steam-tight. It is closed at the top, and the piston [Pg134] rod,
being accurately turned, runs in a steam-tight collar, B,
furnished with a stuffing-box, and is constantly lubricated with
melted tallow. A funnel is screwed into the top of the cylinder,
through which, by opening a stop-cock, melted [Pg135] tallow is
permitted from time to time to fall upon the piston within the
cylinder, so as to lubricate it, and keep it steam-tight. Two
boxes, A A, called the upper and lower steam boxes, contain valves
by which steam from the boiler may be admitted and withdrawn.
These steam boxes are connected by a tube of communication T, and
they communicate with the cylinder at the top and bottom by short
tubes represented in the figure. The upper steam box A contains
one valve, by which a communication with the boiler may be opened
or closed at pleasure. The lower valve box contains two valves.
The lower valve I communicates with the tube T′, leading to the
condenser D, which being opened or closed, a communication is made
or cut off at pleasure, between the cylinder C and the condenser
D. A second valve, or upper valve H, which is represented closed
in the figure, may be opened so as to make a free communication
between the cylinder C and the tube T, and by that means between
the cylinder C, below the piston and the space above the piston.
The condenser D is submerged in a cistern of cold water. At the
side there enters it a tube, E, governed by a cock, which being
opened or closed to any required extent, a jet of cold water may
be allowed to play in the condenser, and may be regulated or
stopped, at pleasure. This jet, when playing, throws the water
upwards in the condenser towards the mouth of the tube T′, as
water issues from the rose of a watering pot. The tube S proceeds
from the boiler, and terminates in the steam box A, so that the
steam supplied from the boiler constantly fills that box. The
valve G is governed by levers, whose pivots are attached to the
framing of the engine, and is opened or closed at pleasure, by
raising or lowering the lever G′. The valve G, when open, will
therefore allow steam to pass from the boiler through the short
tube to the top of the piston, and this steam will also fill the
tube T. If the lower valve H be closed, its circulation beyond
that point will be stopped; but if the valve H be open, the valve
I being closed, then the steam will circulate equally in the
cylinder, above and below the piston. If the valve I be open, then
steam will rush through the tube T′ into the condenser; but this
escape of the steam will be [Pg136] stopped, if the valve I be
closed. The valve H is worked by the lever H′, and the valve I by
the lever I′.
The valve G is called the upper steam valve, H the lower steam
valve, I the exhausting valve, and E the condensing valve.
From the bottom of the condenser D proceeds a tube leading to the
air-pump, which is also submerged in the cistern of cold water. In
this tube is a valve M, which opens outwards from the condenser
towards the air-pump. In the piston of the air-pump N is a valve
which opens upwards. The piston-rod Q of the air-pump is attached
to a beam of wood called a plug frame, which is connected with the
working beam by a flexible chain playing on the small arch-head
immediately over the air-pump. From the top of the air-pump barrel
above the piston proceeds a pipe or passage leading to a small
cistern, B, called the hot well. The pipe which leads to this
well, is supplied with a valve, K, which opens outwards from the
air pump barrel towards the well. From the nature of its
construction, the valve M admits the flow of water from the
condenser towards the air-pump, but prevents its return; and, in
like manner, the valve K admits the flow of water from the upper
part of the air-pump barrel into the hot well B, but obstructs its
return.
Let us now consider how these valves should be worked in order to
move the piston upwards and downwards with the necessary force. It
is in the first place necessary that all the air which fills the
cylinder, the tubes and the condenser shall be expelled. To
accomplish this it is only necessary to open at once the three
valves G, H, and I. The steam then rushing from the boiler through
the steam-pipe S, and the open valve G will pass into the cylinder
above the piston, will fill the tube T, pass through the lower
steam valve H, will fill the cylinder C below the piston, and will
pass through the open valve I into the condenser. If the valve E
be closed so that no jet shall play in the condenser, the steam
rushing into it will be partially condensed by the cold surfaces
to which it will be exposed; but if the boiler supply it through
the pipe S in sufficient abundance, it will rush with violence
through the cylinder and all the passages, and its pressure in
the [Pg137] condenser D, combined with that of the heated air
with which it is mixed, will open the valve M, and it will rush
through mixed with the air into the air-pump barrel N. It will
press the valves in the air-pump piston upwards, and, opening
them, will rush through, and will collect in the air-pump barrel
above the piston. It will then, by its pressure, open the valve K,
and will escape into the cistern B.
Throughout this process the steam, which mixed with the air fills
the cylinder, condenser and air-pumps will be only partially
condensed in the last two, and it will escape mixed with air
through the valve K, and this process will continue until all the
atmospheric air which at first filled the cylinder, tubes,
condenser and air-pump barrel shall be expelled through the valve
K, and these various spaces shall be filled with pure steam. When
that has happened let us suppose all the valves closed. In closing
the valve I the flow of steam to the condenser will be stopped,
and the steam contained in it will speedily be condensed by the
cold surface of the condenser, so that a vacuum will be produced
in the condenser, the condensed steam falling in the form of water
to the bottom. In like manner, and for like reasons, a vacuum will
be produced in the air-pump. The valve M, and the valves in the
air-pump piston will be closed by their own weight.
By this process, which is called _blowing through_, the
atmospheric air, and other permanent gases, which filled the
cylinder, tubes, condenser and air-pump are expelled, and these
spaces will be a vacuum. The engine is then prepared to be
started, which is effected in the following manner:—The upper
steam valve G is opened, and steam allowed to flow from the boiler
through the passage leading to the top of the cylinder. This steam
cannot pass to the bottom of the cylinder, since the lower steam
valve H is closed. The space in the cylinder below the piston
being therefore a vacuum, and the steam pressing above it the
piston will be pressed downwards with a corresponding force. When
it has arrived at the bottom of the cylinder the steam valve G
must be closed, and at the same time the valve H opened. The valve
I leading to the condenser being also closed, the steam [Pg138]
which fills the cylinder above the piston is now admitted to
circulate through the open valve H below the piston, so that the
piston is pressed equally upwards and downwards by steam, and
there is no force to resist its movement save its friction with
the cylinder. The weight of the pump rods on the opposite end of
the beam being more than equivalent to overcome this the piston is
drawn to the top of the cylinder, and pushes before it the steam
which is drawn through the tube T, and the open valve H, and
passes into the cylinder C below the piston.
[Illustration: _Fig._ 22.]
When the piston has thus arrived once more at the top of the
cylinder, let the valve H be closed, and at the same time the
valves G and I opened, and the condensing cock E also opened, so
as to admit the jet to play in the condenser. The steam which
fills the cylinder C below the piston, will now rush through the
open valve I into the condenser which has been hitherto a vacuum,
and there encountering the jet, will be instantly converted into
water, and a mixture of condensed steam and injected water will
collect in the bottom of the condenser. At the same time, the
steam proceeding from the boiler by the steam pipe S to the upper
steam box A, will pass through the open steam valve G to the top
of the piston, but cannot pass below it because of the lower steam
valve H being closed. The piston, thus acted upon above by the
pressure of the steam, and the space in the cylinder below it
being a vacuum, its downward motion is resisted by no force but
the friction, and it is therefore driven to the bottom of the
cylinder. During its descent the valves G, I, and E remained open.
At the moment it arrives at the bottom of the cylinder, all these
three valves are closed, and the valve H opened. The steam which
fills the cylinder above the piston is now permitted to circulate
below it, by the open valve H, and the piston being consequently
pressed equally upwards and downwards will be drawn upwards as
before by the preponderance of the pump rods at the opposite end
of the beam. The weight of these rods must also be sufficiently
great to draw the air-pump piston N upwards. As this piston rises
in the air-pump, it leaves a vacuum below it into which the water
and air collected in the condenser will be drawn through the valve
M, which opens outwards. When the [Pg139] air-pump piston has
arrived at the top of the barrel, which it will do at the same
time that the steam piston arrives at the top of the cylinder, the
water and the chief part of the air or other fluids which may have
been in the condenser will be drawn into the barrel of the
air-pump, and the valve M being closed by its own weight, assisted
by the pressure of these fluids they cannot return into the
condenser. At the moment the steam piston arrives at the top of
the cylinder, the valve H is closed, and the three valves G, I,
and E are opened. The effect of this change is the same as was
already described in the former case, and the piston will in the
same manner and from the same causes be driven downwards. The
air-pump piston will at the same time descend by the force of its
own weight, aided by the weight of the plug-frame attached to its
rod. As it descends, the air below it will be gradually compressed
above the surface of the water in the bottom of the barrel, until
its pressure becomes sufficiently great to open the valves in the
air-pump piston. When this happens, the valves in the air-pump
piston, as represented on a large scale in _fig._ 22., will be
opened, and the air will pass through them above the piston. When
the piston comes in contact with the water in the bottom of the
barrel, this water will likewise pass through the open valves.
When the piston has arrived at the bottom of the air-pump barrel,
the valves in it will be closed by the pressure of the fluids
above them. The next ascent of the steam piston will draw up the
air-pump piston, and with it the fluids in the pump barrel above
it. As the air-pump [Pg140] piston approaches the top of its
barrel, the air and water above it will be drawn through the valve
K into the hot cistern B. The air will escape in bubbles through
the water in that cistern, and the warm water will be deposited in
it.
The magnitude of the opening in the condensing valve E, must be
regulated by the quantity of steam admitted to the cylinder. As
much water ought to be supplied through the injection valve as
will be sufficient to condense the steam contained in the
cylinder, and also to reduce the temperature of the water itself,
when mixed with the steam, to a sufficiently low degree to prevent
it from producing vapour of a pressure which would injuriously
affect the working of the piston. It has been shown, that five and
a half cubic inches of ice-cold water mixed with one cubic inch of
water in the state of steam would produce six and a half cubic
inches of water at the boiling temperature. If then the cylinder
contained one cubic inch of water in the state of steam, and only
five and a half cubic inches of water were admitted through the
condensing jet, supposing this water, when admitted, to be at the
temperature of 32°, then the consequence would be that six and a
half cubic inches of water at the boiling temperature would be
produced in the condenser. Steam would immediately arise from
this, and at the same time the temperature of the remaining water
would be lowered by the amount of the latent heat taken up by the
steam so produced. This vapour would rise through the open
exhausting valve I, would fill the cylinder below the piston, and
would impair the efficiency of the steam above pressing it down.
The result of the inquiries of Watt respecting the pressure of
steam at different temperatures, showed, that to give efficiency
to the steam acting upon the piston it would always be necessary
to reduce the temperature of the water in the condenser to 100°.
Let us then see what quantity of water at the common temperature
would be necessary to produce these effects.
If the latent heat of steam be taken at 1000°, a cubic inch of
water in the state of steam may be considered for the purposes of
this computation, as equivalent to one cubic inch of water at
1212°. Now the question is, how many cubic inches of water at 60°
must be mixed with this, in order that the [Pg141] mixture may
have the temperature of 100°? This will be easily computed. As the
cubic inch of water at 1212° is to be reduced to 100°, it must be
deprived of 1112° of its temperature. On the other hand, as many
inches of water at 60° as are to be added, must be raised in the
same mixture to the temperature of 100°, and therefore each of
these must receive 40° of temperature. The number of cubic inches
of water necessary to be added will therefore be determined by
finding how often 40° are contained in 1112°. If 1112 be divided
by 40, the quotient will be 27·8. Hence it appears, that to reduce
the water in the condenser to the temperature of 100°, supposing
the temperature of the water injected to be 60°, it will be
necessary to supply by the injection cock very nearly twenty-eight
times as much water as passes through the cylinder in the state of
steam; and therefore if it be supposed that all the water
evaporated in the boiler passes through the cylinder, it follows
that about twenty-eight times as much water must be thrown into
the condenser as is evaporated in the boiler.
From these circumstances it will be evident that the cold cistern
in which the condenser and air-pump are submerged, must be
supplied with a considerable quantity of water. Independently of
the quantity drawn from it by the injection valve, as just
explained, the water in the cistern itself must be kept down to a
temperature of about 60°. The interior of the condenser and
air-pump being maintained by the steam condensed in them at a
temperature not less than 100°; the outer surfaces of these
vessels consequently impart heat to the water in the cold cistern,
and have therefore a tendency to raise the temperature of that
water. To prevent this, a pump called the _cold pump_, represented
at L in _fig._ 21., is provided. By this pump water is raised from
any convenient reservoir, and driven through proper tubes into the
cold cistern. This cold pump is wrought by the engine, the rod
being attached to the beam. Water being, bulk for bulk, heavier
the lower its temperature, it follows that the water supplied by
the cold pump to the cistern will have a tendency to sink to the
bottom, pressing upwards the warmer water contained in it. A
waste-pipe is provided, by which this [Pg142] water is drained
off, and the cistern therefore maintained at the necessary
temperature.
From what has been stated, it is also evident that the hot well B,
into which the warm water is thrown by the air-pump, will receive
considerably more water than is necessary to feed the boiler. A
waste-pipe, to carry off this, is also provided; and the quantity
necessary to feed the boiler is pumped up by a small pump, O, the
rod of which is attached to the beam, as represented in _fig._
21., and which is worked by the engine. The water raised by this
pump is conducted to a reservoir from which the boiler is fed, by
means which will be hereafter explained.
We shall now explain the manner in which the machine is made to
open and close the valves at the proper times. By referring to the
explanation already given, it will be perceived that at the moment
the piston reaches the top of the cylinder, the upper steam valve
G must be open, to admit the steam to press it down; while the
exhausting valve I must be opened, to allow the steam to pass to
the condenser; and the condensing valve E must be opened, to let
in the water necessary for the condensation of the steam; and at
the same time the lower steam valve H must be closed, to prevent
the passage of the steam which has been admitted through G. The
valves G, I, and E must be kept open, and the valve H kept closed,
until the piston arrives at the bottom of the cylinder, when it
will be necessary to close all the three valves, G, I, and E, and
to open the valve H, and the same effects must be produced each
time the piston arrives at the top and bottom of the cylinder. All
this is accomplished by a system of levers, which are exhibited in
_fig._ 21. The pivots on which these levers play are represented
on the framing of the engine, and the arms of the levers G′, H′,
and I′, communicating with the corresponding valves G, H, and I,
are represented opposite a bar attached to the rod of the
air-pump, called the _plug frame_. This bar carries certain pegs
and detents, which act upon the arms of the several levers in such
a manner that, on the arrival of the beam at the extremities of
its play upwards and downwards, the levers are so struck that the
valves are opened and closed at the proper [Pg143] times. It is
needless to explain all the details of this arrangement. Let it be
sufficient, as an example of all, to explain the method of working
the upper steam valve G. When the piston reaches the top of the
cylinder, a pin strikes the arm of the lever G′, and throws it
upwards: this, by means of the system of levers, pulls the arm of
the valve G downwards, by which the upper steam valve is raised
out of its seat, and a passage is opened from the steam pipe to
the cylinder. The valve is maintained in this state until the
piston reaches the bottom of the cylinder, when the arm G′ is
pressed downwards, by which the arm G is pressed upwards, and the
valve restored to its seat. By similar methods the levers
governing the other three valves, H, I, and E, are worked.
[Illustration: _Fig._ 23.]
[Illustration: _Fig._ 24.]
The valves used in these engines were of the kind called _spindle
valves_. They consisted of a flat circular plate of bell metal, A
B, _fig._ 23., with a round spindle passing perpendicularly
through its centre, and projecting above and below it. This valve,
having a conical form, was fitted very exactly, by grinding into a
corresponding circular conical seat, A B C D, _fig._ 24., which
forms the passage which it is the office of the valve to open and
close. When the valve falls into its seat, it fits the aperture
like a plug, so as entirely to stop it. The spindle plays in
sockets or holes, one above and the other below the aperture which
the valve stops; these holes keep the valve in its proper
position, so as to cause it to drop exactly into its place.
In the experimental engine made by Mr. Watt at Kinneal, he used
cocks, and sometimes sliding covers, like the regulator described
in the old engines; but these he found very soon to become leaky.
He was, therefore, obliged to change them for the spindle valves
just described, which, being truly [Pg144] ground, and accurately
fitted in the first instance, were not so liable to go out of
order. These valves are also called _puppet clacks_, or _button
valves_.
In the earlier engines constructed by Watt, the condensation was
produced by the contact of cold surfaces, without injection. The
reason of rejecting the method of condensing by injection was,
doubtless, to avoid the injurious effects of the air, which would
always enter the condenser, in combination with the water of
condensation, and vitiate the vacuum. It was soon found, however,
that a condenser acting by cold surfaces without injection, being
necessarily composed of narrow pipes or passages, was liable to
incrustation from bad water, by which the conducting power of the
material of the condenser was diminished; so that, while its outer
surface was kept cold by the water of the cold cistern, the inner
surface might, nevertheless, be so warm that a very imperfect
condensation would be produced.
[Illustration: SOHO, BIRMINGHAM.]
FOOTNOTES:
[19] Eloge, p. 308.
[Pg145]
[Illustration: BIRMINGHAM.]
CHAP. VI.
CORRESPONDENCE OF WATT WITH SMEATON. — FAILURE OF CONDENSATION
BY SURFACE. — IMPROVEMENTS IN CONSTRUCTION OF PISTON. — METHOD
OF PACKING. — IMPROVEMENTS IN BORING THE CYLINDERS. —
DISADVANTAGES OF THE NEW COMPARED WITH THE OLD ENGINES. —
GREATLY INCREASED ECONOMY OF FUEL. — EXPEDIENTS TO FORCE THE
NEW ENGINES INTO USE. — CORRESPONDENCE WITH SMEATON. —
EFFICIENCY OF FUEL IN THE NEW ENGINES. — DISCOVERY OF THE
EXPANSIVE ACTION OF STEAM. — WATT STATES IT IN A LETTER TO DR.
SMALL. — ITS PRINCIPLE EXPLAINED. — MECHANICAL EFFECT
RESULTING FROM IT. — COMPUTED EFFECT OF CUTTING OFF STEAM AT
DIFFERENT PORTIONS OF THE STROKE. — PRODUCES A VARIABLE POWER.
— EXPEDIENTS FOR EQUALISING THE POWER. — LIMITATION OF THE
EXPANSIVE PRINCIPLE IN WATT'S ENGINES. — ITS MORE EXTENSIVE
APPLICATION IN THE CORNISH ENGINES.
(77.) In a letter addressed by Watt to Smeaton, dated April, 1766,
Watt refers to some of these practical difficulties which he had
to encounter. "I have been," says he, "tormented with exceedingly
bad health, resulting from the operation of an anxious mind, the
natural consequence of staking everything [Pg146] upon the cast
of a die; for in that light I look upon every project which has
not received the sanction of repeated success.
"I have made considerable alterations in our engine lately,
particularly in the condenser. That which I used at first was
liable to be impaired, from incrustations from bad water;
therefore we have substituted one which works by an injection. In
pursuing this idea I have tried several kinds, and have at last
come to one, which I am not inclined to alter. It consists of a
jack-head pump, shut at bottom, with a common clack bucket, and a
valve in the cover of the pump, to discharge the air and water.
The eduction steam pipe, which comes from the cylinder,
communicates with this pump both above and below the bucket, and
has valves to prevent anything from going back from the pump to
the eduction pipe. The bucket descends by its own weight, and is
raised by the engine when the great piston descends, being hung to
the outer end of the great lever: the injection is made both into
the upper part of this pump and into the eduction pipe, and
operates beyond my ideas in point of quickness and perfection."
Besides the difficulty arising from incrustation, Watt found the
tubulated condensers, and indeed all other expedients for
condensing by cold surfaces, subject to a fatal objection. They
did not condense instantaneously, and although they were capable
of ultimately effecting the condensation, yet that process was not
completed until a great part of the stroke of the piston was made.
Thus during more or less of the stroke the uncondensed steam
resisted the piston, and robbed the moving power of a part of its
effect. This objection has ever attended condensation by surface.
[Illustration: _Fig._ 25.]
[Illustration: _Fig._ 26.]