(76.)

Thus protected and supported, Watt now directed the whole vigour of his mind to perfect the practical details of his invention, and the result was, the construction on a large scale of the engine which has since been called his Single acting Steam Engine.

It is necessary to recollect, that notwithstanding the extensive and various application of steam power in the arts and manufactures, at the time to which our narrative has now reached, the steam engine had never been employed for any other purpose save that of raising water by working pumps. The motion, therefore, which was required was merely an upward force, such as was necessary to elevate the piston of a pump, loaded with the column of water which it raised. The following then is a description of the improved engine of Watt, by which such work was proposed to be performed:—

Fig. 21.

In the cylinder represented at C ( fig. 21.), the piston P moves 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boxes, A A, called the upper and lower steam boxes, contain valves by which steam from the boiler may be admitted and withdrawn.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.

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.

Fig. 22.

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.

Fig. 23.

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.

SOHO, BIRMINGHAM.