condensers as perfect as that which is maintained in the ordinary
condensers by injection. It is objected, on the other hand, that
without the injection water and the air which accompanies it being
introduced into his condensers, Mr. Hall uses as large and
powerful an air-pump as those which are used in engines of equal
power condensing by injection; that, consequently, the vacuum
which is maintained is produced, not as it ought to be altogether
by the condensation of steam, but by the air-pump drawing off the
uncondensed steam. To whatever extent this may be true, the
efficacy of the machine, as indicated by the barometer-gauge, is
only apparent; since as much power is necessary to pump away any
portion of uncondensed vapour as is obtained by the vacuum
produced by the absence of that vapour.
A tubular condenser of the form proposed by Mr. Hall is represented
in _fig._ 126.; _a_ is the upper part of the condenser to which
steam is admitted from the slide after having worked the piston; _k_
is the section of a thin plate, forming the top of the condenser,
perforated with small holes, in which the tubes are inserted so as
to be steam-tight and water-tight. Water is admitted to flow around
these tubes between the top _k_ and the bottom _d_ of the condenser,
so as to keep them constantly at a low temperature. The steam passes
from _a_ through the tubes to the lower chamber _f_ of the
condenser, where it is reduced to water by the cold to which it has
been exposed. A supply of cold water is constantly pumped through
the condenser, so as to keep the tubes at a low temperature. The
air-pump _g_ is of the usual construction, having valves in the
piston opening upwards, and [Pg460] similar valves in the cover of
the pump also opening upwards. The water formed by the condensed
steam in _f_ is drawn through the foot-valve, and after passing
through the piston-valves, is discharged by the up-stroke of the
piston into the hot well. Any air, or other permanent gas, which may
be admitted by leakage through the tubes of the condenser, or by any
other means, is likewise drawn out by this pump, and when drawn into
the hot well is carried from thence to the feeding apparatus of the
boiler, to which it is transferred by the feed-pump.
[Illustration: _Fig._ 126.]
A provision is likewise made by which the steam escaping at the
safety-valve is condensed and carried away to the feeding cistern.
(214.) One of the remedies proposed for the evil consequences
arising from incrustation is the substitution of copper for iron
boilers. The attraction which produces the adhesion of the
calcareous matter held in solution by salt water to the surface of
iron has no existence in copper, and all the saline and other
alkaline matter precipitated in the boiling water in [Pg461]
copper boilers is suspended in a loose form, and carried off by
the process of blowing out.
Besides the injury arising from the deposition of salt and the
incrustation on the inner surface of boilers, an evil of a
formidable kind attends the accumulation of soot mixed with salt
in the flues, which proceeds from the leaks. In the seams of the
boiler there are numerous apertures, of dimensions so small as to
be incapable of being rendered stanch by any practicable means,
through which the water within the boiler filters, and the salt
which it carries with it mixes with the soot, forming a compound
which rapidly corrodes the boilers. This process of corrosion in
the flues takes place not less in copper than in iron boilers. In
cleansing the flues of a copper boiler, the salt and soot which
was thrown out upon the iron-plates which formed the flooring of
the engine-room, having remained there for some time, left behind
it a permanent appearance of copper on the iron flooring, arising
from the precipitation of the copper which had combined with the
soot and salt in the flues.[36] In this case the leaks from whence
the salt proceeded were found, on careful examination, so
unimportant, that the usual means to stanch them could not be
resorted to without the risk of increasing the evil.
(215.) In the application of the steam-engine to the propulsion of
vessels in voyages of great extent, the economy of fuel acquires
an importance greater than that which appertains to it in
land-engines, even in localities the most removed from coal-mines,
and where its expense is greatest. The practical limit to
steam-voyages being determined by the greatest quantity of coals
which a steam-vessel can carry, every expedient by which the
efficiency of the fuel can be increased becomes a means, not
merely of a saving of expense, but of an increased extension of
steam-power to navigation. Much attention has been bestowed on the
augmentation of the duty of engines in the mining districts of
Cornwall, where the question of their efficiency is merely a
question of economy, but far greater care should be given to this
subject when the practicability of maintaining intercourse by
steam between distant points of the globe will perhaps depend on
the effect produced by a given quantity [Pg462] of fuel. So long
as steam-navigation was confined to river and channel transport,
and to coasting voyages, the speed of the vessel was a paramount
consideration, at whatever expenditure of fuel it might be
obtained; but since steam-navigation has been extended to
ocean-voyages, where coals must be transported sufficient to keep
the engine in operation for a long period of time without a fresh
relay, greater attention has been bestowed upon the means of
economising it.
Much of the efficiency of fuel must depend on the management of
the fires, and therefore on the skill and care of the stokers.
Formerly the efficiency of firemen was determined by the abundant
production of steam, and so long as the steam was evolved in
superabundance, however it might have blown off to waste, the duty
of the stoker was considered as well performed. The regulation of
the fires according to the demands of the engine were not thought
of, and whether much or little steam was wanted, the duty of the
stoker was to urge the fires to their extreme limit.
Since the resistance opposed by the action of the paddle-wheels of
a steam-vessel varies with the state of the weather, the
consumption of steam in the cylinders must undergo a corresponding
variation; and if the production of steam in the boilers be not
proportioned to this, the engines will either work with less
efficiency than they might do under the actual circumstances of
the weather, or more steam will be produced in the boilers than
the cylinders can consume, and the surplus will be discharged to
waste through the safety-valves. The stokers of a marine engine,
therefore, to perform their duty with efficiency, and obtain from
the fuel the greatest possible effect, must discharge the
functions of a self-regulating furnace, such as has been already
described: they must regulate the force of the fires by the amount
of steam which the cylinders are capable of consuming, and they
must take care that no unconsumed fuel is allowed to be carried
away from the ash-pit.
(216.) Until within a few years of the present time the heat
radiated from every part of the surface of the boiler was allowed
to go to waste, and to produce injurious effects on those parts of
the vessel to which it was transmitted. This evil, [Pg463]
however, has been lately removed by coating the boilers,
steam-pipes, &c. of steam-vessels with felt, by which the escape
of heat from the surface of the boiler is very nearly, if not
altogether, prevented. This felt is attached to the boiler-surface
by a thick covering of white and red lead. This expedient was
first applied in the year 1818 to a private steam-vessel of Mr.
Watt's called the _Caledonia_, and it was subsequently adopted in
another vessel, the machinery of which was constructed at Soho,
called the _James Watt_.
The economy of fuel depends in a considerable degree on the
arrangement of the furnaces, and the method of feeding them. In
general each boiler is worked by two or more furnaces communicating
with the same system of flues. While the furnace is fed, the door
being open, a stream of cold air rushes in, passing over the burning
fuel and lowering the temperature of the flues: this is an evil to
be avoided. But, on the other hand, if the furnaces be fed at
distant intervals, then each furnace will be unduly heaped with
fuel, a great quantity of smoke will be evolved, and the combustion
of the fuel will be proportionally imperfect. The process of coking
in front of the grate, which would insure a complete combustion of
the fuel, has been already described (147.). A frequent supply of
coals, however, laid carefully on the front part of the grate, and
gradually pushed backwards as each fresh feed is introduced, would
require the fire-door to be frequently opened, and cold air to be
admitted. It would also require greater vigilance on the part of the
stokers than can generally be obtained in the circumstances in which
they work. In steam-vessels the furnaces are therefore fed less
frequently, fuel introduced in greater quantities, and a less
perfect combustion produced.
When several furnaces are constructed under the same boiler,
communicating with the same system of flues, the process of
feeding, and consequently opening one of them, obstructs the due
operation of the others, for the current of cold air which is thus
admitted into the flues checks the draft and diminishes the
efficiency of the furnaces in operation. It was formerly the
practice in vessels exceeding one hundred horse-power, to place
four furnaces under each boiler, communicating with the same
system of flues. Such an arrangement [Pg464] was found to be
attended with a bad draft in the furnaces, and therefore to
require a greater quantity of heating surface to produce the
necessary evaporation. This entailed upon the machinery the
occupation of more space in the vessel in proportion to its power;
it has therefore been more recently the practice to give a
separate system of flues to each pair of furnaces, or, at most, to
every three furnaces. When three furnaces communicate with a
common flue, two will always be in operation, while the third is
being cleared out; but if the same quantity of fire were divided
among two furnaces, then the clearing out of one would throw out
of operation half the entire quantity of fire, and during the
process the evaporation would be injuriously diminished. It is
found by experience, that the side plates of furnaces are liable
to more rapid destruction than their roofs, owing, probably, to a
greater liability to deposit. Furnaces, therefore, should not be
made narrower than a certain limit. Great depth from front to back
is also attended with practical inconvenience, as it renders
firing tools of considerable length, and a corresponding extent of
stoking room necessary. It is recommended, by those who have had
much practical experience in steam-vessels, that furnaces six feet
in depth from front to back should not be less than three feet in
width, to afford means of firing with as little injury to the side
plates as possible, and of keeping the fires in the condition
necessary for the production of the greatest effect. The tops of
the furnaces almost never decay, and seldom are subject to an
alteration of figure, unless the level of the water be allowed to
fall below them.[37]
(217.) A form of marine engine was some years since proposed and
patented by Mr. Thomas Howard, possessing much novelty and
ingenuity, and having pretensions to a very extraordinary economy
of fuel, in addition to the advantages claimed by Mr. Hall. In Mr.
Howard's engines, the steam, as in Mr. Hall's, is constantly
reproduced from the same water, so that pure or distilled water
may be used; but Mr. Howard dispenses altogether with the use of a
boiler.
A quantity of mercury is placed in a shallow wrought-iron
vessel over a coke fire, by which it is maintained at a [Pg465]
temperature varying from 400° to 500°. The surface exposed to the
fire was computed at three fourths of a square foot for each
horse-power. The upper surface of the mercury was covered by a
very thin plate of iron in contact with it, and so contrived as to
present about four times as much surface as that exposed beneath
the fire. Adjacent to this a vessel of water was placed,
maintained nearly at the boiling point, and communicating by a
nozzle and valve with the chamber immediately above the mercury.
At intervals corresponding to the motion of the piston a small
quantity of water was injected from this vessel, and thrown upon
the plate of iron resting upon the hot mercury. From this it
received not only the heat necessary to convert it into common
steam, but to give it the qualities of highly superheated steam.
In fact, the steam thus produced had a temperature considerably
above that which corresponded to its pressure, and was, therefore,
capable of being deprived of more or less of its heat without
being condensed. (94.) The quantity of water injected into the
steam-chamber was regulated by the power at which the engine was
intended to be worked. The fire was supplied with air by a blower
subject to exact regulation. The steam thus produced was conducted
to a chamber surrounding the working cylinder, and this chamber
itself was enclosed by another space through which the air from
the furnace passed before it reached the flue. By this contrivance
the air imparted its redundant heat to the steam, as the latter
passed to the cylinder, and raised its temperature to about 400°,
the pressure, however, not exceeding 25 lbs. per square inch. The
valves, governing the admission of steam to the piston, were
adapted for expansive action.
The vacuum on the opposite side was maintained by condensation in
the following manner:—The condenser was a copper vessel placed in
a cistern of cold water, and the steam was admitted to it from the
cylinder by an eduction pipe in the usual way. A jet was
introduced from an adjacent vessel filled with distilled water,
and the condensing water and condensed steam were pumped from the
condenser as in common engines. The warm water thus pumped out of
the [Pg466] condenser was drawn through a copper worm, carried
with many coils through a cistern of cold water, so that when it
arrived at the end of this pipe it was reduced nearly to the
temperature of the atmosphere. The pipe was thus brought to the
vessel of distilled water already mentioned, and the water
supplied by it replaced. The water admitted to the condenser
through the condensing jet being purged of air, a small air-pump
was sufficient, since it had only to exhaust the condenser and
tubes at starting, and to remove the air which might be admitted
by leakage. Mr. Howard stated that the condensation took place as
rapidly and perfectly as in the best engines of the common kind.
An engine of this construction was in the spring of 1835 placed in
the government steamer called the _Comet_. It was stated, that
though the machinery was not advantageously constructed, a part of
the engine being old, and not made expressly for a boiler of this
kind, the vessel performed a voyage from Falmouth to Lisbon, in
which the consumption of fuel did not exceed a third of her former
consumption when worked by Boulton and Watt's engines, the former
consumption of coals being about eight hundred pounds per hour,
and the consumption of Mr. Howard's engine being less than two
hundred and fifty pounds of coke per hour.
The advantages claimed for this contrivance were the following:
_first_, the small space and weight occupied by the machinery,
arising from the absence of a boiler; _second_, the diminished
consumption of fuel; _third_, the reduced size of the flues;
_fourth_, the removal of the injurious effects arising from
deposit and incrustation; _fifth_, the absence of smoke.
(218.) The method by which the greatest quantity of practical effect
can be obtained from a given quantity of fuel must, however, mainly
depend on the extended application of the expansive principle. This
has been the means by which an extraordinary amount of duty has been
obtained from the Cornish engines. The difficulty of the application
of this principle in marine engines has arisen from the objections
entertained in Europe to the use of steam of high pressure under the
circumstances in which the engine must be worked at sea. To apply
the expansive principle, it is necessary that the moving power at
the commencement of the stroke shall considerably exceed the
[Pg467] resistance, its force being gradually attenuated till the
completion of the stroke, when it will at length become less than
the resistance. This condition may, however, be attained with steam
of limited pressure, if the engine be constructed with a sufficient
quantity of piston-surface. This method of rendering the expansive
principle available at sea, and compatible with low-pressure steam,
has recently been brought into operation by Messrs. Maudslay and
Field. Their improvement consists in adapting two steam-cylinders in
one engine, in such a manner that the steam shall act simultaneously
on both pistons, causing them to ascend and descend together. The
piston-rods are both attached to the same horizontal cross-head,
whereby their combined action is applied to one crank by means of a
connecting rod placed between the pistons.
[Illustration: _Fig._ 127.]
A section of such an engine, made by a plane passing through the
two piston-rods P P′ and cylinders, is represented in _fig._ 127.
The piston-rods are attached to a cross-head C, [Pg468] which
ascends and descends with them. This cross-head drives upwards and
downwards an axle D, to which the lower end of the connecting rod
E is attached. The other end of the connecting rod drives the
crank-pin F, and imparts revolution to the paddle-shaft G. A rod H
conveys motion by means of a beam I to the rod K of the air-pump
E.
(219.) Connected with this, and in the same patent, another
improvement is included, consisting of the application of a hollow
wrought-iron framing carried across the vessel above the machinery,
to support the whole of the bearings of the crank-shaft. A plan of
this, including the cylinders and paddle-wheel, is represented in
_fig._ 128. The advantages proposed by these improvements are
simplicity of construction, more direct action on the crank, economy
of space and weight of material, combined with increased area of the
piston, whereby a given evaporating power of the boiler is rendered
productive, by extended application of the expansive principle, of a
greater moving power than in former arrangements. Consequently,
under like circumstances, greater power and economy of fuel is
obtained, with the further advantage at sea, that when the engine is
reduced in its speed, either by the vessel being deeply laden with
coal, as is the case at the commencement of a long sea voyage, or by
head winds, more steam may be given to the cylinders, and
consequently more speed imparted to the vessel, all the steam
produced in the boiler being usefully employed.
(220.) Another improvement, having the same objects, and analogous
to the preceding, has been likewise patented by Messrs. Maudslay
and Field. This consists in the adoption of a cylinder of greater
diameter, having two piston-rods P P′, as represented in _fig._
129., of considerable length, connected at the top by a cross-head
C. From this cross-head is carried downwards the connecting rod D,
which drives the crank-pin E, and thereby works the paddle-shaft
S. In this case the paddle-shaft is extended immediately above the
piston, and the double piston-rod has sufficient length to be
above the paddle-shaft when the piston is at the bottom of its
stroke. This improvement is intended to be applied more
particularly for engines for river navigation, the advantages
resulting from [Pg469] it being that a paddle-shaft placed at a
given height from the bottom of the vessel will be enabled to
receive a longer stroke of piston than by any other arrangement
now in use. A more [Pg470] compact and firm connection of the
cylinder with the crank-shaft bearings is effected by it, and a
cylinder of much greater diameter may be applied by which the
expansive action of steam may be more fully brought into play; and
a more direct action of the steam-power on the crank with a less
weight of materials and a greater economy of space may be obtained
than by any of the arrangements of marine engines hitherto used.
[Illustration: _Fig._ 128.]
[Illustration: _Fig._ 129.]
(221.) Mr. Francis Humphrys has obtained a patent for a form of
marine engine, by which some simplification of the machinery is
attained, and the same power comprised within more limited
dimensions. In this engine there is attached to the piston of the
cylinder, instead of a piston-rod, a hollow casing D D (_fig._
130.), which moves through a stuffing-box G, constructed in a
manner similar to the stuffing-box of a piston-rod. In the figure,
this casing is presented in section, but [Pg471] its form is that
of a long narrow slit, or opening, rounded at either end as
exhibited in the plan (_fig._ 131) of the cylinder-cover. The
crank C is driven by the other end of the connecting rod H, the
crank-shaft being immediately above the centre of the piston and
the connecting rod passing through the oblong opening D, and
descending into the hollow piston-rod it is attached to an axis I
at the bottom of the piston. A box or cover K K encloses the
cross-piece or axis I with its bearings, and is [Pg472] attached
so as to be steam-tight to the bottom of the piston. A hollow
space L L is cast in the bottom of the cylinder for the reception
of the box K K, when the piston is at the bottom of the cylinder.
[Illustration: _Fig._ 130.]
[Illustration: _Fig._ 131.]
By this arrangement the force by which the piston is driven in its
ascent and descent is communicated to the connecting rod, not, as
usual, through the intervention of a piston-rod, but directly from
the piston itself by the cross-pin I, and from thence to the crank
C, which it drives without the intervention of beams, cross-heads,
or any similar appendage.
The slide-valves regulating the admission and eduction of steam
are represented at _a_; the rod of the air-pump is shown at _d_,
being worked by a crank placed on the centre of the great crank
shaft.[38]
(222.) To obtain from the moving power its full amount of
mechanical effect in propelling the vessel, it would be necessary
that its force should propel, by constantly acting against the
water in a horizontal direction, and with a motion contrary to the
course of the vessel. No system of mechanical propellers has,
however, yet been contrived capable of perfectly accomplishing
this. Patents have been granted for many ingenious mechanical
combinations to impart to the propelling surfaces such angles as
appeared to the respective contrivers most advantageous. In most
of these the mechanical complexity has formed a fatal objection.
No part of the machinery of a steam-vessel is so liable to become
deranged at sea as the paddle-wheels; and, therefore, that
simplicity of construction which is compatible with those repairs
which are possible on such emergencies is quite essential for safe
practical use.
[Illustration: _Fig._ 132.]