BOILER ACCESSORIES 127
furnace bars themselves. In the Erith stoker there is an air-chamber
underneath the fuel magazine, to which air is brought by fan, or
other means, and it is passed from it through the hollows in the
dead plates, up into the fuel. In the Under-Feed Stoker Co.'s appa-
ratus, the lower portion of the magazine itself forms an air-chamber,
and the air is forced up between the grate bars. It is claimed in
both cases that the air consumption and fuel consumption are under
complete control, from the front of the boiler, in the usual way.
Providing the Air for the Furnace
There are four methods of providing the air that is required by the furnace-by chimney draught, by forced draught, by induced draught, and by the aid of steam jets. All of the methods are simply variations of the same thing. In all of them a certain force is applied to the air, to drive it through the fire bars, the fuel lying. on them, and the flues, etc., beyond. With forced and induced draught the air is under much greater control. With chimney draught the only method of control is by closing or opening the damper, more or less throttling the supply of air.
Chimney Draught
Chimney draught is caused by the difference in the weight of the column of hot gases in the chimney, and that of a similar column of air on the outside of the chimney. Perhaps the matter will be more easily understood by reference to the case of the ventilation of a coal mine. At the present time furnace ventilation is not often found in coal mines, but thirty years ago it was very common. Now the furnace has been almost entirely displaced by fans, very much in the same way that chimney draught for boilers is gradually being dis- placed by one or other of the methods mentioned. In a coal mine there are two vertical shafts, from the surface to the coal seam, a short distance apart, and at the bottom of the shafts there are roads extending into the mine from each of the shafts, the roads being con- nected by cross-roads, working places, etc., and for ventilation the air has to pass down one of the shafts, called the down-cast, along the road leading from it, through the cross-roads, working faces, to the road leading to the other shaft, known as the up-cast, and through the up cast to the surface again. In the days of furnace ventilation, a large furnace was kept continually burning, near the bottom of the up-cast shaft, the furnace being very similar to that of a boiler furnace, but without boiler flues, etc., and its office was to heat the
