Boiler Accessories Furnaces

Another form of bar, shown in Fig. 12, has proved very satisfactory. A and B are two bars, the ends of which are of different depths. These rest at each end on a crank-shaft C. As this is oscillated by the lever G, the alternate bars move up and down, and the clinkers are easily shaken out.

Bridge. The bridge is a large wall or partition at the back of the grate, usually built of firebrick or cast iron, or of ordinary brick covered with firebrick. The bridge separates the grate from the combustion chamber, and causes the gases to come in close contact with the boiler in passing into the combustion chamber. The proper height of the bridge will depend upon the draft. If the space is narrow between the bridge wall and the boiler, more draft will be necessary to carry the gases through. Two or more bridges may sometimes be built in long boilers to keep the gases in contact with the shell as long as possible.

Special Furnaces. Almost any furnace is adapted for the use of anthracite or bituminous coal containing less than 20 per cent of volatile matter; but if there is more than this amount of volatile matter, the heat is likely to be so intense that the fire should not be brought in direct contact with the boiler. If the fuel should contain 40 per cent of volatile matter, the furnace should be surrounded with firebrick and should have a high combustion chamber. Coal is the most common fuel used; but wood, sawdust, and straw are not uncommon fuels. When these are burned, there should be plenty of room in the furnace, and a sufficient supply of air on top of the fuel. Sawdust, shavings, and fine coal may be blown into the furnace by an airblast.

In the West, crude petroleum is becoming a common fuel. Experiments have shown that one pound of crude oil is equivalent in heat units to something less than two pounds of good coal. Oil has many advantages as a boiler fuel. It is clean, gives a uniform heat, is economical, and requires much less attention than coal. There are no ashes to handle, and one man can easily tend two or three times the number of furnaces that he could if burning coal. The fire can be started and stopped instantly; and the supply of air can be so reguted that, unless the boiler is forced to the limit, there will be practically no production of smoke. Whether or not oil is an economical

fuel, will depend upon the local conditions and the market.

Oil fuel is fed into the furnace through a sprayer formed, in some cases of two concentric conical tubes. Compressed air or steam entering through the one tube draws the oil through the other, on the principle of the atomizer, and throws it into the furnace in a fine spray. For marine work, compressed air should be used, as the loss of steam for this purpose would be a matter of considerable consequence. Steam, however, is sometimes used in marine work, in which case the vessel must be equipped with an evaporator to make up the steam thus lost. On land, where fresh water is plenty, steam is usually preferred, and is less expensive in first cost.

Prevention of Smoke. In large cities, where the escape of considerable quantities of smoke is undesirable, several methods have been devised either to consume the smoke or to prevent its formation. The cause of smoke, as we have seen, is an insufficiency in the supply of air, or perhaps a too abundant supply of cold air above the fire; or, again, smoke may be due to the contact of the flame with cold surfaces. An exceedingly high temperature is necessary to consume the finely divided particles of carbon, and anything that tends to chill the flame will cause smoke.

The actual loss caused by the escape of smoke, even when it is dense and black, has been found to be slight, and usually the appliance used for prevention costs more than is saved. The alternate firing of two furnaces which open into a common combustion chamber, or the alternate firing of two sides of the same furnace, produces a slight gain if the proper amount of air is admitted. But if, in order to burn the smoke, the bed in one furnace or on one side of a furnace is allowed to become thin, there will be no gain in efficiency.

The introduction of steam is an efficient method, but it is likely to cause a too rapid rate of combustion.