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03_chap_wang.qxd 05/05/2004 12:48 pm Page 104
104 José Renato Coury et al.

Therefore, the average residence time of the gas inside the cyclone, θ, can be
estimated by
2)
(V + V KD 3
,
θ = s nl H = c c (21)
Q Q
A criterion for choosing the appropriate gas velocity at the entrance, v , is to compare
i
it with the saltation velocity, v , which is the minimum velocity capable of “peeling
s
off” particles from the wall. In principle, velocities larger than the saltation cause resus-
pension of the collected particles and a decrease in efficiency. According to Kalen and
Zenz (22) and Koch and Licht (23), the recommended ratio is

1.20 ≤ v i ≤ 1.35
v
s
in which v is given by
s
.


 4 gµρ p  13  ( bD c ) 04 
v = 2.055     D c 0.067 v i 2 3 (22)
s
 3ρ 2  − ( bD c )] 
13
 [  1 
for the variables in English units (lb , ft, and s). In general, v /v = 1.25 [valid for T = 38°C
m i s
3
and ρ = 2580 kg/m (18)] is adopted for maximizing collection efficiency.
p
2.2.3. The Iozia and Leith Model
The Iozia and Leith model (10,13) departs from the Barth (12) model by proposing
new equations for estimating the diameter and length of the cyclone central axis, the
maximum tangential velocity, and the dependence of these variables on the device
dimensions, based on 26 experiments performed at ambient temperature. The correla-
tion proposed for the collection efficiency is based on the particle Stokes diameter,
which is defined as the diameter of the sphere that has the same terminal velocity of the
particle. The proposed expression can be written as

η = 1 (23)
i [ 1 + D ( 50 D i) β ]

where D is the Stokes diameter of the particle with 50% collection efficiency, D is the
i
50
Stokes diameter of the particle whose collection efficiency is being determined, and β
is an exponent dependent of the cut diameter D .
50
The cut diameter can be estimated by
 9µ Q  0.5
D = (24)
50  2 
 πρ pc max 
Z v
t
The natural length Z in this model is estimated as a function of the diameter of the
c
central axis, d , according to the expression
c
d c = 0.47  ab  −0.25   D  1.4 (25)
e



2
D  D   D 
c c c