Document ybQr4dGZX67QRNq87xzXg3oO4

332 ; CHAPTER 16 ' .1949. Guide. Fuels>and Combustion 333 of gas and fixes a minimum limit to its heat content. The gross or higher calorific value usually ranges between 520 and 545 Btu per cu ft with an average of 535. A given heat value may be mainlined and yet leave considerable latitude in the composition of the gas so that as distributed the composition is not necessarily the same in different districts, nor at successive times in the same district. However, in any community the variations in gas composition are held within suitable limits so that the performance of approved gas appliances will not be adversely affected. 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, and for engineering purposes the values given herewith may be used. T able 7. General Data o r Combustible Elements and Compounds ^Values In table taken chiefly from page 61 of Fuel Flue Gases published by American Cas Association. t>Gas measured a t AO F and &0 in. Ha. . Value from National Bureau <4Standards. Bt Volume, Peb Cent Br Weight, Peb Cent Nitrogen, Nt................................................ 20.9 79.1 23.15 76.85 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, or healing valve 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 s'.