Boiler, in Mechanical Engineering, is a vessel for the generation of steam from water, and is an essential accompaniment to every steam-engine. The build of the boiler depends on the pressure at which the steam is to be produced, on the position it is to occupy, on its being stationary or locomotive, on the nature of the water supplied and of the coal burnt in the furnace, and on other circumstances. Hence the different types of boiler are very numerous, and definite classification is difficult. The efficiency of the boiler is measured by the number of pounds of steam generated per pound of coal employed in the furnace. The coal, or other fuel, should therefore be burnt efficiently, and the boiler should have a large surface in contact with the furnace, the hot gaseous products of combustion passing off to the chimney. The intensity of natural draught is regulated by the height of the chimney, but if this cannot be made sufficiently great, a forced draught is effected by injecting the exhaust steam into the chimney through a contracted nozzle. This we have on an ordinary locomotive, where the chimney cannot be made very long.
The Cornish and Lancashire boilers are the most common forms used for stationary engines. The Cornish boiler is a horizontal cylinder, through which runs another of three-fifths its diameter. A part of the front end of this inner tube is arranged as a furnace, terminated by a transverse bridge, of fire-brick or hollow metal, towards which the fire-bars slope downwards from the front. The steam of hot gases passes along the tube or inside flue to the end, then through external flues in contact with the outside of the boiler, and then up the chimney, at the lower end of which a damper is placed to vary the draught when required. In the Lancashire boiler there are two long internal flues instead of one passing through the shell, the diameter of each being two-fifths that of the shell. Galloway tubes, forming passages for the water from one side of the flue to the other, possess the advantages of increasing the heating-surface, producing beneficial eddies in the flow of gases, and of considerably strengthening the flue. The same advantages are partially gained by the use of corrugated flues.
If instead of one or two large flues a number of small tubes are employed, we have a multitubular boiler, much stronger, having much more heating surface, but more expensive than the simpler form. Such boilers are extensively used for locomotives, marine-engines, and other cases where compactness and economy of fuel have to be considered together. For many small purposes vertical boilers are employed; they are generally tubular.
Boilers are built of plates of mild steel or of wrought-iron, the first being much more extensively used now than formerly, as it may be produced cheaply and of fairly uniform quality. Steel boilers are as strong as wrought-iron boilers of about 1-3/8 times the thickness, and may therefore be made thinner. This is a distinct advantage from the heating point of view, for thick plates do not conduct heat so well as thin plates. The quality of the metal must be well tested, especially for those parts subjected to the action of the flames. The fire-box of a locomotive is made of copper, the tubes of copper, brass or iron.
The chief boiler appendages are the drum, which gives additional steam-space and enables dry steam to enter the steam-pipe, which opens here; the manhole, an opening to the boiler, closed by a tight-fitting bolted cover, for a man to enter when cleaning out or repairing is required; the blow-off cock, near the bottom of the boiler, for the discharge of muddy water and sediment; the feed water pump; the pressure-gauge for showing the pressure of the steam within the boiler, this pressure varying in different cases from 30 to 150 lbs. per square inch; the glass water-gauge, to show the level of the water within; and the safety-valve to provide an exit for the steam when its pressure exceeds a certain limit.