PARTICULATE EMISSIONS 829
monitored. Passage of a particle through the orifice momen-
tarily reduces current to an extent determined by particle
size. The device electronically counts the number of par-
ticles in each of several size ranges, and a size distribution
can then be calculated. The method is capable of giving very
good results, and newer model counters are very fast.
A novel liquid phase sedimentation analyzer is the
Sedigraph (Micrometrics Instrument Corporation, Norcross,
Georgia). The particle sample dispersed in liquid is put into a
sample cell and allowed to settle. Mass concentration is con-
tinuously monitored be attenuation of an X-ray beam, and
this is mathematically related to particle size, X-ray location
and time. The instrument automatically plots particle diam-
eter as cumulative weight percent. The device can cover the
size range from 0.1–10 μ in a single operation, a much wider
range than can be conveniently analyzed by most analyzers.
Laser optics techniques relying on light scattering,
Fraunhofer diffraction, or light extinction are becoming
the method of choice in many applications. The Leeds and
Northrop “Microtrac” and Malvern Instruments Co. laser par-
ticle and droplet sizer are representative of such techniques.
These Instruments can measure particles in a flowing gas
stream, and thus can theoretically be used on line. More often a
collected particulate sample is dispersed in liquid for analysis.
Impingement devices such as the Anderson Impactor, or
in the impactors developed by May or Batelle, may be used
to measure particle sizes in situ in a combined sampling and
sizing operation. As is shown in Figure 11 such a device con-
sists of a series of orifices arranged to give gas jets of increas-
ing velocity and decreasing diameter, which jets impingeon collection plates, Successive stages collect smaller and
smaller particles, and the size distribution of aspirated par-
ticles can be obtained from the weight collected on each stage
and the size “cut point” calibration of the stage. Several stud-
ies of calibrations have been published, 33 - 37 and discrepancies
have been pointed out.^38 Impactors must be operated at con-
stant known gas flow rate and for this reason are not capable
of giving true isokinetic sampling under conditions of fluc-
tuating duct velocity. This is one of the few types of devices
which may be applied to liquid droplets, which coalesce once
collected. It is capable of size determination well below 1 μ
(finer than most devices). Because it eliminates recovery
of particles from a filter and subsequent handling, it can be
useful in measuring distributions at low concentrations.
Most particles emitted to the atmosphere are approxi-
mately spherical so that the exact meaning of “diameter”
is not usually important in the context. For highly irregular
particles a great many different diameters many be defined,
each with particular applications. For purposes of particulate
control equipment, persistence of airborne dusts, and physi-
ological retention in the respiratory tract, the most meaning-
ful diameter is usually the “aerodynamic” diameter, that of
the sphere having the same free fall velocity as the particle
of interest. This is the diameter measured by sedimentation,
eleutriation, and inertial impaction techniques.
A large number of methods are available for expressing
particle size distributions, each having properties of fitting
certain characteristic distribution shapes or of simplifying(^112)
COLLECTION
CUP
SPRING
JET SPINDLE
GASKET
FIGURE 11 Stage of typical cascade impactor
(Monsanto).
FIGURE 10 Air classifier for subsieve particle size
analysis.
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