Heat transfer characteristics of horizontal tube falling film evaporation for desalination

Abstract

An experimental study on heat transfer characteristics of falling film evaporation outside horizontal tube was carried out at low evaporation temperature. The Al-brass tubes with 19 mm outer diameter and 1,600 mm effective length were adopted and arranged vertically in the evaporator. The electric heater was fixed tightly in the test tube for generating uniform heat flux. And thermocouples were installed in the surface of the test tube to measure surface temperatures. The experimental fluids are water and seawater with salinity of 1.5, 3, and 4.5%. The variations of heat transfer coefficient h were studied in detail with saturation temperament ranging from 50 to 70°C, the spray density Γ ranging between 0.017 and 0.087 kg m−1 s−1, and the heat flux q varying from 7.75 to 12.30 kW m−2. The results show that the average heat transfer coefficients of both water and seawater increase with the spray density until about 0.052 kg m−1 s−1. The heat transfer coefficient of fresh water increases with evaporation temperature whereas that of the seawater decreases slightly with the evaporation temperature. The average heat transfer coefficient decreases with salinity. In addition, the circumferential distribution of local heat transfer coefficients and the effect of defoamer on heat transfer are studied.