Large amounts of thermal energy are transferred to for heating or cooling in the industry as well as in the other sectors. Typical examples are crude oil preheating, ethylene plants, exothermic and endothermic reactions and many others. Heat exchangers frequently operate under varying conditions. Their appropriate use in flexible heat exchanger networks as well as maintenance/reliability related calculations require adequate models for estimating their dynamic behaviour. Cell-based dynamic models are very often used to represent heat exchangers with varying arrangements. The paper describes a direct method and a visualisation technique for determining the number of the modelling cells and their size.