Document Nevy6Q6warv8664b6BbammyJV

326 CHAPTER 13 1951 Guide pressure and evaporate when pressure is relieved, obtaining the heat of vaporization from the surrounding air or ground. As butane boils at 32 F, tanks must be buried below the frost line, if used in the colder climates. Propane, which has a boiling point of -- 40 F, is more often used where temperatures fall below freezing. These gases are supplied in small mobile tanks, but when used for heating, a large high pressure tank is usually used and the gas delivered by tank truck to the consumer, much the same as oil. These gases, when mixed with air, are used to augment peak loads of many gas companies, and in smaller cities, where gas manufacturing plants are uneconomical, liquefied petroleum-air mixtures are delivered through the mains in place of manufactured gas. FUNDAMENTAL PRINCIPLES OF COMBUSTION Combustion may be defined as the chemical combination of a substance with oxygen, with a resultant evolution of heat.' The rate of combustion depends partly upon the specific rate of reaction of the combustible sub stance with oxygen, partly upon the rate at which oxygen is supplied, and upon the temperature obtained due to surrounding conditions. . Complete combustion is obtained when all of the combustible elements in the fuel are oxidized with all of the oxygen with which they can combine. All of the oxygen supplied may not be utilized. Perfect combustion is defined as the result of supplying the required amount of oxygen for combination with all of the combustible elements of the fuel, and utilizing all of the oxygen so supplied. The oxygen required for the process of combusion is obtained from air which is a mechanical mixture of oxygen, nitrogen and small amounts of carbon dioxide, water vapor and inert gases. These inert gases are gen erally included with the nitrogen. Hence, the composition of air (percent) may be taken as: 20.9 oxygen and 79.1 nitrogen by volume, and 23.15 oxy gen and 76.85 nitrogen by weight. The combination of oxygen with the combustible elements and com pounds of a fuel is in accordance with fixed laws. In the case of perfect combustion the reactions and resultant combinations are shown in Table 7. The most important condition governing the process of combustion is temperature. It is necessary to bring a combustible substance to its ig nition temperature before it will unite in chemical combination with oxy gen to produce combustion. The ignition temperatures for several of the combustible constituents of fuels are presented in Table 7. HEAT OF COMBUSTION As previously stated, the process of combustion results in the evolution of heat. The heat generated by the complete combustion, of a unit of fuel is constant for a given combination of combustible elements and com pounds, and is known as the heat of combustion, calorific value, dr heating value of the fuel. The heat of combustion of the several substances found in the more common fuels is given in Table 7. The calorific value of a fuel may be determined either by direct measure ment of the heat evolved during combustion in a calorimeter, or it may be computed from the ultimate analysis and the heat of combustion of the several chemical elements in the fuel. When the heating value of a fuel is determined in a calorimeter, the water vapor is condensed and the latent heat of vaporization is included in the heating value of the fuel. The heat ing value so determined is termed the gross or higher heating value, and T able 7. General Data of Combustible Elements and Compounds a S'' ' ? "- Fuels wd tk)nfi`lstion