Water enters the pump through the induction-pipe, _E_, pa.s.ses into the pump-barrel through the valves, _V V_, and issues through the eduction-valves, _T T_, and goes on to the "mains" by the pipe, _G_, above which is seen an air-chamber, which a.s.sists to preserve a uniform pressure on that side the pump. This engine works very smoothly and quietly, is cheap and durable, and has done excellent duty.
Beam pumping-engines are now almost invariably built with crank and fly-wheel, and very frequently are compound engines. The accompanying ill.u.s.tration represents an engine of the latter form.
[Ill.u.s.tration: FIG. 105.--Double-Cylinder Pumping-Engine, 1878.]
[Ill.u.s.tration: FIG. 106.--The Lawrence Water-Works Engine.]
_A_ and _B_ are the two steam-cylinders, connected by links and parallel motion, _C D_, to the great cast-iron beam, _E F_. At the opposite end of the beam, the connecting-rod, _G_, turns a crank, _H_, and fly-wheel, _L M_, which regulates the motion of the engine and controls the length of stroke, averting all danger of accident occurring in consequence of the piston striking either cylinder-head.
The beam is carried on handsomely-shaped iron columns, which, with cylinders, pump, and fly-wheel, are supported by a substantial stone foundation. The pump-rod, _I_, works a double-acting pump, _J_, and the resistance to the issuing water is rendered uniform by an air-chamber, _K_, within which the water rises and falls when pressures tend to vary greatly. A revolving shaft, _N_, driven from the fly-wheel shaft, carries cams, _O P_, which move the lifting-rods seen directly over them and the valves which they actuate. Between the steam-cylinders and the columns which carry the beams is a well, in which are placed the condenser and air-pump. Steam is carried at 60 or 80 pounds pressure, and expanded from 6 to 10 times.
[Ill.u.s.tration: FIG. 107.--The Leavitt Pumping-Engine.]
A later form of double-cylinder beam pumping-engine is that invented and designed by E. D. Leavitt, Jr., for the Lawrence Water-Works, and shown in Figs. 106 and 107. The two cylinders are placed one on each side the centre of the beam, and are so inclined that they may be coupled to opposite ends of it, while their lower ends are placed close together. At their upper ends a valve is placed at each end of the connecting steam-pipe. At their lower ends a single valve serves as exhaust-valve to the high-pressure and as steam-valve to the low-pressure cylinder. The pistons move in opposite directions, and steam is exhausted from the high-pressure cylinder directly into the nearer end of the low-pressure cylinder. The pump, of the "Thames-Ditton" or "bucket-and-plunger" variety, takes a full supply of water on the down-stroke, and discharges half when rising and half when descending again. The duty of this engine is reported by a board of engineers as 103,923,215 foot-pounds for every 100 pounds of coal burned. The duty of a moderately good engine is usually considered to be from 60 to 70 millions. This engine has steam-cylinders of 17-1/2 and 36 inches diameter respectively, with a stroke of 7 feet. The pump had a capacity of about 195 gallons, and delivered 96 per cent. Steam was carried at a pressure of 75 pounds above the atmosphere, and was expanded about 10 times. Plain horizontal tubular boilers were used, evaporating 8.58 pounds of water from 98 Fahr. per pound of coal.
STEAM-BOILERS.--The steam supplied to the forms of stationary engine which have been described is generated in steam-boilers of exceedingly varied forms. The type used is determined by the extent to which their cost is increased in the endeavor to economize fuel by the pressure of steam carried, by the greater or less necessity of providing against risk of explosion, by the character of the feed-water to be used, by the facilities which may exist for keeping in good repair, and even by the character of the men in whose hands the apparatus is likely to be placed.
As has been seen, the changes which have marked the growth and development of the steam-engine have been accompanied by equally marked changes in the forms of the steam-boiler. At first, the same vessel served the distinct purposes of steam-generator and steam-engine. Later, it became separated from the engine, and was then specially fitted to perform its own peculiar functions; and its form went through a series of modifications under the action of the causes already stated.
When steam began to be usefully applied, and considerable pressures became necessary, the forms given to boilers were approximately spherical, ellipsoidal, or cylindrical. Thus the boilers of De Caus (1615) and of the Marquis of Worcester (1663) were spherical and cylindrical; those of Savery (1698) were ellipsoidal and cylindrical.
After the invention of the steam-engine of Newcomen, the pressures adopted were again very low, and steam-boilers were given irregular forms until, at the beginning of the present century, they were again of necessity given stronger shapes. The material was at first frequently copper; it is now usually wrought-iron, and sometimes steel.
The present forms of steam-boilers may be cla.s.sified as plain, flue, and tubular boilers. The plain cylindrical or common cylinder boiler is the only representative of the first cla.s.s in common use. It is perfectly cylindrical, with heads either flat or hemispherical. There is usually attached to the boiler a "steam-drum" (a small cylindrical vessel), from which the steam is taken by the steam-pipe. This enlargement of the steam-s.p.a.ce permits the mist, held in suspension by the steam when it first rises from the surface of the water, to separate more or less completely before the steam is taken from the boiler.
[Ill.u.s.tration: FIG. 108.--Babc.o.c.k & Wilc.o.x"s Vertical Boiler.]
Flue-boilers are frequently cylindrical, and contain one or more cylindrical flues, which pa.s.s through from end to end, beneath the water-line, conducting the furnace-gases, and affording a greater area of heating-surface than can be obtained in the plain boiler. They are usually from 30 to 48 inches in diameter, and one foot or less in length for each inch of diameter. Some are, however, made 100 feet and more in length. The boiler is made of iron 1/4 to 3/8 of an inch in thickness, with hemispherical or carefully stayed flat heads, and without flues. The whole is placed in a brickwork setting. These boilers are used where fuel is inexpensive, where the cost of repairing would be great, or where the feed-water is impure. A cylindrical boiler, having one flue traversing it longitudinally, is called a Cornish boiler, as it is generally supposed to have been first used in Cornwall. It was probably first invented by Oliver Evans in the United States, previous to 1786, at which time he had it in use. The flue has usually a diameter 0.5 or 0.6 the diameter of the boiler. A boiler containing two longitudinal flues is called the Lancashire boiler. This form was also introduced by Oliver Evans. The flues have one-third the diameter of the boiler. Several flues of smaller diameter are often used, and when a still greater proportional area of heating-surface is required, tubes of from 1-1/4 inch to 4 or 5 inches in diameter are subst.i.tuted for flues. The flues are usually constructed by riveting sheets together, as in making the sh.e.l.l or outer portion. They are sometimes welded by British manufacturers, but rarely if ever in the United States. Tubes are always "lap-welded" in the process of rolling them. Small tubes were first used in the United States, about 1785. In portable, locomotive, and marine steam-boilers, the fire must be built within the boiler itself, instead of (as in the above described stationary boilers) in a furnace of brickwork exterior to the boiler. The flame and gases from the furnace or fire-box in these kinds of boiler are never led through brick pa.s.sages en route to the chimney, as often in the preceding case, but are invariably conducted through flues or tubes, or both, to the smoke-stack. These boilers are also sometimes used as stationary boilers. Fig. 108 represents such a steam-boiler in section, as it is usually exhibited in working drawings. Provision is made to secure a good circulation of water in these boilers by means of the "baffle-plates," seen in the sketch, which compel the water to flow as indicated by the arrows.
The tubes are frequently made of bra.s.s or of copper, to secure rapid transmission of heat to the water, and thus to permit the use of a smaller area of heating-surface and a smaller boiler. The steam-s.p.a.ce is made as large as possible, to secure immunity from "priming" or the "entrainment" of water with the steam. This type of steam-boiler, invented by Nathan Read, of Salem, Ma.s.s., in 1791, and patented in April of that year, was the earliest of the tubular boilers. In the locomotive boiler (Fig. 109), as in the preceding, the characteristics are a fire-box at one end of the sh.e.l.l and a set of tubes through which the gases pa.s.s directly to the smoke-stack. Strength, compactness, great steaming capacity, fair economy, moderate cost, and convenience of combination with the running parts, are secured by the adoption of this form. It is frequently used also for portable and stationary engines. It was invented in France by M. Seguin, and in England by Booth, and used by George Stephenson at about the same time--1828 or 1829.
[Ill.u.s.tration: FIG. 109.--Stationary "Locomotive" Boiler.]
Since the efficiency of a steam-boiler depends upon the extent of effective heating-surface per unit of weight of fuel burned in any given time--or, ordinarily, upon the ratio of the areas of heating and grate surface--peculiar expedients are sometimes adopted, having for their object the increase of heating-surface, without change of form of boiler and without proportionate increase of cost.
One of these methods is that of the use of Galloway conical tubes (Fig. 110). These are very largely used in Great Britain, but are seldom if ever seen in the United States. The Cornish boiler, to which they are usually applied, consists of a large cylindrical sh.e.l.l, 6 feet or more in diameter, containing one tube of about one-half as great dimensions, or sometimes two of one-third the diameter of the sh.e.l.l each. Such boilers have a very small ratio of heating to grate surface, and their large tubes are peculiarly liable to collapse. To remove these objections, the Messrs. Galloway introduced stay-tubes into the flues, which tubes are conical in form, and are set in either a vertical or an inclined position, the larger end uppermost. The area of heating-surface is thus greatly increased, and, at the same time, the liability to collapse is reduced. The same results are obtained by another device of Galloway, which is sometimes combined with that just described in the same boiler. Several sheets in the flue have "pockets" worked into them, which pockets project into the flue-pa.s.sage.
[Ill.u.s.tration: FIG. 110.]
Another device is that of an American engineer, Miller, who surrounds the furnace of cylindrical and other boilers with water-tubes. The "fuel-economizers" of Greene and others consist of similar collections of tubes set in the flues, between the boiler and the chimney.
"_Sectional_" boilers are gradually coming into use with high pressures, on account of their greater safety against disastrous explosions. The earliest practicable example of a boiler of this cla.s.s was probably that of Colonel John Stevens, of Hoboken, N. J. Dr.
Alban, who, forty years later, attempted to bring this type into general use, and constructed a number of such boilers, did not succeed. Their introduction, like that of all radical changes in engineering, has been but slow, and it has been only recently that their manufacture has become an important branch of industry.
A committee of the American Inst.i.tute, of which the author was chairman, in 1871, examined several boilers of this and the ordinary type, and tested them very carefully. They reported that they felt "confident that the introduction of this cla.s.s of steam-boilers will do much toward the removal of the cause of that universal feeling of distrust which renders the presence of a steam-boiler so objectionable in every locality. The difficulties in thoroughly inspecting these boilers, in regulating their action, and other faults of the cla.s.s, are gradually being overcome, and the committee look forward with confidence to the time when their use will become general, to the exclusion of older and more dangerous forms of steam-boilers."
The economical performance of these boilers with a similar ratio of heating to grate surface is equal to that of other kinds. In fact, they are usually given a somewhat higher ratio, and their economy of fuel frequently exceeds that of the other types. Their princ.i.p.al defect is their small capacity for steam and water, which makes it extremely difficult to obtain steady steam-pressure. Where they are employed, the feed and draught should be, if possible, controlled by automatic attachments, and the feed-water heated to the highest attainable temperature. Their satisfactory working depends, more than in other cases, on the ability of the fireman, and can only be secured by the exercise of both care and skill.
Many forms of these boilers have been devised. Walter Hanc.o.c.k constructed boilers for his steam-carriage of flat plates connected by stay-bolts, several such sections composing the boiler; and about the same time (1828) Sir Goldsworthy Gurney constructed for a similar purpose boilers consisting of a steam and a water reservoir, placed one above the other, and connected by triangularly-bent water-tubes exposed to the heat of the furnace-gases. Jacob Perkins made many experiments looking to the employment of very high steam-pressures, and in 1831 patented a boiler of this cla.s.s, in which the heating-surfaces nearest the fire were composed of iron tubes, which tubes also served as grate-bars. The steam and water s.p.a.ce was princ.i.p.ally comprised within a comparatively large chamber, of which the walls were secured by closely distributed stay-bolts. For extremely high pressures, boilers composed only of tubes were used.
Dr. Ernst Alban described the boiler already referred to, and its construction and operation, and stated that he had experimented with pressures as high as 1,000 pounds to the square inch.
The Harrison steam-boiler, which has been many years in use in the United States, consists of several sections, each of which is made up of hollow globes of cast-iron, communicating with each other by necks cast upon the spheres, and fitted together with faced joints. Long bolts, extending from end to end of each row, bind the spheres together. (_See_ Fig. 111.)
[Ill.u.s.tration: FIG. 111.--Harrison"s Sectional Boiler.]
An example of another modern type in extensive use is given in Fig.
112, a semi-sectional boiler, which consists of a series of inclined wrought-iron tubes, connected by T-heads, which form the vertical water-channels, at each end. The joints are faced by milling them, and then ground so perfectly tight that a pressure of 500 pounds to the square inch is insufficient to produce leakage. No packing is used.
The fire is made under the front and higher end of the tubes, and the products of combustion pa.s.s up between the tubes into a combustion-chamber under the steam and water drum; hence they pa.s.s down between the tubes, then once more up through the s.p.a.ce between the tubes, and off to the chimney. The steam is taken out at the top of the steam-drum near the back end of the boiler. The rapid circulation prevents to some extent the formation of deposits or incrustations upon the heating-surfaces, sweeping them away and depositing them in the mud-drum, whence they are blown out. Rapid circulation of water, as has been shown by Prof. Trowbridge, also a.s.sists in the extraction of the heat from the gases, by the presentation of fresh water continually, as well as by the prevention of incrustation.
[Ill.u.s.tration: FIG. 112.--Babc.o.c.k and Wilc.o.x"s Sectional Boiler.]
Attempts have been made to adapt sectional boilers to marine engines; but very little progress has yet been made in their introduction. The Root sectional boiler (Fig. 113), an American design, which is in extensive use in the United States and Europe, has also been experimentally placed in service on shipboard. Its heating-surface consists wholly of tubes, which are connected by a peculiarly formed series of caps; the joints are made tight with rubber "grummets."
[Ill.u.s.tration: FIG. 113.--Root Sectional Boiler.]
SECTION II.--PORTABLE AND LOCOMOTIVE ENGINES.
Engines and boilers, when of small size, are now often combined in one structure which may be readily transported. Where they have a common base-plate simply, as in Fig. 114, they are called, usually, "semi-portable engines." These little engines have some decided advantages. Being attached to one base, the combined engine and boiler is easily transported, occupies little s.p.a.ce, and may very readily be mounted upon wheels, rendering it peculiarly well adapted for agricultural purposes.
[Ill.u.s.tration: FIG. 114.--Semi-Portable Engine, 1878.]
The example here shown differs in its design from those usually seen in the market. The engine is not fastened to or upon the boiler, and is therefore not affected by expansion, nor are the bearings overheated by conduction or by ascending heat from the boiler. The fly-wheel is at the base, which arrangement secures steadiness at the high speed which is a requisite for economy of fuel. The boilers are of the upright tubular style, with internal fire-box, and are intended to be worked at 150 pounds pressure per inch. They are fitted with a baffle-plate and circulating-pipe, to prevent priming, and also with a fusible plug, which will melt and prevent the crown-sheet of the boiler burning, if the water gets low.
[Ill.u.s.tration: FIG. 115.--Semi-Portable Engine, 1878.]
Another ill.u.s.tration of this form of engine, as built in small sizes, is seen below. The peculiarity of this engine is, that the cylinder is placed in the top of the boiler, which is upright. By this arrangement the engine is constantly drawing from the boiler the hottest and driest steam, and there is thus no liability of serious loss by condensation, which is rapid, even in a short pipe, when the engine is separate from the boiler.
The engine ill.u.s.trated is rated at 10 horse-power, and makers are always expected to guarantee their machines to work up to the rated power. The cylinder is 7 by 7 inches, and the main shaft is directly over it. On this shaft are three eccentrics, one working the pump, one moving the valves, and the third one operating the cut-off. The driving-pulley is 20 inches in diameter, and the balance-wheel 30 inches. The boiler has 15 1-1/4-inch flues. It is furnished with a heater in its lower portion. The boiler of this engine is tested up to 200 pounds, and is calculated to carry 100 pounds working pressure, though that is not necessary to develop the full power of the engine.
The compactness of the whole machine is exceptional. It can be set up in a s.p.a.ce 5 feet square and 8 feet high. The weight of the 10 horse-power engine is 1,540 pounds, and of the whole machine 4,890 pounds, boxed for shipment. Every part of the mechanism usually fits and works with the exactness of a gun-lock, as each piece is carefully made to gauge.
Portable engines are those which are especially intended to be moved conveniently from place to place. The engine is usually attached to the boiler, and the feed-pump is generally attached to the engine. The whole machine is carried on wheels, and is moved from one place to another, usually by horses, but sometimes by its own engine, which is coupled by an engaging and disengaging apparatus to the rear-wheels.
English builders have usually excelled in the construction of this cla.s.s of steam-engine, although it is probable that the best American engines are fully equal to them in design, material, and construction.
The later work of the best-known English builders has given economical results that have surprised engineers. The annual "shows" of the Royal Agricultural Society have elicited good evidence of skill in management as well as of excellence of design and construction. Some little portable engines have exhibited an economical efficiency superior to that of the largest marine engines of any but the compound type, and even closely competing with that form. The causes of this remarkable economy are readily learned by an inspection of these engines, and by observation of the method of managing them at the test-trial. The engines are usually very carefully designed. The cylinders are nicely proportioned to their work, and their pistons travel at high speed. Their valve-gear consists usually of a plain slide-valve, supplemented by a separate expansion-slide, driven by an independent eccentric, and capable of considerable variation in the point of cut-off. This form of expansion-gear is very effective--almost as much so as a drop cut-off--at the usual grade of expansion, which is not far from four times. The governor is usually attached to a throttle-valve in the steam-pipe, an arrangement which is not the best possible under variable loads, but which produces no serious loss of efficiency when the engine is driven, as at compet.i.tive trials, under the very uniform load of a p.r.o.ny strap-brake and at very nearly the maximum capacity of the machine. The most successful engines have had steam-jacketed cylinders--always an essential to maximum economy--with high steam and a considerable expansion. The boilers are strongly made, and are, as are also all other heated surfaces, carefully clothed with non-conducting material, and well lagged over all. The details are carefully proportioned, the rods and frames are strong and well secured together, and the bearings have large rubbing-surfaces. The connecting-rods are long and easy-working, and every part is capable of doing its work without straining and with the least friction.
In handling the engines at the compet.i.tive trial, most experienced and skillful drivers are selected. The difference between the performances of the same engine in different hands has been found to amount to from 10 to 15 per cent., even where the compet.i.tors were both considered exceptionally skillful men. In manipulating the engine, the fires are attended to with the utmost care; coal is thrown upon them at regular and frequent intervals, and a uniform depth of fuel and a perfectly clean fire are secured. The sides and corners of the fire are looked after with especial care. The fire-doors are kept open the least possible time; not a square inch of grate-surface is left unutilized, and every pound of coal gives out its maximum of calorific power, and in precisely the place where it is needed. Feed-water is supplied as nearly as possible continuously, and with the utmost regularity. In some cases the engine-driver stands by his engine constantly, feeding the fire with coal in handfuls, and supplying the water to the heater by hand by means of a cup. Heaters are invariably used in such cases.
The exhaust is contracted no more than is absolutely necessary for draught. The brake is watched carefully, lest irregularity of lubrication should cause oscillation of speed with the changing resistance. The load is made the maximum which the engine is designed to drive with economy. Thus all conditions are made as favorable as possible to economy, and they are preserved as invariable as the utmost care on the part of the attendant can make them.
These trials are usually of only three or five hours" duration, and thus terminate before it becomes necessary to clean fires. The following are results obtained at the trial of engines which took place in July, 1870, at the Oxford Agricultural Fair:
KEY: A: Number.
B: Diameter.
C: Stroke.
D: Nominal.
E: Dynamometric.
F: Point of cut off.
G: Revolutions per minute.
H: Pounds coal per horse-power per hour.
---------------+-------------+-----+--------------+------+------+---- MAKERS" NAME | CYLINDERS. | | HORSE-POWER. | | | AND +-----+-------+ +-------+------+ | | RESIDENCE. | A | B | C | D | E | F | G | H ---------------+-----+-------+-----+-------+------+------+------+---- | |Inches.| In. | | | | | Clayton, | | | | | | | | Shuttleworth | 1 | 7 | 12 | 4 | 4.42 | ... |121.65|3.73 & Co., Lincoln | | | | | | | | | | | | | | | | Brown & May, | | | | | | | | Devizes | 1 | 7-3/16| 12 | 4 | 4.19 | 11.48|125.65|4.44 | | | | | | | | Reading Iron- | | | | | | | | Works Company, | 1 | 5-3/4 | 14 | 4 | 4.16 | ... |145.7 |4.65 Reading | | | | | | | | ---------------+-----+-------+-----+-------+------+------+------+----