| Summary: | 碩士 === 國立臺灣大學 === 職業醫學與工業衛生研究所 === 92 === Due to the partial charging effect, the collection efficiency of an ESP tends to decrease with decreasing particle size. In other words, the collection efficiencies in terms of number density for nanoparticles of an ESP may be relatively low, although high mass collection efficiency is well achieved by a conventional ESP. The main objective of this study was to search the right types of filter foams that could efficiently collect those fugitive uncharged nanoparticles. In order to conduct the aerosol penetration tests of filter foams, a constant-output aerosol atomizer was used to generate challenge aerosol particles in the size range of 7 to 100 nm. A scanning mobility particle sizer (SMPS 3085) was used to measure the aerosol concentrations upstream and downstream of the ESP unit and/or the filter foams. Among the operation parameters were the foam porosity, foam solidity, foam thickness and filtration velocity.
The results showed that aerosol penetration through filter foams decreased with increasing foam porosity, apparently due shorter interstitial distance and the more surface area for aerosol deposition by diffusion. Aerosol penetration increased with increasing filtration velocity due to shorter retention time within the filter foam. The effect of foam packing density on aerosol penetration was very similar to foam porosity (fiber diameter) because higher packing density means more filter materials and therefore, more surface area for aerosol deposition. To take into account the air resistance together with aerosol penetration, we found that low porosity, low packing and low filtration velocity resulted in higher filter quality factor. The ESP unit tested in this work had a designed flow rate of 100 L/min. For particles smaller than the most penetrating size (about 0.3 �慆), the aerosol penetration through ESP decreased with decreasing aerosol size until the size reached about 15 nm (19%). Aerosol penetration of particles smaller than 15 nm increased due to partial charging. Filter foam (110 ppi, packing density of 0.04 and thickness of 25.4 mm) removed most of the fugitive ESP nanoparticles (penetration from 19% down to 2.5%). The air resistance induced by the ESP was almost negligible. Therefore, ESP is superior to the filter foams from the perspective of filter quality, even for small particles with partial charging effect.
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