Chemical Engineering March 2019 - 56

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
1,000
10,000
Fine dust, d50=17.5 µm
Standard cyclones
Uniflow cyclones
100
1,000
Standard cyclones
Uniflow cyclones
10,000
FIGURE 10. Presented here is an exemplary comparison
of single standard cyclones with single
uniflow cyclones 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 1,000 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 (top) and for fine dust with d50,3 = 17.5 µm
(middle) (For more details, see Ref. 25)
100
1,000
Standard cyclones
Uniflow cyclones
10,000
100
1,000
Gas flow volume flow Q, m3/h
the fractional efficiency curve, measured
and calculated for a standard
cyclone. For the case shown in Figure
5, the curve no longer drops to
zero, but passes a minimum and
increases as the particle size decreases.
That is, finer particles are
increasingly precipitated. This can
be explained by a burying of fine
particles below the solids strand on
the wall deposited immediately after
10,000
entry. The burying of fine particles increases
with increasing solids loading
at the entrance. For no burying to
take place, this would be indicated
by a steadily decreasing course of
the fractional separation efficiency.
Similar behavior is also observed
in uniflow cyclones (Figure 6). Immediately
after entering the device,
particles can form more or less
pronounced strands at the cyclone
wall, depending mainly on the curvature
of the inlet vanes, the solids
concentration and the mean particle
size of the solids feed. In this case,
the fractional efficiency curve passes
a minimum similar to what has been
observed in standard cyclones.
The model for standard cyclones,
as described in the references, assumes
a limited loading capacity of
the gas stream splitting the separation
process into two steps. At any
solids loading, µe, in excess of this
critical loading, µlim, the solids are
immediately separated from the gas
at the inlet to the cyclone. The solids
remaining in the gas are separated
in the cyclone barrel and in the inner
vortex below the gas outlet tube with
a second, inner separation efficiency,
i. Total separation efficiency of the
cyclone is described in Equation (2).
(2)
Uniflow cyclone calculation
Analogous to the model for standard
cyclones, it is assumed that
both separation processes also
occur in uniflow cyclones (Figure 7).
The first separation takes place inside
the swirl vane inserts for swirl
generation and subsequently in the
separation chamber due to exceeding
the limited load ratio, µlim, of the
uniflow cyclone. If the load ratio at
the inlet µe exceeds the limited load
ratio, µlim , the excess mass fraction
will be removed immediately after
the gas jet enters the cyclone, and
only a small fraction that is restricted
by µlim will undergo the centrifugal
separation process in the inner vortex
of the cyclones.
In a deviation from standard cyclones,
uniflow cyclones have a third
separation process that has to be
taken into account. In contrast to
standard cyclones, the solids discharge
in uniflow cyclones is located
close to the gas exit. Furthermore,
a considerable part of the gas flow
passes through the ring chamber
between the cyclone wall and the
vortex
finder
pipe
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Cyclone length, mm
Cyclone diameter Dc,
mm
Separation efficiency
, %
Separation efficiency
,%
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Chemical Engineering March 2019

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