The horizontal-tube falling film evaporation is a widely adopted technique in multiple-effect distillation desalination plant. It has a high heat transfer coefficient under quite small temperature difference. In this paper, an experimental equipment for horizontal-tube falling film evaporation was set up. Experiments were carried out to show how the heat transfer coefficient is affected by different parameters including heat flux, circumference direction of tubes, spray density, evaporation temperature, and experimental fluid. Results indicate that the heat transfer coefficient decreases after a little increase with growth of spray density. The heat transfer coefficient decreases along the tube circumference, but at the bottom of the tube, it shows increasing trend. In addition, a simple comparison between seawater and fresh water in heat transfer coefficient is also provided.

A set of experimental facilities were set up to measure overall heat transfer coefficient of horizontal-tube falling film evaporator with square-pitch bundle. Effect of spray density, saturation temperature, total temperature difference, and inlet steam velocity on the overall heat transfer coefficient K is studied. The tubes are made of HAL77-2A aluminum brass with an outer diameter of 25.4 mm. Fluids inside and outside the tubes are steam and fresh water respectively. The results indicate that growth of spray density and saturation temperature helps to increase the K. The K could also be increased by reducing the total temperature difference. However, the impact of the inlet steam velocity on the K is less significant. Furthermore tube bundle-depth effect and space distribution of local overall heat transfer coefficient in the evaporator are also presented. Based on this investigation, basic engineering design information will be provided to establish the governing parameters for horizontal-tube falling film evaporator in the field of seawater desalination.

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.