Air-cooled condenser steam flow distribution and related dephlegmator design considerations

[摘要] ENGLISH ABSTRACT: The steam-side side operation of a practical air-cooled steam condenser isinvestigated using a combination of CFD, numerical, analytical and experimentalmethods. Particular attention is directed towards the vapor flow distribution in theprimary condensers and dephlegmator performance.Analysis of the vapor flow in the distributing manifold, connecting the steamturbine exhaust to the air-cooled heat exchangers, highlights the importance ofcareful design of the guide vanes in the manifold bends and junctions. Improvedguide vane design and configuration can reduce the steam-side pressure drop overthe manifold and improve the vapor flow distribution, which may be beneficial tocondenser operation.The vapor flow in the primary condensers is shown to exhibit a non-uniformdistribution amongst the heat exchanger tubes. The vapor flow distribution isstrongly linked to the distribution of tube inlet loss coefficients through the heatexchanger bundles. The non-uniform flow distribution places an additionaldemand on dephlegmator performance, over and above the demands of roweffects in the case of multi-row primary condenser bundles. Row effects areshown to account for as much as 70 % of available dephlegmator capacity in thiscase. Simultaneously, inlet loss coefficient distributions can account for up to30 % of dephlegmator capacity. In some situations then, the dephlegmator is fullyutilized under ideal operating conditions and there is no margin of safety to copewith non-ideal operation of the primary condensers. The upstream regions of theprimary condensers are therefore exposed to a high risk of undesirable noncondensablegas accumulation. Reduced dephlegmator capacity due to insufficientejector performance may further compound this problem. Single-row primarycondenser bundles eliminate row effects and thereby significantly reduce thedemands on dephlegmator performance. The use of such bundles in thedephlegmator would also measurably reduce ejector loading. In light of thefindings of this study, it is recommended that single-row bundles be considered asthe primary option for future air-cooled condenser applications.A hybrid (dry/wet) dephlegmator concept is analysed and shown to be able toprovide measurably enhanced dephlegmator performance when operating in wetmode, while consuming only a small amount of water. The enhanceddephlegmator cooling translates to an increase in total air-cooled condensercapacity of up to 30 % at high ambient temperatures in this case. The benefit ofthis enhanced cooling capacity to steam turbine output may be significant. Thehybrid dephlegmator concept therefore offers a simple, cost-effective andsustainable solution to the issue of reduced air-cooled condenser performanceduring hot periods. Careful design of the first and second stage bundleconfigurations in the hybrid dephlegmator is necessary to avoid flooding in thefirst stage during wet operation of the second. Furthermore, the slightly poorerdry-operation performance of the hybrid dephlegmator results in increased risk ofnon-condensable gas accumulation in multi-row primary condensers. Again,single-row primary condenser bundles would lay rest to such concerns.

[发布机构] Stellenbosch University