(79.). The bottom of the piston is a circular plate just so much
less in diameter than the cylinder as is sufficient to allow its
free motion in ascending and descending. A little above its lowest
point this plate begins gradually to diminish in thickness, until
its diameter is reduced to from one to two inches less than that
of the cylinder, leaving therefore around [Pg243] it a hollow
space, as represented in _fig._ 65. The cover of the piston is a
plate similarly formed, being in like manner gradually reduced in
thickness downwards, so as to correspond with the lower plate. In
the hollow space which thus surrounds the piston a packing of
unspun hemp or soft rope, called _gasket_, is introduced by
winding it round the piston so as to render it an even and compact
mass. When the space is thus filled up, the top of the piston is
attached to the bottom by screws. The curved form of the space
within which the hempen packing is confined is such that when the
screws are tightened, that part of the packing which is nearest to
the top and bottom of the piston is forced against the cylinder,
so as to produce upon the two parallel rings as much pressure as
is necessary to render it steam-tight. When by use the packing is
worn down so as to produce leakage, the cover of the cylinder must
be removed, and the screws connecting the top and bottom of the
piston tightened: this will force out the packing and render the
piston steam-tight. This packing is lubricated by melted tallow
let down upon the piston from the funnel inserted in the top of
the cylinder, furnished with a stop-cock to prevent the escape of
steam. The lower end of the piston-rod is formed slightly conical,
the thickest part of the cone being downward. It is passed up
through the piston, and a nut or wedge between the top and bottom
is inserted so as to secure the piston in its position upon the
rod.
[Illustration: _Fig._ 65.]
The process of removing the top of the cylinder for the purpose of
tightening the screws in the piston is one of so laborious a
nature, that the men entrusted with the superintendence of these
machines are tempted to allow the engine to work notwithstanding
injurious leakage at the piston, rather than incur the labour of
tightening the screws as often as it is necessary to do so.
To avoid this inconvenience, the following method of [Pg244]
tightening the packing of the piston without removing the lid of
the cylinder, was contrived by Woolf. The head of each of the
screws was formed into a toothed pinion, and as these screws were
placed at equal distances from the centre of the piston, these
several pinions were driven by a large toothed wheel, revolving on
the piston-rod as an axis. By such an arrangement it is evident
that if any one of the screws be turned, a like motion will be
imparted to all the others through the medium of the large central
wheel. Woolf accordingly formed, on the head of one of the screws,
a square end. When the piston was brought to the top of the
cylinder, this square end entered an aperture made in the under
side of the cover of the cylinder. This aperture was covered by a
small circular piece screwed into the top of the cylinder, which
was capable of being removed so as to render the square head of
the screw accessible. When this was done, a proper key being
applied to the square head of the screw, it was turned; and by
being turned, all the other screws were in like manner moved. In
this way, instead of having to remove the cover of the cylinder,
which in large cylinders was attended with great labour and loss
of time, the packing was tightened by merely unscrewing a piece in
the top of the cylinder not much greater in magnitude than the
head of one of the screws.
This method was further simplified by causing the great circular
wheel already described to move upon the piston-rod, not as an
axis, but as a screw, the thread being cut upon a part of the
piston-rod which worked in a corresponding female screw cut upon
the central plate. By such means, the screw whose head was let
into the cover of the cylinder which turned, would cause this
circular plate to be pressed downwards by the force of the screw
constructed on the piston-rod. This circular plate thus pressed
downwards, acted upon pins or plugs which pressed together the top
and bottom of the cylinder in the same manner as they were pressed
together by the screws connecting them as already described.
METALLIC PISTONS.
(142.) The notion of constructing a piston so as to move
steam-tight in the cylinder without the use of packing of
vegetable [Pg245] matter was first suggested by the Rev. Mr.
Cartwright, a gentleman well known for other mechanical
inventions. A patent was granted in 1797 for a new form of steam
engine, in which he proposed to use the vapour of alcohol to work
the piston instead of the steam of water: and since the principle
of the engine excluded the use of lubrication by oil or tallow, he
substituted a piston formed of metallic rings pressed against the
surface of the cylinder by springs, so as to be maintained in
steam-tight contact with it, independently either of packing or
lubrication. Although the engine for which this form of piston was
intended never came into practical use, yet it is so simple and
elegant in its structure, and forms a link so interesting in the
history of the steam engine, that some explanation of it ought not
to be omitted in this work.
The steam-pipe from the boiler is represented cut off at B (_fig._
66.); T is a spindle-valve, for admitting steam above the piston,
and R is a spindle-valve in the piston; D is a curved pipe forming
a communication between the cylinder and the condenser, which is
of very peculiar construction. Cartwright proposed effecting a
condensation without a jet, by exposing the steam to contact with
a very large quantity of cold surface. For this purpose, he formed
his condenser by placing two cylinders nearly equal in size, one
within the other, allowing the water of the cold cistern in which
they were placed to flow through the inner cylinder, and to
surround the outer one. Thus, the thin space between the two
cylinders formed the condenser.
[Illustration: _Fig._ 66.]
The air-pump is placed immediately under the cylinder, and the
continuation of the piston-rod works its piston, which is solid
and without a valve. F is the pipe from the condenser to the
air-pump, through which the condensed steam is drawn off through
the valve G on the ascent of the piston, and on the descent this
is forced through a tube into a hot well H, for the purpose of
feeding the boiler through the feed-pipe I. In the top of the hot
well H is a valve which opens inwards, and is kept closed by a
ball floating on the surface of the liquid. The pressure of the
condensed air above the surface of the liquid in H forces it
through I into the boiler. When the air accumulates in too great a
degree [Pg246] in H, the surface of the liquid is pressed so low
that the ball falls and opens the valve, and allows it to escape.
The air in H is that which is pumped from the condenser with the
liquid, and from which it was disengaged.
Let us suppose the piston at the top of the cylinder: it strikes
the tail of the valve T, and raises it, while the stem of the
piston-valve R strikes the top of the cylinder, and is pressed
into its seat. A free communication is at the same time open
between the cylinder, below the piston and the condenser, through
the tube D. The pressure of the steam [Pg247] thus admitted above
the piston acting against the vacuum below it, will cause its
descent. On arriving at the bottom of the cylinder, the tail of
the piston-valve R will strike the bottom, and it will be lifted
from its seat, so that a communication will be opened through it
with the condenser. At the same moment, a projecting spring K,
attached to the piston-rod, strikes the stem of the steam-valve T,
and presses it into its seat. Thus while the further admission of
steam is cut off, the steam above the piston flows into the
condenser, and the piston being relieved from all pressure, is
drawn up by the momentum of the fly-wheel, which continues the
motion it received from the descending force. On the arrival of
the piston again at the top of the cylinder, the valve T is opened
and R closed, and the piston descends as before, and so the
process is continued.
The mechanism by which motion is communicated from the piston to
the fly-wheel is peculiarly elegant. On the axis of the fly-wheel
is a small wheel with teeth, which work in the teeth of another
larger wheel L. This wheel is turned by a crank, which is worked
by a cross-piece attached to the end of the piston-rod. Another
equal-toothed wheel M is turned by a crank, which is worked by the
other end of the cross-arm attached to the piston-rod.
One of the peculiarities of this engine is, that the liquid which
is used for the production of steam in the boiler circulates
through the machine without either diminution or admixture with
any other fluid, so that the boiler never wants more feeding than
what can be supplied from the hot well H. This circumstance forms
an important feature in the machine, as it allows of ardent
spirits being used in the boiler instead of water, which, since
they boil at low heats, promised a saving of fuel. The inventor
proposed that the engine should be used as a still, as well as a
mechanical power, in which case the whole of the fuel would be
saved.
[Illustration: _Fig._ 67.]
[Illustration: _Fig._ 68.]
(143.) That part of Cartwright's piston which in the common piston
is occupied by the packing of gasket, already explained (141.),
was filled by a number of rings, one placed within and above
another, and divided into three or four [Pg248] segments. Two
rings of brass were made of the full size of the cylinder, and so
ground as to fit the cylinder nearly steam-tight. These were cut
into several segments A A A (_fig._ 67.), and were placed one
above the other, so as to fill the space between the top and
bottom plates of the piston. The divisions of the segments of the
one ring were made to fit between the divisions of the other.
Within these another series of rings, B B B, were placed,
similarly constructed, so as to fit within the first series in the
same manner as the first series were made to fit within the
cylinder. The joints of the upper series of each set of rings are
exhibited in the plan (_fig._ 67.); the places of the joints of
the lower series are shown by dotted lines; the position of the
rings of each series one above the other is shown in the section
(_fig._ 68.). The joints of the inner series of rings are so
placed as to lie between those of the outer series, to prevent the
escape of steam which would take place by one continued joint from
top to bottom of the packing. The segments into which the rings
are divided are pressed outwards by steel springs in the form of
the letter V, the springs which act upon the outer series of
segments abutting upon the inner series, and those which act on
the inner series abutting upon the solid centre of the piston:
these springs are represented in _fig._ 67.
[Illustration: _Fig._ 69.]
[Illustration: _Fig._ 70.]
(144.) An improved form was given to the metallic piston by
Barton. Barton's piston consists of a solid cylinder of cast iron,
represented at A in section in _fig._ 69., and in plan in [Pg249]
_fig._ 70. In the centre of this is a conical hole, increasing in
magnitude downwards, to receive the piston-rod, in which the
latter is secured by a cross-pin B. A deep groove, square in its
section, is formed around the piston, so that while the top and
bottom of the piston form circles equal in magnitude to the
section of the cylinder, the intermediate part of the body of the
piston forms a circle less than the former by the depth of the
groove. Let a ring of brass, cast iron, or cast steel, be made to
correspond in magnitude and form with this groove, and let it be
divided as represented in _fig._ 70., into four segments C C C C,
and four corresponding angular pieces D D D D. Let the groove
which surrounds the piston be filled by the four segments with the
four wedge-like angular pieces within them, and let the latter be
urged against the former by eight spiral springs, as represented
in _fig._ 69. and _fig._ 70. These springs will abut against the
solid centre by the piston, and will urge the segments C against
the cylinder. The spiral springs which urge the wedges are
confined in their action by steel pins which pass through their
centre, and by being [Pg250] confined in cylindrical cavities
worked into the wedges and into corresponding parts of the solid
centre of the piston, as the segments C wear, the springs urge the
wedges outwards, and the points of the latter protruding, are
gradually worn down so as to fill up the spaces left between the
segments, and thus to complete the outer surface of the piston.
Various other forms of metallic pistons have been proposed, but as
they do not differ materially in principle from those we have just
described, it will not be necessary here to describe them.
[Illustration: ENGINE AT THE CITY SAW MILLS.]
[Pg251]
[Illustration: FURNACE AT THE CITY SAW MILLS.]
CHAP. IX.
CONSTITUENTS OF COAL. — PROCESS OF COMBUSTION. — HEAT EVOLVED
IN IT. — FORM AND STRUCTURE OF BOILER. — WAGGON BOILER. —
FURNACE. — METHOD OF FEEDING IT. — COMBUSTION OF GAS IN FLUES.
— CONSTRUCTION OF GRATE AND ASH-PIT. — MAGNITUDE OF HEATING
SURFACE OF BOILER. — STEAM-SPACE AND WATER-SPACE IN BOILER. —
POSITION OF FLUES. — METHOD OF FEEDING BOILER. — LEVEL GAUGES.
— SELF-REGULATING FEEDERS. — STEAM-GAUGE. — BAROMETER-GAUGE. —
INDICATOR. — COUNTER. — SAFETY-VALVE. — FUSIBLE PLUGS. —
SELF-REGULATING DAMPER. — SELF-REGULATING FURNACE. — POWER AND
DUTY OF ENGINES. — HORSE-POWER OF STEAM ENGINES. — EVAPORATION
PROPORTIONAL TO HORSE-POWER. — SOURCES OF LOSS OF POWER. —
ABSENCE OF GOOD PRACTICAL RULES FOR POWER. — COMMON RULES
FOLLOWED BY ENGINE MAKERS. — DUTY DISTINGUISHED FROM POWER. —
DUTY OF BOILERS. — PROPORTION OF STROKE TO DIAMETER OF
CYLINDER. — DUTY OF ENGINES. — CORNISH SYSTEM OF INSPECTION. —
ITS GOOD EFFECTS. — HISTORICAL DETAIL OF THE DUTY OF CORNISH
ENGINES.
(145.) The machinery which has been explained in the preceding
chapters, consisting of the cylinder with its passages and valves,
the piston-rod, parallel motion, beam, connecting-rod and crank,
together with the condenser, air-pump, and other appendages,
having no source of moving power in themselves, must be regarded
as mere instruments by which the mechanical effect developed by
the furnace and the boiler is transmitted to the working point and
so [Pg252] modified as to be adapted to the uses to which the
machine is applied. The boiler is at once a magazine in which the
moving power is stored in sufficient quantity to supply the
demands of the engine and an apparatus in which that power is
fabricated. The mechanical effect evolved in the conversion of
water into steam by heat, is the process by which the power of the
steam-engine is produced, and space is provided in the boiler,
capacious enough to contain as much steam as is necessary for the
engine, besides a sufficient quantity of water to continue that
supply undiminished, notwithstanding the constant drafts made upon
it by the cylinder: even the water itself, from the evaporation of
which the mechanical power is produced, ought to be regarded as an
instrument by which the effect of the heat of the combustible is
rendered mechanically efficient, inasmuch as the same heat,
applied not only to other liquids but even to solids, would
likewise be productive of mechanical effects. The boiler and its
furnace are therefore parts of the steam-engine, the construction
and operation of which are entitled to especial attention.
(146.) COAL, the combustible almost universally used in
steam-engines, is a substance, the principal constituents of which
are _carbon_ and _hydrogen_, occasionally mixed with sulphur in a
small proportion, and earthy incombustible matter. In different
sorts of coal the proportions of these constituents vary, but in
coal of good quality about three quarters of the whole weight of
the combustible is carbon.
When carbon is heated to a temperature of about 700° in an
atmosphere of pure oxygen, it will combine chemically with that
gas, and the product will be the gas called _carbonic acid_. The
volume of carbonic acid produced by this combination, will be
exactly equal to that of the oxygen combined with the carbon, and
therefore the weight of a given volume of the gas will be
increased by the weight of carbon which enters the combination. It
is found that two parts by weight of oxygen combined with three of
carbon, form carbonic acid. The weight of the carbonic acid,
therefore, produced in the combustion, will be greater than the
weight of the oxygen, bulk for bulk, in the proportion of five to
two, the volume being the same and the gases being [Pg253]
compared at the same temperatures and under equal pressures. In
this combination heat is evolved in very large quantities. This
effect arises from the heat previously latent in the carbon and
oxygen being rendered sensible in the process of combustion. The
carbonic acid proceeding from the combustion is by such means
raised to a very high temperature, and the carbon during the
process acquires a heat so intense as to become luminous; no
flame, however, is produced.
Hydrogen, heated to a temperature of about 1000°, in contact with
oxygen will combine with the latter, and a great evolution of heat
will attend the process; the gases will be rendered luminous, and
flame will be produced. The product of this process will be water,
which being exposed to the intense heat of combustion, will be
immediately converted into steam. Hydrogen combines with eight
times its own weight of oxygen, producing nine times its own
weight of water.
Hydrogen gas is, however, not usually disengaged from coal in a
simple form, but combined chemically with a certain portion of
carbon, the combination being called carburetted hydrogen. Pure
hydrogen burns with a very faintly luminous blue flame, but
carburetted hydrogen gives that bright flame occasionally having
an orange or reddish tinge, which is seen to issue from burning
coals: this is the gas used for illumination, being expelled from
the coal by the process of coking, and conducted to the various
burners through proper pipes.
The sulphur, which in a very small proportion is contained in
coals, is also combustible, and combines in the process of
combustion with oxygen, forming sulphurous acid: it is also
sometimes evolved in combination with hydrogen, forming
sulphuretted hydrogen.
Atmospheric air consists of two gases, azote and oxygen, mixed
together in the proportion of four to one; five cubic feet of
atmospheric air consisting of four cubic feet of azote and one of
oxygen. Any combustible will combine with the oxygen contained in
atmospheric air, if raised to a temperature somewhat higher than
that which is necessary to cause its combustion in an atmosphere
of pure oxygen.
If coals, therefore, or other fuel exposed to atmospheric [Pg254]
air, be raised to a sufficiently high temperature, their
combustible constituents will combine with the oxygen of the
atmospheric air, and all the phenomena of combustion will ensue.
In order, however, that the combustion should be continued, and
should be carried on with quickness and activity, it is necessary
that the carbonic acid, and other products, should be removed from
the combustible as they are produced, and fresh portions of
atmospheric air brought into contact with it; otherwise the
combustible would soon be surrounded by an atmosphere composed
chiefly of carbonic acid to the exclusion of atmospheric air, and
therefore of uncombined oxygen, and consequently the combustion
would cease, and the fuel be extinguished. To maintain the
combustion, therefore, a current of atmospheric air must be
constantly carried through the fuel: the quantity and force of
this current must depend on the quantity and quality of the fuel
to be consumed. It must be such that it shall supply sufficient
oxygen to the fuel to maintain the combustion, and not more than
sufficient, since any excess would be attended with the effect of
absorbing the heat of combustion, without contributing to the
maintenance of that effect.
Heat is communicated from body to body in two ways, by radiation
and by contact.
Rays of heat issue from a heated body, and are dispersed through
the surrounding space in a manner, and according to laws, similar
to those which govern the radiation of light. The heat thus
radiated meeting other bodies is imparted to them, and penetrates
them with more or less facility according to their physical
qualities.
A heated body also brought into contact with another body of lower
temperature, communicates heat to that other body, and will
continue to do so until the temperature of the two bodies in
contact shall be equalised. Heat proceeds from fuel in a state of
combustion in both these ways: the heated fuel radiates heat in
all directions around it, and the heat thus radiated will be
imparted to all parts of the furnace which are exposed to the
fuel.
The gases, which are the products of the combustion, escape from
the fuel at a very high temperature, and consequently, in
acquiring that temperature they absorb a considerable [Pg255]
quantity of the heat of combustion. But besides the gases actually
formed in the process of combustion, the azote forming four fifths
of the air carried through the fuel to support the combustion,
absorbs heat from the combustible, and rises into the upper part
of the furnace at a high temperature. These various gases, if
conducted directly to the chimney, would carry off with them a
considerable quantity of the heat. Provision should therefore be
made to keep them in contact with the boiler such a length of time
as will enable them to impart such a portion of the heat which
they have absorbed from the fuel, as will still leave them at a
temperature sufficient, and not more than sufficient, to produce
the necessary draft in the chimney.
(147.) The forms of boiler which have been proposed as the most
convenient for the attainment of all these requisite purposes have
been very various. If strength alone were considered, the
spherical form would be the best; and the early boilers were very
nearly hemispheres, placed on a slightly concave base. The form
adopted by Watt, called the waggon boiler, consists of a
semi-cylindrical top, flat perpendicular sides, flat ends, and a
slightly concave bottom. The steam intended to be used in boilers
of this description did not exceed the pressure of the external
atmosphere by more than from 3 to 5 lbs. per square inch; and the
flat sides and ends, though unfavourable to strength, could be
constructed sufficiently strong for this purpose. In a boiler of
this sort, the air and smoke passing through the flues that are
carried round it, are in contact at one side only with the boiler.
The brickwork, or other materials forming the flue, must therefore
be non-conductors of heat, that they may not absorb any
considerable portion of heat from the air passing in contact with
them. A boiler of this form is represented in _fig._ 71.
The grate and a part of the flues are rendered visible by the
removal of a portion of the surrounding masonry in which the
boiler is set. The interior of the boiler is also shown by cutting
off one half of the semi-cylindrical roof. A longitudinal vertical
section is shown in _fig._ 72., and a cross section in _fig._ 73.
A horizontal section taken above the level of the grate, and below
the level of the water in the boiler, showing [Pg256] the course
of the flues, is given in _fig._ 74. The corresponding parts in
all the figures are marked by the same letters.
[Illustration: _Fig._ 71.]
The door by which fuel is introduced upon the grate is represented
at A, and the door leading to the ash-pit at B. The fire bars at C
slope downwards from the front at an angle of about 25°, giving a
tendency to the fuel to move from the front towards the back of
the grate. The ash-pit D is constructed of such a magnitude, form,
and depth, as to admit a current of atmospheric air to the
grate-bars, sufficient to sustain the combustion. The form of the
ash-pit is usually wide below, contracting towards the top.
[Illustration: _Fig._ 72.]