Page 77 - Boiler plant and distribution system optimization manual

62 Boiler Plant and Distribution System Optimization Manual
3. Possible ruptured boiler tube or other com-
ponent which is cooling the gases with steam
or water.
4. In negative draft units, cold air may be en-
tering boiler through open door or defective
wall or skin.
THE DANGERS OF CARBON MONOxIDE
An example of the dangers of carbon monox-
ide is evident in a report from Canada. Between
1973 and 1983 there were 293 reports of carbon
monoxide poisoning, including 145 deaths. Also,
combustion systems caused 238 deaths during
the same period.
What is Carbon Monoxide?
It is the product of incomplete combustion
and is a flammable colorless and odorless gas.
Carbon monoxide is about the same density as
the air that we breath. Therefore, easily mixes to
form a deadly atmosphere.
The major hazards of carbon monoxide are
it’s toxicity and flammability. Carbon monoxide
becomes a combustible gas when its concentra-
tion reaches 12.5% by volume (125,000 ppm).
Carbon monoxide is classified, however, as
a chemical asphyxiant which produces a toxic
action by preventing the blood from absorbing
oxygen. Since the affinity of carbon monoxide is
200-300 times that of oxygen in blood, even small
amounts of carbon monoxide in the air will cause
toxic reactions to occur.
If breathed for a sufficiently long time, a
carbon monoxide concentration of only 50 ppm
will produce symptoms of poisoning. As little
as 200 ppm will produce slight symptoms like a
headache or discomfort in just a few hours. A con- tent heat. If this latent heat can be extracted from
centration of 400 ppm will produce a headache
and discomfort in two to three hours. The effect
at higher concentrations may be so sudden that a
person has little or no warning before collapsing.
It should be noted that all of these values are ap- in the flue gas. In flue gas condensing systems the
proximate and vary as to the individual.
To prevent over-firing and under-firing the
fuel supply pressure must be tested from time to
time to insure the firing rate has not shifted be-
cause of a pressure change.
In small boilers or furnaces, a daft measure-
ment is necessary to guard against a gas reversal
where exhaust gases are escaping to the environ-
ment, which is potentially toxic. A small draft
gage will indicate problems, like building ex-
haust fans pulling gases back down the chimney,
obstructions in the exhaust system, down drafts
from high wind conditions and defective stack
covers and other problems.
CHIMNEY EFFECT
When taking draft readings, insure that
the system is warmed up to normal operating
temperatures. As air is warmed, it expands and
the same weight of air will take up more space
becoming lighter. This warm “light” air will rise
up the chimney decreasing the furnace pressure.
If this low pressure is not established, combustion
products may escape. If the draft is too high, and
the hot gases are creating too much negative draft
heat will be lost up the stack.
CONDENSING FLUE GASES TO
IMPROVE EFFICIENCY
Fuel is a hydrocarbon which means that it is
made up of hydrogen and carbon. Carbon burns
dry but each pound of hydrogen that enters into
the combustion process forms about 9 pounds
of water. Now, at the 2,000 to 3,000 degree com-
bustion temperature this water is in the form of
steam and it carries a considerable amount of la-
the exhaust gases, there is an opportunity to raise
efficiency by 10% or more.
Roughly there will be about 970 Btus avail-
able from each pound of water that is condensed
exit temperature is typically about 100 F.