Effect of the tube surface temperature on the seawater fouling process during falling film evaporation outside horizontal tubes

The technology of falling film evaporation outside horizontal tubes has been widely used in thermal desalination plants with advantages of the high heat transfer coefficient and low heat consumption. The seawater fouling during falling film evaporation outside horizontal tubes limits the increment of the top brine temperature and seriously reduces the heat transfer coefficient. This paper presents an experimental study of the heat transfer surface temperature effect on the seawater fouling process during falling film evaporation. Dalian local seawater is selected as the working agent. The scale growing curves under different conditions are obtained by weighting the scale mass at different time during fouling. The morphology and composition of scale layer are analyzed through a scanning electron microscopy and an energy dispersive X-ray spectroscopy. The results show that there exists no significant fouling process for the tube surface temperatures of 50 degrees C and 53 degrees C, but for the tube surface temperature over 53 degrees C, the scale asymptotic value increases significantly. The tube surface temperature plays an important role on the content of Ca and Mg in scale layer. The content of Mg is greater than that of Ca for the tube surface temperature of 70 degrees C, but it is opposite for the tube surface temperatures of 50 degrees C and 60 degrees C. The content of Mg increases with the tube surface temperature. For the tube surface temperature of 70 degrees C, the content of Ca is smaller than that for the tube surface temperatures of 50 degrees C and 60 degrees C.