Filtration and Sampling of Aerosols by Fibrous Filters - Aerosols: Measurement, Dosimetry, and Health Effects - page 459

1

0.8

0.6

0.4

0.2

0

0.1 1 10 100

v (m/s)

FIGURE 17.13 Effect of rebound of aerosol particles on iltration characteristics of a ibrous ilter: Quality
factor ( q f ) versus velocity of particles ( v ). Mean iber diameter is 7 μm, packing density = 0.03, particle
diameter = 0.8 μm. (From Skitovich, V.I. et al., IPG Works, 1976, 21, 8.)

The ibers of the iltering material FPA-70 are made of cellulose diacetate. Hydrodynamic iber
diameter is 5.5 μm and packing density of the iltering material is 0.03. Fan-type models are made
of cylinder tungsten wire with diameter 11.1 μm. The packing density of model #1 is 0.01 and of
model #2 is 0.025.
A critical velocity V crit was found. The exceedance of this velocity leads to increase of penetra-
tion through the fan-type model and the iber ilter and, thus, to reduction in their capture eficiency
by the cylinder. Values of V crit obtained on models #1 and #2 coincide. This means that the lattice
packing density does not affect the rebound process, at least at α < 0.025. Experimental results are
presented in Table 17.2.
As it can be seen from the data presented in Table 17.2 values of V crit shift to lower magnitudes
with the increase of the particle size. The relationship between V crit and r is well described by the
equation

−0 .

V

kp =

Cr

where the constant С = 1.6, if the particle radius r is given in microns and V crit in m/s.
It is easy to notice that the relationships for the cellulose diacetate ibers with hydrodynamic
diameter of 5.5 μm are the same as for the tungsten wire with diameter 11.1 μm. However, the con-
stant С equals 0.78, which is less than for the grids. Obviously, the difference in the constants is
caused by different diameters and material of the cylinder grids and ilter ibers.

TABLE 17.2
Critical Velocities (m/s) for Models 1 and
2 and for Filtering Material FPA-70

Particle Radius ( μ m) 0.36 0.51 0.77 0.98

Models I and II 3.20 2.55 1.90 1.35

Filter FPA-70 1.60 1.25 0.95 0.80