Abstract
A multiple-effect diffusion still in a vacuum container was theoretically analyzed with a one-dimensional steady-state model. The still was heated at Tbtm＝100 ℃ or less from the bottom and the container was evacuated at the saturated water vapor pressure corresponding to Tbtm. The findings are summarized as: (1) The still without the container shows that the coefficient of performance, COP, increases as Tbtm decreases and has a gentle peak at Tbtm ≈ 50 ℃, while the decrease in Tbtm causes a serious decrease in the total evaporation rate, Uv. (2) The still in the vacuum shows that the Uv is much larger, and the COP is higher than when it is without the container. (3) The temperature drop from the top of the still to the ambient air is 15～22 ％ of the total in the vacuum, while it is 2～12 ％ otherwise. (4) A decrease in Tbtm accompanies a decrease in the heat consumed in heating up seawater fed to the still, and the evacuation increases both the heat input to the bottom and the ratios of evaporation heat to total heat through a diffusion layer. These result in the high performance of the still in the vacuum container.
