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FIRE ENGINE. The shell of the boiler, which
may have a great variety of forms, is the general Water.
or outer wall, enclosing a space, part of which, Alcobol.
as occupied by water, is the "water room, Ethir (sulphurie).. Bi-sulphuret of carbon
and the portion above this the “steam room.” Oil of turpentine..
The furnace is the chamber which receives the Chloroforma.
fuel; when within the shell, this is called the For accurate estimates, the specific heats must fire box. The grate or fire grate is that part also be regarded; these by latest results are, of the bottom of the furnace on which lies the for the vapors named in order, .305, .451, .481, fuel; it is constructed of "fire bars” or “grate .157, .506, and .156, that of an equal weight bars," with spaces for admission of air. The of water being 1. Thus far, however, engines part consisting of a plate, without such opendriven by vapors other than steam, and also ings, is the “ dead plate.” The chimney affords compound-vapor engines, have proved too draught and conveys away the final products of expensive through waste of the material, and combustion; the flues or tubes for flame somehave not attained to a decided success. For times open into a chamber at its lower part, the application of hot air as a motor, see called the smoke box or uptake. The chimney, ATMOSPHERIC ENGINE. VI. The Production of flues, or tubes may be controlled by dampers. Steam. The capacity and principle of con- The“ tube plates” receive and are pierced by struction of the boiler or generator of steam the ends of the tubes; they form either part must be determined generally with a view to of the shell, or one side of an internal fire box. the strength of the materials and the laws of The steam chest or dome, upon the upper side combustion and of heat, and especially with of the boiler, is the reservoir for the generated reference to the specific purpose for which the steam, which, as the various valves may allow, engine is intended. In boilers for marine or is supplied from it to the cylinder; or when its locomotive purposes, rapid generation, secu- pressure exceeds the load upon the safety valve, rity, lightness, and compactness should be at- directly escapes through this into the air. The tained, and if needs be at the cost of some waste height of water in the boiler is known by the of fuel ; in those for stationary engines econ- water gauge, usually a strong glass tube standomy is the paramount object. Boilers before ing outside the boiler; or by use of “trythe time of Watt were usually in shape of an cocks," three or more at different heights upon inverted frustum of a cone, with a hemispher- the boiler, and opened by hand. A pressure ical top and slightly concave bottom, and were gauge shows the pressure within the boiler. A set in an arch of brick-work, the fire being ad- vacuum valve is a safety valve opening inward, mitted to the bottom only, or also about the to prevent collapse when the pressure within sides. To make a boiler stronger for its ca- falls below that of the atmosphere. * Mud pacity and with greater heating surface, Watt holes," at or near the bottom, are for the disintroduced the long rectangular form, with charge, when needful, of collections of sedisemi-cylindrical top, the ends flat, the bottom ment; but it is very important, especially in and sides slightly concave, and set in a long marine boilers where sea water is used, to arch; this was called the wagon boiler. The prevent such collections; and for this purpose next transition was to the cylindrical boiler sediment collectors, and various means for prewith hemispherical ends, as giving greater vention of incrustation by previously precipistrength and heating surface. Subsequently tating the saline matters from the water, are a single straight flue, a single flue bent and re- employed. The apparatus for feeding the turning, or two or more flues, were carried boiler with water, especially when the engine through the interior of the boiler; and the fire is at rest, may require to be driven by a sepabeing made directly in these, or the heated air rate and smaller engine; this is then called of the furnace being made to circulate through the “donkey engine.” Feed-water heating, them while the water surrounded the fues, a previously to admission of the supply to the much increased utilization of the heat of com- boiler, may be accomplished in various ways; bastion was the result. From boilers with among them by carrying the water pipe through fues to those traversed by a large number of or about the flues or chimney. Brine pumps small tubes, in which the flame and heated air may be used for clearing the boiler of brine directly from the furnace shall circulate on its or sediment, or a large blow-off cock through way to the chimney, thus exposing to the fire the bottom of the boiler. Scum is removed by & maximum of water surface, the transition a scum cock or surface-blow at the surface of vas a natural one; this form, known as the the water. The importance of keeping the moltitubular or tubular, is best illustrated in water in the boiler always as near as possible the locomotive boiler. The name tubular boil- to a certain height, so as to prevent liabilities er is more correctly given to those boilers in on the one hand of priming, and on the other which the tubes contain water, being sur- of burning or melting its sides or flues, or of rounded by the flame. Flue and tubular boilers explosion, will be very obvious; and a variety are those now most generally in use; they are of devices, some of them already named, have either horizontal or upright. For a description been applied in order to aid and direct the enof Messrs. Lee and Larned's annular boiler, see gineer in this respect. Among the most recent,
and perhaps for safety the most valuable of for some time, the body of water may assume such devices, are those known as “steam the spheroidal condition (see EVAPORATION), alarms,” which are automatic, being caused to and a film of vapor lying between it and the open a safety valve and blow a whistle or ring metal, the latter can become unusually heated; a bell when the water falls below a given level on feeding cooler water or disturbing the fires, in the boiler. (See further, Boiling Point, contact with the metal may be renewed, with COMBUSTION, EVAPORATION, and Fuel.) The sudden generation of steam of extremely high questions of the efficiency of steam boilers, the pressure; 2, by long boiling, without renewal, due proportioning of the fire grate, the flues, the water may become so far freed of air or the heating surface, &c., to the capacity of the other permanent gases that serve to form openboiler and of the engine, and the duty of en- ings within its mass into which vaporization gines, are among those of great interest in the can begin, that its boiling point shall be raised, practical and commercial aspects of the subject, and when reached, a considerable body of the but for which space can only be allowed in the water, as has been observed in the experiments larger and specific treatises. It may be stated of Donny and others, shall burst suddenly into that in large land engines it is customary to steam; 3, superheating may have taken place allow for each horse power of pressure about unsuspected within the dome, and when by 1 square foot of fire surface, 3 cubic feet of fresh feed or rousing the fires the equilibrium furnace room, 10 cubic feet of water, and 10 within the boiler is disturbed, water being cubic feet of steam room. To vaporize the dashed up into the body of superheated steam, water, it is found in England, where coal is or otherwise, the sudden transfer of heat to more cheaply obtained, and economy in its the generation of saturated steam may inuse not so much an object, that from 10 down stantly and very greatly increase the presto 8 or 7 lbs. of coal per cubic foot of water The last named cause could occur at are consumed; in this country, in cases of any time during the running of an engine, good performance, the like effect is often ob- if the steam became superheated and the boiltained with about 6 lbs. of anthracite coal. ing afterward more tumultuous; and any of VII. Steam Boiler Explosions. These accidents these three causes can readily be supposed to are of different sorts; sometimes the metal is occasion some of the explosions of stationary simply rent, steam or water or both escaping; and steamboat boilers, known so often to occur sometimes the boiler or a flue is burst inward, upon first disturbing the water or fire after a a result known as collapse;" but in explo- period of rest of the engine. In all three of sions proper, the boiler is not only ruptured them, moreover, the production of an enorand often thrown from its place, but fragments mously great steam pressure may be almost inof it are usually hurled with terrible force stantaneous; so that the well known principle from the spot, accompanied with escape of of blow or shock would come into play, the steam and scalding water. Of certain peculiar tenacity of the boiler giving way under the theories proposed to account for explosions, suddenness of the impulsion, before the limit one, to the effect that they are due to an elec- of strength of the metal had been reached. trical charge within the boiler, has no support Whatever be the occasion of explosions proper, whatever in facts, and is wholly visionary; a however, their immediate cause is always the second, assigning as the cause the occurrence same-a momentary excess of the pressure of of an explosive mixture of gases within the steam within the boiler over the strength of boiler-hydrogen and oxygen being the only some of its parts. In the best arranged modpossible gases, and their presence in the need- ern boilers, there are 2 or 3 safety valves, at ful quantities in any case being extremely least one of which is placed within the boiler, questionable—is probably not less so. A recent or otherwise beyond the control of the engiprojectile theory of explosions appears to add neer. An advantage of tubular and small-flue nothing to our knowledge of the accident; for boilers is the circulation of the water maintainthough projection of the steam and water does ed in them, which retards or prevents formaoccur, it is the effect of the steam pressure, tion of scale. Another, with water tubes, is and allowed by relative weakness of the boiler. the greatly increased strength to be had even M. Jobard (1861) has, however, shown that with less thickness of material, owing to the many explosions of stationary boilers are owing relatively small diameter of the tubes; and first of all to explosive combination of a sort further, since the same tube resists rupture of fire-damp of hydrogen and air, collecting in from within up to pressures exceeding those the chimney or flues during rest of the engine, that would cause flattening or collapse from and ignited on subsequently disturbing the fires, without, it follows that, with like size and without previous opening of the dampers; the thickness of tubes, water-tube are stronger and first effect is to throw the boiler out of place, safer than flue-tube boilers. The common imand the flow of the water within it over its pression respecting the greater danger of high upper heated surfaces generates a pressure that pressure than of low pressure boilers, requires next rends the boiler itself. Other causes or some qualification. In itself, and in one way, conditions which doubtless often lead really the high pressure is a source of increased risk; to explosions are the following: 1, in a boiler but the boilers generating such steam usually very highly heated, and without fresh feeding admit of being much smaller, and from this cir
camstance are relatively stronger; beside, they the efficient adhesion of smooth driving wheels contain (locomotive boilers excepted) á less had before been discovered ; and Stephenson's body of water and of steam to be projected combination of all these essentials, with his about them in case of explosion. Low pressure direct connection of the piston rods, one on boilers, on the other hand, have in them, when each side, with the propelling wheels, resulted working, a large body of steam and also of wa- in the first really successful locomotive, the ter; when explosion occurs, this water being Rocket, in the renowned “railway year,” 1829. suddenly released from the confining pressure, Timothy Hackworth contributed, before 1830, much of the excess above 212° of heat in both the six coupled wheels and the steam chamber the steam and water instantly goes to the over the boiler, placed the cylinders under the generation of a greatly enlarged volume of boiler, and made other improvements. The steam; and this large body of scalding steam size and weight of the locomotive have been and water is projected through a more con- since that time much increased, and a construcsiderable space about the place of the boiler. tion highly perfected in detail and well adaptThus, the actual destructiveness and fatality ed to the purposes desired has been attained. consequent on explosion of the low pressure Modern English passenger locomotives are of boiler are likely to be the greater, to say no- two general types: 1, the “inside cylinder" thing of the fact that, from some of the pecu- locomotive, having the cylinders within the liar conditions named above, even the projec- framing, under the boiler, with a main driving tile force given to the fragments and contents axle cranked at two points to receive the acof the latter can be, in certain instances, quite tion of the piston rods; 2,"outside cylinder" as great as with the former. Obviously, most locomotives, having the cylinders external to if not all the occasions of boiler explosions are the framing, the axles straight, and the piston aroidable, through, 1, avoiding the forcing of rods attaching each to a crank pin fixed bethe fires; 2, keeping the valves in proper con- tween spokes of a driving wheel, the pin turndition, and in no case over-weighting them; 3, ing with the wheel about its axle, and servsupplying the feed water regularly, constantly, ing as the engine crank. English freight or and in sufficient quantity; 4, in case the plates "goods” locomotives are also of two classes : are discovered, or where they are likely, to be 1, those with 6 wheels of like size, the 3 on over-heated, abstaining from the sudden intro- either side having their crank pins coupled by duction of feed water, drawing or extinguish- horizontal bars called parallel rods, which ing the fires, and blowing off the steam and secure the same crank action upon and movewater.—STEAM CARRIAGE. For information re- ment of all the wheels; 2, those having the specting the invention and early improvement fore wheels smaller, the two back pairs only of the locomotive engine, see RAILROAD, and being of like size and coupled. The power of incidentally Evans, STEPHENSON, and SteveNS. a locomotive, other things being equal, will deThe locomotive engine has so few points of re- pend on the amount of the steam pressure upon semblance to any other as to be essentially a the piston; but a condition favoring is found new application of the same moving power. also in the use of small driving wheels. As The use of steam at very high pressure and each revolution of the wheel to which the piswith rapid travel of piston, allows of a com- ton rod is attached must correspond to one paratively diminutive cylinder; while the high stroke of the piston, it follows that speed of steam and great speed sought require a very travel will depend on the speed of the piston, large boiler and intense fire, to generate such or number of strokes it can make per minute, steam with due rapidity. Intended, as its name but that it is also favored by use of large drivimports, for locomotion, this engine must carrying wheels. After many Auctuations, Amerwith it the water and fuel necessary to its ac- ican practice has recently tended to the adoption; and being subjected to violent strains tion of wheels of somewhat less diameter and shocks, it should, along with the requisite than those used a few years since. American conditions for furnishing the power, embody locomotives are almost invariably outsidegreat strength and compactness of construc- connected, i. e., have their cylinders outside tion; to secure the latter, the engine and boil- the truck or engine frame, as well as nearly er are mounted together upon one carriage, or quite on a level with the axles of the drivthrough the wheels of which the tractive power ing wheels. There are but two general types is to be applied. Evahs and others had placed of construction, those for passenger and for the boiler and engine on one set of wheels; Tre- freight trains. The former have 8 wheels, 4 vithick and Vivian (1802–'3) separated the trac- in front set in a movable frame, the bogie or tioa carriage, or locomotive proper, from those truck, turning on a central pivot to allow of intended to receive the load; and they first running on curves in the road, and 4 larger discharged the exhaust steam from the cylin- ones behind, the drivers, or driving wheels, of der into the chimney, to create draught. equal size, and coupled with parallel rods. George Stephenson, about 1825, applied this The freight locomotives are on 10 wheels, the principle much more successfully, perhaps re- leading 4 in a swivelling truck, and the 6 inventing it, and to him it is usually assigned. back wheels, 3 on a side, coupled as drivers. Seguin in France, and Booth in England, in American locomotives are distinguished also by 1829, furnished the multitubular flue boiler; the cab or house at the back end to protect the
engineer and fireman, with glazed opening in end, the fire must burn briskly, and the neated front to afford a view ahead, and by the larger gases escape rapidly through the flue tubes, size and form of the chimney or smoke stack, 100 to 200 or more in number, to the chimney. a cylinder of wire net within an inverted cone, To secure the required draught through these, with wire net and baffle-plate above to deflect the exhaust pipe from each valve chest is carsparks into the intervening space—an arrange- ried directly into the chimney, and the two ment specially adapted to the use of wood as are united in a pipe called the blast pipe, fuel, and called the spark-arrester. Between which opens at a little height by a contracted the cab and the chimney of a locomotive ap- mouth. The forcible and rapidly recurring pear, also above the boiler, the smaller dome puffs of the waste steam discharged from the in front surmounted with the steam whistle, cylinders out of this pipe into the chimney, the bell, and the dome proper. The “cow- carry with them the surrounding body of air, catcher" or the snow plough may be attached and create a partial vacuum, which can be forward, for the purpose of clearing the track; supplied only by the consequent rush of fresh and in front of the chimney is also affixed a air through the grate, the fire, and the flues. lamp with a parabolic reflector, for lighting Now, the contraction of the blast pipe needful the track in advance at night. The “tender," to secure blast unavoidably keeps back somewhich conveys water and fuel, is a smaller car- what the exhaust steam, and so results in a riage next the locomotive, and on 4, 6, or 8 continued back pressure against the piston, wheels, arranged in two trucks, to facilitate while advancing either way under the last adturning. Its back and sides are occupied by mitted steam charge; and when the speed of the water tank, in horse-shoe form, holding the locomotive, and so of the piston, becomes from 800 to 1,000 gallons of water, while in very great, this back pressure is correspondingthe hollow of this is stowed the fuel, both be- ly increased and rendered well nigh continuous. ing intended for some 15 to 30 or more miles To accomplish successfully by use of the slide
The water tank communicates through valve the required distribution of steam in the pipes, connected by flexible hosing with two cylinders (see Steam ENGINE, IV.) has been small lift-and-force pumps, worked from the perhaps the most difficult problem in locomopiston rod, and the delivery of which can be tive construction, and the one on which the regulated so as to feed continuously or at in- greatest amount of ingenuity has been extervals. From the upper part of the dome pended.. In the case of a question so highly the steam pipe opens out, descending and run- mathematical and technical as this, nothing ning forward within the upper part of the beyond an intimation of the objects sought boiler, thence emerging within the chimney, and of the means employed to attain them where it divides into two branches, that run can be presented; the reader will find the subdown and open each into the valve chest of ject fully discussed in Mr. D. K. Clark's “Railone of the cylinders, each chest lying at the way Machinery" (Glasgow and London, 1855). inner side of its cylinder. Sometimes the First, the valve should be so moved as to addome and commencement of the steam pipe mit and discharge steam when the piston is at are at the forward end of the boiler. In any or near the beginning or end of the half case, while working, the chests are kept filled strokes ; secondly, the valve gear should renwith steam which surrounds the valve—a sin- der a variable expansion, or ratio of cutting gle slide valve moved by one or two eccentrics off, practicable; and thirdly, the valve gear and rods from the driving axle—and the steam must be capable of reversing at will the action is thus continually in readiness to flow into on the piston, so as to reverse the movement either port when uncovered, and to act upon of the engine and carriage. Remembering that side of the piston corresponding. (See that the diameter of the crank circle equals STEAM ENGINE, II.) The steam pipe may have the length of stroke of the piston ; that when in it a throttle-valve, under control of the en- the crank pin and piston rod are in the gineer, through a lever handle and links. Thus horizontal diameter of the crank circle the the governor is rendered unnecessary, as it is piston is at beginning or end of stroke; that also inapplicable. From length of passages, when the crank pin is in the vertical posismallness of apertures, failure of the boiler to tions the piston is about midway in either half keep up the supply, or other causes, the pres- stroke ; and that in a general way admission sure the steam can exert at and on the piston and exhaust should begin to occur about the is always less than that it has within the boil- times when the piston begins or ends a half er; and the necessity and mode of obtaining stroke; it will be seen that, to accomplish the draught still further deduct from the effective first purpose, the valve should begin to open pressure. The double-cased sides of the fire either port to steam and the other to exhaust box, a space of 24 to 4 inches within which, about the time when the crank pin and rod are directly about the fire, is filled with water, in the horizontal positions. Now, rememberand the small flue tubes traversing the cylin- ing further that an eccentric turning on an drical portion of the boiler, conveying flame axle is essentially itself a crank, the distance and hot air toward the chimney, and sur- of the centre of its form from the centre of rounded with water, can generate steam rap- motion being the length of crank, and double idly and of very high pressure. But to this this distance being the diameter of this crank
circle, and also the length of the throw the objects above named is, with any method of revolution of the eccentrio will impart to its valve motion, accomplished by means of a rerod; that the valve moving either way from versing handle at the command of the enits iniddle position will be just beginning to gineer, which by lever and links is made to uncover the ports; and that it will be in such detach the eccentric rod or reverse its action position when the centre of form of the ec- on the valve, and consequently the direction of centric is in the vertical positions in respect to movement of the crank and of the wheels. This the axle, it will be seen that, with a single ec- is employed as an auxiliary means of arresting centric and rod, the first object is attained by the movement of the engine and train at high setting the eccentric so that its radius shall be speed, or for the purpose of backing the en" quartering" or at right angles with the line gine.' The second of the three objects named of the crank pin from the same axle. Thus, above is that which has presented the chief the general result secured is, that whenever difficulty. The outer edges of a slide valve dethe crank is in horizontal position, and the pis- termine the times of admission and of suppreston at beginning or end of stroke has its slow- sion; the inner, the times of release and compresest movement, the eccentric is vertical, and sion. Outside “lap" of valve conspires with the valve in middle of its throw has the most rapid movement, as required for duly opening
L the ports; and vice versa. An engine moving
R at high speed, say 38 miles an hour, or 1,093
R yards a minute, the driving wheels 5 ft. 3 in. in diameter, or about 16 ft. 6 in. in circumfer
b ence, the number of strokes of each piston must be about 200, and of separate or half
W strokes 400 per minute; if the length of each be 18 inches, this gives the piston an average
Vr velocity of 192 yards per minute, or 10 feet per
f second, i. e., more than three times the usual
FIG. 9. speed in stationary engines; thus it will appear "lead" to secure early admission of steam and how important is the proper timing of the steam early and efficient release ; inside lap defers ehanges within the cylinder, and how nice release and prolongs expansion ; while inside must be the adjustments required for economic clearance shortens both expansion and comprescal and perfect working. The third of the sion. Thus, these three elements, the lap, lead,
FIG. 10. and throw of the valve, control the distribution rod, which by a stud or pin within the link can of the steam; and any change in either affects be made to stand at different heights within the distribution in a definable way. Up to about it; Il is the link; e is the fore and e' the back 1843, a variety of complicated expansion gear eccentric, jointly giving movement to the link; for locomotive slide valves had been devised, their centre of motion is at o, their centres of for varying the travel or length of throw of form, c, 0; oc, o c are their radii; f is the fore the valve during stroke, and hence the expan- and b the back eccentric rod, the relative posision; and though the link motion then intro- tions appearing reversed in the two figures; 0 daced has since come into almost universal use, is the crank; o a its radius; ara a half revoyet such devices, with single or with complex lution of the crank; v is the slide valve; pp or cut-off slides, have continued since to be are the steam ports and E the exhaust, to the presented. Of the link motion, the simplest cylinder; n represents the reversing link, h the and most satisfactory solution of the problem, reversing lever; but the mode of reversing is the construction in one form of shifting link is better shown in fig. 9, in which k is the suprepresented in fig. 9, while the mode of action porting link, B a bent lever, with counterpoise will be better understood by aid of the dia- W; RR a rod from the lever B to the reversgram, fig. 10. In the figures, or is the valve ing lever or handle, L, at the hand of the en