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Boiler
A pressurized system in which water is vaporized to steam, the desired end product, by heat transferred from a source of higher temperature, usually the products of combustion from burning fuels. Steam thus generated may be used directly as a heating medium, or as the working fluid in a prime mover to convert thermal energy to mechanical work, which in turn may be converted to electrical energy. Although other fluids are sometimes used for these purposes, water is by far the most common because of its economy and suitable thermodynamic characteristics.The physical sizes of boilers range from small portable or shop-assembled units to installations comparable to a multistory 200-ft-high (60-m) building equipped, typically, with a furnace which can burn coal at a rate of 6 tons/min (90 kg/s). In terms of steam production capacities, commercial boilers range from a few hundred pounds of steam per hour to more than 6,000,000 lb/h (750 kg/s). Pressures range from 0.5 lb/in.2 (3.4 kilopascals) for domestic space heating units to 5000 lb/in.2 (34 megapascals) for some power boilers. The latter type will deliver steam superheated to 1100±°F (593±°C) and reheated to similar values at intermediate pressures. Large units are field-assembled at the installation site but small units (frequently referred to as package boilers) are shop-assembled to minimize the overall boiler price.Boilers operate at positive pressures and offer the hazardous potential of explosions. Pressure parts must be strong enough to withstand the generated steam pressure and must be maintained at acceptable temperatures, by transfer of heat to the fluid, to prevent loss of strength from overheating or destructive oxidation of the construction materials. The question of safety for design, construction, operation, and maintenance comes under the police power of the state and is supplemented by the requirements of the insurance underwriters. The ASME Boiler Construction Code is the prevalent document setting basic standards in most jurisdictions.Being in the class of durable goods, boilers that receive proper care in operation and maintenance function satisfactorily for several decades. Thus the types found in service at any time represent a wide span in the stages of development in boiler technology.The earliest boilers, used at the beginning of the industrial era, were simple vats or cylindrical vessels made of iron or copper plates riveted together and supported over a furnace fired by wood or coal. Connections were made for offtake of steam and for the replenishment of water. Evolution in design for higher pressures and capacities led to the use of steel and to the employment of tubular members in the construction to increase economically the amount of heat-transferring surface per ton of metal. The earliest improvement was the passage of hot gases through tubes submerged in the water space of the vessel, and later, arrangements of multiple water-containing tubes which were exposed on their outer surface to contact with hot gases. See also: Fire-tube boiler; Water-tube boilerThe overall functioning of steam-generating equipment is governed by thermodynamic properties of the working fluid. By the simple addition of heat to water in a closed vessel, vapor is formed which has greater specific volume than the liquid, and can develop increase of pressure to the critical value of 3208 lb/in.2 absolute pressure (22.1 MPa absolute pressure). If the generated steam is discharged at a controlled rate, commensurate with the rate of heat addition, the pressure in the vessel can be maintained at any desired value, and thus be held within the limits of safety of the construction. See also: SteamAddition of heat to steam, after its generation, is accompanied by increase of temperature above the saturation value. The higher heat content, or enthalpy, of superheated steam permits it to develop a higher percentage of useful work by expansion through the prime mover, with a resultant gain in efficiency of the power-generating cycle. See also: SuperheaterIf the steam-generating system is maintained at pressures above the critical, by means of a high-pressure feedwater pump, water is converted to a vapor phase of high density equal to that of the water, without the formation of bubbles. Further heat addition causes superheating, with corresponding increase in temperature and enthalpy. The most advanced developments in steam-generating equipment have led to units operating above critical pressure, for example, 3600–5000 lb/in.2 (25–34 MPa).Superheated steam temperature has advanced from 500±°F (260±°C) to the present practical limits of 1050–1100°F (566–593°C). Progress in boiler design and performance has been governed by the continuing development of improved materials for superheater construction having adequate strength and resistance to oxidation for service at elevated temperatures. For the high temperature ranges, complex alloy steels are used in some parts of the assembly.Steam boilers are built in a wide variety of types and sizes utilizing the widest assortment of heat sources and fuels. See also: Boiler economizer; Boiler feedwater; Boiler feedwater regulation; Marine engineering; Nuclear power; Steam-generating unit
A pressurized system in which water is vaporized to steam, the desired end product, by heat transferred from a source of higher temperature, usually the products of combustion from burning fuels. Steam thus generated may be used directly as a heating medium, or as the working fluid in a prime mover to convert thermal energy to mechanical work, which in turn may be converted to electrical energy. Although other fluids are sometimes used for these purposes, water is by far the most common because of its economy and suitable thermodynamic characteristics.The physical sizes of boilers range from small portable or shop-assembled units to installations comparable to a multistory 200-ft-high (60-m) building equipped, typically, with a furnace which can burn coal at a rate of 6 tons/min (90 kg/s). In terms of steam production capacities, commercial boilers range from a few hundred pounds of steam per hour to more than 6,000,000 lb/h (750 kg/s). Pressures range from 0.5 lb/in.2 (3.4 kilopascals) for domestic space heating units to 5000 lb/in.2 (34 megapascals) for some power boilers. The latter type will deliver steam superheated to 1100±°F (593±°C) and reheated to similar values at intermediate pressures. Large units are field-assembled at the installation site but small units (frequently referred to as package boilers) are shop-assembled to minimize the overall boiler price.Boilers operate at positive pressures and offer the hazardous potential of explosions. Pressure parts must be strong enough to withstand the generated steam pressure and must be maintained at acceptable temperatures, by transfer of heat to the fluid, to prevent loss of strength from overheating or destructive oxidation of the construction materials. The question of safety for design, construction, operation, and maintenance comes under the police power of the state and is supplemented by the requirements of the insurance underwriters. The ASME Boiler Construction Code is the prevalent document setting basic standards in most jurisdictions.Being in the class of durable goods, boilers that receive proper care in operation and maintenance function satisfactorily for several decades. Thus the types found in service at any time represent a wide span in the stages of development in boiler technology.The earliest boilers, used at the beginning of the industrial era, were simple vats or cylindrical vessels made of iron or copper plates riveted together and supported over a furnace fired by wood or coal. Connections were made for offtake of steam and for the replenishment of water. Evolution in design for higher pressures and capacities led to the use of steel and to the employment of tubular members in the construction to increase economically the amount of heat-transferring surface per ton of metal. The earliest improvement was the passage of hot gases through tubes submerged in the water space of the vessel, and later, arrangements of multiple water-containing tubes which were exposed on their outer surface to contact with hot gases. See also: Fire-tube boiler; Water-tube boilerThe overall functioning of steam-generating equipment is governed by thermodynamic properties of the working fluid. By the simple addition of heat to water in a closed vessel, vapor is formed which has greater specific volume than the liquid, and can develop increase of pressure to the critical value of 3208 lb/in.2 absolute pressure (22.1 MPa absolute pressure). If the generated steam is discharged at a controlled rate, commensurate with the rate of heat addition, the pressure in the vessel can be maintained at any desired value, and thus be held within the limits of safety of the construction. See also: SteamAddition of heat to steam, after its generation, is accompanied by increase of temperature above the saturation value. The higher heat content, or enthalpy, of superheated steam permits it to develop a higher percentage of useful work by expansion through the prime mover, with a resultant gain in efficiency of the power-generating cycle. See also: SuperheaterIf the steam-generating system is maintained at pressures above the critical, by means of a high-pressure feedwater pump, water is converted to a vapor phase of high density equal to that of the water, without the formation of bubbles. Further heat addition causes superheating, with corresponding increase in temperature and enthalpy. The most advanced developments in steam-generating equipment have led to units operating above critical pressure, for example, 3600–5000 lb/in.2 (25–34 MPa).Superheated steam temperature has advanced from 500±°F (260±°C) to the present practical limits of 1050–1100°F (566–593°C). Progress in boiler design and performance has been governed by the continuing development of improved materials for superheater construction having adequate strength and resistance to oxidation for service at elevated temperatures. For the high temperature ranges, complex alloy steels are used in some parts of the assembly.Steam boilers are built in a wide variety of types and sizes utilizing the widest assortment of heat sources and fuels. See also: Boiler economizer; Boiler feedwater; Boiler feedwater regulation; Marine engineering; Nuclear power; Steam-generating unit
