provide automatic vibrator operation only at the inception and during an
evacuation cycle. Provide manual override control for hopper vibrators and
evacuation system in hopper area and enclose in (a) case(s) to prevent
accidental energization of systems. Place a warning over the vibrator
manual control with the following inscription:
"WARNING: VIBRATOR CONTROL.
DO NOT ACTIVATE UNLESS HOPPER EVACUATION
SYSTEM IS OPERATING."
2.5.3
Hopper Heater System
Provide a hopper heater system for each precipitator.
2.5.3.1
Hopper Heater System Design
Provide the system complete with all material required for mounting. The
system shall provide a 139 degrees C 250 degree F rise in temperature in
the hopper, in the vicinity of the heaters, during offline and startup
conditions. Size the system to provide a hopper skin temperature of not
less than 177 degrees C 350 degree F when the insulation is in place during
minimum ambient temperatures specified in Section 15080 THERMAL INSULATION
FOR MECHANICAL SYSTEMS. Design the system with a minimum heating safety
factor of 1.1 and a minimum wind heat loss factor of 1.12. Design the
system to provide maximum heater coverage between hopper stiffeners
utilizing modular heaters and flexible blanket or tape heaters for the
hopper throat heating. Heater modules shall cover not less than 33 percent
of the hopper area. Cover the bottom portion of the hopper to the maximum
extent possible, and extend at least 70 percent up the hopper height.
Provide a two zone system. Comprise the lower zone of heaters located on
the bottom one-third of hopper height including the throat heater; the
upper zone shall include the remaining hopper heaters. Use flexible
electric heating blankets or tapes, capable of withstanding 427 degrees C
800 degree F, where modular equipment will not fit. Provide only equipment
designed to withstand natural and induced vibrations, plus shock loadings
normally experienced during operation of the precipitator and ancillary
equipment including manual rapping of the strike plates. Provide an
individually, thermostatically controlled hopper heater system with
adjustable setpoint and include power, control, and alarm components.
Locate the low temperature and control thermocouples in the lower portion
of each heater zone. Heater voltage shall be 480 volts AC. Control
voltage shall be 120 volts AC.
a.
Hopper Heater Module Design: Provide self-contained, modular
heaters. Provide hopper heater modules which have a flexible
heating face to conform to the irregularities of the hopper
surface, providing contact between the heaters and the hopper, and
providing maximum heat transfer. Provide low watt density design
modules with a maximum of 0.0047 watts per square mm 3 watts per
square inch of resistance element and with a minimum of six
parallel resistance paths per heater. Continuous blanket type
elements shall be deemed to meet the multipath requirement. Each
module shall have dual heating elements. Both elements shall
function during startup and offline conditions. To reduce power
consumption and cycling while maintaining the hopper temperature
during online operating conditions, controls shall automatically
switch off one element in the lower zone and both elements in the
upper zone without affecting the remaining element's operation.
The hopper throat blanket heater shall have a single heating
element and shall remain on during startup, offline, and online
SECTION 15862N
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