The successful operation of any petrochemical plant is dependent on the use of several utilities whichmay include electricity, steam, compressed air, cooling media, refrigeration media, nitrogen, condensateand fuel gas. These utilities form a significant portion of the fixed cost associated with running a plant.Utility optimisation has not received much attention until recently, driven by rising energy costs, stricterenvironmental policies, more competitive markets, and the threat of climate change. The generation,preparation, and transportation of utilities require energy and therefore should be optimised to reducelosses and improve operating efficiency. One example of such a utility is a cooling water system. Thispaper describes the modelling of a dual circuit induced draft cooling water system for control and opti-misation purposes. The derived model is verified with plant data indicating promising results. The modelis represented in a steady-state algebraic form as well as a dynamic state-space form. This provides aconvenient basis for simulation studies and controller/optimiser design.