PURPOSE: To clean up the multiple connected tubes of heat exchanger at one time by a method wherein, when a grind cleanser is carried inside the tube by means of pressurized nitrogen gas throughout the overall length of continuous tube, it is specified so that the inlet flow rate and the outlet flow rate of said gas may perform the optimum cleaning operation of the tube inside.

CONSTITUTION: A heat exchanger 50 to be cleaned up is opened fitting the ends of tube 52 with U type connecting tubes 54 and connecting the tubes to each other to form a long continuous tube. At this time, the length of one tube shall be specified so that the outlet flow rate may perform the optimum cleaning operation causing no thinning wall by the outlet flow rate calculating the inlet flow rate and the tube length depending upon the inlet pressure and the charge of the pressurized nitrogen gas as well as the flow resistance depending upon the constituents of scale. A grind cleanser such as small steel balls etc. may be carried and forwarded to one end 56 of one tube so far constituted by pressurized nitrogen gas to perform the cleaning operation.