Abstract: A liquid inlet opening of a cyclone filter is connected to a discharge opening of a first pump adapted to pump up untreated liquid coolant contained in a reservoir. The cyclone filter separates the liquid coolant fed by the first pump into clean liquid and dirty liquid as it swirls the liquid coolant. A clean liquid outlet of the cyclone filter is connected to the suction opening of a second pump so that liquid coolant with its pressure increased in the second pump is fed under a high pressure to a machine tool. A dirty liquid return pipe is connected to a dirty liquid outlet of the cyclone filter, with the end of the dirty liquid return pipe submerged in the liquid contained in the reservoir. A flow-directing orifice adjacent the clean liquid outlet of the cyclone filter ensures the dirty liquid flows into the dirty liquid return pipe.