This paper studies the energy dynamics of process networks and/or staged processes in the presence of large energy input and output flows. A generic model structure is developed for such networks and singular perturbation arguments are used to document that the variables in the energy balance evolve over a short time horizon while the variables in the material balance may exhibit both fast and slow transients. Nonlinear reduced order models for the fast and slow dynamics that can be used for model based controller design are also derived. A high-purity distillation column and a reactor with an external heat exchanger are used as simulation case studies to illustrate the theoretical results. (C) 2008 Elsevier Ltd. All rights reserved.