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Thermal fouling studies : Computations on roughness effects, modifications of a test loop and tests... Mayo Abad, Orestes

Abstract

The thermal fouling data of Watkinson were recomputed on the assumption that increases in pressure drop were caused entirely by roughness of the fouling deposit and not at all by blockage. The results indicate that roughness played some role in the sand-water runs, but no role in the gas oil runs. The anomalous increases in overall heat transfer coefficient with degree of fouling in the first two sand-water runs were thus explained by the effect of roughness on liquid film heat transfer coefficient. The original loop was modified, the principal change being the replacement of the manual temperature recording system by a digital Data Logging System. Fluid and wall temperatures, as well as differential pressure, could thus be automatically recorded as a function of time. Better control of independent operating variables such as fluid velocity, inlet temperature and heat flux could therefore be achieved, and the effects of temporary fluctuations in these operating conditions could be observed and separated from fouling effects. Thermal fouling runs were made in the modified heat transfer loop on samples of Bayer Process spent liquor from the ALCAN aluminum refinery in Arvida, P.Q. The only discernible trends in the results were increases in rate and degree of fouling with increasing heat flux to a maximum value, followed by a sharp decrease, at liquor velocities less than 5 ft/sec, and the absence of thermally detectable fouling at higher velocities. The unreproducibi1ity of the results is believed to be attributable, at least in part, to changes in chemical composition of the liquor throughout the experiments.

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