[Illustration: WATER TUBE BOILER.—Fig. 26.]
A popular form of steam boiler in use in the United States and Europe
is what is called the water tube boiler. This term is applied to a
class of boiler in which the water is contained in a series of tubes,
of comparatively small diameter, which communicate with each other and
with a common steam-chamber. The flames and hot gases circulate between
the tubes and are usually guided by partitions so as to act equally on
all portions of the tubes. There are many varieties of this type of
boiler of which the cut illustrates one: in this each tube is secured
at either end into a square cast-iron head, and each of these heads
has two openings, one communicating with the tube below and the other
with the tube above; the communication is effected by means of hollow
cast-iron caps shown at the end of the tubes; the caps have openings in
them corresponding with the openings in the tube heads to which they
are bolted.
In the best forms of the water tube boilers, it is suspended entirely
independent of the brick work from wrought iron girders resting on iron
columns. This avoids any straining of the boiler from unequal expansion
between it and its enclosing walls and permits the brick work to be
repaired or removed, if necessary, without in any way disturbing the
boiler. This design is shown in Fig. 26.
The distinguishing difference, which marks the water tube boiler
from others, consists in the fact that in the former the small tubes
are filled with water instead of the products of combustions; hence
the comparison, frequently made, between water-tube and _fire tube_
boilers—the difference has been expressed in another way, “Water-tube
vs. shell boilers,” but the principle of steam production in both
systems remains the same; the heat from the combustible is transferred
to the water through the medium of iron plates and in both, the
furnaces, steam appliances, application of the draught, etc., is
substantially the same. In another important point do the systems
agree, _i.e._, in the average number of pounds of water evaporated
per lb. of combustible; it is in the thoroughness of construction
and skillfulness of adaptation to surroundings that produce the best
results. Water tube or sectional boilers, have been made since the
days of James Watt, in 1766, in many different forms and under various
names. Owing, however, to the imperfection of manufacture the system,
as compared to shell boilers, has been a failure until very recently;
various patterns of water-tube boilers are now in most favorable
and satisfactory use. The advantages claimed for this form of steam
generator are as follows:
1. Safety from disastrous explosions, arising from the division of
the contents into small portions, and especially from details of
construction which make it tolerably certain that the rupture will be
local instead of a general violent explosion which liberates at once
large masses of steam and water.
2. The small diameter of the tubes of which they are composed render
them much stronger than ordinary boilers.
3. They can be cheaply built and easily repaired, as duplicate pieces
can be kept on hand. The various parts of a boiler can be transported
without great expense, trouble or delay; the form and proportions of a
boiler can be suited to any available space; and, again, the power can
be increased by simply adding more rows of tubes and increasing the
grate area.
4. Their evaporative efficiency can be made equal to that of other
boilers, and, in fact, for equal proportions of heating and grate
surfaces, it is often a trifle higher.
5. Thin heating surface in the furnace, avoiding the thick plates
necessarily used in ordinary boilers which not only hinder the
transmission of heat to the water, but admit of overheating.
6. Joints removed from the fire. The use of lap welded water tubes with
their joints removed from the fire also avoid the unequal expansion of
riveted joints consequent upon their double thickness.