It is obvious, however, that this form of boiler adds greatly to the area of the water surface contact, and in that particular is a great improvement.
The Water Tube Boiler.—In the early days of the development of boilers, the universal practice was to have the products of combustion pass through the flues or the tubes. But quick generation of steam, and high pressures, necessitated a new type. This was accomplished by connecting an upper, or steam drum, with a lower, or water drum, by a plurality of small tubes, and causing the burning fuel to surround these tubes, so that the water, in passing upwardly, would thus be subjected to the action of the fuel.
This form of boiler had two distinct advantages. First, an immense surface of water could be provided for; and, second, the water and steam drums could be made very small, diametrically, and thus permit of very high pressures.
In [Fig. 6], which is designed to show a well known type of this structure, A A, represent the water drums and B, the steam drum. The water drums are separated from each other, so as to provide for the grate bars C, and each water drum is connected with the steam drum by a plurality of tubes D.
It will thus be seen that a fire box, or combustion chamber, is formed between the two sets of tubes D, and to retain the heat, or confine it as closely as possible to the tubes, a jacket E is placed around the entire structure.
The ends of the water and steam drums are connected by means of tubes F, shown in side view, [Fig. 7], for the return or downward flow of the water. The diagrams are made as simple as possible, to show the principal features only. The structure illustrated has been modified in many ways, principally in simplifying the construction, and in providing means whereby the products of combustion may be brought into more intimate contact with the water during its passage through the structure.