Part 4 (1/2)
The machine was thus made capable of working uninterruptedly for a period of time only limited by its own decay.
Savery never fitted his boilers with safety-valves, although it was done earlier by Papin; and in deep mines he was compelled to make use of higher pressures than his rudely-constructed boilers could safely bear.
Savery's engine was used at a number of mines, and also for supplying water to towns; some large estates, country houses, and other private establishments, employed them for the same purpose. They did not, however, come into general use among the mines, because, according to Desaguliers, they were apprehensive of danger from the explosion of the boilers or receivers. As Desaguliers wrote subsequently: ”Savery made a great many experiments to bring this machine to perfection, and did erect several which raised water very well for gentlemen's seats, but could not succeed for mines, or supplying towns, where the water was to be raised very high and in great quant.i.ties; for then the steam required being boiled up to such a strength as to be ready to tear all the vessels to pieces.” ”I have known Captain Savery, at York's buildings, to make steam eight or ten times stronger than common air; and then its heat was so great that it would melt common soft solder, and its strength so great as to blow open several joints of the machine; so that he was forced to be at the pains and charge to have all his joints soldered with spelter or hard solder.”
Although there were other difficulties in the application of the Savery engine to many kinds of work, this was the most serious one, and explosions did occur with fatal results. The writer just quoted relates, in his ”Experimental Philosophy,” that a man who was ignorant of the nature of the engine undertook to work a machine which Desaguliers had provided with a safety-valve to avoid this very danger, ”and, having hung the weight at the further end of the steelyard, in order to collect more steam in order to make his work the quicker, he hung also a very heavy plumber's iron upon the end of the steelyard; the consequence proved fatal; for, after some time, the steam, not being able, with the safety-c.o.c.k, to raise up the steelyard loaded with all this unusual weight, burst the boiler with a great explosion, and killed the poor man.” This is probably the earliest record of a steam-boiler explosion.
Savery proposed to use his engine for driving mills; but there is no evidence that he actually made such an application of the machine, although it was afterward so applied by others. The engine was not well adapted to the drainage of surface-land, as the elevation of large quant.i.ties of water through small heights required great capacity of receivers, or compelled the use of several engines for each case. The filling of the receivers, in such cases, also compelled the heating of large areas of cold and wet metallic surfaces by the steam at each operation, and thus made the work comparatively wasteful of fuel. Where used in mines, they were necessarily placed within 30 feet or less of the lowest level, and were therefore exposed to danger of submergence whenever, by any accident, the water should rise above that level. In many cases this would result in the loss of the engine, and the mine would remain ”drowned,” unless another engine should be procured to pump it out. Where the mine was deep, the water was forced by the pressure of steam from the level of the engine-station to the top of the lift. This compelled the use of pressures of several atmospheres in many cases; and a pressure of three atmospheres, or about 45 pounds per square inch, was considered, in those days, as about the maximum pressure allowable. This difficulty was met by setting a separate engine at every 60 or 80 feet, and pumping the water from one to the other. If any one engine in the set became disabled, the pumping was interrupted until that one machine could be repaired. The size of Savery's largest boilers was not great, their maximum diameter not exceeding two and a half feet. This made it necessary to provide several of his engines, usually, for a single mine, and at each level. The first cost and the expense of repairs were exceedingly serious items. The expense and danger, either real or apparent, were thus sufficient to deter many from their use, and the old method of raising water by horse-power was adhered to.
The consumption of fuel with these engines was very great. The steam was not generated economically, as the boilers used were of such simple forms as only could then be produced, and presented too little heating surface to secure a very complete transfer of heat from the gases of combustion to the water within the boiler. This waste in the generation of steam in these uneconomical boilers was followed by still more serious waste in its application, without expansion, to the expulsion of water from a metallic receiver, the cold and wet sides of which absorbed heat with the greatest avidity. The great ma.s.s of the liquid was not, however, heated by the steam, and was expelled at the temperature at which it was raised from below.
Savery quaintly relates the action of his machine in ”The Miner's Friend,” and so exactly, that a better description could scarcely be asked: ”The steam acts upon the surface of the water in the receiver, which surface only being heated by the steam, it does not condense, but the steam gravitates or presses with an elastic quality like air, and still increasing its elasticity or spring, until it counterpoises, or rather exceeds, the weight of the column of water in the force-pipe, which then it will necessarily drive up that pipe; the steam then takes some time to recover its power, but it will at last discharge the water out at the top of the pipe. You may see on the outside of the receiver how the water goes out, as well as if it were transparent; for, so far as the steam is contained within the vessel, it is dry without, and so hot as scarcely to endure the least touch of the hand; but so far as the water is inside the vessel, it will be cold and wet on the outside, where any water has fallen on it; which cold and moisture vanish as fast as the steam takes the place of the water in its descent.”
After Savery's death, in 1716, several of these engines were erected in which some improvements were introduced. Dr. Desaguliers, in 1718, built a Savery engine, in which he avoided some defects which he, with Dr. Gravesande, had noted two years earlier. They had then proposed to adopt the arrangement of a single receiver which had been used by Savery himself, as already described, finding, by experiment on a model which they had made for the purpose, that one could be discharged three times, while the same boiler would empty two receivers but once each. In their arrangement, the steam was shut back in the boiler while the receiver was filling with water, and a high pressure thus acc.u.mulated, instead of being turned into the second receiver, and the pressure thus kept comparatively low.
[Ill.u.s.tration: FIG. 14.--Papin's Two-Way c.o.c.k.]
In the engine built in 1718, Desaguliers used a spherical boiler, which he provided with the lever safety-valve already applied by Papin, and adopted a comparatively small receiver--one-fifth the capacity of the boiler--of slender cylindrical form, and attached a pipe leading the water for condensation into the vessel, and effected its distribution by means of the ”rose,” or a ”sprinkling-plate,” such as is still frequently used in modern engines having jet-condensers.
This subst.i.tution of jet for surface-condensation was of very great advantage, securing great promptness in the formation of a vacuum and a rapid filling of the receiver. A ”two-way c.o.c.k” admitted steam to the receiver, or, being turned the other way, admitted the cold condensing water. The dispersion of the water in minute streams or drops was a very important detail, not only as securing great rapidity of condensation, but enabling the designer to employ a comparatively small receiver or condenser.
The engine is shown in Fig. 15, which is copied from the ”Experimental Philosophy” of Desaguliers.
[Ill.u.s.tration: FIG. 15.--Engine built by Desaguliers in 1718.]
The receiver, _A_, is connected to the boiler, _B_, by a steam-pipe, _C_, terminating at the two-way c.o.c.k, _D_; the ”forcing-pipe,” _E_, has at its foot a check-valve, _F_, and the valve _G_ is a similar check at the head of the suction-pipe. _H_ is a strainer, to prevent the ingress of chips or other bodies carried to the pipe by the current; the cap above the valves is secured by a bridle, or stirrup, and screw, _I_, and may be readily removed to clear the valves or to renew them; _K_ is the handle of the two-way c.o.c.k; _M_ is the injection-c.o.c.k, and is kept open during the working of the engine; _L_ is the chimney-flue; _N_ and _O_ are gauge-c.o.c.ks fitted to pipes leading to the proper depths within the boiler, the water-line being somewhere between the levels of their lower ends; _P_ is a lever safety-valve, as first used on the ”Digester” of Papin; _R_ is the reservoir into which the water is pumped; _T_ is the flue, leading spirally about the boiler from the furnace, _V_, to the chimney; _Y_ is a c.o.c.k fitted in a pipe through which the rising-main may be filled from the reservoir, should injection-water be needed when that pipe is empty.
Seven of these engines were built, the first of which was made for the Czar of Russia. Its boiler had a capacity of ”five or six hogsheads,”
and the receiver, ”holding one hogshead,” was filled and emptied four times a minute. The water was raised ”by suction” 29 feet, and forced by steam pressure 11 feet higher.
Another engine built at about this time, to raise water 29 feet ”by suction,” and to force it 24 feet higher, made 6 ”strokes” per minute, and, when forcing water but 6 or 8 feet, made 8 or 9 strokes per minute. Twenty-five years later a workman overloaded the safety-valve of this engine, by placing the weight at the end and then adding ”a very heavy plumber's iron.” The boiler exploded, killing the attendant.
Desaguliers says that one of these engines, capable of raising ten tons an hour 38 feet, in 1728 or 1729, cost 80, exclusive of the piping.
Blakely, in 1766, patented an improved Savery engine, in which he endeavored to avoid the serious loss due to condensation of the steam by direct contact with the water, by interposing a cus.h.i.+on of oil, which floated upon the water and prevented the contact of the steam with the surface of the water beneath it. He also used air for the same purpose, sometimes in double receivers, one supported on the other. These plans did not, however, prove satisfactory.
Rigley, of Manchester, England, soon after erected Savery engines, and applied them to the driving of mills, by pumping water into reservoirs, from whence it returned to the wells or ponds from which it had been raised, turning water-wheels as it descended.
Such an arrangement was in operation many years at the works of a Mr.
Kiers, St. Pancras, London. It is described in detail, and ill.u.s.trated, in Nicholson's ”Philosophical Journal,” vol. i., p. 419.
It had a ”wagon-boiler” 7 feet long, 5 wide, and 5 deep; the wheel was 18 feet in diameter, and drove the lathes and other machinery of the works. In this engine Blakely's plan of injecting air was adopted. The injection-valve was a clack, which closed automatically when the vacuum was formed.
The engine consumed 6 or 7 bushels of good coals, and made 10 strokes per minute, raising 70 cubic feet of water 14 feet, and developing nearly 3 horse-power.
Many years after Savery's death, in 1774, Smeaton made the first duty-trials of engines of this kind. He found that an engine having a cylindrical receiver 16 inches in diameter and 22 feet high, discharging the water raised 14 feet above the surface of the water in the well, making 12 strokes, and raising 100 cubic feet per minute, developed 2-2/3 horse-power, and consumed 3 hundredweight of coals in four hours. Its duty was, therefore, 5,250,000 pounds raised one foot per bushel of 84 pounds of coals, or 62,500 ”foot-pounds” of work per pound of fuel. An engine of slightly greater size gave a duty about 5 per cent. greater.
When Louis XIV. revoked the edict of Nantes, by which Henry IV. had guaranteed protection to the Protestants of France, the terrible persecutions at once commenced drove from the kingdom some of its greatest men. Among these was Denys Papin.
It was at about this time that the influence of the atmospheric pressure on the boiling-point began to be observed, Dr. Hooke having found that the boiling-point was a fixed temperature under the ordinary pressure of the atmosphere, and the increase in temperature and pressure of steam when confined having been shown by Papin with his ”Digester.”
Denys Papin was of a family which had attached itself to the Protestant Church; but he was given his education in the school of the Jesuits at Blois, and there acquired his knowledge of mathematics. His medical education was given him at Paris, although he probably received his degree at Orleans. He settled in Paris in 1672, with the intention of practising his profession, and devoted all his spare time, apparently, to the study of physics.