A newly built 50-zone rubber-curing oven suffered severe distortion and metal-work buckling during startup. One major cause of the failure was the differential expansion of the metals due to the sudden application of full power to the electric heaters (and the consequent severe rate of temperature increase). Two solutions come to mind:
Limit the Startup Power. The discrete controllers had a built-in power limit feature. This feature could be used to bring each zone up to temperature more slowly by setting the power limit to some percentage of full power. This often is an acceptable, cheap solution, provided that the power limit is later reset high enough to achieve the full working temperature. However, in a multi-zone system, this method is unlikely to ensure that all zone temperatures will track each other.
- Substitute New Program/Controllers. Expensive but not necessary. Luckily, the controllers used in the oven, while not programmable, had a simple, built-in ramp-to-setpoint feature. Itt was easy to wake it up and put it to work -- just set the target temperature and the degrees-per-minute ramp rate. On startup, all zone setpoints find and start at the current temperature (usually room temperature), then ramp up to the target temperature at the rate selected. Because the ramp is accurately controlled, the temperatures stay together, minimizing differential thermal expansion between zones.
Some typical products needing ramp-and-soak control are glass, ceramic and heavy metal parts. A slow ramp minimizes the risk of distortion or cracking due to differential thermal expansion within the work. It also helps to avoid temperature overshoot as the ramp stops and becomes a fixed temperature for a specified dwell time (sometimes called the soak segment). At this stage, the dwell time is set long enough to ensure that the parts attain a uniform temperature throughout and maintain it long enough to complete that stage of the processing.
Sensors may be required on or inside the work to ensure that the actual material is receiving the specified temperature and time processing. A chart recorder or the data acquisition part of an integrated multi-zone control package would monitor the sensors.A program/controller is a controller and setpoint generator in one unit. In a typical program, the setpoint can ramp, step or hold at one value. Here, the setpoint takes off up Ramp 1 (R1), holds at Level 1 (L1) for Dwell Time (D1), and so on, until the time/temperature program is completed. The controller forces the process temperature follow the setpoint.
There are many types of controllers on the market. Some come packaged in a 48 mm square case with a user interface consisting of three pushbuttons and two digital displays. Complexity increases up through to multi-zone models with more user controls and a graphic panel showing the program plus digital displays and annotations.
Controller manufacturers have applied much thought and ingenuity to meeting the needs of heat processors across different industries. Major advances continue in control and communication techniques and in configuring controllers to handle the wide range of inputs, sensors, engineering units and final control elements.In the next issue, I'll look some of features available in program controllers.