Review of Developments in the Use of Fouling Models in Pre-Heat Train Design and Operational Analysis

Models exist for the prediction of both the thermal and hydraulic effects of fouling on heat exchanger performance. In recent years these models have been applied in the analysis of fouling on the tube-side of shell-and-tube exchangers, in exchangers fitted with tube inserts, and in welded plate exchangers (Compabloc). Most of these models have been developed through the analysis of experiments conducted in the laboratory. Consequently, they apply to point conditions of temperature and wall shear. Both temperature and wall shear vary over the surface of industrial exchangers. Two means of overcoming this limitation have been developed. The paper commences with a review of these developments. Fouling rates are dependent on the chemistry of the crude oil being processed. Consequently, the parameters needed for the application of the models need to be established through the analysis of the performance of operational exchangers. Techniques for undertaking this analysis are outlined. Means of considering fouling during the design of heat exchangers are reviewed. It is concluded that the use of fouling models provides the only valid approach. Using such models the affect of exchanger geometry on long term performance can be determined. The benefits of using tube inserts can be established. The effects of changes in operational conditions (inlet temperatures and fluid flow-rates) on exchanger fouling can be established. Exchangers can be designed in order to achieve a required performance throughout a pre-defined operating period. Handling “uncertainty” is an important part of exchanger design and relevant techniques are outlined. Fouling in pre-heat trains is dependent upon the structure of the train. Procedures for analysing the effect of structure on fouling rate (and its hydraulic effects) are described. Finally, the authors describe how information on fouling rates can be used in identifying when heat exchangers should be cleaned and how de-salter temperature should be controlled.