Document wg2YKn6B6xqJvdzwanZwyNwO6

168 Chapter 8 1945 Guide prevent the fire frqm_burning^too_rapidly. In orderJ:o_ob.tain_the..same "interval of attention as with other fuels a deep fuel bed always should be maintained when burning coke. The grates should be shaken only slightly in mild weather and should be shaken only until the first red particles drop from the grates in cold weather. The best size of coke for general use, for small fire-pots where the fuel depth is not over 20 in., is that which passes over"a 1 in. screen and through a 1J4 in. screen. For large fire-pots where the fuel can be fired over 20 in. deep, coke which passes over a 1 in. screen and through a.3 in. screen can be used, but a coke of uniform size is always more satisfactory. Large sizes of coke should be either mixed with fine sizes or broken up before using. FURNACE VOLUME The principal requirements for a hand-firedfurnace are that it shall have enough grate area and correctly proportioned combustion space. The amount of grate area required is dependent upon the desired combustion rate. The furnace volume is influenced by the kind of coal used. Bituminous coals, on account of their long-flaming characteristic, require more space in which to burn the gases of combustion completely than do the coals low in volatile matter. For burning high volatile coals provision should be made for mixing the combustible gases thoroughly, so that combustion is complete before the gases come in contact with the relatively cool heating surfaces. An abrupt change in the direction of flow tends to mix the gases of combustion more thoroughly. Anthracite requires com paratively little combustion space. CLASSIFICATION OF OILS The Commercial Standard Specifications for Fuel Oils (CS 12-40) of the U. S. Department of Commerce are given in Table 7. These speci fications conform with American Society for Testing Materials Tentative Specifications for Fuel Oils D396-38T. The specific gravity of oil is of interest in its relationship to the calorific value and these data are given in Table 8. COMBUSTION OF OIL With oil, as with any kind of fuel, efficient heat production requires that all combustible matter in the fuel shall be completely consumed and that it shall be done with a minimum of excess air. The combustion of oil is a rather rapid chemical reaction. Excess air provides an over supply of oxygen so that all of the oil will be completely oxidized and thus produce all the heat possible. The use of unreasonable quantities of air in excess of theoretical combustion requirements results in lowered efficiencies due to increased stack losses. Such losses, if not accompanied by unburned products of combustion, may be offset somewhat by in creasing the secondary heating surfaces of the heat absorbing medium, boiler or furnace. Oil is a highly concentrated fuel composed mainly of hydrogen and carbon. In its liquid form oil cannot burn. It must be converted into a gas or vapor by some means. If the excess air is to be kept within efficient limits, air must be supplied in. carefully regulated quantities. The air and oil vapor must be thoroughly mixed to get a rapid and complete chemical reaction. The better the mixing, the less excess air will be T a b l e 7. D e t a il e d R eq u ir em en ts for F u e l Oils 11 Combustion and Fuels 169