Filtration and Sampling of Aerosols by Fibrous Filters - Aerosols: Measurement, Dosimetry, and Health Effects - page 442

This value is dimensional [ L ] and depends on the iltration conditions. The higher the value of q f , the
better the ilter suits various conditions of air puriication.
The major task of the theory of gas iltration is to calculate the capture coeficient for a iber of a
given radius r p , depending on velocity U , viscosity μ, temperature Т , and pressure Р of air, existence
of external forces F i , electrical ield, ilter parameters—packing density α, mean iber diameter
2 a and their dispersion σ, ilter internal structure:

(17.5)

η η = ( r , ρ , U , μ , T P F a , , , , α σ ε , , )

p i

where
ρ is the density of particles
ε is the parameter of inhomogeneity, characterizing the ilter structure

Besides, the capture coeficient depends both on the particle shape, its electrostatic charge q ,
its dielectric permittivity and on charges present on the ibers, their dielectric permittivity, and
the ibers' cross-sectional shape. In nonstationary iltration (deposit accumulates on ibers) the
capture coeficient depends on the number of deposited particles and the deposit packing density
as well.

17.3 AEROSOL PARTICLE CAPTURE MECHANISMS IN FIBROUS FILTERS
There exist several physical mechanisms of aerosol particle capturing by ilter ibers. First, it should
be mentioned that the so-called “sieve” mechanism (screening particles larger than the ilter cell) is
not the main mechanism gas of particle capturing.
For low iltration velocity diffusive capture by iber ilters dominates. Aerosol particles do not
move along streamlines lowing round an obstacle (in our case, the iber) but become displaced
from them due to incessant collisions with gas molecules (Figure 17.2). The smaller the size of the
particle and its velocity, the higher the probability of collision. The diffusive capture coeficient can
be more than 1.
If a particle moves along a streamline lowing in the immediate vicinity of a iber, then the
increase of the particle size leads to its capture probability increase. This mechanism is called the
“interception effect.”
Large and heavy particles can be displaced from the streamline due to its own inertia. The
probability of collision of aerosol particle with ibers depends on the Stokes number (St) and the
Reynolds (Re) number. There exists the critical value of St crit , below which the inertial deposition
does not appear [27].
For cylindrical ibers St crit ≤ 0.25 [28,29]. Particle capture due to inertia is the dominant
mechanism of micron and submicron aerosols capturing during high velocity iltration. Regularities
of inertia deposition are used for aerosol particle fractioning according to their dimensions (see the
following).
If a particle and a ilter iber are charged, then the probability of the particle capture due to the
Coulomb interaction increases. In some iltering installations, particles are intentionally charged in
the corona ield in order to increase iltration eficiency (in the atmosphere the charges on the ine
particle are distributed according to the Boltzmann law and, as a rule, their amount does not exceed
1-2 elementary charges). For example, iltering material ibers, obtained by electrospinning, are
being charged directly while producing. They are widely used, therefore, in disposable means for
respiratory system personal protection [20].
Gravity affects aerosol particle deposition at low velocity of air through ilter. This effect is
substantial for heavy particles and can be most clearly observed during iltration from “below
to up” or from “up to bottom.” In the irst case the integrate capture coeficient is increasing,