Perfect combustion is the result of
supplying just the right amount of oxygen to unite with all the
combustible constituents of the fuel, and utilizing in the combustion
all of the oxygen so supplied that neither the fuel nor the oxygen
may be left over.
Complete combustion, on the other hand,
results from the complete oxidation of all the combustible
constituents of the fuel, without necessarily using all the oxygen
that is left over. Obviously, if extra oxygen is supplied, it must be
heated and will finally leave the boiler carrying away at least part
of the heat, which is thereby lost.
If perfect combustion could be obtained
in a boiler there would be no such waste or loss of heat. The more
nearly complete combustion can approach a perfect combustion, the
loss will occur in the burning of a fuel. The problems of design and
operation of a boiler are contained in obtaining as nearly as
possible perfect combustion.
When fuels are burned, they not only
produce the combustion products indicated in the chemical equations
listed above. More importantly, they also produce heat. The heat will
cause the temperature of the gases and the surrounding parts to rise.
The distinction between temperature and heat must be clearly understood. Temperature defines the intensity, that is, how hot a substance is, without regard to the amount of heat that substance may contain.
For example, some of the boiling water
from a kettle may be poured into a cup; the temperature of the water
in the kettle and the cup may be the same, but the amount of heat in
the greater volume of water in the kettle is obviously several times
the amount of heat contained in the water in the cup.
If two bodies are at different
temperatures, heat will tend to fl ow from the hotter one to the
colder one, just as a fluid such as water tends to flow from a higher
to a lower level.
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