Document NerbB51e5KL6XjoRvEpdY8q0b

American Society of Heating and Ventilating Engineers Guide, 1936 Semi-bituminous coal is fired as bituminous coal, and because of its caking characteristics it requires practically the same attention. 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 recom mended that the slides in the firing door be kept closed, as the thinner fuel bed around the sides allows enough air to get through. Burning Coke Coke is a very desirable fuel and usually will give satisfaction as soon as the user learns how to control the fire. Coke ignites and burns very, rapidly with less draft than anthracite coal. 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 more rapidly than an. anthracite fire 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. 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. Since coke weighs only about half as much as anthracite per cubic foot only about half as much can be put in the firepot, so it will benecessary to fire oftener. 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 \x/i 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. Dustless Coal The practice of treating the more friable coals to allay the dust they create is increasing. The coal is sprayed with a solution of calcium chloride or a mixture of calcium and magnesium'chlorides. Both these salts are very hygroscopic and their moisture under normal atmospheric conditions keeps the surface of the coal damp, thus reducing the dust during delivery and in the cellar, and obviating the necessity of sprinkling the coal in the bin. The coal is sometimes treated at the mine, but more usually-by the local distributor just before delivery. The solution is 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. Pulverized Coal Installations of pulverized coal burning plants in heating boilers are of the unit type, in which the pulverized coal is delivered into the furnace immediately after grinding, together with the proper amount of preheated 478 Chapter 27--Fuels and Combustion air. With this apparatus, where the necessary furnace volume is ob tainable, high efficiencies can be obtained. A 150-hp boiler has generally been considered the smallest size for which pulverized fuel is feasible. Complications are introduced if an installation with a single boiler has to take care of very light loads. Hand Firing Hand firing is the oldest and the most widely used method of burning coal for heating purposes. To keep the fuel bed in proper condition where hand firing is used, the following general rules should be observed: 1. Remove ash from fuel bed by shaking the grates whenever fresh fuel is fired. This removes ash from the fire, enables the air to reach the fuel, and does away with the for mation of clinker which is melted ash. t 2. Supply the boiler with a deep bed of fuel. Nothing is gained by attempting to fire a small amount of fuel. A deep bed-of fuel secures the most economical results. 3. Remove ash from ashpit at least once daily. Never allow ash to accumulate up to the grates. If the ash prevents the air from passing through, the grate bars will burn out and much clinker trouble will be experienced. The principal requirements for a hand-fired,furnace are that it shall have enough grate at;ea and 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 com bustion 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. OIL Uniform oil specifications were prepared in 1929 by the American Oil Burner Association, in cooperation with the American Petroleum Institute, the U. S. Bureau of Standards, the American Society for Testing Materials and other interested organizations. Oil fuels were classified into six groups, as indicated by Table 3. When these specifications were prepared, it was generally accepted that the first three grades were adapted to domestic use, while the last three were suitable only for commercial and industrial burners." Today domestic installations may use No. 4 oil of the so-called heavy oil group, when and if said oil very closely follows the specifications of No. 3. Up-to-date listing by the Underwriter's Laboratories should be referred to before a No. 4 grade of fuel is used which merely meets Commercial Standards CS 12-35. Since the specifications as originally drawn provide for maximum limits only for the several grades, this differentiation has not proved stable. Realizing how unsatisfactory it is to have specifications which permit the substitution of one grade for another, the U. S. Bureau of Standards in cooperation with the American Society for Testing Materials is figuring 479