PURPOSE: To obtain the titled heat exchanger, easy in removing non-condensed gas, installation and maintenance, by a method wherein an evaporating tube group, a condensing tube group and upper and lower headers are divided to form a plurality of enclosed vessels while counter-flow tubes for extracting the non- condensed gas accumulated in respective enclosed vessels are provided.

CONSTITUTION: The end part of the upper header 30B of the condensing tube group 30 is partitioned by a porous partitioning 25 to form a gas reservoir 27 while the condensing tube 32 below the partition is utilized as the counter-flow tube for extracting the non-condensed gas. Operating medium is supplied to an arbitrary liquid level in the evaporating tube group 20 from a pipeline 7 through a valve 8 while air in the heat exchanger is released from the valve 9 passing through the pipe 5 and the non-condensed gas in the condensing tube group 30 is released to the outside of the system through the gas reservoir 27 and valve 26A after passing through the counter-flow tube 32. Transportation, assembly and maintenance of the tube groups are facilitated by forming small tube groups into a block.