a semi-analytical model to predict the pressure drop of clean pleated high-efficiency particulate air filters

The present article outlines the development of a semi-analytical model devoted to predict the pressure drop induced by clean pleated high-efficiency particulate air (HEPA) filters. Both experimental measurements and numerical simulations are used to characterize the velocity field in the pleat channel. On this basis, a semi-analytical model is derived to determine the gas flow within the pleat channel. This analytical formulation is used to predict the air pressure evolution according to filtration velocity in the pleat. This model is then validated on the basis of comparisons with measurements found in the scientific literature for different kinds of HEPA filters with different pleat geometries. This model is easy to use, fast to run compared to standard computational fluid dynamics (CFD) approaches, and is in good agreement with the experimental results.     

Clogging modeling in pleated filters for gas filtration

A model of clogging of a pleated filter in gas filtration is presented. The model is obtained by combining a semi-analytical model of the flow in a pleated filter and an empirical model of clogging of the planar filter medium applied locally along the pleated filter channels. The model takes into account the formation of a filtration cake of variable thickness at the porous wall of the pleated filter entrance channels and the resulting evolution of the entrance channels aperture distribution during the filtration/clogging process. Based on the numerical predictions, two main clogging scenarios are identified and analyzed. The optimum pleat density, defined as the pleat density maximizing the filter capacity, is determined and shown to be greater than the pleat density minimizing the pressure drop for a clean filter. Predictions of the evolution of overall pressure drop across the pleated filter due to clogging are compared with experimental data.     

烧结金属过滤式除尘技术研究

以烧结金属微孔材料为研究对象,分析了过滤层介质不均匀的影响。不均匀的滤层设计能减小压降,提高反吹清洗的效率;使用Fluent6.1的多孔介质单元模型模拟过滤元件内的二维流动,探讨过滤层为V形褶皱几何结构中一些参数变化对过滤的影响,最后对过滤的设计优化进行了探讨,研究结果对烧结金属过滤器的理论和设计有参考价值。     

Effizienz von Luftfiltern bei hohen relativen Feuchten und Beaufschlagung mit Wassertröpfchen

Filter media of E12 filters were exposed to NaCl particles and high relative humidity. Afterwards the particle separation efficiency of DEHS (DEHS: di(2‐ethylhexyl) sebacate) and pressure drop were measured according to DIN EN 1822 part 3. The results of new filter media and those at different loadings were compared to determine the influence of relative humidity or water‐soluble impurities like salt. In addition, scanning electron microscope pictures were taken to achieve a better understanding of the particle behavior inside the filters. The results are compared to filter media aged in real applications.     

Development and evaluation of multilayer air filter media

Three types of multilayer air filter media were developed and evaluated. Two other existing filters were also used for comparison of filter performance. The pressure drop, the collection efficiency, and the dust-holding capacity of the tested filters were measured, and the internal structure of the filter media was analyzed by using a scanning electron microscope. The multilayer filters tested in this study are composed of pre-surface layer, surface layer, and substrate layer. Among those layers, the surface layer is mainly responsible for particle collection. As a test result, it was found that the thickness of a surface layer has the greatest effect on filtration performance of a multilayer air filter. Additionally, filtration velocity and electrostatic forces should be considered together as important parameters for multilayer air filter design.     

空气滤清器过滤阻力多元关联式的研究

为减低空气滤清器过滤阻力,对空气滤清器流场特性进行的实验测试和模拟计算,旨在于为空气滤清器的优化设计提供相关的理论指导和依据。采用CFD模拟技术,结合实验测试的方法,建立了空气滤清器的CFD模拟的两维和三维模拟模型,计算机模拟计算结果与实验测试结果吻合,计算得出空气滤清器的过滤阻力与其各自影响因素的变化规律,通过数值模拟计算结果的回归分析,给出了空气滤清器总成过滤阻力与其显著影响因素的单因素关联式。利用大量的过滤压降与不同滤清器结构参数、过滤速度和过滤介质特性参数的模拟数据,回归得出总成过滤阻力与入口速度、滤芯褶数、进气管长、进气管径、出气管长、出气管径和滤纸厚度的无量纲的多元关联式。     

基于CFD-DEM方法的净化器流场模拟与结构优化

针对空气净化器能耗高的问题,使用离散元方法(DEM)在吸附滤网中建立随机堆积柱形活性炭模型,采用计算流体力学(CFD)方法对空气净化器内部流场进行数值模拟,在模拟与实验验证的基础上,考察了压降最小、流场最均匀的吸附滤网结构。结果表明,空气净化器压降主要发生在轴向,活性炭吸附滤网中回流、沟流现象严重,流体阻力是其他两种滤网的3倍。边数对多边形填充孔结构吸附滤网内压降与流场均匀性无影响,当孔结构改为圆形时,压降减小约52 Pa,节能18.4%(49 W);当孔直径由8 mm增至12 mm,压降减小约48 Pa,节能19.4%(45 W);滤网间距对空气净化器压降无影响,圆形、小孔径的吸附滤网内流场最均匀。     

Saturation models of oleophilic coalescing filters

Fibrous coalescing filters are widely used in a series of processes such as compressed air purification, engine crankcase ventilation, processing and cutting, and are used to remove liquid aerosol particles in the airflow. Pressure drop and ef?ciency of coalescence filters are greatly affected by saturation. It is of importance to establish the relationship among saturation, filter media parameters and operating conditions, which is helpful to optimize the filter design. Coalescence ?lters composed of thin glass fibrous media with micron fiber diameters are widely used in industry, while the saturation of which cannot be accurately predicted by the existing saturation models. This work investigated the relationship between pressure drop and saturation of multi-layered filters with different oleophilic filter media. In this study, there was no sharp boundary between wetting and non-wetting regions within filter media, thus a filter was regarded as a whole capillary system. According to the Jump-and-Channel model and capillary theory, a saturation model was developed. Compared with a large number of published literature data, it is found that when the saturation value is greater than 0.2, the predicted value is in good agreement with the experimental results, and the relative deviation is ≤20%. With the decrease in saturation, the boundaries become more and more obvious between the wetting and non-wetting regions. In this case, there is no need to modify to the capillary radius in the developed model. However, the developed model was also limited by the need for the channel pressure drop measurement, which should be solved in further work.     

亲油型聚结滤芯饱和度预测模型研究

纤维聚结滤芯广泛用于压缩空气净化、发动机曲轴箱通风、加工和切割等一系列工艺过程中,用于除去气流中的液体气溶胶颗粒。由于聚结滤芯饱和度对于过滤效率及阻力具有重要影响,因此建立饱和度与滤材参数及操作条件之间的关系将有助于优化滤芯结构并提高过滤性能。目前实际工业用聚结滤芯通常由多层微米级玻璃纤维材料组成,然而现有计算模型无法用于此类滤芯的饱和度预测。因此,本文基于多种常用亲油型聚结滤芯压降及饱和度实验测试结果,根据“跳跃-通道”模型及毛细管理论建立了新的饱和度预测模型。通过与大量已发表文献数据对比发现,当饱和值大于0.2时,预测值与实验结果吻合度较好,相对偏差≤20%。随着饱和度的降低,滤芯润湿区域和非润湿区域之间界限逐渐明显,此时无需对毛细管半径进行修正。然而,新模型仍然要依靠压降测量值进行计算,这一问题需在后续工作中加以解决。     

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.     

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     

Measurement of the Nanoparticles Distribution in Flat and Pleated Filters During Clogging

It is currently admitted that for each filtration process using pleated filters, at least three steps can be distinguished: depth and surface filtration, which are common to flat filters, and surface reduction. This step is caused by inefficient filling of the pleat due to the filter geometry. For combustion aerosol, it has been proved that this third step strongly depends on the filtration velocity resulting in an increase of the resistance when air flow decreases. This observation leads one to think that Brownian diffusion, higher for low velocities, could influence the clogging dynamic of a pleated filter.

In this article, a protocol derived from the dust cake preparation method published by Schmidt is developed. The aim of this study is to measure the aerosol penetration inside a filter media as well as in a pleat using a scanning electronic microscope and energy dispersive X-ray spectroscopy elementary detection. This method has also been extended to the study of pleated filters to measure the particle distribution inside the pleat. Filters were loaded with nanoparticles in order to evaluate the specificity of the diffusional regime on the clogging of pleated HEPA filters. For pleated filters, two filtration velocities were investigated: 2.5 and 0.2 cm/s.

Copyright 2014 American Association for Aerosol Research     

Experimental study of cake formation on heat treated and membrane coated needle felts in a pilot scale pulse jet bag filter using optical in-situ cake height measurement

Pulse-jet bag filters are frequently employed for particle removal from off gases. Separated solids form a layer on the permeable filter media called filter cake. The cake is responsible for increasing pressure drop. Therefore, the cake has to be detached at a predefined upper pressure drop limit or at predefined time intervals. Thus the process is intrinsically semi-continuous. The cake formation and cake detachment are interdependent and may influence the performance of the filter. Therefore, understanding formation and detachment of filter cake is important. In this regard, the filter media is the key component in the system. Needle felts are the most commonly used media in bag filters. Cake formation studies with heat treated and membrane coated needle felts in pilot scale pulse jet bag filter were carried out. The data is processed according to the procedures that were published already [Powder Technology, Volume 173, Issue 2, 19 April 2007, Pages 93-106]. Pressure drop evolution, cake height distribution evolution, cake patches area distribution and their characterization using fractal analysis on different needle felts are presented here. It is observed that concavity of pressure drop curve for membrane coated needle felt is principally caused by presence of inhomogeneous cake area load whereas it is inherent for heat treated media. Presence of residual cake enhances the concavity of pressure drop at the start of filtration cycle. Patchy cleaning is observed only when jet pulse pressure is too low and unable to provide the necessary force to detach the cake. The border line is very sharp. Based on experiments with limestone dust and three types of needle felts, for the jet pulse pressure above 4 bar and filtration velocity below 50 mm/s, cake is detached completely except a thin residual layer (100-200 μm). Uniformity and smoothness of residual cake depends on the surface characteristics of the filter media. Cake height distribution of residual cake and newly formed cake during filtration prevails. The patch size analysis and fractal analysis reveal that residual cake grow in size (latterly) following regeneration initially on the base with edges smearing out, however, the cake heights are not leveled off. Fractal dimension of cake patches boundary falls in the range of 1-1.4 and depends on vertical position as well as time of filtration. Cake height measurements with Polyimide (PI) needle felts were hampered on account of its photosensitive nature.     

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.