THE STEAM GENERATOR
The most widely known and utilized source of power is the steam engine.
Before its discovery wind and water were the only available means,
except the muscular power of man, horses and other animals, which was
used with the crudest sort of contrivances.
In primitive days men did not value their time, so they laboriously
performed the work which machinery now does for us.
The steam engine, like everything else which man has devised, was a
growth, and, singular as it may seem, the boiler, that vital part of the
organism, was, really, the last to receive due consideration and
improvement.
As the boiler is depended upon to produce the steam pressure, and since
the pressure depends on the rapid and economical evaporation of water,
the importance of the subject will be understood in treating of the
steam engine.
Water as an Absorbent of Heat.--Water has the capacity to absorb a
greater amount of heat than any other substance. A pewter pot, which
melts at 500 degrees, will resist 2000 degrees of heat if it is filled
with water, since the latter absorbs the heat so rapidly that the
temperature of the metal is kept near the boiling point of water, which
is 212 degrees.
Notwithstanding the great heat-absorbing qualities of water, a large
portion of the heat of the fuel passes through the flues and escapes
from the stack. This fact has caused inventors to devise various forms
of boilers, the object being to present as large an area of water as
possible to the heat of the burning fuel. How that was accomplished we
shall try to make plain.
Classification of Boilers.--Numerous types of boilers have been devised,
the object being, in all cases to evaporate the largest amount of water
with the minimum quantity of fuel. All boilers may be put under two
general heads, namely, those which contain a large quantity of water,
and those which are intended to carry only a small charge.
In the first division the boilers are designed to carry a comparatively
small pressure, and in the latter high pressures are available.
Mode of Applying Heat.--The most important thing to fully understand is
the manner in which heat is applied to the boiler, and the different
types which have been adapted to meet this requirement.
The Cylindrical Boiler.--The most primitive type of boiler is a plain
cylindrical shell A, shown in Fig. 3, in which the furnace B is placed
below, so that the surface of the water in contact with the fire area is
exceedingly limited.
[Illustration: _Fig. 3. Primitive Boiler._]
In such a type of boiler it would be impossible for water to extract
more than quarter the heat of the fuel. Usually it was much less. The
next step was to make what is called a return tubular type in which the
heat of the burning gases is conveyed to the rear end of the boiler, and
then returned to the front end through tubes.
Fig. 4 shows this construction. The head of the shell holds the ends of
a plurality of tubes, and the products of combustion pass through the
conduit, below the boiler to the rear end, and are conducted upwardly to
the tubes. As all the tubes are surrounded by water, it will absorb a
large amount of the heat as the gases move through, and before passing
out of the stack.
[Illustration: _Fig. 4. Return Tubular Boiler._]
[Illustration: _Fig. 5. Cornish, or Scotch Boiler._]
The Cornish Boiler.--One of the most important inventions in the
generation of steam was the Cornish boiler, which for many years was the
recognized type for marine purposes. It had the advantage that a large
amount of water could be carried and be subjected to heat at all times.
Aside from that it sought to avoid the great loss due to radiation.
It will be seen from an examination of Fig. 5 that the shell is made
very large, and its length does not exceed its diametrical measurement.
Two, and sometimes three, fire tubes are placed within the shell, these
tubes being secured to the heads. Surrounding these fire tubes, are
numerous small tubes, through which the products of combustion pass
after leaving the rear ends of the fire tubes.
In these boilers the tubes are the combustion chambers, and are provided
with a grating for receiving the coal, and the rear ends of the tubes
are provided with bridge walls, to arrest, in a measure, the free exit
of the heated gases.
These boilers would be very efficient, if they could be made of
sufficient length to permit the water to absorb the heat of the fuel,
but it will be seen that it would be difficult to make them of very
great length. If made too small diametrically the diameter of the fire
boxes would be reduced to such an extent that there would not be
sufficient grate surface.
It is obvious, however, that this form of boiler adds greatly to the
area of the water surface contact, and in that particular is a great
improvement.
[Illustration: _Fig. 6. Water Tube Boiler: End View._]
The Water Tube Boiler.--In the early days of the development of boilers,
the universal practice was to have the products of combustion pass
through the flues or the tubes. But quick generation of steam, and high
pressures, necessitated a new type. This was accomplished by connecting
an upper, or steam drum, with a lower, or water drum, by a plurality of
small tubes, and causing the burning fuel to surround these tubes, so
that the water, in passing upwardly, would thus be subjected to the
action of the fuel.
This form of boiler had two distinct advantages. First, an immense
surface of water could be provided for; and, second, the water and steam
drums could be made very small, diametrically, and thus permit of very
high pressures.
In Fig. 6, which is designed to show a well known type of this
structure, A A, represent the water drums and B, the steam drum. The
water drums are separated from each other, so as to provide for the
grate bars C, and each water drum is connected with the steam drum by a
plurality of tubes D.
It will thus be seen that a fire box, or combustion chamber, is formed
between the two sets of tubes D, and to retain the heat, or confine it
as closely as possible to the tubes, a jacket E is placed around the
entire structure.
The ends of the water and steam drums are connected by means of tubes F,
shown in side view, Fig. 7, for the return or downward flow of the
water. The diagrams are made as simple as possible, to show the
principal features only. The structure illustrated has been modified in
many ways, principally in simplifying the construction, and in providing
means whereby the products of combustion may be brought into more
intimate contact with the water during its passage through the
structure.
[Illustration: _Fig. 7. Water Tube Boiler: Side View._]
As heretofore stated, this type of boiler is designed to carry only a
small quantity of water, so that it is necessary to have practically a
constant inflow of feed water, and to economize in this respect the
exhaust of the steam engine is used to initially heat up the water, and
thus, in a measure, start the water well on its way to the evaporation
point before it reaches the boiler.
Various Boiler Types.--The different uses have brought forth many kinds
of boilers, in order to adapt them for some particular need. It would
be needless to illustrate them, but to show the diversity of structures,
we may refer to some of them by their characteristics.
Compound Steam-Boiler.--This is a battery of boilers having their steam
and water spaces connected, and acting together to supply steam to a
heating apparatus or a steam engine. These are also made by combining
two or more boilers and using them as a feed water heater or a
superheater, for facilitating the production of steam, or to be used for
superheating steam.
The terms _feed water heater and super heater_ are explained in chapter
III.
Locomotive Steam-Boiler.--This is a tubular boiler which has a contained
furnace and ash pit, and in which the gases of combustion pass from the
furnace directly into the horizontal interior tubes, and after passing
through the tubes are conveyed directly into the smoke box at the
opposite ends of the tubes. The name is derived from the use of such
boilers on locomotive engines, but it is typical in its application to
all boilers having the construction described, and used for generating
steam.
Vertical Steam-Boiler.--This is a form of construction in which the
shell, or both the shell and the tubes, are vertical, and the tubes
themselves may be used to convey the products of combustion, or serve
as the means for conveying water through them, as in the well known
water tube type.
This form of boiler is frequently used to good advantage where it is
desired to utilize ground space, and where there is sufficient head
room. Properly constructed, it is economical as a steam generator.
From the foregoing it will be seen that the structural features of all
boilers are so arranged as to provide for the exposure of the largest
possible area of water to a heated surface so that the greatest amount
of heat from the fuel may be absorbed.