In all thermal systems, there are four major parts:
- The Workload. This is the material to be heated or cooled. It generally is associated with a cycle time or a time rate of passage over or through the heated part.
- The Heat Source. Either electrical resistance or fuel-fired heat provides a heat source.
- The Heat Transfer Medium. This is the solid, liquid or gas that transfers the heat to the energy load.
- The Controlling Device. Sensing and controlling devices control the amount of heating and maintain a specific temperature for the load.
Placement of each of these components must be considered with respect to the others. Of the four major parts of a closed-loop system, the sensor’s location will play a major role, provided the other elements of the system have been properly selected. Placement of the sensor in relationship to the workload and heat source can compensate for various types of energy demands from the workload. Sensor placement can limit the effects of thermal lags in the heat transfer process. The controller can only respond to the temperature changes it “sees” through feedback from the sensor location. For this reason, sensor placement will influence the ability of the controller to regulate temperature to a desired setpoint.
Be aware that sensor placement cannot compensate for inefficiencies in the system caused by long delays in thermal transfer. Also, remember that in most thermal systems, temperature will vary from point to point.
Sensor in a Static System. A system is referred to as “static” when there is a slow thermal response from the heat source, slow heat transfer and minimal changes in the workload. When the system is static, placing the sensor closer to the heat source will keep the heat fairly constant throughout the process. In this type of system, the distance between the heat source and the sensor is small (minimal thermal lag). Therefore, the heat source will cycle frequently, reducing the potential for overshoot and undershoot at the workload. With the sensor placed at or near the heat source, it can quickly sense temperature changes and maintain tight control.
Sensor in a Dynamic System. “Dynamic” systems are those with a rapid thermal response from the heat source, rapid heat transfer and frequent changes in the workload. When a system is dynamic, placing the sensor closer to the workload will enable the sensor to “see” the load temperature change faster and allow the controller to take the appropriate output action more quickly. However, in this system type, the distance between the heat source and sensor is notable, causing thermal lag or delay. Therefore, the heat source cycles will be longer, causing a wider swing between the maximum (overshoot) and minimum (undershoot) temperatures at the workload.
It is recommended that the electric controller selected for this situation include the PID features (anticipation and offset ability) to compensate for these conditions. With the sensor at or near the workload, it can quickly sense temperature rises and falls.
Sensor in a Combination Static/Dynamic System. When the heat demand fluctuates and creates a system between static and dynamic, place the sensor halfway between the heat source and workload to divide the heat transfer lag times equally. Because the system can produce some overshoot or undershoot, it is recommended that the electronic controller selected for this situation include the PID features to compensate for these conditions. This sensor location is most practical in the majority of thermal systems.
Sensor Placement Examples in Solids. The various methods of contact heat transfer can be categorized into conduction (typically solid or liquid) and convection (typically a liquid or gas). A conduction system might consist of the heat source in direct contact with the heat transfer medium, which in turn heats the load. Expect different system responses depending upon different temperature sensor locations.
- Best. The heat source is close to the work and the sensor is close to both the heat source and the work. This short heat conduction path minimizes thermal lag (figure 1).
- Practical. The heat source is distant from the work and the sensor is located between the source and the work. The longer heat conduction path increases thermal lag in the system, but being located mid-way, the sensor can respond to work or heat source changes without excessive lag (figure 2).
- Poor. The heat source is close to the work and the sensor is distant from both the heat source and the work. The sensor is too far from the heat conduction path to respond to temperature changes without excessive lag. The sensor is also located too far from the workload (figure 3).