Investigation of Shell and Tube Heat Exchanger Tube Inlet Wear by Computational Fluid Dynamics

Abstract Computational fluid mechanics is still a developing art in the prediction of erosion/corrosion often due to lack of suitable mathematical models to represent physical processes. In the present work, new flow modelling strategies have been developed for flow simulation in the inlet header of side entry shell and tube heat exchangers used in alumina plants and minerals processing plants for heat recovery. The traditional approach of treating the tube bundle as a porous plug observing Darcy’s friction law has been replaced by modelling of all tubes individually and application of an inertial model for flow resistance. This better reflects the much higher velocities for the approach into the tube bundle as only about one third of the tube sheet is open area. It also includes the effect of flow separation within the inlet to some tubes which is a flow feature not able to be simulated with the porous plug approach. The traditional use of symmetry for the inlet geometry has been found to suppress the inherent asymmetry found in this flow associated with the large flow expansion from the inlet nozzle to the inlet header space. The results indicate that tube inlet wear is spread across the tube sheet more extensively than indicated by porous plug modelling. The asymmetry of flow uncovered by the new modelling technique leads to less regular wear patterns which may help to explain the difficulty of recognizing clear cut patterns of wear in processing plants. Areas of high angle of flow into tubes, variations of mass flow distributions across tube sheets and a flow component across the geometrical symmetry plane are identified. High flow angles have been found to correlate well with flow separation inside tube entrances where previous studies have shown excessive tube wear to occur. Regions where flow separation within tubes occurs have also been found to correlate well with low static pressure within the tubes downstream of the tube entrances.

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