The MED-TVC distillers developed by HHI had the capacities of 1, 2.2, 3.5 and 4.4 migd. The thermal performance of each distiller was also tested by steady state simulation program based on the first and second law of thermodynamics. The detailed range of operation conditions of the distiller is verified with the performance ratios, distillate productions, steam consumption rates, thermo-compressor entrainment ratios, top brine temperatures and temperature differences across effects. This paper presents exergy analysis to identify the potential for improving system efficiency. The exergy losses due to irreversibility in subsystems of each MED-TVC desalination system were evaluated. This exergy analysis showed that most of the specific exergy losses in the MED-TVC desalination system occurred in the TVC and the effect. The amount of exergy destruction in TVC and effects accounts for more than 70% of the total amount. The improved design for TVC in HHI models was considered by design program. The increase of TVC entrainment ratio to 120% resulted in about 12% reduction of total heat transfer area. Also the proposed TVC entrainment ratio value has been applied to the design of HHI's MED-TVC systems.