Abstract: Disclosed is a device that comprises a cylindrical member, a viscous member which is applied to the cylindrical face of the cylindrical member to provide a viscous layer, a contact member, which contacts the cylindrical face, and a contact area varying member which varies the contact area, wherein the contact area is variable to change braking force for revolutions.

Abstract: A method for removing soot or the like adhered to heat transfer tubes of a heat exchanger by providing a steel ball scatterer above the heat transfer tubes and intermittently scattering steel balls towards the heat transfer tubes. The improvements reside in that the steel ball scattering rate is set to a small initial rate at the commencement of the scattering operation and thereafter is increased either in a stepwise manner or continuously. Preferably, within a main body casing of the heat exchanger, a plurality of steel ball collision preventing plates having their central portions extending convexly upwards are provided between the steel ball scatterer and the heat transfer tube group, in order to prevent fins of the heat transfer tubes from being damaged by steel balls falling from the steel ball scatterer and directly colliding against the fins.