PURPOSE: To enable accumulation of sludge to be reduced, eliminate a cleaning of a collector as well as various accompanying facilities and further enable the cleaning operation to be rapidly completed by a method wherein a separate box type sludge collector is fixed between an outer pipe enclosing a group of heat transfer pipes and a gas-water separating device.

CONSTITUTION: A separate box type sludge collector is removably attached between an outer cylinder 5 enclosing a group 8 of heat transfer pipes and a gas-water separator 6. In this type of heat exchanger, an upper part of the outer cylinder 5 is provided with a support and the separate box type sludge collector may be removably attached to the support. In addition, the collector may be divided into each of upper stage, middle stage and lower stage, respectively, and fixed. In this way, arrangement of the separate box type sludge collector causes iron substances brought from the supplied water to become magnetite particles (sludge). The sludge is collected by the collector, so that accumulation of the sludge on a pipe plate can be reduced.