832PARTICULATE REMOVAL
TYPES AND CHARACTERISTICS OF CLEANING
EQUIPMENTThree principal considerations enter into the selection of
particulate removal equipment. Cleaning efficiency must be
sufficient to meet the desired particulate emission levels, but
efficiency much beyond this lower limit is usually unwar-
ranted. Equipment operating characteristics, such as tem-
perature and pressure drop limitations, must be compatible
with the specific process application. Economic factors such
as purchase and operating cost, delivery time, and reliability
must also be considered.
Cleaning performance of specific particulate removal
equipment is perhaps most usefully expressed in the form
of a grade efficiency curve. This is the relationship of dust
particle size (or a related property such as free-fall veloc-
ity) versus corresponding collection efficiency for that size
particle. Overall collection efficiency is obtained by integrat-
ing the product of weight fraction increment times collection
efficiency at that particle size over the cumulative weight
fraction from 0 to 1. Since collection efficiency is highly
dependent on particle size, this approach permits calculation
of overall efficiency for any particle size distribution of dust
entering the device. This is a more useful way of expressing
performance than is the use of adjectives such as high effi-
ciency or low efficiency.
Representative grade efficiency curves are shown in
Figure 1. The figure should not be used for design pur-
poses, because for a given type of equipment the exact
placement of a curve depends on design characteristics and
operating conditions. The figure is useful for preliminary
screening and evaluation of relative merit of different types
of collectors.
Extensive coverage of operating principles and design
methodology is given by Stern,^1 Brauer and Varma,^2 Theodore
and Buonicore,^3 and Licht.^4 More recent comprehensive treat-
ments have included those by Cooper and Alley^5 , Heumann^6
and Schneller^7. Manufacturers literature and specifications
have been conveniently collected^8 for many types of particu-
late collectors. The recent NAPCA study^9 is perhaps the most
comprehensive attempt at a direct comparison of different
types of equipment from the standpoint not only of opera-
tion and efficiency, but also of economics. This chapter will
attempt to survey the features, performance, and selection of
some of the major types of equipment in current pollution
control applications.CyclonesCyclones are one of the cheaper and simpler dust collectors
available, but they have a relatively low efficiency unless
used with coarse dust. A basic cyclone is shown in Figure 2.
Dusty gas enters the tangential inlet, typically at velocities of
50–100 ft/sec. Gas flows in a helical path, first downward in an
annulus and then upward in the center, passing out the top of
the cyclone through the outlet pipe. Particles are driven to the
cyclone walls by centrifugal forces on the order of 100 g and
exit from the dust outlet at the bottom of the cyclone cone.
In many applications cyclone alone can give satisfac-
tory particulate removal. For more stringent requirements
cyclones are often used in series with other equipment with
complementary characteristics. For example, cyclones can
operate at almost any dust loading that a gas stream can
carry and become increasingly efficient at higher loadings
so that they are useful as pre-cleaners for equipment such as
filters that would be choked by a high dust loading. Cyclone
efficiency increases with gas throughput in direct contrast to
precipitators, and thus they may serve to damp out effects of
changing operating conditions.
Cyclone Performance The conceptual simplicity of
cyclone operation has encouraged considerable theoretical
analysis of particle collection efficiency. The usual approach
is to calculate the acceleration the particle experiences based
on inlet velocity and cyclone barrel diameter, and compute
a terminal velocity of particles in a radial direction based
on Stokes Law. A maximum distance a particle must travel
before being collected at the cyclone wall is selected (usu-
ally the width of the cyclone inlet), and a gas residence time
is calculated. The result is that one can calculate a critical
particle diameter, the minimum size that can, according to
such oversimplified theory, be collected at 100% efficiency
as well as efficiencies for smaller parties. Stern^10 has com-
pared several such calculations. 11 – 17 A more recent review by
Strauss^18 has included references. 19 – 23
All of the above mentioned equations employ extremely
simplified models of gas flow. They assume instantaneous
particle acceleration and no interaction of particles as well
as absence of non-idealities such as particle re-entrainment.
For such reasons this purely theoretical approach has not been
very successful in predicting quantitative cyclone efficiencies.
It gives, however, an acceptable basis for scaling of an experi-
mental efficiency to other operating conditions and somewhat
different sizes and geometries of cyclone. A semi-empiricalC016_002_r03.indd 832C016_002_r03.indd 832 11/18/2005 1:06:45 PM11/18/2005 1:06:45 P