BOILER

the name applied to any ve el or dron for boiling large quantities of liquor, but commonly used as the designation of a metallic ve el, in which water is converted into steam by the action of fire, the steam being intended by its expansive force to give motion to the piston of a steam-engine, or to be used for a variety of manufacturing purposes. A very common construction of boiler is that which oonsiBts of an outward shell inclosing the furnace, combustion chamber, boiler tubes or flues, smokebox, ., and having the steam-chest placed on top of it. Boilers with a large number of these t^nal tubes are called multi tubular. It is not lutely nece ary that the boiler should inclose furnace and its connected tubes; but as it ii of utmost importance to obtain the largest extent heating surface with the smallest expenditure of this is one of the most common forms of the boiler, and is the only one used for marine engines. shell of the boUer usually consists of iron plates etted together, sometimes of sheet copper, and in cent times occasionally of steel plat^. Cylindrical or spherical boilers, or parts of boilers, are the strongest naturally, and parts that deviate considerably from these diapes require to be strengthened by or struts. About f ths of an inch is the thickne usually given to iron boiler-plates of the low-pre ure description; but the flat ends of cylindrical boilers must be made one-half thicker than the body. Steel boilers, however, do not require to be of so great thickne , the tenacity of steel being 1'6 that of The furnace or fire-box po e es a prate, or grated bottom, consisting of wrought-iron bars placed a short distance apart, and supported with suitable bearers. — ^The fire-door closes the opening through which the furnace is fed with fuel, and is pierced with a number of small holes, for the admi ion air above the fuel to a ieit combustion. — ^The plate is fitted to the furnace front, and is mainly for supporting the bars. In land boilers this plate G21 elemade of great breadth, the fresh coal is laid on it, and pushed forward as the fuel is consumed. — ^The ath-pU in is immediately under the fire-grate; the front ia fitted with hinge-doors to regulate the admi ion of air underneath the grate, and to act as a damper when all required. — The bridge is a vertical wall placed at the back of the furnace bars, over which the flame pa es on its way to the chinmey. — ^The jlame'Chamber, behind the bridge, the flues, and tubes pa iog through the boiler, are traversed by the fiiune and heated and gases from the furnace, and thus the water in the boiler is exposed to a large extent of heating surface. of — The $mol'€'box is intended to receive the waste heat which is taken advantage of in superheating the steam. — ^The tubes pa ing through the boiler have their ends fitted into the tube-plaUtf which are placed at the front and back of the boiler; the holes for the of tubes are accurately set out and drilled; the tubes are fitted tightly, and are then expanded with a tool for the purpose. — ^The manhole ia an opening placed at the most convenient part of the boiler, intended most, to admit a man into the interior of the boiler for inspection and cleaning. The boiler is supplied with water by means of a pump worked by tiie engine; there is likewise a steam pump provided for filling the boilers when the engine is not working. In locomotives and in some into other engines the pump is dispensed with, and the injector is used to throw the water into the boiler. St. There is, however, this disadvantage attending the use of the injector, viz. that the water supplied by oom it is cold, wlule the steam pump can throw water in into the boiler at nearly the boiling-point, thus effect all ing a saving in fuel/— The steam-pipe conducts the steam from the boiler to the engine, and is fitted caul with a valve, called the throttle-veUve, for regulating most by hand the supply of the steam. Biefore the steam is conducted to the engine it pa es into the iteamchetl, where it deposits any watery particles it may have taken up. — ^There are generally one or two Kifety-valveB fitted to each boiler, the use of which is to allow the steam to escape when the pre ure becomes too great. In marine boilers these are circular valves, held down by weights suspended from them inside, and having a lever and shaft, with handle outside the boiler for lifting the valves by hand. the in- For stationary land boilers the safety-valve is held absodown by means of a loaded lever, with a weight that the can be adjusted at pleasure; while for locomotive the boilers a spring and lever is used, with thumb-screw of for compre ing or releasing the spring when the full fuel, pre ure is required, or when the steam requires to be blown off. — ^The vacuum-valve is a valve fitted to The flat-shaped boilers; it opens inwards to admit the rivair, and prevent the po ibility of the boiler collapsreing, in case the pre ure of the steam within should fall below the pre ure of the atmosphere without. The presaure-gavge indicates the difference between the pre ure of the steam in the boiler and the atties mospheric pre ure. The gauge most generally used is Bourdon's. It consists e entially of a curved, flattened tube, into which the steam is admitted, and which becomes more or le curved, according as the pre ure is le or greater. — ^The vxUer-gauge shows a whether the water in the boiler is kept high enough. For this purpose there is generally a cast-iron pipe iron. projecting from the front of the boiler, extending very from the steam space to about the crown of the furnace it; is fitted with a strong vertical gla -tube, communicating by means of plug-taps and other connections with the water in the boiler, and thus of clearly showing its height. — Marine boilers are supdead plied with scum and blow-off cocks. The former is used connected with an internal pipe, having a trumpet is mouthpiece for oollectiDg the scum and froth, and with an external pipe for blowing it overboaid; while the blow-off cock aJlowv the brine to escape through the bottom of the ship, which is fitted with a valve termed the Kingston valve. — ^The doihrng of a boiler is an outer casing of some non-conducting substance, to prevent the radiation of heat from the boiler. One of the substances most commonly used is felt, which is covered with lagging, or narrow strips of wood, secured to the boiler with screws, while over all there is laid a casing of lead to prevent moisture affecting the plates. I^d boilers are protected with a covering of fire-brick, felt, or other non-conductor, and in some instances sand is used with advantage. There are a variety of boilers now used for land engines. Plain cylindrical boilers vnth hemispherical ends are the most common, and far outnumber any other form in use. Their construction renders them very strong, as the whole of the iron is subjected to tensional streos. The boiler rests on a structure of brick-work, having the furnace underneath, with a return flue all round, which is lined with fire-bricks^ so as to resist the action of the flame. The Cornish boiler differs materially from the foregoing, having both ends quite flat, with one or more internal flues running through and through, the furnaces being placed in the flues, with return flues all round. Others have two furnaces meeting in one combustion-chamber, which allows of alternate firing, a plan well calculated for the prevention of smoke nuisance. The internal flues are usually stayed with a number of conical stay tubes, with the water freely pa ing through them. The flame warps around these tubes, and is thus broken up and its pa age delayed, so that the gases are thoroughly mixed, and, along with the smoke, are fully utiUzed for heating purposes. All these boilets require to be bedded in brick-work, and surrounded with flues built of the same material; there are, however, many horizontal seU-contained boilers used for land purposes, chiefly consisting of the locomotive da , having no outward flues. The locomotive boiler has an inside fire-box made of copper, which is surrounded with an outside sheU of wrought iron, leaving a water space all round. This is dosed at the bottom with a bar of wrought iron, or a flanged plate, which unites the inside and outside shells together; the bottom of the inside firebox being of course quite open for the reception of the fire-lmrB. As the water space round the fire-box is in free communication with the other parts of the boiler, the sides and top are subjected to the full steam pre ure, which tends to collapse the inside box, ami burst the outside shell; consequently the flat sides require to be securely stayed, as likewise the top of Uie inside fire-box. The body of the boiler is of a cylindrical form, securely rivetted to the fire-box at one end, having a tube-plate at the Hmoke-box end; between this tube-plate and the firebox small bnuu tubes are arranged, honey-combing, as it were, the water space in the body of the boiler, and giving hundreds of square feet of heating surface. By this means the flame and heated gases pa from Uie fire-box through the tubes, and thus supply heat to the water in the boiler, while the waste heat pa es into the smoke-box, and then up the chimney. A draught is created in the chimney by means of the waste steam from the cylinders, which rushes into it through a pipe, and so creates a partial vacuum. To effect the complete combustion of the coal in the locomotive boiler, various arrangements are in uae. Among these, currents of air, inducted by means of steam, and entering the fire-box above the coal, have been tried with succe . The air i sun ducted through stay-tubes placed on the ■ides of the fire-box, between iiie inaide and outside shells; a pipe fitted with small nooIeB is arranged in connection with the stay-tubes, these nozzles being just so many miniature steam blast-pipes, and acting in the same way as the main blast-pipe placed in the chimney. Each by creating a partial vacuum, induces a current of air to pa into the fire-box, and thus supplies the oxygen of the atmosphere, so nece ary to ^ect comfdete combustion. Other arrangements aim at delaying the flame in the fire-box. With that object in view, the fire-box is made longer, projecting into the bo^ of the boiler, and has a separate combustion-chamber in front of it. The flame Mid heated gases pa through tubes, or apertures formed with fire-brick tiles, pLhoed between the firo-box and combustion-chamber; and these fire-brick tiles having the property of retaining the heat when the fire is bright, and giving it off when the fire is dull, or in the act ot firing, thus serve as equalizers of temperature. In some cases air is admitted through stay tubes in the front of the fire-box, which flows in with considerable force when the engine is running; while nearly all the fire-doors in the locomotive boiler are pierced with a number of small holes through which the air flows in, and is then deflected on the top of the fuel with a baffle-plate. Another form of boiler extensively used for general purposes is the vertical type, arranged with tubes, flues, pots, These boilers are of a cylindrical form and quite self-contained, and as such they are very convenient where space ia an object, and, moreover, they are very strong and capable of bearing great pre ure. Watei>tube vertical boilers have met with much favour. A series of small tubes, hermetically sealed at bottom, are fitted to the tube plate, pa ing downwards into the fire-box, and having an internal tube in each. By this arrangement only a thin film of water is exposed to the heating surface. As the steam globules are formed, they ascend, displacing the water in the annular space; while the water at the top descends through the internal tube and fills up the cavity. Tlie chinmey in these boilers is placed at the centre; there is a large aperture at the centre of the top of the fire-box for the uptake leading to the chimney. This leaves a centre space which is not filled up with tubes, so it is nece ary to suspend a fire-brick lump from the top of the uptake. Thus the flame is distributed amongst the tubes, otherwise a great part of the heat would pa up the chimney to waste. Other boilers of this cla are not inclosed in a metallic shell, the tubes being much larger, and so connected as to form a series of boilers, the whole being encased with brick-work, furnace, flues, These tubes are capable of sustidning great pre ure, and it is considered they tend greatly to the prevention of serious boiler explosions, as in all probability one tube would give way before the othoa, thus relieving the boiler from pre ure. Flue-boilers, or boilers with large heating-tubes, for marine purposes, have become a thing of tbe past, although at times they are used for drei^ing-ve els, the feedwater being generally very muddy, and flue boilers having an advantage in being more easily cleansed from deposit. For ocean steam-ships a spedes of overhead flue-boiler has been extensively lued. The flues are quite narrow, formed of plate iron strengthened with stays a few indies ^>art. These stays act as heat-conductors to a considerable extent. The furnaces are placed underneath the flues, as in ordinary multi tubular arrangements, and are generally made quite round. The multi tubular-boiler is certainly the greatest improvement we have had in steam producers, and has conduced in a great measure to the succe of the mercantile marine, as well as having been of the utmost advantage to the royal xiavy. For tlie latter there are two claBses, high and low, the former having the tubes arranged over the fomaoeay the latter having tiiem azranged alongside, the top row being level with the crown of the fomaces. Tida arrangement is adopted for veeselB of light draught gines, round boilers are now extensively used, with tubes on the return principle. The shell and furnaces are quite round, and are capable of sustaining, without stays, the great steam-pre ure now universally acknowledge^ aa the one thing needful effect a saving in fueL But all the flat parts these boilers require to be very strongly stayed, hi similar manner as the locomotive boiler. The take, or part above the boiler for conveying smoke into the chimney, is of a circular f onn, tached from the boiler. The steam is admitted inside, and is superheated in its pa age to cylinder of the ste«m-engine. 268