MIL-HDBK-1003/6
Rigid Frame Type. In this type of electrostatic precipitator the
6.3.5
discharge electrodes are not tensioned by weights, but are rigidly supported
by attachment to a discharge electrode frame. In some cases, the use of wires
is completely eliminated by substituting sharply pointed structural members in
the discharge electrode frame. This design has been in common use outside the
United States and is sometimes referred to as the European design. It is now
coming into use in the United States as an alternative to the weighted wire
design.
Flow Considerations. The inlet and outlet duct of the precipitator
6.3.6
should provide for uniform flow with the flow evenly distributed across the
precipitator. The use of turning vanes and/or distribution plates is often
necessary to reduce turbulence. To reduce the risk of uneven flow in the
design, model studies are sometimes advisable. The additional cost of the
model study may be advisable in some cases which involve unique designs or
unusually abrupt changes in flow direction near the precipitator and under
such conditions, the designer considers the risk of the uncertainty to be
unacceptable.
Electrical Sectionalization. The number of chambers, fields, power
6.3.7
supplies per field, and bus sections per field must be determined based on
reliability, efficiency, and process requirements. Sectionalization, which is
the number of power supplies per installation, produces improved performance,
reliability and maintenance flexibility by using several smaller power
supplies in lieu of fewer large power supplies.
Hoppers. For ash handling facilities, consider the number and
6.3.8
types of hoppers, hopper material, hopper slope or wall angle, and ash
conveying system. Adequate access to all compartments, chambers, hoppers,
ducts, and between the fields should be provided for maintenance. These
access openings must be provided with a positive means of preventing entry
while the high voltage electrodes, which are inside, are energized.
Rapper Systems. It is necessary to rap or vibrate the discharge
6.3.9
and collection electrodes to remove collected ash. It is important that this
system be capable of creating sufficient acceleration forces at the electrode
surface where the ash collects to dislodge the accumulated material. The
transmission of these forces from the rapper or vibrator to the electrodes is
just as important as the rapper or vibrator itself. To assure adequate
cleaning action and to minimize re-entrainment of ash into the gas stream, the
electrode area served by each rapper or vibrator should be minimized. Rapping
and vibrating controls should be adjustable for duration, frequency, and
intensity and should prevent operation of more than one rapper or vibrator at
a time.
Process Variables. The electrostatic precipitator must be matched
6.3.10
to the process using the common design parameters of gas flow, temperature,
and pressure, as well as the fuel (coal) analysis and resulting ash chemistry,
dust concentration, particle size distribution, moisture content and flow
distribution. Consideration should be given to the future variability of the
fuel supply. Figure 20 demonstrates how the performance of a precipitator
designed for 99 percent collection efficiency is reduced when coal with a low
sulfur content is substituted for the specified fuel.
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