Studies On Heat Transfer With High Fractional Noncondensable Gas In Dewvaporation Desalination Process

Dewvaporation desalination is a novel desalination process, in which air is used as a carrier gas to carry water vapor from feed saline water and to form fresh water by condensation. It is unavoidable that there is high fractional noncondensable gas in the condensation process. Sharply decreasing heat-transfer coefficient, the high fractional noncondensable gas has significant effect on the condensation process in the dewvaporation desalination.In this research, tubular columns with shell and tube structure were designed and constructed to evaluate the performances of the condensation process for the dewvaporation desalination. The process is related to steam-air mixed gas condensation in the annulus around vertical condensing columns with copper tubes 40 mm O. D. and 10 mm O. D., respectively, for heat exchange (Column I and Column II). The mixed gas inlet temperatures ranges from 92 to 68 oC with inlet air mass fraction from 0.35 to 0.80.In order to investigate the effect of mixture local film coefficient along condensing tube, temperatures of the gas-steam mixture and the copper tube outside surface were measured at 0.26 m intervals along the condensing column I. The results show that the mixed gas local film coefficient decreases with the distance from mixed gas inlet under the same Reynolds number of inlet mixed gas. The local mixed gas film coefficient varied from 200 to 1900 W/m2·oC in the experiment.Column II with different length were used to investigate the effect of column length on overall heat transfer coefficient, the length is varied from 2.5 m, 2.0 m, 1.6 m, 1.3 m to 1.0 m. The experimental results show that the condensing column length affects overall heat transfer coefficient seriously. Under the same mixed gas inlet Reynolds number, the overall heat transfer coefficient decreases with the condensing column length increases. From the top of the condensing column to the bottom, the local Reynolds number decreases caused by dehumidification, and thus, the local condensing film coefficient decreases. The shorter the condensing column is, the larger the heat transfer coefficient at the same operation conditions. It is proposed that the column length should not be longer than 2.0 m for small dewvaporation desalination plants like that of our experiments.It has been well established that the presence of noncondensable gas in vapor can greatly deteriorate the condensation process. The overall heat transfer coefficient varied...