Page 69 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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

292 Aerosols

the resolution of the mobility analyzer itself. This requires, possible to pass the impaction plate through the particle-
however, that the fraction of aerosol carrying unit charge laden air (external flow). The effectiveness of particle col-
be known precisely. Aerosol concentration measurement lection in the latter arrangement is comparable to that of
using electrical effects requires a method of detecting the conventional impactors. In operation, these impactors nor-
charged aerosol. This is usually done by measurement mally consist of impaction plates (or cylinders) mounted at
of an electrical current on a grounded collector with at- the ends of rotating arms. As the arms are rotated through
tachment and charge transfer of a particle. In addition, the air, particles are impacted onto the collection surface.
electrical sizing methods require a precipitator or classi- The size of the particles collected depends on the speed
fier by which the particles of different electric mobilities and width (or diameter) of the impaction surface as well as
are separated before detection. Various approaches have the size and density of the particles. These devices can be
been discussed, including condenser ion counters, denud- used to collect particles larger than 10–20 µm in diameter.
ers, and ion capture devices. Thus, for the collection of large particles, which may be
difficult to sample efficiently in a conventional impactor,
this type of impactor is a suitable alternative.
2. Inertial Impaction
In the methods discussed so far, continuous observations
3. Centrifugation
in terms of particle size have been involved, giving de-
tailed information on the particle concentration–size dis- The deposition of particles can be achieved by introducing
tribution but limited detail on particle morphology. An external forces normal to the flow of an aerosol. This is
important requirement for aerosol experimentation is the basically the principle of size separation devices employ-
ability to sample and collect particles with size segrega- ing centrifugal forces acting on the particles. Two types
tion. One such method of sampling uses the variation of of particle samplers have emerged in this group. The first
inertial impaction with mass (or size). Devices that have are cyclones, which are passive in nature, inducing spin-
been designed for this purpose are called impactors. They ning air motion and forcing particles to move outward to
operate on the idea that a large particle tends to collide a collection surface. The second are centrifuges in which
with a surface when particle-laden air is directed to a sur- air is spun mechanically, causing particle migration and
face, while small particles follow the gas flowing around deposition on the outer walls of the device.
the collector. In a typical device, the air is forced through a Experimental investigations to determine the aerody-
converging nozzle and ejected onto a plate oriented normal namic equivalent particle size for nonspherical particles
to the gas flow. The gas streamlines bend sharply inside, led to the design of centrifugal instruments to resolve in-
while particles with sufficient inertia hit the plate. The ba- dividual submicron particle deposition by the influence of
sic design parameters of the impactor are the nozzle throat an external force. The first aerosol particle size spectrome-
diameter or width and the distance from the nozzle exit ter actually providing a continuous size spectrum in terms
plane to the plate. of aerodynamic diameters was built in 1950 by Sawyer
By operating several impactor stages at different flow and Walton. Their centrifugal device, called a conifuge,
conditions, one can classify the aerosol particles into sev- deposited the particles according to their aerodynamic
eral size ranges from which the size distribution is deter- diameter in a size range between 0.5 and 30 µm on the
mined. These single stages can be operated in a parallel or outer wall of a rotating conical annular duct.
in a series arrangement. In the parallel flow arrangement, In reviewing the situation of centrifugal size spectrom-
each of the stages classifies the airborne particles at dif- etry and after assessing the limitations of a semidisper-
ferent cutoff sizes, so that the difference in the amount of sive, cone-shaped, helical-duct, aerosol centrifuge, work-
the deposit on any two stages gives the quantity of parti- ers suggested that the performance of the conifuge-type
cles in the particular size interval defined by the respective size spectrometers could be improved by employing ring-
cutoff sizes of the two stages. In the series arrangement, slit aerosol entrances in modified designs featuring slen-
also known as the cascade impactor, the aerosol stream der cones or cylindrical annular ducts. It was anticipated
is passed from stage to stage with continually increasing that ring-slit aerosol inlets would permit increased sam-
velocities and decreasing particle cutoff sizes. Thus, each pling rates as desired for many practical purposes. The
stage acts as a differential size classifier. Of the two flow cylindrical design would have the additional benefitoffa-
systems, the cascade arrangement is by far the most pop- cilitating an exact theoretical performance evaluation. An
ular, as is evident from the large number of commercial actual instrument of the latter kind was subsequently built
cascade impactors currently available. in 1968 by Berner and Reichelt. They showed that the ex-
In the conventional impactor, the jet is formed in a noz- perimental deposit patterns did, in fact, follow theoretical
zle (internal flow) and then impacts onto a plate. It is also predictions.