Page 202 - Air Pollution Control Engineering

04_chap_Wang.qxd 05/05/2004 1:15 pm Page 181
Electrostatistic Precipitation 181

From Eqs. (45)–(48), we obtain

N = Q vDH = (20 000 ) (100/ )(0 25. )( ) =10 80 ducts
,
d
N = r H L = (09 )( ) ( ) =10 / 3 3 sections
.
p
s
a
D = N D = ( )(0 25 ) = 20 m
80
.
d
t
L = N L + ( N − ) 1 L + L + L out = ( )( ) + (3 − )(0 3 ) + 4 + 4 = 17 6 m
.
.
33
1
s
t
s
in
s
p
2
A = 2HL p N N = ( )( )( )( )( ) =14 400 m (>14,000 m 2 )
21
8
0
,
3
0
3
d
a
s
3.4. Electrode Systems
Good precipitator design provides for definite structural relationships between the
electrode systems: discharge electrode and collecting electrode. The type and position-
ing of the discharge and collecting electrodes can be major factors in the operation and
maintenance of an ESP. As illustrated in Fig. 6, the discharge electrode system consists
of a high-voltage duct, feed-through support insulator, tension support insulator, upper
support grid, discharge electrode vibrator and wires, lower alignment grid, and weight
tension. The discharge wires energized negatively are usually designed of round, 12-
gage steel spring wires with sharp edges to facilitate the formation of a corona around
them. They are reinforced at the top and bottom to ensure good electrical contact and to
resist mechanical and electrical erosion. The discharge wires are taut by weights and posi-
tioned through guides to prevent excess swaying. The wires tend to be high-maintenance
items. Corrosion can occur near the top of the wires because of air leakage and acid
condensation. Moreover, long weighted wires tend to oscillate. The middle of the wire
can approach the collecting plates quite closely, causing increased sparking and wear.
Some types of discharge wire are illustrated in Fig.10.
The collecting electrode system is designed to have maximum collecting surface,
high-sparkover voltage characteristics, no tendency to buckle or warp, resistance to
corrosion, and aerodynamic shielding of collecting surfaces to minimize re-entrainment
of the collected particles. Standard planar electrodes are usually made of cold-rolled
steel sheets to ensure flatness. Collecting electrode panels are grouped within the pre-
cipitator housing to form independently suspended and independently rapped collecting
electrode modules, and they are rapped periodically by electromechanical means.
Because the collecting electrodes are generally cleaned by the rapping and dropping
of collected particles by gravity, they have to be separated at a sufficient distance for
the free fall of the particles. This leads to widening the distance between electrodes,
which, in turn, requires a higher voltage to produce the desired corona discharge. The
dimensions of the electrode systems are fixed largely by the required voltage and area of
electrode surface per unit volume of gas. Some types of collecting plate are illustrated
in Fig.10.
3.5. Power Requirements
The power requirements for an electrostatic precipitator vary with collection effi-
ciency. It is important for the power supply to deliver a unidirectional current to the