Page 207 - Air Pollution Control Engineering

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186 Chung-Shin J. Yuan and Thomas T. Shen

the particles are removed by vibrating or rapping the collector plates. For dry ESPs, this
is a critical step in the overall performance because improperly adjusted or operating
rappers can cause re-entrainment of collected particles or sparking because of excessive
particulate buildup on the collection plates or discharge electrodes. In normal operation,
dust buildup of 6–25 mm is allowed before rapping of a given intensity is initiated. In
this way, collected material falls off in large clumps that would not be re-entrained. If
rapping is initiated more frequently or if the intensity of rapping is lowered, the result-
ing smaller clumps of particulate matter are more likely to be re-entrained, reducing the
collection efficiency of the ESPs. Optimal adjustment of the ESP can best be made by
direct visual inspections through sight ports.
In the ESP, the collected particles or droplets on collecting plates are generally
removed by rapping or by washing. The design of electromechanical rapping
includes an electric motor drive, pneumatic drive, or magnetic impulse drive. The
selection of the type and number of rappers varies among manufacturers and with the
characteristics of particles being collected. Modern precipitators are designed for two
independent electromechanical rapping systems: one for keeping the high-voltage
discharge electrodes continuously clean, and the other for sequential rapping of the
collecting electrode modules.
If rapping does not provide for complete cleaning of the electrodes, particulate
collection efficiency of the precipitator may decrease in the course of operation
owing to particulate buildup. As a consequence, the condition of inadequate rapping
may require an increase of corona power input in order to maintain the level of par-
ticulate collection efficiency. Successful rapping depends mainly on a certain range
of particulate resistivity at various temperatures. For certain particles, the applica-
tion of an adhesive to the collecting electrodes is necessary. In this case, the removal
of the accumulated material can only be accomplished by a washing procedure. After
washing is completed, the adhesive fluid again is applied before the unit is put back
in operation.
In most American designs, the collecting plates are rapped by a falling weight. The
intensity of the rap can be easily adjusted by varying the height from which the weight
is dropped or by adjusting the acceleration field strength. In a typical European design,
rapping is accomplished by hammers connected to a motor rotating at a constant speed.
Thus, to adjust the rapping intensity, the hammers must be changed physically. Generally,
2
2
one rapping unit is designed and provided for every 110–150 m (or 1200–1600 ft ) of
collection area (28). Both designs allow for convenient adjustment of the rapping interval
varying from 1 to 10 min.
Hoppers are designed to catch the falling particles as well as to provide space for
temporary storage. Most hoppers have a pyramidal shape that converges to either a round
or square discharge. Hopper walls must be steeply sloped (at least 60% slope) to pre-
vent dust caking and bridging. In addition, hoppers are often heat traced because warm
dust flows much better than cold dust. In general, approx 60–70% of the collected dust
can be removed through the first inlet set of the hoppers. However, in the case of fail-
ure of the first electrical set, the dust load is then transferred to the next downstream
hopper. Therefore, liberal sizing of the hoppers is recommended. Proper support structure
must be provided so that a hopper will not collapse when filled with dust. The discharge