A number of quantitative models of wet pressing have been proposed in the literature but none of them take into account the observed rate dependence of the structural pressure. This rate dependence, which is ascribed to the flow resistance of the water in the fibre walls, should be incorporated in wet pressing models.
This paper presents a hydrodynamic model of wet pressing that includes the rate dependence of the structural pressure and thus takes into account the flow resistance of the intra-fibre water. The model is general in the sense that it can predict changes in the state of the fibre web due to a pressure pulse of arbitrary shape, under a wide range of web saturation and temperature conditions. The model can predict the effect of high press-roll temperatures.
The predictive capability of the model has been studied with the help of data from the third press of a pilot paper machine. For wet pressing at ambient temperatures, the model gives good predictions of solids content over wide ranges of machine speeds, grammages and linear loads and its predictive capability is quite adequate for most engineering applications.
For high press roll temperatures, the predictions are satisfactory over a wide range of press roll temperatures and linear loads but some significant systematic discrepancies occur. These predictions can probably be improved by using better estimates of the hardening curves at high temperatures. Once calibrated, the model can give considerable information about changes in the state of the web as it passes through the nip and some insights about the mechanisms that are active.