Thermal Vapor Compressors are the main part of desalination systems and directly influence the overall efficiency of the systems. In order to improve the design of a Thermal Vapor Compressor (TVC), it is necessary to understand the flow field inside such devices. This study will be presented in two separate parts. The main objective of the first part is to prepare and validate a numerical method to investigate the flow pattern within a conventional TVC. Two numerical schemes on the basis of axisymmetric and three dimensional assumptions are implemented to analyze the performance parameters. The results of both models are compared with the experimental measurements obtained from an industrial desalination unit. It is shown that the axisymmetric model is quite capable of producing similar results to three-dimensional model, and both results are in acceptable agreements with the experimental data. Verification of the numerical method enables us to introduce a new design methodology in the second part of this study. (C) 2012 Elsevier Ltd. All rights reserved.