Biomedical Engineering Reference
In-Depth Information
Table 2.2
Values of
d
50
(
m) and GSD
stage
for the size-fractionating stages
of the ACI at
Q
= 28.3 L/min
Stage
μ
Nominal
d
50
Calibration
d
50
Calibration GSD
stage
0
9.0
9.0
1.15
1
5.8
6.0
1.17
2
4.7
5.7
1.20
3
3.3
3.1
1.22
4
2.1
2.1
1.20
5
1.1
0.9
1.23
6
0.7
0.6
1.21
7
0.4
ND
ND
ND
not determined
nozzle diameters from nominal were possible as the result of imperfections during
manufacture as well as the expected result of wear (corrosion/plugging) in normal
use [
23
]. Later in this chapter, the importance of having an accurate understanding
of each stage collection efficiency curve profile will be explained, since an under-
standing of the issues involved is essential in the context of abbreviating full-
resolution impactors by the process of removing intermediate stages that is at the
heart of the AIM concept.
Returning to the basic impactor theory, Marple and Liu [
16
] and Rader and
Marple [
24
] identified that the value of
St
50
, should be close
to 0.49 for well-designed circular-profiled nozzles, where differences in particle
inertia dominate the size-separation process. The compendial CIs include only
round nozzles, although other geometries, such as a slit, are possible. However, at
least two other parameters have a secondary influence on the particle size-separation
performance of a CI. The ratio of nozzle-to-collection surface distance/nozzle
diameter (
S
/
W
) describes the geometry of the stage. The value of
√
St
at
E
50
, defined as
√
St
50
is unaffected
by small variations in
S
/
W
if
S
/
W
is between 1.0 and 10.0. At the same time, the
dimensionless flow Reynolds number (
Re
f
), defined as
√
r
g
UW
Re
f
=
(2.10)
h
should ideally be in the range 500-3,000 to minimize the value of
GSD
stage
. However,
this criterion may be overstringent [
25
], as indicated by the summary of the ACI
design information presented in 1998 by Marple et al. for the ACI [
26
], where sev-
eral stages do not meet this criterion yet function effectively as size fractionators at
28.3 L/min (Table
2.3
).
The ratio of nozzle throat length (
T
) to
W
can also influence size-separation effi-
ciency, decreasing
St
50
with increasing
T
/
W
[
16
]. However, the effect is likely to be
small with commercially available impactors, where
T
/
W
is typically <10 [
25
].
Cross-flow, induced by air exiting the nozzles near the center of the nozzle
plate and flowing outwards past other air jets located near the periphery of the
nozzle cluster, can prevent the air jets near the edge of the cluster from reaching
√
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