When the fuel is well ignited, its temperature will be far above that of ignition. While combustion is taking place, if the temperature of the elements is lowered (by whatever means) below that of ignition, combustion will become imperfect or will cease, causing waste of fuel and the production of a large amount of soot.

Since it is the purpose to develop into heat all the latent energy in the fuel, it is important that the temperature of the fuel be kept as high as practical. The maximum temperature attainable will depend generally on four factors:

1. It is impossible to achieve complete combustion without an excess amount of air over the theoretical amount of air required, and the temperature tends to decrease with the increase in the amount of excess air supplied.

2. If this excess air be reduced to too low a point, incomplete com- bustion results and the full amount of heat in the fuel will not be liberated.

3. With high rates of combustion, so much heat can be generated that, in a relatively small space, even if the excess air is reduced to the lowest possible point, the temperatures reached may damage the containing vessel.

4. Contrary-wise, if the containing vessel is cooled too rapidly (by whatever means), the temperature of the burning fuel may be lowered resulting in poor efficiency.

The rate of combustion, therefore, affects the temperature of the fire, the temperature increasing as the combustion rate increases, provided that relation of fuel to air is maintained constant.

Temperatures of 3000°F may be reached at high rates of combustion and low amounts of excess air and may cause severe damage to heat resisting materials and other parts of the containing vessel.

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