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30.11.2017

Only picking by hand is gentler: The separation of coarse and small particles in tea manufacturing

Autumn is tea season. So, consumers only want to brew actual tea leaves and not some plant stems or even bits of plastic or metal. For the manufacturers, it is incredibly important to separate off these foreign objects before the tea is packaged. But not just foreign bodies, the smallest fragments of the leaves that get in the cup whilst the teabag is brewing are also a nuisance. For an untroubled tea-drinking experience, the manufacturer uses a screening machine at the end of their manufacturing process to separate off all the undesirable bits. However, not every screening process is suitable for the delicate tea leaves. If the screen movements are too intense then the delicate tea leaves can be broken up which increases the amount of small particles in the screening trough, as well as the amount of production waste. In order to avoid this, screening processes that are especially gentle to products are needed. That is reflected in a current task carried out by J. Engelsmann AG for one of the largest tea manufacturers in Germany. The different types of tea had to be separated from small dust particles and fine debris, as well as from large foreign bodies, as gently as possible, before being packaged.

Searching for a gentle screening process

After the tests in the Engelsmann technical center it quickly became clear that the JEL Freischwinger was the best screening machine to handle this job, thanks to its gentle operation. With the long-stroke screening machine, the screen trough moves in a consistent, horizontal oscillating movement. This type of screen movement ensures that the product that requires screening only moves in a horizontal direction and therefore stays on the screen mesh. In contrast to this, there are vibration screening machines, for example, which move the entire screen trough at a high-frequency oscillation with the help of an unbalance motor. It works both horizontally and vertically which causes the product being screened to jump up and down. With these types of machine the screening is indeed very efficient, however, the delicate tea leaves would be broken up.

The intensity of the oscillation of the long-stroke screening machines can be controlled by the length of the screen (20, 30, 40 or 60 mm) and the oscillation frequency (280-420 min-1 depending on the adjusted stroke), which means that the machine can be perfectly adapted to the product being screened or in this case the particular type of tea being screened. A further advantage of the long-stroke principle is the energy-saving momentum. The electronic motor that is used transfers power to the flywheels, which match the size of the screen trough exactly, via a V-belt. The thrust rods that are connected to the flywheels now control its horizontal movement. After only 15 seconds, the trough is kept in motion with only 10-20 percent of the nominal current, which represents a significant conservation of energy, especially for large screening machines. For the tea manufacturer, the Freischwinger is also a large screening machine. In order to reach the demanded throughput rate of 2,000 kg/h, a very large screen of almost 7 meters in length was used on a screening surface of 16 m², divided into two screening levels, the upper deck and the lower deck.

First, the tea leaves are spread across the screening surface that is equipped with a separator sheet which then transports them to the upper screening level. Thanks to this sheet, the flow of product is spread out to all sides so that the entire screen surface is optimally utilized. The screen mesh on the upper deck is covered with a relatively large mesh of 4-12 mm, in order to separate off all coarse particles, such as coarse tea leaves, remnants of plants such as wooden stalks, as well as foreign bodies such as gloves that were accidentally packed by the tea harvesters. All these coarse particles remain on the mesh and are eventually transported to the coarse product outlet of the machine. Below that lies the screen deck where the small particles are separated. There, the screen mesh is covered with a mesh size of 0.2-1.4 mm. Fine dust particles and tea leaves that are too small or broken are screened off at this level and transported to the fine product outlet of the machine. The good products from the machine are eventually channeled to a third outlet, to be portioned and packaged in the next step of the process.

Full capacity thanks to continuous cleaning

Small pieces of tea leaves that are the exact same size as the mesh on the screen can get stuck and clog up the mesh, which can have a negative effect on the throughput rate. Therefore, the JEL Freischwinger is also equipped with a dedusting system that uses rubber balls to clean the screens. With this type of screen insert cleaning, rubber balls that bounce off of a woven wire mesh are used on the underside of the inserts. The rubber balls are set in motion by the horizontal oscillation of the screen trough and continuously rub on the underside of the screen mesh and in doing so, loosen the stuck particles from the individual mesh openings. For this specific application, the long-stroke screen was additionally equipped with Makrolon windows along the screen trough. These can be opened by quick release clamps, so that the dust that builds up on the lower deck screen can be vacuumed off without the operator having to dismantle the upper screen insert. The dust tight tea-screening machine was designed in accordance with German accident prevention regulations (UVV) and features large inspection and assembly hatches which allow easier access to all areas of the screen trough. In order to keep downtimes to a minimum and to maximize the operational availability of the screening machine, it is important, particularly for screening machines with large screen surfaces, that the screen inserts can be changed quickly and easily. The cover of the JEL Freischwinger that features viewing windows is therefore made from lightweight aluminum. After the cover is folded up and kept open with pneumatic springs, the screen inserts can be changed in just a few steps with the help of the attached quick release clamps.

So, the JEL Freischwinger offers many advantages for operators who are looking for a screening process that is gentle to their delicate products. This screening machine covers a wide spectrum of particle sizes, from 40 μm to 20 mm and is available with screen sizes ranging from 0.24 m² to 24 m², suitable for both small and large tasks.