Cross flow filter device provided for filtering a raw material comprising particles in a medium, said cross flow filter device comprising: a first longitudinal filter element having a central longitudinal axis; a second longitudinal filter element installed substantially concentrically inside said first longitudinal filter element, said second longitudinal filter element being a separate filter element to said first longitudinal filter element; a housing surrounding said first longitudinal filter element; a raw material inlet provided for providing said raw material to said first and second longitudinal filter elements (1, 2); a concentrate outlet provided for a concentrate comprising said raw material enriched in particles; a first filtrate outlet provided for a first filtrate which has passed through the first longitudinal filter element and which comprises said medium depleted in particles; and a second filtrate outlet provided for a second filtrate which has passed through the second longitudinal filter element and which comprises said medium depleted in particles; characterised in that said first longitudinal filter element has a first constant inner width and the second longitudinal filter element has a second constant outer width which is smaller than said first constant inner width, the ratio between said second constant outer width and said first constant inner width being at least 65%, said first and second widths (w1, w2) of said first and second longitudinal filter elements (1, 2) defining a space therebetween; in that said raw material inlet is connected to said space to supply said raw material thereto, the filtrate passing through the filter media of the first longitudinal filter element from said space to the outside thereof and through the filter media of the second longitudinal filter element from said space to the inside thereof; and in that the raw material inlet and the concentrate outlet are aligned with said central longitudinal axis. a first longitudinal filter element having a central longitudinal axis; a second longitudinal filter element installed substantially concentrically inside said first longitudinal filter element, said second longitudinal filter element being a separate filter element to said first longitudinal filter element; a housing surrounding said first longitudinal filter element; a raw material inlet provided for providing said raw material to said first and second longitudinal filter elements (1, 2); a concentrate outlet provided for a concentrate comprising said raw material enriched in particles; a first filtrate outlet provided for a first filtrate which has passed through the first longitudinal filter element and which comprises said medium depleted in particles; and a second filtrate outlet provided for a second filtrate which has passed through the second longitudinal filter element and which comprises said medium depleted in particles; characterised in that said first longitudinal filter element has a first constant inner width and the second longitudinal filter element has a second constant outer width which is smaller than said first constant inner width, the ratio between said second constant outer width and said first constant inner width being at least 65%, said first and second widths (w1, w2) of said first and second longitudinal filter elements (1, 2) defining a space therebetween; in that said raw material inlet is connected to said space to supply said raw material thereto, the filtrate passing through the filter media of the first longitudinal filter element from said space to the outside thereof and through the filter media of the second longitudinal filter element from said space to the inside thereof; and in that the raw material inlet and the concentrate outlet are aligned with said central longitudinal axis.