To partition simply and surely a passage for both of dry and wet air from a wet passage and permit the generation of desired amount of dry air upon generating white smoke by a method wherein a partitioning plate for partitioning the passage for both of dry and wet air from the wet passage is provided while the periphery of contacting part between a sprinkler bar, penetrating the partitioning plate, and the partitioning plate is sealed so as to be water-tight.

A plurality of neighboring air passages in a heat exchanger are defined into at least two systems and one passage is used as the passage for both of dry and wet air S while the other passage is used as the wet type passage W. In this case, a sprinkling bar 15, spraying sprinkling water or circulating water uniformly into the passages for both of dry and wet air S and the wet type passages W, is arranged so as to be extended horizontally across the widthwise direction of an outdoor air intake. The partitioning plates 16 for partitioning the passages S from the passages W are drooped from an upper water tank 10 and are provided with a through hole 17, through which the sprinkling bar 15 is penetrated horizontally. An elastic sleeve 18 for sealing a gap between the peripheral surface of the sprinkling bar 15 and the periphery of the through hole 17 for respective partitioning plates so as to be water-tight is provided on the periphery of respective through holes 17.