part of the grate nearest to the fire-door, called the dead plates:
there it is submitted to the process of coking, by which the gases
and volatile matter which it contains are expelled, and being
carried by a current of air, admitted [Pg257] through small
apertures in the fire-door over the burning fuel in the hinder part
of the grate, they are burnt. When the fuel in front of the grate
has been thus _coked_, it is pushed back, and a fresh feed
introduced in front. The coal thus pushed back soon becomes vividly
ignited, and by continuing this process, the fuel spread over the
grate is maintained in the most active state of combustion at the
hinder part of the grate. By such an arrangement, the smoke produced
by the combustion of the fuel may be burnt before it enters the
flues. The flame and heated air proceeding from the burning fuel
arising from the grate, and rushing towards the back of the furnace,
passes over the _fire-bridge_ E, and is carried through the flue F
which passes under the boiler. This flue (the cross section of which
is shown in _fig._ 73., by the dark shade put under the boiler) is
very nearly equal in width to the bottom of the boiler, the space at
the bottom of the boiler, near the corners, being only what is
sufficient to give the weight of the boiler support on the masonry
forming the [Pg258] sides of the flue. The bottom of the boiler
being concave, the flame and heated air as they pass along the flue
rise to the upper part by the effects of their high temperature, and
_lick_ the bottom of the boiler from the fire-bridge at E to the
further end G.
[Illustration: _Fig._ 73.]
[Illustration: _Fig._ 74.]
At G the flue rises to H, and turning to the side of the boiler at
I I, conducts the flame in contact with the side from the back to
the front; it then passes through the flue K across the front, and
returns to the back by the other side [Pg259] flue L. The side
flue is represented, stripped of the masonry, in _fig._ 71., and
also appears in the plan in _fig._ 74., and in the cross section
in _fig._ 73. The course of the air is represented in _fig._ 74.
by the arrows. From the flue L the air is conducted into the
chimney at M.
By such an arrangement, the flame and heated air proceeding from
the grate are made to circulate round the boiler, and the length
and magnitude of the flues through which it is conducted should be
such, that when it shall arrive at the chimney its temperature
shall be reduced, as nearly as is consistent with the maintenance
of draught in the chimney, to the temperature of the water with
which it is in contact.
The method of feeding the furnace, which has been described above,
is one which, if conducted with skill and care, would produce a
much more perfect combustion of the fuel than would attend the
common method of filling the grate from the back to the front with
fresh fuel, whenever the furnace is fed. This method, however, is
rarely observed in the management of the furnace. It requires the
constant attention of the stokers (such is the name given to those
who feed the furnaces). The fuel must be supplied, not in large
quantities, and at distant intervals, but in small quantities and
more frequently. On the other hand, the more common practice is to
allow the fuel on the grate to be in a great degree burned away,
and then to heap on a large quantity of fresh fuel, covering over
with it the burning fuel from the back to the front of the grate.
When this is done, the heat of the ignited coal acting upon the
fresh fuel introduced, expels the gases combined with it and,
mixed with these, a quantity of carbon, in a state of minute
division, forming an opaque black smoke. This is carried through
the flues and drawn up the chimney. The consequence is, that not
only a quantity of solid fuel is sent out of the chimney
unconsumed, but the hydrogen and other gases also escape unburned,
and a proportional waste of the combustible is produced; besides
which, the nuisance of an atmosphere filled with smoke ensues.
Such effects are visible to all who observe the chimneys of
steam-vessels, while the engine is in operation. When the furnaces
are thus filled with fresh fuel, a large volume of [Pg260] dense
black smoke is observed to issue from the chimney. This gradually
subsides as the fuel on the grate is ignited, and does not
reappear until a fresh feed is introduced.
This method of feeding, by which the furnace would be made to
consume its own smoke, and the combustion of the fuel be rendered
complete, is not however free from counteracting effects. In
ordinary furnaces the feed can only be introduced by opening the
fire-doors, and during the time the fire-doors are opened a volume
of cold air rushes in, which passing through the furnace is
carried through the flues to the chimney. Such is the effect of
this in lowering the temperature of the flues, that in many cases
the loss of heat occasioned is greater than any economy of fuel
obtained by the complete consumption of smoke. Various methods,
however, may be adopted by which fuel may be supplied to the grate
without opening the fire-doors, and without disturbing the supply
of air to the fire. A hopper built into the front of the furnace,
with a moveable bottom or valve, by which coals may be allowed to
drop in from time to time upon the front of the grate, would
accomplish this.
(148.) In order to secure the combustion of the gases evolved from
the coals placed in the front of the grate, it is necessary that a
supply of atmospheric air should be admitted with them over the
burning fuel. This is effected by small apertures or regulators,
provided in the fire-doors, governed by sliding-plates, by which
they may be opened or closed to any required extent.
A patent has recently been granted to Mr. Williams, one of the
directors of the City of Dublin Steam Navigation Company, for a
method of consuming the unburned gases which escape from the
grate, and are carried through the flues. This method consists in
introducing into the flue tubes placed in a vertical position, the
lower ends of which being inserted in the bottom of the flue are
made to communicate with the ash-pit, and the upper ends of which
are closed. The sides and tops of these tubes are pierced with
small holes, through which atmospheric air drawn from the ash-pit
issues in jets. The oxygen supplied by this air immediately
combines with the carburetted hydrogen, which [Pg261] having
escaped from the furnace unburned is carried through the flues at
a sufficient temperature to enter into combination with the oxygen
admitted through holes in the tubes. A number of jets of flame
thus proceed from these holes, having an appearance similar to the
flame of a gas-lamp.
It is evident that such tubes must be inefficient unless they are
placed in the flues so near the furnace, that the temperature of
the unburned gases shall be sufficiently high to produce their
combustion.
(149.) The magnitude of the grate and ash-pit must be determined by
the rate at which the evaporation is required to be conducted in the
boiler and the quality of the fuel. It must be a matter of regret,
that the proportions of the various parts of steam-engines, with
their boilers and furnaces, have not been determined by any exact or
satisfactory experiments; and those who project and manufacture the
engines themselves, are not less in ignorance on those points than
others. With coals of the common quality a certain average
proportion must exist between the necessary magnitude of the
grate-surface and the quantity of water to be evaporated in a given
time in the boiler. But what that proportion is for any given
quality of fuel, is at present unascertained. Each engine-maker
follows his own rule, and the rule thus followed is in most cases a
matter of bare conjecture, unsupported by any experimental evidence.
Some engine-makers will allow a square foot of grate-surface for
every cubic foot of water per hour, which is expected to be
evaporated in the boiler; others allow only half a square foot: and
practice varies between these limits. Bituminous coals which melt
and cake, and which burn with much flame and smoke, must be spread
more thinly on the grate than other descriptions of fuel, otherwise
a considerable quantity of combustible gases would be dismissed into
the flues unburnt. Such coals therefore, other circumstances being
the same, require a larger portion of grate-surface; and the same
may be said of coals which produce clinkers in their combustion, and
form lumps of vitrified matter on the grate, by which the spaces
between the grate-bars are speedily closed up. When such fuel is
used, the grate-bars require to be frequently raked [Pg262] out,
otherwise the spaces between them being obstructed, the draught
would become insufficient for the due combustion of the fuel.
To facilitate the raking out of the grate, the bars are placed
with their ends towards the fire-door: they are usually made of
cast-iron, from two to two inches and a half wide on the upper
surface, with intervals of nearly half an inch between them. The
bars taper downwards, their under surfaces being much narrower
than their upper, the spaces between them thus widening, to
facilitate the fall of the ashes between them. The grate-bars
slope downwards from the front to the back. The height of the
centre of the bottom of the boiler, above the front of the grate,
is usually about two feet, and about three feet above the back of
it. The concave bottom of the boiler, however, brings its surfaces
at the slide closer to the grate.
(150.) Between the evaporating power of the boiler, and the
magnitude of surface it exposes to the action of the furnace,
there is a relation which, like that of the grate surface, has
never been ascertained by any certain or satisfactory experimental
investigation; much less have the different degrees of efficiency
attending different parts of the boiler-surface been determined.
That part of the surface of the boiler immediately over and around
the grate, is exposed to the immediate radiation of the burning
fuel, and is therefore probably the most efficient in the
production of steam. The tendency of flame and heated air to rise,
would naturally bring them in the flues into closer contact with
those parts of the boiler-surface which are horizontal in their
position, and which form the tops of the flues, than with those
which are lateral or vertical in their position, and which form
the sides of the flues. In a boiler constructed like that already
described, the flue-surface therefore, which would be most
efficient, would be the concave bottom of the boiler extending
from the fire-bridge to its remote end. In some boilers,
especially those in which steam of high pressure is produced, the
form is cylindrical, the middle flue being formed into an
elliptical tube the greater axis of which is horizontal from end
to end of the boiler. It seems doubtful, however, [Pg263] whether
in such a boiler the heat produces any useful effect on the water
below the flue, the water above being always at a higher
temperature, and therefore lighter than that below, and
consequently no currents being established between the upper and
lower strata of the water.
It was considered by Mr. Watt, but we are not aware on what
experimental grounds, that from eight to ten square feet of
heating surface were sufficient to produce the evaporation of one
cubic foot of water per hour. The practice of engine-makers since
that time has been to increase the allowance of heating surface
for the same rate of evaporation. Engine-builders have varied very
much in this respect, some allowing twelve, fifteen, and even
eighteen square feet of surface for the same rate of evaporation.
It must, however, still be borne in mind, that whether this
increased allowance did or did not produce the actual evaporation
imputed to it, has not been, as far as we are informed, ever
accurately ascertained. The production of a given rate of
evaporation by a moderate heat diffused over a larger surface,
rather than by a fiercer temperature confined to a smaller
surface, is attended with many practical advantages. The plates of
the boiler acted upon by the fire are less exposed to oxydisation,
and the boiler will be proportionally more durable.
(151.) Besides presenting to the action of the fire a sufficient
surface to produce steam at the required rate, the capacity of the
boiler must be proportioned to the quantity of water to be
evaporated. The space within the boiler is appropriated to a
twofold purpose: 1st, To contain the water to be evaporated; 2dly,
To contain a quantity of ready made steam for the supply of the
cylinder. If the space appropriated to the steam did not bear a
considerable proportion to the magnitude of the cylinder, the
momentary expansion of the steam passing to the cylinder from the
boiler at each stroke would reduce the pressure of the steam in a
great proportion, and unless the pressure in the boiler were
considerably greater than that which the steam is intended to have
in the cylinder, the pressure in the latter would be reduced below
the proper amount. The proportion of the [Pg264] steam space in
the boiler to the magnitude of the cylinder has been very
variously estimated, nor can it be said that any practical rule of
a general kind has been adopted. It is held by some that the
steam-space will be sufficient if it contain five times the
quantity of steam consumed at each stroke, while others maintain
that it should contain at least ten times that quantity, and
opinions vary between these limits.
(152.) The proportion of water-space in the boiler to its
evaporating power should also be regulated, so that the
introduction of the feed at a comparatively low temperature may
not unduly chill the water in the boiler. Supposing the feed to be
introduced in a low pressure boiler at the temperature of 100°,
and that the necessary temperature within the boiler be 225°, the
quantity of water it contains should be about five times the
quantity evaporated, and therefore also five times the quantity
introduced through the feed per hour. For every cubic foot of
water per hour therefore, intended to be evaporated by the boiler,
water-space for five cubic feet should be provided. It is,
however, right to repeat that this (like almost every other so
called rule) is the result not of any exact general calculation,
but one deduced from the custom which has obtained among the
manufacturers of steam-engines.
(153.) The surface of the water in the boiler should always be
above the range of the flues. When the heated air in the flues
acts upon a part of the boiler within which water is contained,
the water within receiving an increased temperature becomes, bulk
for bulk, lighter than the strata of water above it, and ascends.
It is replaced by the descending strata, which, in their turn
receiving increased temperature, rise to the surface; or if the
action of the heat convert the water into steam, the bubbles of
steam rise to the surface, fresh portions of water continually
coming into contact with the boiler-plates on which the heated air
or flame acts. By this process the boiler-plates are continually
cooled, either by being successively washed by water at a lower
temperature, or by the heat taken from them becoming latent in the
steam bubbles formed in contact with them. But if the heat act
[Pg265] upon a part of the boiler containing steam within it,
which steam being a slow recipient of heat, and no currents being
established, nor any phenomenon produced in which heat is rendered
latent, the heat of the fire communicated to the boiler-plates
accumulates in them, and raises their temperature to an injurious
degree. The plates may by this means be softened, so as to cause
the boiler to burst, or the difference between the expansion of
the highly heated plates thus exposed to fire in contact with
steam and that of the plates which are cooled by contact with
water, may cause the joinings of the boiler-plates to open, and
the boiler to leak. By whatever means, therefore, the boiler be
fed, care should be taken that the evaporation should not be
allowed to reduce the level of the water in it below the highest
flue.
(154.) As the water by which the boiler is fed must always have a
much lower temperature than that at which the boiler is
maintained, the supply of the feed will have a constant tendency
to lower the temperature of the water, and this tendency will be
determined by the proportion between the magnitude of the feed and
the quantity of water in the boiler.
Since it is requisite that the level of the water in the boiler
shall not suffer any considerable change, it is evident that the
magnitude of the feed must be equal to the quantity of water
evaporated. If it were less, the level of the water would
continually fall by reason of the excess of the evaporation over
the feed; and if it were greater, the level would rise by the
accumulation of water in the boiler. If therefore the quantity of
water-space allowed in the boiler be five times the volume of
water evaporated per hour, the quantity introduced by the feed per
hour, whether continuously or at intervals, must be of the same
amount. Since the process of evaporation is continuous, the
variation of level of water in the boiler will be entirely
dependent on the intervals between the successive feeds. If the
feed be continuous, and always equal to the evaporation, then the
level of the water in the boiler will undergo no change; but if
while the evaporation is continuous the feed be made at intervals,
then the change of level of water in the boiler as [Pg266] well
as its change of temperature, will be subject to a variation
proportional to the intervals between the successive feeds. It is
manifest, therefore, that the feed should either be uninterrupted
or be supplied at short intervals, so that the change of level and
temperature of the water in the boiler should not be considerable.
(155.) Different methods have been, from time to time, suggested
for indicating the level of the water in the boiler. We have
already mentioned the two gauge-pipes used in the earlier
steam-engines (31.), and which are still generally continued.
There are, however, some other methods which merit our attention.
[Illustration: _Fig._ 75.]
A weight F (_fig._ 75.), half immersed in the water in the boiler,
is supported by a wire, which, passing steam-tight through a small
hole in the top, is connected by a flexible string, or chain,
passing over a wheel W, with a counterpoise A, which is just
sufficient to balance F when half immersed. If F be raised above the
water, _A_ being lighter will no longer balance it, and F will
descend pulling up A, and turning the wheel W. If, on the other
hand, F be plunged deeper in the water, A will more than balance it,
and will pull it up, so that the only position in which F and A will
balance each other is, when F is half immersed. The wheel W is so
adjusted, that when two pins placed on its rim are in the horizontal
position, the water is at its proper level. Consequently it follows,
that if the water rise above this level, the weight F is lifted and
A falls, so that the pins come into another position. If, on the
other hand, the level of the water fall, F falls and A rises, so
that the pins assume a different position. Thus, in general, the
position of the pins becomes an indication of the quantity of water
in the boiler.
(156.) Another method is to place a glass tube (_fig._ 76.), with
one end T entering the boiler above the proper level, and the
other end T′ entering it below the proper level. It must [Pg267]
be evident that the water in the tube will always stand at the
same level as the water in the boiler, since the lower part has a
free communication with that water, while the surface is submitted
to the pressure of the same steam as the water in the boiler. This
and the last-mentioned gauge have the advantage of addressing the
eye of the engineer at once, without any adjustment; whereas the
gauge-cocks must be both opened, whenever the depth is to be
ascertained.
[Illustration: _Fig._ 76.]
These gauges, however, require the frequent attention of the
engine-man; and it becomes desirable either to find some more
effectual means of awakening that attention, or to render the
supply of the boiler independent of any attention. In order to
enforce the attention of the engine-man to replenish the boiler
when partially exhausted by evaporation, a tube was sometimes
inserted at the lowest level to which it was intended that the
water should be permitted to fall. This tube was conducted from
the boiler into the engine-house, where it terminated in a
mouth-piece or whistle, so that whenever the water fell below the
level at which this tube was inserted in the boiler, the steam
would rush through it, and issuing with great velocity at the
mouth-piece, would summon the engineer to his duty with a call
that would rouse him even from sleep.
[Illustration: _Fig._ 77.]
(157.) In the most effectual of these methods, the task of
replenishing the boiler should still be executed by the engineer;
and the utmost that the boiler itself was made to do, was to give
due notice of the necessity for the supply of water. The
consequence was, among other inconveniences, that the level of the
water was subject to constant variation.
To remedy this a method has been invented, by which [Pg268] the
engine is made to feed its own boiler. The pipe G (_fig._ 77.),
which leads from the hot water pump, terminates in a small cistern
C in which the water is received. In the bottom of this cistern, a
valve V is placed, which opens upwards, and communicates with a
feed-pipe, which descends into the boiler below the level of the
water in it. The stem of the valve V is connected with a lever
turning on the centre D, and loaded with a weight F dipped in the
water in the boiler in a manner similar to that described in
_fig._ 75., and balanced by a counterpoise A in exactly the same
way. When the level of the water in the boiler falls, the float F
falls with it, and pulling down the arm of the lever raises the
valve V, and lets the water descend into the boiler from the
cistern C. When the boiler has thus been replenished, and the
level raised to its former place, F will again be raised, and the
valve V closed by the weight A. In practice, however, the valve V
adjusts itself by means of the effect of the water on the weight
F, so as to permit the water from the feeding-cistern C to flow in
a continued stream, just sufficient in quantity to supply the
consumption from evaporation, and to maintain the level of the
water in the boiler constantly the same.
By this arrangement the boiler is made to replenish itself, or,
more properly speaking, it is made to receive such a supply, as
that it never wants replenishing, an effect which no effort of
attention on the part of an engine-man could produce. But this is
not the only good effect produced by this contrivance. A part of
the steam which originally left the boiler, and having discharged
its duty in moving the piston, was condensed and reconverted into
water, and lodged by the air-pump in the hot well (_fig._ 77.), is
here again restored to the source from which it came, bringing
back all the unconsumed portion of its heat preparatory to being
once more put in circulation through the machine.
The entire quantity of hot water pumped into the cistern C, is not
always necessary for the boiler. A waste-pipe may be provided for
carrying off the surplus, which may be turned to any purpose for
which it may be required; or it may be discharged into a cistern
to cool, preparatory to [Pg269] being restored to the cold
cistern, in case water for the supply of that cistern be not
sufficiently abundant.
[Illustration: _Fig._ 78.]
(158.) Another method of arranging a self-regulating feeder is
shown in _fig._ 78. A is a hollow ball of metal attached to the
end of a lever, whose fulcrum is at B. The other arm of the lever
C is connected with the stem of a spindle-valve, communicating
with a tube which receives water from the feeding-cistern. Thus,
when the level of the water in the boiler subsides, the ball A
preponderating over the weight of the opposite arm, the lever
falls, the arm C rises and opens the valve, and admits the feeding
water. This apparatus will evidently act in the same manner and on
the same principles as that already described. [Pg270]