Filtration and Sampling of Aerosols by Fibrous Filters - Aerosols: Measurement, Dosimetry, and Health Effects - page 461

TABLE 17.4
Filtering Belt Characteristics

Penetration Index by
0.15 μ m Particles
at 120 cm/s (%)

Penetration Index by
0.34 μ m Particles
at 170 cm/s (%)

Grade of
Analytical Belt

Average Pore
Diameter ( μ m)

Resistance
at 1 cm/s (Pa)

Millipore FSLW 1.5 110 0.01 0.001

LFAS-2 2.0 20 10 5

LFAS-4 1.1 40 0.1 0.05

LFS-2 analytical iltering belt is developed mainly for use in radiometric and spectrometric
instruments for measuring of volumetric activity and nuclide composition of radioactive aerosols.
LFS-2 belt consists of two ibrous layers. The upper (front) iltering layer consists of ultrathin ibers
with diameter of 0.3-0.5 μm. The lower layer fulills the role of a substrate (space frame) and con-
sists of glued to each other ibers having diameter of 5-7 μm. The operation layer is additionally
covered with thick ibers to enable its ixing to the substrate.
Ultrathin ibers of the main iltering layer build up a structure having package density of
0.07-0.1. One can consider the ibrous structure as a porous layer having an average size of pores
of 2.0 μm.
LFS-2, NEL-3, and NEL-4 belts were developed about 40 years ago but until now remain reli-
able and effective means of capturing and monitoring of radioactive aerosols of different origin.
Nevertheless, their relatively low effectiveness in relation to particles of the size range of 100-
200 nm at iltration rates of about 1 m/s does not allow reliable evaluation of volumetric activity of
aerosols containing particles of that size.
Aerosol particles containing daughter products of radon it exactly into that size range. Such
radionuclides as iodine and ruthenium can be discovered at the smallest particles [118]. A new
iltering material LFAS-2 is developed for the analysis of such aerosols and upgraded ilters AFA-
RSP as well as pilot belts LFAS-2 are developed on its base [117]. LFAS-2 differs from LFS-2 by
the presence of thinner ibers (0.1-0.5 μm) used for manufacture of a front iltering layer. Apart
from that, one managed to avoid using of thick ibers for ixing the front layer to the space frame by
implementing a special process method. LFAS-4 belt contains more of thinner ibers, which brings
about enhanced effectiveness of particle capturing.
As is evident from Table 17.4 the LFAS-4 belt practically equals to one of the best analytical belts
FSLW of Millipore Company by its iltering characteristics [119] but its hydrodynamic resistance is
essentially lower (Figure 17.14).

17.11 USING FIBROUS FILTERS FOR DETERMINATION
OF PARTICLE SIZE DISTRIBUTION
The problem of determination of aerosol size distributions is solved by different methods based
on the use of correlation of the measured physical property with the size of a particle. Widely
known are precise but time-consuming analytical methods under optical or electronic micros-
copy usage. But it is dificult to determine which chemical elements or materials are carried
by particles of this or that size if the aerosol was originated from several sources. The identi-
cal problem appears when using particle size distribution instruments based on light diffusion
[120]. Impactors [121] mostly often used for analysis of particle size distribution of aerosols have
relatively low productivity of sampling. That is why they are not so useful for the cases when
concentration of the analyzed aerosols is extremely low in spite of their relatively high precision
of distribution by size fractions.