TYPES OF HEAT EXCHANGERSDOE-HDBK-1018/1-93Heat ExchangersCross flow,as illustrated in Figure 5, exists when one fluid flows perpendicular to thesecond fluid; that is, one fluid flows through tubes and the second fluid passes around thetubes at 90 angle. Cross flow heat exchangers are usually found in applications whereone of the fluids changes state (2-phase flow). An example is a steam system'scondenser, in which the steam exiting the turbine enters the condenser shell side, and thecool water flowing in the tubes absorbs the heat from the steam, condensing it into water.Large volumes of vapor may be condensed using this type of heat exchanger flow.Figure 5 Cross Flow Heat ExchangerComparisonoftheTypesofHeatExchangersEach of the three types of heat exchangers has advantages and disadvantages. But of the three,the counter flow heat exchanger design is the most efficient when comparing heat transfer rateper unit surface area. The efficiency of a counter flow heat exchanger is due to the fact that theaverage T (difference in temperature) between the two fluids over the length of the heatexchanger is maximized, as shown in Figure 4. Therefore the log mean temperature for acounter flow heat exchanger is larger than the log mean temperature for a similar parallel orcross flow heat exchanger. (See the Thermodynamics, Heat Transfer, and Fluid FlowFundamentals Handbook for a review of log mean temperature). This can be seen by comparingthe graphs in Figure 3, Figure 4, and Figure 5. The following exercise demonstrates how thehigher log mean temperature of the counter flow heat exchanger results in a larger heat transferrate. The log mean temperature for a heat exchanger is calculated using the following equation.ME-02Rev. 0Page 6

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