Water Removal
Suction applied at the perforated couch roll represents the final dewatering mechanism in the forming section of a paper machine before the wet web transfers to the press section. Optimizing the couch vacuum ensures that the sheet achieves the necessary wet-web strength to bridge the open draw without tearing. This vacuum level must be carefully controlled to balance water removal with energy consumption.
Web Transfer
Maintaining high wet-tensile strength prevents sheet breaks and reduces machine downtime at the wet-to-dry transition. Inadequate couch vacuum leaves the paper sheet too wet, forcing the press section to do extra dewatering work which increases the steam demand in the dryer section. The moisture content of the sheet entering the first press is a primary bottleneck for overall machine speed.
Operating Boundary
Continuous monitoring of the vacuum level prevents sheet crushing and minimizes fabric wear. Too much couch vacuum can cause the wet sheet to stick to the roll, leading to sheet breaks or web defects. Conversely, insufficient suction fails to extract free water from the sheet, which results in crushing in the nip of the first press where the sheet experiences rapid compression.
Because different pulp furnishes release water at different rates, the operator must tune the vacuum levels to prevent structural disruption of the newly formed sheet.
