Page 70 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

P1: FWD Revised Pages
Encyclopedia of Physical Science and Technology En001c-14 May 7, 2001 18:25

Aerosols 293

In the following years, a variety of ring-slit centrifuges culating the number of particles of a given size deposited
of the conifuge concept as well as the first spiral duct per unit area or unit filter length, as the sample depends on
centrifuge were built and tested. A comparison of the per- flow rate, porosity per hole or filter diameter, temperature,
formance tests of these devices indicated that from almost pressure, presence of condensation or chemically reactive
all practical viewpoints the concept of the spinning spiral vapors, electrical fields, and so on. The overall filter col-
duct was superior to the other designs. lection efficiency, combined with the pressure drop or flow
The theoretical basis of the cylindrical centrifuge is a resistance, is a crucial characterization parameter for the
straightforward application of force balance on particles in selection of appropriate filters for air purification.
the annulus. If the centrifugal force acting on the particle Particles are deposited from a gas layer adjacent to
is constant, the length from the entrance where a particle the substrate. Deposition takes place by convective dif-
of given size is deposited is proportional to the aerosol fusion and interception. Thus, the complex pattern of flow
flow rate, but inversely proportional to rotation speed and through a filter becomes a key to calculating its efficiency.
the square of the particle radius. These relationships are In principle, one calculates the flow through a fibrous fil-
borne out by deposition experiments using particles of ter in terms of a superposition of flow around a cylindrical
known radius and density. array, taking into account the mutual interactions between
fibers using the packing density.
The character of flow through a fibrous mat can be seen
4. Diffusion and Filtration
by examining the drag force on a unit fiber length in terms
Collection on porous filter media is perhaps the most ef- of pressure drop across the filter.
ficient means of particle removal. Aerosol filtration is an The superposition of electrostatic forces on particle be-
effective means of air purification, while at the same time havior near a filter mat can have appreciable influence on
it has been widely used for sampling airborne material for filtration efficiency. The deposition patterns can take on
mass and chemical composition determination. A wide significant treeing or branching of agglomerates on indi-
variety of filter media is available, ranging from fibrous vidual fibers. This aerodynamically distorts the cylindrical
mats of relatively inert material to porous membranes. collector surface and branches the surface area, as well as
Fibrous mats and model filter arrays appear microscopi- distorting the electrical field around the collector.
cally as stacks of overlaid cylinders, where the cylinders For air-monitoring purposes, gravimetric measures of
may be smooth or rough. In contrast, the membrane me- total mass concentration from filters, combined with
dia are plastic films with microscopic holes of nearly uni- chemical assessment, generally require a relatively large
form size; nuclepore filters, for example, are produced of amount of sample. Also, as will be seen later, separate
sheetsofpolyester,andtheholesareintroducedbyneutron samples free of influence of chemical interactions during
bombardment. collection are of interest. A device for monitoring appli-
Fibrous filters are the most economical and effective de- cations was developed in the 1970s that improves on the
vices for the purification of air from suspended particles. high-volume sampler. The device is called dichotomous
This purification is achieved with minimal loss of pump- sampler. It collects particulate material in two size groups,
ing energy associated with flow resistance, compared with between 2 and 5 µm diameter and less than 2 µm diameter.
other types of filters. The porosity of such materials is 85– Segregation of very large particles (>10 µm) is readily
2
99%, and fiber diameter varies from 10 to 10 −2 µm. achieved by design of an inlet shroud, which restricts entry
The advantage of membrane filters is that particles do of particles larger than 10 µm diameter. Separation of the
not become imbedded in the filter medium. Thus, individ- coarse and finely divided particles is achieved by a method
ual particles are readily identifiable and characterized mi- called virtual impaction. In principle, this method avoids
croscopically on the filter surfaces. Furthermore, certain such difficulties as particle bounce-off, re-entrainment,
kinds of chemical analysis, such as X-ray fluorescence and nonuniform deposition. In addition, it provides a sep-
analysis, readily can be done in situ with minimal effects aration of large and small particles such that they cannot
of filter interference on the membrane substrates. chemically interact with one another after collection on a
Sampling devices range from simple filter holders to substrate. This sophistication in sampling is important for
sequential configurations for automated routine air moni- characterizing chemically unstable particles in air.
toring of many samples in series. Membrane filters can be Virtual impaction uses the principle of inertial separa-
obtained in different pore sizes, so that they can be used tion, but the impaction plate is replaced by a zone of rel-
in series as particle size fractionators. atively stagnant air below the nozzle. The virtual surface
The theory of filtration is a direct application of prin- formed by deflecting streamlines gives separation condi-
ciples of Brownian diffusion discussed previously. The tions similar to those in conventional impactors. Large
objective of the theory is to provide a framework for cal- particles travel straight through into the low-flow region,