A vapour power generating system for generating power by using heat from a source of heat. The system has a closed circuit for a working fluid, and includes a heat exchanger assembly (1) for heating the fluid under pressure with heat from the source, a separator (8) for separating the vapour phase of the heated fluid from the liquid phase thereof, an expander (14) for expanding the vapour to generate power, a condenser (17) for condensing the outlet fluid from the expander (14), a feed pump (F) for returning condensed fluid from the condenser (17) to the heater and a return path for returning the liquid phase from the separator to the heater. The liquid phase of the working fluid contains a lubricant which lubricant is soluble or miscible in the liquid phase and a bearing supply path (21) is arranged to deliver liquid phase pressurised by the feed pump (F) to at least one bearing for a rotary element of the expander.