Abstract: A fluid-circulation system, specifically a heat exchanger with entrance and exit ducts interconnected by a bank of tubes, is provided in its exit duct with an extractor for intercepting generally spherical cleaning bodies of sponge rubber or the like which are to be returned to the inlet duct via a bypass connection for recirculation through the tubes. The extractor comprises two substantially symmetrical screens, converging downward in the direction of fluid flow, which direct the oncoming cleaning bodies into a narrow collecting box having a sloping bottom near one or more outlets forming part of the bypass connection. The two screens have generally planar confronting surfaces and are swingable about respective transverse axes into a downwardly diverging position in which their opposite surfaces are exposed to the flow for cleansing purposes during a regeneration phase.