BOILER ACCESSORIES: FIRING Starting the Fire. The fireman should first ascertain the waterlevel; as the gauge-glass is not always reliable, on account of impurities, foam, etc., the gauge-cocks should be tried. In a battery of boilers, the gauge-cocks of each should be opened, for the water may not stand at the same level in each. The safety-valve should be raised slightly from its seat. If the fire has been banked over night, open the draughts, and rattle down the ashes and clinkers from the grate. In case the fire has been allowed to go out, a new one may be started if the gauge-glass shows the proper amount of water, and the valves work well.
If anthracite coal is used, first throw a thin layer of coal all over the grate, then place a piece of wood across the mouth of the furnace just inside the door and lay other pieces of wood at right angles to the cross-piece with the ends resting on it. This allows a space under the wood for air. Now throw on coal until the wood is covered: The fire may be started with oily cotton waste, shavings, or any combustible material.
Keep the furnace door open and the draught-plate closed until the wood is burning freely, which causes the flame to pass over and through the coal and to ignite it. The fire is then spread or pushed back evenly over the furnace bars; the furnace door closed; the ashpit door opened, as the draught requires; and more coal added when necessary. If bituminous coal is used, do not spread a thin layer over the grate bars under the wood.
The fire at the start should be slow, to cause gradual, uniform heating of the water and various parts of the boiler. If steam is raised too rapidly, enormous strains are set up, due to unequal expansion, thereby causing leakage at joints, and perhaps rupture.
If the boiler is of the water-tube type, steam may be raised more rapidly, because the amount of water is less and the joints are usually placed at some distance from the intense beat of the fire.
The fire being started, the method of adding coal depends upon the fireman, the kind of coal, the type of boiler, and the rate of combustion. There are three general methods of firing—spreading, alternate or side firing, and coking.
Spreading is accomplished by placing small amounts of coal uniformly over the entire surface of the grate at short intervals. By this method, the coal is thrown just where it is wanted and then not disturbed. The fire should be hollowed in the center; that is, it
should be thicker at the sides. Good results are obtained from this method, since the fire can be kept in the right condition at all times, if the coal is of the right sort. During the operation of firing, the door should be kept open as little as possible, or the fire will be cooled by the entrance of cold air. For a short time, while the coal is giving off gas, the draught-plate of the furnace door should be opened, in order that sufficient air may be admitted above the coal to burn the hydrocarbons.
When the alternate or side firing method is used, coal is spread so as to cover one side of the fire completely at one firing, leaving the other side bright. At the next firing, the bright side is covered. The hydrocarbons given off by the fresh coal are burned by the hot gases from the incandescent coal. This method is superior to spreading, because the entire furnace is not cooled off by the addition of fresh fuel.
Side firing is most advantageous with two furnaces leading to a common combustion chamber. The furnaces are fired at regular intervals with moderate charges of coal, and the draught-plates are opened while the coal is giving off gas.
The two systems described above are best adapted to anthracite coal, since it burns with comparatively little smoke.
With bituminous coal, which is soft and burns with considerable smoke, the coking method is used. The coal is piled on the grate just inside the door, and allowed to coke from 15 to 30 minutes. During this time, the hydrocarbons are driven off and burned by the heat from the fire. In order fully to accomplish this, air must be admitted above the grate through the draught-plates of the furnace door. The coke is then pushed backward over the fire, and a new supply placed on the front of the grate. The air admitted prevents the forming of carbon monoxide gas and smoke. At the same time, however, it cools the furnace somewhat and reduces the rate of evaporation; but this objection is not serious unless a boiler must be worked to its maximum capacity in order to furnish the required amount of steam. If this is the case, economy is sacrificed to rapidity, for a low rate of combustion is usually more economical than a high rate.