Opening the Operating Window of Impulse Drying - I: The Effect of Ambient Pressure at Nip Opening, 1997 Engineering Conference Proceedings

Application of increased ambient pressure during and after the nip opening process of impulse drying inhibits sheet delamination.

Handsheets, simulating 205 g/m 2 linerboard with hydrodynamic specific surface area of 29.5 m 2 /g and solids content of 35%, were impulse dried at press surface temperatures from 120 to 330°C. When the nip opened to one atmosphere ambient pressure, sheets delaminated at and above a press surface temperature of 140°C. At successively higher press surface temperatures, it was found that the ambient pressure at nip opening could be raised to a level above which sheet delamination did not occur. This “critical” ambient pressure was found to be a function of press surface temperature, and ranged up to 411 kPa (abs) for a press surface temperature of 330°C.

Moisture ratio change was a linear function of press surface temperature and was independent of ambient pressure at nip opening. As long as the sheets were not delaminated, their strength was also independent of ambient pressure at nip opening.