Current Approaches to APSD Measurements of OIPs Based on Inertial Impaction - Good Cascade Impactor Practices, AIM and EDA for Orally Inhaled Products - page 23

Fig. 2.6 Hypothetical single-stage impactor showing trajectories of particles that are collected
and those that miss the collection substrate

Subsequently, Newton's equation of motion is used to model the passage of
different-sized particles through the ideal impactor. The process may ultimately be
extended to evaluate the particle transport and size fractionation through impactor
stages having various nozzle dimensions (diameter, W , and length, L ) as well as
several nozzle-to-collection surface distances ( S ) [ 16 ]. Flow is normally assumed to
be laminar within the stages of the CI. However, if gas flow and particle transport
through the induction port inlet are also being modeled, an appropriate model of
turbulence such as the low Reynolds number (LRN) approach can be intro-

duced, based on its ability to accurately predict pressure drop, velocity profiles, and
shear stress for transitional and turbulent flows [ 17 ].
Returning to the idealized single-nozzle (jet) impactor (Fig. 2.5 ), St is related to
W through the expression

κ - ω

2

r CdU

W

(2.6)

pcp

St =

18 h

in a self-consistent set of units, based on d p . U is the linear velocity of the particle,
which can be considered the same as the surrounding local air velocity when the
flow rate through the impactor is constant. The Cunningham slip correction factor,
C c , that takes into account the faster settling of particles whose size is close to that
of the mean free path of the surrounding gas is defined by (2.4) and (2.5).
The particle collection efficiency ( E ) of an ideal impactor stage, expressed as a
percentage, will increase in a stepwise manner between limits of 0-100%. In prac-
tice, for a well-designed stage, E is a monotonic sigmoidal function of either St or
d p that increases steeply from E ≈ 0% to >95%, reaching its maximum steepness