EXPERIMENTAL STUDY OF DUST FILTRATION IN SURFACE-TYPE NONWOVENS

ABSTRACT

Surface type nonwovens are widely used in industrial dust control. Recently, they have been utilized in some engine air filtration applications as automotive filters, heavy-duty engine self-cleaning filters or safety filters. Because of their mechanical strength and regenerative ability they are a perfect material for applications where filter replacement is a problem. On the other hand, the random distribution of fibers and needle punching may result in pinhole formation during dust loading, especially at high aerosol velocities. As a result, the seepage mechanism is common in applications involving fine solid aerosols.

In the inertia dominated region, the collection efficiency of particles depends on the adhesion probability. When particle momentum increases, the efficiency decreases. In general, there is no agreement between filtration theory and experiment when the Stokes number is greater than one.

Filter efficiency increases with dust loading when the filter medium is a good dust cake supporter. In this case, dust reentrainment, causing seepage, may occur at high aerosol velocities and pressure drops. In contrast, reentrainment in nonwovens can take place even at lower aerosol velocities and dust loadings. It is difficult to predict conditions favorable for dust reentrainment and pinhole formation. This process depends on media geometry, dust particle size distribution, and aerosol flow parameters.

This paper discusses filter performance of surface-type nonwovens exposed to polydisperse dusts. Filter efficiency and pressure drop are discussed as functions of aerosol velocity, dust loading, and dust particle size distribution.     

Influence of leaks in surface filters on particulate emissions

Compliance with severe limit values of dust emissions is a main characteristic of surface filters. This characteristic is due to the high particle collection efficiency of surface filters. Beside regular operation it is necessary to consider phenomena such as a "pinhole" bypass through leaks in surface filters to ensure the above mentioned compliance with the limit values at all times. Experimental research has been carried out to observe and understand the "pinhole" bypass through leaks and the behaviour of pinholes over filtration time. To work out the influence of different filtration conditions the parameters pinhole diameter, filter face velocity and dust cake thickness were varied. The results can be explained by formulas usually used to calculate volumetric flow rates of orifice gauges. The experiments and the calculations lead to the conclusions that bigger pinholes decrease the collection efficiency and higher filter face velocities increase the collection efficiency of pinholed filter media.     

EXPERIMENTAL STUDY OF NONWOVEN FILTER PERFORMANCE USING SECOND ORDER ORTHOGONAL DESIGN

This paper presents the results and conclusions from an experimental investigation concerning nonwoven precleaner performance in non-stationary filtration

Results were obtained using filters with chemically cured polyacrylonitrile fibers having diameters of 3.5-20.5 denier, packing densities ranging from 0.0188 to 0.0612, filter thicknesses between 5.5 and 22.5 mm, and aerosol velocities from 0.24 to 2.36 m/s. The experiment was planned using second order orthogonal design. Since the precleaners were developed for ventilation systems and engines of off-road vehicles, the test dust was modelled from the properties of contaminants found in road tests.     

DESIGN AND CALIBRATION OF A STACK SAMPLER FOR OIL FLY ASH

A particle sampling system was developed for the collection of Fly ash the stacks of ail-fired electrical generation plants. The apparatus has a cyclone separator which is designed to provide a cutpoint of approximately 2.5 µ followed by a pulse jet fabric filter. Flow rate is a nominal 226 L/min (8 cfm).

The system was tested in the laboratory with monodisperse aerosols to determine the fractionation characteristics of the cyclone and the inlet probe. Efficiencies of three candidate filters were examined with clean media, with a dust cake and with the dust cake removed. Tests were conducted using monodisperse polystyrene aerosols of 0.50, 0.76 and 1.09 µ

The pulse Jet apparatus was checked by operating the system at a flow rate of 200 L/min, loading the filter with AC fine test dust, and determining the stable filter pressure drop as a function of pulse jet air pressure.     

COMPARISONS OF MECHANICAL MELT-BLOWN FILTERS AND ELECTROSTATICALLY ENHANCED SPLIT-FIBER FILTERS FOR CABIN AIR FILTRATION APPLICATIONS

In this paper, compared are filtration characteristics of two different filters made from uncharged melt-blown media and electrostatically charged split-fiber media. The split-fiber filters have stronger electrostatic charges in the media than the melt-blown filter, which in turn enhances the initial fractional efficiency. The compared parameters are fractional efficiencies, dust holding capacities, incremental fractional efficiencies, and pressure drop. The advantages and disadvantages of these filters for cabin air filtration applications are discussed based on the experimental results obtained using SAE-fine dust and sodium chloride test aerosol. The optimization process of the filter selection is also discussed.     

PARTICULATE CONCENTRATION IN AUTOMOBILE PASSENGER COMPARTMENTS

An experimental study of the dust concentration in automobile passenger compartments was conducted to understand how driving conditions and a vehicle's ventilation system can affect cabin air quality. The results of this study are essential for designing appropriate cabin air filters and developing proper test procedures.

The particle size distribution of atmospheric contaminants entering the vehicle's passenger compartment through air registers and in the occupants' breathing zone was measured using an optical particle counter. The concentration and size distribution of aerosol in the cabin was found to be dependent on traffic conditions, road surface, environmental conditions (e.g., weather) and ventilation mode. In all cases over 90% of the particles were smaller than 1 µm. The vehicle's HVAC system (evaporator core) reduces the concentration of aerosol entering the car interior, especially larger particles. Commercial vehicle ventilation filters decrease particle concentrations in a driver's breathing zone for all particle sizes. However, the reduction in the submicron particle range is not sufficient to substantially improve air quality in the passenger cabin.     

BENEFITS OF SERIAL LAYER MICROFILTRATION

Many filtration applications are of necessity being driven toward finer and finer particulate removal and improved permeate quality. Finer particle removal requires filters with smaller pore sizes and absolute pore size guarantees, and the finer the pores in the membrane filter, the sooner it will clog slowing its throughput rate and reducing its useful life. Engineers will frequently design processes involving several separate filtration and clarification steps to allow the process to run larger batches or for extended periods of operation without, system shutdown for filter changes. Membrane manufacturers who produce the final filters used at the end of the filtration train have resorted to a familiar technique in many liquid filtration and purification applications to achieve the desired final fluid quality level at reasonable filtration rates and final filter life by designing and producing filter elements with two membrane layers.

It is the objective of this paper to illustrate simple techniques used by a microporous nylon membrane manufacturer to select and optimize the prefilter membranes to be used in a dual-layered membrane pleated cartridge filter element. The membranes are thin polymeric sheets of nylon supported internally with a nonwoven polyester cloth that is coated with the nylon polymer in such a way that fluid passing through the filter contacts only the nylon and does not contact the polyester.     

LOCAL EFFICIENCY MEASUREMENTS APPLICABLE TO BOTH AUTOMOTIVE ENGINE AND CABIN FILTRATION

Presented herein are velocity and particle concentration profiles that were measured in planes just upstream and downstream of an engine air intake filter (AF3192) with a Laser Doppler Velocimeter. These detailed measurements allow the computation of local filter efficiency, with overall efficiency being an integral of the local efficiencies. Measurements were performed for nominal air flow rates ranging from 34 m³/hr to 268 m³/hr with monodisperse polystyrene latex spheres of 0.966, 5.3 and 10.2 micron diameters. Two test housings were employed in the research -- the SAE J726 housing (for engine tests) and a small angle diffuser housing that is similar to the SAE J1669 housing (for cabin filtration tests). The SAE housing generally had a higher filtration efficiency than the small angle diffuser housing, and the SAE housing overall efficiency's variation with air flow rate was flatter than that of the diffuser housing. As expected from Stokes number considerations, the large particles had higher filtration efficiencies than the small particles, and the overall filtration efficiencies of 30% to 80% were within the range predicted by theoretical models and gravimetric testing. Work will continue with more particle sizes, SAE dust, various contaminant loading (dirty filters), charged/charge neutralized systems, and housings more representative of actual installed [in the automobile] geometries.     

粉尘对空气滤清器原始过滤效率测试结果的影响

为了比较国内外不同试验粉尘对空气滤清器原始过滤效率测试结果的影响,说明试验粉尘对于过滤效率测试的重要性,对国内外标准所规定的试验粉尘的粒径分布和形貌特征进行测试分析。结果表明:在相同实验条件下,以石英砂作为试验粉尘测得的原始过滤效率为99.35%,以A2细灰作为试验粉尘测得结果为99.10%;从粉尘粒径分布和形貌特征两方面讨论造成原始过滤效率测试结果不一致的原因。     

OPTIMIZATION OF NONWOVEN FILTER PERFORMANCE FOR INTERMEDIATE REYNOLDS NUMBERS

The selection of a nonwoven air filter structure that is optimal for specific conditions is not simple and is affected by many factors. Traditionally, the selection process is based on experience and professional instinct. However, this approach to decision-making lacks an objective basis for making comparisons between alternative structures. In order to optimize the effectiveness of the filter performance, the decision-making process must be placed on a rational and objective basis.

In this paper, a general approach to a multicriteria optimization is discussed. Linear and nonlinear models are analyzed as well as compound criteria obtained by various combinations of simple criteria. An essential component of the optimization model is the criterion function, which is a mathematical expression of the optimization criterion deduced from the goals. Each criterion represents a filter characteristic, such as filter efficiency, pressure drop, flow rate, dust-holding capacity and, in some cases, economic indices.

The formulation of the optimization problem involves transforming filter performance into an equivalent mathematical model. A set of equations was developed to describe initial efficiency and pressure drop as well as efficiency and pressure drop for dust loaded nonwoven media at intermediate Reynolds numbers.

Using a simple mathematical method, the optimal structure of a nonwoven filter for a given set of conditions was obtained. A compound criterion can also be used as a quality factor in order to make comparisons between filters of different structures.     

CONVENTIONAL AIR CLEANERS VERSUS ELECTROSTATIC AIR CLEANERS IN HEPA AND INDOOR APPLICATIONS

In spite of extensive research and development work illustrating the significant advantages of Electrically Stimulated Filters (ESF's), this technology has not been commercially successful. This paper addresses this subject by analyzing conventional filter and ESF cost effectiveness in HEFA, clean room and indoor air cleaning applications.

A new cost effective form of ESF technology has been considered: a single (charging and collection) field device. The laboratory performance of five commercially available conventional filters and the new ESF is characterized in terms of efficiency versus particle size, flow rate versus pressure drop and dirt loading characteristics. Using this information and the application requirements, the cost effectiveness of the ESF has been established with respect to conventional filters. Both initial and operating costs are discussed. The results show that ESF technology is cost effective for indoor applications even where space constraints are not important. Space constraints make the ESF even more attractive. However, in HEPA applications, the ESF has a significant overall cost disadvantage. Further, due to potential reliability problems, this technology has not been successful in this application.     

OILWELL COMPLETION FLUID SOLIDS INTERACTION WITH AN UNCONSOLIDATED OIL RESERVOIR

This work addresses the problem of modelling, detection and evaluation of the mechanical interaction between oilwell completion fluid particles and granular reservoir rocks. These interactions contribute to a reservoir productivity reduction known as the “permeability damage” phenomenon.

Fundamental concepts of cake filtration and filter medium filtration were used to formulate mathematical models of particles capturing: gradual pore blocking, screening, and straining. The models provide theoretical bases for simple diagnostic plots in which the linearity of a plot becomes an indication of a prevailing mechanism of particles' capture.

Laboratory tests were run by pumping a completion fluid through synthetic simulated cores of unconsolidated sandstone gas reservoir samples from the Adriatic Sea. By using diagnostic plots and grade passing efficiency curves, the effects of total solids concentration particle size on the type and efficiency of the capture mechanism were investigated.

The study revealed the existence of three mechanisms of particle capture and the applicability of diagnostic plots for their detection. The onset of the straining mechanisms was attributed mainly to the total solids concentration while the duration of the gradual pore blockage phase was correlated with both particle size and concentration.     

EFFECT OF SURFACTANT ADDITION ON THE MICROSTRUCTURE OF COAL FILTER CAKES

Currently, water removal from fine coal fractions is affected by vacuum filtration of a coal slurry followed by thermal drying. Due to high operating costs and the potentially hazardous nature of the drying operation, alternate methods are sought.

This work investigated the change of filter cake microstructure resulting from one such method, namely, surfactant addition to the coal slurry. By impregnating a coal filter cake with an epoxy resin, micrographic analysis of the cake structure could be made by using an image analyzer with the aid of quantitative stereology. This analysis provided a particle and pore size distribution of the filter cake which is fundamental to the understanding of the dewatering mechanism.

Three surfactants were investigated in this work: non-ionic Triton X-114, anionic Aerosol-OT, and cationic dodecyl pyrindinium chloride. The behavior of Triton X-114 and DPC was similar; they both enhanced dewatering by reducing the amount of particle segregation while increasing the pore size of the cake. Conversely, Aerosol-OT, while exhibiting enhanced dewatering characteristics, did not significantly alter the filter cake structure. At low concentrations all three surfactants exhibited little change in particle and pore size distributions when compared to a filter cake formed at the same conditions without surfactant.

Additionally, a linear relationship was shown to exist between the volume-surface mean diameters of the particles and that of the pores. Similar relationships are also presented for the geometric mean diameter and the geometric deviation. The best correlation between particles and pores was obtained from coal cakes formed with Aerosol-OT.     

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.     

EFFECT OF LOADING WITH AIM OIL AEROSOL ON THE COLLECTION EFFICIENCY OF AN ELECTRET FILTER

Loading of an electret filter changes the distribution of electrical field in the filter from its preloading condition, and, therefore, affects the filtration efficiency of the filter. Liquid droplets collected on electret filters cause degradation of the electrostatic enhancement of filtration efficiency because of charge neutralization and the formation of a dielectric coating over die charged fibers. In this study, calculations were made for the penetration of aerosol particles through a spun-type electret filter as a function of the particulate loading. An assumption was made that each charge collected neutralized one charge of opposite polarity on the fibers of the filter. It was also assumed that the electrostatic charges present on the particles followed the Boltzmann equilibrium charge distribution. The decrease in fiber charge and resulting increase in penetration were calculated as a function of time and of total particulate loading on the filter. The calculated penetrations were compared with experimental measurements of loading on a spun fiber electret filter challenged with monodisperse liquid droplets of bis-Ethylhexyl Sebacate with equilibrium charge distribution and with zero charge. The rate at which the penetration increased was found to be the same for particles with zero charge as for particles with equilibrium charge distribution. For 1 um diameter droplets with equilibrium charge the theory predicted complete discharge of the filter at a loading of around 200 g/m². Experimentally, only about 0.3 g/m² was required. This difference indicates the presence of additional mechanisms for the discharge of the fibers.     

A COMPARISON OF TWO NaCl TEST SYSTEMS USED FOR MEASURING FILTER EFFICIENCY

The filter efficiency of materials to be used in respiratory protective devices is commonly measured by using a solid NaCl particle challenge and a flame ionization detector. The Moore's BS 4400 Bench Rig is an example of such equipment.

Recently, TSI, Inc. introduced their Model 8110. This system also produces a NaCl particle challenge. It uses a laser diode detector.

The filter efficiencies of two different filter materials were determined in a comparative study utilizing several Moore's and TSI systems. The results indicate that the two test systems are relatable and that the between-machine consistency of the Model 8110 is better than the Moore's system.     

PRELIMINARY PERFORMANCE TESTS OF A PULSED JET PLEATED PAPER CARTRIDGE FILTER SYSTEM

A pulsed jet pleated paper cartridge filter system was tested for particle removal efficiency and operating stability using agricultural limestone as the test dust. The test system consisted of 6 pleated paper filter units arranged in parallel, each unit having an effective filtration area of 18.1 square meters. The system was operated at constant pressure drops of 6.25, 7.50 and 8.75 kPa (2.5, 3.0 and 3.5 inches of water), with face velocities of 0.78 to 1.04 m/ min (2.5 to 3.4 feet per minute) and dust loadings ranging from 0.5 g/ m to 2.5 g/ m³. Penetration through the media appeared to be relatively independent of dust loading, and efficiencies were in the 99.95 + % range. Slightly lower efficiencies were found for particles having diameters of 0.3 to 1.0 micrometers. A measure of the redeposition of dust pulsed from the filter was required to describe the effects on the pulse rate caused by changes in system flow and pulsing set point. Increased redeposition was found to occur with increasing flow rate, causing an increase in the pulse rate required to maintain operation of the system at a pulsing set point. Overall, the reverse pulse jet pleated paper cartridge filter system displayed extremely high particle removal efficiency in a compact unit that operated with low differential pressure.     

Choice of Filtration Methods for Monitoring the Dissolution of Frusemide

Although the British Pharmacopoeia indicates the need for filtration during the removal of samples in the course of dissolution tests, there is little guidance as to the type of filters that should be used to clarify fluids during their removal. On the basis of results obtained using frusemide as a model drug, the following recommendations are made regarding the use of filters in dissolution testing.

[1] If membrane filters are to be used to clarify dissolution fluids, filters with the smallest diameter and the highest pore size as practically permissible should be utilized to minimise drug loss into the filters.

[2] Whatman Paper filters, or equivalent, are probably to be preferred to membrane filters since they do not seem susceptible to drug sorption.

[3] The membrane type should be varied to the pH of the dissolution fluid. At pH 5.8, cellulose acetate filters sorbed less frusemide than cellulose nitrate filters but with 0.1M HC1 more sorption occurred into the cellulose acetate filters.     

EFFECT OF LOADING WITH AIM OIL AEROSOL ON THE COLLECTION EFFICIENCY OF AN ELECTRET FILTER

ABSTRACT

Loading of an electret filter changes the distribution of electrical field in the filter from its preloading condition, and, therefore, affects the filtration efficiency of the filter. Liquid droplets collected on electret filters cause degradation of the electrostatic enhancement of filtration efficiency because of charge neutralization and the formation of a dielectric coating over die charged fibers. In this study, calculations were made for the penetration of aerosol particles through a spun-type electret filter as a function of the particulate loading. An assumption was made that each charge collected neutralized one charge of opposite polarity on the fibers of the filter. It was also assumed that the electrostatic charges present on the particles followed the Boltzmann equilibrium charge distribution. The decrease in fiber charge and resulting increase in penetration were calculated as a function of time and of total particulate loading on the filter. The calculated penetrations were compared with experimental measurements of loading on a spun fiber electret filter challenged with monodisperse liquid droplets of bis-Ethylhexyl Sebacate with equilibrium charge distribution and with zero charge. The rate at which the penetration increased was found to be the same for particles with zero charge as for particles with equilibrium charge distribution. For 1 um diameter droplets with equilibrium charge the theory predicted complete discharge of the filter at a loading of around 200 g/m². Experimentally, only about 0.3 g/m² was required. This difference indicates the presence of additional mechanisms for the discharge of the fibers.     

PRELIMINARY PERFORMANCE TESTS OF A PULSED JET PLEATED PAPER CARTRIDGE FILTER SYSTEM

ABSTRACT

A pulsed jet pleated paper cartridge filter system was tested for particle removal efficiency and operating stability using agricultural limestone as the test dust. The test system consisted of 6 pleated paper filter units arranged in parallel, each unit having an effective filtration area of 18.1 square meters. The system was operated at constant pressure drops of 6.25, 7.50 and 8.75 kPa (2.5, 3.0 and 3.5 inches of water), with face velocities of 0.78 to 1.04 m/ min (2.5 to 3.4 feet per minute) and dust loadings ranging from 0.5 g/ m to 2.5 g/ m³. Penetration through the media appeared to be relatively independent of dust loading, and efficiencies were in the 99.95 + % range. Slightly lower efficiencies were found for particles having diameters of 0.3 to 1.0 micrometers. A measure of the redeposition of dust pulsed from the filter was required to describe the effects on the pulse rate caused by changes in system flow and pulsing set point. Increased redeposition was found to occur with increasing flow rate, causing an increase in the pulse rate required to maintain operation of the system at a pulsing set point. Overall, the reverse pulse jet pleated paper cartridge filter system displayed extremely high particle removal efficiency in a compact unit that operated with low differential pressure.