MS Thesis Presentation by Wyly Gilfoil
Friday, April 29, 2005
(Dr. David Orloff, Chair)
"Smart Water Receiver for Use in the Wet Press Section of a Paper Machine"
When the paper web and press felt enter a nip in the press section of a paper machine, both the paper web and felt are compressed. Water is forced from the paper sheet into the press felt due to a hydrodynamic pressure gradient between the sheet and felt. Water enters the uppermost batt layer of the felt and continues into the void volume of the base fabric of the felt. Some of the water then leaves the felt and enters grooves in the press roll, while some water remains in the felt. Water not only flows through the felt in the transversal z-direction, but also flows through the felt in the machine and cross-machine directions.
On the exit side of the nip, the pressure imposed on the sheet-felt system by the rolls begins to decrease. Both the paper web and press felt begin to expand, and a vacuum is created in the web and felt. The vacuum in the web is stronger than that in the felt, and thus water and air tend to flow from the felt back into the sheet, causing rewet. Three mechanisms that contribute to rewet have been proposed: 1) film splitting between the paper web and press felt, 2) capillary forces in the web drawing water from the felt into the web, and 3) the pressure differential between the web and felt during expansion. Numerous alterations of the press felt have been made in an attempt to reduce the rewet of the paper web up on exit from the press nip.
The objective of this project was to design and test under flow conditions similar to those in a press nip a “smart” water receiver to be used in the press section of a paper machine. In this manner, the feasibility of such a water receiver was to be determined. The purpose of this water receiver is to accept water that is pressed from the paper web in a nip and prevent the return of this water to the paper web upon exit from the nip. Thus, the smart water receiver allows flow through the felt in the positive z-direction of the felt (away from the paper web) and not in the negative z-direction (towards the paper web). The smart water receiver concept utilizes a layer of micro-check valves incorporated into the press felt to perform in the desired manner. A mathematical model and lab-scale prototype were created in order to predict the behavior of such a design in the press nip.