The many types of boilers may be classed under two general divisions, viz.:

(1) Shell or fire-tube boilers.

(2) Pipe or water-tube boilers.

The shell boiler is made in several forms, of which the locomotive and the cylindrical or Scotch marine types are familiar examples. In general design the shell boiler is in the form of a cylindrical shell which is a reservoir for the water and steam. Attached to the shell, and more or less surrounded by it are (1) a fire box, or one or more cylindrical steel furnaces and their com-bustion chambers, and (2) a nest of tubes opening from the fire box or combustion chambers into the smoke pipe.

The water-tube boiler consists of an assemblage of straight or bent tubes, the ends of which open into water and steam reservoirs usually designated respectively as headers and drums. The water and steam are contained in these tubes, headers and drums, and the boiler is surrounded by a sheet-steel casing which confines the fire and smoke within its limits. A space for the furnace is provided under the tubes and the flame and hot gases pass among the tubes to reach the smoke pipe.

The shell boiler held supremacy for many decades as a steam generator after the steam engine came into use, but demand for the economy of higher steam pressures has gradually brought into use many types of the water-tube boiler, which is particularly adapted to standing high pressures. This change has taken away much work from the shop for building shell boilers. The building of water-tube boilers consists in a great measure of the assembling of the products of other shops and plants, and of the making of certain parts by the cold or hot pressing of mild steel plates to special forms, leaving very little work to be done, for this type of boiler, by the methods and machines used in the building of shell boilers.

The many patented types of water-tube boilers have brought about the building of these boilers as special work, and many of the processes of forming the headers and other parts of a particular type of boiler are unique and ingenious methods of hot and cold pressing and of welding by aid of electric, furnace, and gas blow-pipe heat.

In some patented boilers the parts are made up more or less of cast steel, or even of cast iron for pressures not over about 100 pounds.

Many shell boilers for high and low pressures are still built for marine, locomotive, and stationary uses, also the larger drums and some other parts of water-tube boilers are built in the shop equipped for shell-boiler work, hence the boiler shop and its equipment continue to be an essential part of a general manufacturing plant.

The improvement in recent years of boiler steel enables shell boilers to be built for higher pressures than formerly.