Thesis (MEng)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: A novel induced draft hybrid (dry/wet) dephlegmator (HDWD) is introduced which can enhance the performance of dry air-cooled condenser systems and a model is developed to simulate its performance. The HDWD consists of two stages of cooling with the steam flow in series and the air flow in parallel through both stages. The first stage consists of downwardly inclined finned tube bundles, similar to conventional air-cooled condenser bundles, and the second stage comprises horizontal bare tube bundles of which the outer surface can selectively be operated dry or wet by spraying it with deluge water. A comparison of the HDWD with other existing and new concepts reveals the significant advantages that this technology has over other technologies. This thesis is a continuation of the work done by Heyns (2008), who introduced a forced draft HDWD and investigated its performance, and Owen (2013), who conducted a parametric investigation on the induced draft HDWD to improve the basic design proposed by Heyns (2008). In his thermo-flow model, Heyns (2008) conducted a thermal analysis assuming equal air flow through both stages and a constant vapor temperature inside the bundles, while Owen (2013) solved the thermal, steam-side pressure drop and draft equations. Both their models make use of empirical correlations by Gaddis & Gnielinski (1985), Zukauskas (1987), Mizushina et al. (1967) and Niitsu et al. (1967) for the performance characteristics. The current model solves the thermal, steam-side pressure drop and draft equations of the HDWD with better accuracy in the steam-side pressure drop calculation. The airside heat and mass transfer and loss coefficient correlations found in literature were found to deviate significantly from each other resulting in uncertainty regarding their suitability. Based on the recommendations by Owen (2013), a bare tube test bundle with 19 mm outer diameter tubes arranged with a triangular pitch of 38 mm is therefore designed, manufactured and tested to investigate the performance characteristics of the bundle experimentally under dry and wet operating conditions to evaluate the correlations from literature. The experimental data confirmed the applicability of the correlations of Gaddis & Gnielinski (1985) and Zukauskas (1987) to predict the dry HDWD performance, since they deviated by 7% and 5% respectively from the experimental data in die applicable range. The heat transfer during wet operation is slightly over estimated within 5% with the correlations of Mizushina et al. (1967) in the range of application. The air-side pressure 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 used in the current model. Using the current HDWD model indicates significant performance enhancement using tubes with a smaller diameter in the delugeable bundle compared to the larger tubes of Heyns (2008) and agrees very well with the model of Owen (2013). / AFRIKAANSE OPSOMMING: ‘n Hibriede geïnduseerde trek deflegmator is voorgestel wat die werksverrigting van huidige lugverkoelde kondenserstelsels kan verbeter en ‘n model is ontwikkel wat die werksverrigting simuleer. Die deflegmator bestaan uit twee stadiums van verkoeling met die stoomvloei in serie en die lugvloei in parallel deur beide stadia. Die eerste stadium bestaan uit skuins, afwaartse finbuisbundels, soortgelyk aan konvensionele lugverkoelde kondenser-finbuisbundels, en die tweede stadium bestaan uit horisontale kaalbuisbundels waarvan die buite-oppervlak selektief droog of nat bedryf kan word deur dit met verkoelingswater te benat. ‘n Vergelyking tussen die deflegmator en ander huidige en nuwe konsepte toon merkbare voordele wat die tegnologie bo die ander tegnologië het. Die tesis is ‘n voortsetting van die werk van Heyns (2008), wie ‘n hibriede geforseerde trek deflegmator voorgestel en die werksverrigting daarvan ondersoek het, en Owen (2013) wat ‘n parametriese studie op die hibriede geïnduseerde trek deflegmator gedoen het om die basiese ontwerp voorgestel deur Heyns (2008) te verbeter. In sy termovloei-model, het Heyns (2008) die termiese analise ondersoek met die aanname van gelyke hoeveelhede lugvloei deur beide stadia met ‘n konstante damp-temperatuur binne die bundels, terwyl Owen (2013) die termiese, stoomkant-drukval and trekvergelykings opgelos het. Beide hul modelle maak van die empiriese korrelasies van Gaddis & Gnielinski (1985), Zukauskas (1987), Mizushina et al. (1967) en Niitsu et al. (1967) gebruik vir die werkverrigtingskarakteristieke. Die huidige model los die termiese, die stoomkant-drukval asook die trekvergelykings van die deflegmator op met verbeterde akkuraatheid van die stoomkant-drukval berekeninge. Die lugkant warmte- en massaoordrag en verlieskoëffisiënt korrelasies wyk baie van mekaar af wat lei tot onsekerhede rakende hul toepasbaarheid. Gebaseer op die voorstelle van Owen (2013) is ‘n kaalbuisbundel met 19 mm buite-diameter buise met ‘n driehoekige steek van 38 mm dus ontwerp, vervaardig en getoets om die werksverrigtingskarakteristieke van die bundel eskperimenteel te ondersoek onder droë en nat toestande om die korrelasies van die literatuur te evalueer. Die eksperimentele data bevestig die toepaslikheid van die korrelasies van Gaddis & Gnielinski (1985) en Zukauskas (1987) om die droë werksverrigting van die deflegmator te voorspel, aangesien hul met 7% en 5% onderskeidelik afwyk van die eksperimentele data in die toepaslike bestek. Die warmteoordrag tydens natwerking is ietwat oorskat binne 5% met die korrelasies van Mizushina et al. (1967) in die bestek van toepassing. Die lugkant-drukval tydens natwerking is onderskat met 29 tot 39% deur Niitsu et al. (1967) en die korrelasie verkry vanaf die eksperimentele resultate is dus gebruik in die huidige model. Deur gebruik te maak van die huidige model van die deflegmator dui merkbare werksverrigting verbetering aan deur van buise met ‘n kleiner diameter in die benatbare bundel te gebruik in vergelyking met die groter buise van Heyns (2008) en stem baie goed ooreen met die model van Owen (2013).

Thesis (MEng) -- Stellenbosch University, 2014. / ENGLISH ABSTRACT: Water collection systems are used in wet-cooling towers (WCTs) with fill over which water to be cooled flows and in evaporative air-cooled heat exchangers (EACHEs) having deluged tube bundles over which water flows to transfer heat from the tubes. They collect and remove water falling from the bottom of the fill or heat exchanger tube bundle while allowing air to pass with limited flow resistance. There are basically two types of collection systems, categorized as trough and basin systems. Trough systems comprise of multiple, evenly spaced parallel troughs extended in one direction with inclined capture plates to direct water into the troughs. The water collects in the troughs and drains under gravity to a collecting tank or manifold from where it is pumped to the sprayers. In basin systems water falls directly into an open basin under gravity where it is collected and can be pumped to the sprayers. The hybrid (dry/wet) dephlegmator (HDWD) for air-cooled steam condensers, which is being developed at Stellenbosch University as discussed by Heyns (2008), Owen (2013) and Anderson (2014), requires a more effective water collection system with a lower pressure drop than what current designs have to offer. In this thesis, an existing trough system is systematically evaluated by means of various high speed camera and flow experiments to determine its performance characteristics and to find ways to improve its catchment effectiveness. A modification is proposed which increases the effectiveness from approximately 92% to 100 % with a relatively small increase in pressure drop, by adding a small deflector plate and reducing the spacing between adjacent troughs. Experimental data and CFD models are used to determine the performance characteristics of the modified trough system for design purposes. For comparison, various basin system designs are proposed and investigated using CFD, where the baseline model is validated by means of experimental data of rectangular cooling tower inlets by Kröger (2004). Rectangular cooling tower inlets are relevant as they have a similar re-circulating flow pattern to what is seen with the water basin inlets at the vicinity of the inlet wall. The numerical data is correlated to determine empirical relations for the flow performance characteristics. The comparison between the modified trough and the different basin systems yields that the trough system requires significantly less fan and pumping power than basin systems and the costs are also expected to be less. / AFRIKAANSE OPSOMMING: Wateropvangstelsels word gebruik in nat koeltorings en verdamping verkoelers, waar waterdruppels in teen-vloei is met n lugstroom. Die stelsel vang die water wat val op en verwyder dit uit die lugstroom met 'n minimale lugweerstand. Daar is twee wateropvangstelsels wat as volg gekategoriseer word: trog stelsel en wateropvangbak stelsel. 'n Trog stelsel bestaan uit eweredige gespasieerde trôe parallel aan mekaar met 'n skuinsvlak om water in die kanaal op te vang. Water dreineer as gevolg van swaartekrag na 'n opvangs tenk waarvan dit terug gepomp word na die sproeiers. In 'n wateropvangbak stelsel val water direk in die bak en word terug gepomp na die sproeiers. 'n Lugverkoelde kondenserstelsels met 'n hibriede (droog/nat) deflegmator wat huidiglik by die Stellenbosch Universiteit ontwikkel word benodig 'n meer effektiewe wateropvangstelsel met 'n lae drukval. In hierdie tesis word bestaande trog stelsels sistematies geëvalueer deur middel van verskeie toetse met 'n hoë spoed kamera en lugvloei eksperimente om die werkverrigtingskarakteristieke te bepaal en om die opvangsvermoë te verbeter. 'n Verbetering word voorgestel wat die opvangsvermoë verbeter van ongeveer 92 % na 100 % met 'n relatiewe klein verhoging in die drukval deur 'n klein leilem by te voeg en die spasiëring tussen trôe te verminder. Eksperimentele data en BVD (berekeningsvloeidinamika) modelle word gebruik om die werkverrigtingskarakteristieke te bepaal van die nuwe trog stelsel vir ontwerps doeleindes. Die modelle is bekragtig deur gebruik te maak van literatuur van inlaatverlies korrelasies vir reghoekige koeltorings, soos gedoen deur Kröger (2004). Die inlaat van reghoekige koeltorings het soortgelyke vloeipatrone en vloeiwegbreking soos wat by die inlaat van die wateropvangbak stelsel gesien word. Die numeriese data word gebruik om 'n vergelyking vir die vloei karakteristieke te bepaal. 'n Vergelykende studie tussen die nuwe trog stelsel en die wateropvangbak stelsel het bewys dat die trog stelsel noemenswaardig minder waaier drywing en pomp drywing benodig.