Clogging of Industrial High Efficiency Particulate Air (HEPA) Filters in Case of Fire: From Analytical to Large-Scale Experiments

The Institute of Radioprotection and Nuclear Safety (IRSN in French) is conducting research on the impact of a fire on the behaviour of containment devices such as high efficiency particulate air (HEPA) pleated filters for radioactive materials. This work aims to study the clogging of HEPA filters in case of fire involving realistic materials (polymers making up gloves boxes, waste treatment solvent, hydraulic oil, solid material mixtures making up a trash bin, electrical cables, and cabinets) used in nuclear facilities, from the medium to large scale. The clogging kinetics of industrial pleated HEPA filters is monitored by measuring the pressure drop of the filters and the filtered air temperature at a given filtration velocity (from 0.23 to 2.1 cm/s). Upstream HEPA filters, combustion aerosols are characterized in terms of size distribution, mass concentration, composition, and particle morphology using, respectively, a DMS500 (Cambustion^LTD), glass fiber filter sampling, and transmission electron microscope analysis of particles deposited on TEM grids. Particles emitted denote well-known fractal morphology, are composed of carbonaceous primary particles with diameters ranging from 31 nm to 48 nm and showing an high clogging efficiency. An empirical relationship has been successfully applied to the obtained results for a larger range of fuels, filtration velocities and fire conditions.

Finally, experiments have been performed on a large-scale facility, using full-scale fire scenarios (electrical cabinet, constant, and variable filtration velocity) and a reasonable agreement was observed with our empirical relationship. At this scale, particles appear to be compact, with a complex composition and diameters close to 220 nm with a lower clogging efficiency.

Copyright 2014 American Association for Aerosol Research     

Modeling the pressure drop behavior of cleanable dust filters during pressure-controlled operation

Cleanable dust filter media are typically used in huge baghouse filter apparatuses. Thereby, the regeneration by back-pulsing from the clean gas side is done by either time-controlled or pressure-controlled operation, whereas the latter is more common. Hence, the need for a detailed knowledge of the clogging and filtration mechanisms during long time operation of a pressure-controlled filter aging arises.

A mathematical model describing the pressure drop evolution during time-controlled filter aging has been developed. The core of the developed model is the concept of dust masses that distribute themselves on a specific particle deposition area inside and on the surface of the filter medium. By altering this particle deposition area, various clogging mechanisms, occurring during an aging procedure, are covered by the model.

In this work, the model was adapted and coefficient parameters adjusted for pressure-controlled filter regeneration operation. A multitude of pressure-controlled test runs were performed in a specially designed filtration apparatus. From these tests, process-specific parameters were regressed and used to model the respective pressure drop curves. These model pressure drop curves show good accordance both quantitatively and qualitatively to experimental data and give a detail view on different clogging mechanisms.     

An integrative model for the filtration efficiencies in realistic tests with consideration of the filtration velocity profile and challenging particle size distribution

Many well-established models can be applied to calculate the filtration efficiencies. In these models the filtration velocity and challenging particle size are assumed to be known accurately. However, in realistic filtration tests, the filtration velocity has profiles dependent on the filter holder geometry and experimental conditions; the challenging particles have size distributions dependent on the instruments and operation conditions. These factors can potentially affect the measured filtration efficiency and lead to discrepancies with the models.

This study aims to develop an integrative model to predict the filtration efficiencies in realistic tests by incorporating the effects of the filtration velocity profile and challenging particle size distribution classified by a differential mobility analyzer (DMA) into the existing filtration models. Face velocity profile is modeled with fluid mechanics simulations; the initial generated particle size distribution, the particle charging status and the DMA transfer function are modeled to obtain the challenging particle size distribution. These results are then fed into the filtration models. Simulated results are compared with experimental ones to verify the model accuracy. This model can be used to reduce filtration test artifacts and to improve the experimental procedure.

The results reveal that the face velocity upstream the filter exhibits high degree of homogeneity not affecting the filtration efficiency if the filter pressure drop is not very low. The generated particle size distribution and the DMA selection size window could influence the challenging particle size distribution and therefore the measured filtration efficiency.

Copyright © 2017 American Association for Aerosol Research     

Comparative Testing and Evaluation of Nine Different Air Samplers: End-to-End Sampling Efficiencies as Specific Performance Measurements for Bioaerosol Applications

Accurate exposure assessments are needed to evaluate health hazards caused by airborne microorganisms and require air samplers that efficiently capture representative samples. This highlights the need for samplers with well-defined performance characteristics. While generic aerosol performance measurements are fundamental to evaluate/compare samplers, the added complexity caused by the diversity of microorganisms, especially in combination with cultivation-based analysis methods, may render such measurements inadequate to assess suitability for bioaerosols. Specific performance measurements that take into account the end-to-end sampling process, targeted bioaerosol and analysis method could help guide selection of air samplers.

Nine different samplers (impactors/impingers/cyclones/ electrostatic precipitators/filtration samplers) were subjected to comparative performance testing in this work. Their end-to-end cultivation-based biological sampling efficiencies (BSEs) and PCR-/microscopy-based physical sampling efficiencies (PSEs) relative to a reference sampler (BioSampler) were determined for gram-negative and gram-positive vegetative bacteria, bacterial spores, and viruses.

Significant differences were revealed among the samplers and shown to depend on the bioaerosol's stress–sensitivity and particle size. Samplers employing dry collection had lower BSEs for stress-sensitive bioaerosols than wet collection methods, while nonfilter-based samplers showed reduced PSEs for 1 μm compared to 4 μm bioaerosols. Several samplers were shown to underestimate bioaerosol concentration levels relative to the BioSampler due to having lower sampling efficiencies, although they generally obtained samples that were more concentrated due to having higher concentration factors.

Our work may help increase user awareness about important performance criteria for bioaerosol sampling, which could contribute to methodological harmonization/standardization and result in more reliable exposure assessments for airborne pathogens and other bioaerosols of interest.

Copyright 2014 American Association for Aerosol Research     

Modeling of nonstationary processes during separation of multicomponent isotope mixtures

The article presents a mathematical model for calculation of nonstationary hydraulic and separation processes in a gas centrifuge (GC) cascade for separation of multicomponent isotope mixtures. The model has been applied to calculate the parameters of nonstationary processes in a GC cascade for separation of krypton, germanium and tungsten isotopes. As a result, the specifics of the excess holdup distribution along the cascade stages has been identified, and variations of the isotope concentrations in a nonstationary process have been revealed. The data obtained show that the proposed mathematical model is able to adequately describe nonstationary hydraulic processes in GC cascades for separation of multicomponent isotope mixtures.

Highlights:

Mathematical model of cascade for separation of multicomponent isotope mixture has been developed.

The model verification has been done.

The isotope transient regularities into cascade during nonstationary processes has been identified.     

The Use of Reverse Osmosis in the Removal of PAHs from Municipal Landfill Leachate

Polycyclic aromatic hydrocarbons (PAHs) are relatively well-known organic pollutants and due to their carcinogenic and mutagenic properties their presence in the environment still attracts a lot of attention.

According to literature reports and own research, PAHs presence in wastewaters is common. It was confirmed that PAHs are the components of municipal landfill leachate. Membrane techniques are one of the most interesting ways of removing PAHs from leachate.

The purpose of this article is to monitor PAHs concentration changes during the membrane (reverse osmosis - RO) leachate treatment processes. In the first stage of testing leachates were filtrated on the sand bed (pre-filtration). After the pre-filtration they were directed to the membrane module for the main filtration.

Sixteen PAHs listed by EPA were analyzed. The results with information on PAHs concentration in leachate samples were presented using HPLC with fluorescence detection (FLD). The changes in PAHs concentration were determined in leachate samples before and after pre-filtration as well as after RO. The decrease of PAHs concentration in the samples was observed after these processes. The total concentration of 16 PAHs in raw municipal landfill leachates amounted to 23.64–26.95 μg/L. The research confirmed the high efficiency in removal of PAHs while using a reverse osmosis (59–72%). Including the pre-filtration, the overall level of removed PAHs reached 81–86%. The average PAHs concentration after pre-filtration and RO was in the 4.46–4.99 μg/L range. The municipal landfill leachate with a high concentration of PAHs should be cleaned before it is discharged into the environment.     

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     

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     

Ozone Treatment in Cooling Water Systems

Ozone treatment for preventing the biofouling in cooling water systems is investigated.

In the fresh water system, the separating effect of the ozonated water on the microorganisms such as the sphaerotilus and the zoogloea which adhere to the piping and form the slime is recognized. When the ozonated water is supplied intermittently to the piping without stopping the flow of the cooling water, a constant volume of cooling water can be maintained. At the velocity of 1 m/sec, the amount of metal corrosion produced by the ozonated water is reduced on the mild steel, increased on the copper and does not change on the cast iron, when compared with that produced by the water containing no ozone.

In the seawater system, since many substances are oxidized by the ozone, the same treatment as that in the fresh water system cannot be applied. However, if the seawater in the cooling system can be replaced with ozone-containing air intermittently once a week, the adhesion of organisms such as barnacles and mussels to the piping can be prevented without having a bad influence on the other living oceanic organisms.     

Immunological Examinations in Patients with Chronic Conditions under Administration of Ozone/Oxygen Mixtures

In four groups of patients with chronic conditions receiving an ozone application (intravascular administration of ozone/oxygen mixtures with or without autohemotherapy), the immunoglobulins IgA, IgG and IgM were determined in serum; in groups 1 and 2, lysozyme and vitamin A values also were measured. The groups were broken down as follows:

Group 1: 19 patients with carcinoma of the cervix. Group 2: 20 patients with carcinoma of the ovaries. Group 3: patients with chronic conditions (80% patients with rheumatoid arthritis), shortterm study (34 patients). Group 4: patients with chronic conditions (80% patients with rheumatoid arthritis), long–term study (47 patients). A further breakdown into statistical categories in groups 1 and 2 showed no significant changes in the immunoglobulins examined, including the vitamin A and lysozyme values.

A short-term influence in group 3 became evident through a significant increase in immunoglobulin G, whereas no significant decrease was observed in all immunoglobulins in group 4. An immunosuppressive effect of the ozone applied could not be detected in any of the 4 groups.     

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     

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     

Effect of different application techniques on microshear bond strength of self-etch adhesives to dentin

Objectives: To investigate the effect of different self-etch adhesive systems application techniques: active or passive in a single or double layer on adhesive–dentin microshear bond strength.

Methods: Occlusal surfaces of 48 extracted human molars were ground to expose flat superficial dentin surfaces. Specimens were randomly divided into two main groups according to the tested self-etch adhesive system either: One-step self-etch (Adper^TM easy-one) or two-step self-etch (Adper^TM SE Plus). Each adhesive system was applied on the prepared dentin surfaces followed one of these techniques: (1) Passive application of a single layer, (2) Active application of single layer, (3) Passive application of double adhesive layer (with light curing in between), and (4) Active application of double adhesive layers. Resin composite was packed inside micro-tubes fixed on the bonded dentin surfaces and light cured for 40 s. All specimens were stored in artificial saliva either for 24 h or 3 months before testing. Microshear bond strength test was employed using a universal testing machine at a crosshead speed of 0.5 mm/min.

Results: Adper^TM SE Plus showed higher significant microshear bond strength in compared with Adper^TM easy-one. For both adhesive systems active application showed higher significant microshear bond strength to dentin than passive application. Double application of adhesive systems showed lower microshear bond strength than single application.

Conclusion: Active application of self-etch adhesives could improve the dentin microshear bond strength. Double application with curing in between the layers did not improve the bond strength to the tested adhesive.     

Effect of desensitizers on the microleakage of previously restored Class V resin composite restorations

Objectives: The aim of this in vitro study was to evaluate the effect of different desensitizers’ application on the microleakage of previously restored Class V composite resin restorations.

Materials and methods: Class V cavities were prepared on the buccal surfaces of 40 extracted human third molars. Forty box-shaped cavities were divided into four groups, based on the desensitizers used (n = 10). All teeth were restored with the same bonding agent and composite material. No desensitizer was applied in the control group. In the experimental groups, BisBlock, Gluma and Universal bonding agents were the desensitizers. The desensitizers were applied after completion of composite restorations according to manufacturers’ instructions. All specimens were then thermocycled at 5–55 °C, with a 10-s dwell time for 500 cycles. The samples were then immersed in 0.5% methylene blue dye for 24 h, sectioned into two equal halves, evaluated for microleakage using a stereomicroscope at 30× magnification and scored on a scale of 0–3. The data were analysed using the Kruskal–Wallis test at the significance level p < 0.05.

Results: There were no significant differences in microleakage after desensitizer application (p > 0.05). However, based on the obtained numerical values in our study, while the BisBlock and bonding groups showed lower microleakage at the occlusal margin, BisBlock, Gluma and bonding group showed lower microleakage at the gingival margin compared to the control group.

Conclusions: The application of desensitizers as a post-treatment option could be considered an advisable procedure to minimize microleakage.     

Do phosphoric acid esters affect the immediate enamel bond strengths?

Purpose: To investigate the effects of phosphoric acid esters (PAEs)-containing primers on the micro-tensile bond strengths (MTBS) of etched enamel, the micro-morphologies of the resin–enamel interfaces, and the enamel surfaces.

Materials and Methods: Thirty-three bovine incisors were used in this study. After the labial enamel surfaces were highly polished, they were etched, water sprayed, and air-dried. Afterward, the enamel surfaces were treated with or without (control) one of three PAEs-containing primers (40%MDP, Ivoclar Vivadent; Resulcin AquaPrime A + B, DMG; Xeno V, Dentsply), water sprayed, and air-dried. Subsequently, an adhesive Heliobond (Ivoclar Vivadent) was applied and resin composite (Core Build-up, Bisco) was placed incrementally. They were prepared into multiple beams of about 1 × 1 × 8 mm for MTBS tests. The enamel surfaces and resin–enamel interfaces were analyzed by SEM and TEM. The data were analyzed by two-way ANOVA and LSD test.

Results: Using PAEs-containing primers could increase the enamel MTBS (p < 0.05). The primer MDP produced higher MTBS than the primer Resulcin AquaPrime A + B and Xeno V. The SEM findings revealed the primed enamel surfaces were covered with a thin or glue-like layer of monomer-calcium salts of PAEs and the residual enamel crystallites, and various micro-porosities were detected within the hybrid layers. The TEM findings revealed the hydroxyapatite crystallites were tightly covered by the adhesive, the adhesive mixed with resin composite, or sparse irregular enamel crystallites.

Conclusion: The application of PAEs-containing primers on the etched enamel substrate could significantly increase the immediate enamel bond strengths. However, this effect was dependent on the individual PAEs-containing primer used.     

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     

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     

Opto-aerodynamic focusing of aerosol particles

We describe a new method for focusing and concentrating a stream of moving micron-sized aerosol particles in air. The focusing and concentrating process is carried out by the combined drag force and optical force that is generated by a double-layer co-axial nozzle and a focused doughnut-shaped hollow laser beam, respectively. This method should supply a new tool for aerosol science and related research.

Copyright © 2018 American Association for Aerosol Research     

Aerosol Measurement by Induced Currents

We introduce a new electrical measurement technique for aerosol detection, based on pulsed unipolar charging followed by a non-contact measurement of the rate of change of the aerosol space charge in a Faraday cage. This technique, which we call “aerosol measurement with induced currents,” has some advantages compared to the traditional method of collecting the charged particles on either an electrode or with a particle filter. We describe the method and illustrate it with a simple and miniature (shirt-pocket-sized) instrument to measure lung-deposited surface area. Aerosol measurement by induced currents can also be applied to more complex devices.

Copyright 2014 American Association for Aerosol Research