Chemical Engineering March 2019 - 58

let gas velocity. The pressure drop is
determined as the difference in static
pressures between the inlet duct and
the outlet duct. All cyclone configurations
have been calculated for two
dusts with different particle-size distributions
(Table 1) and a particle density
of 2,700 kg/m3.
Uniflow cyclones for solving this
FIGURE 12. The photos show swirl-vane inserts
installed in standard cyclones for pressure
recovery. The left image is a vortex finder with
swirl-vane inserts (dia. = 2,300 mm (90.6 in.) in
a recirculating cyclone at a power plant, operated
at 900°C [15]. The right image is of manufactured
swirl-vane inserts (dia. = 650 mm (25.6 in.) to be
installed in decoking gas cyclones applied in a
steamcracker, operated at 450°C (842°F)
linked within this calculation model.
In addition, the model allows the
calculation of the pressure loss of a
uniflow cyclone for any positions of
the measuring point for the pressure
in the gas outlet line, for example, at
positions only a few inner diameters
behind the gas outlet tube opening
(that is, at a point where the gas flow
still has a high vorticity). This is particularly
useful for comparing calculation
results with experimental data, as
it is often measured within short distances
to the gas outlet tube opening.
For details of the pressure drop
calculation, see Ref 24.
Comparing cyclone types
The calculation models for standard
cyclones and for uniflow cyclones
described above allow comparisons
between cyclone types in a systematic
way. With respect to industrial
applicability, a key question is what
amount of particles both cyclone
types can remove from specified
gas-solids flows. To address this
question, properly designed uniflow
cyclones and standard cyclones for
purifying
four
different gas volume
flows between 24 and 6,640 m3/h
(air at 20°C, 1.013 bars) carrying 2 g/
m3 dust are compared. All uniflow cyclones
are geometrically similar and
operate at the same static pressure
drop of 4,000 Pa. The same applies
to the four standard cyclones. In this
example comparison, all considered
cyclones are installed in a piping
system with the same inlet and outproblem
have diameters between 30
mm (24 m3/h) and 500 mm (6,640
m3/h), whereas standard cyclones
need diameters between 54 mm (24
m3/h) and 900 mm (6,640 m3/h),
(Figure 9). The total lengths (including
the length of the swirl generator)
vary between 110 mm and 1,830
mm for uniflow cyclones, and between
150 mm and 2,580 mm for
standard cyclones.
Thus, in the present case, uniflow
cyclones are about 40% smaller in diameter
and about 30% shorter than
standard cyclones for purifying the
same gas-solid flows at a given pressure
drop, under typical conditions.
Note that an increase in the length
of a uniflow cyclone beyond a value
of Lc/Dc ~ 3 (Figure 8) does not improve
its separation efficiency [22].
This constitutes an essential difference
compared to standard cyclones,
where the
separation efficiency
increases with increasing
separator height, since the cut-off
size decreases with increasing height
below the vortex finder (as long as
this height does not exceed a critical
value where the inner vortex starts to
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Chemical Engineering March 2019

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