The characteristics of a retorted furnace cause the furnace temperature to be 5 to 10 degrees hotter than inside the retort. This is called a thermal gradient or Delta Temperature (Delta T).
Many metallurgists prefer to control their retorted furnaces using a thermocouple placed next to, or into the workpiece, or inside the retort. This however can cause undesirable effects such as excessive heater temperatures and overshoot of the desired setpoint.
Manufacturers often use a furnace thermocouple placed near the furnace heaters in order to get around these problems. The ideal solution is to use a controller that has two separate control loops, each with its own thermocouple input. One loop will use the furnace thermocouple that is located near the heaters and the other loop will use the workpiece / retort thermocouple. The control loop with the lowest output demand will be used to control the furnace temperature.
Reduces maximum heater temperatures, increases heater life.
Guarantees that the workpiece follows the required setpoint profile.
Optimizes startup and settling times.
Automatic switching from workpiece control, to chamber temperature control as conditions require. Provides a method of controlling maximum delta T.
The ability to change the maximum delta T as the controller progresses through its setpoint program.
A simple effective method to control a furnace.
For this application Eurotherm has implemented a control technique called override control. Override control consists of two control loops, each with it's own input and setpoint, but they share a common control output. The lowest output of the two loops is directed to the common output circuit. This type of control strategy is available across a wide range of Eurotherm controllers and programmers. This example uses the 2604 controller/programmer.
Figure 1 shows a simple override controller. The main and override control outputs are fed to a low signal selector. The override setpoint is set to a value somewhere above the normal operating setpoint, but below any safety interlocks or unexpected values. There is one auto manual switch for both loops. In manual mode the control outputs of both loops track the actual output therefore ensuring bumpless transfer when auto is selected.
Although the 2604 is multi-loop controller it needs only one control loop to implement override. Each loop is capable of being set-up as an override control loop. Two profiles can be used in the setpoint programmer, one to set the furnace setpoint and the other to set the workpiece/retort setpoint. In this manner the setpoints and their relationship to each other can change as the process is being run.
Alternatively, one of the profiles can set the workpiece/retort setpoint and the other can be used to set the Delta T between the retort and the furnace.
This type of control strategy is available across a wide range of Eurotherm controllers and programmers.