The common methods of heat exchanger design are based on the assumption that the heat capacity of each fluid stream remains constant throughout the exchanger. Heat exchangers with variable specific heat, however, can be analyzed only by either numerical techniques or one of several approximate methods. While these approximate methods are generally satisfactory, they fail when fluid heat capacities change in such a way that the ratio of heat capacity rates changes from a value less than unity to one greater than unity along the exchanger. Such a situation results in a “pinch point,” or point of lowest temperature approach, inside the exchanger rather than at one of the ends. Complete information on possible pinch points is necessary for rating and sizing of these exchangers, particularly in high-Ntuunits. In this article a mathematical model is given to predict the existence and location of pinch points without computing complete temperature profiles. A step-by-step procedure is given for computer Simulation of heat exchanger performance with variable heat capacity of the working fluids.
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