PURPOSE: To enhance the efficiency and production of a plant by a method wherein heat transmitting pipes are dispersedly arranged in plural shelves so that the heat transmitting pipes are prevented from the effect of the static pressure due to the depth of water immersion, in an evaporator for use in a low thermal heat power generation system.

CONSTITUTION: The fluid fed from a pump 14 is led to the uppermost shelf 18 and allowed to flow from left to right, as indicated by the arrow. During flowing in this way, the working fluid exchanges heat with the surface-layer sea water flowing in heat transmitting pipes 16, whereby part of the working fluid is evaporated; the unevaporated portion of the working fluid flows down from the right end of the shelf 18 to the right end of the next shelf 19, and flows toward the left end of the shelf 19, undergoing heat exchange. In this way the working fluid flows down the shelves sequentially. In this case, the liquid surface level in the evaporator is detected by a detector 23 and kept constant. Accordingly, the effect of static pressure due to the depth of immersion is eliminated, the heat transfer on the side of the working fluid outside of the pipes is equal to the sum of the heat transfer by boiling of pool and the heat transfer by forced convection, resulting in an increased coefficient of heat transfer and enhanced efficiency and production.