Boiler Accessories Furnaces

Another arrangement to prevent the escape of smoke is that by which the coal is distilled in a small furnace which is separate from the boiler. The coke and gases thus made are burned in the furnace of the steam boiler. This device is not altogether satisfactory, on account of the loss of heat from the detached furnace. Rather than add any smoke-prevention device, anthracite or coke mad• be used instead of bituminous coal.

Many engineers and business men consider a good fireman to be the best smoke preventer.

Down-Draft Furnaces.In order to increase economy and capacity, or to prevent smoke, a down-draft furnace is sometimes used. In this type of furnace, there are two grates, one a foot or more above the other. Fresh coal is fed to the upper grate, and, as it becomes partially consumed, falls through to the grate below, where the combustion is completed. The draft is downward through the tipper grate, and upward through the lower, because the connection to the chimney is from the space between the grates. The volatile gases are carried down through the bed on the upper grate, and are burned in the space below it, where they meet the hot air drawn upward from the lower grate. A large proportion of the air for combustion enters the door at the upper grate. Tests on the Hawley furnace show that 30 to 45 pounds of coal per square foot per hour can be burned with good results.

In the furnace made by the Hawley Down-Draft Boiler Company, the grates are formed of a series of water tubes opening at the ends into steel drums, shown in Figs. 13 and 14, which are connected with the boiler. Fig. 13 shows this furnace attached to a horizontal, multitubular boiler. It may be applied to both tubular and watertube boilers with good results, and is advantageous to boilers of insufficient heating surface, and when inferior fuels are burned. It is claimed that this attachment insures complete combustion, small amount of ashes on account of the second grate, good water circulation, and increased economy and capacity.

The Hollow Arch. Among boiler accessories specially adapted for use on locomotives because of their intense draft, the hollow arch has recently come into prominence. Its principle is simply that of a conduit providing a passage for the admission of heated air to the firebox above the fire, in addition to the air that comes up through the grate from below in the ordinary way. Its object is to keep the

supply of oxygen at all times sufficient in quantity, and at the proper temperature, to insure a practically perfect combustion of the unconsumed carbon and hydrocarbon gases which are ordinarily wasted and lost in the form of black smoke pouring from the stack. It thus insures an economy of fuel and a proportional reduction in operating expense.

The problem of securing complete combustion of fuel on a loco motive, is one that presents peculiar difficulties. The quantity of fuel to be burned is so large, and the firing space relatively so small, that the conditions usually are unfavorable for economical combustion. A ton of average bituminous coal contains about 1,000 pounds of pure carbon, 700 pounds of hydrocarbon gases, and 300 pounds of noncombustible matter or ash. The 1,700 pounds of carbon and hydrocarbons require about 300,000 cubic feet of air for their complete combustion. In the ordinary method of burning coal on a locomotive, fully 90 per cent of this air—or 270,000 cubic feet per ton of fuel burned—must be drawn up through the grate-bars and firebed. This is practically impossible without forcing the draft to such an extent that the fire will be pulled off the grates, and more or less of the unburned coal carried away through the flues and stack. The result is that the supply of air actually used is, as a general thing, insufficient for perfect combustion, and the combustible carbon smoke and gases pass out of the stack without giving up all of their heat to the water in the boiler. The energy they contain is simply wasted.

How, then, can this be prevented? In other words, since the quantity of air that comes through the grate:; is insufficient, how can we get enough air to the fuel without interfering with the fire? It must be let in above the fire; but it will not do to admit cold air, which, as every fireman knows, would act as a damper on the fire, retarding combustion, and increasing rather than preventing smoke and loss of energy. The air to be admitted to the fire must first be heated to as near the ignition point as possible.