This value is of particular importance in waste heat work and it is regrettable that there is such a variation in the different experiments. Mallard and Le Chatelier determined values considerably higher than any given in [Table 28] . All things considered in view of the discrepancy of the values given, there appears to be as much ground for the use of a constant value for the specific heat of air at any temperature as for a variable value. Where this value is used throughout this book, it has been taken as 0.24.
Air may carry a considerable quantity of water vapor, which is frequently 3 per cent of the total weight. This fact is of importance in problems relating to heating drying and the compressing of air. [Table 29] gives the amount of vapor required to saturate air at different temperatures, its weight, expansive force, etc., and contains sufficient information for solving practically all problems of this sort that may arise.
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Column 5 = barometer pressure of 29.921, minus the proportion of this due to vapor pressure from column 4.
COMBUSTION
Combustion may be defined as the rapid chemical combination of oxygen with carbon, hydrogen and sulphur, accompanied by the diffusion of heat and light. That portion of the substance thus combined with the oxygen is called combustible. As used in steam engineering practice, however, the term combustible is applied to that portion of the fuel which is dry and free from ash, thus including both oxygen and nitrogen which may be constituents of the fuel, though not in the true sense of the term combustible.
Combustion is perfect when the combustible unites with the greatest possible amount of oxygen, as when one atom of carbon unites with two atoms of oxygen to form carbon dioxide, CO2. The combustion is imperfect when complete oxidation of the combustible does not occur, or where the combustible does not unite with the maximum amount of oxygen, as when one atom of carbon unites with one atom of oxygen to form carbon monoxide, CO, which may be further burned to carbon dioxide.
Kindling Point—Before a combustible can unite with oxygen and combustion takes place, its temperature must first be raised to the ignition or kindling point, and a sufficient time must be allowed for the completion of the combustion before the temperature of the gases is lowered below that point. [Table 30], by Stromeyer, gives the approximate kindling temperatures of different fuels.