A dewatering screw press is a screw press that separates liquids from solids. A screw press can be used in place of a belt press, centrifuge, or filter paper. It is a simple, slow moving device that accomplishes dewatering by continuous gravitational drainage. Screw presses are often used for materials that are difficult to press, for example those that tend to pack together. The screw press squeezes the material against a screen or filter and the liquid is collected through the screen for collection and use.
History
An example of a dewatering press is a wine press. Dating back to Roman times, these machines worked similarly to the modern screw press but possessed some disadvantages which have been corrected and improved within modern presses. The ancient wine press only allowed for grapes to be juiced in batches and often a thick cake would form against the screen, making it difficult for the juice to flow through the screen and be collected for wine. Most modern screw presses allow for a continuous flow of material by surrounding the screw with a screen, which also helps to avoid the build up of a layer of solid material on the screen. One modern approach even removes the screen in favor of a system of fixed and moving rings, which often eliminates solids buildup entirely.
The most commonly known screw press of this design is said to have been invented by famous Greek mathematician Archimedes and is known as the screw conveyor. The screw conveyor consists of a shaft, which is surrounded by a spiral steel plate, similar in design and appearance to a corkscrew. This design is used in a multitude of screw presses. There are some machines of this and also of similar design that are not screw presses at all - they do not separate solids from liquids but are instead used to fuse them together. An example of this is a mold-filling machine. Plastic pellets are inserted at one end and heat is applied, melting the pellets and discharging them into a mold. Another example is known as a cooker-extruder and is used in the production of snack foods such as pretzels and more.
Design
Most screw presses can have dilute materials pumped directly into the screw press, although pre-thickening sometimes improves the performance of the press. This is typically done with a static or sidehill screen, a rotating drum screen, belt press, or a gravity table.
Patented in 1900, Valerius Anderson's interrupted flight design is most commonly used as opposed to the continuous flight design. Anderson, upon studying the continuous flight design, noticed that it led to co-rotation and a less efficient job being done dewatering, especially with softer materials. He solved this by putting interruptions on the flights of the screw. The interruptions allowed for the materials to stop moving forward between interruptions along the shaft and also allows for an adequate buildup of the material before it is pushed through the screw press to container that catches the material. This allowed for a better job at the dewatering and a consistent cake material being released.
The interrupted flight design screw presses uses were broadened from just soft or mushy materials to include most materials screw presses were used for because unlike the continuous design screw presses the interrupted flight design did not require constant feed or consistency of material. If either were diminished in the continuous design so would production of the dewatered product, in order to avoid this while maintaining the continuous flight design a larger and heavier press with variable speed settings was a necessity; the press also entailed the need of an operator.
The interrupted flight design eliminated the need for consistency as the compression of the screw did not change as the material did not progress through the screw until a sufficient amount of the material had formed, as described above. This also eliminates the need for changing speed and an operator. The design allows for self-correction and efficiency that is unavailable with the continuous design. It allowed for a more economically effective screw press that has been used for more than just slimy or slippery materials.
After a period of time and its initial patent, resistor teeth were added to the presses where there was no flighting in order to increase the agitation of the materials adding to the limitation of the tendencies of co-rotation within the press