In the manufacture of candy, a boiler for processing the candy mixture has to be heated with steam. The temperature of the candy mixture is the setpoint and must be controlled as precisely as possible. The solution that was chosen by a candy manufacturer is cascade control using the JUMO IMAGO 500 screen-led controller.
As can be seen from the schematic diagram of the process, the raw mixture of candy or jelly is transported through a pipe system into a boiler where it is boiled. The boiler is heated with steam. The steam is not in direct contact with the medium but the raw mixture is pumped through a closed pipe system inside the boiler. The transfer of heat closely follows the heat exchanger principle.
Saturated steam has a fixed pressure for a given temperature. This steam pressure relationship can be found in graphic or tabular form in any collection of physics formulae. Cascade control makes use of this relationship. The required temperature of the medium is defined as the setpoint (x), for example 140°C. The boiler pressure (xH) is influenced by the subordinate (slave) controller. In this way, the temperature and steam pressure are linked within the cascade control.
Since the subordinate controller has to process the manipulating variable (y1) of the master controller as its setpoint (w1), it is necessary to normalise the output level. The steam pressure relationship makes it necessary to normalise the output level, for instance in the range from 0 to 500 kPa (max. 150°C) which is sufficient for this application.
Cascade control is used because steam is the decisive element for heating up the candy mixture and the heat transfer to the candy mixture is very sluggish, the significantly faster pressure alterations are also evaluated within the cascade control. The cascade controller thus responds much faster, by reacting to pressure changes and deviations, which has a beneficial effect on the temperature of the mixture. The characteristics and advantages of the master controller for precise control of the target value are maintained when the parameters are selected carefully.
Another reason for cascade control being used is production cannot be started up during the preheating of the boiler because the temperature in the piping system is too low. Since no candy mixture is being transported during this phase, it is not possible to measure the temperature with temperature sensors. The master controller is therefore switched over to manual control during the warming-up phase of the system. In this phase, the master controller can be used to output a freely chosen manual output level (y1). The manual output level of the master controller determines the setpoint (w1) for the subordinate slave controller. This feature is used for warming up the system. A required setpoint (w1) for the boiler pressure can now be set by varying the manual output level (y1) for the master controller. The customer can read the target pressure directly on the display, since the setpoint (w1) takes account of the normalisation.
When the pressure (and thus the required system temperature) has been reached, the production process can then be started. At this stage the controller is switched back to automatic operation. This changeover could also be triggered by a limit comparator if it is required by the customer.
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