STEAM BOILERS.
A steam-boiler is a mechanical arrangement in which heat, evolved by the combustion of fuel, is absorbed by a liquid (generally water), whose tension is thereby increased. When used in connection with a steam engine, the water is converted into steam, whose tension is used to pro duce mechanical motion by impelling a piston. Water is always con verted into steam in boilers used for motive power, but not always in boilers used for heating purposes; it is with the former only that we shall deal here. The word "generator" would be more appropriate than " boiler," as any pot or kettle may constitute a boiler.
ClassVicalion and are made of cast iron, wrought iron, and steel, and, for special purposes, small portions of them are made of sheet copper. They may be classified as spherical, cylindrical, egg shaped, wagon-top, haystack, etc. One of the earliest forms was spherical, the fire being underneath, and this was followed by a flat-bottomed, curved topped cylinder surrounded with mason-work flues. Boilers are generally cylindrical or are made up of cylinders and cylindrical tubes, although there are varieties principally composed of small spheres suitably joined. Other forms, as the wagon-top, haystack, etc., have been abandoned, the cylinder and the sphere being chosen for their simplicity of construction, and for the reason that those shapes best resist deformation from internal pressure.
Plain Cylindrical simplest type is the plain cylinder so set in brickwork as to receive the furnace-heat only upon its lower surfaces. A development of this is so set as to give the gases of combustion passage first under the cylindrical "shell" from front to back, then along one side from back! to front, then along the other side from front to back, where they are allowed to pass into the chimney or stack or into a flue leading thereto. In all these forms the boiler proper is a plain cylinder merely supplied with the necessary man-holes and hand-holes for inspection, cleaning, and repair, with 21n inlet for water and an outlet for steam, and with fittings and attachments designed to increase safety and con venience.
red development of the plain cylinder in the direction of complexity has the entire length of the cylindrical shell from head to head a large cylindrical passage or flue, in which is the grate and through which the gases of combustion pass from front to back, next along the outside from back to front, and then underneath from front to back. This is called a "Cornish boiler" (61. 73, fig. 4). A modification of this (the " Lancashire," Jigs. 7, 8) has two lengthwise and parallel cylindrical flues, with a grate in each, in which flues the gases of combustion take a course somewhat similar to that taken by those in the Cornish boiler. One advantage of the Cornish and Lancashire types is that they may be set tip without brick-work. Boilers in which the grate is contained in the flue or flues are called " internally-fired " boilers, of which the Galloway (pi. 73, figs. II, 12) is distinguished by having vertical and inclined con ical water-tubes extending at right angles to the passage of the gases of combustion. It has greater capacity and economy than have the Cornish and Lancashire types, of which it is a development. It is used consider ably in England, and is being introduced into the United States. Properly, it should have the internally-fired flue of elliptical cross-section. It has the advantage of not allowing sediment to collect in the tubes. Another development of the plain cylindrical boiler has four or more flues, smaller than those in which internal firing is conducted, and through which the gases of combustion of a fire under the shell pass from back to front after going from front to back underneath. Such a generator is generally termed simply a "four-flue," ''six-flue," etc., boiler. Its flues are from 4 to S inches across, according to the diameter of the shell.