The direct steam generation in parabolic trough collectors is an option to further improve the efficiency and costs of solar thermal power plants. After its operability has been demonstrated, a refined, cost-oriented design of some specific components is in order. One of such components is the separator in the collector field used for the re-circulation mode. This paper presents the first results of an investigation of options for compact and cost effective devices. It outlines the physical mechanisms of phase separation and inspects several models found in the literature for their applicability to the case under study. The focus is on T-junctions, which offer potential cost advantages over more voluminous constructions. (C) 2003 Elsevier Ltd. All rights reserved.