TM 5-810-15
Air supply is maintained by modulating the FD fan
characterized based on load to obtain the proper
and primary air fan inlet vanes or dampers to
primary-to-overfire air flow ratio. The upper over-
maintain pressure in the FD fan outlet duct.
fire air dampers are closed below 50 percent load.
(3) Combustion air flow measurement. Accu-
The primary air controller setpoint is low limited by
rate combustion air flow measurement is also
a minimum primary air setpoint. Each primary and
important in combustion control systems for ACFB
upper air flow controller compares measured air
boilers. Measurement of primary air flow, overfire
flow to setpoint. The controller output becomes the
air flow, stripper cooler air flow, and coal chute air
demand to its respective air flow control damper.
flow will be measured with a venturi section or air
All air flow measurements should be temperature
flow element in the ductwork to the various
corrected. Furnace exit gas temperature should be
equipment. The design pressure differential pres-
monitored and at high temperature alarmed to
sure across the air flow elements will be not less
allow the operator to make the proper air flow
than 2-inches wg at full load. The transmitters
adjustment to bring the temperature back to
selected for primary air flow and secondary air flow
normal. A bias adjustment normally is provided for
will be differential pressure transmitters that are
each controller.
(5) Furnace bed inventory control and solids
accurate in the range of differential pressure
cooler temperature, air flow and spray water con-
developed by the flow elements.
(4) Primary and overfire air flow control. Fig-
trol. Furnace bed inventory control requires re-
ure 9-6 shows the primary and overfire air flow.
moval of excess bed material from the furnace. The
The firing rate demand signal serves as an index for
solids cooler cools the excess bed material to a
air flow demand. The fuel feed signal and firing rate
temperature which allows it to be disposed of via
demand signal are cross limited by a high selector
the ash system. Solids are removed from the
to serve as the setpoint for the total air flow
furnace either by operator action or automatically
controller. The output of the total air flow
on high furnace plenum pressure. The furnace bed
controller becomes the setpoint for the primary and
static pressure, total furnace differential pressure,
overfire air flow controllers. The setpoint is
9-8
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