|A Boiler or Steam Generator is a device used to create steam by applying heat energy to water. Although the definitions are somewhat flexible, it can be said that older steam generators were commonly termed boilers and worked at low to medium pressure (1–300 psi/0.069–20.684 bar; 6.895–2,068.427 kPa), but at pressures above this it is more usual to speak of a steam generator.
A boiler or steam generator is used wherever a source of steam is required. The form and size depends on the application: mobile steam engines such as steam locomotives, portable engines and steam-powered road vehicles typically use a smaller boiler that forms an integral part of the vehicle; stationary steam engines, industrial installations and power stations will usually have a larger separate steam generating facility connected to the point-of-use by piping. A notable exception is the steam-powered fireless locomotive, where separately generated steam is transferred to a receiver (tank) on the locomotive.
The source of heat for a boiler is combustion of any of several fuels, such as wood, coal, oil, or natural gas. Nuclear fission is also used as a heat source for generating steam. Heat recovery steam generators (HRSGs) use the heat rejected from other processes such as gas turbines.
Steam boilers are used where steam and hot steam is needed. Hence, steam boilers are used as generators to produce electricity in the energy business. Besides many different application areas in the industry for example in heating systems or for cement production, steam boilers are used in agriculture as well for soil steaming.
TYPES OF BOILERS
Auxiliary boilers on Navy ships may be divided into two groups: FIRE-TUBE BOILERS and WATER-TUBE BOILERS.
Fire-tube boilers are generally similar to Scotch marine or locomotive boilers. In this type of boiler, the gases of combustion pass through tubes that are surrounded by water. There are a number of auxiliary boilers of the fire-tube type in use in diesel-driven ships. Figure illustrates a cutaway view of the fire-tube boiler shown in figure.
Water-Tube, Natural-Circulation Boilers
Water-tube, natural-circulation boilers consist basically of a steam drum and a water drum connected by a bank of generating tubes. The two drums are also connected by a row of water tubes, which forms a water-cooled sidewall opposite the tube bank. The water-wall tubes pass beneath the refractory furnace floor before they enter the water drum. In natural-circulation boilers, the steam and water drums are connected by several tubes of larger diameter, called DOWNCOMERS or WATER TUBES (not shown). These tubes are positioned away from the flow of hot gases of combustion. Refractory is also used to protect these downcomers from contact with the combustion gases.