The tubes slope upwards
from the chimney end, where the heat is less, to the fire-door end, where
the heat is fiercer, and a good circulation is thus assured. The Babcock
and Wilcox boiler is the highest development of this system, which has
proved very successful, and may be recommended for model boilers of all
sizes. The heating surface may be increased indefinitely by multiplying the
number of tubes. If a solid fuel-coal, coke, charcoal, etc.-fire is used,
the walls of the casing should be lined with asbestos or fire-clay to
prevent the metal being burnt away.
[Illustration: FIG. 79--Side and end elevations of a small water-tube
boiler.]
The horizontal boiler has an advantage over the vertical in that, for an
equal diameter of barrel, it affords a larger water surface, and is,
therefore, less subject to "priming," which means the passing off of minute
globules of water with the steam. This trouble, very likely to occur if the
boiler has to run an engine too large for it, means a great loss of
efficiency, but it may be partly cured by making the steam pass through
coils exposed to the furnace gases on its way to the engine.
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