The steam engine, like everything else which man has devised, was a growth, and, singular as it may seem, the boiler, that vital part of the organism, was, really, the last to receive due consideration and improvement.
As the boiler is depended upon to produce the steam pressure, and since the pressure depends on the rapid and economical evaporation of water, the importance of the subject will be understood in treating of the steam engine.
Water as an Absorbent of Heat.—Water has the capacity to absorb a greater amount of heat than any other substance. A pewter pot, which melts at 500 degrees, will resist 2000 degrees of heat if it is filled with water, since the latter absorbs the heat so rapidly that the temperature of the metal is kept near the boiling point of water, which is 212 degrees.
Notwithstanding the great heat-absorbing qualities of water, a large portion of the heat of the fuel passes through the flues and escapes from the stack. This fact has caused inventors to devise various forms of boilers, the object being to present as large an area of water as possible to the heat of the burning fuel. How that was accomplished we shall try to make plain.
Classification of Boilers.—Numerous types of boilers have been devised, the object being, in all cases to evaporate the largest amount of water with the minimum quantity of fuel. All boilers may be put under two general heads, namely, those which contain a large quantity of water, and those which are intended to carry only a small charge.
In the first division the boilers are designed to carry a comparatively small pressure, and in the latter high pressures are available.
Mode of Applying Heat.—The most important thing to fully understand is the manner in which heat is applied to the boiler, and the different types which have been adapted to meet this requirement.
The Cylindrical Boiler.—The most primitive type of boiler is a plain cylindrical shell A, shown in [Fig. 3], in which the furnace B is placed below, so that the surface of the water in contact with the fire area is exceedingly limited.
