Working Principle: It takes the filter paper as the filter element, the waste oil is injected into the machine under high oil pressure, which produced by the oil pump. The impurity is hold up by the superimposed filter paper. When the value of oil pressure excess to a setting value or the filter paper is wet, we just need to change the new filter paper, then it can work normally. The wet filter paper can reuse again after drying by a special dryer.