Roles of Press Felt in Press Section of Paper Machine

Abstract

The functions required of press felt for paper machines are mainly water removal, web conveyance, and paper surface finish. The water removal performance of felts is closely related to steam consumption during the paper drying process and is a very important factor in determining energy consumption costs.Studies on water removal at the press section have been carried out by many researchers. The mechanism of water removal published by Wahlström in 1960 is the theory that is met with wide acceptance even today. Wahlström studied the process of water removal at the press nip which he divided into four phases and established a press model equation for sheet moisture ratio after the press nip based on peak pressure, flow resistance, evenness of pressure distribution, and rewetting.Nip peak pressure is the force required for pressing the sheet and squeezing the water out and is determined by machine and felt factors. Flow resistance is the resistance which occurred when the water flows at the cell walls, in between fibers, and at the inside of the felt while the pressure is loaded to both the sheet and felt. Even pressure distribution plays an important role in water removal at the press, especially in the case of a low basis weight sheet.Rewetting is water transference from the felt to sheet at the exiting side of the nip, caused by volume expansions of both the felt and sheet, capillary action, and water film splitting. Although water removal performance of felts plays an important role in cost reduction of paper production, felts need to have other critical functions. Therefore, it is very important for us feltmakers to have close discussions with paper makers on felt performance for designing the most suitable felts.