Approaches of finite-time thermodynamics in conceptual design of heat exchange systems

Abstract

Authors give a review of the methods of finite-time thermodynamics in problems of obtaining the limiting capabilities of two- and multi-flow heat exchange systems. The boundaries of their physical reachability are constructed. The case of flows with changing heat capacity and, in particular, flows with a change in phase state is considered. The conditions for minimum heat exchange dissipation are obtained for different kinetics at fixed heat transfer coefficient and heat load. The concept of thermodynamically equivalent heat exchange systems is introduced and a method for transition from a multi-flow system to an equivalent two-flow heat exchanger in which heat capacities of the flows depend on temperature is proposed. Based on the obtained results, an algorithm for synthesis of multi-flow heat exchange systems is proposed. The synthesis takes into account the limitations on the temperatures of all or part of the flows. The synthesis of the system involves the calculation and minimization of the number of two-flow heat exchangers and the selection of the contact structure, the values of flow parameters, the distribution of contact surfaces and heat loads between two-flow heat exchange cells. In the design procedure the limitations on the temperatures of all or part of the flows are taken into account. Using thermodynamic criteria instead of technical and economic ones allows us to radically simplify the problem of designin the system.