Page 201 - Air Pollution Control Engineering

04_chap_Wang.qxd 05/05/2004 1:15 pm Page 180
180 Chung-Shin J. Yuan and Thomas T. Shen

ducts is very crucial for proper operation of an ESP, as are uniform plate spacing, prop-
er discharge electrode arrangement, trueness of plates, slopes of hoppers, adequate
numbers of electrical sections, and other features inside the ESP (26,27). Using the prac-
tical design parameters given in Table 1 and the basic understanding of ESP configuration,
we can specify the geometry of an ESP. The overall width of the precipitator is virtually
equal to the number of ducts for gas flow as follows (7):
N = Q vDH (45)
d
where N is the number of ducts, Q is the total volumetric gas flow rate, v is the linear gas
d
velocity in the ESP, D is the width of ducts between two collecting plates, and H is the
height of the plates (8–15 m). The overall length of the precipitator is given as follows (7):
L = N L + ( N − ) 1 L + L + L out (46)
in
s
s
t
p
s
where L is the overall length, N is the number of electrical sections in the direction of
t s
gas flow (2–8), L is the length of the collecting plate (1.0–4.0 m), L is the spacing
p s
between electrical sections (0.5–2.0 m), L is the length of the inlet section (3–5 m),
in
and L is the length of the outlet section (3–5 m).
out
The overall height of an ESP could be 1.5–3.0 times the plate height because of hop-
pers, superstructure, controls, and so forth. The number of electrical sections depends on
the aspect ratio (the ratio of overall plate length to plate height) and plate dimensions.
However, the number of electrical sections must be sufficient to provide the minimum
collection area required but not a great excess of area. The number of electrical sections
can be estimated by
N = r H L
s a p (47)
where N is the number of electrical sections and r is the aspect ratio.
s a
When the numbers of ducts and sections are specified, the actual overall plate area
can be calculated by
A = 2 HL N N d (48)
p
s
a
In general, the performance of ESP can be improved with increasing sectionalization
because of more accurate alignment and spacing for smaller sections and more stable
rectifier sets operating at higher voltages. Large numbers of electrical sections allow for
meeting the overall efficiency targets even if one or more sections are inoperable.
However, adding extra sections increases the capital cost.
Example 8
3
Estimate the overall width and length of an ESP designed for treating 20,000 m /min of
2
gas with total plate area of 14,000 m . Assume the plates are available in 8-15 meters high
and 3 meters long.
Solution

3
Given: Q = 20,000 m /min and A = 14,000 m 2
From Table 1, we select H = 10 m, D = 0.25 m, L = 3 m, L = 0.3 m, L = 4 m,
p s in
L = 4 m, v = 100 m/min, and r = 0.9.
out a