A History of the Growth of the Steam-Engine Part 24

Corliss, who made it a feature of the Corliss valve-gear in 1849. In the year 1855, N. T. Greene introduced a form of expansion-gear, in which he combined the range of the Sickels beam-motion device with the expansion-adjustment gained by the attachment of the governor, and with the advantages of flat slide-valves at all ports--both steam and exhaust.

Many other ingenious forms of expansion valve-gear have been invented, and several have been introduced, which, properly designed and proportioned to well-planned engines, and with good construction and management, should give economical results little if at all inferior to those just named. Among the most ingenious of these later devices is that of Babc.o.c.k & Wilc.o.x, in which a very small auxiliary steam-cylinder and piston is employed to throw the cut-off valve over its port at the instant at which the steam is to be cut off. A very beautiful form of isochronous governor is used on this engine, to regulate the speed of the engine by determining the point of cut-off.

In Wright's engine, the expansion is adjusted by the movement, by the regulator, of cams which operate the steam-valves so that they shall hold the valve open a longer or shorter time, as required.

Since compactness and lightness are not as essential as in portable, locomotive, and marine engines, the parts are arranged, in stationary engines, with a view simply to securing efficiency, and the design is determined by circ.u.mstances. It was formerly usual to adopt the condensing engine in mills, and wherever a stationary engine was required. In Europe generally, and to some extent in the United States, where a supply of condensing water is obtainable, condensing engines and moderate steam-pressures are still employed. But this type of engine is gradually becoming superseded by the high-pressure condensing engine, with considerable expansion, and with an expansion-gear in which the point of cut-off is determined by the governor.

[Ill.u.s.tration: FIG. 97.--Corliss Engine.]

[Ill.u.s.tration: FIG. 98--Corliss Engine Valve-Motion.]

The best-known engine of this cla.s.s is the Corliss engine, which is very extensively used in the United States, and which has been copied very generally by European builders. Fig. 97 represents the Corliss engine. The horizontal steam-cylinder is bolted firmly to the end of the frame, which is so formed as to transmit the strain to the main journal with the greatest directness. The frame carries the guides for the cross-head, which are both in the same vertical plane. The valves are four in number, a steam and an exhaust valve being placed at each end of the steam-cylinder. Short steam-pa.s.sages are thus secured, and this diminution of clearance is a source of some economy. Both sets of valves are driven by an eccentric operating a disk or wrist-plate, _E_ (Fig. 98), which vibrates on a pin projecting from the cylinder. Short links reaching from this wrist-plate to the several valves, _D D_, _F F_, move them with a peculiarly varying motion, opening and closing them rapidly, and moving them quite slowly when the port is either nearly open or almost closed. This effect is ingeniously secured by so placing the pins on the wrist-plate that their line of motion becomes nearly transverse to the direction of the valve-links when the limit of movement is approached. The links connecting the wrist-plate with the arms moving the steam-valves have catches at their extremities, which are disengaged by coming in contact, as the arm swings around with the valve-stem, with a cam adjusted by the governor. This adjustment permits the steam to follow the piston farther when the engine is caused to "slow down," and thus tends to restore the proper speed. It disengages the steam-valve earlier, and expands the steam to a greater extent, when the engine begins to run above the proper speed. When the catch is thrown out, the valve is closed by a weight or a strong spring. To prevent jar when the motion of the valve is checked, a "dash-pot" is used, invented originally by F. E. Sickels.

This is a vessel having a nicely-fitted piston, which is received by a "cus.h.i.+on" of water or air when the piston suddenly enters the cylinder at the end of the valve-movement. In the original water dash-pot of Sickels, the cylinder is vertical, and the plunger or piston descends upon a small body of water confined in the base of the dash-pot.

Corliss's air dash-pot is now often set horizontally.

[Ill.u.s.tration: FIG. 99.--Greene Engine.]

In the Greene steam-engine (Fig. 99), the valves are four in number, as in the Corliss. The cut-off gear consists of a bar, _A_, moved by the steam-eccentric in a direction parallel with the centre-line of the cylinder and nearly coincident as to time with the piston. On this bar are tappets, _C C_, supported by springs and adjustable in height by the governor, _G_. These tappets engage the arms _B B_, on the ends of rock-shafts, _E E_, which move the steam-valves and remain in contact with them a longer or shorter time, and holding the valve open during a greater or less part of the piston-stroke, as the governor permits the tappets to rise with diminis.h.i.+ng engine-speed, or forces them down as speed increases. The exhaust-valves are moved by an independent eccentric rod, which is itself moved by an eccentric set, as is usual with the Corliss and with other engines generally, at right angles with the crank. This engine, in consequence of the independence of the steam-eccentric, and of the contemporary movement of steam valve-motion and steam-piston, is capable of cutting off at any point from beginning to nearly the end of the stroke. The usual arrangement, by which steam and exhaust valves are moved by the same eccentric, only permits expansion with the range from the beginning to half-stroke. In the Corliss engine the latter construction is retained, with the object, in part, of securing a means of closing the valve by a "positive motion," should, by any accident, the closing not be effected by the weight or spring usually relied upon.

[Ill.u.s.tration: FIG. 100.--Thurston's Greene-Engine Valve-Gear.]

The steam-valve of the Greene engine, as designed by the author, is seen in Fig. 100, where the valve, _G H_, covering the port, _D_, in the steam-cylinder, _A B_, is moved by the rod, _J J_, connected to the rock-shaft, _M_, by the arm, _L K_. The line, _K I_, should, when carried out, intersect the valve-face at its middle point, under _G_.

The characteristics of the American stationary engine, therefore, are high steam-pressure without condensation, an expansion valve-gear with drop cut-off adjustable by the governor, high piston-speed, and lightness combined with strength of construction. The pressure most commonly adopted in the boilers which furnish steam to this type of engine is from 75 to 80 pounds per square inch; but a pressure of 100 pounds is not infrequently carried, and the latter pressure may be regarded as a "mean maximum," corresponding to a pressure of 60 pounds at about the commencement of the period here considered--1850.

Very much greater pressures have, however, been adopted by some makers, and immensely "higher steam" has been experimented with by several engineers. As early as 1823, Jacob Perkins[88] commenced experimenting with steam of very great tension. As has already been stated, the usual pressure at the time of Watt was but a few pounds--5 or 7--in excess of that of the atmosphere. Evans, Trevithick, and Stevens, had previously worked steam at pressures of from 50 to 75 pounds per square inch, and pressures on the Western rivers and elsewhere in the United States had already been raised to 100 or 150 pounds, and explosions were becoming alarmingly frequent.

[88] Perkins was a native of Newburyport, Ma.s.s. He was born July 9, 1766, and died in London, July 30, 1849. He went to England when fifty-two years of age, to introduce his inventions.

Perkins's experimental apparatus consisted of a copper boiler, of a capacity of about one cubic foot, having sides 3 inches in thickness.

It was closed at the bottom and top, and had five small pipes leading from the upper head. This was placed in a furnace kept at a high temperature by a forced combustion. Safety-valves loaded respectively to 425 and 550 pounds per square inch were placed on each of two of the steam-pipes.

Perkins used the steam generated under these great pressures in a little engine having a piston 2 inches in diameter and a stroke of 1 foot. It was rated at 10 horse-power.[89]

[89] It was when writing of this engine that Stuart wrote, in 1824: "Judging from the rapid strides the steam-engine has made _during the last forty years_ to become a universal first-mover, and from the experience that has arisen from that extension, we feel convinced that every invention which diminishes its size without impairing its power brings it a step nearer to the a.s.sistance of the 'world's great laborers,' the husbandman and the peasant, for whom, as yet, it performs but little. At present, it is made occasionally to tread out the corn. What honors await not that man who may yet direct its mighty power to plough, to sow, to harrow, and to reap!"

The progress of the steam-engine during those forty years does not to-day appear so astounding. The sentiment here expressed has lost none of its truth, nevertheless.

In the year 1827, Perkins had attained working pressures, in a single-acting, single-cylinder engine, of upward of 800 pounds per square inch. At pressures exceeding 200 pounds, he had much trouble in securing effective lubrication, as all oils charred and decomposed at the high temperatures then unavoidably encountered, and he finally succeeded in evading this seemingly insurmountable obstacle by using for rubbing parts a peculiar alloy which required no lubrication, and which became so beautifully polished, after some wear, that the friction was less than where lubricants were used. At these high pressures Perkins seems to have met with no other serious difficulty.

He condensed the exhaust-steam and returned it to the boiler, but did not attempt to create a vacuum in his condenser, and therefore needed no air-pump. Steam was cut off at one-eighth stroke.

In the same year, Perkins made a compound engine on the Woolf plan, and adopted a pressure of 1,400 pounds, expanding eight times. In still another engine, intended for a steam-vessel, Perkins adopted, or proposed to adopt, 2,000 pounds pressure, cutting off the admission at one-sixteenth, in single-acting engines of 6 inches diameter of cylinder and 20 inches stroke of piston. The steam did not retain boiler-pressure at the cylinder, and this engine was only rated at 30 horse-power.[90]

[90] Galloway and Hebert, on the Steam-Engine. London, 1836.

Stuart follows a description of Perkins's work in the improvement of the steam-engine and the introduction of steam-artillery by the remark:

" ... No other mechanic of the day has done more to ill.u.s.trate an obscure branch of philosophy by a series of difficult, dangerous, and expensive experiments; no one's labors have been more deserving of cheering encouragement, and no one has received less. Even in their present state, his experiments are opening new fields for philosophical research, and his mechanism bids fair to introduce a new style into the proportions, construction, and form, of steam-machinery."

Perkins's experience was no exception to the general rule, which denies to nearly all inventors a fair return for the benefits which they confer upon mankind.

Another engineer, a few years later, was also successful in controlling and working steam under much higher pressures than are even now in use. This was Dr. Ernst Alban, a distinguished German engine-builder, of Plau, Mecklenburg, and an admirer of Oliver Evans, in whose path he, a generation later, advanced far beyond that great pioneer. Writing in 1843, he describes a system of engine and boiler construction, with which he used steam under pressures about equal to those experimentally worked by Jacob Perkins, Evans's American successor. Alban's treatise was translated and printed in Great Britain,[91] four years later.

[91] "The High-Pressure Steam-Engine," etc. By Dr. Ernst Alban.

Translated by William Pole, F. R. A. S. London, 1847.

Alban, on one occasion, used steam of 1,000 pounds pressure. His boilers were similar in general form to the boiler patented by Stevens in 1805, but the tubes were horizontal instead of vertical. He evaporated from 8 to 10 pounds of water into steam of 600 to 800 pounds pressure with each pound of coal. He states that the difficulty met by Perkins--the decomposition of lubricants in the steam-cylinder--did not present itself in his experiments, even when working steam at a pressure of 600 pounds on the square inch, and he found that less lubrication was needed at such high pressures than in ordinary practice. Alban expanded his steam about as much as Evans, in his usual practice, carrying a pressure of 150 pounds, and cutting off at one-third; he adopted greatly increased piston-speed, attaining 300 feet per minute, at a time when common practice had only reached 200 feet. He usually built an oscillating engine, and rarely attached a condenser. The valve was the locomotive-slide.[92] The stroke was made short to secure strength, compactness, cheapness, and high speed of rotation; but Alban does not seem to have understood the principles controlling the form and proportions of the expansive engine, or the necessity of adopting considerable expansion in order to secure economy in working steam of great tension, and therefore was, apparently, not aware of the advantages of a long stroke in reducing losses by "dead-s.p.a.ce," in reducing risk of annoyance by hot journals, or in enabling high piston-speeds to be adopted. He seems never to have attained a sufficiently high speed of piston to become aware that the oscillating cylinder cannot be used at speeds perfectly practicable with the fixed cylinder.

[92] Invented by Joseph Maudsley, of London, 1827.

Alban states that one of his smallest engines, having a cylinder 4-1/2 inches in diameter and 1 foot stroke of piston, with a piston-speed of but 140 to 160 feet per minute, developed 4 horse-power, with a consumption of 5.3 pounds of coal per hour. This is a good result for so small an amount of work, and for an engine working at so low a speed of piston. An engine of 30 horse-power, also working very slowly, required but 4.1 pounds of coal per hour per horse-power.

The work of Perkins and of Alban, like that of their predecessors, Evans, Stevens, and Trevithick, was, however, the work of engineers who were far ahead of their time. The general practice, up to the time which marked the beginning of the modern "period of refinement," had been but gradually approximating that just described. Higher pressures were slowly approached; higher piston-speeds came slowly into use; greater expansion was gradually adopted; the causes of losses of heat were finally discovered, and steam-jacketing and external non-conducting coverings were more and more generally applied as builders became more familiar with their work. The "compound engine"

was now and then adopted; and each experiment, made with higher steam and greater expansion, was more nearly successful than the last.

Finally, all these methods of securing economy became recognized, and the reasons for their adoption became known. It then remained, as the final step in this progression, to combine all these requisites of economical working in a double-cylinder engine, steam-jacketed, well protected by non-conducting coverings, working steam of high pressure, and with considerable expansion at high piston-speed. This is now done by the best builders.

One of the best examples of this type of engine is that constructed by the sons of Jacob Perkins, who continued the work of their father after his death. Their engines are single-acting, and the small or high-pressure cylinder is placed on the top of the larger or low-pressure cylinder. The valves are worked by rotating stems, and the loss of heat and burning of packing incident to the use of the common method are thus avoided. The stuffing-boxes are placed at the end of long sleeves, closely surrounding the vertical valve-stems also, and the water of condensation which collects in these sleeves is an additional and thorough protection against excessively high temperature at the packing. The piston-rings are made of the alloy which has been found to require no lubrication.

Steam is usually worked at from 250 to 450 pounds, and is generated in boilers composed of small tubes three inches in diameter and three-eighths of an inch thick, which are tested under a pressure of 2,500 pounds per square inch. The safety-valve is usually loaded to 400 pounds. The boiler is fed with distilled water, obtained princ.i.p.ally by condensation of the exhaust-steam, any deficiency being made up by the addition of water from a distilling apparatus. Under these conditions, but 1-1/4 pound of coal is consumed per hour and per horse-power.

THE PUMPING-ENGINE in use at the present time has pa.s.sed through a series of changes not differing much from that which has been traced with the stationary mill-engine. The Cornish engine is still used to some extent for supplying water to towns, and is retained at deep mines. The modern Cornish engine differs very little from that of the time of Watt, except in the proportions of parts and the form of its details. Steam-pressures are carried which were never reached during the preceding period, and, by careful adjustment of well-set and well-proportioned valves and gearing, the engine has been made to work rather more rapidly, and to do considerably more work. It still remains, however, a large, costly, and awkward contrivance, requiring expensive foundations, and demanding exceptional care, skill, and experience in management. It is gradually going out of use. This engine, as now constructed by good builders, is shown in section in Fig. 101.

A comparison with the Watt engine of a century earlier will at once enable any one to appreciate the extent to which changes may be made in perfecting a machine, even after it has become complete, so far as supplying it with all essential parts can complete it.

[Ill.u.s.tration: FIG. 101.--Cornish Pumping-Engine, 1880.]

In the figure, _A_ is the cylinder, taking steam from the boiler through the steam-pa.s.sage, _M_. The steam is first admitted above the piston, _B_, driving it rapidly downward and raising the pump-rod, _E_. At an early period in the stroke the admission of steam is checked by the sudden closing of the induction-valve at _M_, and the stroke is completed under the action of expanding steam a.s.sisted by the inertia of the heavy parts already in motion. The necessary weight and inertia is afforded, in many cases, where the engine is applied to the pumping of deep mines, by the immensely long and heavy pump-rods.

Where this weight is too great, it is counterbalanced, and where too small, weights are added. When the stroke is completed, the "equilibrium valve" is opened, and the steam pa.s.ses from above to the s.p.a.ce below the piston, and an equilibrium of pressure being thus produced, the pump-rods descend, forcing the water from the pumps and raising the steam-piston. The absence of the crank, or other device which might determine absolutely the length of stroke, compels a very careful adjustment of steam-admission to the amount of load. Should the stroke be allowed to exceed the proper length, and should danger thus arise of the piston striking the cylinder-head, _N_, the movement is checked by buffer-beams. The valve-motion is actuated by a plug-rod, _J K_, as in Watt's engine. The regulation is effected by a "cataract," a kind of hydraulic governor, consisting of a plunger-pump, with a reservoir attached. The plunger is raised by the engine, and then automatically detached. It falls with greater or less rapidity, its velocity being determined by the size of the eduction-orifice, which is adjustable by hand. When the plunger reaches the bottom of the pump-barrel, it disengages a catch, a weight is allowed to act upon the steam-valve, opening it, and the engine is caused to make a stroke. When the outlet of the cataract is nearly closed, the engine stands still a considerable time while the plunger is descending, and the strokes succeed each other at long intervals.

When the opening is greater, the cataract acts more rapidly, and the engine works faster. This has been regarded until recently as the most economical of pumping-engines, and it is still generally used in freeing mines of water, and in situations where existing heavy pump-rods may be utilized in counterbalancing the steam-pressure, and, by their inertia, in continuing the motion after the steam, by its expansion, has become greatly reduced in pressure.

In this engine a gracefully-shaped and strong beam, _D_, has taken the place of the ruder beam of the earlier period, and is carried on a well-built wall of masonry, _R_. _F_ is the exhaust-valve, by which the steam pa.s.ses to the condenser, _G_, beside which is the air-pump, _H_, and the hot-well, _I_. The cylinder is steam-jacketed, _P_, and protected against losses of heat by radiation by a brick wall, _O_, the whole resting on a heavy foundation, _Q_.

The Bull Cornish engine is also still not infrequently seen in use.

The Cornish engine of Great Britain averages a duty of about 45,000,000 pounds raised one foot high per 100 pounds of coal. More than double this economy has sometimes been attained.

[Ill.u.s.tration: FIG. 102.--Steam-Pump.]

A vastly simpler form of pumping-engine without fly-wheel is the now common "direct-acting steam-pump." This engine is generally made use of in feeding steam-boilers, as a forcing and fire pump, and wherever the amount of water to be moved is not large, and where the pressure is comparatively great. The steam-cylinder, _A R_, and feed-pump, _B Q_ (Fig. 102), are in line, and the two pistons have usually one rod, _D_, in common. The two cylinders are connected by a strong frame, _N_, and two standards fitted with lugs carry the whole, and serve as a means of bolting the pump to the floor or to its foundation.

The method of working the steam-valve of the modern steam-pump is ingenious and peculiar. As shown, the pistons are moving toward the left; when they reach the end of their stroke, the face of the piston strikes a pin or other contrivance, and thus moves a small auxiliary valve, _I_, which opens a port, _E_, and causes steam to be admitted behind a piston, or permits steam to be exhausted, as in the figure, from before the auxiliary piston, _F_, and the pressure within the main steam-chest then forces that piston over, moving the main steam-valve, _G_, to which it is attached, admitting steam to the left-hand side of the main piston, and exhausting on the right-hand side, _A_. Thus the motion of the engine operates its own valves in such a manner that it is never liable to stop working at the end of the stroke, notwithstanding the absence of the crank and fly-wheel, or of independent mechanism, like the cataract of the Cornish engine.

There is a very considerable variety of pumps of this cla.s.s, all differing in detail, but all presenting the distinguis.h.i.+ng feature of auxiliary valve and piston, and a connection by which it and the main engine each works the valve of the other combination.

[Ill.u.s.tration: FIG. 103.--The Worthington Pumping-Engine, 1876.

Section.]

[Ill.u.s.tration: FIG. 104.--The Worthington Pumping-Engine.]

In some cases these pumps are made of considerable size, and are applied to the elevation of water in situations to which the Cornish engine was formerly considered exclusively applicable. The accompanying figure ill.u.s.trates such a pumping-engine, as built for supplying cities with water. This is a "compound" direct-acting pumping-engine. The cylinders, _A B_, are placed in line, working one pump, _F_, and operating their own air-pumps, _D D_, by a bell-crank lever, _L H_, connected to the pump-buckets by links, _I K_. Steam exhausted from the small cylinder, _A_, is further expanded in the large cylinder, _B_, and thence goes to the condenser, _C_. The valves, _N M_, are moved by the valve-gear, _L_, which is actuated by the piston-rod of a similar pair of cylinders placed by the side of the first. These valves are balanced, and the balance-plates, _R Q_, are suspended from the rods, _O P_, which allow them to move with the valves. By connecting the valves of each engine with the piston-rod of the other, it is seen that the two engines must work alternately, the one making a stroke while the other is still, and then itself stopping a moment while the latter makes its stroke.