The installation of parallel risers between the cooling-air cooler and the low-pressure drum in the heat recovery steam generators used for model Vx4.3 gas turbines poses several thermohydraulic questions which could not be resolved by analysis. A test system was designed at the DANTON test facility whose configuration was based on the planned geometry of the full-scale system in order to determine through a series of experiments the thermohydraulic properties of two-phase flow in unheated, parallel risers inclined at an upward angle of 10°. The overall system operated in accordance with the principle of natural circulation.