Document 3QqJbBz8OEVDvR4BbV364Kwn3

344 CHAPTER 14 1954 Guide ture obtained due to surrounding conditions. This is combustion in its :simplest form. All solid, as well as liquid and even gaseous fuels generally Contain several combustible elements in combination with others which, jdependii'ig on their nature, affect, oxygen requirements and thus govern jthe combustion process. For a continuous reaction, as in heating processes, jit is necessary to establish an effective balance between rates of removing Iheat and of supplying fuel and air or oxygen to keep the reaction going. [In establishing such a balance, consideration must be given to the removal ior venting of products of combustion, so that the entire process is one of flow wherein draft conditions in the combustion space are important. i Complete combustion is obtained when all combustible elements in a fuel are oxidized by all of the oxygen with which they will combine. All oxygen or air supplied is generally not utilized, and this excess portion is commonly referred to as excess oxygen or excess air. Excess air is usually ;expressed as a percentage of the of air required for perfect combustion. Perfect combustion results when the exact amount of oxygen required ifor complete combustion of all elements of a fuel is supplied and utilized.. jThe percentage of carbon dioxide contained in the products of combustion) [from such a reaction is obviously the maximum attainable and,is referred To as the ultimate CO? or maximum theoretical percentage of carbon dioxide:, 'This condition of perfect combustion, without having excess air or oxygen left from the reaction, is seldom, if ever, realized in practice. Most types.; of heating equipment must be sufficiently flexible in performance to pro-); vide complete combustion with not only variations in the quality of a fueliv but also changes in the rate at which it is supplied. This situation makes;; it advisable, from a practical standpoint, to insure complete combustion but not perfect combustion in the sense expressed above, To attempt to doji so would undoubtedly result eventually in unsatisfactory performance) especially from a safety standpoint.' Consequently, common types.;,of heating equipment.are usually designed, installed, and adjusted to operate; with some excess air. The exact percentage of such air depends on the type of fuel being utilized, as well as anticipated variations in its quantity; and quality. Despite these practical limitations, however, it should not? be inferred that common types of fuels cannot be utilized economical}^ Reference to flue loss charts such as Figs. 6 and 7 for gas burning equip) ment and to the air requirements discussion in Chapter 15 shows that; reasonable quantities of excess air can be used without appreciable reduc-; tions in operating efficiencies. ip'i Oxygen combines with the combustible elements and compounds of. tpYF fuel in accordance with fixed laws. The reactions and resultant products,? of perfect combustion of common fuel constituents are set forth in Table-T; All of the oxygen required for combustion is normally obtained from -tj?e, surrounding air, which is a mechanical mixture of nitrogen and oxygen with small amounts of carbon dioxide, water vapor and inert gases. For pc tical combustion calculations, air is considered to consist of 20.9 percent oxygen and 79.1 percent nitrogen by volume, and 23.15 percent oxyg1?,' and 76.85 percent nitrogen by weight. The nitrogen, being inert; pas?& through the reaction without change. Table 7 gives the air quantijjef, corresponding to the oxygen required for perfect combustion. fc; Air supplied to the combustion reaction is in most instances introduce*} in two ways. Primary air is introduced through or with the fuel,;} secondary air is supplied to the flames issuing from the fuel. : V;. Incomplete combustion is obtained when any of the combustible.elem. r. are not completely oxidized in the combustion reaction. This concha. - Fuels and Combustion combustion. _F_o,,r.,,e,,xuaAmiupele, ma uhysduraollcyarobnoen omf atyhenopt_ro_odx_uidciztse ocof minpcdloemnteaplzyleatrtecc carbon dioxide and water, as indicated in Table 7, but may also form al cohols, ketones, aldehydes, or carbon monoxide depending on where and how the reaction is interrupted. Too low-a temperature (such as may be caused by flame impingement on a cold-surface), a poor oxygen supply to the flames (due to insufficient or poorly located air supply, or smothering %C0, , % EXCESS . AIR ' rn FLUE GASES . 1 IN FLUE OASES 1 % excess/ur :%ca 15 a -I i ~6O0 " | ]-*a rr %ruy.Loss WT4NE AND PROPANE *oo_ -v :- 400- \ 30Q_ -I i a* <0 3 uj 7* * Z o iAdapted from American 0as Association Laboratories Fine Loss Charts. Fig. 7. Alignment Chart fob Calculation of Flue Losses fob Butane, Propane, Coke Oven, and Natural Gases by products of combustion not properly vented), or insufficient mixing 01 the air and fuel, are the primary causes of incomplete combustion. 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 B constant for a given combination of combustible elements and com pounds, and is known as the heat of combustion, calorific value, or heeding value of the fuel. The heat of combustion of the several substances found ln 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 pveral chemical elements in the fuel. When the heating value of a fuel * determined in a calorimeter, the water vapor is condensed and the latent cat of vaporization is included in the heating value of the fuel. The heat-