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heat exchanger[′hēt iks‚chānj·ər]
a device in which heat is transferred from one to another fluid (or fluids) or between a fluid and the surface of a solid. The process of heat transfer from one fluid to another is one of the most important and most widely used processes in technology. For example, the production of steam in boiler units is based on the transfer of heat from the products of the combustion of organic fuel to water.
According to the operating principle used, heat exchangers are divided into three types: recuperative, regenerative, and direct-contact. There also exist heat exchangers in which a fluid is heated (or cooled) by an “internal” source of heat (or cold).
In recuperative heat exchangers, two flowing fluids at different temperatures are separated by a solid wall. Heat transfer occurs through convection in the fluids, through conduction in the wall (seeCONVECTIVE HEAT EXCHANGE), and through radiative transfer if at least one of the fluids is a radiating gas. Examples of recuperative heat exchangers include steam generators, preheaters, and evaporators. Some designs of recuperative exchangers are shown in Figure 1.
In regenerative heat exchangers, the same heating surface is alternately exposed to the hot and the cold fluid—that is, the surface first receives heat and is heated and then gives up heat and is cooled. A typical example of a regenerator is the hot-blast stove in a blast furnace.
Since the heat transfer in recuperative and regenerative heat exchangers occurs at the surface of a solid, they are called surface heat exchangers. In direct-contact, or contact, heat exchangers, the transfer of heat occurs with the fluids in direct contact. Cooling towers, in which water is cooled by atmospheric air, are heat exchangers of this type.
Heat exchangers with an internal source of heat or cold involve the use of just one fluid. Nuclear reactors and electric heaters are heat exchangers of this type.
The calculation of the thermal quantities characterizing a heat exchanger reduces to the simultaneous solution of the heat-balance and heat-transfer equations. A distinction is made between design calculations, which are necessary to determine the heat-transfer surface area and are carried out in the design of new heat exchangers, and check calculations, which are carried out to determine the quantity of heat transferred and the final temperatures of the fluids for a known heat-transfer surface area.
Heat exchangers are used extensively in thermal power engineering. Examples are air preheaters, superheaters, economizers, and condensers. Other areas where heat exchangers are used include the chemical and food industries.
REFERENCESKichigin, M. A., and G. N. Kostenko. Teploobmennye apparaty i vyparnye ustanovki. Moscow-Leningrad, 1955.
Kays, W. M., and A. L. London. Kompaktnye teploobmenniki, 2nd ed. Moscow, 1967. (Translated from English.)
Kasatkin, A. G. Osnovnye protsessy i apparaty khimicheskoi tekhnologii, 9th ed. Moscow, 1973.
I. N. ROZENGAUZ
A device used to transfer heat from a fluid flowing on one side of a barrier to another fluid (or fluids) flowing on the other side of the barrier.
When used to accomplish simultaneous heat transfer and mass transfer, heat exchangers become special equipment types, often known by other names. When fired directly by a combustion process, they become furnaces, boilers, heaters, tube-still heaters, and engines. If there is a change in phase in one of the flowing fluids—condensation of steam to water, for example—the equipment may be called a chiller, evaporator, sublimator, distillation-column reboiler, still, condenser, or cooler-condenser.
Heat exchangers may be so designed that chemical reactions or energy-generation processes can be carried out within them. The exchanger then becomes an integral part of the reaction system and may be known, for example, as a nuclear reactor, catalytic reactor, or polymerizer.
Heat exchangers are normally used only for the transfer and useful elimination or recovery of heat without an accompanying phase change. The fluids on either side of the barrier are usually liquids, but they may also be gases such as steam, air, or hydrocarbon vapors; or they may be liquid metals such as sodium or mercury. Fused salts are also used as heat-exchanger fluids in some applications.
Most often the barrier between the fluids is a metal wall such as that of a tube or pipe. However, it can be fabricated from flat metal plate or from graphite, plastic, or other corrosion-resistant materials of construction.
Heat exchangers find wide application in the chemical process industries, including petroleum refining and petrochemical processing; in the food industry, for example, for pasteurization of milk and canning of processed foods; in the generation of steam for production of power and electricity; in nuclear reaction systems; in aircraft and space vehicles; and in the field of cryogenics for the low-temperature separation of gases. Heat exchangers are the workhorses of the entire field of heating, ventilating, air-conditioning, and refrigeration. See Conduction (heat), Convection (heat), Cooling tower, Evaporator, Heat transfer, Vapor condenser