Conference Dates

May 18-22, 2003

First Page



The data collected during experiments in water loops on the fouling of heat exchange surfaces with magnetite particles often display a dip in the deposit accumulation curve at about 10 – 20 hours into the experiment. Generally the dips are small enough to be considered part of the scatter and may not even be discernible in long runs with considerable amounts of deposit. We have examined this phenomenon in a series of short experiments lasting up to a maximum of 40 hours. The test section of the recirculating water loop used for the study contained a heat exchanger tube of Alloy-800. Synthetic magnetite colloids were suspended in the coolant, which was usually maintained at 60°C or 90°C, and their deposition onto the tube was monitored for different conditions of pH, flow rate and heat flux. For a pH where deposition should be dominated by transport processes, deposition curves all displayed a “knee” at about 5 – 10 hours, regardless of whether the tube was heated or not. The curve shape can be explained in terms of a layered growth of the deposit, the first layer conditioning the surface for the growth of subsequent layers. First-order phenomenological coefficients in a simple model reproduce the curves quite well.