PARTICLE CHARGING AT LOW PRESSURES
BENJAMIN Y. H. LIU and KENNETH T. WHITBY
University of Minnesota, Minneapolis, Minnesota
ABSTRACT
The electric
charge imparted to particles at reduced pressures has
been measured for particles ranging from 0.02 to 2.0 u and for pres-
sures ranging from 0.01 to 1.0 atm with an improved particle- mobility
analyzer of cylindrical geometry. The solid spherical particles of
methylene blue dye are charged by positive ions in electric fields of
various intensities. The measured particle charge is generally higher
than the values predicted by White’s equations, and the difference in-
creases with decreasing pressure. Typically, the measured charge at
0.01 atm is 100% higher than the values predicted by the diffusion-
charging equation and is slightly less than one order of magnitude
higher than what the field-charging equation predicts.
INTRODUCTION
The electrical charging of small particles in the micron and Submicron size range has been the subject of numerous theoretical and
experimental studies. It is an important factor in the design of elec-
trostatic precipitators, which are among the most effective devices for
removing small airborne particulate matter. Electrostatic precipitators
have been used extenSively as air or gas filters and, more recently,
as atmospheric dust samplers at both ground levels and high altitudes
for radioactive-fallout studies.
Particle-charging experiments have been reported by Hewitt,!
Penney and Lynch,’ White,’ Cochet,* Drozin and La Mer,°* Langer,® and
others, and considerable particle-charging data are now available for
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