Chemical Engineering March 2019 - 57

through the gas outlet pipe. Due to
these characteristics, particles carried
into the ring chamber can be
re-entrained back into the gas outlet.
The re-entrainment rate depends
on the cyclone geometry and on the
operation data. The efficiency of the
gas-particle separation within the
ring chamber is denoted by RC.
Finally, the collection efficiency
of the bunker has to be taken into
account, as in standard cyclones.
Thus, the total separation efficiency,
as well as the fractional separation
efficiency of the cyclone, are functions
of those four single separation
efficiencies (Figure 7).
The method for
calculating
the
separation in the inner vortex has
been described in detail elsewhere
[23, 24]. For low loadings (µ close
to 0.001), this separation mechanism
is dominant and its efficiency
is expected to be close to the total
separation efficiency. With increasing
solids loadings, the wall separation
becomes more and more important
and already below a solids loading
of 0.01, the wall separation can
become the dominant separation
mechanism, depending mainly on
the cyclone size, the tangential velocity
at the inlet and on the size distribution
and density of the particles.
Analogous to the calculation
model for reverse-flow cyclones, the
pressure drop of uniflow cyclones is
calculated as the difference of the
total pressures between a position in
front of the cyclone inlet o and a position
m far beyond the opening of the
gas outlet where the swirl strength of
the vortex flow in the gas outlet has
decreased to approximately zero
due to wall friction (Figure 8). As in
standard reverse-flow cyclones, the
total pressure drop is divided into
three parts: the pressure drop in the
inlet, ∆pinlet; the pressure drop in the
separation chamber, ∆pe, between
the mean entrance radius and the
position i at the vortex finder radius
rVF; and the pressure drop, ∆pi, in
the gas outlet tube, including the
inlet pressure drop at the tube inlet
between the position i and the meaFIGURE
11. This is a 3-D drawing of swirl-vane
inserts for pressure drop reduction in a cyclone
surement position m. At low loadings
(below about 0.01), the third pressure-loss
component, ∆pi, accounts
for the major part of the total pressure
drop. The exact value depends
on the geometry and operating data
of the cyclone. At high loads (greater
than 1), ∆pi is still roughly 50% of the
total pressure drop.
The pressure drop calculation
method is based on the same gas
velocity field that is used to calculate
the separation efficiency. Therefore,
both calculation quantities are closely
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Chemical Engineering March 2019

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