Experimental Research Of Seawater Fouling Process On Falling-film Evaporation Outside Horizontal Tube

As a kind of complex dilute solution, the process of seawater fouling on the heat transfer surface are influenced by numerous factors. It is scaling on the surface of heat transfer tube in the evaporators of desalination the mian factor that limites the top brine temperature, which also affects the heat transfer efficiency of total equipment. Therefore, the study on the process of scaling on the heat transfer surface will contribute a lot to reduce the fouling on heat transfer surface.The atmospheric testbed of fouling process on falling-film evaporation outside a horizontal tube was bulit to do the experiment under the simulated conditions in actual engineering. Seawater(concentration to be3.5%and7%) was chosen to be the test fluid with the spraying density to be0.03kg/m·s,0.06kg/m· s and0.08kg/m· s, as well as the temperature of heat transfer surface to be50℃,53℃,60℃,63℃,70℃and73℃, respectively. In this paper, the influences of spraying density, temperature of heat transfer surface and concerntration were discussed to analyzed the induction period of fouling, the scaling rate, the time to reach the balance of surface scaling and the gradual value of scaling through the curves of deposition growth. And the weight increment of heat transfer tube was measured by precision balance. The results indicated that the temperature of heat transfer surface was the main influence factor while the spraying density and the concrntration had synergistic effect in fouling on the heat transfer surface. The contact angle was measured using a drop shape analysis (DSA) instrument for three kinds of tube material(aluminum, aluminum brass and titanium) in order to calculate the surface free energy of the tubes. It was shown in the paper that the material with low surface free energy had better effect on preventing scale deposition.Suppressed ion chromatography method was used to measure the ionic concentration so as to analyze the possible production on the heat transfer surface, which were suppose to be CaCO3and Mg (OH)2.Scanning electron microscopy (SEM) in combination with energy dispersive X-ray spectroscopy (EDXS) and X Ray Fluorescence (XRF) provide qualitative information on structural and chemical characteristics/properties of the scale, especially about composition, crystal structure, crystal size and orientation. The results indicated that there was a little aluminum in the production due to the washout of the liquid. The temperature affected the content of Mg(OH)2in. the production. When the temperature of heat transfer suifae was low, CaCO3(aragonite) appeared to be the main component in the scale production. With the temperature of heat transfer surfae incresed, the content of Mg(OH)2(brucite) growed. Mg(OH)2(brucite) became the main component after the temperature of heat transfer surfae over70℃. And the quantitity of scale particles were on account of the concerntration of sea water. When the deposition happened, the tube surface was covered mainly with CaCO3(aragonite) and Mg(OH)2(brucite). The flaky brucite forms a thin base layer on the tube surface. Rod-shaped aragonite crystals grew up on this base layer. At last, aragonite crystals developped to be the stable scale layer together with the brucite.