Page 196 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

6.4 Cyclone 171

6.4.2 Pressure Drop of Cyclone

Pressure drop is the second important cyclone performance indicator, after col-
lection efficiency. The pressure drop across a Lapple cyclone can be estimated by:
!
2
q u HW
g g
DP ¼ K ð6:64Þ
2 D 2
e
which contains the design dimensions of H; W and D e . For the coefficient K,a
value in the range of 16–18 is suggested, with K ¼ 16 as a recommended value [4].
A cyclone is capable of reducing dust concentrations in a gas stream from
3
3
several g/m to below 0.1 g/m . Cyclones can also be applied for removing water
from oil at oil fields or solids from water. They are considered effective as low-cost
preseparators for gas cleanup purposes.

6.4.3 Other Cyclone Models

Many analytical or semi-empirical models have been developed to predict the
collection efficiencies of reverse flow cyclones under laminar or complete mixing
assumptions. In these theoretical analyses, dimensionless geometric parameters are
frequently defined. Dirgo and Leith [9] have summarized the models that were
developed prior to 1985. It was stated that Lapple’s cut-size theory based on time
flight approach was widely cited in North American literature, while Barth’s theory
based on static particle approach was more often referred to in Europe. Both the-
ories are based on laminar flow assumption. The well-known Leith–Licht model
[22] developed in the 1970s was based on the assumption that flow was turbulent
and uncollected particles were completely and uniformly mixed. Barth’s theory and
Leith–Licht theory were closest to Dirgo and Leith’s experimental results [9]
obtained from a Stairmand high efficiency cyclone.
Several uniflow cyclone models have been published too. Most researchers
assume that increasing the separation length favors the solids separation efficiency,
as the residence time increases allowing more particles to migrate to the wall of the
cyclone. Summer et al. [32] reported an optimum separation length of around 1
cyclone diameter. Gauthier et al. [12] found that the optimum length increased with
the inlet air velocity. These assumptions were validated by experimental results of
large particle separation in a small uniflow cyclone with a tangential inlet. The
diameter of the cyclone was 50 mm and the particles had a mean diameter of 29 μm.
These models, however, might not apply for separation of fine particles in cyclones
handling high airflow rates, where high turbulence exists.
Ogawa et al. [24] analyzed the separation mechanism of fine solid particles for a
uniflow cyclone and demonstrated that particle cut size could be smaller than 5 μm.
However, this theory is also based on small scale models. Their outer diameters