Study On Aluminum Alloy Heat-transfer Tube In Horizontal Tube Falling Film Evaporation Desalination

Horizontal tube falling film evaporation is now widely used in low temperaturemulti-effect desalination (LT-MED). But the cost of the LT-MED equipment is veryhigh. Aluminum alloy has low cost, good thermal conductivity and corrosionresistance.The application of Aluminum alloy tube is an effective way to reduce thecost of LT-MED technology, so it has a good prospect in desalination field.The studyof aluminum alloy heat-transfer tube is of great significance.The article firstly studied the corrosion resistance of5052aluminum alloy indifferent seawater conditions by simulating the environment of LT-MED evaporationdesalination. Potentio-dynamic polarization test and scanning electron microscope(SEM) technology are used to do the study. The results show that, in low temperaturevacuum environment, when the pH value of seawater is neutral, the corrosion currentdensity of the aluminum alloy is the lowest, and the corrosion rate is the lowest.When the pH value increases or reduces, the corrosion potential of aluminum alloy ismore negative,the corrosion current density increases, and the corrosion resistance ofthe aluminum alloy gets worse. The corrosion potential of5052aluminum alloydecreases and the corrosion current density increases with the increase of temperatureand salinity. The corrosion behavior of5052aluminum alloy becomes more seriously.The corrosion resistance of the aluminum alloy got worse.When there are oxygendissolved in seawater, passivation film formed on the aluminum alloy surface.Thecorrosion potential increases and the corrosion current density decreases with theincrease of the dissolved oxygen concentration. The corrosion resistance of thealuminum alloy becomes better.The corrosion potential of aluminum alloy increasesand corrosion current density decreases with the adding of the corrosion scaleinhibitor, improving the corrosion resistance of the aluminum alloy in seawater. Atthe operating condition of60℃,3.5%salinity, pH7and adding corrosion scaleinhibitor B with5mg/L, the corrosion rate of aluminum alloy5052can be controlledat about0.004mm/a, showing good corrosion resistance.Then a horizontal tube falling film evaporation apparatus with5052aluminumalloy tube as heat-transfer tube was designed and set up. The heat-transfer performance of the evaporator in LT-MED was studied with actual seawater as rawmaterial. The effects of sprinkling density, evaporation boiling point, evaporationtemperature difference, salinity and non-condensable gas concentration on theevaporation heat-transfer coefficients outside the tube, the condensing heat transfercoefficient inside the tube and the total heat-transfer coefficient of desalinationprocess were studied. The results show that,within the test scope, the evaporationheat-transfer coefficients and the total heat-transfer coefficients increase with theincrease of sprinkling density. All the coefficients increase with the increase of theevaporation boiling point. But the condensing heat-transfer coefficient and the totalheat-transfer coefficient decreases with the increase of evaporation temperaturedifference. The condensing heat-transfer coefficient and the total heat-transfercoefficient decreases sharply when there is non-condensable gas mixed in the steaminside the heat-transfer tube.Salinity has little effect on the heat-transfercoefficient.When controlling the non-condensable gas’concentration, the heat-transfercoefficients are more than3500W/(m~2·℃). The horizontal tube falling filmevaporator shows good heat-transfer performance.All the experiment study provides technical basis and design parameters for theapplication of5052aluminum alloy heat-transfer tube in desalination equipmentprocess and relavant engineering design.