Effects of relative humidity and particle type on the performance and service life of automobile cabin air filters

Cabin air filters are the main barrier for protecting automobile passengers from on road particulate matter. There are many studies about the evaluation of their performance in terms of filtration efficiency. However, the knowledge about the loading capacity of them is still lacking. Meanwhile, there has been no quantitative method to estimate the proper filter service life time. This study focuses on testing the loading capacity of different types of cabin air filters under the conditions of different relative humidity values and particle types. The results indicate that when the relative humidity increases, the activated carbon coated filters can adsorb significant amounts of water with no significant increase of the pressure drop. The normal fibrous filters show in contrast negligible water adsorbance. Compared with the filters loaded by Arizona road dust only, loading the filters by Arizona road dust and soot particles simultaneously will result in the steeper loading curves as well as the shift of most penetrating particle size to the smaller diameter. Finally, a new method to estimate the proper service life time of the cabin air filters is suggested based on the loading curves.

© 2016 American Association for Aerosol Research     

Air sampling filtration media: Collection efficiency for respirable size-selective sampling

The collection efficiencies of commonly used membrane air sampling filters in the ultrafine particle size range were investigated. Mixed cellulose ester (MCE; 0.45, 0.8, 1.2, and 5 μm pore sizes), polycarbonate (0.4, 0.8, 2, and 5 μm pore sizes), polytetrafluoroethylene (PTFE; 0.45, 1, 2, and 5 μm pore sizes), polyvinyl chloride (PVC; 0.8 and 5 μm pore sizes), and silver membrane (0.45, 0.8, 1.2, and 5 μm pore sizes) filters were exposed to polydisperse sodium chloride (NaCl) particles in the size range of 10–400 nm. Test aerosols were nebulized and introduced into a calm air chamber through a diffusion dryer and aerosol neutralizer. The testing filters (37 mm diameter) were mounted in a conductive polypropylene filter-holder (cassette) within a metal testing tube. The experiments were conducted at flow rates between 1.7 and 11.2 l min^?1. The particle size distributions of NaCl challenge aerosol were measured upstream and downstream of the test filters by a scanning mobility particle sizer (SMPS). Three different filters of each type with at least three repetitions for each pore size were tested. In general, the collection efficiency varied with airflow, pore size, and sampling duration. In addition, both collection efficiency and pressure drop increased with decreased pore size and increased sampling flow rate, but they differed among filter types and manufacturer. The present study confirmed that the MCE, PTFE, and PVC filters have a relatively high collection efficiency for challenge particles much smaller than their nominal pore size and are considerably more efficient than polycarbonate and silver membrane filters, especially at larger nominal pore sizes.     

Polyacrylonitrile microscaffolds assembled from mesh structures of aligned electrospun nanofibers as high-efficiency particulate air filters

Electrospun polyacrylonitrile (PAN) fibers of very small diameters have potential for integration into filters capable of increasing the particle filtration efficiency. To fulfill the requirements for high-efficiency particulate air (HEPA) filters with a reasonable pressure drop, we generated aligned electrospun PAN fibers through pre-alignment at various rotation rates and subsequent solvent vapor annealing (SVA) under a loading. We evaluated the properties of microscaffold filters assembled from aligned electrospun PAN fibers in the form of linear, square, and triangular multiple meshes. The microscaffolds featuring multiple square meshes exhibited dramatically increased filtration efficiency without a significant pressure drop. A nine-layer cross-ply structure provided a filtration efficiency of 99.98% for 0.25-μm particles at a face velocity of 10 cm s^?1; its filtration quality factor was the highest among all of the tested microscaffolds. Thus, HEPA filters featuring a low packing density can be achieved using PAN fibers.

© 2016 American Association for Aerosol Research     

Porous polyurethane foam for use as a particle collection substrate in a nanoparticle respiratory deposition sampler

Porous polyurethane foam was evaluated to replace the eight nylon meshes used as a substrate to collect nanoparticles in the Nanoparticle Respiratory Deposition (NRD) sampler. Cylindrical (25 mm diameter by 40 mm deep) foam with 100 pores per inch was housed in a 25-mm-diameter conductive polypropylene cassette cowl compatible with the NRD sampler. Pristine foam and nylon meshes were evaluated for metals content via elemental analysis. The size-selective collection efficiency of the foam was evaluated using salt (NaCl) and metal fume aerosols in independent tests. Collection efficiencies were compared to the nanoparticulate matter (NPM) criterion and a semi-empirical model for foam. Changes in collection efficiency and pressure drop of the foam and nylon meshes were measured after loading with metal fume particles as measures of substrate performance. Substantially less titanium was found in the foam (0.173 µg sampler^?1) compared to the nylon mesh (125 µg sampler^?1), improving the detection capabilities of the NRD sampler for titanium dioxide particles. The foam collection efficiency was similar to that of the nylon meshes and the NPM criterion (R² = 0.98, for NaCl), although the semi-empirical model underestimated the experimental efficiency (R² = 0.38). The pressure drop across the foam was 8% that of the nylon meshes when pristine and changed minimally with metal fume loading (～19 mg). In contrast, the pores of the nylon meshes clogged after loading with ～1 mg metal fume. These results indicate that foam is a suitable substrate to collect metal (except for cadmium) nanoparticles in the NRD sampler.

Copyright © 2016 American Association for Aerosol Research     

Silicone sampling tubes can cause drastic artifacts in measurements with aerosol instrumentation based on unipolar diffusion charging

Aerosol instrumentation based on unipolar diffusion charging has become popular in the recent years. These instruments can be made very small, making them suitable as personal monitors. In many applications, including personal monitoring, the use of flexible sampling tubes is required. We found that degassing from these sampling tubes can alter the gas composition of the aerosol, which changes the ion properties in the unipolar charger. As a result, the particle concentrations, measured with a unipolar diffusion charger are biased. The strongest effect was found with new, conductive silicone tubes, because of the degassing of siloxanes. Results obtained with one miniature diffusion size classifier unit were by a factor of approximately two too low. Partector and Nanoparticle Surface Area Monitor showed in principle the same behavior, but to a lower degree. Other tube materials were found to have even the opposite effect, i.e., the measured concentrations increased, when measuring through a tube. The largest observed increase was, however, only approximately 14%. Tygon? tubes were found to be the best compromise considering particle losses and effect on the diffusion charger.

Copyright © 2016 American Association for Aerosol Research     

Modeling of submicron particle filtration in an electret monolith filter with rectangular cross-section microchannels

Electret monolith filters have the advantage of low pressure drop and high filtration efficiency. In such filters, the filtration of submicron aerosol particles occurs as air passes through millions of microchannels. This article investigates the flow and filtration mechanisms in a representative rectangular microchannel of an electret monolith filter. An improved incompressible lattice Boltzmann method with Bhatnagar–Gross–Krook (traditionally shortened as LBGK) and lattice velocity D3Q15 model is employed to simulate no-slip and slip flows in the rectangular microchannels of a monolith filter. We considered mono-disperse submicron particles and one-way coupling (particle motion was affected by the flow, but the presence of particles did not affect the flow). Based on flow computations, the effects of key dimensionless parameters (Reynolds number, Knudsen number, Stokes number and the dimensionless length of the channel) on the total capture efficiency of mono-disperse submicron particles were investigated. Our results indicate that the optimal monolith filter should be characterized by a Knudsen number between 0.022 and 0.044, and that the dimensionless length of the channel should be between 4 and 8.

Copyright © 2016 American Association for Aerosol Research     

Characterization of filter performance under low-pressure operation

Particulate gas filters are a critical element in the purification systems used to ensure defect-free manufacturing in semiconductor industry. In atomic layer deposition (ALD) processes, these filters are typically operated under sub-atmospheric pressure conditions, but their filtration characteristics are, often, only known at atmospheric pressure. In this study, performance of a metal filter that is typically used in low-pressure ALD precursor delivery systems is studied experimentally and theoretically down to 4.5 kPa. The experimental procedure was designed to minimize the presence of multiply charged particles in the test aerosol for different operating pressures and flowrates. The experimental results suggest that most penetrating particle size only slightly varies with pressure, but the shape of the penetration curve and the maximum value of the penetration changes significantly with pressure. The experimental data are used to test predictions of filter performance at low pressures made using classical theory. The comparison results suggest that the combination of classical theory and manufacturer-specified parameters results in large errors in calculated penetration values at low pressures. Accurate predictions are seen to be possible for particle Stokes numbers less than 0.1, when an inhomogenous filtration model is used in combination with effective filter parameters that are obtained from experimental measurements of filter efficiency and pressure drop at atmospheric pressures.

Copyright © 2016 American Association for Aerosol Research     

Modeling unsteady filtration performance of pleated filter

The dust loading has a significant influence on the transient performance of air filters. This study developed two models based on the Lagrangian and Eulerian methods to simulate the unsteady filtration process in the pleated filter. The flow field through the filter was calculated by solving the Navier-Stokes equation with the DES-SA turbulence model. The filter media and the cake layer were modeled as the porous zone. The Lagrangian method tracked the particle trajectories to model the particle motion, but the Eulerian model treated the particle as continuous phase. Two cell models were proposed to simulate the transient particle deposition and the cake layer growth on the filter medium surface for the Lagrangian and Eulerian methods, respectively. The simulated results were validated by the available experimental data. Both of the methods could provide relative accurate results with acceptable error. But the computing speed of Eulerian model was faster than the Lagrangian method. Otherwise, the new developed Eulerian model was used to investigate the effect of dust loading on the optimal design of pleated filter.

© 2016 American Association for Aerosol Research     

Filtration efficiency for bacterial aerosol through breathing filters

Tests determining the filtration efficiency for a bacterial test aerosol of Bacillus subtilis spores through filter cartridges of domestic materials and through filter cartridges where the principal filtering layer was an electrostatic material from Freudenberg company were carried out to determine the efficiency of purifying air from microorganisms using breathing filters. Tests of the breathing filters were prepared and carried out according to a procedure developed at the SRC FMBA of Russia. Spores of B. subtilis were elliptical and small (0.7 × 1.3 μm). Their concentration in the aerosol was (2.76-5.00)⋅10⁷ spore/m³. The tests were carried out with linear filtration rate 17 cm/s.     

Analytical expressions for predicting capture efficiency of bimodal fibrous filters

In this work, a series of numerical simulations are formulated for studying the performance (collection efficiency and pressure drop) of filter media with bimodal diameter distributions. While there are numerous analytical expressions available for predicting performance of filters made up of fibers with a unimodal fiber diameter distribution, there are practically no simple relations for bimodal filters. In this paper, we report on the influence of the fiber diameter dissimilarity and the number (mass) fraction of each component on the performance of a bimodal filter. Our simulation results are utilized to establish a unimodal equivalent diameter for the bimodal media, thereby taking advantage of the existing expressions of unimodal filters for capture efficiency prediction. Our results indicate that the cube root relation of Tafreshi, Rahman, Jaganathan, Wang, and Pourdeyhimi (2009) offers the closest predictions for the range of particle diameters, coarse fiber number (mass) fractions, fiber diameter ratios, and solid volume fractions (SVF) considered in this work. Our study revealed that the figure of merit (FOM) of bimodal filters increases with increasing fiber diameter ratios for Brownian particles (d_p<100 nm), and decreases when challenged with larger particles. It has also been shown that when increasing the ratio of coarse fibers to fine fibers, FOM increases for Brownian particles, and decreases for larger particles.     

Nanoparticle collection efficiency of capillary pore membrane filters

The surface and overall collection efficiencies of capillary pore membrane filters were measured for sub-micrometer particles. Collection efficiencies were derived from the surface loadings of particles on filters measured by scanning electron microscopy and from airborne particle concentrations measured with a scanning mobility particle sizer. Tests used filters with nominal pore diameters of 0.4 and 0.8 μm and face velocities of 3.7 and 18.4 cm/s. Surface collection efficiencies were below 100% for particles smaller than 316 nm and below 55% for particles smaller than 100 nm. Overall collection efficiencies reached as low as 45% for 70 nm particles. For nanoparticles, collection efficiencies overall were substantially higher than those to the filter surface, indicating that deposition occurs to a large extent inside the filter pores. These results underscore the need to account for surface collection efficiency when deriving airborne concentrations from microscopic analysis of nanoparticles on capillary pore membrane filters.     

Collection efficiency of sintered ceramic filters made of submicron spheres

The performance of filters made of sintered submicron alumina particles was evaluated. The filter has a high collection efficiency and high pressure drop, requiring the development of a special measuring system for its evaluation. The system consists of a polydisperse NaCl particle generator, a differential mobility analyzer (DMA), an ejector to supply aerosols for testing filters with high pressure drop, and a mixing-type condensation nucleus counter (CNC) capable of obtaining a stable reading of very low concentration particles. Penetrations as low as 10⁻⁹ can be measured in the particle diameter range of 0.02-0.14 μm. Two filters made by sintering 0.60 and 0.84 μm alumina particles were evaluated. The experimental data collected served as the basis of theoretical development. Following the single fibre theory, the filter penetration is calculated by using a single sphere as the element. Both the diffusion and interception collection mechanisms were taken into account. The resulting equation gives a general trend of efficiency curves as a function of the parameters involved, e.g. the test aerosol size, packed particle size and filtration velocity. However, it is not sufficiently accurate for providing quantitative performance results.     

Penetration of nanoparticles through fibrous filters perforated with defined pinholes

This paper presents experimental data for penetration of nanoparticles in the 4–30 nm size range through fibreglass filters perforated with defined pinholes at 5 and 15 cm/s. Results show that, when the filter is damaged, penetration increases with decreasing particle diameter. This lower performance is more significant for filters with high airflow resistance. A model explaining the experimental data has been designed based on balanced, laminar airflow inside the pinhole; these conditions were confirmed by detailed study. The experimental points agreed very closely with this model.     

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.     

Silica nanoparticles as a tool for fluorescence collection efficiency enhancement

In this work we demonstrate enhancement of the fluorescence collection efficiency for chlorophyll-containing photosynthetic complexes deposited on SiO₂ spherical nanoparticles. Microscopic images of fluorescence emission reveal ring-like emission patterns associated with chlorophyll-containing complexes coupled to electromagnetic modes within the silica nanoparticles. The interaction leaves no effect upon the emission spectra of the complexes, and the transient behavior of the fluorescence also remains unchanged, which indicates no influence of the silica nanoparticles on the radiative properties of the fluorophores. We interpret this enhancement as a result of efficient scattering of electromagnetic field by the dielectric nanoparticles that increases collection efficiency of fluorescence emission.     

Determination of the retention efficiency of industrial air filters for particle sizes in the submicron range

Particle size distribution up to a maximum diameter of 0.1 μm has been produced in the presence of short-lived tracer Na-24 using an aerosol generator. Industrial air filters with a cross-section of 100 cm² have been exposed to very high particle fluxes such that particulate breakthroughs down to 10⁻⁸ can be assessed by measuring the activity ratio of the air before and after going through the filter.

This method, which involves installing an analytical filter (absolute filter) after the test filter, has a high accuracy and makes it possible to determine particle breakthrough under the same conditions of air flow velocity and pressure loss as in industrial processes. In comparison with on-the-spot-methods, e.g. the condensation nucleus counter, the above method includes passage across the entire filter area and excludes secondary aerosols.     

An Experimentally Fitted and Simple Model for the Pores in Nuclepore Membranes

Abstract

The porosities (percentage of empty volume over the total volume) of several Nuclepore membranes are measured by means of a pycnometric method which is shown. If cylindrical pores are assumed, the porosities can be calculated from the surface pore densities and mean pore radii, both measured by microscopy. The disagreement between these two methods implies that the pores are not cylindrical in shape. A model is proposed that assumes an internal pore radius, r_r different from the external one, r_e (mean pore radius). When it is assumed that there is a mean angle, φ, between the pores and the membrane surface, this angle can be calculated if we assume that the experimental surface pore density is the maximum one compatible with the model. From a comparison of calculated and experimental φ, the maximization of the surface pore density can be tested.     

Measurement and prediction of filtration efficiency evolution of soot loaded diesel particulate filters

We here present laboratory based experimental and theoretical methods to characterize the filtration efficiency (FE) behavior of diesel particulate filters (DPFs) exposed to soot laden gas streams. Sensitivity of the FE behavior on filter microstructure and geometry properties have been studied, along with the impact of the hydrodynamic and aerosol flow conditions (flow rate, temperature, aerosol characteristics). Evolution of FE with soot load is reported from clean filter FE to maximum efficiency (100%), as the deposited soot in the filter wall itself acts as the filtering medium. The theoretical model considers different mechanisms (Brownian diffusion, particle interception and inertia) of soot capture and their impact on number based and mass based FE. The predictions from the theoretical model are in good agreement with experimental observations over a broad range of filter microstructure. Sensitivity of FE evolution on bare and coated filters has been reported, along with the impact of ash loading of the filters. Methods presented here are useful in determining the performance of DPFs under well-defined laboratory conditions and their extension to dynamic field conditions. These are also useful in determining filter properties for obtaining high FE and low pressure drop.