The essential features of locomotive boilers are dictated by the duties
which they have to perform under peculiar conditions. The size and the
weight are limited by the fact that the boiler has to be transported
rapidly from place to place, and also that it has to fit in between
the frames of the locomotive; while at the same time, the pressure of
the steam has to be very great in order that with comparatively small
cylinder the engine may develop great power; moreover, the quantity of
water which has to be evaporated in a given time is very considerable.
To fulfil these latter conditions a large quantity of coal must be
burned on a fire grate of limited area; hence intense combustion is
necessary under a forced blast. To utilize advantageously the heat
thus generated, a large heating surface must be provided and this can
only be obtained by passing the products of combustion through a great
number of tubes of small diameter.
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.
[Illustration: 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.