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Electrostatistic Precipitation 185

outlet duct connections, inspection doors, and insulator casings. A variety of construction
materials are used to meet specific gas and particulate properties, operating practices, and
other factors. Construction materials for the electrostatic precipitator can be either rein-
forced concrete supports, frame with brick wall, or steel throughout. Steel construction
permits shop fabrication before on-site installation. Concrete shells are not recommended
°
when the operating temperature exceeds 260 C.
The insulation of an ESP is also one of the basic design requirements in order to
prevent the ESP from the condensation of water vapor and acidic gases. Condensation
of moisture and acidic gases on the outer shell walls results mainly from the contact of
cool ambient air, which could cause potential corrosion of construction materials. Some
electrically controlled heating blankets are usually used for this design. In addition,
corrosion-resistant materials are generally used when the effluent gas stream of an
industrial process contains corrosive gases.
The most common material used in ESP construction is carbon steel, in cases where
the gas stream contains high concentrations of SO or where liquid–gas contact areas
3
are involved, stainless steel may be required. However, by keeping the emission stream
temperature above the dew point and by insulating the ESP (the temperature drop
°
across an insulated ESP should not exceed 20 F), the use of stainless steel should not
be necessary.

3.8. Flue Gas Conditioning

As mentioned earlier in this section, gas temperature, moisture content, and chemical
composition of flue gas have strong influence on particulate resistivity and, thus, on the
collection efficiency of particles. However, the gas temperature often cannot be decreased
owing to the possibility of acid and moisture condensation. Ducting can sometimes be
arranged such that the gas temperature entering the ESP can be raised, but this procedure
is usually quite costly. Therefore, chemical conditioning of flue gas becomes one of the
most practical approaches. Several flue gas conditioning systems are commercially
available and work quite well with reasonably small expense.
Flue gas conditioning is extremely important for the improvement of particulate
resistivity. Flue gas can be conditioned by adjusting gas temperature and moisture con-
tent, by adding sulfur trioxide and ammonium to the gas stream, and by varying sulfur
content of fuel. Among them, moisture conditioning can be accomplished by stream
injection or by liquid water spray into the dusty gas stream. Proper spray nozzle
design, spacing, and careful temperature control are crucial. If too much water is
injected, the particles will cake on the interior of the ESP (27) and increase the
sparkover rate, which results in the decrease of particulate collection efficiency in an
ESP (7). Figure 7 illustrated the effectiveness of adding moisture to cement kiln dust
exhaust streams. In addition to the temperature and humidity of the gas stream, chem-
ical compounds such as SO , NH , and NaCl are commonly used as conditioning
3 3
agents.
3.9. Removal of Collected Particles
Particles accumulating on the collecting plates must be removed periodically. In wet
ESPs, the liquid flowing down the collector surface removes the particles. In dry ESPs,