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158 Chung-Shin J. Yuan and Thomas T. Shen

The type of corona that is produced depends on the polarity of the discharge elec-
trode. If the discharge electrode is positive, negative ions are accelerated toward the
electrode, causing the breakdown of gas molecules, with the result that positive ions
are repelled outward from the discharge electrode in the form of a corona glow.
Conversely, if the discharge electrode is negative, positive ions are accelerated toward
the discharge electrode and negative ions are repelled from the discharge electrode to
produce a corona discharge.
One has the choice of applying either a positive or a negative potential to the discharge
electrode. The negative potential generally yields a higher current at a given voltage than
the positive, and the sparkover voltage (voltage at which complete breakdown of the gas
dielectric occurs), which sets the upper limit to the operating potential of the precipita-
tor, is also usually higher; in addition, a positive corona tends to be sporadic and unstable.
A negative corona is usually used in industrial precipitators, whereas a positive corona,
because of its lower ozone generation properties, is used in domestic and commercial
air conditioning.
When the corona-starting voltage is reached, the ions repelled from the discharge
electrodes toward the collecting electrodes constitute the only current in the entire space
outside the corona glow. This interelectrode current increases slowly at first and then more
rapidly with increasing voltage. As sparkover is approached (i.e., as complete breakdown
of the gas dielectric occurs), small increments in voltage give sizable increases in current.
2
Typical corona currents are of the order of 0.1–5.0 mA/m of collecting electrode area.
Sparkover does not occur infrequently; in general, a frequency of sparkover less than 100
times per minute is acceptable.

2.2. Electrical Field Characteristics
The fundamental differential equation that describes the field distribution between
two electrodes is Poisson’s equation, which expressed vectorially is
⋅
∇Ε = σ K (1)
i 0
where σ is the ion space-charge density (i.e., the quantity of electrical charge per unit
i
volume of the space between the discharge and collecting electrodes) and K is the per-
0
mittivity of free space (8.85×10 −12 F/m). Applying Eq. (1) to the simplest precipitator
geometry, the coaxial wire–cylinder combination, the following is obtained:
( 1 r)(ddr )( ) =rE σ i K 0 (2)

where r is the radial distance from the cylinder axis. Prior to the onset of the corona, σ
1
is zero. Taking V as the potential at the wire surface r and grounding the cylinder (i.e.,
0 0
V = 0 at r = r ), integration of Eq. (2) yields the electrostatic field
1
E = V 0 (3)
ln r r )
r ( 1 0
Equation (3) shows that the finer the discharge electrode, the greater will be the field
strength at the surface of that electrode. Furthermore, as r increases, i.e., at point distant
from the discharge electrode, the field strength decreases.