Page 433 - Boiler_Operators_Handbook,_Second_Edition

418 Boiler Operator’s Handbook

Also don’t expect oxygen trim to be a cure all. per fire with all the stove air inlets shut to heat up the
There is a definite lag in time between the change of an stack before starting a wood fire.
air to fuel ratio on a burner and the appearance at the The air that is pulled into the draft hood from the
analyzer cell of a gas sample that is the result of that room goes up the stack. It cools the stack gases which
change. If the controls aren’t aligned properly to main- lowers the natural draft until there isn’t a significant dif-
tain an air to fuel ratio with load changes don’t expect ference between the pressure in the room and the pres-
the oxygen trim controls to correct that. If you see the sure under the hood. Since the boiler outlet is under the
trim controller output change with load that’s what it’s hood the pressure at the outlet and the boiler inlet dif-
trying to do. You should also be aware that the oxygen fers by the natural draft through the boiler. If the hood
measured at the analyzers didn’t necessarily come wasn’t there the pressure at the boiler outlet could vary
through the burners; this is particularly true on induced so much that it could blow out the fire, as on cold days,
draft and balanced draft boilers where the furnace and or be so high that you wouldn’t get enough air for clean
boiler passes are at a lower pressure than atmospheric combustion. That draft hood stabilizes a fixed fire opera-
and air can leak in at several points after the burners. tion to ensure maintenance of the air to fuel ratio.
I still have friends asking me about problems with
their gas fired appliances and discover that they went
FIRING RATE CONTROL—CO TRIM
a little overboard with one of those insulating blankets
that the home stores are pushing. They plugged up the
One solution to problems with oxygen analyzers
opening to the hood!
sensing oxygen that didn’t come through the burners is
Instead of a draft hood we’ll occasionally use a
to control based on another parameter. Carbon monox-
barometric damper. That’s a single bladed, usually
ide, the result of incomplete combustion and a gas that is
round, pivoted just above its center, damper that sepa-
always present in some minute quantities in flue gas can
rates the boiler room and the stack. These usually have
be sensed and controlled. The original oxygen analyzer
stops on them so the damper will not swing out at the
problems still hold for analyzing CO and there’s the
bottom into the boiler room. As the stack draft increases
problem of its finite quantity.
the difference in boiler room pressure and the stack base
We normally control at about 50 parts per million,
forces the damper open and cold air from the boiler
that is 0.005%, a very little amount of gas in the whole
room slips into the stack to cool the stack gases and re-
and hard to measure accurately. Large utility boilers fre-
duce the draft. If the draft gets too low the damper closes
quently use it to resolve the problems with casing and
down to restrict the flow of cold boiler room air into the
ductwork leakage. The operating modes are the same as
stack. The stack temperature then increases to raise the
for oxygen trim. I’ve never installed one but more mod-
draft. Those dampers usually have a weight mounted on
ern analyzers could change all that.
a stud to adjust them, by screwing the weight in and out
you change the pressure required to open the damper.
DRAFT CONTROL Barometric dampers or some other means of con-
trolling draft is essential on systems with two or more
Many small boilers use natural draft and a natural boilers attached to the same stack. Draft is always a
means of draft control. The gas fired hot water heater in balance between the differential pressure produced by
a house is one. Most use a draft hood, nothing more than natural draft and the resistance to gas flowing up the
an open box over the outlet of the boiler. Natural draft stack. Double the flow of gas, by firing two boilers in-
up the stack produces a difference in pressure between stead of one, and the resistance to flow up the stack will
the bottom of the hood and the rest of the room so the increase by a factor of four. It’s obvious that the pressure
space under the hood is negative with respect to the at the base of the stack will differ considerably so the
room. You know, and usually check to ensure, that there flow of combustion air and flue gases through the boil-
is a negative there by holding a match or lighter near the ers will too. It’s impossible to maintain air to fuel ratios
bottom edge of the hood. If there’s a draft, air flowing in boilers with a common stack unless you have draft or
from the room into the hood will pull the flame into the metering controls.
hood. I check draft on my wood stove before lighting it Barometric dampers do a fair job but they also
by holding a flame near the top of the charging door and require a lot of additional air supplied to the boiler
open the door a little; the draft almost always pulls the room. In the winter you have to heat that air or worry
flame down into the stove. If it doesn’t, I use a newspa- about freezing some pipes. If you can’t get exhaust air