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A thermal desalination system with a flash drum, a spray-nozzle system, and a condenser under vacuum is designed. Theoretical yield of water vapor in the vacuum flash drum has been evaluated based on conservation equations of mass and energy in the range from 0.35 % to 8.37% at temperature of saline feed water from 40 °C to 60 °C, salinity from 0.035 to 0.04 kg/kg, feed flow rate of 40000 kg/day, and supersaturation from 2 °C to 50 °C. A thermal-fluid model has been adopted to simulate the dynamics of saline water droplets for flash evaporation under vacuum. At a moderate droplet velocity, the model predicts time scales for evaporation from 0.5 s to 1 s with yield of flashing vapor from 69.24 % to 84.59 % at droplet radius from 100 μm to 150 μm. The yield increases with the increase in supersaturation and feed temperature, whereas it increases appreciably with the decrease in droplet size. The design is based on the evaporation time scale and the theoretical yield. Both droplet dynamics for flash evaporation and theoretical yield have been validated with results from the literature.