A theoretical study consisting of two parts is conducted on triple concentric tube heat exchangers. This paper presents the first part of the study and deals with mathematical modelling. The model includes the derivation and possible solutions of the governing differential equations for both counter-flow and parallel-flow arrangements. In the second part of the study, which is under its way for publication, the results of several case studies will be presented. (C) 1998 Elsevier Science Ltd.