PURPOSE: To prevent the overheat of a heat transfer tube as well as the deterioration of dryness of steam by a method wherein the rows of heat transfer tubes are provided between respective upper headers and lower headers, which are installed in parallel, while an upper communicating tube, connecting respective upper headers mutually, is provided to regulate pressures in respective upper headers and respective rows of heat transfer, tubes.

CONSTITUTION: Upper headers 1 and lower headers 2 are arranged in parallel and the rows 3 of heat transfer tubes are provided between a pair of upper headers 1 and the lower headers 2 in one row while an upper communicating tube 9, connecting respective upper headers 1, is provided. When a pressure difference is generated between respective rows 3 of heat transfer tubes and respective upper headers 1, steam is sent from an upper header 1, whose pressure is high, into another upper header 1, whose pressure is low, through the upper communicating tube 9 and the pressure difference, generated due to the difference of steam generating amounts, can be reduced whereby the differences of water level, which are generated in every rows 3 of heat transfer tubes, are reduced. In this case, the level of water can be regulated in accordance with conditions in respective rows 3 of heat transfer tubes even when differences of heating amount are generated in every rows 3 of heat transfer tubes.