Wetted Wall Cyclones for Bioaerosol Sampling

A wetted wall bioaerosol sampling cyclone with an aerosol sampling flow rate of 1250 L/min and a continuous liquid outflow rate of about 1 mL/min was developed by upgrading an existing system. The aerosol-to-hydrosol collection efficiency curve for the upgraded device was shown to have a cutpoint of 1.2 μ m aerodynamic diameter (AD) and an average collection efficiency of 90% over the size range of 2 to 10.2 μ m AD. Tests with near-monodisperse cells and clusters of Bacillus atrophaeus (aka BG) spores showed an average aerosol-to-hydrosol collection efficiency of 98% over the size range from 1.7 to 9.8 μ m AD. Pressure drop across the cyclone, which is also the ideal specific power, was 5.5 kPa (22 inches H₂O). Stokes scaling was used to design geometrically similar cyclones with nominal air sampling flow rates of 100 and 300 L/min. Extensive tests were performed with the 100 L/min unit and check tests with the 300 L/min. Results with the scaled units showed similar, although somewhat lower collection efficiencies than the 1250 L/min device, but with lower consumption of liquid and lower pressure losses. For the 100 L/min cyclone, the cutpoint of the aerosol-to-hydrosol efficiency curve was 1.2 μ m AD, and the average collection efficiency for single cells and clusters of BG spores was 86% over a size range of 1.2 to 8.3 μ m AD. Also, for the 100 L/min cyclone, typical output liquid flow rates were 100 μ L/min, and the pressure loss was 1.6 kPa (6.4 inches H₂O).     

Characterization of the Velocity Field in a Small,Cylindrical, Low Reynolds Number Aerosol Sampling Cyclone

A laser Doppler velocimeter capable of simultaneously measuring two components of velocity was used to measure the flow field in a 88.9-mm diameter, cylindrical, reverse-flow, multiport cyclone. A third component was measured by reorienting the cyclone. The components measured were those in the radial, axial, and tangential directions with the tangential component being measured in both orientations. The ratio of velocities was ? _θ:? _z:? _R = 100:10:1. At a flow-rate Reynolds number, N _RQ= (4pQ)/(πμD _c), of approximately 860, values of turbulence intensity for each component within the cyclone were on the order of 10% when determined as the component standard deviation normalized to the mean flow. The vortex structure is dependent upon Reynolds number.     

In-Line Impactor Inlet for Bioaerosol Sampling

The proof of concept of a novel in-line real impactor (IRI) for preseparation of large particles in ambient inlets was demonstrated with a 1,250 L/min design. Numerical simulations predicted a cutpoint Stokes number 0.3 for a ratio of jet-to-plate spacing to jet width (S/W) of 2.0 and 0.5 for a ratio of 4.0. This variation in cutpoint Stokes number allows minor adjustments in cutpoint for a given device size. Experimental benchmark tests support the prediction of a shift in cutpoint with S/W. Inlet systems with flow rates of 100 and 400 L/min were designed by Stokes scaling of the 1,250 L/min IRI and integrating the lower flow devices with an existing inlet aspiration section and an insect screen. Experiments with the inlet system were conducted in a wind tunnel with particles from 3 to 20 μm aerodynamic diameter (AD) and wind speeds of 2, 8, and 24 km/h. A nominal cutpoint of approximately 11 μm AD was selected to accommodate bioaerosol sampling needs, and the wind tunnel results showed the average cutpoints of the 100 and 400 L/min inlet systems at the three wind speeds were 11.2 and 11.6 μm AD, respectively. Stand-alone tests with the 100 and 400 L/min IRIs were conducted where dry dusts (Arizona road dust/fine and coarse) were impacted on three types of collection surfaces (dry, grease-coated, and oil-soaked porous surfaces) to characterize solid particle carryover. The oil-soaked porous media allowed the least carryover of large solid particles.     

A Method for Particle Size Amplification by Water Condensation in a Laminar,Thermally Diffusive Flow

A new method is presented for the enlargement of particle size through condensation of water vapor in a laminar, thermally diffusive flow. The method involves the introduction of an air flow at temperature T _i into a wet-walled tube at a temperature T_w > T_i . This approach yields higher supersaturation values than either mixing or cold-walled condensers when operating between the same temperature extremes. Model results for the saturation profiles within the condensing region show that the peak supersaturations are reached along the centerline of the flow, and that the activation efficiency curves are steeper for large temperature differences when the cutpoint diameter is smaller. Experiments conducted with three types of aerosol, oleic acid (a water-insoluble oil), a mixture of oxalic acid and sulfate, and with ambient laboratory aerosol confirmed that condensational growth is achieved with this approach, although experimental cutpoints are somewhat higher than predicted for wettable particles.     

System for Precise Control of Volumetric Flow Rate during Sampling with a Cascade Impactor

Calculations were made with efficiency curves developed for the micro-orifice impactor (MOI) to estimate errors in mass collected on individual stages due to fluctuations in the flow rate during sampling of submicrometer particles. The sizes of these errors depend on the size distribution of the sampled aerosol and the level of flow rate fluctuation. For a log-normally distributed particle population, mass errors due to flow rate fluctuations were bimodally distributed about stages with cutpoints near the aerosol Mass Median Aerodynamic Diameter (MMAD). The largest errors occurred uniformly on the stage with the smallest cutpoint (here, 0.059 w m). These errors were asymmetric with respect to sign, which leads to a net error for a randomly fluctuating flow rate. In general, mass errors increased with decreasing geometric standard deviation ( σ g ) and were substantially greater for populations with 0.5 w m MMADs than for those with 0.2 w m MMADs. The largest net errors for the former were 4, 110, and 560% for σ g of 1.2 and flow rate fluctuations of - 1, - 5, and - 10%, respectively, but decreased to 0.03, 0.9, and 4%, respectively, for a σ g of 1.9. Flow rate fluctuations, therefore, lead to a positive bias in the geometric standard deviation inferred from the measured masses and reduce the user's ability to interpret differences in size distributions. To minimize these effects, we developed and tested a system for controlling the volumetric sampling rate through a MOI at 30 LPM with a precision of 0.06% (600 ms averaging; 0.67% for 5 ms averaging), a level of precision that allows for accurate relative calibration between flow systems and for which errors from flow rate fluctuations are reduced to <1%, even for a very narrow aerosol ( σ g 1.2). Mass errors for an uncontrolled field test were as large as -60%, but these were reduced to <0.22% in a comparable controlled field test. In two replicate tests of the system, agreements between stage masses collected on MOI stage 7 ( D 50 = 0.173 w m) of two simultaneously operated flow-controlled impactors sampling 0.2 w m diameter monodisperse test particles were 0.997 and 0.996, although differences as large as 4% were observed for some stages. The system is suitable for use with standard "Federal Reference Method" samplers.     

Analysis of the efficiency of reverse flow cyclones

The efficiency of a 152 mm cyclone was determined for inlet velocities in the range 6.1 — 15.5 m/s, for particle sizes from 0.6 μm to 4.7 μm, using monodisperse methylene blue-uranine aerosol particles. The efficiency of a geometrically similar 76 mm cyclone was also measured for a few conditions of inlet velocity and particle size. These data and others were correlated by linear regression analyses on a function of cyclone Stokes number and cyclone Reynolds number. It was concluded that in the range of variables spanned by the experimental data, the flow fields within the cyclones depended on Reynolds number, and hence that contrary to the assumptions of most theoretical models, cyclone efficiency is not exclusively a function of Stokes number.     

Influence of Air Pollutants in the 7Be Size Distribution of Atmospheric Aerosols

ABSTRACT

The atmospheric behavior of ⁷Be aerosols was studied by using 1-ACFM cascade impactors. The activity distribution of Be measured by gamma spectrometry (E _γ=477 keV), was largely associated with submicron aerosols in the accumulation mode (0.4–2.0 μm). The activity median aerodynamic diameter, AM AD ranged from 0.62 to 1.00 yam (average 0.80 μm). The geometric standard deviation, σ_g ranged from 1.87 to 2.50 (average 2.22). Low AMADs of ⁷Be aerosols have been observed at locations characterized with relatively low pollution. Some dependency of AMADs on height has been also observed. In near marine environment the ⁷Be activity size distribution was observed in higher size range of aerosol particles (AMAD 0.82 μm).     

Size Measurements of Latex Particles by Laser Aerosol Spectrometer

Size measurements of PSL (polystyrene latex) particles in a size range from 0.109 to 0.330 μm were made by laser aerosol spectrometer (PMS, LAS-X). The results were compared with those by electron microscopy. For example, the geometric standard deviation, σ_g, of nominally 0.176-μm PSL particles was measured as 1.05, assuming that their sizes distribute log-normally. The value of 1.05 was very close to 1.02 measured by electron microscopy. It was found that the spectrometer had very high size resolution, although the size resolution of the light scattering type spectrometer has been said to be poor. For some samples of PSL particles, however, there were large differences between particle sizes measured by LAS-X and those by electron microscopy. It was also found that LAS-X had a problem in calibration of size response curve.     

The Logistic Function and Cyclone Fractional Efficiency

The relationship between fractional efficiency and particle size for a cyclone can be described using a logistic equation: efficiency = 1/(1 + (d ₅₀/d)^β. To use this equation, cyclone cut diameter, d ₅₀, and the logistic slope parameter, β, must be known. Although cyclone cut diameter can be predicted from equations previously published, no method has been available to predict β. Collection efficiency was measured for particles between 1.4 and 7.4 μm in diameter using pilot-scale apparatus to determine the dependence of d ₅₀ and β on cyclone dimensions and operating conditions. An equation for β was developed from a statistical analysis of these data. Although important questions remain concerning the extent to which the p equation can be generalized beyond the experimental conditions for which it was developed, predictions from this equation seem to allow better prediction of fractional efficiency for data presented here and taken from similar experiments in the literature than is found using the efficiency theories of Lapple (1950), Barth (1956), Leith-Licht (1972), or Dietz (1981).     

Analysis of Aerosolized Particulates of Feedyards Located in the Southern High Plains of Texas

The objective of this study was to quantify, size, and examine the composition of particulates found in ambient aerosolized dust of four large feedyards in the Southern High Plains. Ambient air samples (concentration of dust) were collected upwind (background) and downwind of the feedyards. Aerosolized particulate samples were collected using high volume sequential reference ambient air samplers, PM ₁₀ and PM _2.5 , laser strategic aerosol monitors, cyclone air samplers, and biological cascade impactors. Weather parameters were monitored at each feedyard. The overall (main effects and estimable interactions) statistical (P < 0.0001) general linear model statement (GLM) for PM ₁₀ data showed more concentration of dust (μg/m ³ of air) downwind than upwind and more concentration of dust in the summer than in the winter. PM _2.5 concentrations of dust were comparable for 3 of 4 feedyards upwind and downwind, and PM _2.5 concentrations of dust were lower in the winter than in the summer. GLM (P < 0.0001) data for cascade impactor (all aerobic bacteria, Enterococcus spp, and fungi) mean respirable and non-respirable colony forming units (CFU) were 676 ± 74 CFU/m ³ , and 880 ± 119 CFU/m ³ , respectively. The PM ₁₀ geometric mean size (±GSD) of particles were analyzed in aerosols of the feedyards (range 1.782 ± 1.7 μm to 2.02 ± 1.74μm) and PM _2.5 geometric mean size particles were determined (range 0.66 ± 1.76 μm to 0.71 ± 1.71 μm). Three of 4 feedyards were non-compliant for the Environmental Protection Agency (EPA) concentration standard (150 μg/m ³ /24 h) for PM ₁₀ particles. This may be significant because excess dust may have a negative impact on respiratory disease.     

Performance Characteristics of the Aerosol Collectors of the Autonomous Pathogen Detection System (APDS)

This research analyzes the physical performance characteristics of the aerosol collectors of the autonomous pathogen detection system (APDS) that was recently developed by the Lawrence Livermore National Laboratory. The APDS is capable of continuous and fully autonomous monitoring for multiple airborne threat organisms and can be used as part of a monitoring network for urban areas and major public gatherings. The system has already been successfully tested with airborne Bacillus anthracis and Yersinia pestis biowarfare agents. The APDS aerosol collection system consists of a PM-style cap to remove large particles and a low-pressure drop virtual impactor preconcentrator positioned in front of a wetted-wall cyclone. The aerosol collectors operate at flow rates as high as 3750 l/min and collect airborne particles into 4 ml of liquid for subsequent detection. In our tests we determined the overall collection efficiency of the system by measuring the difference between inlet and outlet particle concentrations. The tests were performed with polydisperse oleic acid and monodisperse polystyrene latex (PSL) particles (0.6–3.1 µ m), and for three values of the major air flow rates in the virtual impactor (1760, 2530, and 3300 l/min), two values of the product, or cyclone, flow rates (375 and 450 l/min), and two different volumes of collection liquid (4 and 6 ml). We found that the cutoff size (d₅₀ ) of the entire collection system varied from 1.5 to 2.0 µ m when collecting PSL particles, with 3.1 µ m PSL particles being collected with efficiency of approximately 85%. When collecting oleic acid particles the d₅₀ of the entire system varied from 1.1 to 1.6 µ m. The concentration rates of the aerosol collection system were found to increase with increasing overall collection flow rate and approached one million per minute at the highest tested flowrates. Such high concentrating rates and high air sample volumes make the APDS collection system highly suitable for detecting low concentrations of airborne pathogens.     

Pressure drop and separation efficiency in a flooded hydrocyclone

The pressure drop and separation efficiency were measured of a hydrocyclone operating under flooded underflow or no air-core conditions. The solids were glass spheres with a geometric number average diameter of 13 μm and a geometric standard deviation of 1.69. Water was the fluid. Other fixed variables included the solids feed concentration of 2265 mg/L, the glass hydrocarbon configuration following the optimal Rietema recommendations and the temperature. The variables were feed flowrate 0.44 to 0.63 L.s¹ and volume split of 2.4:1 to 6.1:1; a factorial design was used. The pressure drop, ΔP_s, was 4 to 7 times larger than that expected for air core operation and was correlated in terms of the capacity ratio Q/(ΔP_s)^0.47. The pressure drop was relatively independent of volume split. The separation efficiency was a much stronger function of volume split than of feed flowrate. The D_p(50) diameter at a feed flowrate of 0.63 L.s⁻¹ lies between 4.3 to 9.2 μm; of the seven correlations available for predicting D_p(50), the approach of Rietema (1961) gives the best results. The experimentally measured reduced efficiency was less sharp than that reported by Yoshioka and Hotta (1955) and by the theory of Uematu et al. (1962).     

Eliminating the main source of instability and turbulence in TSI's 3071 DMA

The transfer function of TSI's widely used 3071 Differential Mobility Analyzer (DMA) widens drastically when the flow rate Q of sheath gas exceeds 30–40 lit/min, limiting its ability to resolve very small particles. This flow instability is unexpected at the prevailing relatively small Reynolds number (Re < 400). Here, we note that the rings holding the laminarization screens penetrate into the flow channel, generating unsteady vortices. A screen step exists not only on the outer screen region, but also on the inner screen region. Using a new step-free screen, no critical transition is observed up to the highest flow rate achieved of Q = 103 lit/min. The original DMA widens the flow cross-section in the mixing region where the aerosol joins the sheath gas. The flow deceleration then arising at small aerosol input flow rates introduces another source of transfer function broadening, which, however, has negative resolution effects only at Q > 60 lit/min. This feature is suppressed here by modifying a single inlet piece. Although the two flow improvements implemented increase the resolving power in the analysis of very small particles, a substantial non-ideality of unclear origin remains: the best resolving power R found with electrosprayed ions of the protein Immunoglobulin is R = 13.9 for the trimer (IgG)₃, and 12 for the monomer, even at a sheath/aerosol flow ratio of 100.

© 2017 American Association for Aerosol Research     

Effect of geometric configuration on the collection efficiency of axial flow cyclones

The particle collection efficiencies of axial flow cyclones with eight different geometric configurations, operated at 50 lpm aerosol flowrate, have been evaluated in this study. The geometric variation of test cyclones includes the optional addition of an upside-down cup, two vortex finder lengths, and two cyclone base shapes. Under various configurations, the cutoff aerodynamic particle size of axial flow cyclones changed from 272 to 448 nm. Our study shows that configuration effects on the collection efficiency of axial flow cyclones are different from those of tangential flow cyclones. The observation of different geometric effects on particle collection by axial and tangential flow cyclones is attributed to the flow pattern difference between cyclones of two types. It is further concluded that the optimal configuration for axial flow cyclones is with an abrupt contraction base, without an upside-down cup and with an increased vortex finder length. A simple model combining the model of Leith and Licht (1972) and the tubing loss in 90° bends at high Reynolds numbers has also been proposed to predict the particle collection efficiency curve of the optimal axial flow cyclone among those tested.     

Laboratory and Ambient Particle Density Determinations using Light Scattering in Conjunction with Aerosol Mass Spectrometry

A light scattering module has been integrated into the current AMS instrument. This module provides the simultaneous measurement of vacuum aerodynamic diameter (d _va) and scattered light intensity (R_LS) for all particles sampled by the AMS above ～180 nm geometric diameter. Particle counting statistics and correlated chemical ion signal intensities are obtained for every particle that scatters light. A single calibration curve converts R_LS to an optical diameter (d _o). Using the relationship between d _va and d _o the LS-AMS provides a real-time, per particle measurement of the density of the sampled aerosol particles. The current article is focused on LS-AMS measurements of spherical, non-absorbing aerosol particles. The laboratory characterization of LS-AMS shows that a single calibration curve yields the material density of spherical particles with real refractive indices (n) over a range from 1.41 < n < 1.60 with an accuracy of about ±10%. The density resolution of the current LS-AMS system is also shown to be 10% indicating that externally mixed inorganic/organic aerosol distributions can be resolved. In addition to the single particle measurements of d _va and R_LS, correlated chemical ion signal intensities are obtained with the quadrupole mass spectrometer. A comparison of the particle mass derived from the physical (R_LS and d _va) and chemical measurements provides a consistency check on the performance of the LS-AMS. The ability of the LS-AMS instrument to measure the density of ambient aerosol particles is demonstrated with sample results obtained during the Northeast Air Quality Study (NEAQS) in the summer of 2004.     

Numerical Performance Simulation of a Wetted Wall Bioaerosol Sampling Cyclone

Computational fluid dynamics techniques are used to study the performance of an axial flow bioaerosol sampling cyclone that continuously collects particles onto a flowing liquid film. A special shell-volume concept was developed to study formation and development of the liquid film on the inner wall of the cyclone. For a previous version of the cyclone, simulations demonstrated the presence of a ring of liquid in the region just upstream of the liquid skimmer that was suspected of causing liquid carryover into the exhaust air stream and degradation in aerosol collection efficiency. This ring was eliminated by re-design of the cyclone. For the upgraded version of the cyclone, CFD was used to successfully predict aerosol collection efficiency and cyclone pressure drop. The simulations reveal a complex flow evolution inside the cyclone. Stream-tubes are used to describe a significant narrowing of the width of the airflow as it enters the cyclone and an inward displacement of the flow as it travels in the axial direction. The particle deposition occurs primarily in a region that is subtended approximately by the length of the rectangular entrance slot and the first half turn of the flow in the cyclone. Cutpoint Stokes number is about 0.05 and the cutpoint particle size is about 1 μm aerodynamic diameter. At a flow rate of 1250 L/min, the pressure drop across the cyclone is 5.6 kPa (22 inches of water).     

Development of a Multi-Stage Axial Flow Cyclone

A prototype multi-stage cyclone system consisting of an impaction inlet and five axial flow cyclone stages has been developed to classify simulants of Lunar and Martian dusts for various research and development needs of NASA's space exploration missions. Individual axial flow cyclone stages can be either independently operated with an inline connection to other particle devices or cascaded together for particle separation and collection. The impaction inlet and first three cyclone stages were designed to operate at the flowrate of 50 lpm under pressure close to ambient. The last two cyclone stages were designed to operate under low pressure conditions to separate particles with diameters less than 200 nm. Due to the limited vacuum capacity of the pump used, the flowrates of last two cyclone stages were restricted to 11.0 and 1.0 lpm when operating the assembled prototype. The impaction inlet and each cyclone stage of the prototype were experimentally calibrated, and the cutoff particle sizes were 11.3 μm, 0.97 μm, 550 nm, 255 nm, 109 nm, and 40 nm.

It was further found that in general the flow Reynolds (Re) and particle Stokes numbers (StK) were critical parameters to characterize the performance of the axial flow cyclone stages, and the relationship between Re and the dimensionless cutoff size (√StK was established. In addition, the collection efficiency curves are shifted to a smaller size range with a decrease of the cyclone pressure. However, using √StK as the abscissa and keeping the same Re, the particle collection curves at different pressures can be merged into one. This study also found that the upstream pressure should be used to calculate StK instead of the average of upstream and downstream pressures of the test cyclone stage.     

A Versatile Flat-Deposit Impactor-Type Aerosol Collector Part 2: Calibration and Quantitative Study

The SPAL collector is a versatile impactor-type aerosol collector with rotatable substrate plates and radially aligned slot orifices. The airflow, controlled with a critical orifice at the last stage, becomes choked for an exit pressure of 0.5 atm or less. Monodisperse latex and methylene blue aerosols sampled on coated and uncoated aluminum foil substrates were used to determine the particle cutpoint diameters of each stage (9.0-0.07 mu m aed), loss on walls (3-7% for 0.054 mu m particles), and overall efficiency (97% from 0.3 to 7 mu m). Quantitative measurements were made with a spectrophotometer after recovering aerosols with a water and surfactant solution and with a semiautomated particle counting system using phase contrast microscopy. The experimental cutpoints and mea sured interstage pressures compare well with theory.     

CFD Study on the Effect of Hydrocyclone Structure on the Separation Efficiency of Fine Particles

Abstract

Effects of geometric structure parameters of 10 mm-diameter hydrocyclones on the particle separation efficiency are studied using computational fluid dynamics (CFD). The fluid velocity profiles and particle trajectories are simulated using RFLOW software with a standard isotropic k-ε turbulent model. The JIS standard CaCO₃-17 particles are adopted as a particulate sample in simulations and experiments. Comparing the simulated results with experimental data, a maximum deviation about 20% in partition curves occurs for 5–10 µm particles. However, fairly good agreements for the cut-size predictions and the fish-hook phenomenon are obtained. The simulated cut-size d ₅₀ is only 2 µm larger than that measured in experiments, while the value of d ₁₀₀ can be accurately predicted. An increase in overflow diameter or a decrease in underflow diameter leads to a lower separation efficiency but a clearer separation sharpness due to lower fluid underflow rate. A short-and-wide rectangular inlet is more efficient for particle separation than a tall-and-narrow one. An inclined inlet conduit plays an inessential role on the efficiency improvement but gains a 2 µm reduction in d ₁₀₀. Comparing the simulated results, the hydrocyclone used in the experiments of this study exhibits a higher separation sharpness than the Rietema type and a higher efficiency than the Bradley type based on the same operation capacity and hydrocyclone size.     

DIMENSIONLESS REPRESENTATION OF PARTICLE SEPARATION CHARACTERISTIC OF CYCLONES

In particular the collection efficiencies were measured as a function of flow rate, cyclone dimensions and particle size. For this purpose a fast, accurate and problem adapted measuring technique has been used, which enables the determination of grade efficiency curves by measuring the size distributions in the cyclone up- and downstream with optical particle counters. The extended experimental data from this parameter study were analysed by the methods of dimensional analysis and theory of models. An evaluation of all measuring results for two cyclone designs has been resulted in an empirical, nondimensional correlation of the collection characteristic, a dimensionless grade efficiency curve. Deviating from geometric similarity this correlation includes a variation of cyclone outlet diameter. Grade efficiencies of the cyclones are a definite function of the dimensionless numbers Stokes and Reynolds number and of the dimensionless cyclone outlet diameter. Analysis of own and published data has shown that this experimental correlation includes the influence of the temperature and that cyclone body diameter do not influence efficiency. The influence of cyclone height on flow behaviour and collection characteristic could be quantified as well. The range, in which prediction of collection efficiencies is possible, is marked in a state diagram Reynolds number versus dimensionless cyclone height.