Steam, Its Generation and Use | Page 8

and rises by its
levity, while colder portions come to be heated in turn, thus setting up
currents in the fluid.

Now, when all the water has been heated to the boiling point
corresponding to the pressure to which it is subjected, each added unit
of heat converts a portion, about 7 grains in weight, into vapor, greatly
increasing its volume; and the mingled steam and water rises more
rapidly still, producing ebullition such as we have noticed in the kettle.
So long as the quantity of heat added to the contents of the kettle
continues practically constant, the conditions remain similar to those
we noticed at first, a tumultuous lifting of the water around the edges,
flowing toward the center and thence downward; if, however, the fire
be quickened, the upward currents interfere with the downward and the
kettle boils over (Fig. 1).
[Illustration: Fig. 1]
If now we put in the kettle a vessel somewhat smaller (Fig. 2) with a
hole in the bottom and supported at a proper distance from the side so
as to separate the upward from the downward currents, we can force the
fires to a very much greater extent without causing the kettle to boil
over, and when we place a deflecting plate so as to guide the rising
column toward the center it will be almost impossible to produce that
effect. This is the invention of Perkins in 1831 and forms the basis of
very many of the arrangements for producing free circulation of the
water in boilers which have been made since that time. It consists in
dividing the currents so that they will not interfere each with the other.
[Illustration: Fig. 2]
But what is the object of facilitating the circulation of water in boilers?
Why may we not safely leave this to the unassisted action of nature as
we do in culinary operations? We may, if we do not care for the three
most important aims in steam-boiler construction, namely, efficiency,
durability, and safety, each of which is more or less dependent upon a
proper circulation of the water. As for efficiency, we have seen one
proof in our kettle. When we provided means to preserve the
circulation, we found that we could carry a hotter fire and boil away the
water much more rapidly than before. It is the same in a steam boiler.
And we also noticed that when there was nothing but the unassisted
circulation, the rising steam carried away so much water in the form of

foam that the kettle boiled over, but when the currents were separated
and an unimpeded circuit was established, this ceased, and a much
larger supply of steam was delivered in a comparatively dry state. Thus,
circulation increases the efficiency in two ways: it adds to the ability to
take up the heat, and decreases the liability to waste that heat by what is
technically known as priming. There is yet another way in which,
incidentally, circulation increases efficiency of surface, and that is by
preventing in a greater or less degree the formation of deposits thereon.
Most waters contain some impurity which, when the water is
evaporated, remains to incrust the surface of the vessel. This
incrustation becomes very serious sometimes, so much so as to almost
entirely prevent the transmission of heat from the metal to the water. It
is said that an incrustation of only one-eighth inch will cause a loss of
25 per cent in efficiency, and this is probably within the truth in many
cases. Circulation of water will not prevent incrustation altogether, but
it lessens the amount in all waters, and almost entirely so in some, thus
adding greatly to the efficiency of the surface.
[Illustration: Fig. 3]
A second advantage to be obtained through circulation is durability of
the boiler. This it secures mainly by keeping all parts at a nearly
uniform temperature. The way to secure the greatest freedom from
unequal strains in a boiler is to provide for such a circulation of the
water as will insure the same temperature in all parts.
3rd. Safety follows in the wake of durability, because a boiler which is
not subject to unequal strains of expansion and contraction is not only
less liable to ordinary repairs, but also to rupture and disastrous
explosion. By far the most prolific cause of explosions is this same
strain from unequal expansions.
[Illustration: Fig. 4]
[Illustration: 386 Horse-power Installation of Babcock & Wilcox
Boilers at B. F. Keith's Theatre, Boston, Mass.]
Having thus briefly looked at the advantages of circulation of water in

steam boilers, let us see what are the best means of securing it under the
most efficient conditions We have seen in our kettle that one essential
point was that the currents should be kept from interfering with