BOILER, in steam engineering, a closed vessel for the generation of steam under pres sure. In days when steam pressures did not exceed a few pounds to the square inch, many forms of boiler were used, that are now out of the on account of the intrinsic weakness of their forms. At the present time, when steam pressures are often carried as high as 150 or 250 pounds to the square inch, the strictest attention must be paid to every trifling detail of design and construction, in order to ensure the safety of the structure. The fanci ful shapes that prevailed in the days of 'Watt and other early steam engineers have perforce disappeared, and given place to a limited num ber of standard types that have been found to be capable of withstanding the severe condi tions of modern practice. The types at present in use may be divided into two general classes, according as they are °internally fired" or °ex ternally fired)); that is, according as the fire which furnishes the energy for the formation of steam is contained within the general con tour of the boiler, or is situated externally to it. Internally fired boilers are the rule in England, but a large majority of the boilers in use in the United States are fired externally.
Internally Fired Boilers.— The Cornish and Lancashire boilers are the commonest in ternally fired types. Each consists of a cylin drical shell with flat ends or °heads." In the Cornish type the boiler is traversed from end to end by a large flue, which is often corru gated, to increase its strength. The fire is built within this flue, upon a suitable grate at one end of the boiler; and the gaseous products of combustion, after passing through the flue, are returned along the outside of the shell, so as to give up still more of their heat to the water in the boiler. The large flue in the Cornish type is an element of weakness, since the tendency of a flue to collapse through the action of an external pressure increases very rapidly with the diameter of the flue. To guard against collapse, the long flue is often provided with strengthening rings, which are riveted to it externally at short intervals. The Lan
cashire boiler differs from the Cornish type chiefly in having two comparatively small flues in the place of a single large one. Such a con struction is intrinsically stronger, and since there is a fire in each of the flues, the fuel can be replenished, and the fires cleaned, alter nately. This implies a greater steadiness of pressure, and less strain upon the boiler from the chilling action of the comparatively cold air that enters and strikes against the heated flue-walls when the fire doors are opened. The Cornish boiler is cheaper to build, and the Lancashire boiler is harder to fire, owing to the smaller size of its flues. The Galloway boiler does not differ in any essential particular from the Cornish or Lancashire types, except that its flues are crossed by conical-shaped water tubes, which serve the double purpose of increasing the heating surface, and of stif fening the flues that they traverse. The conical shape is adopted for the cross-tubes chiefly on account of the ease with which tubes of this form can be put in position, by passing the flange of the smaller end through the opening to which the larger end is to be riveted.
The Scotch, or cylindrical marine boiler, shown in Fig. 1, is a very common type in marine practice. It contains several furnaces (three in the illustration), which are usually corrugated. These furnace-flues do not pass through the entire length of the boiler, as in the Cornish and Lancashire types, but each is connected, within the boiler, to a separate "combustion chamber." The products of com bustion pass from the furnace back into the combustion chamber, and then return to the front end of the boiler through banks of small tubes which occupy the water space of the boiler, above the furnace. A "breeching" (or hood) of sheet steel, secured to the front of the boiler, then receives them, and conducts them to the stack.