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Title: Reflux condensation enhancement
Author: Jibb, Richard J.
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2000
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The use of tubular reflux condensers in the place of conventional arrangements may provide significant improvements in mass transfer performance. However, industrial application is limited because of the low vapour velocities required to avoid flooding. It has been suggested that the mass transfer performance can be improved, without violating the flooding limit, by the application of "HiTRAN®", a wire matrix insert, manufactured by Cal Gavin Ltd. This work represents a theoretical and experimental investigation of reflux condensation inside vertical tubes to determine the potential of this system. A heat transfer test rig has been designed, built and commissioned in the UMIST Pilot Plant for this purpose. A series of experiments have been performed on this equipment using pure steam and steam/air mixtures, enabling a comparison of the performance of a plain tube, and the same tube fitted with a low density HiTRAN element. For condensation of pure steam the condensate film dominates the heat transfer resistance, and plain tube experiments indicate that there is little interaction between the vapour and liquid phases. As a result, correlations for the prediction of condensate heat transfer resistance in co-current flow may be applied to predict performance. By contrast with the HiTRAN element the pressure drop and liquid hold up were greatly increased. The element, as installed, provided little or no benefit in terms of improved performance, possibly because the increased mixing is offset by an increased hold up of liquid at the tube wall. For the condensation of steam air mixtures, where there is a significant gas side resistance, the capacity of the test heat exchanger is increased by the use of HiTRAN inserts. However, the increase was limited by the reduction in temperature driving force caused by the increased pressure drop. Despite the large increase in pressure drop, no obvious change in the conditions required to produce flooding of the system was observed. The film model, due to Colburn and Hougen (1933), was found-to be successful in predicting the heat load and condensate flowrate to within ±10%, for both systems, steam and steam-air. However, comparisons of the measured wall temperature profile, and axial heat load distribution indicate that the film model does not give good predictions for conditions at the top of the heat exchanger where the liquid mass flow is very small. For the HiTRAN element the film model provides a good approximation of the local heat transfer resistance along the entire heat exchanger, although the accuracy is limited by the poor prediction of pressure drop. The same model has been extended to predict the performance of the test rig for the case of reflux condensation of methanol/water mixtures, where the most important mass transfer benefits of tube inserts are expected to be realised.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available