A multiple-effect evaporator is used to evaporate water from sugar juice in a series of pressure vessels. Steam is used for evaporation in the first vessel, and vapor from all vessels, except the last one is used for evaporation in subsequent vessels. In order for evaporator surfaces to be used efficiently, juice entering the evaporator should be at the saturation temperature. Therefore, incoming juice with a low temperature must be heated in a juice heater. The heating medium in the juice heater is vapor bled from the evaporator. It is apparent that the multiple-effect evaporator and the juice heater interact through mass and energy balances. Previous investigations have focused on only the multiple-effect evaporator, and paid little attention to the juice heater. This paper presents the model of the interaction between the multiple-effect evaporator and the juice heater. The model is used to investigate how variations of surfaces in the evaporator and the juice heater affect the two performance parameters of the system, which are the amount of sugar juice processed by the system and the ratio of the amount of water evaporated from sugar juice in the evaporator to the amount of high-pressure steam required by the evaporator.