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The water removal process in a conventional roll press nip occurs when the sheet and one or two felts pass between two rolls pressed together, rotating in opposite directions. The amount of water removed in a press nip depends on two factors. Nip pressure and dwell time. The time the sheet is exposed to that pressure.
Water removal from the sheet occurs in two phases. During the compression phase, air is forced out of the sheet in felts. And the nip pressure forces water from the sheet into the felts. During the expansion phase, the felts continue to absorb water from the sheet. The sheet reabsorbs some water or rewets as it exits the nip. To minimize rewetting, the sheet is separated from the felts as soon as possible after the nip.