It is important to position the temperature sensors in the sensor arrays to get access to the complete steam balance inside the refining zone. The sensor array is mounted between two refining segments, see Figure 1, to get a fast dynamic response. Placing the sensors in the refining segments have been tested in the early days with no success as the response become slow which ultimately can affect the control performance negatively.

The stagnation point is important to track as the temperature maximum, see Figure 2, can be used when stabilizing the fiber pad inside the refining zone. The reason is a consequence of the fact that the temperature at the stagnation point is almost unaffected by the consistency inside the refining zone. This has been proven for most types of refiners independent on which refining segments the costumer use. To illustrate that, assume that a positive step change in production rate, dilution water feed rate and the hydraulic pressure. As seen in Figure 3, the process gain Kij is negligible near the maximum temperature i.e. the maximum temperature is almost unaffected by changes in the dilution water feed rate.

Moreover, the shape of the temperature profile is strongly dependent on the refining segment taper and pattern. For instance, when the refining zone contraction point is closer to the centrum the temperature maximum is found at that contraction point. In those cases where the taper is negligible, the temperature maximum can be found near the periphery of the refining segments which results in more backward flowing steam. When the steam balance is vital for process stabilization and the customers want more forward flowing steam, the temperature profile can be a good tool when selecting type of segments.

The temperature maximum can be control within ± 1 C, which means that many operating points within the operating window can be reached. This is important as the stabilization of the profile provide the costumer with a larger interval for fiber development. This is shown in Figure 4 where two serially linked refiners are controlled using the temperature profile control concept TCtrl. The red dots refers to “manual mode” while the black dots corresponds to the “control mode”. This resulted in more than 50% reduced pulp quality variation in CSF, mean fiber length and shives.

For further reading, please download "Refiner Optimization and Control Part III: Natural".