Michael S. Parsons

High-efficiency particulate air filter test stand and aerosol generator for particle loading studies

This manuscript describes the design, characterization, and operational range of a test stand and high-output aerosol generator developed to evaluate the performance of 30 x 30 x 29 cm(3) nuclear grade high-efficiency particulate air (HEPA) filters under variable, highly controlled conditions. The test stand system is operable at volumetric flow rates ranging from 1.5 to 12 standard m(3)/min. Relative humidity levels are controllable from 5%-90% and the temperature of the aerosol stream is variable from ambient to 150 degrees C. Test aerosols are produced through spray drying source material solutions that are introduced into a heated stainless steel evaporation chamber through an air-atomizing nozzle. Regulation of the particle size distribution of the aerosol challenge is achieved by varying source solution concentrations and through the use of a postgeneration cyclone. The aerosol generation system is unique in that it facilitates the testing of standard HEPA filters at and beyond rated media velocities by consistently providing, into a nominal flow of 7 standard m(3)/min, high mass concentrations (approximately 25 mg/m(3)) of dry aerosol streams having count mean diameters centered near the most penetrating particle size for HEPA filters (120-160 nm). Aerosol streams that have been generated and characterized include those derived from various concentrations of KCl, NaCl, and sucrose solutions. Additionally, a water insoluble aerosol stream in which the solid component is predominantly iron (III) has been produced. Multiple ports are available on the test stand for making simultaneous aerosol measurements upstream and downstream of the test filter. Types of filter performance related studies that can be performed using this test stand system include filter lifetime studies, filtering efficiency testing, media velocity testing, evaluations under high mass loading and high humidity conditions, and determination of the downstream particle size distributions.

Evaluation of the Effect of Media Velocity on Filter Efficiency and Most Penetrating Particle Size of Nuclear Grade High-Efficiency Particulate Air Filters

High-efficiency particulate air (HEPA) filters are widely used to control particulate matter emissions from processes that involve management or treatment of radioactive materials. Section FC of the American Society of Mechanical Engineers AG-1 Code on Nuclear Air and Gas Treatment currently restricts media velocity to a maximum of 2.5 cm/sec in any application where this standard is invoked. There is some desire to eliminate or increase this media velocity limit. A concern is that increasing media velocity will result in higher emissions of ultrafine particles; thus, it is unlikely that higher media velocities will be allowed without data to demonstrate the effect of media velocity on removal of ultrafine particles. In this study, the performance of nuclear grade HEPA filters, with respect to filter efficiency and most penetrating particle size, was evaluated as a function of media velocity. Deep-pleat nuclear grade HEPA filters (31 cm × 31 cm × 29 cm) were evaluated at media velocities ranging from 2.0 to 4.5 cm/sec using a potassium chloride aerosol challenge having a particle size distribution centered near the HEPA filter most penetrating particle size. Filters were challenged under two distinct mass loading rate regimes through the use of or exclusion of a 3 μ m aerodynamic diameter cut point cyclone. Filter efficiency and most penetrating particle size measurements were made throughout the duration of filter testing. Filter efficiency measured at the onset of aerosol challenge was noted to decrease with increasing media velocity, with values ranging from 99.999 to 99.977%. The filter most penetrating particle size recorded at the onset of testing was noted to decrease slightly as media velocity was increased and was typically in the range of 110–130 nm. Although additional testing is needed, these findings indicate that filters operating at media velocities up to 4.5 cm/sec will meet or exceed current filter efficiency requirements. Additionally, increased emission of ultrafine particles is seemingly negligible.

Evaluating Performance of High Efficiency Mist Eliminators

Processing liquid wastes frequently generates off gas streams with high humidity and liquid aerosols. Droplet laden air streams can be produced from tank mixing or sparging and processes such as reforming or evaporative volume reduction. Unfortunately these wet air streams represent a genuine threat to HEPA filters. High efficiency mist eliminators (HEME) are one option for removal of liquid aerosols with high dissolved or suspended solids content.HEMEs have been used extensively in industrial applications, however they have not seen widespread use in the nuclear industry. Filtering efficiency data along with loading curves are not readily available for these units and data that exist are not easily translated to operational parameters in liquid waste treatment plants.A specialized test stand has been developed to evaluate the performance of HEME elements under use conditions of a US DOE facility. HEME elements were tested at three volumetric flow rates using aerosols produced from an iron-rich waste surrogate. The challenge aerosol included submicron particles produced from Laskin nozzles and super micron particles produced from a hollow cone spray nozzle. Test conditions included ambient temperature and relative humidities greater than 95%.Data collected during testing HEME elements from three different manufacturers included volumetric flow rate, differential temperature across the filter housing, downstream relative humidity, and differential pressure (dP) across the filter element. Filter challenge was discontinued at three intermediate dPs and the filter to allow determining filter efficiency using dioctyl phthalate and then with dry surrogate aerosols. Filtering efficiencies of the clean HEME, the clean HEME loaded with water, and the HEME at maximum dP were also collected using the two test aerosols.Results of the testing included differential pressure vs. time loading curves for the nine elements tested along with the mass of moisture and solid material on each element at final dP. Plots of overall filtering efficiencies for DOP (spherical aerosol) and dry surrogate (aspherical aerosols) at specified dPs were computed for each filter. Filtering efficiencies were determined as a function of particle size. Curves were also generated showing the most penetrating particle size as a function of dP.A preliminary set of tests was conducted to evaluate spray location, duration, pressure, and wash volume for in-place cleaning the interior surface (reducing dP) of the HEME element. A variety of nozzle designs were evaluated and test results demonstrated the potential to overload the HEME (saturate filter medium) resulting in very high dPs and extensive drain times. At least one combination of spray nozzle design, spray location on the surface of the element, and spray time/pressure was successful in achieving extension of operational life.© 2013 ASME

Lifecycle Testing Results of Dimple Pleat Radial Flow HEPA Filters Under Ambient and Elevated Conditions

The Institute for Clean Energy Technology (ICET) at Mississippi State University (MSU) has developed testing capabilities for evaluating the American Society of Mechanical Engineers (ASME) Code on Nuclear Air and Gas Treatment (AG-1) section FK radial flow HEPA filters. These high efficiency particulate air (HEPA) filters are typically used at sites within the DOE complex to control particulate matter emissions. The goal of this testing was to provide information related to the performance of radial flow HEPA filters that use dimple pleat separators. Prior to this testing, insufficient data were available to determine the lifecycle performance of these filters. Two different types of dimple pleated radial flow HEPA filters were tested: a safe change design and a remote change design. Filters were tested at both ambient and elevated conditions of temperature and relative humidity. The challenge aerosols were Alumina (Al(OH)3), Carbon black, and Arizona road dust. The performance of these filters at ambient conditions was determined by the filters’ mass loading capacity and filtering efficiency. Elevated condition performance was evaluated by the filters’ resistance to elevated temperature and relative humidity. The results indicated that the challenge aerosol had a large impact on the total loading capacity of the filters. Testing at elevated conditions of temperature and relative humidity revealed an issue with the strength of the dimple pleats. The dimples softened when challenged with elevated conditions, which led to filter failure due to a cascading reduction in free flow area and increase in differential pressure. Copyright 2013 American Association for Aerosol Research

Large-scale generic test stand for testing of multiple configurations of air filters utilizing a range of particle size distributions.

The Institute for Clean Energy Technology (ICET) at Mississippi State University has developed a test stand capable of lifecycle testing of high efficiency particulate air filters and other filters specified in American Society of Mechanical Engineers Code on Nuclear Air and Gas Treatment (AG-1) filters. The test stand is currently equipped to test AG-1 Section FK radial flow filters, and expansion is currently underway to increase testing capabilities for other types of AG-1 filters. The test stand is capable of producing differential pressures of 12.45 kPa (50 in. w.c.) at volumetric air flow rates up to 113.3 m(3)/min (4000 CFM). Testing is performed at elevated and ambient conditions for temperature and relative humidity. Current testing utilizes three challenge aerosols: carbon black, alumina, and Arizona road dust (A1-Ultrafine). Each aerosol has a different mass median diameter to test loading over a wide range of particles sizes. The test stand is designed to monitor and maintain relative humidity and temperature to required specifications. Instrumentation is implemented on the upstream and downstream sections of the test stand as well as on the filter housing itself. Representative data are presented herein illustrating the test stands capabilities. Digital images of the filter pack collected during and after testing is displayed after the representative data are discussed. In conclusion, the ICET test stand with AG-1 filter testing capabilities has been developed and hurdles such as test parameter stability and design flexibility overcome.

A test stand for the evaluation of high efficiency mist eliminators

High efficiency mist eliminators (HEME) are airstream filtering elements primarily used to remove liquid and solid aerosols. HEME elements are designed to reduce aerosol load on downstream high efficiency particulate air filters and to have a liquid particle removal efficiency of 99.5% for aerosols as small as 1 μm in size. The test stand described herein is designed to evaluate the loading capacity and filtering efficiency of a single HEME element. The loading capacity was determined with or without use of a water spray cleaning system to wash the interior surface of the element. The HEME element is challenged with a liquid waste surrogate using Laskin nozzles and large dispersion nozzles. The waste surrogate used was a highly caustic solution with both suspended and dissolved solids representative of actual exposures at mixed, hazardous, and radiological, waste treatment facilities. The filtering efficiency performance was determined by challenging the element with a dried waste surrogate aerosol and di-octyl phthalate intermittently during the loading process. Capabilities of the test stand and representative results obtained during testing are presented.

Factors Influencing the Performance and Lifetime of Fibrous Glass and Metal Media HEPA Filters

High efficiency particulate air (HEPA) filters are used in a variety of nuclear applications as final air treatment units. The design of air filtration systems in nuclear facilities that will function well requires a significant amount of knowledge about the challenge conditions that these filters will be exposed to. Additionally, risk assessments conducted as a review of these systems need also to be based on knowledge of filter challenge parameters during upset conditions that may be used as design basis conditions. This paper presents a summary of findings of factors that influence the performance lifetime of conventional fibrous glass media filters. These factors include aerosol challenge (particle size distributions and mass concentrations), media velocities, wetting conditions, and changes in gas density. These data are correlated to design considerations for new systems and to process upset conditions used in risk assessments, particularly those involving fires. Data from filter testing activities are also compared to filter loading models and to literature information regarding aerosol emission rates from combustion of various materials. Additional data are provided relating the performance of metal media HEPA filters under conditions that exceed maximum ranges for fibrous glass filters. These discussions focus on selection factors between these two categories of filter units.Copyright

HEPA Filter Performance Under Adverse Conditions

This study involved challenging nuclear grade high-efficiency particulate air (HEPA) filters under a variety of conditions that can arise in Department of Energy (DOE) applications such as: low or high RH, controlled and uncontrolled challenge, and filters with physically damaged media or seals (i.e., leaks). Reported findings correlate filter function as measured by traditional differential pressure techniques in comparison with simultaneous instrumental determination of up and down stream PM concentrations. Additionally, emission rates and failure signatures will be discussed for filters that have either failed or exceeded their usable lifetime. Significant findings from this effort include the use of thermocouples up and down stream of the filter housing to detect the presence of moisture. Also demonstrated in the moisture challenge series of tests is the effect of repeated wetting of the filter. This produces a phenomenon referred to as transient failure before the tensile strength of the media weakens to the point of physical failure. An evaluation of the effect of particle size distribution of the challenge aerosol on loading capacity of filters is also included. Results for soot and two size distributions of KCl are reported. Loading capacities for filters ranged from approximately 70 g of soot to nearly 900 g for the larger particle size distribution of KCl.Copyright

Evaluation of the Effects of Media Velocity on HEPA Filter Performance

Section FC of the ASME AG-1 Code addresses glass fiber HEPA filters and restricts the media velocity to a maximum of 2.54 cm/s (5 ft/min). Advances in filter media technology allow glass fiber HEPA filters to function at significantly higher velocities and still achieve HEPA performance. However, diffusional capture of particles < 100 nm is reduced at higher media velocities due to shorter residence times within the media matrix. Therefore, it is unlikely that higher media velocities for HEPA filters will be allowed without data to demonstrate the effect of media velocity on removal of particles in the smaller size classes. In order to address this issue, static testing has been conducted to generate performance related data and a range of dynamic testing has provided data regarding filter lifetimes, loading characteristics, changes in filter efficiency and the most penetrating particle size over time. Testing was conducted using 31 cm × 31 cm × 29 cm deep pleat HEPA filters supplied from two manufacturers. Testing was conducted at media velocities ranging from 2.0–4.5 cm/s with a solid aerosol challenge composed of potassium chloride. Two set of media velocity data were obtained for each filter type. In one set of evaluations, the maximum aerosol challenge particle size was limited to 3 μm, while particles above 3 μm were not constrained in the second set. This provided for considerable variability in the challenge mass mean diameter and overall mass loading rate. Results of this testing will be provided to the ASME AG-1 FC Committee for consideration in future versions of the HEPA standard. In general, the initial filter efficiency decreased with increasing media velocity. However, initial filter efficiencies were generally good in all cases. Filter efficiency values averaged over the first ten minute of the loading cycle ranged from 99.970 to 99.996%. Additionally, the most penetrating particle size was observed to decrease with increasing media velocity, with initial values ranging from 194 to 134 nm.Copyright