Addressing constraints in the factory rate-control system

Although there are extraction benefits in maintaining a constant first-mill speed, there are inevitably capacity constraints that prevent that speed from being continuously maintained. This paper presents modifications to a conventional cane-rate control system that have been implemented at Tully to address the constraints. In order to maximise cane rate, it is generally desirable to operate at a relatively high first-mill speed. With feeding station constraints, a high first-mill speed may result in occasions when the feeding station is unable to deliver sufficient cane to sustain the desired prepared cane level in the first-mill chute. During these occasions, best performance is likely to be achieved by reducing the turbine speed so that the prepared cane level in the first-mill chute can be sustained at its setpoint. The best approach identified to achieve this objective was a single chute-level controller that utilises the first half of its control output to increase the speed of the prepared cane elevator and the second half of its control output to reduce the speed of the first mill. Both bagasse-handling and juice-processing constraints require the ability to reduce first-mill speed on occasions. Utilising a cane-fibre rate controller to prevent excessive cane-fibre rate has proven an effective method of reducing the incidence of bagasse system chokes. Varying the speed of the first mill to control the juice level in the mixed-juice tank has likewise been effective. Key words Mill control, cane rate, chute level, chute height, mill speed, elevator speed