boiler, apparatus designed to convert a liquid to vapour. In a conventional steam power plant, a boiler consists of a furnace in which fuel is burned, surfaces to transmit heat from the combustion products to the water, and a space where steam can form and collect. A conventional boiler has a furnace that burns a fossil fuel or, in some installations, waste fuels. A nuclear reactor can also serve as a source of heat for generating steam under pressure.
Boilers were built as early as the 1st century ad by Hero of Alexandria but were used only as toys. Not until the 17th century was serious consideration given to the potential of steam power for practical work. The first boiler with a safety valve was designed by Denis Papin of France in 1679; boilers were made and used in England by the turn of the 18th century. Early boilers were made of wrought iron; as the advantages of high pressure and temperature were realized, manufacturers turned to steel. Modern boilers are made of alloy steel to withstand high pressures and extremely high temperatures.
Most conventional steam boilers are classed as either fire-tube or watertube types. In the fire-tube type, the water surrounds the steel tubes through which hot gases from the furnace flow. The steam generated collects above the water level in a cylindrically shaped drum. A safety valve is set to allow escape of steam at pressures above normal operating pressure; this device is necessary on all boilers, because continued addition of heat to water in a closed vessel without means of steam escape results in a rise in pressure and, ultimately, in explosion of the boiler. Fire-tube boilers have the advantage of being easy to install and operate. They are widely used in small installations to heat buildings and to provide power for factory processes. Fire-tube boilers are also used in steam locomotives.
In the watertube boiler, the water is inside tubes with the hot furnace gases circulating outside the tubes. When the steam turbogenerator was developed early in the 20th century, modern watertube boilers were developed in response to the demand for large quantities of steam at pressures and temperatures far exceeding those possible with fire-tube boilers. The tubes are outside the steam drum, which has no heating surface and is much smaller than in the fire-tube boiler. For this reason, the drum of the watertube boiler is better able to withstand higher pressures and temperatures. A wide variety of sizes and designs of watertube boilers are used in ships and factories. The express boiler is designed with small water tubes for quick generation of steam. The flash boiler may not require a steam drum, because the tubes operate at such high temperatures that the feed water flashes into steam and superheats before leaving the tubes. The largest units are found in the central-station power plants of public utilities. Units of substantial size are used in steel mills, paper mills, oil refineries, chemical plants, and other large manufacturing plants.