_MOTIVE MACHINERY._
In this class belong--
Steam-engines.
Caloric or air engines.
Water-wheels or water-engines.
Wind-wheels or pneumatic engines.
These four types comprehend the motive-power in general use at the
present day. In considering different engines for motive-power in
a way to best comprehend their nature, the first view to be taken
is that they are all directed to the same end, and all deal with
the same power; and in this way avoid, if possible, the impression
of there being different kinds of power, as the terms water-power,
steam-power, and so on, seem to imply. We speak of steam-power,
water-power, or wind-power; but power is the same from whatever
source derived, and these distinctions merely indicate different
natural sources from which power is derived, or the different means
employed to utilise and apply it.
Primarily, power is a product of heat; and wherever force and
motion exist, they can be traced to heat as the generating
element: whether the medium through which the power is obtained be
by the expansion of water or gases, the gravity of water, or the
force of wind, heat will always be found as the prime source. So
also will the phenomenon of expansion be found a constant principle
of developing power, as will again be pointed out. As steam-engines
constitute a large share of the machinery commonly met with, and as
a class of machinery naturally engrosses attention in proportion,
the study of mechanics generally begins with steam-engines, or
steam machinery, as it may be called.
The subject of steam-power, aside from its mechanical
consideration, is one that may afford many useful lessons, by
tracing its history and influence, not only upon mechanical
industry, but upon human interests generally. This subject is often
treated of, and both its interest and importance conceded; but no
one has, so far as I know, from statistical and other sources,
ventured to estimate in a methodical way the changes that can be
traced directly and indirectly to steam-power.
The steam-engine is the most important, and in England and America
best known among motive agents. The importance of steam contrasted
with other sources of motive-power is due not so much to a
diminished cost of power obtained in this way, but for the reason
that the amount of power produced can be determined at will, and
in most cases without reference to local conditions; the machinery
can with fuel and water be transported from place to place, as in
the case of locomotives which not only supply power for their own
transit, but move besides vast loads of merchandise, or travel.
For manufacturing processes, one importance of steam-power rests
in the fact that such power can be taken to the material; and
beside other advantages gained thereby, is the difference in the
expense of transporting manufactured products and the raw material.
In the case of iron manufacture, for example, it would cost ten
times as much to transport the ore and the fuel used in smelting
as it does to transport the manufactured iron; steam-power saves
this difference, and without such power our present iron traffic
would be impossible. In a great many manufacturing processes steam
is required for heating, bleaching, boiling, and so on; besides,
steam is now to a large extent employed for warming buildings, so
that even when water or other power is employed, in most cases
steam-generating apparatus has to be set up in addition. In many
cases waste steam or waste heat from a steam-engine can be employed
for the purposes named, saving most of the expense that must be
incurred if special apparatus is employed.
Other reasons for the extended and general use of steam as a power,
besides those already named, are to be found in the fact that no
other available element or substance can be expanded to a given
degree at so small a cost as water; and that its temperature will
not rise to a point injurious to machinery, and, further, in the
very important property of lubrication which steam possesses,
protecting the frictional surfaces of pistons and valves, which it
is impossible to keep oiled because of their inaccessibility or
temperature.
The steam-engine, in the sense in which the term is employed, means
not only steam-using machinery, but steam-generating machinery or
plant; it includes the engine proper, with the boiler, mechanism
for feeding water to the boiler, machinery for governing speed,
indicators, and other details.
An apprentice must guard against the too common impression that
the engine, cylinder, piston, valves, and so on, are the main
parts of steam machinery, and that the boiler and furnace are
only auxiliaries. The boiler is, in fact, the base of the whole,
that part where the power is generated, the engine being merely
an agent for transmitting power from the boiler to work that is
performed. This proposition would, of course, be reached by any one
in reasoning about the matter and following it to a conclusion, but
the fact should be fixed in the mind at the beginning.
When we look at a steam-engine there are certain impressions
conveyed to the mind, and by these impressions we are governed in
a train of reflection that follows. We may conceive of a cylinder
and its details as a complete machine with independent functions,
or we can conceive of it as a mechanical device for transmitting
the force generated by a boiler, and this conception might be
independent of, or even contrary to, specific knowledge that we at
the same time possessed; hence the importance of starting with a
correct idea of the boiler being, as we may say, the base of steam
machinery.
As reading books of fiction sometimes expands the mind and enables
it to grasp great practical truths, so may a study of abstract
principles often enable us to comprehend the simplest forms
of mechanism. Even Humboldt and Agassiz, it is said, resorted
sometimes to imaginative speculations as a means of enabling them
to grasp new truths.
In no other branch of machinery has so much research and experiment
been made during eighty years past as in steam machinery, and,
strange to say, the greater part of this research has been directed
to the details of engines; yet there has been no improvement made
during the time which has effected any considerable saving of
heat or expense. The steam-engines of fifty years ago, considered
as steam-using machines, utilised nearly the same proportion of
the energy or power developed by the boiler as the most improved
engines of modern construction--a fact that in itself indicates
that an engine is not the vital part of steam machinery. There is
not the least doubt that if the efforts to improve steam-engines
had been mainly directed to economising heat and increasing
the evaporative power of boilers, much more would have been
accomplished with the same amount of research. This remark,
however, does not apply to the present day, when the principles of
steam-power are so well understood, and when heat is recognised
as the proper element to deal with in attempts to diminish the
expense of power. There is, of course, various degrees of economy
in steam-using as well as in steam-generating machinery; but so
long as the best steam machinery does not utilise but one-tenth
or one-fifteenth part of the heat represented in the fuel burned,
there need be no question as to the point where improvements in
such machinery should be mainly directed.
The principle upon which steam-engines operate may be briefly
explained as follows:--
A cubic inch of water, by taking up a given amount of heat,
is expanded to more than five hundred cubic inches of steam,
at a pressure of forty-five pounds to the square inch. This
extraordinary expansion, if performed in a close vessel, would
exert a power five hundred times as great as would be required
to force the same quantity of water into the vessel against this
expansive pressure; in other words, the volume of the water when
put into the vessel would be but one five-hundredth part of its
volume when it is allowed to escape, and this expansion, when
confined in a steam-boiler, exerts the force that is called
steam-power. This force or power is, through the means of the
engine and its details, communicated and applied to different kinds
of work where force and movement are required. The water employed
to generate steam, like the engine and the boiler, is merely an
agent through which the energy of heat is applied.
This, again, reaches the proposition that power is heat, and heat
is power, the two being convertible, and, according to modern
science, indestructible; so that power, when used, must give off
its mechanical equivalent of heat, or heat, when utilised, develop
its equivalent in power. If the whole amount of heat represented
in the fuel used by a steam-engine could be applied, the effect
would be, as before stated, from ten to fifteen times as great as
it is in actual practice, from which it must be inferred that a
steam-engine is a very imperfect machine for utilising heat. This
great loss arises from various causes, among which is that the
heat cannot be directly nor fully communicated to the water. To
store up and retain the water after it is expanded into steam, a
strong vessel, called a boiler, is required, and all the heat that
is imparted to the water has to pass through the plates of this
boiler, which stand as a wall between the heat and its work.
To summarise, we have the following propositions relating to steam
machinery:--
1. The steam-engine is an agent for utilising the power of heat and
applying it to useful purposes.
2. The power of a steam-engine is derived by expanding water in a
confining vessel, and employing the force exerted by pressure thus
obtained.
3. The power developed is as the difference of volume between the
feed-water forced into the boiler, and the volume of the steam that
is drawn from the boiler, or as the amount of heat taken up by the
water.
4. The heat that may be utilised is what will pass through the
plates of the boiler, and be taken up by the water, and is but a
small share of what the fuel produces.
5. The boiler is the main part, where power is generated, and the
engine is but an agent for transmitting this power to the work
performed.
6. The loss of power in a steam-engine arises from the heat carried
off in the exhaust steam, loss by radiation, and the friction of
the moving parts.
7. By condensing the steam before it leaves the engine, so that the
steam is returned to the air in the form of water, and of the same
volume as when it entered the boiler, there is a gain effected by
avoiding atmospheric pressure, varying according to the perfection
of the arrangements employed.
Engines operated by means of hot air, called caloric engines,
and engines operated by gas, or explosive substances, all act
substantially upon the same general principles as steam-engines;
the greatest distinction being between those engines wherein the
generation of heat is by the combustion of fuel, and those wherein
heat and expansion are produced by chemical action. With the
exception of a limited number of caloric or air engines, steam
machinery comprises nearly all expansive engines that are employed
at this day for motive-power; and it may be safely assumed that a
person who has mastered the general principles of steam-engines
will find no trouble in analysing and understanding any machinery
acting from expansion due to heat, whether air, gas, or explosive
agents be employed.
This method of treating the subject of motive-engines will no
doubt be presenting it in a new way, but it is merely beginning at
an unusual place. A learner who commences with first principles,
instead of pistons, valves, connections, and bearings, will find
in the end that he has not only adopted the best course, but the
shortest one to understand steam and other expansive engines.
(1.) What is principal among the details of steam
machinery?--(2.) What has been the most important improvement
recently made in steam machinery?--(3.) What has been the result
of expansive engines generally stated?--(4.) Why has water proved
the most successful among various expansive substances employed
to develop power?--(5.) Why does a condensing engine develop
more power than a non-condensing one?--(6.) How far back from
its development into power can heat be traced as an element in
nature?--(7.) Has the property of combustion a common source in
all substances?