Page:International Library of Technology, Volume 93.djvu/90

 rule has yet been devised by which to compute the heat value of any chemical compound from its formula and the heat values of the elements of which it is composed. Hence, the heat values of compounds must be found by a separate determination for each one in the laboratory. The heat developed by the combustion of some of the commoner fuels and gases is given in Table IV. In the case of carbon, the heat developed by its complete combustion, forming CO2, and the heat of its partial combustion to CO, are given; also the heat of combustion of CO to CO2.

26. Heat Value of a Mixture. — The heat value of a mixture may be found from the heat values of the substances of which it is composed and the percentage of each substance. If h1, h2, h3, etc. represent the heat values of the substances forming the mixture, and p1, p2, p3, etc. represent the percentage of each substance, the heat value of the mixture will be represented by the following formula: $$h_m = p_1 h_1 + p_2 h_2 + p_3 h_3 +$$ etc.

Example. — A certain gas has the following composition:

Constituents of Gas Per Cent. Hydrogen, H 20 Marsh gas, CH4 70 Acetylene, C2H2 10

What is the heat value per cubic foot of the mixture?

Solution.— Referring to Table IV, the heat values per cubic foot of these gases are seen to be 327, 1,010, and 1,464 B. T. U., respectively. Apply the formula just given. p1 = .20, p2 = .70, and p3 = .10. Also, h1 = 327, h2 = 1,010, and h3 = 1,464. Substituting, hm = .20 × 327 + .70 × 1,010 + .10 × 1,464 = 65.4 + 707 + 146.4 = 918.8 B. T. U. Ans.

27. Temperature of Combustion. — The theoretical temperature of the combustion of a given fuel can easily be calculated. Making no allowance for losses of heat, and supposing that just enough air is furnished for the combustion, burning carbon should have a temperature about 4,940° above zero; while burning hydrogen should have a temperature of about 5,800° above zero. In practice, these temperatures are never attained, on account of heat losses. Heat usually escapes so rapidly through the cylinder walls, and so much