Thermo-hydraulic channelling in parallel heat exchangers subject to fouling

The thermal and hydraulic impacts of fouling by crude oil on the tube side of shell-and-tube heat exchangers is analysed for cases where a centrifugal pump and control valve determine the throughput. For co- and counter-current heat exchangers where the fouling layer thickness does not vary markedly over the tube length a new method of calculating the mean fouling rate was derived based on (i) a 'threshold' fouling rate model and (ii) an estimated linear profile in film temperature along the tubes. Over-design of exchangers, using guidelines such as those provided by TEMA, is shown to be capable of exacerbating fouling problems. The new mean is also used to evaluate the operation of parallel heat exchangers, such as are often employed at the hot end of a preheat train; a phenomenon that we call 'thermo-hydraulic channelling' is identified. It is shown that this may substantially impair network performance but that its effect may be countered by introducing a simple retrofit option of flow measurement and control. (C) 2008 Elsevier Ltd. All rights reserved.