10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Measurement of the Fractional Deposition Efficiency of Full Scale HVAC and HEPA Filters for Nanoparticles ≥ 4 nm
CHRISTOF ASBACH, Tobias Schuldt, Frank Schmidt, Wolfgang Mölter-Siemens, Ana Maria Todea, IUTA, Duisburg, Germany
Abstract Number: 734 Working Group: Indoor Aerosols
Abstract Concerns regarding adverse effects of nanoparticles, e.g. concerning human health upon inhalation, or product quality upon deposition on clean production equipment or the eventual product, such as semiconductor chips, have increasingly been raised over the recent years. Consequently, these particles have to be removed from the supply air for e. g. residential or office buildings or cleanrooms. It is well known from filtration theory that such small particles are efficiently captured in filters because of their high diffusivity. Methods for determining the fractional deposition efficiency are only available and well established for filter media, whereas no such method has existed up to now for full scale HVAC filters. This is mainly due to the lack of suitable particle generators that provide such small particles in sufficient concentrations to test the filters at flow rates of up to several thousand cubic meters per hour. The high efficiency of filters for nanoparticles necessitates very high upstream concentrations in order to still be able to measure the downstream particle size distribution. If the particles are (quasi-) monodisperse, then it would also suffice to only measure the total particle concentration downstream, which can be done even for very low concentrations.
In the present work, we used a flame based nanoparticle generator (FG2, MoTec Konzepte, Bochum, Germany), in which a sodium chloride solution is continuously fed into a hydrogen-oxygen flame, where it evaporates. Upon cooling, the sodium chloride vapor nucleates to produce very small particles at very high concentrations. The so-produced aerosol is mixed with the filtered air in a test rig according to EN 779/ISO 16890 (also comparable to ASHRAE 52.2), in which the test filter is installed. The modal particle diameter can be adjusted between approximately 4 nm and 70 nm via the sodium chloride concentration in the solution, the feed rate of the solution, as well as the residence time of the freshly produced aerosol in the injection system before it mixes with the high flow rate. The generated aerosols are quasi-monodisperse (according to VDI 3491-1) with geometric standard deviations mostly between 1.3 and 1.5 and particle concentrations upstream of the filter in the range of 1e6 1/cm³ to 1e7 1/cm³. To extend the tested size range up to 100 nm or to increase the monodispersity of the evaluated NaCl particles, the aerosol particles in the flow sampled from the test rig can be classified with a differential mobility analyzer (DMA) at a fixed voltage. If information on the collection efficiency for particles >100 nm is desired, a DEHS test aerosol can be used. A correction procedure to account for multiply charged particles upon classification with a DMA has been developed and successfully tested. Several new and artificially aged filters ranging from F7 (according to EN 779, comparable with MERV 13 according to ASHRAE 52.2) to H13 (according to EN 1822) have been tested using the newly developed method and set up. The results typically compare well with those obtained from media samples. The experimental set up and test procedure will be presented along with the data from the filter evaluations.