Performance of ventilation filtration technologies on characteristic traffic related aerosol down to nanocluster size

Near traffic routes and urban areas, the outdoor air particle number concentration is typically dominated by ultrafine particles. These particles can enter into the nearby buildings affecting the human exposure on ultrafine particles indoors. In this study, we demonstrate an aerosol generation system which mimics the characteristic traffic related aerosol. The aerosol generation system was used to determine the size-resolved particle filtration efficiencies of five typical commercial filters in the particle diameter range of 1.3–240 nm. Two different HEPA filters were observed to be efficient in all particle sizes. A fibrous filter (F7) was efficient at small particle sizes representing the nucleation mode of traffic related aerosol, but its efficiency decreased down to 60% with the increasing particle size. In contrast, the filtration efficiency of an electrostatic precipitator (ESP) increased as a function of the particle size, being more efficient for the soot mode of traffic related aerosol than for the nucleation mode. An electret filter with a charger was relatively efficient (filtration efficiency >85%) at all the observed particle sizes. The HEPA, F7 and electret filters were found to practically remove the particles/nanoclusters smaller than 3 nm. All in all, the filtration efficiencies were observed to be strongly dependent on the particle size and significant differences were found between different filters. Based on these results, we suggest that the particulate filter test standards should be extended to cover the ultrafine particles, which dominate the particle concentrations in outdoor air and are hazardous for public health.

Copyright © 2017 American Association for Aerosol Research     

Effects of size and shape on filtration of TiO2 nanoparticles

Titanium dioxide (TiO₂) is one of the most widely used nanoscale materials to date and could result in human exposures. The main objective of this study was to perform detailed characterization of TiO₂ agglomerate particles and how these properties influence particle penetration in a screen filter. Transmission electron microscope (TEM) photos showed compact agglomerates of nanoscale primary particles. The projected area diameter was close to the mobility diameter, where the length was about 25% larger than the mobility diameter. The mean aspect ratio of TiO₂ agglomerate was constant between 1.39 and 1.55. Using the tandem differential mobility analyzer-aerosol particles mass analyzer (DMA-APM) technique, we were able to measure aerodynamic diameter, mass, and fractal dimension. The value of fractal dimension based on mass and mobility diameter was 2.8. Penetration of classified TiO₂ particles through a screen filter was measured. Penetration increased with increasing mobility diameter and flow rate indicating that diffusion and interception were the main filtration mechanism. The measured physical dimensions, mobility diameter, and aerodynamic diameter were used in a single-fiber filtration theory for the fan model filter to predict the penetration of TiO₂ particles. The interception parameter was the key to estimate the penetration. Experimental penetration data were in best agreement with the model in which the maximum length was used to calculate the interception model. This result was consistent with the assumption that the rotation time of a non-spherical particle of small aspect ratio was much less than the transport time for the particle to pass through the filter fiber.

© 2017 American Association for Aerosol Research     

Silver-incorporated poly vinylidene fluoride nanofibers for bacterial filtration

An anti-bacterial filter was developed using poly vinylidene fluoride (PVDF) nanofibers using electrospinning method blended with silver nanoparticles (AgNO₃) of varying weight percentages of filler. Polypropylene (PP) non-woven substrate was used as base material for collecting the nanofibers. It also acted as a barrier to protect the fibers. UV-visible spectroscopy and fourier transform infra red spectroscopy confirmed the uniform dispersion of silver nanoparticles throughout the nanofibers. The experiment was designed using Box–Behnken statistical tool through three different variables namely, PP non-woven sheets (GSM), electrospinning time (hours), concentration of silver (wt%) in 15 runs. Surface morphology was analyzed using scanning electron microscopy and atomic force microscopy. Thermogravimetric analysis was performed for the analyses of mass decomposition of the material. Bacterial filtration efficiency and anti-bacterial activity studies were tested against Staphylococcus aureus and Escherichia coli for both PVDF + 0?wt% Ag fibers and PVDF-Ag nanofibers. This research shows the bacterial filtration efficiency for the prepared PVDF-Ag nanofibers as 99.86%. The prepared nanofilter was shown providing greater possibilities towards the application for clean air management.

© Copyright 2019 American Association for Aerosol Research     

Optimization of centrifugally spun thermoplastic polyurethane nanofibers for air filtration applications

While our knowledge of fiber formation by using conventional nanofiber spinning techniques has increased to a considerable extent, there are still few studies on centrifugal spinning either in academia or in the industry. Centrifugal spinning is a comparatively new method of producing fibers having nano- or microscale diameters. In this study, three main parameters (nozzle orifice diameter, rotational speed, polymer concentration) of centrifugal spinning were optimized to produce air filter media from thermoplastic polyurethane nanofibers. The effect of concentration of polymer solution was found to be a major contributor in TPU fibers optimization estimating 77.5%. After the optimization studies, the average fiber diameter of nanofiber sample produced at optimum conditions (22G needle as an orifice, 4000 rpm, and 10 wt% concentration of polymer solution) was 205 ± 84 nm. Aerosol filtration performance of the produced webs was analyzed. Filtration efficiency of the optimized sample was found to be 99.4% for 0.3 µm particle size at an expense of 98 Pa pressure drop.

Copyright © 2018 American Association for Aerosol Research     

Centrifugally spun silica (SiO2) nanofibers for high-temperature air filtration

In this study, silica-based nanofibers were produced via centrifugal spinning (C-spin) and subsequent calcination. The produced heat resistant media was challenged with NaCl nanoparticles to investigate their filtration performance. To obtain inorganic SiO₂ nanofibers, C-spun organic PVP–TEOS nanofibers were calcinated at 300–600?°C. Effects of solution concentration and calcination temperature on crystallinity, morphology and air filtration performance of nanofibers were investigated. Scanning electron microscopy (SEM) analysis was performed to analyze fiber diameter and morphology of nanofibrous webs. Differential thermal analysis (DTA) was realized for the thermal behavior of samples. Moreover, X-ray diffraction (XRD) and Fourier transform infrared spectra (FTIR) analysis were realized for further characterization. In addition to the chemical and morphological analysis, the ductility of the samples was investigated via tensile tests. Finally, calcinated webs were challenged with 0.4?μm salt particles to analyze their filtration performance. The calcinated 5?wt% TEOS/PVP silica nanofiber webs were more brittle due to three times lower precursor content. Therefore, flexibility (percent elongation) of 15?wt%TEOS/PVP sample was nearly five times higher than 5?wt%TEOS/PVP sample. The calcinated 15?wt%TEOS/PVP sample showed the highest filtration performance among all the silica nanofibers. The average fiber diameter of the optimized web was found to be the lowest, which is around 521?±?308?nm, which resulted in enhanced filtration efficiency around 75.89%.

Copyright © 2019 American Association for Aerosol Research     

An artifact-minimizing method for total dust sampling and chemical characterization of industrial high-temperature aerosols

Conventional methods for total dust sampling from industrial high-temperature aerosols containing condensable species, e.g., in boilers of municipal solid waste incinerators (MSWI), are always influenced by condensation artifacts. Therefore, we extended the scope of a previously developed probe intended for size-fractionated aerosol sampling with reduced artifacts and employed it for total dust measurements. The dust is collected on quartz fiber filters, which are gravimetrically evaluated and chemically analyzed by energy dispersive X-ray spectroscopy and wet-chemical methods. First measurements in the boiler of an MSWI confirm that the probe is also suitable for artifact-minimized total dust sampling. The data are consistent with results from measurements with the size-fractionating method conducted in parallel. By combining the results of both sampling methods, we reveal the average chemical composition of the submicron particles in the aerosol, which is not accessible by one of the two methods alone.

© 2017 American Association for Aerosol Research     

Experimental investigation and numerical modeling of the orientation angle of silver nanowires passing through polyester filters

In the present study, we measured the penetration of silver nanowires with the mobility diameter in the range from 200 to 400 nm through two different types of polyester filters: the screen filter with the solidity of 0.505 and fibrous filter with the solidity of 0.278. The orientation angles of silver nanowires passing through the single layer and multi-layers of polyester filter were experimentally estimated on the basis of the single fiber efficiency theory. In the case of the screen filter, the orientation angle obtained by fitting the experimental data for single layer was found to be close to 40?, indicating a random orientation of nanowires near the filter. However, the fibrous filter has the orientation angle much larger than 40?. The orientation angle can be affected by inhomogeneity of the filter. In particular, in the case of the fibrous filter, the solidity and fiber diameter may affect the orientation angle. For multi-layers of both screen and fibrous filters, it is difficult to determine the typical orientation angle and the fibrous filter tends to have a larger orientation angle than the screen filter. In addition, we carried out numerical simulations on the penetration of silver nanowires through the five layers of screen filter and the single layer of both screen and fibrous filters. Numerical prediction was carried out by using the three-dimensional numerical model determined by solidity and thickness of fibrous filter. Numerical predictions are highly congruent with experimental results and theoretical prediction.

© 2017 American Association for Aerosol Research     

Application of centrifugal filter to aerosol size distribution measurement

In the present work, the centrifugal filter proposed by the authors was applied to classify aerosol particles followed by the detection of total mass or number concentrations so as to measure the size distribution of aerosol particles. The structure and operating condition of the centrifugal filter were optimized in order to attain sharp separation curves with various cut-off sizes between 0.3 and 10 μm. The aerosol penetrating the centrifugal filter at various rotation speeds was measured with a photometer to determine the total mass concentration. The virtue of this system is that the cut-off size is varied just by scanning the rotation speed of filter and that it can be applied to the measurement of high concentration aerosols without dilution by choosing an appropriate filter medium. As a result, the centrifugal filter was successfully applied to measure the size distribution of solid particles in size ranging from 0.3 to 10 μm.

Copyright © 2017 American Association for Aerosol Research     

Using partial flow dilution to measure PM mass emissions from light-duty vehicles

Partial flow dilution (PFD) offers a number of benefits relative to conventional full flow dilution tunnels for motor vehicle PM emissions measurement in terms of measurement variability, footprint, and cost. Its implementation into automotive emissions test cells depends on its ability to record PM mass emissions equivalent to the current constant volume sampling (CVS) dilution tunnel approach. The present work examines factors critical to successful application of PFD sampling and compares vehicle PM emissions measured simultaneously by PFD and CVS tunnel. The combination of a current technology commercial PFD system and ultrasonic flow meter fulfill the two principal requirements of accurate exhaust flow measurement and fast time response for proportional sampling. PM mass emissions measured by PFD versus CVS systems satisfy a 1.03 ± 0.03 regression for the regulatory FTP test cycle, and comparably good agreement for the supplemental US06 drive cycle. Both dilution approaches are amenable to the single filter approach newly allowed under EPA regulations; however, the PFD sample weighted approach has the capability to provide a roughly 35% lower variability relative to the flow weighted approach used in the CVS method. Whereas partial flow dilution has seen application in heavy duty engine measurements, the present work demonstrates its feasibility for light duty chassis dynamometer testing.

Copyright © 2018 Ford Motor Company     

Detachment of adhered particles from a cloth surface subjected to a rod strike

A mechanical impulse can cause adhered particles to detach from a surface. For various purposes, particle detachment may need to be enhanced or restricted. Unlike rigid solids, cloth material can be deformed or bent by a mechanical impulse. However, neither the cloth deformation nor the induced turbulent airflow has been well studied. This investigation experimentally measured the detachment of Arizona test dust (ATD) from cloth segments. The vertical margins of each cloth segment were fastened to a frame, and the cloth surface with the ATD adhered to the reverse side was struck with a rod. The cloth motion, induced airflow, and particle detachment were recorded by a high-speed camera. In addition, the displacement and acceleration of the cloth were monitored with a laser distance sensor. The mass percentages of detached particles from the cloth and the particle residual were weighed. Several factors that affected particle detachment were compared. The results revealed that the particle detachment was caused by a combination of the vibrating motion of the cloth surface, the hydrodynamic action of the induced turbulent airflows, and the particle agglomeration when the cloth was bent. A strike could even leave fewer residual particles when a much higher surface dust load had initially adhered to the cloth.

Copyright © 2019 American Association for Aerosol Research     

声波与化学团聚协同作用对转炉微细粉尘团聚的影响

针对转炉烟气中微细粉尘的难处理问题,采用声波与化学团聚协同处理方法,在蒸发冷却器内对转炉烟气粉尘进行预处理,以提高微细粉尘的除尘效率。采用单因素实验和正交实验方法研究了絮凝剂的种类[丙烯酰胺(PAM)、羧甲基纤维素钠(CMC)和黄原胶(XTG)]、絮凝剂浓度、声波频率、声波驻场时间等不同影响因素下的微细粉尘的团聚效果。单因素实验结果表明：三种絮凝剂的团聚效果由大到小依次为PAM>CMC>XTG,絮凝剂浓度为0.1 g/L时团聚效果最好,声波频率为33 KHz时作用效果较为显著,声波驻场停留时间为15 s时效果最佳;声波与化学团聚协同作用效果好于化学团聚和声波团聚单独作用效果。通过正交实验得出,当浓度为0.1 g/L、声波频率33 KHz、声波驻场停留时间15 s的条件下,团聚效果最显著,峰值粒径由原始的3.311 μm提升至43.59 μm,对应的静电除尘器的除尘效率可达到97%。研究成果可为提高钢铁工业烟气中微细粉尘的去除工艺开发提供可靠的基础数据,为工业烟气中微细粉尘的去除效率提高奠定了基础。     

A novel-charged fibrous media characterized by higher efficiency and lower pressure drop

Abstract

A new porous fiber shape was produced, one with a longitudinal notch. This new shape increases the surface area and therefore the overall number of pores, which in turn leads to a higher per-fiber surface charge density. Filters with charge generally show advantageous lower pressure drops when compared with the same filtration efficiencies. Although previous studies have hypothesized that increasing the surface area of porous fibers can improve charge capacity, but few experiments have proven this concept. In this study, two different micron-sized fibers were fabricated via electrospinning: smooth (“normal”) fibers and the novel notched fibers with surface pores. The fiber profile and voltage drop of the two fibers were measured, and the penetration rate of the two types of fibers before and after charging was measured with aerosol particles ranging from 25 to 478?nm in size. The results showed that the specific surface area of the notched fiber was 47.44?±?0.67 m² g^?1, larger than that of the smooth fibers (2.64?±?0.08 m²g^?1). The notched fiber was charged by negative corona discharge, and the surface capacitance was measured to be ?51.2 µC m^?2 when the thickness was 0.19?mm, which was also larger than that of the smooth fibers (?38.2 µC m^?2). When charged, the value of the average quality factor of the notched fibers increased from 0.030 to 0.068?Pa^?1. Overall, the notched fibers demonstrated better filtration efficiency. We concluded that this novel fiber shape delivers both low penetration rates and low pressure drops.

Copyright © 2019 American Association for Aerosol Research     

Particle charge-size distribution measurement using a differential mobility analyzer and an electrical low pressure impactor

We introduce a particle charge-size distribution measurement method using a differential mobility analyzer and an electrical low pressure impactor in tandem configuration. The main advantage of this type of measurement is that it is suitable for a wide range of particle sizes, from approximately 30 nm up to a micrometer, and for high charge levels, which have been problematic for previously used methods. The developed charge measurement method requires information on the particle effective density, and the accuracy of the measurement is dependent on how well the particle effective density is known or estimated. We introduce the measurement and calculation procedures and test these in laboratory conditions. The developed method has been tested using narrow and wide particle size distributions of a known density and well-defined particle charging states. The particles have been produced by the Singly Charged Aerosol Reference (SCAR) and an atomizer and charged with the previously well-characterized unipolar diffusion chargers used in the Nanoparticle Surface Area Monitor (NSAM) and in the Electrical Low Pressure Impactor (ELPI+). The acquired charge-size distributions are in good agreement with the reference values in terms of the median charge levels and widths of the charge distributions.

Copyright © 2017 American Association for Aerosol Research     

Collection efficiency of airborne fibers on nylon mesh screens with different pore sizes and configurations

Aerodynamic behavior of airborne fibers including high-aspect ratio particles plays an important role in aerosol filtration and lung deposition. Fiber length is considered to be an important parameter in causing toxicological responses of elongate mineral particles, including asbestos, as well as one of the factors affecting lung deposition. In order to estimate the toxicity of fibers as a function of fiber length, it is required to separate fibers by length and understand mechanisms related to fiber separation for use in toxicology studies. In this study, we used nylon mesh screens with different pore sizes as a separation method to remove long fibers and measured screen collection efficiency of glass fibers (a surrogate for asbestos) as a function of aerodynamic diameter with the aim to prepare toxicology samples free of long fibers and/or harvest long fibers from the screen. Two screen configurations ([i] without a laminar flow entrance length, and [ii] with the entrance length) were tested to investigate the effect of screen pore size (10, 20, and 60 µm) and screen configuration on collection efficiency of fibers. Screen collection efficiency (η) was obtained based on measurements of downstream concentrations of a test chamber either without or with a screen. The results showed that screen collection efficiency increases as screen pore size decreases from 60 to 10 µm for both cases with and without entrance lengths. For the screen configuration without entrance length, higher collection efficiency was obtained than the case with entrance length probably due to increased impaction caused by the close proximity of inlet to screen. In addition, the difference between the collection efficiencies for the different configurations was small in the aerodynamic size range below 3 µm while it increased in the size range from 3 to about 7 µm, indicating that as large aerodynamic diameter is associated with longer fibers, some differential selection of fibers is possible. Modified model collection efficiency for 10 and 20 µm screens based on the interception predicts well the measured data for the case with entrance length, indicating that the fiber deposition on these screens occurs dominantly through the interception mechanism in the micrometer size range under a given flow condition.     

Filtration efficiency analysis of fibrous filters: Experimental and theoretical study on the sampling of agglomerate particles emitted from a GDI engine

Fibrous filters are commonly used for aerosol purification and sampling. The filtration efficiency has been extensively studied using standard aerosol generators, yet the literature on experimental data and theoretical study concerning the filtration of agglomerates from real engines remains scarce. A filtration efficiency test system was developed to determine the filtration efficiency of two types of filters (uncoated and fluorocarbon coated) loaded by particulate matter (PM) emissions from a gasoline direct injection (GDI) engine. The experimental results showed that the filtration efficiency in terms of PM mass and number increased over time for both types of filters. The fractional efficiency (penetration efficiency) curves for the test fibrous filters rendered a U-shaped curve for particle sizes from 70 to 500 nm, and the most penetrating particulate size (MPPS) decreased over time. A small fraction of accumulation mode particles with the size between 70 nm to 500 nm penetrated the filters while almost all nucleation mode particles with the size below 50 nm were captured by the filters. The filtration efficiency derived from an empirical model based on classical single-fiber theory for laden filters generally agreed with the experimental data for the first 500 s, but suffered a significant deviation by approximately one order of magnitude at 948 s. A better estimate of the filtration efficiency trend with the maximum deviation of about 20% (except for large particles at the high end of the measurement spectra) was obtained by using a revised model which incorporates the effects of the increase in filter solidity, local velocity, dynamic shape factor and effective total length of fibers during the filtration process.

© 2017 American Association for Aerosol Research     

Experimental and modeling assessment of a novel automotive cabin PM2.5 removal system

Poor air quality inside vehicles and its impact on human health is an issue requiring attention, with drivers and passengers facing levels of air pollution potentially greater than street-side outdoor air. This paper assesses the potential effectiveness of a car cabin filtration system to remove fine particulate matter PM_2.5 and improve air quality for car passengers. The study was conducted as a practical evaluation coupled to a model implementation. First, the effectiveness of PM_2.5 filter material was investigated in a chamber experiment under a range of environmental and loading conditions using a realistic automotive auxiliary scrubber. Second, implementation of such a system was evaluated in a full air flow 3D computational fluid dynamical model configured for a realistic cabin and ventilation system, and related to the chamber results through a simple decay model. Additionally, performance of low-cost dust sensors was evaluated as potential cabin monitoring devices. The experiment and modeling support the feasibility of a robust system which could be integrated into automotive designs in a straightforward manner. Results suggest that an auxiliary scrubber in the rear of the cabin alone would provide suboptimal performance, but that by incorporating a PM_2.5 filter into the main air handling system, cabin PM_2.5 concentrations could be reduced from 100?µg m^?3 to less than 25?µg m^?3 in 100?s and to 5?µg m^?3 in 250?s. A health impact assessment for hypothetical occupational driver populations using such technology long term showed considerable reductions in indicative PM_2.5 attributable mortality.

Copyright © 2018 The Authors. Published with license by Taylor & Francis Group, LLC     

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