Experiment Study Of Vapor Condensation And Heat Transfer In Vacuum Horizontal Tubes

Low temperature multi-effect distillation is one of the primary desalination technologies at present, and falling film evaporation on horizontal tubes represents the most advanced technology of evaporator in low temperature multi-effect distillation. It is advantageous as an effective heat transfer mechanism with high heat transfer rates, small temperature differences, and high transfer performance at low temperature, and also can carry out multi-effect evaporation within lesser temperature difference and enhance gained output ration. Furthermore, it also can make use of the low quality heat energy. So it now has been widely used in desalination devices, refrigeration systems, and petrochemical plants etc.Condensation in horizontal tubes is one of the primary heat transfer processes in the falling film evaporator for multi-effect distillation, and it has decisive effect on heat transfer effect and caloric calculation of evaporator. For equipment of the multi-effect distillation seawater desalination, the processes of condensation in each effect evaporator are all running in high vacuum, but there is lack of experimental and investigating result under corresponding conditions. The objectives of this paper is mainly focusing on some aspects as follows: establish the experimental equipment for condensation heat transfer in vacuum horizontal tubes, experimentally survey and observe the processes of condensation heat transfer in horizontal tubes with vacuum, and learn the characteristics in the processes, gained the mainly factors which have important effect on the heat transfer coefficient and flowing resistance in order to supply help for seawater desalination projects.In this Experiment, the test part was conducted using a 10m length and 25.4mm outer diameter HAL77-2A type Al-brass tube which is averagely divided five segments with a 300mm length viewport between every two segments in order to observe flow pattern. And the experimental investigation was conducted concerning condensation heat transfer coefficient and flowing resistance. Results were attained about the condensation heat transfer coefficient and flowing resistance with the effect of cooling water flux, condensation temperature, vapor velocity and temperature difference between vapor and cooling water. The experimental equipment can keep a good vacuum state in order to get the saturated vapor from evaporator and get the vapor condensed in testing tubes. Through the viewports, the flow patterns were observed clearly. The experimental results indicate that there are primary factors greatly affecting condensation and heat transfer including condensation temperature, vapor velocity and difference in temperature, and the cooling water flux has less effect on heat transfer. And the heat transfer coefficient decreases with the growth of temperature difference, and increases with the growth of saturation temperature and vapor velocity; Well then, the pressure drop is weakened with the growth of saturation temperature, and strengthened with growth of vapor velocity. About the flow patterns, results show the transition from mist flow to annular flow and stratified flow along the axial direction.Experimental work was conducted for saturated steam with saturation temperature ranging from 40 to 65℃, vapor velocity from 40 to 85 m/s and temperature difference from 2.5 to 8.5℃. Accordingly, the load of cooling water spans from 0.211 to 0.336kg/s. From the result to know, the condensing heat transfer coefficient ranges from 4500 to 13000 W /(m~2·K), and unit linear pressure drop from 10 to 290 Pa/m.