Design Parameters. In the design of electrostatic precipitators
several parameters come into use in evaluating and comparing different designs
and configurations. The most common parameters include:
a) Collection Efficiency -- The weight of the particulate
collected per unit time divided by the weight of the particulate entering the
precipitator during the same unit time expressed as a percentage.
b) Specific Collection Area (SCA) -- The ratio of the total
collecting surface area in square feet to the gas flow rate expressed in
thousands of actual cubic feet per minute. Generally, the larger the SCA the
better the collection efficiency. The typical range is 350 to 450 square feet
(32.5 to 41.8 square meters) per acfm (0.0283 cubic meters).
c) Electrode Area per Rapper -- The collection area, expressed as
square foot or square meter, rapped by each rapper. Generally, the smaller
the electrode area per rapper, the better the collection efficiency.
d) Corona Power Density -- The ratio of the total power input in
watts to the total gas flow rate in thousands of actual cubic feet per minute.
Generally, the higher the corona power density the better the collection
efficiency. Typical power densities are 1 to 2 watts per square foot.
e) Aspect Ratio -- The ratio of total length to the height of
collector surface. Generally, the higher the aspect ratio, the better the
collection efficiency. Typical aspect ratios are 1.0 to 1.5.
f) Treatment Time or Residence Time -- This parameter reflects
the theoretical time a particle is exposed to the electric field within the
precipitator. It is the ratio of effective length of the collector surface in
the direction of gas flow expressed in ft to the velocity of the gas flow in
ft/sec. Generally, the longer the treatment time the better the collection
efficiency. Typical gas velocities are 4 to 6 ft/sec (1.22 to 1.83 m/sec)
with treatment times of 8 to 12 seconds.
Application. Scrubbers are used to remove acid gases such as SO2
and HCl from flue gas streams. Scrubbers have an added benefit of removing
some NOx, but they are not applied specifically for NOx control. Dry flue gas
desulfurization (FGD) is applicable for controlling SO2 when the required
removal rate does not exceed 85 percent efficiency. Wet scrubbers are used
when the S2 inlet loading is high, or when the required collection efficiency
exceeds 85 percent.