A thermo vapor compressor is simply a steam ejector employed in a multi effect desalination process. A greater understanding of flow phenomena inside an ejector plays an important role in its performance improvement. In this paper, CFD investigation has been carried out to study the flow structure inside a steam ejector. This research revealed the influence of operating pressures and ejector geometries on the flow structure and the performance of a steam ejector. The CFD results were verified with available experimental data. The angle of the converging duct as the geometry parameter was varied as 0$^{circ}$, 0.5$^{circ}$, 1$^{circ}$, 2$^{circ}$, 3.5$^{circ}$ and 4.5$^{circ}$. The best performance was obtained by the ejector with converging duct angle of 1$^{circ}$.