Conclusions and outlook
Heat exchangers ? Definitions and general principles

In this article, we have clarified the field we have analyzed. We have shown the generality of what we have called a heat exchanger and we have restricted this domain to the set of two-fluid heat exchangers in the stationary state. The notions of heat capacity flow rate, logarithmic mean temperature difference (LMDT), efficiency, number of transfer units (NUT) and overall heat exchange coefficient were clarified. The two basic flow configurations (co-current and counter-current) were described. It was emphasized that the counter-current configuration will be used as the reference configuration for sizing exchangers with other configurations. A summary of the main definitions is given in Table 2 .