POWER February 2011 - 47

INSTRUMENTATION & CONTROL
4. Dual arrangement. With this approach, a portion of tempering air is injected upstream of
the oxidation catalyst and the balance of the tempering airflow is injected into the section between
the oxidation and reduction catalyst. In this arrangement, the efficiency of the oxidation catalyst is
maximized and its overcooling at partial loads is prevented. Courtesy: Peerless Mfg. Co.
TE
100
TE
200
TE
300
TE
400
TE
500
TE
600
and the head increases, thus some energy
savings are achieved.
The other blower capacity control is the
variable speed control. With this control
both the flow and the head are reduced. This
method is more efficient than mechanical
capacity control. In both cases, the response
from the tempering blower will be different
for small and large changes.
In addition, the effects of increased or
decreased tempering airflow are influenced
by the mixing efficiency of tempering air
and exhaust gas. Response time will be site
specific, depending upon the particular exhaust
system dimensions and the exhaust
flow to tempering airflow ratio.
Inject at Multiple Points. A third
Flue gas
Perforated
plate
TE
20
Oxidation
catalyst
Reduction
catalyst
Tempering air
A feedforward control system will adjust
tempering airflow, and corrective action
is taken before the output temperature
deviates from the setpoint. The required
amount of tempering air can also be determined
from thermodynamic calculations,
taking into account exhaust gas operating
conditions and the ambient air temperature.
In this control the amount of tempering
air will be adjusted with any detected
load swings prior to observing any temperature
change at the reduction catalyst.
Because some of the process disturbances
are not measured, and some may even be
unknown, the feedforward control is coupled
with feedback control to compensate
for errors.
A feedback control system is the simplest
system to control the tempering air
blower output. The average temperature of
the operating exhaust temperature, measured
as shown in Figure 3, is fed to a
controller and compared with the setpoint.
The setpoint is typically about 850F. The
control system governs the amount of temFebruary
2011 | POWER
pering air injected into the exhaust system.
The feedback system lags behind changes
in the exhaust temperature. The corrective
action is not taken until the output temperature
deviates from the setpoint. However,
this system is ideal where the system
oscillations are minimal. The feedback
control strategy requires some time before
the exhaust gas temperature stabilizes and
steady state operation is achieved.
Also note that each proportional-integrative-derivative
(PID) control loop must
be tuned based on the required speed of
response and the absolute value of temperature
error when a disturbance occurs. The

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POWER February 2011

Table of Contents for the Digital Edition of POWER February 2011

Contents
POWER February 2011 - Cover1
POWER February 2011 - Cover2
POWER February 2011 - Contents
POWER February 2011 - 2
POWER February 2011 - 3
POWER February 2011 - 4
POWER February 2011 - 5
POWER February 2011 - 6
POWER February 2011 - 7
POWER February 2011 - 8
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POWER February 2011 - Cover3
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