Evaluation of the performance characteristics of a hybrid (dry/wet) induced draft dephlegmator

[摘要] ENGLISH ABSTRACT: A novel induced draft hybrid (dry/wet) dephlegmator (HDWD) is introducedwhich can enhance the performance of dry air-cooled condenser systems and amodel is developed to simulate its performance. The HDWD consists of twostages of cooling with the steam flow in series and the air flow in parallel throughboth stages. The first stage consists of downwardly inclined finned tube bundles,similar to conventional air-cooled condenser bundles, and the second stagecomprises horizontal bare tube bundles of which the outer surface can selectivelybe operated dry or wet by spraying it with deluge water. A comparison of theHDWD with other existing and new concepts reveals the significant advantagesthat this technology has over other technologies. This thesis is a continuation ofthe work done by Heyns (2008), who introduced a forced draft HDWD andinvestigated its performance, and Owen (2013), who conducted a parametricinvestigation on the induced draft HDWD to improve the basic design proposedby Heyns (2008). In his thermo-flow model, Heyns (2008) conducted a thermalanalysis assuming equal air flow through both stages and a constant vaportemperature inside the bundles, while Owen (2013) solved the thermal, steam-sidepressure drop and draft equations. Both their models make use of empiricalcorrelations by Gaddis & Gnielinski (1985), Zukauskas (1987), Mizushina et al.(1967) and Niitsu et al. (1967) for the performance characteristics. The currentmodel solves the thermal, steam-side pressure drop and draft equations of theHDWD with better accuracy in the steam-side pressure drop calculation. The airsideheat and mass transfer and loss coefficient correlations found in literaturewere found to deviate significantly from each other resulting in uncertaintyregarding their suitability. Based on the recommendations by Owen (2013), a baretube test bundle with 19 mm outer diameter tubes arranged with a triangular pitchof 38 mm is therefore designed, manufactured and tested to investigate theperformance characteristics of the bundle experimentally under dry and wetoperating conditions to evaluate the correlations from literature. The experimentaldata confirmed the applicability of the correlations of Gaddis & Gnielinski (1985)and Zukauskas (1987) to predict the dry HDWD performance, since they deviatedby 7% and 5% respectively from the experimental data in die applicable range.The heat transfer during wet operation is slightly over estimated within 5% withthe correlations of Mizushina et al. (1967) in the range of application. The air-sidepressure drop during wet operation is underestimated by 29 to 39% by Niitsu et al.(1967) and the correlation obtained from the experimental results is therefore usedin the current model. Using the current HDWD model indicates significant performance enhancement using tubes with a smaller diameter in the delugeablebundle compared to the larger tubes of Heyns (2008) and agrees very well withthe model of Owen (2013).

[发布机构] Stellenbosch University