THE GROWTH OF THE STEAM BOILER.
After the first crude forms, such as that used in connection with the Baranca and Newcomen engine, and numerous others, the steam boiler which came into very general use was the plain cylinder boiler. An illustration is given of this in figures [14] and [15].
It consists of a cylinder A, formed of iron plate with hemispherical ends B. B. set horizontally in brick work C. The lower part of this cylinder contains the water, the upper part the steam. The furnace D is outside the cylinder, being beneath one end; it consists simply of grate bars e e set in the brick work at a convenient distance below the bottom of the boiler.
Fig. 14.
Fig. 15.
The sides and front of the furnace are walls of brick work, which, being continued upwards support the end of the cylinder. The fuel is thrown on the bars through the door which is set in the front brick work. The air enters between the grate bars from below. The portion below the bars is called the ash pit. The flame and hot gases, when formed, first strike on the bottom of the boiler, and are then carried forward by the draft, to the so-called bridge wall o, which is a projecting piece of brick work which contracts the area of the flue n and forces all the products of combustion to keep close to the bottom of the boiler.
Thence the gases pass along the flue n, and return part one side of the cylinder in the flue m ([fig. 15]) and back again by the other side flue m to the far end of the boiler, whence they escape up the chimney. This latter is provided with a door or damper p, which can be closed or opened at will, so as to regulate the draught.