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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