does not render either necessary or possible that great regularity
of action which is indispensable in a steam-engine applied to the
purposes of manufacture. The agitation of the surface of the sea
will cause the immersion of the paddle-wheels to be subject to
great variation, and the resistance produced by the water to the
engine will undergo a corresponding change. The governor,
therefore, and other parts of the apparatus, contrived for giving
to the engine that great regularity required in manufactures, are
omitted in nautical engines, and nothing is introduced save what
is [Pg447] necessary to maintain the machine in its full working
efficiency.
[Illustration: _Fig._ 120.]
[Illustration: _Fig._ 121.]
To save space, marine boilers are constructed so as to produce the
necessary quantity of steam within the smallest possible
dimensions. With this view a more extensive surface in proportion
to the capacity of the boiler is exposed to the action of the
fire. The flues, by which the flame and heated air are conducted
to the chimney, are so constructed that the heat may act upon the
water on every side in thin oblong shells or plates. This is
accomplished by constructing the flues so as to traverse the
boiler backwards and forwards several times before they terminate
[Pg448] in the chimney. Such an arrangement renders the expense of
the boilers greater, but their steam-producing power is
proportionally augmented, and experiments made by Mr. Watt, at
Birmingham, have proved that such boilers with the same
consumption of fuel will produce, as compared with common land
boilers, an increased evaporation in the proportion of about three
to two.
[Illustration: _Fig._ 122.]
[Illustration: _Fig._ 123.]
The form and arrangement of the water-spaces and flues in marine
boilers may be collected from the sections of the boilers used in
some of the government steamers, exhibited in _figs._ 121, 122,
123. A section made by a horizontal plane passing through the
flues is exhibited in _fig._ 121. The furnaces F communicate in
pairs with the flues E, the air following the course through the
flues represented by the arrows. The flue E passes to the back of
the boiler, then returns to the front, then to the back again, and
is finally carried back to the front, where it communicates at C
with the curved flue B, represented in the transverse vertical
section, _fig._ 122. This curved flue B finally terminates in the
chimney A. There are in this case three independent boilers, each
worked by two furnaces communicating with the same system of
flues; and in the curved flues B, _fig._ 122., by which the air is
finally conducted through the chimney, are placed three
independent [Pg449] dampers, by means of which the furnace of
each boiler can be regulated independently of the other, and by
which each boiler may be separately detached from communication
with the chimney. The letters of reference in the horizontal
section, _fig._ 121., correspond with those in the transverse
vertical section, _fig._ 122., E representing the commencement of
the flues, and C their termination.
[Illustration: _Fig._ 124.]
A longitudinal section of the boiler made by a vertical plane
extending from the front to the back is given in _fig._ 123.,
where F, as before, is the furnace, G the grate-bars sloping
downwards from the front to the back, H the fire-bridge, C the
commencement of the flues, and A the chimney. An elevation of the
front of the boiler is represented in _fig._ 124., showing two of
the fire-doors closed, and the other two removed, displaying the
position of the grate-bars in front. Small openings are also
provided, closed by proper doors, by which access can be had to
the under side of the flues between the foundation timbers of the
engine for the purpose of cleaning them.
Each of these boilers can be worked independently of the others.
By this means, when at sea, the engine may be worked by any two of
the three boilers, while the third is being cleaned and put in
order. In all sea-going steamers multiple boilers are at present
provided for this purpose.
In the boilers here represented the flues are all upon the same
level, winding backwards and forwards without passing one above
the other. In other boilers, however, the flues, [Pg450] after
passing backwards and forwards near the bottom of the boiler, turn
upwards and pass backwards and forwards through a level of the
water nearer its surface, finally terminating in the chimney. More
heating surface is thus obtained with the same capacity of boiler.
The most formidable difficulty which has been encountered in the
application of the steam-engine to sea-voyages has arisen from the
necessity of supplying the boiler with sea-water instead of pure
fresh water. The sea-water is injected into the condenser for the
purpose of condensing the steam, and it is thence, mixed with the
condensed steam, conducted as feeding water into the boiler.
(209.) Sea-water holds, as is well known, certain alkaline
substances in solution, the principal of which is muriate of soda,
or common salt. Ten thousand grains of pure sea-water contain two
hundred and twenty grains of common salt, the remaining
ingredients being thirty-three grains of sulphate of soda,
forty-two grains of muriate of magnesia, and eight grains of
muriate of lime. The heat which converts pure water into steam
does not at the same time evaporate those salts which the water
holds in solution. As a consequence it follows, that as the
evaporation in the boiler is continued, the salt, which was held
in solution by the water which has been evaporated, remains in the
boiler, and enters into solution with the water remaining in it.
The quantity of salt contained in sea-water being considerably
less than that which water is capable of holding in solution, the
process of evaporation for some time is attended with no other
effect than to render the water in the boiler a stronger solution
of salt. If, however, this process be continued, the quantity of
salt retained in the boiler having constantly an increasing
proportion to the quantity of water, it must at length render the
water in the boiler a saturated solution—that is, a solution
containing as much salt as at the actual temperature it is capable
of holding in solution. If, therefore, the evaporation be
continued beyond this point, the salt disengaged from the water
evaporated instead of entering into solution with the water
remaining in the boiler will be precipitated in the form of
sediment; and if the process be continued in the [Pg451] same
manner, the boiler would at length become a mere salt-pan.
But besides the deposition of salt sediment in a loose form, some
of the constituents of sea-water having an attraction for the iron
of the boiler, collect upon it in a scale or crust in the same
manner as earthy matters held in solution by spring-water are
observed to form and become incrusted on the inner surface of
land-boilers and of common culinary vessels.
The coating of the inner surface of a boiler by incrustation and
the collection of salt sediment in its lower parts, are attended
with effects highly injurious to the materials of the boiler. The
crust and sediment thus formed within the boiler are almost
non-conductors of heat, and placed, as they are, between the water
contained in the boiler and the metallic plates which form it,
they obstruct the passage of heat from the outer surface of the
plates in contact with the fire to the water. The heat, therefore,
accumulating in the boiler-plates so as to give them a much higher
temperature than the water within the boiler, has the effect of
softening them, and by the unequal temperature which will thus be
imparted to the lower plates which are incrusted, compared with
the higher parts which may not be so, an unequal expansion is
produced, by which the joints and seams of the boiler are loosened
and opened, and leaks produced.
These injurious effects can only be prevented by either of two
methods; first, by so regulating the feed of the boiler that the
water it contains shall not be suffered to reach the point of
saturation, but shall be so limited in its degree of saltness that
no injurious incrustation or deposit shall be formed; secondly, by
the adoption of some method by which the boiler may be worked with
fresh water. This end can only be attained by condensing the steam
by a jet of fresh water, and working the boiler continually by the
same water, since a supply of fresh water sufficient for a boiler
worked in the ordinary way could never be commanded at sea.
(210.) The method by which the saltness of the water in the boiler
is most commonly prevented from exceeding a certain [Pg452] limit
has been to discharge from the boiler into the sea a certain
quantity of over-salted water, and to supply its place by
sea-water introduced into the condenser through the injection-cock
for the purpose of condensing the steam, this water being mixed
with the steam so condensed, and being, therefore, a weaker
solution of salt than common sea-water. To effect this, cocks
called _blow-off cocks_, are usually placed in the lower parts of
the boiler, where the over-salted, and therefore heavier, parts of
the water collect. The pressure of the steam and incumbent weight
of the water in the boiler force the lower strata of water out
through these cocks; and this process, called _blowing out_, is,
or ought to be, practised at such intervals as will prevent the
water from becoming over salted. When the salted water has been
blown out in this manner, the level of the water in the boiler is
restored by a feed of corresponding quantity.
This process of blowing out, on the due and regular observance of
which the preservation and efficiency of the boiler mainly depend,
is too often left at the discretion of the engineer, who is, in most
cases, not even supplied with the proper means of ascertaining the
extent to which the process should be carried. It is commonly
required that the engineer should blow out a certain portion of the
water in the boiler every two hours, restoring the level by a feed
of equivalent amount; but it is evident that the sufficiency of the
process founded on such a rule must mainly depend on the supposition
that the evaporation proceeds always at the same rate, which is far
from being the case with marine boilers. An indicator, by which the
saltness of the water in the boiler would always be exhibited, ought
to be provided, and the process of blowing out should be regulated
by the indications of that instrument. To blow out more frequently
than is necessary is attended with a waste of fuel; for hot water is
thus discharged into the sea while cold water is introduced in its
place, and consequently all the heat necessary to produce the
difference of the temperatures of the water blown out and the feed
introduced is lost. If, on the other hand, the process of blowing
out be observed less frequently than is necessary, then more or less
incrustation and deposit [Pg453] may be produced, and the injurious
effects already described ensue.
As the specific gravity of water holding salt in solution is
increased with every increase of the strength of the solution, any
form of hydrometer capable of exhibiting a visible indication of
the specific gravity of the water contained in the boiler would
serve the purpose of an indicator, to show when the process of
blowing out is necessary, and when it has been carried to a
sufficient extent. The application of such instruments, however,
would be attended with some practical difficulties in the case of
sea-boilers.
The temperature at which a solution of salt boils under a given
pressure varies considerably with the strength of the solution;
the more concentrated the solution is, the higher will be its
boiling temperature under the same pressure. A comparison,
therefore, of a steam-gauge attached to the boiler, and a
thermometer immersed in it, showing the pressure and the
temperature, would always indicate the saltness of the water; and
it would not be difficult so to graduate these instruments as to
make them at once show the degree of saltness.
If the application of the thermometer be considered to be attended
with practical difficulty, the difference of pressures under which
the salt water of the boiler and fresh water of the same
temperature boil, might be taken as an indication of the saltness
of the water in the boiler, and it would not be difficult to
construct upon this principle a self-registering instrument, which
would not only indicate but record from hour to hour the degree of
saltness of the water. A small vessel of distilled water being
immersed in the water of the boiler would always have the
temperature of that water, and the steam produced from it
communicating with a steam-gauge, the pressure of such steam would
be indicated by that gauge, while the pressure of the steam in the
boiler under which pressure the salted water boils might be
indicated by another gauge. The difference of the pressures
indicated by the two gauges would thus become a test by which the
saltness of the water in the boiler would be measured. The two
pressures might be made to act on opposite ends of the same column
of [Pg454] mercury contained in a siphon tube, and the difference
of the levels of the two surfaces of the mercury would thus become
a measure of the saltness of the water in the boiler. A
self-registering instrument founded on this principle formed part
of the self-registering steam-log which I proposed to introduce
into steam-vessels some time since.
(211.) The Messrs. Seaward of Limehouse have adopted, in some of
their recently constructed engines, a method of indicating the
saltness of the water, and of measuring the quantity of salted
water or brine discharged, by blowing out. A glass-gauge, similar
in form to that already described in land engines (156.), is
provided to indicate the position of the surface of the water in
the boiler. In this gauge two hydrometer balls are provided, the
weight of which in proportion to their magnitude is such that they
would both sink to the bottom in a solution of salt of the same
strength as common sea-water. When the quantity of salt exceeds
5/32 parts of the whole weight of the water, the lighter of the
two balls will float to the top; and when the strength is further
increased until the proportion of salt exceeds 6/32 parts of the
whole, then the heavier ball will float to the top. The actual
quantity of salt held in solution by sea-water in its ordinary
state is 1/32 part of its whole weight; and when by evaporation
the proportion of salt in solution has become 9/32 parts of the
whole, then a deposition of salt commences. With an indicator such
as that above described, the ascent of the lighter hydrometer ball
gives notice of the necessity for blowing out, and the ascent of
the heavier may be considered as indicating the approach of an
injurious state of saltness in the boiler.
[Illustration: _Fig._ 125.]
The ordinary method of blowing out the salted water from a boiler
is by a pipe having a cock in it leading from the boiler through
the bottom of the ship, or at a point low down at its side.
Whenever the engineer considers that the water in the boiler has
become so salted that the process of blowing out should commence,
he opens the cock communicating by this pipe with the sea, and
suffers an indefinite and uncertain quantity of water to escape.
In this way he discharges, according to the magnitude of the
boiler, from two to six tons [Pg455] of water, and repeats this
at intervals of from two to four hours, as he may consider to be
sufficient. If, by observing this process, he prevents the boiler
from getting incrusted during the voyage, he considers his duty to
be effectually discharged, forgetting that he may have blown out
many times more water than is necessary for the preservation of
the boiler, and thereby produced a corresponding and unnecessary
waste of fuel. In order to limit the quantity of water discharged,
Messrs. Seaward have adopted the following method. In _fig._ 125.
is represented a transverse section of a part of a steam-vessel; W
is the water-line of the boiler, B is the mouth of a blow-off
pipe, placed near the bottom of the boiler. This pipe rises to A,
and turning in the horizontal direction, A C is conducted to a
tank T, which contains exactly a ton of water. This pipe
communicates with the tank by a cock D, governed by a lever H.
When this lever is moved to D′, the cock D is open, and when it is
moved to K, the cock D is closed. From the same tank there
proceeds another pipe E, which issues from the side of the
[Pg456] vessel into the sea governed by a cock F, which is
likewise put in connection with the lever H, so that it shall be
opened when the lever H is drawn to the position F′, the cock D′
being closed in all positions of the lever between K and F′. Thus,
whenever the cock F communicating with the sea is open, the cock D
communicating with the boiler is closed, and _vice versâ_, both
cocks being closed when the lever is in the intermediate position
K. By this arrangement the boiler cannot, by any neglect in
blowing off, be left in communication with the sea, nor can more
than a ton of water be discharged except by the immediate act of
the engineer. The injurious consequences are thus prevented which
sometimes ensue when the blow-off cocks are left open by any
neglect on the part of the engineer. When it is necessary to blow
off, the engineer moves the lever H, to the position D′. The
pressure of the steam in the boiler on the surface of the water W
forces the salted water or brine up the pipe B A, and through the
open cock C into the tank, and this continues until the tank is
filled: when that takes place, the lever is moved from the
position D′ to the position F′, by which the cock D is closed, and
the cock F opened. The water in the tank flows through the pipe E
into the sea, air being admitted through the valve V, placed at
the top of the tank, opening inwards. A second ton of brine is
discharged by moving the lever back to the position D′, and
subsequently returning it to the position F′; and in this way the
brine is discharged ton by ton, until the supply of water from the
feed which replaces it has caused both the balls in the indicator
to sink to the bottom.
(212.) A different method of preserving the requisite freshness of
the water in the boiler has been adopted by Messrs. Maudslay and
Field, and introduced with success into the Great Western and
other steam-vessels. Pumps called _brine-pumps_ are put into
communication with the lower part of the boiler, and so
constructed as to draw the brine therefrom, and drive it into the
sea. These brine-pumps are worked by the engine, and their
operation is constant. The feed-pumps are likewise worked by the
engine, and they bear such a proportion to the brine-pumps that
the quantity of salt discharged in a given time in the brine is
equal to the quantity of salt [Pg457] introduced in solution by
the water of the feed-pumps. By this means the same actual
quantity of salt is constantly maintained in the boiler, and
consequently the strength of the solution remains invariable. If
the brine discharged by the brine-pumps contains 5/32 parts of
salt while the water introduced by the feed-pumps contains only
1/32 part, then it is evident that five cubic feet of the feeding
water will contain no more salt than is contained in one cubic
foot of brine. Under such circumstances the brine-pumps would be
so constructed as to discharge 1/5 of the water introduced by the
feed-pumps, so that 4/5 of all the water introduced into the
boiler would be evaporated, and rendered available for working the
engine.
To save the heat of the brine, a method has been adopted in the
marine engines constructed by Messrs. Maudslay and Field similar
to one which has been long practised in steam-boilers, and in
various apparatus for the warming of buildings. The current of
heated brine is conducted from the boiler through a tube which is
contained in another, through which the feed is introduced. The
warm current of brine, therefore, as it passes out, imparts a
considerable portion of its heat to the cold feed which comes in;
and it is found that by this expedient the brine discharged into
the sea may be reduced to a temperature of about 100°.
This expedient is so effectual that when the apparatus is properly
constructed, and kept in a state of efficiency, it may be regarded
as nearly a perfect preventive against the incrustation, and the
deposition of salt in the boilers, and is not attended with any
considerable waste of fuel.
(213.) About the year 1776, Mr. Watt invented a tubular condenser,
with a view to condense the steam drawn off from the cylinder
without the process of injection. This apparatus consisted of a
number of small tubes connecting the top and bottom of the
condenser, arranged in a manner not very different from that of
the tubes which traverse the boiler of a locomotive engine. These
tubes were continually surrounded by cold water, and the steam, as
it escaped from the cylinder passing through them, was condensed
by their cold surfaces, and collected in the form of water in a
reservoir below, from [Pg458] whence it was drawn off by a pump
in the same manner as in engines which condensed by injection. One
of the advantages proposed by this expedient was, that no
atmospheric air would be introduced into the condenser, as is
always the case when condensation by injection is practised. Cold
water, which is injected, has always combined with it more or less
common air. When this water is mixed with the condensed steam, the
elevation of its temperature disengages the air combined with it,
and this air circulating to the cylinder, vitiates the vacuum. One
of the purposes for which the air-pump in condensing steam-engines
was provided, and from which it took its name, was to draw off
this air. If, however, a tubular condenser could be made to act
with the necessary efficiency, no injection water would be
introduced for condensation, and the pump would have no other duty
except to remove the small quantity of water produced by the
condensed steam. That water being subsequently carried back to the
boiler by the feed-pumps, a constant system of circulation would
be maintained, and the boiler would never require any fresh supply
of water, except what might be necessary to make good the waste by
leakage and other causes.
This contrivance has been of late years revived by Mr. Samuel Hall
of Basford, near Nottingham, with a view to supersede in marine
engines the necessity of using sea-water in the boilers. Mr. Hall
proposes to make marine boilers with fresh water to condense the
steam without injection, by a tubulated condenser, and to provide
by the distillation of sea-water the small quantity of fresh water
which would be necessary to make good the waste. These condensers
have been introduced into several steam-vessels: in some they have
been continued, and in others abandoned, and various opinions are
entertained of their efficacy. I have not been able to obtain the
results of any satisfactory experiments on them, and cannot
therefore form a judgment of their usefulness. Mr. Watt abandoned
these condensers from finding that the condensation of the steam
was not sufficiently sudden, and that consequently at the
commencement of the stroke the piston was subject to a resistance
which [Pg459] injuriously diminished the amount of the moving
power, whereas condensation by jet was almost instantaneous, and
the efficiency of the piston throughout the entire stroke was more
uniform.
Mr. Watt also found that a fur collected around the tubes of the
condenser, so as to obstruct the free passage of heat from the
steam to the water of the cold cistern; and that, consequently,
the efficiency of the condenser was gradually impaired, and could
only be restored by frequent cleansing.