Melt Filtration at Constant Pressure
Filter elements can be cleaned very effectively and with little pressure variation in a newly developed piston-type screen changer. The time during which the backflushed material is removed is relatively long. As a result, the pressure variations in the process are kept within a very narrow range.
Based on the proven principle of the piston-type screen changer, Kreyenborg GmbH has developed a novel filtration concept for heavily contaminated melts. The filter elements can be cleaned via backflushing during the ongoing production process with little pressure variation, reports Stefan Wöstmann, Manager of Applications Technology at Kreyenborg, in der August issue of the magazine Kunststoffe.
During extrusion of plastics, the pressure and volumetric flow rate (throughput) should be kept as constant as possible. Screen changers based on backflushing, however, take some melt from the process to clean the filter element. As a result, the pressure downstream from the screen changer drops. Relatively large amounts of backflushed material are needed especially when the polymer melts are heavily contaminated.
In the K-SWE-4K-V-RS screen changer, the melt is directed through four screens. On the material inlet side, the melt stream is divided into four channels, each of which leads to a screen chamber. After filtration in the screen-bearing piston, these divided streams are recombined in the housing. As soon as the filter elements must be cleaned via backflushing, the screen moves into the backflushing position. Next, a backflushing piston is slowly introduced into the discharge channel of this screen in order to compress the melt. The close fit of the backflushing piston in the discharge channel prevents pressure spikes on the material outlet side. When the backflushing valve opens, the compressed material expands abruptly, releasing the contaminants from the filter element. At the same time, the backflushing piston is shifted hydraulically to the front end position at an adjustable speed in order to divert the contaminants through the flushing channel and flushing valve.
After flushing, the screen-bearing piston returns to the production position and the backflushing piston is returned to the back end position by the hydraulically assisted melt pressure. Removal of material from the process thus occurs over a longer period of time. The result is little pressure variation in the process.
Additional benefits are found in the rugged construction of the components, the large active filter area, simple operation and the economy of the concept. By using four filtering chambers, the active filter area can be increased to provide an economical alternative to rotary screen changing technology while offering the same constant pressure.
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