Environmental Engineering Reference
In-Depth Information
17.7  NONSTATIONARY FILTRATION AND OPTIMIZATION PROBLEMS
The dificulties of principal character at solving the problems of determination of a pressure drop
Δ p growth, eficiency growth, and estimation of time expectancy before clogging of a ilter with
solid particles are related to the necessity of taking into account numerous additional factors and,
irst of all, those related to variation of the low ield while accumulating sediment. An approach
to the solution of this problem proved to be possible for the irst time due to approximating a dust-
covered iber to a cylinder with a co-axial porous permeable membrane [82,93]. The distribution of
deposited particles through the ilter depth and the reverse inluence of a deposit growing on ibers
on the low ield in the proximity of the ibers is taken into account in the proposed approach. Such
an approach is used for calculation of pre-ilters [94,141].
A correlation of packing density of a deposit layer on the ilter surface and radius of particles is
obtained for calculation of surface clogging of a high-eficiency ilter [95]. It is demonstrated that
a 3D layer of submicron particles building at low velocity U = 1 cm/s on the front surface of a ilter
has a high-porous structure with the maximal packing density equal to 0.15. Approaches based on
ilters clogging kinetic research are proposed for optimizing the ilter parameters in a two-stage air
puriication system composed of a pre-ilter and a inishing high-eficiency ilter [94,96,97].
In conclusion, we note that different attempts are being continuously made to solve the problem
of creating high-eficiency ilters with low resistance to the low on the basis of nanoibers with
special coniguration. For example, it is experimentally demonstrated in Ref. [98] that “nano-mous-
tache” of nanotubes developed on ilter ibers noticeably increase ilter eficiency at a relatively low
increase of its resistance. Quantitative theory of iltration with such ilters was developed earlier
in the work [93], where it is demonstrated that quality criterion of a loose ilter may considerably
increase at the initial stage if to coat ibers with a permeable porous shell consisting, for example,
of ultrathin “whiskers.” Such iltering materials alongside with nanoiber membranes may ind wide
application in industry and in analytical iltration of dust-loaded gases.
17.8  ANALYTICAL FILTRATION
The theory of air sampling by ilters with a purpose to analyze aerosol particles is one of the most
important directions of the iltration theory that incorporates evaluation of particle capture com-
pleteness at a ilter and that of separation of particles using ilters depending on ilter parameters and
sampling conditions. We have mentioned previously a diffusive method of particles' determination.
In this section we shall consider the traditional approach to sampling description based on deposi-
tion particles on ibers of a ilter due to their inertia.
The high rate iltration is poorly known theoretically until now despite the wide use of analyti-
cal ilters. There are a number of experimental studies of iltering materials eficiency in inertial
mode of particles capture [12,99-103], including by FP iltering materials [103-106]. Equations for
calculating inertial capture factors are obtained from experiments with real and model ilters but
one can acknowledge full absence of universal functional connections for a wide range of structural
characteristics, velocities, densities, and sizes of aerosol particles. It is worth noting that iguring out
of a character of inertial deposition dependence on Stokes numbers under low conditions at Re ∼ 1
is required for justiication of the evaluation of aerodynamic diameter of radioactive particles by the
multilayer ilter method (MFM) [107].
Inertia deposition of aerosol particles onto the cylindrical obstacle is one of the earliest study
trends in the area of aerosols mechanics. Applied to the theory of iltration, the inertia deposition
was being studied for long time on the basis of the Lamb low ield in the vicinity of a separate iber
at low Reynolds numbers. Later, the Kuwabara [31] cell model with the Stokes low ield [25] was
used for considering neighboring ibers inluence. Experimental and theoretical (mainly numerical)
research was carried out for low low velocity that conforms to the Stokes low condition, Re << 1
[42]. In this case the pressure drop in the ilter is strictly linearly dependent on the low velocity.
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