Constructal Optimization of the Coupling Between a Hot and a Cold Stream for Power and Refrigeration

The first part of this paper reviews some basic results regarding the extraction of power from a hot stream. The second part of the paper outlines the thermodynamic optimization of the coupling between a hot and a cold stream for refrigeration under a total heat transfer area constraint, via constructal theory. The refrigeration system is driven by a hot stream of single-phase fluid that is subsequently discharged into the ambient. The irreversibility is due to three heat exchangers and the discharging of the used stream. It is shown that the thermodynamic optimum is pinpointed by an optimal ratio between the mass flow rates of the hot stream and the stream that is heated by the hot stream, and by an optimal distribution of the heat exchanger area inventory among the three heat exchangers of the installation. The robustness of the optima found is investigated regarding to the variation of the design and operating parameters. The connection between constructal theory and the thermodynamic optimization employed in this paper is the end product: the optimal matching of the streams. It represents configuration (architecture, geometry) derived from principle. If design is approached in this manner (constructal theory), design becomes science, not art.