TM 5-810-15
(3) Combustion air flow measurement. Accu-
to develop the demand signal for the feeder master.
rate combustion air flow measurement is also
The feeder master demand signal is applied to all
important in metering type combustion control
feeders which have duplicate controls. Therefore,
systems for gas/oil fired boilers. A venturi section
as firing rate demand is increased or decreased the
or air foil flow element should be provided in the
feeder speed is increased or decreased. Coal chute
ductwork between the FD fan and the burner
air flow compares measured air flow to load flow
windbox or a piezometering may be installed at the
to operate the coal chute air damper. A feed
FD fan inlet. The flow element will be designed to
forward signal based on rate of change is also used
provide a design differential pressure across the
to modulate the coal chute air control damper. Coal
flow element of not less than 2 inches wg at full
feeder and coal chute air damper control is shown
load conditions. The flow transmitter selected for
in figure 9-5.
(2) Combustion air control. Combustion air
combustion air flow will be a differential pressure
transmitter that is accurate in the range of
flow in ACFB systems consists of primary air flow,
differential pressure developed by the flow element.
overfire air flow, stripper cooler air flow, and main
(4) Oil atomization. The oil to the burner will
fuel chute air flow. Primary air is introduced below
be atomized utilizing steam or compressed air. A
the bed and keeps the fuel and bed in suspension.
control valve installed in the atomizing steam or air
Overfire air is delivered by the FD fan and is
line will be controlled to maintain the atomizing
utilized at loads above 50 percent to control
medium pressure above the oil supply pressure to
furnace exit gas temperature. The stripper cooler
the burner.
air flow is utilized to cool the excess bed material
h. Atmospheric circulating fluidized bed (ACFB)
which is removed in the stripper section. Main fuel
boiler.
chute air flow is utilized to sweep the fuel tube to
(1) Fuel flow control. Main fuel flow in an
the combustor. Total air flow is the sum of primary
ACFB system is established by fuel flow through
air, overfire air, stripper cooler air, and coal chute
the feeder to the combustor. The volumetric rate of
air flow. All air can be supplied by the FD fan or
fuel flow is directly related to feeder speed. The
separate primary air and FD fans may be utilized.
coal feed demand speed utilizes the lower of the
The primary air, overfire air and stripper cooler are
total air flow or firing rate demand as the set
controlled by positioning the appropriate damper.
point and compares the set point to total fuel flow
9-7
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