Variable reducing pitch screw

In this design, the pitch decreases with axial distance as opposed to the screw shown in Fig. 8.32(a). The reducing pitch causes a lateral compression of the material in the screw channel as a result, the normal compression from the reducing channel depth can be reduced or eliminated altogether. In fact, many of these variable reducing pitch screws maintain the same channel depth along the entire length of the screw. 

Variable pitch extruder screw with reducing pitch... 

Fig. 6.4-38 sho vs the functional components. At the inlet on top of the equipment, the slightly pre-mixed feed components enter first a mixing and predensification screw. Pressure builds-up by the changes in shaft diameter, sometimes variable pitch of the screw flights, and by a collar at the end of the screw shaft or a pressure piece , both forming an annular space with reduced area through which the material must pass. Such a screw conveyor, mixer, and processor is often called expander. 

In the press process, the seed from which oil is to be removed is cracked and crushed to produce flakes about 0.25 mm thick, which are cooked at temperatures up to 104 °C for 15-20 minutes. The temperature is then raised to about 110-115 °C until the moisture content is reduced to about 30 g/kg. The material is then passed through a perforated horizontal cylinder in which revolves a screw of variable pitch that gives pressures up to 40 MN/m. The residue from screw pressing usually has an oil content of 25-40 g/kg. The cylindrical presses used for extraction are called expellers and the method of extraction is usually referred to as the expeller process. 

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