A Comprehensive Guide to Sheet Rewetting for press felt

A Comprehensive Guide to Sheet Rewetting-Pay Close Attention!
Sheet rewetting specifically refers to the phenomenon occurring during the papermaking press section where, after the wet sheet leaves the press nip, water previously removed into the press felt is reabsorbed by the sheet due to capillary action. Simply put, it is a process of "wicking back" water after dewatering-a phenomenon that must be strictly avoided in papermaking. Causes of sheet rewetting: Rewetting arises from a combination of factors, primarily categorized as follows: Basic physical mechanism: The essence of rewetting is the transfer of capillary water. When press pressure is released, the fiber structures of both the sheet and the felt undergo elastic recovery, creating new micropores. Because the fiber pores in the sheet are finer than those in the felt, they generate stronger capillary forces-acting like thinner "straws"-that actively draw water back from the felt into the sheet. Operational and equipment factors: If the wet sheet does not separate from the felt promptly after pressing, the sheet continues to absorb water remaining in the felt; the longer the contact time, the more severe the rewetting. Structural characteristics of the press felt: Felt design is crucial for controlling rewetting. Poorly designed felts-such as those with improper layering or unbalanced proportions-reduce resistance to rewetting, leading to significant moisture regain. An ideal felt should form "inverted-funnel" drainage channels to facilitate effective water removal. Impact of sheet speed (machine speed): Rewetting issues are more pronounced on low-speed paper machines. Experimental data shows that at a speed of 900 m/min, post-nip rewetting increases the moisture content of newsprint by no more than 1.5 percentage points; at 1,000 m/min, the amount of rewetting increases with the duration of contact between the sheet and the felt. However, at speeds exceeding 1,200 m/min, if the separation distance is short, the impact of rewetting on sheet dryness becomes minimal. Impact on Finished Paper Quality and Production: Re-wetting directly increases the moisture content of the paper web entering the dryer section, leading to a series of issues: Increased energy consumption and reduced profitability-in the press section, for every 1% increase in web dryness, energy consumption in the dryer section can be reduced by approximately 5%; re-wetting effectively negates the dewatering achieved during pressing, significantly increasing steam consumption. Induction of paper defects-uneven moisture distribution increases the likelihood of deformation (such as wrinkling or curling) during drying, compromising paper flatness. Reduced physical strength-abnormally high moisture content weakens inter-fiber bonding, directly reducing the paper's surface strength; during subsequent printing, this can cause surface "picking" (where fibers are pulled up by the ink), potentially leading to large-scale surface delamination in severe cases. Measures to Mitigate Re-wetting: The key to mitigating re-wetting lies in reducing the contact time between the paper sheet and the press felt, as well as optimizing felt performance. Primary measures include: Optimizing press felt design and selection-Selecting the right felt type: Choose felts with uniform, fine surfaces; studies show that felts with fine, uniform surfaces can reduce re-wetting levels from 15–20 g/m² to approximately 5 g/m² compared to rough-surfaced felts. Improving structural design: Require suppliers to design felts with superior re-wetting resistance, such as those featuring "inverted-funnel" drainage channels or novel elastomeric structures. Refined operation and process adjustments-Achieving rapid separation: The wet paper sheet should be separated from the felt immediately after exiting the press nip; this is the most direct and effective method for reducing re-wetting. Enhancing felt dewatering: Installing or adjusting the opening of felt suction boxes downstream of the press nip (e.g., setting the top felt suction box opening to 40%–60% and the bottom felt to 80%–100%) helps lower felt moisture content, thereby reducing the source of re-wetting. Maintaining cleanliness: Regularly clean felts and press rolls to ensure clean surfaces and optimal dewatering performance. Application of Advanced Technologies
On certain production lines-particularly high-speed paper machines-technologies such as hot pressing (which raises the sheet temperature to enhance dewatering) or precise re-wetting (conditioning) after drying are employed to control the final moisture content and smoothness of the finished paper.