TYPES OF HEAT EXCHANGERSDOE-HDBK-1018/1-93Heat ExchangersParallel flow DTlm =(20080oF) (145120oF)ln(20080oF)(145120oF)61oFInserting the above values into heat transfer Equation (2-2) for the counter flow heatexchanger yields the following result.Q70BTUhrft2F(75ft2) (72F)Q3.8x105BTUhrInserting the above values into the heat transfer Equation (2-2) for parallel flow heatexchanger yields the following result.Q70BTUhrft2F(75ft2) (61F)Q3.2x105BTUhrThe results demonstrate that given the same operating conditions, operating the same heatexchanger in a counter flow manner will result in a greater heat transfer rate thanoperating in parallel flow.In actuality, most large heat exchangers are not purely parallel flow, counter flow, or cross flow;they are usually a combination of the two or all three types of heat exchangers. This is due tothe fact that actual heat exchangers are more complex than the simple components shown in theidealized figures used above to depict each type of heat exchanger. The reason for thecombination of the various types is to maximize the efficiency of the heat exchanger within therestrictions placed on the design. That is, size, cost, weight, required efficiency, type of fluids,operating pressures, and temperatures, all help determine the complexity of a specific heatexchanger. One method that combines the characteristics of two or more heat exchangers and improves theperformance of a heat exchanger is to have the two fluids pass each other several times withina single heat exchanger. When a heat exchanger's fluids pass each other more than once, a heatexchanger is called a multi-pass heat exchanger. If the fluids pass each other only once, the heatexchanger is called a single-pass heat exchanger. See Figure 6 for an example of both types.Commonly, the multi-pass heat exchanger reverses the flow in the tubes by use of one or moresets of "U" bends in the tubes. The "U" bends allow the fluid to flow back and forth across thelength of the heat exchanger. A second method to achieve multiple passes is to insert baffleson the shell side of the heat exchanger. These direct the shell side fluid back and forth acrossthe tubes to achieve the multi-pass effect.ME-02Rev. 0Page 8
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