TM 5-810-15
ized coal fired and ACFB boilers. Variations in the
allows the operator to load the boiler over its
steam pressure setpoint adjust the fuel flow input.
complete operating range. The fuel valves and air
Since steam flow is directly related to heat release
damper are operated by the same drive through a
of the fuel, and because a relationship can be
mechanical linkage. The gas and oil valves include
established between heat release and combustion
cams which are adjusted at start up to maintain
air requirements, steam flow can be used as an
proper fuel air ratio over the operating range of the
index of required combustion air. Note however
boiler. Parallel positioning systems with fuel/air
that this relationship is true only at steady load
ratio control as shown in figure 9-2(b) are suitable
conditions.
for use on gas/oil and stoker fired units. This
c. System categories. Combustion controls can
system allows the operator to adjust the fuel/air
be further divided into two categories within the
ratio for the entire load range of the boiler. The
basic types: positioning control and metering con-
addition of steam flow correction of air flow to
trol, as shown in figure 9-2.
parallel positioning with fuel/air ratio control
(1) Positioning control. Positioning systems
creates a system suitable for use on ACFB, gas/oil
require that the final control elements move to a
and pulverized coal fired units as shown in figure 9-
preset position in response to steam pressure
2(c). This system uses variation of steam pressure
variations from a setpoint. Series positioning con-
from a setpoint to initially control fuel and air
trol will not be covered here since it is only used on
inputs. The system recorrects combustion air flow
very small boilers operating at constant loads.
using steam flow as a setpoint for air flow, since
Parallel positioning systems that use a mechanical
steam flow is a function of fuel Btu input (inferred
jackshaft to simultaneously position fuel feed and
fuel flow). This system relates directly to a series-
air flow from a single actuator apply to packaged
parallel type control with the addition of a feed-
type gas/oil fired boilers in the 20,000 to 70,000
forward signal from the steam pressure controller
pph size and is shown in figure 9-2a. This system
to the combustion air control.
9-3
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