The forced draught in a locomotive boiler is obtained by causing the steam from the cylinders, after it has done its work, to be discharged into the chimney by means of a pipe called the blast pipe; the lower portion of this consists of two branches, one in communication with the exhaust port of each cylinder. As each puff of steam from the blast pipe escapes up the chimney it forces the air out in front of it, causing a partial vacuum, which can only be supplied by the air rushing through the furnace and tubes.
The greater the body of steam escaping at each puff, and the more rapid the succession of puffs, the more violent is the action of the blast pipe in producing a draught, and consequently this contrivance regulates the consumption of fuel and the evaporation of water to a certain extent automatically, because when the engine is working its hardest and using the most steam, the blast is at the same time most efficacious.
LOCOMOTIVE BOILER.—Fig. 27.
The blast pipe is perhaps, the most distinctive feature of the locomotive boiler, and the one which has alone rendered it possible to obtain large quantities of steam from so small a generator. The steam blast of a locomotive has been compared to the breathing apparatus of a man, and has rendered the mechanism described nearer a live thing than any other device man has ever produced.
On account of the oscillations, or violent motions to which the boiler of locomotive engines are subject, weighted safety-valves are not possible to be used and springs are used instead to hold the valves in place.
The locomotive form of steam boiler is sometimes used for stationary engines, but owing to extra cost and increased liability to corrode in the smaller passage they are not favorites.
DESCRIPTION OF PAGE ILLUSTRATION.
In [fig. 27], F B represents the fire box or furnace; F D, fire door; D P, deflector plate; F T P, fire box tube plate; F B R S, fire box roof stays; S T P, smoke box tube plate; S B, smoke box; S B D, smoke box door; S D, steam dome; O S, outer shell; R S V, Ramsbottom safety-valve; F, funnel or chimney.