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Dewatering screw pressA 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
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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.[1]
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
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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
Options
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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.
Capacity
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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
Drive
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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. A vertical design was popular in the 1800s through the 1950s but they are no longer made. Most screw presses are currently built with the screws in a horizontal configuration. One newer version uses an angled screw design to reduce floor footprint and press cake moisture.[2]
Compressive Mechanisms
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Compression is created within the screw press by increasing the inner shaft diameter of the screw. For example, if a 16" screw press has a 6" shaft at the start, the flights on the screw will be 5" tall. If this 6" shaft diameter is then increased to 12" at the discharge, the fights will be only 2" tall at this point. Thus compression is applied as the material is being pressed from a 5" opening through a 2" space.
This compression can also be achieved tightening the separation of the flights of the screw. If at the inlet, the pitch is 16", the material thus will move 16" with each revolution. If it is then decreased to 8" at the point of discharge, the material will move 8" per revolution. This results in there being more volume forced into the press than there is being forced out of the press at a time. This creates the desired compression and pushes the liquid through the screen.
Another way to achieve compression is to place a cone at the point of discharge. This can also be called a choke, stopper, or door. In many designs it is bolted into a fixed position, making a fixed, smaller opening which the material must pass through. More commonly found however, the screw press has the cone pushed into the point of discharge via a hydraulic or air cylinder.
Specialized types
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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.
Classification
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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.
Expellers
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Oil expellers are used to squeeze the fat out of soybeans, peanuts, sunflower seeds, canola (rape seeds), and other oil seeds. The 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.
Removal of free liquid
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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.
Applications
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Pulp and paper industries
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Pulp and paper industries remove water within cellulose fiber.
Sewage disposal
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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.
Food processing
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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.
Fish and orange peel dewatering often provide maximum yield when dewatered within a press of the interrupted flight design and with the addition of steam begin injected into the material. Commonly steam injection holes are drilled into the resistor teeth of the press close to the screw's shaft.
PET bottles "polyethylene terephthalate" is the preferred packaging for soft drinks, fizzy drinks, juice and water. This results in large waste problems. For breweries, large volumes of discharged products need to be destroyed regularly in order to eliminate the risks of the bottles being resold again. For waste collectors handling and transport is difficult and expensive, as there is a large discrepancy between weight and volume[3]
Manufacturers of ice-cream have a need to destroy returned goods with expired date and faulty manufacture to prevent the products being sold by mistake.[4]
Dairies destroy returned goods such as yogurt and other dairy products with expired date and faulty manufacture.[5]
Chemical industry
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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.[6]
Cosmetics: To many manufacturers and brands the challenge is often that they have large amounts of discarded products, that needs 100% destruction to ensure no reselling on the black market.[7]
References
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Screw Press. A striking press whose force is derived from a spindle with steep-pitched threads; an early press that remained in use for nearly four centuries before being replaced by modern presses. The first screw presses derived their power from men, working in teams – pushing on opposite ends of a long balance beam and walking around the press – later by horses or by water power; thus their capability was limited to striking coins and medals of limited diameter (certainly less than 40 millimeters, but more often less than 25mm). Not only was the screw press used for striking, see illustration by Benvenuto Cellini who used one in 1530, but was also used for hubbing and adapted for blanking.
Prior to the screw press, coins were made with a mill and screw, or simply, milled or mill coinage. The development of the milled edge, but this was an incorrect term. The screw press formed the Source of power changed. To fly-press, because of the momentum of its spindle advancing down and up, and the beams rapidly flying around the press. (Hand-operated fly presses of varrying sizes are still in use in some modern metalworking plants. They can be used to try out new equipment before it is installed in a power press.)Later the power came from water power, and by the end of the 18th century, by steam power. Although the How A Screw Press Was Operated Manually "A coin setter], seated in a CH17 {1953} p 164. How the screw press was used. Three (or more) men were required to operate a screw press. One sat facing the press and manually fed the blanks and removed the coins coin setter. (Automatic feeding and delivery was installed on screw presses following 1790.)Two or four men above him – called the spinners – pressed the balance beam at opposite ends pressing in opposite directions. As the men pushed against the beam the spindle on the press moved downward the housing containing the History of the screw press. The concept of screw press had been used in mid 1400s for both printing and taschenwerke), the screw press thrived and was widely used for C1 {1540} BiringuccioC24 {1948} MasonC32 {1954} Singer, 3:338-41.C36 {1956} ForsterC60 {1974} Goldman.C64 {1978} Adams.C67 {1988} Cooper, Chapter 6, pp 51-60.C69 {1995} DeLoreyCH17 {1953} Craig, p 164.NC5 {1964} Peck, p 141.NC11 {1988} Breen, p 708.NE40 {1984} Junge, p 227.Prior to the screw press, coins were made with a blow from a sledge to a punch (containing one design) on a blank laid on a die in the lower position called the pile (containing the opposite design). This coinage is known collectively as hammered and produced by a moneyer . With the development of the screw press, coins and medals truly became struck (see striking).The process of striking in a screw press was calledor simply,orThe development of the rolling mill not only gave this coinage their name but also gave coins more uniformity in their thickness After the invention of the collar (in 1555) struck coins had an edge , unlike hammered coins. Striking with a collar formed an edge that could be either smooth or have reeding . Infrequently this reeding was calledbut this was an incorrect term. The screw press formed the edge , but when it was operated by man the coins were usually of small diameter and had shallow relief To strike larger coins (or deeper relief) more pressure was required of the press. The balance beam became longer, weights were added to the end to effect more pressure, and the number of men increased or horses used. This style of the screw press was called abecause of the momentum of its spindle advancing down and up, and the beams rapidly flying around the press. (Hand-operated fly presses of varrying sizes are still in use in some modern metalworking plants. They can be used to try out new equipment before it is installed in a power press.)Later the power came from water power, and by the end of the 18th century, by steam power. Although the knuckle-joint press was invented in 1817 by Diedrich Uhlhorn in Germany, it did not immediately replace the screw press. It was learned that it was easier to electrify a knuckle-joint press by electric motors than to electrify a screw press. Thus the screw press was only phased out of production with the rise of electricity and electric motors (following 1890).Even so, screw presses were still used for striking medals well into the 20th century (even at the Paris Mint). Screw presses finally were replaced when hydraulic presses (developed during World War I) became widely available thereafter. See presses and pressroom practice "A coining machine [screw press] resembles in a massive way the old-fashioned press used for copying letters or embossing seal on papers. The dies were inserted near floor level and the lower fixed to the foundations; the upper die to the base of a vertical column [spindle] with a spiral engaged to the housings. Two arms , each 6 feet long, loaded at the tip with 300 pounds of lead and furnished with two ropes, projected from the top of the column at about waist-height. "Normally four labourers, hauling on these ropes with the utmost violence of which they were capable, sent the column and die spinning down on a blank; as it rebounded it was [brought] back by a lighter rope to prevent double- striking, and wound up for a fresh blow . A [pressman orseated in a pit before the opening, flicked the struck coin away with his middle finger and with index and thumb set a fresh blank on the lower die . "The ponderous machines worked at the astonishing pace of a blow every two seconds. The toil was exhausting; a crew of seven was assigned to a press, of whom three rested while the four worked a twenty-minute spell. They were reckoned to labour at the press for five hours only of the working day of ten; the rest of the day was spent in fetching and carrying and odd jobs." –John Craig, The Mint {1953} p 164.Three (or more) men were required to operate a screw press. One sat facing the press and manually fed the blanks and removed the coins after they were struck . Sometimes he sat in a pit that allowed him to get closer to the striking . He was called the(Automatic feeding and delivery was installed on screw presses following 1790.)Two or four men above him – called the– pressed the balance beam at opposite ends pressing in opposite directions. As the men pushed against the beam the spindle on the press moved downward the housing containing the die . It was easy at first until the die met the blank , then it required great strength to press even harder to advance the die into the blank forming the design . With practice the team of men could get their rhythm synchronized until the momentum drove the spindle down and back in rapid succession.Longer arms of the beam and heavier weights at the end gave them greater leverage to increase the pressure for the final squeeze forcing the dies into the blank as far as necessary. To achieve even greater pressure it required more men on each end of the beam. The larger the diameter of the coin the greater the pressure was needed. A crown size coin could require as many as eight men, four on each beam. (The use of a horse could replace as many as four or six men.)After they pressed the coin with as great a force as they could apply, the spinners reversed themselves and pressed on the beam in the opposite direction to raise the die housing on the spindle. Sometimes they attached leather straps to the beam to pull the beam back, if they preferred to do that, since raising the die housing did not require great strength to return it to open position.The concept of screw press had been used in mid 1400s for both printing and squeezing fruits and olives. The first screw press for striking numismatic items was developed by an Italian, Donato Bramante (1444-1514) in 1506 for striking lead seals for Pope Julius II (1503-13). This principle was described and illustrated by Benvenuto Cellini in his work on goldsmithing and he used a screw press in 1530 for striking his own lead seals for Pope Clemente VII (1523- 34).In Augsburg, Germany in 1550, Max (or Marx) Schwab improved on the screw press and attempted to sell this to the mint in Venice. Unsuccessful there he did exhibit his "engines" (a term used for screw presses, hubbing presses and rolling mills) to a French ambassador leading to the purchase by King Henry II, for the royal mint in Paris (delivered in January 1551). Later Spanish emissaries visited Schwab to learn of his techniques and machines.The screw press was installed at the Paris Mint . An employee there, Eloye Mestrel, learned its operation very well , but became dissatisfied with the lack of acceptance of the new press by moneyers who feared their hammer technique would be replaced. Mestrel fled, in 1561, to England and set up a screw press at the London Mint . Following this early use in Italy, Germany, France and England, use of the screw press spread to other mints throughout Europe. Despite an attempt to use roller dies to impress blank strip , later to be cut out (the process known asthe screw press thrived and was widely used for striking .No major improvements occurred until 1786 when an Italian engineer, Francesco Comelli, at the Bologna Mint invented a collar and ejection system to be used with the screw press. This would strike coins and shape the edge with one pressing. A few years later, two engineers at the Paris Mint devised a way to feed the blanks, drop the blank in the collar strike and eject the piece , then remove it in one cycle while the press was manually operated. Jean-Pierre Droz and Philippe Gengembre were these innovators.It was for Boulton and Watt, who hired Droz away from the Paris Mint in 1789, to devise a way to incorporate all these innovations and run the press by their steam powered engines. This was an outstanding undertaking and a landmark in the history of coining!Boulton and Watt made full use of the screw press, with Comelli's collar and Droz's automatic feed and delivery systems, with the power of their steam engines. Not only did they manufacturing screw presses for their own private mint in Birmingham, and by custom striking with these presses, but also to manufacturing these presses for other mints.Thus the screw press struck coins for nearly 400 years (1505-1898), medals were struck by the screw press well into the 20th century (1530-1914). The press was also utilized to do blanking , and even hubbing until the end of the 19th century. The invention of the Uhlhorn knuckle-joint press , in 1817 in Germany, and its improvement, by Thonnelier in 1836 in France, did not entirely replace the screw press.It was two events that greatly reduced the usefulness of the screw press: (1) it was learned that a knuckle-joint press with a large fly wheel could be electrified easier than a screw press when electricity became commercially available (after 1883), and (2) improvements to the hydraulic press which proved far more useful for hubbing (and for striking large medals). Screw presses are still in use in tool and die shops for pressing where only a moderate pressure is required, even into the 21st century.References:{1540} Biringuccio{1948} Mason{1954} Singer,:338-41.{1956} Forster{1974} Goldman.{1978} Adams.{1988} Cooper, Chapter 6, pp 51-60.{1995} DeLorey{1953} Craig, p 164.{1964} Peck, p 141.{1988} Breen, p 708.{1984} Junge, p 227.
excerpted with permission from
An Encyclopedia of Coin and Medal Technology
For Artists, Makers, Collectors and Curators
COMPILED AND WRITTEN BY D. WAYNE JOHNSON
Roger W. Burdette, Editor
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