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TECHNICAL

The optimization of sheet transfer from the forming section into the dryers

O. Kääpä (Dipl.-Ing.) Vice President Sales, Heimbach GmbH & Co. KG, olli.kaapa@heimbach.com
J. Mäenpää, Sales Manager Runtech Systems Oy, jouni.maenpaa@runtech.fi

Summary
The job of the press section is to create maximum sheet dewatering with the best sheet characteristics and first class runnability. When future paper machines are designed for even higher speeds, when “more difficult” raw materials are used and paper grades become ever lighter, then the operation of the “zone between pick-up from the forming section up to the pick-up into the dryer section“ will determine to what extent success is achieved in protecting the sheet.

Modern press sections will increasingly be planned under those conditions which enable a significantly improved sheet transfer. But also in existing machines sheet transfer conditions can be substantially improved as shown in this report.

Introduction
Modern high speed paper machines are nowadays being built for speeds in the 1800-2000 m/min range. Productivity increases at present are in many cases achieved by increasing the machine speed. Therefore, it can be assumed that also in the future efforts will continue to be made to increase the paper machine speeds.

A speed increase necessarily requires an increase in the sheet tension. If the sheet on its way from the forming section to the dryers is not well supported by the machine clothing, its wet tensile strength is not sufficient to withstand the increased tension. Additionally, stock developments – increased filler content, more recycled fibre, new polymers etc., combined with the tendency to lower basis weights – combine to increase the necessity for better sheet control.

Therefore as precondition for the achievement of a lasting and successful speed increase, available technology is increasingly highlighted: “The Optimization of Sheet Transfer from the Forming Section into the Dryers.“

The freshly formed sheet is largely supported and carried from the headbox, through the forming section and the press section up to the last press by the fabrics and felts of the paper machine clothing. However, on many machines there are clothing free zones, the so-called “free” or “open draws”. In some cases they can already be found in the press section between a third and a free-standing fourth press, respectively between the press section and the dryers. Through these zones of open draw the sheet must be conveyed, “transferred” into the dryer section.

This involves the risk that the fresh, still wet, sheet – unsupported by clothing – does not have the necessary tensile strength, and as a result, it breaks. Any further increase in speed to avoid this problem is impossible. A speed reduction may even be unavoidable in the interests of overall runnability.

For these reasons the reduction or elimination of the open draws is recommended. In addition the subject of “Transfer Optimisation“ involves further actions for the solution of other aspects of the total problem. In the following the status quo of the transfer, the possibilities for its improvement and further development are considered.

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