Document wjr82kpo0dzqwJjjYkEEm8r6

HEATINC VENTILATING AIR CONDITIONING CUIDE 1944 decrease and friability increases in the order named. In general, the lighter and more friable cokes ignite and burn the more easily. Low-temperature cokes are produced at low coking temperatures, and only a portion of the volatile matter is distilled off. Cokes as made by various processes under develop ment have contained from 10 to 15 per cent volatile matter. In general, these cokes ignite and burn more readily than high-temperature cokes. The properties of various low-temperature cokes may differ more than those of the various high-temperature cokes because of the differences in the quantities of volatile matter and because some may be light and others briquetted. Petroleum cokes, which are obtained by coking the residue left from the distillation of petroleum, vary in the amount of volatile matter they contain, but all have the common property of a very low ash content, which necessitates the use of refractory pieces to protect the grates from being burned. FIRING METHODS FOR ANTHRACITE An anthracite fire should never be poked or disturbed, as this serves to bring ash to the surface of the fuel bed where it may melt into clinker. Egg size is suitable for large fire-pots (grates 24 in. and over) if the fuel can be fired at least 16 in. deep. For best results this coal should be fired deeply. Stove size coal is the proper size of anthracite for many boilers and furnaces used for heating buildings. It burns well on grates at least 16 in. in diameter and 12 in. deep. The fuel should be fired deeply and uniformly. Chestnut size coal is in demand for fire-pots up to 20 in. in diameter, with a depth of from 10 to 15 in. Pea size coal is often an economical fuel to burn. When fired carefully, pea coal can be burned on standard grates. Care should' be taken to shake the grates only until the first bright coals begin to fall through the grates. The fuel bed, after a new fire has been built, should be increased in thickness by the addition of small charges until it is at least level with the sill of the fire-door. A satisfactory method of firing pea coal consists of drawing the red coals toward the front end and-piling fresh fuel toward the back of the fire-box. Pea size coal requires a strong draft and therefore the best results generally will be obtained by keeping the choke damper open and regu lating solely by means of the cold air check and the air inlet damper. As a precaution against clinker, adjust the air inlet damper so that it can. never be completely closed. Buckwheat size coal for best results requires more attention than pea size coal, and in addition the smaller size of the fuel makes it more difficult to burn on ordinary grates. Greater care must b,e taken in shaking the grates than with the pea coal on account of the danger of the'fuel faUing through the grate. In house heating furnaces the coal should be fired lightly and more frequently than pea coal. Wien banking a buckwheat coal fire it is advisable after coaling to expose a small, spot of hot fire by putting a straight poker down through the bed of fresh coal. This will serve to ignite the gas that will be distilled from the fresh coal and prevent delayed ignition within the fire-pot, which in some cases, depending upon the thickness of the bed of fresh coal, is severe enough to blow open the doors and dampers of the furnace. Where frequent attention can be given 176 CHAPTER 8. COMBUSTION AND FUELS and care exercised in manipulation of the grates this fuel can be burned satisfactorily without the aid of any special equipment. In general it will be found more satisfactory with buckwheat coal to maintain a uniform heat output and consequently to keep the system warm all the time, rather than to allow the system to cool off at times and then to attempt to burn the fuel at a high rate while warming up. A uni form low fire' will minimize the clinker formation and keep the clinker in an easily broken up condition so that it readily can be shaken through the grate. Forced draft and small mesh grates are frequently used for burning buckwheat anthracite.' For greater convenience, domestic stokers are used. Buckwheat anthracite No. 2, or rice size, is used principally in stokers of the domestic, commercial and industrial type. No. 3 buckwheat anthracite, or barley, has no application in domestic heating. Standard anthracite sizing specifications are shown in Table 64. FIRING METHODS FOR BITUMINOUS COAL A commonly recommended procedure for firing domestic heating units, called the side-bank method, requires the movement of live coals to one side or the back of the grate, and placing the fresh fuel charge on the opposite side. The results are a more uniform release of volatile gases, and the subjection of these gases to the high temperature of the red coals. If the fresh charge is covered with a layer of fine coal, still better results may be obtained because of slower release of volatile matter. Bituminous coal should never be fired over the entire fuel bed at one time. A portion of the glowing fuel should always be left exposed to ignite the gases leaving the fresh charge. The importance of firing bituminous coal in small quantities at short intervals is discussed in a U. S. Bureau of Mines technical paper5. Better combustion is obtained by this method in that the fuel supply is main tained more nearly proportional to the air supply. If the coal is of the caking kind the fresh charge will fuse into one solid mass which can be broken up with the stoking bar and leveled from 20 min to one hour after firing, depending on the temperature of the fire-box. Care should be exercised when stoking not to bring the bar up to the surface of the fuel as this will tend to bring ash into the high temperature zone at the top, of the fire,, where it will melt and form clinker.. The stoking bar should be kept as near the grate as possible and should be 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. <Approved and adopted by Anthracite Committee, State Street Building, Harrisburg, Pa. sLoc. Cit. Note 1. 177