Plate Gap Estimation based on Physical Refining Measurements, 2007 International Mechanical Pulping Conference

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To estimate the plate gap in refiners, reliable process measurements besides good physical models are required. The model used in this study is developed to cover estimations of the thermodynamic work as well as the refining work simultaneously. The model has been applied to a full-scale industrial first stage refiner and various operating conditions have been studied to verify its robustness. According to the model the estimated plate gap as well as the spatial refining work varies significantly, also during time sequences where the motor load is almost the same. The fiber concentration in the refining zone varies with different operating points as well. Moreover, the fluctuations in the fiber concentration are related to the local pressure gradients which are derived from temperature measurements inside the refining zone. These disturbances affect the radial steam velocity and thereby the fiber velocity substantially. This means that the commonly used control concept of keeping constant specific energy should be reconsidered since the residence time in different parts of the refining zone can vary over time and thereby influence the pulp quality.