Drainage and venting in a swirl vane moisture separator application

Moisture separation, such as that required to dry the steam generated in a recirculating boiler, is often performed in stages. In a typical recirculating nuclear steam generator, there are three serial stages of separation: centrifugal (primary), natural (gravitational), and impact (chevron dryer) separation. In describing the behavior of such moisture separators, care must be taken to distinguish among these stages. And in designing multi-stage moisture separators, care must be taken to avoid interference between one stage and the others. In particular, drainage from natural separation (stage 2) should be kept separate from the partially dried steam produced by stage 1. Significant reduction in the moisture level of steam ingested by the third stage can be achieved by separating steam venting and liquid drainage, as opposed to using mixed liquid/vapor relief. In fact, the drainage and venting arrangement can dominate the performance of the separator overall. Under certain circumstances, quantification of primary separation efficiency alone becomes difficult, and the influence of the first stage itself is of no more importance than the vent/drain arrangement between the first and second stages. A series of swirl vane separator experiments demonstrating the concept are described, and comparison between separated and mixed relief is made.