Page 120 - Air Pollution Control Engineering

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Cyclones 99

Fig. 2. Cyclone entries: (A) tangential; (B) swirl vane; (C) half scroll; (D) full scroll.

Attention here will be focused on the reverse-flow cyclone, with tangential entry,
which is by far the most common geometry found in practice. The main dimensions
of the device are illustrated in Fig. 3 and will be referred to in the remaining of this text:
the cylindrical body has a diameter D and a height h; the conical section has a height Z
c
and ends in the solids outlet of diameter B; the gas entry can be either circular (diameter
D ) or rectangular (height a, width b); the exit duct, also called vortex finder, has a
in
diameter D and starts at a distance S from the top of the cylindrical body; the cyclone
e
has a total height H.
In the cylindrical body, where the entrance and the exit of the gaseous current are
placed, the gas begins a spiral movement down, creating a centrifugal field, intensified
in the conical part, that impels the particles in the direction of the wall of the equipment,
where they are collected. There is a stagnated region close the wall, as a result of the
laminar sublayer, which allows the fall of the collected particles into the reservoir situ-
ated in the base of the equipment. In opposition to the centrifugal force, there is the drag
force caused by the radial movement of the gas stream toward the central axis of the
cyclone, and the turbulence of the gaseous current, whose combined effect is to carry
the noncollected particles to the exit duct (8,9).
The movement of the gas is constituted by an external downward vortex and an
internal upward vortex (10). The intermediate region between these vortexes defines
m
the central axis of the cyclone, with height and diameter Z or h and d , respectively,
c c
delimited by the diameters of the exits B and D . In the external vortex, the tangential
e