Experimental Research Of Seawater Fouling Outside Titanium Tube In The Process Of Horizontal Tube Falling Film Evaporation

In a multiple-effect distillation desalination plant, seawater fouling outside horizontal tube seriously reduces the heat transfer rate and the evaporating temperature is usually limited to 70? to avoid fouling. The application of titanium tube is helpful to solve the fouling problem and make it possible to improve the top brine temperature in the multiple-effect distillation desalination.In this paper, a horizontal tube falling film evaporation experiment system was designed and built. Real seawater was chosen as working fluid. An experimental study was conducted on the fouling characteristics of titanium tube in the process of horizontal tube falling film evaporation.In this experiment, spray seawater temperatures are 70??80? and 90? respectively, temperature differences are 3 ? and 5 ?, spray density is 0.03kg/(m·s), salinities of seawater are 3%26%and 8%. Growth curves of scale under various operating conditions are obtained by weighing method. Macro morphology of heat transfer surface under various operating conditions is taken by digital camera. The effect of spray seawater temperature, temperature difference and seawater salinity on the growth process and scale mass on the titanium tube surface are studied by analyzing growth curves and macro morphology. Under the condition of seawater salinities 3%?6% and 8% respectively, the asymptotic value of scale mass increases with rising of spray seawater temperature. When the spray seawater temperature is 80 ?, temperature difference has little effect on asymptotic value of scale mass; when the spray seawater temperature is 90 ?, the asymptotic value of scale mass increases with rising of temperature difference whatever seawater salinity is. When the spray seawater temperature is 70 ?, seawater salinity has little effect on asymptotic value of scale mass; while when spray seawater temperature are 80 ? and 90 ?, the asymptotic value of scale mass increases with rising of seawater salinity.Micro morphology of scale under various operating conditions was tested by scanning electron microscope(SEM)after a test period of 100 h. Main compositions of scale are obtained by energy dispersive X-ray detector(EDX) after a test period of 100 h. The effect of spray seawater temperature, salinity of seawater and tube material on micro morphology and main compositions of scale are analyzed by the test results. Under the condition of spray seawater temperature t=70? and 80 ?, only a little CaCO3?Mg(OH)2 formed on the tube surface. Under the condition of spray seawater temperature t=90 ?, when the seawater salinity C=3%, the main compositions of scale are CaCO%?Mg(OH)2, and the percentage of CaCO3 is lower. When the seawater salinity C=6%, a little CaSO4 starts to form on the tube surface besides CaCO3?Mg(OH)2. When the seawater salinity C=8%, no Mg(OH)2 is found, the percentage of CaCO3 is very low while the the percentage of CaSO4 is extremely high. The increment of salinity of seawater greatly accelerates the formation of CaSO4.