THE IMPACT OF PRESS FELT DESIGN ON PAPER MACHINE PRESS DEWATERING

Using data from a pilot press and an extended Decreasing Permeability Model (DPM) of wet pressing, the effect of press felt structure on moisture content after pressing has been explored. The model enables separation of the effects of in-nip dewatering, equilibrium moisture, and rewet on final moisture, where rewet is considered to be water expelled from the web but remaining with it upon separation from the press felt. It was found that differences in press felt structure for a given furnish had no significant effect on the model coefficients for dewatering, but a significant effect on rewet. This led to the conclusion that differences in moisture after pressing caused by press felts in rolling nip pressing of lightweight grades arise largely from rewet, not from local pressure differences during pressing that affect dewatering.