Determination of the fractional deposition efficiency of full scale HVAC and HEPA filters for nanoparticles ≥ 4 nm

C. Asbach*, W. Mölter-Siemens, A.M. Todea, T. Institut für Energie- und Umwelttechnik e. V. (IUTA); T. Schuldt, F. Schmidt, University Duisburg-Essen, Germany

Nanoparticles can be of concern for human health and product quality. They consequently 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. While methods for determining the fractional deposition efficiency of filter media are available and well established, no such method has existed up to now for full scale HVAC filters. This is mainly due to the lack of a suitable particle generator that provides 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, 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 106 cm-3 to 107 cm-3. To extend the tested size range up to 100 nm or to increase the monodispersity of the evaluated particles, the aerosol particles in the flow sampled from the test rig can be classified with a differential mobility analyzer at a fixed voltage. Several new and artificially aged filters ranging from F7 (according to EN779) to H13 (according to DIN EN 1822) have been tested with this setup (see Figure 1). In most cases, the results 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...

Session: G17 - HVAC-Systems
Day: 15 March 2018
Time: 14:45 - 16:00 h

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