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A screw press is a continuously operating machine used to squeeze liquid from a mass of fibrous solids, such as in the recovery of animal or vegetable oils, or further to dewater a pasty filter cake. It consists of a screw rotating inside a perforated or slotted shell, so that the mass is pressed against the inside of the shell. The solids must be coarse or stiff enough so that they do not pass through the holes in the shell, while the expressed liquids are extruded through the perforations in the shell and collected in a surrounding casing.
The press can be operated with its axis vertical or horizontal. Pressure is increased along the length of the press either by reducing the screw pitch, or by increasing the shaft diameter or by reducing the diameter of the shell, and can be controlled by varying the size of the outlet for the squeezed solids. The shaft may be hollow to permit steam heating, and wash liquid can be added along the screw to increase recovery.
The expressive forces are high so that screw presses are rugged machines, and the shell can only have fairly coarse perforations. Accordingly, the liquid produced in a screw press will almost certainly need a stage of filtration after the press.
A similar function to that of the screw press, i.e. the expression of liquids contained in natural products, can be achieved by means of rollers, or vertically mounted discs (rotating on axes slightly inclined down from the horizontal so that there is a
pinch zone at the bottom of the rotation) – but neither is a filtration process.