PURPOSE: To clean a heat exchanger by stopping the flow of a gas every minute without the stopping of the flow of all of the gas by a method in which the cell of the heat exchanger is divided into plural sections, the inside of any one of gas ducts connected to the heat exchanger is divided into gas flow paths in the same number as the cells, and dampers are provided to the inside of the gas flow paths.

CONSTITUTION: A low-temperature cell 1 and a high-temperature cell 2 have each a 4-stage stack structure of cylindrical cells. In each cell, the heating pipes 3a of heat pipes are laid, and water-washing nozzles 4a are provided on the uppermost stage heat pipe of each cell. The inside of any one of high-temperature gas ducts 10 is divided into gas flow paths in the same number as the cell 2 by partition walls 12a, and dampers 11a are set in each of the gas flow paths. Even in the case of the cell 1, like the cell 2, the outer edges of both-end openings of stacked cells are connected to the low-temperature gas ducts 8, and partition wall and dampers 9a are set in any one of the ducts 8.