Page 183 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

158 6 Separation of Particles from a Gas

6.3 Electrostatic Precipitation

The model analysis of electrostatic precipitation is very similar to that for the
gravity settling chamber as discussed in Sect. 6.2 except that the driving force is
now not gravitational but electrical. And the electrical field is arranged horizontal
rather than vertical. Consider a flow through a pair of vertical plates H apart from
each other with length L and depth b into the paper.
By replacing the gravitational settling velocity in Eq. (6.16) above with the
terminal precipitating speed V E , we can get the equation for laminar condition as
follows:
V E L V E Lb V E A

g d p ¼ ¼ ¼ ðLaminarÞ ð6:23Þ
UH UHb Q
where A is the area of one plate collecting particles.
Following the similar analysis for gravitational setting chamber, we can also get
the efficiency for complete mixing condition.

V E A

g d p ¼ 1 exp ðTurbulentÞ ð6:24Þ
Q
where the terminal precipitating speed, V E , in the electrical force field can be
determined by equating the electrical force on the particles and the drag force,
3pV E d p l
qE ¼ ð6:25Þ
C c
where q is the charge carried by the particles (columns) and E is the electric field
intensity (V/m). This equation leads to

qEC c
V E ¼ ð6:26Þ
3pd p l

Equation (6.26) shows that the precipitation speed of a particle depends on the
charge carried by the particle, q, and the strength of the electrical field, E. They are
determined as follows.

6.3.1 The Electric Field Intensity

The intensity of an electric field, E; is determined by the electrode geometry and the
voltage difference that is applied between the electrodes.