Page 430 - Boiler_Operators_Handbook,_Second_Edition

Controls 415

erably in its hydrogen to carbon ratio so it wasn’t pos- function generator. Modern digital controllers have a
sible. I encouraged them to use an online analyzer that similar application except you simply enter numbers
could be used to automatically adjust the fuel air ratio instead of measuring a plastic or aluminum plate and
for the gas being burned and incorporate that into an cutting it to get the desired shape to produce the output.
equivalent fuel value applied to the firing rate controls It’s another blessing of microprocessor based controls
but I don’t know if it was ever done. One tricky part of that you can easily change them, you don’t have to cut a
that application would be providing a means of account- new cam if you made a mistake at one point.
ing for the time delay between the analyzer’s response On jackshaft controlled boilers the trim is accom-
to the waste gas values and the time it takes for the gas plished by adjusting the linkage connecting the fan
to get from the analyzer to the burner. damper to the shaft so changes in the relative position
In earlier times we had a separate setpoint genera- of damper and jackshaft alter the air to fuel ratio. The
tor for the gas controller with a low signal selector that adjustment has to be made in a manner that maintains
would reduce the gas firing rate when the fuel flow was some relationship to firing rate because a change in
at minimum. These systems are more dependent on damper position near maximum fire that would be con-
the contracts with your fuel suppliers than any other sidered minimal can be major change in air flow when
parameter so, if you have one, realize that you’re trying the burner is at low fire. Once again microprocessor
to use up fuel you’ve paid for without using any more— based controls serve to recognize those problems and
which normally costs an arm and a leg. correct for them but, if you have an older system, you
should be aware that the same correction at high fire has
to have a much smaller effect on air flow at low fire; the
FIRING RATE CONTROL—OXYGEN TRIM same rules for linearity exist.
Oxygen trim control of parallel positioning sys-
Oxygen trim controls actually measure and con- tems (including steam flow/air flow, inferential and full
trol excess air. The oxygen content of the flue gas is metering) should use a multiplier to change the relation-
controlled but it’s an indicator of excess air. An ana- ship of fuel valve and fan damper position for oxygen
lyzer samples the flue gas in the furnace or at the outlet trim control. That way any change in the two signals is
of the boiler to determine the amount of oxygen in the proportional to load. Multipliers are not an easy device
gas. The analyzer transmits a proportional signal to a to make for pneumatic systems so many use a simple
controller which then changes the firing rate controls to bias adjustment, adding to or subtracting from the sig-
alter the fuel to air ratio to maintain the oxygen content nal to the damper positioner to trim the air to fuel ratio
at a setpoint. and maintain an oxygen setpoint. On inferential and
Almost any oxygen trim system you encounter will full metering controls the air flow signal is modified by
not have a simple oxygen setpoint because the amount the oxygen trim so the output of the transmitter should
of excess air required does vary with load. In most boil- be multiplied by the correcting output of the oxygen
ers the excess air can be held constant at loads over 50% trim controller to change it proportionally over the load
of maximum but it has to increase almost exponentially range.
as the load decreases (see the section on burners for rea- These controls became acceptable during my later
sons). The common approach is to generate an oxygen years in the business and represented another step for-
setpoint that, for all loads up to about 50% is a function ward in technology and reduced manufacturing costs.
of the boiler master signal or the steam flow signal. Originally only utility boilers could be equipped with
I prefer steam flow because it produces higher oxy- oxygen trim control because the analyzers required al-
gen requirements when increasing the firing rate of a cold most constant maintenance and recalibration. Hot wire
boiler. It’s when more excess air is needed to complete analyzers which combined a flue gas sample with some
combustion because the furnace and refractory are not hydrogen and heated it until the hydrogen burned were
as hot so the flame temperatures are lower. The common the first analyzers to prove partially reliable and low
approach is to use a function generator which allows the enough in cost to be used in industrial plants.
technician setting up the control to produce an output The paramagnetic analyzer which used the differ-
that bears no mathematical relationship to the input. ence in oxygen content of a gas to disturb a magnetic
Data collected during firing tests on the boiler (to field then followed. Both required drawing a sample of
determine the necessary amount of excess air at each the flue gas from the boiler or stack and conditioning
load) can be used to determine how to cut a cam in the it before analysis. They used water systems to cool the