NUMERICAL CALCULATION OF PARTICLE COLLECTION IN ELECTRICALLY ENHANCED FIBROUS FILTERS

The collection of particles In an electrostatically enhanced fibrous filter Is Investigated numerically. Individual particle pathlines are computed within a model filter. The model filter Is composed of an array of parallel cylinders arranged In a regular pattern. The effect of such forces as electrophoresis, dielectrophoresis, Brownian motion, and particle Inertia Is considered. It Is shown that the model can predict the Influence of these forces on the collection efficiency of a fibrous filter, and that the model Is able to match available data of enhanced collection to a reasonable degree of accuracy.     

Modelling of filtration processes of fibrous filter media

This work presents a stochastic approach, based on Monte Carlo method, to simulate liquid filtration processes through non-woven fibrous materials. The real filter material is represented as a multilayer medium with a network of multiply connected pores. To describe the deposition and resuspension of particles on and from the filter medium, the following four mechanisms were considered: particle capture by sieving, patricle capture by fibers; particle capture by blocked pores; and particle re-entrainment. The particle capture by fibers and blocked pores, and particle re-entrainment depend on the balance between the adhesion and removal forces. The adhesion forces for particles of diameter smaller than 20 μm were determined through the concept of London-Van Der Waals forces. For particles of diameter greater than 20 μm, gravitational forces were considered. Three-dimensional random flow was assumed to stimulate the particles motion through the multilayer medium. The pressure drop across the filter medium was calculated as the sum of the pressure drop across the clean filter plus the pressure drop due to the deposited particles.A FORTRAN Program was developed to implement the filtration process model. For a wide range of typical filtration conditions, the calculated filter efficiencies predicted the experimental results with a percent difference between 0.5 and 19.3 depending on the particle size. The filter material capacities were predicted with an average discrepancy of 23.0%     

THEORETICAL STUDY OF A NEW MATERIAL FOR FILTERS: AEROGELS

Aerogels are highly porous materials made by removing the liquid phase of a dilute suspension of solids by taking off the vapor under super-critical temperature and pressure conditions. The aerogel structure has a skeleton of nanometer-size elements forming pores that are tens of nanometers in dimension. In some cases the structure of the aerogel arises from clusters having fractal geometry. The collection efficiency and pressure drop for such material is predicted here from models in the literature. A quality factor is used to compare aerogel material with other filter materials. The quality factor relating gas filtration efficiency with pressure drop is predicted to be unusually high for these materials in comparison with other common fibrous filter media, predicting lower flow resistance for aerogels at equivalent particle collection efficiency levels.     

A STAGGERED ARRAY MODEL OF A FIBROUS FILTER WITH ELECTRICAL ENHANCEMENT

A staggered array of parallel circular cylinders is proposed as a model for a fibrous filter. The flow field within the array is numerically solved utilizing Stokes' approximation. Electrical enhancement of the filter is modelled using the method of images. Predictions of pressure drop and collection efficiency are compared to experimental data and it is shown that while the model overpredicts the pressure drop it can give reasonable estimates of collection efficiency.     

Numerical investigation of dilute aerosol particle transport and deposition in oscillating multi-cylinder obstructions

The transport and deposition of aerosol particles through a fibrous filter is encountered in many natural and industrial processes. As the filtration performance for a stationary filter has been extensively studied in the literature, the present work focuses on the effect of fiber oscillation in a filter where the fibers are allowed to vibrate periodically. The transport and deposition of dilute aerosol particles in such a system is simulated using an efficient numerical model, where an iterative immersed-boundary lattice Boltzmann method is applied to solve the background flow with finite-size moving fibers, and the motion of aerosol particles is then tracked by a one-way coupling Lagrangian approach. In the present scheme, the no-slip boundary condition at the fiber surface can be exactly enforced with an iterative approach and the numerical stability is improved by adopting the MRT collision model. After the model validation in the two special cases of flow over an oscillating fiber in a quiescent fluid and particle capture by a stationary fiber, the filtration performance of an oscillating multi-fiber filter is investigated to study the effects of fiber number, arrangement and vibration mode. It is found that the oscillating motion of fiber has significant influence on the filtration performance. For a single fiber, with larger oscillation amplitude, the distribution ranges of the release position and impact angle of captured particles both increase. On the other hand, a larger fiber oscillation frequency tends to reduce the width of release position but increase the width of impact angle of deposited particles. Furthermore, the collection efficiency is found to be linearly related to the oscillation amplitude or frequency. For multiple fibers, the collection efficiency always increases with larger fiber number, but it is a non-monotonic function of the arrangement parameters, i.e., the longitudinal and transverse spacings, and the vibration parameters such as the amplitude, frequency and vibration mode. It is interesting to find that the in-phase mode can usually lead to excellent collection efficiency.     

AN EVALUATION OF THE KUWABARA MODEL

The Kuwabara model was developed to provide an approximate method of calculating the forces experienced by randomly distributed spheres or parallel circular cylinders in Stokes' flow. Although not specifically designed for the purpose, Kuwabara's model has frequently been used to describe the flow surrounding the individual fibers of a fibrous filter. In this paper an attempt is made to evaluate this application by comparing the Kuwabara flow field to that obtained by a numerical solution of Stokes' flow over an array of cylinders.     

静电增强纤维动态过滤的理论研究

为了推导、预测滤料间有外加静电场时过滤器效率和压降的变化并且分析各参数如纤维厚度、颗粒直径、电场强度对效率和阻力的影响,提出了建立在D．Thorns建立的数学模型的基础上的静电增强纤维动态过滤效率和压降的计算方法,首先计算了单纤维的过滤效率和压降,然后计算纤维过滤器稳定状态的过滤效率和压降,从而计算了纤维过滤器动态过滤时的效率和压降,得出了重要结论：粒子直径越小,压力损失越大；电场强度越大,过滤效率越大,压力损失越小；增加纤维厚度能显著提高过滤器的过滤效率。     

Fabrication of a carbon/polytetrafluoroethylene nanoparticle-coated antistatic bag filter using air-assisted electrospraying

Fabric bag filters have been widely used as air pollution control equipment owing to their high collection efficiency. However, the low efficiency of the fabric filtration method against fine dust and the high operating cost owing to the high-pressure drop have restricted its effective application. In addition, the filling of the fabric of bag filters by the passage of gas and deposited charged dust particles results in the generation of a static electricity that can cause a fire. To solve this problem, this study aimed to develop an antistatic filter by electrospraying carbon nanoparticles. The surface properties and conductivity of the antistatic filter as a function of the mixing ratio were compared, and the particle filtration performance as a function of the coating type was confirmed by evaluating the filter filtration performance. The results revealed that among the samples that exhibited conductivity, the sample with a mixing ratio of 1:1 exhibited the highest resistance value and best hydrophobicity. In addition, the filter performance evaluation revealed that the coated filter exhibited enhanced filtration performance compared to the uncoated filter at all particle ranges.     

Buildup of particles on fibers in a high field-high gradient separator

In previous publications the performance of a high field-high gradient magnetic filter was calculated using a particle trajectory model to obtain the capture cross section of the fibers. The configuration of the particle buildup was assumed to be constant without considering the forces on the particle after impinging on the fiber. We now calculate the equilibrium configuration of particle buildup on the edge of a flat ribbon considering only the magnetic and viscous forces and including the effect of the fluid boundary layer. The configurations obtained as a function of field and flow are consistent with the few approximate direct observations reported in the literature. The performance of the filter, calculated using the particle trajectory model, now modified by this equilibrium buildup configuration, is compared to previously reported experimental results on separation of CuO from Al2O3slurries. A somewhat better fit is now obtained over the entire range of fluid velocity and fields, but an adjustable parameter affecting the viscous drag on the collected particles is required.     

Simulation of electrically enhanced fibrous filtration

Experimental investigation of the details of particle deposition in fibrous filters is extremely difficult due to the nature of the filter media. Typically, fibrous filters are composed of fibers that are only a few microns in diameter. The fibers are randomly distributed in a plane that is approximately at right angles to the flow direction, with a mean spacing of the order of 10–100 fiber diameters. Numerical simulation of the filtration process offers an attractive alternative to the experimental approach. In the paper such a simulation is described. The present study considers the influence of an electric field on dendrite formation. Three-dimensional graphical representations of simulations with and without an electric field are presented, and the differences in dendrite structure are discussed. A comparison with a two-dimensional model is also made.     

Hot-gas flow and particle transport and deposition in a candle filter vessel

Hot-gas flow and particle transport and deposition in an industrial filtration system are studied. The special example of the Siemens-Westinghouse filter vessel at the Power System Development Facility at Wilsonville, Alabama is treated in detail. This tangential flow filter vessel contains clusters of 91 candle filters, which are arranged in two tiers. The upper tier containing 36 candle filters is modeled by six equivalent filters. Seven equivalent filters are used in the computational model to represent the 55 candle filters in the lower tiers. The Reynolds stress turbulent model of FLUENT™ code is used, and the gas mean velocity and root mean square fluctuation velocities in the filter vessel are evaluated. The particle equation of motion used includes drag and gravitational forces. The mean particle deposition patterns are evaluated and the effect of particle size is studied. The computational results indicatethat large particlesof the order of 10 μm or larger are removed from the gas due to the centrifugal forces exerted by rotating flow between the shroud and the refractory.     

Analytical expressions for predicting performance of aerosol filtration media made up of trilobal fibers

Despite the widespread use of fibrous filtration media made up of trilobal fibers (referred to as trilobal media here), no mathematical formulations have yet been developed to predict their collection efficiency or pressure drop. In this study, we model the cross-section of a trilobal fiber with three overlapping ellipses separated from one another by a 120° transformation. We generate 2-D models representing the internal structure of trilobal filters having fibers with different dimensions and aspect ratios, and used them to predict pressure drop and collection efficiency of trilobal filter media. This information is then utilized to define an equivalent medium with circular fibers for each trilobal filter. Our results indicate that the circumscribed circle of a trilobal fiber can serve as an equivalent circular diameter, and therefore be used in the existing empirical/semi-empirical correlations that have previously been developed for predicting performance of filters with circular fibers. We have also proposed easy-to-use expressions that can be used with our equivalent circumscribed diameters for calculating the pressure drop of trilobal media.     

Modeling filtration performance of elliptical fibers with random distributions

The fibrous media with elliptical cross sections may improve the filtration performance, however, current researches mainly focus on the capture mechanisms of a single elliptical fiber, and the fibrous media with randomly distributed fibers are rarely involved. In this work, a 2D numerical model was developed to predict the pressure drop and particle penetration for the fibrous filter composed of randomly distributed elliptical fibers. The results show that a big solid volume fraction of filter increases the effective collision area, and enhances the capture at a low face velocity. The particle penetrations through the fibers with the diameter of 5 μm are conspicuously weaker than those with the diameter of 10 μm, especially at big solid volume fractions and high face velocities. The blunt elliptical fibers restrain the penetration more effectively than the circle ones when the solid volume fraction is high. Though the blunt fibers lead to a large drag force, the increased pressure drop cannot improve the filtration performance at low solid volume fractions. In most cases, the slim elliptical fibers can enhance the filtration performance. A bigger aspect ratio of elliptical fibers leads to a low quality factor, showing the capture efficiency increases with the penalty of a high pressure drop.     

A numerical model of fibrous filters containing deposit

This work is aimed at developing numerical methods to model the effects that particle deposit collected by fibrous filters has upon the flow field within the filter and hence upon further deposition. A numerical model of the flow field has been developed using the Boundary Element Method. The model contains a two-dimensional single fibre with the boundary conditions applied on the computational domain accounting for the neighbouring fibres. A layer of porous material is assumed to cover the fibre modelling deposited particulate. The width of the porous layer, which varies along the fibre surface, is defined by particle behaviour determined in earlier work, [Dunnett SJ, Clement CF, A numerical study of the effects of loading from diffusive deposition on the efficiency of fibrous filters. J Aerosol Sci 2006; 37: 1116–1139]. Once the flow field is known the motion of the particles in the flow is modelled. Small particles are considered where diffusion is the main mechanism by which they are captured by the fibres. Various situations are considered and the flow and particle behaviour investigated. In particular the effect of existing porous deposit on the fibres upon further deposition is studied.     

Cycle Optimization Involving the Use of Filter Aids

A method for optimizing the quantity of filter aid powders used In constant rate pumping operations In plate and frame, recessed plate, and leaf filters Is presented. In contrast to the usual assumption that there Is a fixed fraction of filter aid admix which yields the best results, we demonstrate that the optimum amount of filter aid varies with cake thickness, washing and dead times, and use of a membrane for expression or simply pressing out residual slurry In a filter press at the end of filtration. The specific problem treated in this paper relates to the adaptation of a constant rate pump to an existing filter equipped with cake squeezing membranes. The separation is limited by a specified pressure, and the maximum cake thickness Is fixed by the filter geometry.

The optimum fraction of filter aid rises as cake thickness Increases.     

Optimal design and performance evaluation of a metal fiber filter for capturing radioactive aerosols

ABSTRACT

Conventional high-efficiency particulate air (HEPA) filters made of glass fiber media are prone to recycling problem and restrictions in extreme environmental condition such as high flow rate, high temperature, and fire. Therefore, metal fiber filters with minimal maintenance can replace conventional HEPA filters. The objective of the study is to evaluate the theoretical and experimental characteristics of a SUS316L metal fiber filter made from the fiber diameter of 8 µm. Theoretical modeling for predicting the collection efficiency of the radioactive aerosol is performed on the metal fiber as a function of particle size, filter thickness, and flow rate. Comparison between the experimental and theoretical results demonstrates that they are in good agreement. Consequently, the model is later utilized for performance optimization of the metal fiber filter. Also the metal filter for collecting the radioactive aerosol is optimized at the particle collection efficiency of 99.97% in most penetrating particle size (MPPS) of region 0.3 µm which complies with the standards established for conventional glass fiber HEPA filters.     

ROOM-TYPE AIR CLEANER DESIGN CRITERIA AND PROTOTYPE PERFORMANCE

Two rules-of-thumb for minimum performance of a room-type air cleaner have been developed from consideration of a first order model for room air quality. By adopting a criterion that the use of an air cleaner should cause the particle concentration to be at least cut in half, the rule-of-thumb for a room with no smokers is that the product of filter efficiency and flow rate should be ≥.8 m³/min (≥30 cfm). If the particle concentration is dominated by smokers or other sources, the product of filter efficiency and filter flow rate should be = m³ /min (= 100 cfm)

Tests were conducted to determine the efficiencies of candidate filter media. The selected media, Filtrete G-0115, has a fractional efficiency for 1 μm particles of 97 percent when clean, and an efficiency of 78 percent when fully loaded. This drop in efficiency is due to the masking of the electrets on the surfaces of the filter fibers.

A fibrous filter room-type air cleaner was designed to perform in accordance with the rules-of-thumb. When operated with a clean filter, the maximum flow rate is 3.2 m³/min and, when operated with a fully loaded filter, the maximum flow rate is 1.8 m³/min. The system has a multispeed fan which will provide lower flow rates.     

Feasibility study of particle filters for mobile station receivers

The application of an equalisation method for mobile station terminals using particle filters is presented. To use particle filters for equalisation, a mathematical model is shown which allows the transmitted symbols to be represented as the state of a stochastic system, which can be estimated by particle filters. The authors propose an equaliser structure with particle filters for application in mobile station receivers, especially for GSM/EDGE (global system for mobile communications/enhanced data rate for GSM evolution). Several improvement strategies, which help to obtain better estimation results with a lower number of particles, are discussed. In addition, performance evaluations of the particle filter equaliser for GSM/EDGE are presented.     

ROOM-TYPE AIR CLEANER DESIGN CRITERIA AND PROTOTYPE PERFORMANCE

ABSTRACT

Two rules-of-thumb for minimum performance of a room-type air cleaner have been developed from consideration of a first order model for room air quality. By adopting a criterion that the use of an air cleaner should cause the particle concentration to be at least cut in half, the rule-of-thumb for a room with no smokers is that the product of filter efficiency and flow rate should be ≥.8 m³/min (≥30 cfm). If the particle concentration is dominated by smokers or other sources, the product of filter efficiency and filter flow rate should be = m³ /min (= 100 cfm)

Tests were conducted to determine the efficiencies of candidate filter media. The selected media, Filtrete G-0115, has a fractional efficiency for 1 μm particles of 97 percent when clean, and an efficiency of 78 percent when fully loaded. This drop in efficiency is due to the masking of the electrets on the surfaces of the filter fibers.

A fibrous filter room-type air cleaner was designed to perform in accordance with the rules-of-thumb. When operated with a clean filter, the maximum flow rate is 3.2 m³/min and, when operated with a fully loaded filter, the maximum flow rate is 1.8 m³/min. The system has a multispeed fan which will provide lower flow rates.     

Numerical Model for particle deposition and loading in electret filter with rectangular split-type fibers

A simulation model for electret filter made of split type fibers has been developed to study the filtration efficiency as well as the particle loading process. The filter was assumed to be composed of rectangular fibers arranged in staggered array in which the flow field, the electrostatic field and the collection mechanisms were determined by numerical simulation. Single fiber efficiencies under various filtration conditions were calculated and compared with results obtained from semi-empirical expressions derived from experimental results. Influences of particle charge, fiber charge and orientation of fiber on the collection efficiency were evaluated. Finally the particle loading process was studied using the present model. Dendrite growth of particles in equilibrium charge state was simulated. The mechanical efficiency compensation effect was studied by a series of simulations. It is found that the loading of 1.5 m or larger particles has a significant mechanical collection compensation to the loss in electrostatic efficiency; while for 0.4 m particles such compensation is slow and insignificant.