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.
Fig. 110.
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 shell, 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 shell 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-passage.
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 class 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.