Static and Dynamic Modelling of the Drying Section of a Paper Machine
Abstract: The paper making process involves chopping of wood, cooking and forming, followed by dewatering in a vacuum section, press section and drying section. Several models for the most important parts of the paper machine drying section are presented in this thesis to describe phenomena at steady state and during state transitions. New control strategies applicable/useful for both steady-state control and for grade changes were derived. The predicted paper temperature was verified by surface measurements. These measurements also confirmed the steep moisture profile predicted by the model. The paper moisture and temperature profile determine the tension profile in the sheet. This profile showed that the critical section with respect to paper delamination is located in the first third of the machine. These predictions agreed well with machine data from a delamination case in the machine studied. During steady-state operation it is important to maintain sufficiently good quality of the paper sheet. The moisture ratio is one property which is often controlled using the steam flow valve position. Disturbances at the inlet of the machine occur at various speeds due to upstream variations. During state transitions it is important to achieve an acceptable outlet moisture ratio as fast as possible. Two control systems are presented in this thesis: one based on the mid-ranging technique and the other on a feedforward control system. Both control systems use the air dew point and the air impingement velocity to control rapidly occurring disturbances. The waste in production during a grade change could be reduced by 60% with the mid-ranging technique. The feedforward method showed potential in reducing the effect of moisture disturbances. More work is, however, needed to extend the use of the feedforward method to improve machine performance during grade changes.
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