Scaling of heated tubes by recirculated evaporatively cooled water is shown to be a two-stage production process. The mass of scalant precipitated (lost from solution) is governed by the mass entering or being generated in the system, water chemistry, temperature and time. The mass depositing (adhering to a heated surface) is controlled by the mass precipitated, influenced by hydrodynamic transport and shear processes, pH, temperature rise and time. (C) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.