As heretofore stated, this type of boiler is designed to carry only a small quantity of water, so that it is necessary to have practically a constant inflow of feed water, and to economize in this respect the exhaust of the steam engine is used to initially heat up the water, and thus, in a measure, start the water well on its way to the evaporation point before it reaches the boiler.
Various Boiler Types.—The different uses have brought forth many kinds of boilers, in order to adapt them for some particular need. It would be needless to illustrate them, but to show the diversity of structures, we may refer to some of them by their characteristics.
Compound Steam-Boiler.—This is a battery of boilers having their steam and water spaces connected, and acting together to supply steam to a heating apparatus or a steam engine. These are also made by combining two or more boilers and using them as a feed water heater or a superheater, for facilitating the production of steam, or to be used for superheating steam.
The terms feed water heater and super heater are explained in chapter III.
Locomotive Steam-Boiler.—This is a tubular boiler which has a contained furnace and ash pit, and in which the gases of combustion pass from the furnace directly into the horizontal interior tubes, and after passing through the tubes are conveyed directly into the smoke box at the opposite ends of the tubes. The name is derived from the use of such boilers on locomotive engines, but it is typical in its application to all boilers having the construction described, and used for generating steam.
Vertical Steam-Boiler.—This is a form of construction in which the shell, or both the shell and the tubes, are vertical, and the tubes themselves may be used to convey the products of combustion, or serve as the means for conveying water through them, as in the well known water tube type.
This form of boiler is frequently used to good advantage where it is desired to utilize ground space, and where there is sufficient head room. Properly constructed, it is economical as a steam generator.