The necessary thickness of a bed for the best results, is found by experiment. It depends on the draught and the kind of coal used. If the former is strong, and the coal in large lumps, the 'bed may. be thick (about one foot); but if the draught is weak, or if the coal is small, the bed must be thin (about three or four inches), so that suffident air may pass through. In marine and locomotive work, with forced draught, the bed must be very thick to get a large coal consumption per square foot per hour. With the same draught, bituminous coal can be fired more thickly than anthraciteAfter finding from experiment the best thickness for the bed, keep it at that thickness. Always keep the bed of uniform thickness, and never let the fire burn holes in the bed, and do not let the rear of the grate become bare. If a larger amount of 'steam is required, fire smaller quantities at more frequent intervals. Do not fire a large amount of coal, and wait for the pressure to rise The firing of fresh coal chills the furnace and temporarily retards combustion. The coal should be fired in small quantities and as quickly as possible. Keep the fire free from ashes and clinkers, but do not clean the fires oftener than is necessary.
Four tools are used for cleaning the fire—the slice-bar; the prickbar; the clinker hook, sometimes called the devil's claw; and the hoe or rake.
The slier-bar is a long, straight bar, with the end flattened. It is used to break up clinkers by thrusting it between the grate and the fire. It is also used to break up caking coal. The prick-bar is similar to the slice-lair, except that the end is bent at right angles like a hook. To remove ashes, the prick-bar is run along, up between the grate bars, from underneath. This bar is often made with detachable hook, so that the end may be replaced when burned off. The clinker hook, or claw, is used to haul the fire forward. The hoe, or rake, is used to draw out cinders, to haul the fire forward, etc.
In cleaning the fire, the fireman first looks to the water and steam. There should he enough water and sufficient steam pressure to last during cleaning. 'rhea lie breaks up the clinkers with the slice-bar, and removes the ashes with the prick-bar. If necessary, he pushes the fire to the rear, thoroughly cleans the front of the grate bars, and then hauls it forward and cleans the back of the furnace bars. Some firemen clean one side at a time, instead of first the front and then the rear. The fire should be allowed to burn down before cleaning; but sufficient fuel, called chaff, should be left to start the fire quickly. Before cleaning, partly close the dampers, so that the amount of cold air admitted will be small. For this reason and to prevent loss of pressure, clean as rapidly as possible.
Banking the fire depends upon the condition of the fire, the fireman himself, and the length of time it is to remain banked. First clean and place all the coal in a small space at the bridge; then cover with fresh coal to a depth depending on the length of time the fire is to remain banked. Then close all dampers and open the door. Some firemen cover the front of the furnace bars with ashes.
To start from a banked fire, first examine the condition of the water-level, steam pressure, safety-valves, etc. Then clean the fire with the slice-bar, and rattle down the ashes with the prick-bar. After spreading the coal evenly over the grate, cover with a thin layer of coal, and open the dampers.