Performance and thermo-mechanical cost evaluation of API 661 air-cooled heat exchangers

[摘要] ENGLISH ABSTRACT: The optimal design of a heat exchanger for a specified heat transfer, pressure dropand set of ambient conditions entails minimising space, weight, material usage andoverall cost. However, the variables which influence the performance as well as theoverall cost of a heat exchanger are not related in a simple way and it is not obviouswhich variables play the most important roles (Perry & Green, 1997:11-44).Air cooled heat exchangers (ACHEs) are normally designed in three stages, bydifferent experts in the field, and with the aid of specially designed software. Thisproject combines these thermal, mechanical and cost estimation processes into a MSExcel model which makes it easier to see the influence that design parameters haveon the overall cost of the heat exchanger.A thermal model was created to design an API 661 (2006) ACHE. The results fromthis model compared well with those of HTRI Xchanger Suite 6.0 software, withHTRI being more conservative in its design mode.A mechanical design model was then developed, which uses as inputs the outputs ofthe thermal design. The output from this mechanical design model is the minimummaterial thicknesses based on the stress criteria of Appendix 13 of ASME VIII div 1(2007) Boiler and Pressure Vessel Code.An experiment on a finned tube bundle was performed in a wind tunnel facility todetermine performance characteristics and compare these to existing correlations inliterature. The results showed that both the heat transfer coefficient (h) and losscoefficient (Eu) correlations proposed by Ganguli et al. (1985) closely predict themeasured data, and were consequently used in the thermal design model. During thisexperiment it was also shown that the tube bundle reached 8 % - 9 % of its allowableinternal fouling factor, due to rust build up inside the tubes, and in a testing period ofonly nine days.The thermal and mechanical models were then combined with a cost estimationprocess to perform both a thermal and mechanical parametric study. The thermalstudy showed that to obtain an optimal solution, the design must attempt to maximisethe length, increase the width rather than the number of bays, make use of twobundles per bay with fewer but larger fans and employ a large number of tube rowswith the least number of tube passes. These guidelines were used to create an initialdesign; Excel Solver was then applied to locate the optimum combination of bundlelength and width that result in the minimum heat exchanger cost.Two mechanical considerations were investigated, both requiring additional weldingand thus increased welding cost. Firstly the use of stay plates result in reducedrequired plate thicknesses according to the stress criteria since it provides additionalstiffness in the header box design. Secondly the use of more (but smaller) nozzles asopposed to less (but larger) nozzles was also considered. The mechanical parametricstudy showed no specific trends, but both considerations should still be checked as itcan be cost beneficial in a specific design.

[发布机构] Stellenbosch University