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 but slow moving device that accomplishes dewatering by continuous gravity drainage. Screw presses are often used for materials that tend to pack or are difficult to press. The screw press squeezes the material against a screen or filter and the liquid is collected through the screen.
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An example of this is a wine press. Dating back to Roman times, these machines worked similarly to the modern screw press but possessed some disadvantages. The 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. The modern screw press allows 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.
The most commonly known screw press of this design is said to have been invented by Archimedes and is called the screw conveyor. The screw conveyor consists of a shaft about which is a spiral steel plate, similar to a corkscrew. This design is used in a multitude of screw presses. There are some machines of this design that are not screw presses at all - they do not separate solids from liquids but are 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.
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[1] interrupted flight design is the most widely used screw press. The press minimized co-rotation by putting interruptions on the flights of the screw. The shaft of the screw press is also tapered to force the material against the screen.
The buildup of press cake moisture is controlled by a discharge door or cone. Screw presses possess different options that include perforated/slotted screens, a rotating cone, hard surfacing on the screw, and supplemental screen surface in the inlet hopper on the face of the cone. The standard construction for screw presses is of stainless steel with a carbon steel frame on the larger presses.
The specific details of the design of a screw press depend on the material however. The configurations, screw speeds, screens for maximum outlet consistency, including an excellent capture rate vary per material. Most screw presses are designed to feed material that has a 40-60% water make up. The length and diameter ratio of the screw press also depends on the material. The range of the capacity of a screw press, according to French Oil,[2] is 500 kilograms per hour to 12 metric tons per hour. They also have gearbox and machine designs that range from 150 horsepower to 1500 horsepower.
Larger presses use a foot-mounted gearbox while smaller presses use a hollow-shaft gearbox. Currently, nearly all presses are driven by electric motors due to their reliable and low cost frequency drives. The electric motors replaced the previously popular hydraulic motor drives. All screw presses are currently built with the screws in a horizontal configuration. A vertical design was popular in the 1800s through the 1950s but they are no longer made.
Some other types of presses are vapor-tight presses, and twin-screw presses. Vapor-tight presses are used during the production of soybean protein concentrate (SPC), citrus and apple pectin, bioresin, and Xanthan gum. Twin-screw presses contain two overlapping compression screws. This is more complicated on a mechanical level because the screws must remain synchronized in order for them to work properly. These are often used for slippery materials and feature an internal shredding action.
There are two major kinds of screw presses of this design. One type, known as Expellers ®, removes water from fibrous material, while the other removes free liquid from a material.
Oil Expellers ® are used to squeeze the fat out of soybeans, peanuts, sunflower seeds, canola (rape seeds), and other oil seeds. The Oil Expeller ® works by exerting extremely high pressures which convert the fat in seeds into a liquid oil. Once the oil is liquefied the oil flows through the screen and is collected.
Screw presses that are used to free liquid from material are commonly used in the pulp and paper industries, municipal biosolids, septage and grease trap sludge, food production, food waste, manure, and also within the chemical industry.
Pulp and paper industries remove water within cellulose fiber.
Biosolids are dewatered and heated through a specific process which includes raising the pH to a level of 12. Septage and grease trap sludge is dewatered with a simple screw press of the above stated design. Nutrient management programs dewater hog and cow manure for sale and commercial use.
Alcohol solutions are squeezed from foods with screw presses (such as soybeans, protein, pectin, and xanthan gum.) Food processing factories use screw presses to separate water from waste streams and convert the solid into animal feeds. For example sugar beet pulp, orange peel, and spent grain.
Within the chemical industry screw presses are used for “ABS, sodium alginate and carrageenan, synthetic rubber, synthetic resin, hydrated polymer, naphthalene, elastomeric adhesive, color film emulsion, CmC, pharmaceuticals” and more.[3]