Biomedical Engineering Reference
In-Depth Information
Table 2.3
Original design characteristics of the ACI at 28.3 L/min (
From
[
26
]
—used with permission
)
Stage
0
1
2
3
4
5
6
7
d
50
a
(
μ
m)
9.0
5.8
4.7
3.3
2.1
1.1
0.65
0.43
Re
f
163
221
110
141
188
292
394
782
S
/
W
b
0.4
0.54
2.37
3.05
4.07
6.32
8.54
8.54
X
c
-
-
1.2
0.93
0.69
0.44
0.33
0.16
a
Nominal cut-point sizes
b
The nominal value of
S
is 1.6 mm (see Table
2.5
)
the impaction plate with multinozzle designs [
28
]. Under these circumstances,
particles that would otherwise be collected are instead captured by the cross-flow
and transferred beyond the collection surface. Increased interstage losses as well as
bias towards smaller sizes can therefore result with multistage impactors. The
dimensionless cross-flow parameter (
X
c
) is defined as
nW
D
X
=
4
(2.11)
c
c
where
n
is the number of nozzles per stage having nominal nozzle diameter,
W
, and
D
c
is the diameter of the overall cluster of nozzles in a multinozzle impactor stage.
Values of
X
c
are listed in Table
2.3
for the ACI calibration by Vaughan [
19
].
It follows from its definition that
X
c
for a given stage remains unaltered if the
design of the CI is fixed irrespective of design flow rate, as will be seen later is the
case with any of the Marple-Miller model 150/160 variants or the NGI. This param-
eter should be <1.2 to avoid cross-flow-related problems that can lead to unaccept-
ably high internal wall losses. This criterion is achieved for all of the commonly
encountered CIs used with inhaler testing, with the notable exception of stage 2 of
the ACI, where
X
c
is 1.2.
In 2000, Nichols and Smurthwaite [
29
] and Nichols et al. [
30
] extended the use
of the ACI to higher flow rates primarily for the purpose of performance-testing
DPIs. At 60 L/min, stage 7 is removed from the bottom of the stack and replaced by
a new stage “-1” that is located immediately above stage “0.” If the ACI is to be used
at 90 L/min, stage 6 is also removed and replaced by a further new stage “-2”
(Table
2.4
).
Stage-1 comprises 96 nozzles, each with 4.5-mm diameter, whereas stage-2 con-
tains 95 nozzles, each with 5.5-mm diameter. Importantly, in 2004, Byron et al.
published definitive data for current production ACIs, superseding performance
data developed from previous versions of this CI as the result of drawing changes to
make manufacture of this impactor a more robust process [
31
]. These changes,
together with the alternative high flow rate configurations, modify the original ACI
design characteristics given in Table
2.3
to those summarized in Table
2.5
.
Small differences and discrepancies remain between various reports of the val-
ues of parameters in the ACI. For example, one manufacturer's user guide estab-
lishes the
d
50
of stage 5 at 90 L/min to be 0.44
m (BGI, Waltham, MA, USA).
Although this value appears to be unsupported by a technical reference, it is rather
μ
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