Francis Lieber.

Library of universal knowledge. A reprint of the last (1880) Edinburgh and London edition of Chambers' encyclopaedia, with copious additions by American editors (Volume 13) online

. (page 184 of 203)
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ing D, and fovces the piston along to the other end. The
current of steam from the boiler is then allowed to pass
into the other end of the cylinder through the opening C,
and forces the piston back again to its original position,
Fie. 2. and so on. But it is obvious that while this return-

motion is going on, the steam previously admitted at D

must be allowed some exit, or the piston could not be forced back. The manner of this
exit constitutes the difference between the two principal classes of engines, according as
the steam is allowed simply to rush out into the atmosphere, or is conducted into a sepa-
rate vessel, and there "condensed."

The simplest way in which steam can be used in a cylinder is at the same time the
most wasteful. It consists in filling each end of the cylinder alternately fuH of steam
direct from the boiler, and having the full boiler pressure, and thus forcing the piston
along in exactly the same way as that in which it would have to be forced were water
the fluid used instead of steam. We have said this is wasteful; let us examine the
reasons. If we imagine the cylinder to have a capacity of 7 cubic ft., thn, if it be filled
entirely with steam from the boiler at 60 Ibs. pressure, it will contain just one pound-
weight of steam.* The total heat in this pound of steam, as given in the table, is equiv-
alent to 1171 thermal units, f When the piston has reached" the end of its siroke, the
steam contained in the cylinder is thus in itself a great storehouse of work, for each of
these thermal units is equivalent to 772 "foot-pounds" of mechanical energy. But
instead of utilizing this force, at the moment when the cylinder is full of stenm the
one opening is put into communication with the boiler, the other opening with the
atmosphere, and the steam immediately rushes out of the cylinder, and dissipates its
contained energy through the air.

It must be remembered that although the steam, when allowed to go into the atmos-
phere, is immediately reduced to the pressure corresponding to the temperature of the
air (which in ordinary cases would be only a fraction of a pound per square inch), still the
full pressure of the atmosphere itself will nlways be acting on the back of the piston during
its stroke; and that therefore, to find the force with which the piston is being pushed
along, we must subtract that pressure from the steam-pressure. On the one side of the
piston will be the atmosphere with its uniform pressure of nearly 15 Ibs. per square

* These flffiires are near approximations only, as will be seen from the table in article STEAM.

t A "thermal unit" is the quantity of heat necessary to raise, through 1 Fahr., the temperature of
a pound of water at its temperature of max. density viz., 39.1 Fahr., and m <y be considered, without
much error, as the quantity of heat necessary to raise a pound of water 1 Fahr., at *iy place on the
thermornetric scale.




795



Steam.



inc-h, and on the other side the steam pressure of 60 pounds. The effective pressure thus
will be 6015, or 45 Ibs. per square inch only.

Let us now consider the somewhat more economical case of ;ui engine in which the
steam is first used as described above, but afterward, instead of being allowed to pass
into the atmosphere, is conducted through a pipe into a closed vessel, and there con-
densed. The process commonly called condensation, and associated with the idea of
liquefaction, consists in essence merely of the subtraction from steam of a portion of its
sensible heat. This reduction of temperature has a double effect on the steam first, the
liquefaction of a part of it; and then, the reduction of the rest to the pres-ure corre-
sponding to the reduced temperature. (It will be remembered that steam exists at all
temperatures.) It is not possible to do one of these things without the other, and this
fact lies at the bottom of a correct conception of what is called by engineers a '' vacuum."
What is commonly called " vacuum" simply means pres; ure less than the atmospheric
pressure; and, in the case of steam-engines, a vacuum generally implies a pressure of
between 2 ami 4 Ibs. per square inch that is, from a seventh to a fourth of the ordinary
pressure of the air. The most common way of condensing steam is by bringing it into
contact either with a jet of cold water, or with .surfaces kept continually cool by a current
of water. In either case, directly the steam is brought into contact with the waier, or
cooling surface, it transfers to it the larger portion of its sensible heat. During this
process, the greater part of the steam is liquefied, and the remainder retains only such a
pressure as corresponds to its greatly reduced temperature'.

The advantages possessed by a condensing over a non-condensing engine will now be
obvious. When the piston is being forced from one end of the cylinder by steam
entering through the other, the force on the hack of the piston resisting its motion in
that, direction, instead of being equal to the pressure of the atmosphere, is only the
pressure of the steam in the condenser, or about 1 Ib. per square inch. The net ell'c ctive
force is therefore 601, or 59 Ibs., instead of 0015, or 45 pounds. In actual practice
these figures would be modified, because, from various causes, such a low back
pressure as 1 or 15 Ibs. above zero (in condensing and non-condensing engines respec-
tively) is never obtained, but the principle remains the same.

We have supposed that our cylinder, when full of steam, contained just 1 pound-
weight at 60 Ibs. pressure. Let us now find out how much useful work this pound of
steam I- as done for us, and we will then show how the same weight may be made to do
a great deal more, by utilizing more of its great store of heat. Let us suppose that the
area of the cylinder is 2 sq. ft., while its length (the stroke of the pi-ton) is 3i feet. li
will thu^ have a capacity of 7 cubic ft., as before assumed. In the first <asc described,
we should have a pressure of 45 Ibs. per square inch exerted on an area of 388 eq.
in., through a distance of 3A feet. This is equal to 45,360 foot-pounds of work. In
the second case, we have a pressure of 59 Ibs. per square inch on the same area, and
through the same distance. This is equal to 59,472 foot-pounds of work, or about fa
of the total heat supplied by the fuel.* We may now proceed to examine the way in
which the same weight of steam, generated by the consumption of an identical weight
of fuel, may be made to perform many times more work by " workimr expansively."

One of the properties possessed by steam, in common with all oilier gases, is a ten-
dency to expand indefinitely. In article STEAM we mentioned and illustrated the fact
that its pressure varied nearly inversely as its volume. For simplicity's sake we shall
here assume that steam is a perfect gas, and follows Boyle's law, the pressure varying
exactly inversely as the volume. We shall now describe the way in which this expansi
bility of steam is taken advantage of by the engineer. If we have a cylinder of the same
area as before, but of twice the length, but only intend to admit 1 Ib. <f steam into
it at a time, it will be necessary, when the piston has traveled 3A ft. of its stroke, to
shut the entrance valve, so as to prevent more steam entering; this is called "cutting
off" the steam. The piston^ however, still continues its motion in the same direction as
before, propelled by the internal separative energy among the particles of steam. But
as it is pressed forward, the space occupied by the steam is always increasing, and its
pressure always decreasing in proportion, until at length, when the piston has reached
the end of its stroke, the steam occupies exactly double its original volume viz., 14
cubic ft. , and is reduced in pressure to half its original pressure viz., to 30 Ibs. per
square inch. We have thus, during + he first half of the stroke, a constant pressure on
the piston of 60 Ibs. per square inch, and during the second half a pressure gradually
decreasing from 60 to 30 pounds. The mean pressure during this second half of the
stroke will be found on calculation to be almost exactly 40 pounds. Let us now. in the
same way as before, sec what work we have been able to get out of our pound of steam
by expanding it in this way. In the first half of the stroke we have ;"><). 17'_ > foot-pounds
of'work exactly as before, and then we have in addition a mean pressure of 401, or 39
Ibs. per square inch, exerted over 288 sq. in. for a distance of iH feet. Thi- equals
39.312 foot-pounds, making a total of 98,784 footpounds of work obtained from the
steam which only gave us 59,472 before. The economy of working expansively, how-

* For simplicity's sako, we have here assumed that the water in the boiler has to be raised from 38*
to 2!W. a:nl evaporated ar that temperature. If the water were supplied at 212, then the work Uooa
would be about -? 3 instead of -fa of the total heat.



Steam.



796



ever, goes much further than this. If the cylinder had been four times its original
length, and the steam had been cut off at the same point as before (which would then be.
quarter instead of half stroke), we should have obtained from the 1 Ib. of steam 144,845
foot-pounds of work.' If we had goue still further, and expanded the pound of steam
into eight times its original volume, we should have obtained no less than 179,984 foot-
pounds of work, which is more than three times as much as at first.* All modern
engines are worked more or less on this principle of expansion, and the general ten-
dency seems to be every year to adopt higher initial pressures, and larger ratios of

i expansion.

Having thus briefly sketched the history of the steam-engine, and the theory of its

'action, we may now proceed to some consideration of its mechanism. Fig. 3 represents
Watt's "double-acting" condensing engine, which we have already mentioned. By
"double-acting engine" we mean an engine such as was sketched in fig. 2, in which tha
steam acts on both sides of the piston instead of only on one, as in Newcomen's engine.
Watt's engine, though not of the form now generally used, contains all the parts now
considered essential; and we may therefore describe it before saying anything about
these parts in detail. The steam from the boiler passes direct to the valve-chest v, which
is simply a long box attached to the cylinder a. In this chest are placed valves, whicli




FIG. 3.

are so regulated as to open communication between the boiler, cylinder, and conflenser,
in such a way that when the top of the cylinder is op*en to the boiler, the bottom com-
municates with the condenser, and vice versd. When the steam has done its work, it
passes out through the bent pipe into the condenser/, wh<ye it is met by a jet of water
(not shown in the engraving), and condensed, as before explained, g is a pump called
the air-pump, which" continually draws away the contents of the condenser, and dis-
charges them into a cistern h, called the hot-well. A small force pump, j, draws part of
the water from this cistern, and sends it back again to the boiler, there to be reconverted
into steam, while the rest of the water is allowed to run to waste. A suction-pump k,
supplies water to the large tank round the condenser, and also for the condensing jet.
Inside the cylinder are the piston and the rod (called the piston-rod) connecting it with
the beam bb. In Newcomen's engine, the rod bad only to pull the beam down and not
to push it up; it could, therefore, be connected to it by a chain, as shown in fig. 1. In
the double-acting engine, the piston-rod is required both to pull and to push the beam,
so that the chain is no longer admissible. It is obvious that as the head of the rod must
move in a straight line, while every point in the beam describes an arc of a circle, the
two cannot be rigidly connected. Watt invented the arrangement of rods shown in
fig. 1, by which the piston-rod head is always guided in a straight line, while the end
of the beam is left free to pursue its own course. This is called a "parallel motion."

* In actual working, owing to various causes such as imperfect action of the valves, radiation from
the cylinder, bad vacuum, etc. the work obtained from the steam is not more than .65 to .75 of that
given in this paragraph.



797



Steam.




The end of the oeam furthest from the cylinder is connected by a rod ce, called a con-
necting-rod, to the crank I, which is firmly fixed on the shaft; and by this means the
reciprocating motion of the beam is converted into the rotary motion of the "crank-
shaft" r. The governor m, and the fly-wheel ee, will be explained further on.

The cylinder and its pixtmi are both made of cast iron. The former is very accurately
bored in a lathe, and ought always to be covered outside with non-conducting material
to prevent radiation of heat. It is frequently inclosed in another cylinder, and the
annular space, or "jacket" between them filled with steam from the boiler, principally
with the object of preventing liquefaction in the cylinder, which is fatal to economical
working. The openings for the entrance or discharge of the steam are called ports. |

The valve or valves which regulate the admission of steam to the cylinder vary very,
much in construction and design. In ordinary engines one valve, called a tHide-rnh-e,
does the whole work for each cylinder in a way which we shall explain by the aid of
fig. 2. This figure shows the valve in t%vo positions namely, those corresponding to the
times when the piston is at the middle of its stroke, going
in the two different directions c and d are the ports; bia
the " exhaust port," or opening through which the steam
- to the condenser; and a is the slide-valve working
inside the steam-chest (not shown). The sketch to the left
shows the position of the vu\ve when the piston is moving
upward. The steam enters the cylinder through d, as
shown by the arrows, while the steam in the other end is
free to rush out by c, under the valve, and through b into
the condenser. By the time the piston has reached the
same position, going in the opposite direction, .the valve is
in the position shown in the right-hand sketch, and the
motion of the steam is exactly reversed. When it is de-
sired to "cut off" the steam earlier than half-stroke, a
separate valve, called an expansion valve (of which there are
innumerable varieties) is generally used. The rod to which
the piston is attached is called the piston-rod, and the rod
which actually drives the crank the connecting-rod. In FIG. 4.

Watt's engine, and similar machines, thesa are connected to opposite ends of a beam, but
in the common type of engine the two rods are directly attached.

Thefly-wkeeliB a large wheel fixed on the crank shaft, and having a very heavy rim.
As it revolves, this contains. it'>r?<l up in itself, a great quantity of energy, and so
equalizes the motion of the shaft, and, by ni<>ring some of the energy, enables the
engine to pass the " dead-points," or points at jvhich the connecting-rod and crank are
in a line.

The condenser is simply a cast-iron box of any convenient shape. The water for con-
densing the steam is introduced into it in a jet in such a way that its particles mix with
the steam at once on entering, and condense it almost instantaneously.

The goceriw is an ingenious application by Watt of mechanism fong used in water-
mills. Its object is to make the engine to a great extent regulate its own speed, so that
it shall neither be pulled up altogether by a sudden increase of load, nor "race" when
any part of its load is suddenly removed. It consists essentially of a spindle or uprisrht
rod. with a pully, by which it is caused to revolve, fixed on it. Two levers are pivoted
on a pin near the top of the spindle, and at the lower end of each is fixed a heavy cast-
iron ball. When the engine is running at its proper speed, the balls revolve wfth the
:;pindle in the position shown; but if that speed be increased, the centrifugal force causes
them to fly outward, and consequently upward; and conversely, if it be decreased,
they fall downward toward the center. At the upper end of the spindle is a system of
levers, by which the raising of the balls tends to close, and their lowering to open, the
throttU-mlre. This valve is simply a disk of metal placed in the steam-pipe near the cyl-
inder. The further, therefore, it is opened, the greater the amount of steam admitted
to the cylinder, and <v>v rer.^i. and so the tendency of the engine to alter its speed arising
from causes extraneous to itself, is just balanced by the alteration made in the amount
of steam admitted through the throttle-valve. In order that economy as well as regu-
larity of working may be attained, the governor should, however, be so arranged as to
control the "cut-off" instead of throttling the steam.

The "Cornish"' engine, so called from the fact that it is principally used in the Cor-
nish mines, resembles Watt's engine in general appearance. Like Newcomen's engine,
it is used exclusively for pumping, and has no rotary motion, and it is virtually siir.fi"-
acting; but unlike his, the steam pressure, and not that of the atmosphere, actually docs
the w r ork. It is not easy to say why Cornish engines have remained so long in their
original form. They are economical of fuel, owing to the great expansion used, but
the same expansion could also be used with many other forms of engine. They aro very
costly, and extremely heavy and unwieldy, and it seems probable that it is only preju-
dice which stands in the way of their being superseded by small engines running at high
speeds, which would do the same work as economically, and with arnica smaller outlay
in first cost.

Engines in which the piston roil and connecting-rod are directly attached are called



Steam.



798



direrf-m'.t-iiir] engines, of which the horizontal engine is the most common type, and for
ali ordinary purposes is rapidly superseding every other form of stationary engine. It
possesses the merits of having great simplicity and few working parts, and of all these
parts being easily accessible to the engine-driver; and at the same time any required
degree of economical working can be obtained in it as well as in any other form. It
was for a longtime only used as a non-condensing (or so-called " high-pressure") engine,
but is now generally made with a condenser attaclied.

Two other forms of direct-acting engines have been much used in their day, but are
now being rapidly abandoned except under special circumstances; these are called
respectively the "oscillating" and the "trunk" engine. In the former (which has
rarely been used except for marine engines), the crank-shaft is above the cylinder, the
piston-rod head, is attached to the crank-pin, and the connecting-rod is dispensed with.
by allowing the cylinder to oscillate on large hollow centers palled trunnions, and so to
adapt itself td the various positions of the crank-pin. In the "trunk" engine, the piston-
rod becomes a hollow cylinder or trunk, large enough to allow the connecting-rod to
vibrate inside it. The latter is then attached at one end to the crank-pin, as usual, and
at the other to a pin fixed in the piston.

An immense amount of ingenuity has been expended in devising engines in which
the rotary motion of the shaft is obtained directly from the piston without the interven-
tion of reciprocating parts. These machines are called rotary engines; they have never
corae into general use, and most of tliem have been defective in construction, as well as
founded on a dynamical misconception.

In locomotive engines it is necessary that the whole machinery should be compressed
into the smallest possible bulk, and this necessity is the cause of their principal pecul-
iarities. The engine itself is much the same as an ordinary horizontal engine, and has




Fia. 5.



two cylinders placed side by side near the front of the locomotive. These cylinders
are sometimes placed inside the main framing, which runs the whole length of the
engine, and sometimes outside it, each plan having certain advantages. Fig. 5 is an
outline section of .an "inside cylinder" goods locomotive belonging to the Midland
railway company. At the back of the locomotive is the fire-box a the bottom of which
is formed by the grate b. Fuel is introduced by the door c. The fire-box is inclosed
in a casing d, and the space between is filled with water. This space communicates
freely with the barrel ee of the boiler, a long wrought-iron cylinder. From the back of
the fire-box numerous small tubes traverse the boiler (through the water) to the smoke-
box/, and conduct the products of combustion to the chimney g. The steam pipe k is
led away from near the top of the dome h, and fitted with a regulator valve I. At m
are a pair of spring safety-valves. Both cylinders discharge their steam through the
vertical blast-pipe p, and by this means a sufficient draught is caused, notwithstanding
the small height of the chimney. The cylinders r are placed in the bottom of the smoke-
box, and partly inclosed in it.

In all marine engines, except the very smallest, two cylinders are used, working
cranks at right angles to each other, so as to equalize the motion as far as possible, it
being almost impossible to use a fly-wheel of sufficient weight for that purpose on board
ship. In vessels of war, where it is essential that all the machinery should be kept
below the water-line, horizontal engines are used, often of the "trunk" type. In mer-
chant vessels, however, and in all cases where there is no necessity for the machinery
being kept low down in the ship, the form known as the "steam-hammer" engine, or



799



Steam.



some modification or it, is now almost universally adopted. These engines derive tbeir
nume from their resemblance (in their earlier designs) to Mr. Masmyth's steam-hammer,
the form of which seems to have suggested their arrangement. They are direct acting,
but the cylinders arc inverted, and placed right above the propeller shaft.

The two greatest improvements in the modern steam engine ihc surface condenser
and the wini'ivnm.! < //,'//// have been brought to perfection chiefly in connection with
marine engines, and we may therefore mention them here. In the surface-condenser,
the steam is condensed by contact with the surface of a great number of small tubes,
through which a current of cold sea water is kept constantly flowing. By this means
the condensing waier and the condensed are kept separate, the former beyjg returned
to the sea, ami the latter only sent into the hot-well. The boiler, therefore, is continu-
ally fed with distilled water, and the wasteful process of "blowing off," to get rid of
the uuvaporizable matter which would otherwise be deposited iu the boiler, is rendered
unnecessary.

In " compound'' engines, the two- cylinders are of unequal size the larger, called
the low pressure cylinder, having from three to four times the capacity of the smaller or
high-pressure cylinder. The steam from the boiler is admitted into the latter in the
usual way, and cut oil generally at from | to f of the stroke; and after doing its work
there, it is conducted to the large cylinder, where its reduced pressure, by acting on an
increased area, docs as much work as in the other cylinder, and thence to the condenser.
This system of engine has several notable advantages among which are that the inter-
nal stresses are more uniform than in ordinary engines; that leakage past the piston
becomes of less importance; and that for any given large measure of expansion, the
mechanism of the engine is much more simple and less liable to get out of order than
for the same degree of expansion carried out independently in two cylinders.

The 11 ";/, Doni !>>/ Wtnia-ciiffthcx. This is estimated in two ways as horse-power, and
as duty, and the iirst expression includes two things nominal and indicated horse
power. Thirty-three thousand foot-pounds of work done per minute is called one horse-
power, this being considered by Watt as the maximum force which a strong horse could
exert. The nontimtl horse-power of an engine has long ceased to be any expression of
the actual power it exerts; it is only used as a kind of commercial standard (a very defi-
cient one) for the sale and purchase of engines, and is generally made to depend entirely
on the diameter of the cylinder.

The indicated horse power is the most useful measure we have of the work done by
an engine. It expresses, however, the total work done by the steam on the piston, and



Online LibraryFrancis LieberLibrary of universal knowledge. A reprint of the last (1880) Edinburgh and London edition of Chambers' encyclopaedia, with copious additions by American editors (Volume 13) → online text (page 184 of 203)