heat exchanger

a device for transferring the heat of one substance to another, as from the exhaust gases to the incoming air in a regenerative furnace.

[1900-05]

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Any of several devices that transfer heat from a hot to a cold fluid.

In many engineering applications, one fluid needs to be heated and another cooled, a requirement economically accomplished by a heat exchanger. In double-pipe exchangers, one fluid flows inside the inner pipe, and the other in the annular space between the two pipes. In shell-and-tube exchangers, many tubes are mounted inside a shell; one fluid flows in the tubes and the other flows in the shell, outside the tubes. Special-purpose devices such as boilers, evaporators, superheaters, condensers, and coolers are all heat exchangers. Heat exchangers are used extensively in fossil-fuel and nuclear power plants, gas turbines, heating and air conditioning, refrigeration, and the chemical industry. See also cooling system.

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▪ energy conversion

      any of several devices that transfer heat from a hot to a cold fluid. In many engineering applications it is desirable to increase the temperature of one fluid while cooling (cooling system) another. This double action is economically accomplished by a heat exchanger. Among its uses are the cooling of one petroleum fraction while warming another, the cooling of air or other gases with water between stages of compression, and the preheating of combustion air supplied to a boiler furnace using hot flue gas as the heating medium. Other uses include the transfer of heat from metals to water in atomic power plants and the reclaiming of heat energy from the exhaust of a gas turbine by transferring heat to the compressed air on its way to the combustion chambers. Heat exchangers are used extensively in fossil-fuel and nuclear power plants, gas turbines, heating and air-conditioning, refrigeration, and the chemical industry. The devices are given different names when they serve a special purpose. Thus boilers, evaporators, superheaters, condensers, and coolers may all be considered heat exchangers.

 Heat exchangers are manufactured with various flow arrangements and in different designs. Perhaps the simplest is the concentric tube or double-pipe heat exchanger shown in Figure 1—>, in which one pipe is placed inside another. Inlet and exit ducts are provided for the two fluids. In the diagram the cold fluid flows through the inner tube and the warm fluid in the same direction through the annular space between the outer and the inner tube. This flow arrangement is called parallel flow. Heat is transferred from the warm fluid through the wall of the inner tube (the so-called heating surface) to the cold fluid. A heat exchanger can also be operated in counterflow, in which the two fluids flow in parallel but opposite directions. Concentric tube heat exchangers are built in several ways, such as a coil or in straight sections placed side by side and connected in series.

 The most common type of heat exchanger is the shell-and-tube type illustrated in Figure 2—>. It utilizes a bundle of tubes through which one of the fluids flows. These tubes are enclosed in a shell with provisions for the other fluid to flow through the spaces between the tubes. In most designs of this type, the free fluid flows roughly perpendicular to the tubes containing the other fluid, in what is known as a cross-flow exchange. In nuclear reactors (nuclear reactor) fuel rods may replace the tubes, and the cooling fluid flowing around the rods removes the heat generated by the fission process.

 

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