| Summary: | 碩士 === 國立臺灣大學 === 職業醫學與工業衛生研究所 === 85 === Under the new NIOSH certification system, the filter will be
classified as Type I, 95% efficient, Type II, 99% efficient, or
Type III, 99.97% efficient against solid (N), liquid (P), or
both solid and liquid (R) particulates. The maximum initial
resistance shall not exceed 35 mm H2O at the flow rate of 85 L/
min during inhalation. The particle size distribution of the
test aerosols shall have a smaller count median diameter and
geometric standard deviation than that of the European standard.
In addition to reviewing the filter test protocols, this study
also examined the filtration mechanisms, in theory, and tested
ten filter media forfiltration efficiency and air resistance. In
order to conduct the filter performance testing, a constant-
output aerosol nebulizer was used to generate submicrometer-
sized particles; a ultrasonic atomizing nozzle wasused to
generate micrometer-sized particles; a 10-mCi Krypton 85
radiation source was used to neutralize the particles to
Boltzmann charge equilibrium; a scanning mobility particle sizer
(SMPS) was used to measure the aerosol penetration for particles
smaller than 0.8 micrometer; an aerosizer was used for measuring
particle larger than 0.8 micrometer. The results showed
thatthere is still a gap between the filtration theory and the
experiment Under normal circumstances, the air flow
resistance across the filter media increases with increasing
mass loading. At the same time, the captured particles cause the
filtration efficiency and filter quality to change. The increase
in air flow resistance depends not only on the filter media
characteristics such as type, porosity, fiber diameter,
thickness and pore size, but also on the nature of the loading
dust. To study solid particle filter loading in the interception
and impaction capture regime, the loading characteristics of
five different types of filter media are tested with three
different size distributions of PST particles. In order to
better define the process of filter loading with solid aerosols,
the nature of the loading curve must be studied first. These
data may be helpful for consumers to select the proper filter
media. The plots of pressure drop against sampling time display
three regions: an initial region of slow increase, a transition
region and a final linear region after dust formation point. The
flow resistance across the media increase with increasing
fraction of fine particles. For the filter with larger pore
size, there are pinholes together with the dust cake. It affects
both the filtration and air flow resistance characteristics of
the dust cake. In principle, the loading curves should parallel
to each other after dust formation point. However, the results
showed that some of the loading curves meet and cross. This
phenomenon needs to be studied further.
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