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Based on: Industry Standards and Best Practices

Languages: English

Sample Transcript

The water removal process in a conventional roll press nip occurs when the sheet and one or two felts pass between two rolls pressed together, rotating in opposite directions. The amount of water removed in a press nip depends on two factors. Nip pressure and dwell time. The time the sheet is exposed to that pressure. Water removal from the sheet occurs in two phases. During the compression phase, air is forced out of the sheet in felts. And the nip pressure forces water from the sheet into the felts. During the expansion phase, the felts continue to absorb water from the sheet. The sheet reabsorbs some water or rewets as it exits the nip. To minimize rewetting, the sheet is separated from the felts as soon as possible after the nip.

Wet Pressing Theory

Training Time: 18 minutes

This module begins with a description of the objectives of the wet pressing process on a paper machine. Then, it discusses how water is removed from the sheet in press roll nips. It explains the purpose of a wet felt in wet pressing and how wet felts are designed. This module will give you a better understanding of how water is removed from the sheet during the wet pressing process.

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Modern press felts consist of a woven base fabric onto which a non-woven fibrous batt component is needled

The felts are separated from the sheet as quickly as possible to prevent rewetting

Pressing removes water from both sides of the sheet

Learning Objectives

  • Describe the three objectives of wet pressing
  • Identify and describe the pressing phases in a press nip
  • Identify the key process and equipment variables that impact wet pressing
  • Identify the sheet properties impacted by wet pressing
  • Explain how to minimize sheet rewetting
  • Describe the purpose and design of wet felts
  • Explain how uhle boxes aid in water removal at the press
  • Explain how steam boxes aid in water removal at the press
  • Describe the types of rolls used in wet presses
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Wet Pressing Theory FAQs

What are the objectives of wet pressing?
Wet pressing removes water from the sheet, consolidates the sheet, and improves the wet strength of the web so it can remain intact in open draws.

How does wet pressing affect sheet properties?
Pressing increases density, surface smoothness, and dry and wet strength properties. Bulk, caliper, and stiffness typically decrease.

What are the phases of water removal in presses?
Water removal can be divided into compression and expansion phases.

What is the purpose of the wet felt?
Wet felts act as a transfer device or belt to move the wet sheet without damage, absorb water from sheet, and maintain an even, smooth surface for the wet sheet.

What types of rolls are used for pressing?
Wet pressing can be accomplished by using solid rolls, variable crown rolls, suction rolls, and shoe presses.

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