The most familiar form of boiler is, perhaps, the cylindrical tubular boiler. It consists of a cylindrical shell closed at the ends by circular plat es. and has it number of tubes extending lengthwise of the boiler and terminating in the end plates. These tubes are open at both ends and through them pass the gases of combustion. The grate or furnace is placed underneath the front end of the boiler-shell, and from it the gases pass back under the shell to its rear end. thence up and forward through the tubes to the front end. and thence out of the ehhuney. In this passage the hot gases give up a portion of their heat to the water surrounding the tubes inside of the shell. Technically classified, this boiler is a horizontal, externally tired, fire-tube shell boiler. Another form of horizontal shell boiler frequently used has the numerous tire tubes replaced by two large cylindrical flues. This type boiler is less (llieient than the tubular boiler. but its greater simplicity and its aeeessihility for cleaning give it advantages, especially where the feed-water is bad. The fine boiler has several modifications which are known by special names. The 'Lancashire boiler has two dues which are crossed at. by conieal-shaPed tidies whose purpose is to provide an internal braving for the flues and to add to the heating surfa•e. In this boiler the grates are placed inside the flues at the front: or, teehnically classified, it is an internally tired, horizontal, double-tine. shell boiler. The ftallo •ay boiler has two flues at its front end like the Lancashire boiler, but a short distance hack of the grate these tines converge into one large thus which is crossed by •aterdubes like those used in the Lancashire boiler. The Cornish boiler has one large instead of two smaller flues. Both the (1illt)•ay and Cornish boilers arc' internally tired.
Vertical boilers consist of a eylindrieal shell set on end. with the grate at the bottom and fire-tubes running vertically. In large stationary boilers of this type the boiler is mounted on a masonry foundation, but in small portable boil ers. such as are commonly used for hoisting engines, the boiler is sot on a east-iron base which contains the grate and ash-pit complete. Some of the larger stationary vertical boilers built at present are front 20 feet to :10 feet high. Fire-engine boilers are always of the vertical type. See FutE-Exwx ES.
The Scotch marine boiler is a cylindrical shell boiler, but little, if any, longer than its diame ter, resembling a large drum in shape. It is set. horizontal. In the lower half of the boiler there are from two to fotr tines which termi nate in a combustion chamber at the rear, :Ind in the part there are a large number of tire-tubes terminating in the combustion chamber and in the uptake to the smokestack at the trout. Double-ended boilers of this type have furnaces at each end. and resemble two single ended boilers placed back to hack. Sometimes a common combustion chamber is used for both sets of furnaces in a double-ended Scotch boiler, but it is considered to be better practice to have a separate combustion chamber for each set of furnaces. In recent years. as has previously
been stated, water-tube boilers of various forms have been substituted for Seidel) boilers, particu la•ly for naval vessels. The locomotive holler consists essentially of a rectangular fire-box and a cylindri•al barrel through which numerous fire-tubes extend front the fire-box to the smoke box. The latter forms a continuation of the barrel from which the products of combustion pass- tip the stnokest a ek.
All the boilers so far described have been of the fire-tube type—that is, the tubes and dues passing through the boiler-shell have been for the conveyance of hot gases from the furnace. The idea of using tubes and flues is to provide a greater heating surface, and thus use the heat from the fuel more thoroughly and generate steam more rapidly. 'l'o veonomize heat and increase the rapidity of raising steam still fur ther, the •ater-tube boiler lens been developed, and is now being extensively used. In water-tube boilers there are two or more shells or drums connected by tubes of small diameter. These drums and tubes are tilled with water and the hot gases from the furnace pass around and between the tubes. and so heat the water con tained within them. A great variety of water tube boiler: are used. some having only one drum, and others having as many as four drums. The tubes connecting these drums vary from perfectly straight tubes to tubes bent to various degrees of curvature, approaching in some boilers a complete spiral. Water-tube boilers may be made light, powerful, and compact, and are well adapted for forced draught; steam may be raised rapidly from cold water or a sudden demand for increased pressure quickly met. For these rea son., particularly, they are largely used on war vessels. especially of the torpedo-boat and tor pedo-boat-destroyer classes, though most vessels in the merchant marine continue to employ the Scotch boiler. The water-tube boiler is also ex tensively employed in large power-generating plants on land.
Boiler-settings are the supports of masonry or metal upon which the boiler rests and by which it is oftentimes wholly or partly inclosed. Large stationary boilers usually have masonry settings, while portable and semi-portable boilers have settings of cast-iron. Locomotive boilers can hardly he said to have settings unless the locomotive frame upon which they are carried may be said to be one. Marine boilers are seated on plates and castings, which are built into the bottom framing of the ship. In exter nally tired boilers the setting usually con tains the furnace. (See Boiler fittings or accessories comprise steam-pipes and blow-of7 pipes, safety. cheek, and gate valves for various purposes (see VALVES) ; dampers and regulators; feed pumps and feed-water heaters; steam-traps and steam-separator:, and steam gauges and water-glasses, besides a number of minor contrivanees for one purpose or another in the operation of the boiler.