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Simply a tube

Clarification and screen sections with the same diameter make worm/screen decanters more productive
Simply a tube

The standard screen decanters in the Conthick series are mainly employed in the production of bulk materials, such as potassium chloride, sodium carbonate or fertilisers. These machines are designed for throughputs of 20 to 25 t/h. Yet the demand for centrifuges with higher throughputs is on the up. This motivated Siebtechnik’s decision to develop its Turbo Screen Decanters in a tube version for handling up to 100 t of liquid/solids mixes.

The standard screen decanters in the Conthick series rank among the time-tested machines in Siebtechnik’s range. They take the form of two-stage centrifuges. The first stage is a very generously dimensioned solid bowl. This is where clarification and/or sedimentation of the liquid/solids mix occur. Attached to the cylindrical solid bowl is a cone, which discharges into a comparatively short screen section. A thick solids cake forms on the surface of this small-volume section. Over and above this, the lowest G forces are generated as a result of the small drum screen in this part of the machine. These two factors necessarily prolong the drying time required or reduce the drying effect with short residence times.

Large screen section
“The standard screen decanter is, as ever, well positioned in the market“, asserts Horst Dietschreit, Siebtechnik’s Technical Manager Centrifuges. “Admittedly, the short drum screen with its small diameter means that processes with mass throughputs cannot always be managed optimally. What’s more, when manufacturing bulk goods, machines with a higher throughput are frequently demanded“. This was reason enough for Siebtechnik’s engineers to develop a more powerful machine – the tube version of the Turbo Screen Decanter. From the design viewpoint, it is an optimised variant of the proven and tested Turbo Screen Decanter. Turbo Screen Decanters are continuously operating, two-stage, solid-bowl worm/screen centrifuges. The first stage is a cylindrical clarification drum that takes the form of a solid bowl, tapering to the end. Connected to this is the screen section as the second stage. In contrast to the Conthick machines, however, the drum screen has a significantly larger diameter than the clarification drum. “This enables up to 20 to 30 % higher G forces to be attained than on the dry beach of the clarification drum“, Dietschreit explains. “In the transition from the clarification to the screen drum, the solids cake is broken open. As a result of the much larger surface available, the pre-dried solids can be distributed in a thin layer inside the drum screen. This means that in the event of high throughputs combined with high G forces and long residence times, a solid matter is formed with a small layer thickness and therefore good residual moisture content“.
So far, Siebtechnik has installed 23 Turbo Screen Decanters throughout the world. Their main application is reprocessing polymers. In the standard version, these centrifuges are suitable for solids with a particle size greater than 40 µm. With feed concentrations of 25 to 30 %, they achieve a maximum throughput of 50 t/h, depending on the product.
The next development stage
In the Turbo Screen Decanter, the clarification drum has a smaller diameter than the screen drum. “In order to increase the throughput further, it seemed prudent to develop a machine in which the two drums have the same diameter, in other words to boost the performance of the clarification section. The bottom line is a tube that accommodates both decanter stages“, says Dietschreit, describing the basic idea behind the Turbo Screen Decanter in tube design (also known simply as the “Tube”).
This new centrifuge had its premiere at Achema 2009. The heart of the decanter is a continuous tube, into which a cone is welded, separating the clarification and screen sections. The machine works as follows: the liquid/solids mix to be processed arrives in the decanter’s clarification section and is concentrated. The solids cake now forming migrates towards the cone. During the transition to the screen section, the latter causes the solids cake to break up. In the screen section, the solid matter is distributed in a thin layer inside the drum and finally dried. The tube with the clarification and screen sections is double-mounted. It is driven by a heavy-duty planetary gear developed by Siebtechnik. “The outer parts of the gear“, Dietschreit continues, “drive the drums in direct traction, while the internal parts are responsible for the worm drive. In this way, the worm and the decanter drum can operate at different speeds. We have realised this drive concept for all centrifuges working with a worm conveyor“.
The tube version of the Turbo Screen Decanters achieves a throughput of up to 100 t/h. The solids concentration in the feed can be 50 % or more. These tube centrifuges come with a drum diameter of 1000, 1100, 1200 or 1400 mm. The machines are designed for process temperatures up to a maximum of +150 °C and are optionally available in a gas-proof and pressure-resistant version. If they are destined for use in hazardous areas, the decanters – like all Siebtechnik machines – can be equipped with an Atex package.
Safe and almost maintenance-free
A frictionless safety clutch protects the machine against overload. It does this by mechanically separating the mechanical forces between the drum and the worm conveyor. As a result, the speed of the worm conveyor is adjusted instantaneously to the drum speed and the prod-uct transport inside the machine is brought to a halt.
In large centrifuges like the tube version of the Turbo Screen Decanter, wear is a factor that should not be disregarded. The threads of the worm conveyor and the screens are subjected to severe loads due to the abrasive properties of many solids as well as the high centrifugal forces.
According to Dietschreit, the resultant short maintenance intervals would en-danger the machines‘ efficiency. Measures are thus needed to protect against wear. Carrier systems are welded onto the threads of a worm conveyor developed by Siebtechnik engineers. These systems are provided with pins, to which the ceramic elements are stuck or bonded. The worm threads are protected from abrasion in this way. “The service life of the worm conveyor“, Dietschreit adds, “is prolonged by as much as 10,000 operating hours. The clarification section of the centrifuges can also be protected against abrasion in a similar manner“.
Completely new paths were trodden when designing the drum screen. The tube has square openings, into which screen cassettes are pushed from outside like drawers. Clamping rings secure the cassettes in position. “This is an enormous benefit for the user“, Dietschreit comments. “When a cassette is changed, components weighing several tons no longer have to be moved and mounted. The cassettes with the screens are simply pulled out and replaced with new ones.“ This construction also has advantages for the process. The same machine can be equipped with different screen segments. “What is even more important“, he sums up, “is that – compared to a standard screen decanter – the available screen surface is five times larger. This leads to a substantial improvement in the drying effect”.(le)
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