The authors' objective is to simulate a steam-generating system when the real parameters of the process can be measured only at a few locations. In this context it is necessary to define a functional structure and to model the longitudinal distribution of the physical phenomena. To reconstruct the real parameter distributions, it is necessary to take into account the heat and mass balance equations. These equations are elaborated with the help of a control volume. The resulting models of the transportation and transformation elements are used as a first step to define the system concept. Part 2 will be devoted to the numerical simulation.