Theory of Operation B-5
Electr o s t atic Cla s s i f ier
The purpose of the classifier is to extract a known size fraction of
submicrometer particles from the incoming polydisperse aerosol.
In the classifier, the aerosol enters a charger (either the 3088 or
3077/3077A) which exposes the aerosol particles to high concentrations of
ions of both positive and negative polarity. The particles and ions undergo
frequent collisions due to the random thermal motion of the ions. The
particles quickly reach a state of charge equilibrium, in which the particles
carry a known bipolar charge distribution.
The charged aerosol passes from the neutralizer into the main portion of
the Differential Mobility Analyzer (DMA), shown in Figure B-3 and
Figure B-4. The DMA contains two concentric metal cylinders. The
polydisperse aerosol (q
a
) and sheath air (q
sh
) are introduced at the top of
the Classifier and flow down the annular space between the cylinders. The
aerosol surrounds the inner core of sheath air, and both flows pass down
the annulus with no mixing of the two laminar streams. The inner cylinder,
the collector rod, is maintained at a controlled negative voltage, while the
outer cylinder is electrically grounded. This creates an electric field
between the two cylinders.
The electric field causes positively charged particles to be attracted
through the sheath air to the negatively charged collector rod. Particles are
precipitated along the length of the collector rod (see Figure B-3 and
Figure B-4). The location of the precipitating particles depends on the
particle electrical mobility (Z
p
), the classifier flow rates, and the classifier
geometry. Particles with a high electrical mobility are precipitated along the
upper portion of the rod; particles with a low electrical mobility are collected
on the lower portion of the rod. Particles within a narrow range of electrical
mobility exit with the monodisperse air flow (q
m
) through a small slit located
at the bottom of the collector rod. These particles are transferred to a
particle sensor to determine the particle concentration. The remaining
particles are removed from the classifier via the excess sheath air flow (q
e
).
To Next Page
Loading...
Need help?
General
