Experimental Investigation On Heat Transfer Of Horizontal-tube Falling Film Evaporator

Falling film evaporation outside horizontal tubes is used in the equipments of low temperature multi-effect desalination for its advantages. Using this technology, small heat driving force is required. Meanwhile, it is suitable for low temperature evaporation and has high heat transfer efficiency.An experimental system for heat transfer characteristics of the falling film horizontal-tube evaporator is designed and built. Length of tubes of the evaporator is selected 8 meters. Heat exchange tubes are arranged by two patterns that are regular triangle arrangement and rotated square arrangement. For an elementary experiment, fresh water is selected as water outside tubes. During experiment, by changing experimental conditions such as water spray density, water evaporation temperature, steam flow rate inside tubes, etc., parameters are obtained and then come out the factors that have influence on horizontal tube falling film evaporation heat transfer coefficient as well as variation laws of heat transfer and heat transfer rate in two directions of heat transfer tubes after the measurement together with analysis of those data. Design basis for horizontal tube falling film evaporator are provide by this study.Experimental results show that, in the spray density range is 0.03kg/(m·s)～0.10kg/(m·s), the average heat transfer coefficient of horizontal tube falling film evaporator increases with the water spray density. The average heat transfer coefficient of the evaporator also increases with the evaporation temperature. The steam flow rate inside tube has little influence on heat transfer coefficient of the evaporator. In the direction along the length of tubes, the variation laws of heat transfer coefficient depends on that inside the tube, and the variation laws have slightly differences at different evaporation temperatures. Along the column direction of the evaporator, the variation heat transfer laws of heat transfer coefficient are determined by the change outside of horizontal tubes and they slightly reduce in this direction.