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The present paper describes the design of switching moving boundary models for two-phase flow evaporators and the development of an object-oriented library written in the Modelica language. The main idea is to design basic models for each flow state: subcooled liquid, two-phase flow and superheated steam, applying the conservation laws. The design must be done in order that the basic models can be interconnected to create complete, flooded or dry-expansion evaporators depending on the particular case. The basic models should consider the balances of mass, energy and momentum, meanwhile the evaporator models should include mechanisms to switch between them, for example from a complete evaporator to a flooded evaporator in case that the superheated region becomes extinct. Several models currently exist although none of them include all the desirable features.