Document 1QgVqNxXq3p8DDZ5ovYjzrO4E

HEATINC VENTILATING AIR CONDITIONING GUIDE 1940 raised only enough to break up the fuel. With fuels requiring stoking it may not be necessary to shake the grates, as the ash is usually dislodged during stoking. It is acknowledged that it may be difficult to apply the outlined methods to domestic heating boilers of small size, especially when frequent attendance is impractical. The adherence to these methods insofar as practical, however, will result in better combustion. The output obtained from any heater with bituminous coal will usually exceed that obtainable with anthracite, since bituminous coal bums more rapidly than anthracite and with less draft. Bituminous coal, however, will require frequent attention to the fuel bed, because it burns unevenly, even though the fuel bed may be level, forming holes in the fire which admit too much air, chilling the gases over the fuel bed and reducing the available draft. FIRING METHODS FOR SEMI-BITUMINOUS COAL The Pocahontas Operators Association recommends the central cone method of firing, in which the coal is heaped on to the center of the bed forming a cone the top of which should be level with the middle of the firing door. This allows the larger lumps to fall to the sides, and the fines to remain in the center and be coked. The poking should be limited to breaking down the coke without stirring, and to gently rocking the grates. It is recommended that the slides in the firing door be kept closed, as the thinner fuel bed around the sides allows enough air to get through. FIRING METHODS FOR COKE Coke ignites less readily than bituminous coal and more readily than anthracite and burns rapidly with little draft. In order to control the air admitted to the fuel it is very important that all openings or leaks into the ashpit be closed tightly. A coke fire responds rapidly to the opening of the dampers. This is an advantage in warming up the system, but it also makes it necessary to watch the dampers more closely in order to prevent the fire from burning too rapidly. In order to obtain 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 firepots where the fuel depth is not over 20 in., is that which passes over a 1 in. screen and through a in. screen. For large firepots 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. PULVERIZED COAL Although several pulverized coal burning units for domestic heating plant firing have been developed, none has attained extended use. Two general methods of adaptation have been employed, one where the coal is pulverized by the unit at the furnace and one where the coal is delivered to the home in pulverized form. 172 CHAPTER 9. COMBUSTION AND FUELS 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 practically no combustion space. DUSTLESS TREATMENT OF COAL The practice of treating the more friable coals to allay the dust they create is increasing. The coal is sprayed with petroleum products, par ticularly the lighter oils, a solution of calcium chloride or a mixture of calcium and magnesium chlorides. The latter salts are very hygroscopic and their moisture under normal atmospheric conditions keeps the surface of the coal damp, thus reducing the dust during delivery in the cellar, and obviating the necessity of sprinkling the coal in the bin. The coal is usually treated at the mine, but sometimes by., the local distributor just before delivery. The salt solutions are sprayed' under high pressure, using from 2 to 4 gal or from 5 to 10 lb of the salt per ton of coal, depending on its friability and size. Oil for the dustless treatment of coal is also applied under high pressure, in concentrations of 1 to 8 qt per ton of coal, depending upon the characteristics of the coal and oil. CLASSIFICATION OF OILS The Commercial Standard Specifications for Fuel Oils (CS 12-38) 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, composed of carbon and hydrogen, will be completely oxidized and thus produce all the heat possible. The use of unreasonable quantities of air in excess of theoretical combustion require ments results in lowered efficiencies due to increased stack losses. Such 173