Chemical Engineering March 2019 - 55

Wall separation e Vortex separation i
DC

DCore
Ringchamber
separation RC
Swirl vane inserts
for swirl generation
LC
DVF
DC
Collection in
the bunker b
PO
FIGURE 7. In a uniflow cyclone, the separation process has several
components, as shown here
for example, Ref. 14 and 15).
Similarly, industrial practice would
benefit from a reliable analytical calculation
model for uniflow cyclones.
Previous analytical approaches to
calculate the separation efficiency
of uniflow cyclones are mostly
based on the idea of a sedimentation
process that takes place under
the effect of centrifugal force. This
concept insufficiently takes into account
the drag force on the particles
of the gas flowing inward toward the
gas outlet. Since this drag force has
a decisive influence on the cyclone
separation efficiency, the models do
not correctly reflect fundamental correlations,
such as the dependence of
the separation capacity on the vortex-finder
diameter or the cyclone
length. In addition, the re-entrainment
of already separated particles
from the cyclone wall into the clean
gas and the influence of the solids
loading on the performance data are
not taken into account.
Against this background, a new
analytical model was developed for
the calculation of the separation efficiency
and pressure drop of uniflow
cyclones for practical design work.
This model uses the same physical
concepts as the equlibrium orbit
model cited above. This approach
makes sense because the principle
of particle separation in both types of
cyclones is the same: particle separation
occurs through outward centrifugal
forces generated by the swirl
flow, reduced by the inward drag
forces of the gas flowing to the gas
outlet. The novel model has been
validated with extensive experimental
data [16-22].
TABLE 1. RRSB PARTICLE SIZE
DISTRIBUTIONS OF THE CONSIDERED
DUSTS
Dust
Semicoarse dust 0.5
Fine dust
0.2
dmin dmax d50,3 n
300
200
50.5
17.5
0.9
0.95
PInlet
Pe
Pe
Calculation method for standard
cyclones. In standard cyclones, the
separation efficiency depends crucially
on the solids loading of the flow
at the cyclone inlet. Solids loading is
defined as the ratio of dust mass
flow to gas mass flow, as shown in
Equation (1).
µe = M
*
s / M
*
g
(1)
The swirling flow can - similar to the
gas flow in pneumatic transport -
carry only a very limited dust load,
the so-called limited loading. If the
inlet loading (µe) exceeds the limited
loading (µlim), the surplus particles
are deposited on the wall of the sepA
100
90
80
70
60
50
10
100
90
80
70
60
50
10
1,000
800
600
400
200
10
3,000
2,500
2,000
1,500
1,000
500
10
Semicoarse
dust, d50=50.5µm
Standard cyclones
Uniflow cyclones
100
Fine dust, d50=17.5 µm
Standard cyclones
Uniflow cyclones
100
1,000
Standard cyclones
Uniflow cyclones
10,000
1,000
800
600
400
200
100
1,000
Standard cyclones
Uniflow cyclones
10,000
3000
2500
2000
1500
1000
500
100
1,000
Gas flow volume flow Q, m3/h
10,000
10
10
1000
10000
100
90
80
70
60
50
10
Pi
Pi
FIGURE 8. This diagram shows the pressure drop in a uniflow cyclone
aration chamber immediately after
entry of the flow into the cyclone.
Thus, in the cyclone inlet region,
a first separation stage takes place
(also called wall separation). All
particles carried by the vortex flow
then undergo the second separation
stage: the separation in the
inner vortex of the cyclone. Figure 4
shows the wall separation very impressively
in a standard cyclone with
a diameter of 800 mm. With increasing
solids loading at the cyclone inlet,
an increasing portion of the incoming
solids is deposited immediately after
entry to the cyclone wall.
Figure 5 illustrates the influence of
the solids loading on the course of
B
100
90
80
70
60
50
10
Semicoarse dust, d50=50.5µm
Standard cyclones
Uniflow cyclones
100
1000
Fine dust, d50=17.5 µm
Standard cyclones
Uniflow cyclones
100
1,000
Standard cyclones
Uniflow cyclones
10,000
10000
Pm
DVF
Gas outlet
100
1,000
Standard cyclones
Uniflow cyclones
10,000
100
1,000
Gas flow volume flow Q, m3/h
FIGURE 9. These graphs compare single standard cyclones and single uniflow cyclones (a) and four parallel
uniflow cyclones (b) for purifying different gas volume flows (24 m3/h, 266 m3/h, 1,000 m3/h, 2,400
m3/h and 6,640 m3/h. All cyclones have the same pressure drop of about 4,000 Pa, and are geometrically
similar, thus having the same inlet and outlet velocities. The main dimensions are shown in the two lower
figures. Separation efficiencies are calculated for semicoarse dust with d50,3 = 50.5 µm (at the top) and
for fine dust with d50,3 = 17.5 µm (at the middle) (For more details, see Ref. 25)
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
MARCH 2019
55
10,000
Cyclone length, mm
Cyclone diameter Dc,
mm
Separation efficiency
, %
Separation efficiency
 (%)
Cyclone length, mm
Cyclone diameter Dc,
mm
Separation efficiency
, %
Separation efficiency
 (%)

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Chemical Engineering March 2019

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