Environmental Engineering Reference
In-Depth Information
Figure 12.9
Two-stage electronic air cleaner particle collection.
electrons and creating positive ions and additional electrons. The process
produces a corona discharge near ionizing wires.
Positive ions become attached to airborne particles by deposition pro-
cesses. As a consequence, these particles become positively charged. The
magnitude of the charge on individual particles depends on the number of
charges deposited. Particles with high surface areas gain more charge and
therefore have a higher probability of being collected.
Charged particles flow with the airstream into a collector section (second
stage) consisting of a series of vertically placed, parallel, thin, metal plates.
Alternate plates may be positively and negatively charged by a high DC
voltage (6000 V). Positively charged particles are attracted to a negatively
charged plate. The magnitude of the electrostatic force acting on a particle
depends on its charge, the distance between plates, and the voltage applied.
Particles deposited on collection plates lose their original charge and
take on the charge of the collecting surface, where they remain attached to
the plate and other collected particles by molecular adhesion and cohesion.
As particle buildup occurs, electrostatic forces diminish in magnitude. Col-
lection efficiency decreases as a consequence. Collection plates must be
cleaned of their accumulated dust load periodically to restore their initial
collection efficiency. Hot water is typically used to clean collection plates; in
some systems washing is done automatically.
Electronic air cleaner collection efficiencies depend on particle migration
velocity, collection surface area, travel path length (distance through a col-
lection field), and air flow rate. Migration velocity is directly proportional
to a particle's charge and the strength of the electric field. In electronic air
cleaners used for indoor applications, the travel path length is relatively
short, typically 6 to 12” (15.2 to 30.5 cm). In industrial applications, travel
path length may be 20 to 25' (6.1 to 7.6 m) or longer, with collection efficien-
