Modelling of forming and densification of fibre mats in twin-wire formers

In the forming section of modern paper machines, water is removed from the suspension at low loads applied through the tension in curved wires and by vacuum application. This paper presents a physically based model of the forming and densification of fibre mats in twin-wire formers. The model can calculate the effect of complex pressure profiles that are generated through the forming section. It was developed from mass and momentum balances of the fibre and liquid phases, the fibre mat stress-porosity relation and an expression for the permeability as a function of the porosity. The fibre mat stress porosity relation used is rate- independent and accounts for the generally observed hysteresis. Simulations have been conducted to study the effects of blade pulses, wire tension and beating. The effect of sequential blade pressure pulses after the forming roll on the dewatering and the concentration gradients could be characterised. The simulations also exhibited rewetting by expansion when the fibre mats left the forming roll. Increasing wire tension resulted in increased dewatering, but the rate of increase diminished rapidly with increasing tension. The simulation results also indicated that beating has large influence on dewatering.