BOILER ACCESSORIES 139
Forced Draught
By forced draught is understood the passage of air under pressure through the boiler furnace, and it may be accomplished in two ways. The stoke-hold may be closed, and the air that is to pass into the furnace may be forced into the stoke-hold under a certain pressure, the ashpits, etc., being left in the ordinary conditions. The stokehold may be left under ordinary conditions, and the ashpit may be closed and the air for the fires delivered under pressure to each ashpit. The closed ashpit system is necessarily the most convenient for the great majority of boilers on shore, but on board ship the closed stoke-hold system, or, as it is called in America, the closed fireroom system, has also been adopted, particularly in the case of men-of-war. The objection to the closed stoke-hold system is the fact that the men working in it are exposed to the air pressure, and that it is more difficult to maintain a large space, such as the stokehold, closed, than the smaller space formed by the ashpit. One objection to the closed ashpit system is the number of connections that have to be made froin the pipe bringing the air to the different furnaces, and the difficulty of delivering the air uniformly to all parts of the furnace. There is a tendency to blow holes in the fuel, and to cause larger heating at certain grate-bars than at others, and to blow out the ashes, etc., into the boiler-room. In the case of the closed stoke-hole system, providing that the air pressure is not high, the arrangement is good, inasmuch as it provides ventilation for the stoke-hold as well as air for the furnaces. Whichever system is adopted, whether the closed ashpit system or the closed fire-room system, the air pressure is produced by means of a fan placed conveniently to the fire- or boiler-room, and having a duct or other arrangement leading to a supply of fresh air, the fan drawing the air from the fresh supply, and passing it either directly into the stoke-hold or into a pipe leading to the different ashpits. A modification of the forced-draught system provides for heating the air on its way into the ashpit or boiler-room. It will be obvious that the system is not applicable to the case of the closed stoke-hold, inasmuch as the men would be subject to higher air temperatures than they are at present. The economy of the system is, however, considerable.
It will be understood that with forced draught the fan is required to produce the necessary pressure to drive the air into the furnace, through the fuel and the hot gases which are formed, through the boiler flues, or their equivalent in water-tube boilers, and also to force portion of the necessary pressure the hot gases up the chimney. will be provided, wherever there is a chimney of any appreciable
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