HEAT EXCHANGERS

Thermodynamics Heat Transfer and Fluid Flow Volume 2 of 3
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HEAT EXCHANGERS Heat Transfer HEAT EXCHANGERS Heat exchangers are devices that are used to transfer thermal energy from one fluid to another without mixing the two fluids. EO 1.11 DESCRIBE the difference in the temperature profiles for counter-flow and parallel flow heat exchangers. EO 1.12 DESCRIBE the differences between regenerative and non-regenerative heat exchangers. EO 1.13 Given the temperature changes across a heat exchanger, CALCULATE the log mean temperature difference for the heat exchanger. EO 1.14 Given the formulas for calculating the conduction and convection heat transfer coefficients, CALCULATE the overall heat transfer coefficient of a system. Heat Exchangers The transfer of thermal energy between fluids is one of the most important and frequently used processes in engineering. The transfer of heat is usually accomplished by means of a device known as a heat exchanger. Common applications of heat exchangers in the nuclear field include boilers, fan coolers, cooling water heat exchangers, and condensers. The basic design of a heat exchanger normally has two fluids of different temperatures separated by some conducting medium. The most common design has one fluid flowing through metal tubes and the other fluid flowing around the tubes. On either side of the tube, heat is transferred by convection. Heat is transferred through the tube wall by conduction. Heat exchangers may be divided into several categories or classifications. In the most commonly used type of heat exchanger, two fluids of different temperature flow in spaces separated by a tube wall. They transfer heat by convection and by conduction through the wall. This type is referred to as an "ordinary heat exchanger," as compared to the other two types classified as "regenerators" and "cooling towers." An ordinary heat exchanger is single-phase or two-phase. In a single-phase heat exchanger, both of the fluids (cooled and heated) remain in their initial gaseous or liquid states. In two-phase exchangers, either of the fluids may change its phase during the heat exchange process. The steam generator and main condenser of nuclear facilities are of the two-phase, ordinary heat exchanger classification. HT-02 Page 30 Rev. 0

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