Heat-transfer film coefficients of falling film horizontal tube evaporators

The film thickness distribution and average thickness outside a horizontal test tube were measured with a film-thickness meter. The effects of liquid load, evaporation boiling point, temperature difference and tube diameter on the film thickness, the waving intensity and, as a result, the heat-transfer film coefficients outside the horizontal tube were studied. Experiments of the heat-transfer coefficient of horizontal-tube evaporation were conducted with various operation parameters. Equations for the evaporation heat-transfer coefficient were set up by regression of experimental data. The conclusion was made that when 200<Re<2500, the growth of liquid load Gamma helped to increase total heat-transfer coefficient K; K could also be increased by means of reducing piping diameter, increasing the evaporation boiling point t(2) and reducing the total temperature difference Deltat. The steam flow velocity v(s) must remain at a certain value, but the increment of v(s) would be useless for multi-effect distillation. These results provide a basis for further developing a high efficiency heat-transfer process and novel evaporators.