Studies of Vacuum Dewatering on Low Grammage Paper Webs

Sammanfattning: A laboratory scale apparatus to simulate high-vacuum dewatering of low grammage paper sheets, typically for tissue products has been constructed. Vacuum level, vacuum pulse dwell time and frequency of single vacuum pulses are in the range typical for a Through Air Drying (TAD) tissue machine. The vacuum pulse dwell time was varied from 0.5 to 3 ms and the vacuum level from 10 to 60 kPa. For the 50 and 60 kPa series the solids contents for all curves leveled out at 23 and 24 % after about 2 ms. The laboratory apparatus has been validated by comparing results with data obtained from a pilot paper machine. The laboratory data correlated well with the data from the pilot paper machine.



Using the new apparatus, the effects of fabric parameters on sheet solids content during the vacuum dewatering of a low grammage sheet has been studied. The pressure drop over the fabric did not explain the difference in sheet solids content among the fabrics. Instead caliper, void volume and air permeability were shown to have a great impact on the dewatering of the sheet.



The correlation between sheet solids content and caliper, void volume and air permeability may be due to the fact that these fabric parameters control the amount of water that the fabrics can hold during a vacuum pulse and that can be rewetted into the sheet after a vacuum pulse. The theory of rewetting was supported in several experiments and further strengthened in comparisons with the literature.



If fabric caliper could be reduced to zero, the sheet solids content theoretically should level out at 28-34 % after a vacuum pulse dwell time of 0.5 ms at a vacuum level of 30 kPa in the case of 17 g/m2 sheets made of kraft pulp. That is expected in accordance to the theory of viscous drag and implies that dewatering by viscous drag between the penetrating air and the water in the intra-fiber pores start already after a vacuum pulse dwell time of 0.5 ms. The difference in the actual sheet solids content after a vacuum pulse and the theoretical solids content of 28-34 % can be explained as being due to rewetting from the fabric to the sheet