Rapid milk cooling control with varying water and energy consumption

A control system for rapid milk cooling plant connected to a variable flow milking machine is presented. The plant consisted of a pre-cooler in the first stage that utilised ground water as a cooling medium and an ice bank that provides ice chilled water for the second cooling stage. The control system comprised of two proportional integral derivative controllers applied to each cooling stage in tandem. The set point of the first controller was the desired milk pre-cooling temperature while the set point of the second controller was the desired final milk temperature. Eight different precooling set points (13 °C-20 °C) were tested for feedback and feedback-feedforward controller configurations. Selection of low temperature pre-cooling set points resulted in larger volumes of ground water being consumed in the first stage per unit milk in comparison to the selection of higher temperature set points (three times higher). However, low pre-cooling temperatures resulted in less ice storage utilisation and therefore less power consumption. Introduction of a feedforward loop to the controllers reduced the disturbance from the varying milk flow and by doing so reduced the final milk temperature deviation from the set point. Optimum water utilisation rates were calculated for varying water cost at the current price of electricity. These points represent the ideal combination of ground water and power consumption per unit milk to produce the most financially efficient means of cooling. Potential cost reductions of up to 34.5% through the selection of the ideal water rates were discovered.