Quantcast Chapter 6. Cyclones and Multicyclones

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TM 5-815-1/AFR 19-6
be handled and high collection efficiencies are needed
a multiple of small diameter cyclones are usually
The cyclone is a widely used type of particulate collec-
nested together to form a multicyclone. A unit of this
tion device in which dust-laden gas enters tangentially
type consists of a large number of elements joined
into a cylindrical or conical chamber and leaves
together with a common inlet plenum, a common
through a central opening. The resulting vortex motion
outlet plenum, and a common dust hopper. The
or spiraling gas flow pattern creates a strong
multicyclone elements are usually characterized by
centrifugal force field in which dust particles, by virtue
having a small diameter and having axial type inlet
of their inertia, separate from the carrier gas stream.
vanes. Their performance may be hampered by poor
They then migrate along the cyclone walls by gas flow
gas distribution to each element, fouling of the small
and gravity and fall into a storage receiver. In a boiler
diameter dust outlet, and air leakage or back flow from
or incinerator installation this particulate is composed
the dust bin into the cyclones. These problems are
of fly-ash and unburned combustibles such as wood
offset by the advantage of the multicyclone's increased
char. Two widely used cyclones are illustrated in figure
collection efficiency over the single high efficiency
cyclone unit. Problems can be reduced with proper
plenum and dust discharge design. A typical fractional
Cyclone types
efficiency curve for multi-cyclones is illustrated in
a. Cyclones are generally classified according to
figure 6-6.
their gas inlet design and dust discharge design, their
e. Wet or irrigated cyclone. Cyclones may be oper-
gas handling capacity and collection efficiency, and
ated wet in order to improve efficiency and prevent
their arrangement. Figure 6-2 illustrates the various
wall buildup or fouling (See fig. 6-7). Efficiency is
types of gas flow and dust discharge configurations
higher for this type of operation because dust particles,
employed in cyclone units. Cyclone classification is
once separated, are trapped in a liquid film on the
illustrated in table 6-1.
cyclone walls and are not easily re-entrained. Water is
b. Conventional cyclone. The most commonly used
usually sprayed at the rate of 5 to 15 gallons per 1,000
cyclone is the medium efficiency, high gas throughput
cubic feet (ft3) of gas. Wet operation has the additional
(conventional) cyclone. Typical dimensions are illus-
advantages of reducing cyclone erosion and allowing
trated in figure 6-3. Cyclones of this type are used
the hopper to be placed remote from the cyclones. If
primarily to collect coarse particles when collection
acids or corrosive gases are handled, wet operation
efficiency and space requirements are not a major con-
may result in increased corrosion. In this case, a
sideration. Collection efficiency for conventional
corrosion resistant lining may be needed. Re-
cyclones on 10 micron particles is generally 50 to 80
entrainment caused by high values of tangential wall
velocity or accumulation of liquid at the dust outlet can
c. High efficiency cyclone. When high collection
occur in wet operation. However, this problem can be
efficiency (80-95 percent) is a primary consideration in
eliminated by proper cyclone operation. Wet operation
cyclone selection, the high efficiency single cyclone is
is not currently a common procedure for boilers and
commonly used (See figure 6-4). A unit of this type is
usually smaller in diameter than the conventional
cyclone, providing a greater separating force for the
Cyclone collection efficiency
same inlet velocity and a shorter distance for the parti-
a. Separation ability. The ability of a cyclone to
cle to migrate before reaching the cyclone walls. These
separate and collect particles is dependent upon the
units may be used singly or arranged in parallel or
particular cyclone design, the properties of the gas and
series as shown in figure 6-5. When arranged in paral-
the dust particles, the amount of dust contained in the
lel they have the advantage of handling larger gas vol-
gas, and the size distribution of the particles. Most
umes at increased efficiency for the same power con-
efficiency determinations are made in tests on a geo-
sumption of a conventional unit. In parallel they also
metrically similar prototype of a specific cyclone
have the ability to reduce headroom space require-
design in which all of the above variables are
ments below that of a single cyclone handling the same
accurately known. When a particular design is chosen
gas volumes by varying the number of units in opera-
it is usually accurate to estimate cyclone collection
efficiency based upon the cyclone manufacturer's
d. Multicyclones. When very large gas volumes must


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