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Comparison of two fouling probes Fetissoff, Peter Eugene


Studies of heat exchanger fouling are conveniently carried out using small scale probes. The proliferation of probe designs has raised the question of whether radically different probes give comparable results. In this work, the performance of two fouling probes was compared quantitatively in an experimental rig in which they were connected in parallel. A solution of styrene in n-heptane was continuously circulated through .the probe assemblies for periods ranging from 12 hours to 70 hours. Styrene was selected as the foulant since its polymerization to polystyrene can be initiated thermally. One of the probes was a hot wire probe (HWP), based on the UOP Monirex Fouling Test. The heated surface was provided by a 0.14 mm diameter stainless steel 304 coiled wire. It was positioned within a rectangular duct of cross-section 40 mm x 13 mm, perpendicular to the fluid flow. The surface temperature of the wire was determined via wire resistance measurements. The other probe was the Portable Fouling Research Unit (PFRU), designed and manufactured by HTRI. The assembly was a stainless steel 304 heater rod of 10.74 mm outer diameter, surrounded by a 7/8 in. stainless steel 304 tube of 19.74 mm inner diameter, through which the fluid flows. The rod was internally heated by a Nichrome electrical resistance coil packed in magnesium oxide, which provided a heated section 101.6 mm long. The surface temperature was determined using thermocouples imbedded in the wall of the rod, 76.2 mm from the upstream end of the heated section. In the comparative fouling tests, the heat flux, initial surface temperature, bulk fluid temperature, fluid composition and thus initial heat transfer coefficient were maintained essentially the same for both probes. The flowrates of the two probes were radically different, with turbulent, high-velocity flow in the PFRU and laminar, low-velocity flow in the HWP. However, residence times over the heated sections were similar. In twelve of the thirteen runs, fouling was thermally and visually detected on both probes. Nucleate boiling occurred on the probe surfaces during these twelve runs. In the last run, under the conditions of low surface temperature, low heat flux and no boiling, only the HWP exhibited fouling. Both probes exhibited similar fouling behaviour - an induction period, followed by a rise in the fouling resistance, which in most cases levelled off to an asymptotic value. In runs at 11-12% styrene and heat fluxes greater than 400 KW/m² , the fouling resistance of the PFRU, after first levelling off, exhibited an abrupt decline. This decline was attributed to thermal degradation at the high surface temperatures of the fouled probe. There was evidence that the deposits on the probes were not solid but behaved like viscous fluids. This was not unexpected since surface temperatures exceeded the melting point of polystyrene. Thus, the PFRU' deposits conformed to the hydrodynamics of the flow, with striations in the heated section deposit and thin "streamline" deposits on the non-heated sections. The HWP deposit accumulated at the upstream end of the wire coil in the form of globules. Increasing the flowrate decreased the HWP asymptotic fouling resistance, but had no significant effect on the PFRU asymptotic fouling resistance. The rate of fouling and the asymptotic resistance increased with the styrene concentration, however the HWP was affected less than the PFRU. The HWP and PFRU fouling curves were not identical in absolute terms, but on a run-to-run basis they paralleled each other in terms of induction time, the rate of fouling, and the asymptotic fouling resistance. In terms of reproducibility, simplicity of construction and the number of problems, the HWP was superior to the PFRU.

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