In this study, a mathematical model was developed for multiple-effect evaporators. These evaporators have cocurrent, countercurrent and parallel flow operation options. Each operation was investigated with and without pre-heaters. The effect of pre-heating on evaporation process was investigated from the point of energy economy. A sugar factory's data was used with the applied models as a case study. The results obtained for pre-heated and non-pre-heated situations were compared with each other. The maximum COP is found as 3.33 for countercurrent operation with pre-heating. The minimum COP is 2.57 for parallel flow operation without pre-heating. (c) 2006 Elsevier Ltd. All rights reserved.