Chemical Engineering March 2019 - 54

A
B
Gas outlet
Swirl vane inserts
Gas outlet
C
C
D
A
B
Solids outlet
Solids outlet
FIGURE 3. Diagram (a) shows a standard cyclone with tangential inlet, and diagrams (b), (c) and (d) show
uniflow cyclones with axial inlets. A horizontal orientation is shown in (b), vertical downward in (c) and
vertical upward orientation in (d)
the available pressure drop is low.
* If volume and pressure drop are
not restricted, standard cyclones
are more efficient than uniflow
cyclones, but require significantly
more volume. The efficiencies of
both cyclone types approach each
other with decreasing gas volume
flow and with increasing pressure
drop, particle size and particle
density of the feed.
Cyclone operation
Cyclone separators have been in industrial
use for over 100 years and
still are the subject of intensive research
and development (see Ref. 2
and 3, for example). A detailed description
of physical fundamentals,
designs, planning and commissioning,
operation and maintenance of
cyclone separators is given in Ref. 4.
The two essential performance
parameters of a cyclone are its separation
efficiency and its pressure
drop. The latter determines its en100
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0.1
1
Particle
size, µm
FIGURE 5. The graph shows measured [11] and calculated (according to Ref.
12) fractional efficiency curves of a standard cyclone for natural gas purification
from solid particles. Cyclone diameter is 220 mm (8.7 in.), cyclone length
(pure cylindrical shape) is 1,200 mm (47.2 in.), gas volume flow is 300 m3/h,
solids loading (lb solids / lb gas) is 0.00056, and separation efficiency is 94.3%
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ergy consumption. Goals of the cyclone
design are generally to achieve
the maximum possible separation
efficiency with the lowest possible
pressure drop for a given task. In
addition, cyclone operation seeks
to minimize erosion of cyclone walls
by abrasive particles and prevent
deposits on the cyclone walls in the
case of adhesive particles, especially
in areas of the cyclone with low flow
velocity or flow detachment. And all
of these design parameters ideally
should be achieved with a limited
construction volume.
In a cyclone separator, the gas to
be purified from particles is set into
a vortex flow with high vorticity (Figure
1). Due to the density difference
between the particles and the gas,
the particles sediment, under the action
of centrifugal force, outward in
the radial direction against the cyclone
wall, where they move with the
downwards rotating gas flow (only
at high loads due to gravity) and are
Measure
Calculation
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Particle size, µm
FIGURE 6. This graph shows the measured fractional efficiency of a uniflow
cyclone for collecting limestone particles (mass mean diameter d50 = 20 µm
and d10 = 3 µm). The cyclone diameter is 192 mm (7.6 in.), the cyclone length
(pure cylindrical shape) is 1,500 mm (59.1 in.), the gas volume flow is 1,000
m3/h, solids loading: 0.0016 lb solids / lb gas, separation efficiency is 87.5%,
pressure drop: 3,250 Pa (0.47 psi) (see Ref. 20)
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FIGURE 4. Images of dust strands and dust mats
in a cyclone with a diameter of Dc = 800 mm, vortex
finder diameter DVF = 285 mm with different
loadings of washing powder (d50 = 500 µm): a) µe
= 0,1, b) µe = 1 , c) µe = 10 (see Ref. 9)
transported to the solids outlet. The
cleaned gas exits the separation
chamber, after reversal of direction,
concentrically
to the cyclone axis
located at the gas outlet tube, also
called the vortex finder.
The swirl is generated by introducing
the particle-laden gas stream
into the cyclone with a tangential
component. This is realized either by
a tangential slot, a spiral inlet or by
an axial inlet with swirl vanes inclined
to the vertical axis of the cyclone
(Figure 2).
Calculation models
For the computation of the performance
data of standard cyclones,
there are proven analytical models
with high accuracy, which in practice,
are often advantageous over numerical
simulations with long computation
times. One of those models is
based on an equlibrium orbit concept
according to Ref. 5 and is described
in Ref. 2-3 and 5-13. This model has
been applied successfully in a broad
range of industrial applications (see
Fractional efficiency, %
Fractional efficiency, %

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Chemical Engineering March 2019

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