Flow Limitation
Maximum volumetric throughput of liquid stock that a papermaking headbox or drainage element can handle limits the overall production rate of a forming section. This limit, known as the hydraulic capacity, dictates how much water can be pumped through the system while maintaining uniform fibre distribution. If the paper machine operates beyond this threshold, the stock flow becomes turbulent and creates sheet weight defects.
Correct matching of pump flow to the machine’s capacity is essential for high-speed operation.
Drainage Boundary
Sufficient dewatering capability on the forming fabric prevents sheet crushing in the subsequent press nip. When the hydraulic capacity of the foil boxes is exceeded, excess water remains in the sheet, causing it to tear. High-speed packaging lines require effective drainage systems to handle the large volumes of water needed for uniform formation.
System Design
Expanding the flow limits of existing machinery involves upgrading headbox piping, manifold systems, or stock pumps. A carefully sized system ensures that the hydraulic capacity remains above the maximum operating speed of the paper machine. When designing or modifying a machine, engineers calculate the fluid velocities through the distribution manifold to avoid low-flow zones where fibres might settle and clump together.
These calculations prevent the formation of streaks in the paper web and minimize cross-direction variations. Maintaining this balance ensures high print quality on the finished board.
