Abstract: A vapor generating apparatus operating with a small quantity of fluid which comprises in a single unit a heater including a plurality of similarly shaped tubes arranged in one or several coaxially extending layers each comprised of a number of successive sets of adjacent coils for circulating a heat transfer fluid, and a boiler including a bundle of vertically extending tubes for circulating the heat transfer fluid discharged from the heater, said tubes lying within the liquid to be evaporated, the boiler being arranged with a vapor/liquid separator connected at the vapor outlet and control means adapted such that a continuous flow of dry vapor at the separator outlet without a separation plane between liquid and vapor phases being formed.