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.
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 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.
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).