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 filmthickness 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 ' helped to increase total heattransfer coefficient K; K could also be increased by means of reducing piping diameter, increasing the evaporation boiling point t2 and reducing the total temperature difference )t. The steam flow velocity vs must remain at a certain value, but the increment of vs would be useless for multi-effect distillation. These results provide a basis for further developing a high efficiency heat-transfer process and novel evaporators.

Experiments were conducted on heat-transfer film coefficients of falling film horizontal-tube evaporators inside a smooth red copper tube. We draw the following conclusions. 1. When 200<Re<2500, the increment of the liquid load helps to increase the total heat-transfer ' ) * * : D F — — — — — — — — Liquid load, kg.m!1.s!1 Difference; pressure difference Thickness of liquid film, m Average thickness of liquid film, m Thermal conductivity, W.m!2.°C!1 Mucosity, Pa.s Density, kg.m!3 Surface charge distribution, C.m!2; liquid surface tension, N.m!2 Superscript + — Dimensionless symbol Subscripts G L — Condensation component inside the tube — Evaporation component outside the tube — Steam component — Liquid component