The main purpose of this investigation was to study the mechanisms of mixed salt crystallisation fouling on heat transfer surfaces during convective heat transfer and sub-cooled flow boiling conditions. In the present investigation, the effects of various operating parameters such as solution composition and hydrodynamics of the system, on crystallisation fouling of mixtures of calcium sulphate and calcium carbonate have been studied experimentally. After clarification of the effects of operating parameters on the deposition process, the results of the experiments were used to develop a mechanistic model for prediction of fouling resistances, caused by crystallisation of mixed salts, under convective heat transfer and sub-cooled flow boiling conditions. Model predictions were compared with the measured experimental data when calcium sulphate and calcium carbonate form deposits on the heat transfer surface simultaneously. Deviations ranging from 6% to 25% were observed which confirm the suitability of the model. (c) 2005 Elsevier Ltd. All rights reserved.