A. Initial forming zone
Easily placed to utilise the power jet and achieve correct initial drainage, the board supports the forming fabric in the landing area of the jet. Good structural rigidity resists the stress generated by the impact of the jet that might cause changes in the sheet formation.
B. Formation zone
Hydrofoil – Varioline box
To avoid fibre flocculation which might affect the good formation of the sheet, this type of drainage element controls the turbulence according to the number, position and profile of the foils.
C. Low and medium vacuum zone
Vacuumfoil – Duoflow
This type of drainage element removes the water from the pulp by vacuum when hydrodynamic forces in the formation zone are insufficient to ensure adequate drainage.
D. High vacuum zone
Suction box – Bivac – Trivac – Quadrivac
High level vacuum boxes are called for when the dry level of the sheet increases and it is necessary to apply a progressively higher vacuum level to overcome drying resistance.
A multi-vacuum box provides greater drainage efficiency than the use of a number of a single boxes where the atmospheric pressure fluctuates from one box to the next.
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