Best Practices in CONTROL

Submitted by Jon Irvine
on Tue, 02/03/2015
Considering the number of variables present, gaining precise control of a process heating application can be a difficult task. In industries with a low tolerance for irregularities and a high demand for consistent quality, controlling the temperature of a substance from the start to the finish of a process is an absolute necessity. To properly implement a system in which temperatures can be controlled and monitored accurately, the end user or solutions provider must first gain insight into the desired outcome of the process and use technical knowledge to create an effective system designed to match the unique process it is controlling.

For instance, in an application utilizing a power controller that simply turns the heat source on or off, a complex automation system is required to maintain precise control over the temperature of the substance involved in the process. If the process is not controlled correctly, mistakes can easily be made, and the final product can be severely tarnished. One of the best ways to ensure that a process heating application is fully monitored and controlled from start to finish is to utilize an advanced automation system.

It is important to use a temperature controller with PID (proportional-integral-derivative) capability to achieve the tightest control. In critical process heating applications, the operation may involve multiple controls networked into a programmable logic controller (PLC). This network involves sensors that are able to determine the environment of the application, including the external and internal temperatures of the heated media as well as any other relevant measurements that may affect the outcome of the process. Based on the input from these sensors, the PLC can determine automatically when heat needs to be added to maintain a consistent temperature or achieve a change in temperature if desired.

For example, take a process in which a solid material must be cured by heating slowly at a certain rate and then kept at that temperature. Difficulties can arise at several points during this operation due to the unpredictability of certain materials interacting with each other as well as any malfunctions that may result from a substandard process control system.

The first step in creating an effective system to perform this process is for the end user to fully communicate all of the needs and requirements of the application to the technical solutions provider. Without thorough background knowledge of the entire process and the desired outcome, a workable solution cannot be executed effectively. A piece of vital information that should be communicated during this phase is the type or types of material involved in the process. This will allow the technical solutions provider to determine the correct equipment to perform the operations without worrying about any chemical incompatibility or damage to the heated media.

Based on the information provided by the end user, the technical solutions provider then will create a sensor placement layout to communicate each piece of necessary information back to the controller or PLC. At the bare minimum, this will include:
  • Placement of one sensor on the media or in the process stream to measure the material temperature.
  • Placement of one sensor on the heat source itself to measure the heat output temperature and protect against an overtemperature scenario.
An additional benefit of placing a sensor directly on the heat source is avoiding any damage that can result from harmful chemicals attacking the heating source at certain temperatures. It is common for materials to become more aggressive at higher temperatures. If the PLC requires more input information from the sensors in order to accurately control the process, the technical solutions provider should select and place those sensors appropriately.

A preliminary step to sensor placement is correct sensor selection. In precisely controlled processes, it is crucial to select a sensor that best fits the conditions and requirements of the application. An easy way to make this decision is to work from a list of questions designed to narrow down the type of sensor that will work best. These questions can include:
  • What materials will the sensor be exposed to?
  • Will the sensor be exposed to any extreme temperatures or pressures?
  • How dense is the material?
  • How precise does the measurement need to be?
  • How quickly does the sensor need to react to changes in the process?
  • What is the temperature range of the process?
By considering the specific process and evaluating each sensor based on the present criteria, a process engineer can make an informed decision that will ensure the best possible outcome.

After the sensors have been selected and placed in their proper locations, the technical solutions provider should network each sensor back to the power controller. In an electric heating system, it is recommended to use a silicon-controlled rectifier (SCR) power controller due to its high-speed power switching system. This device intelligently controls the heating process by turning the heat source on and off very quickly — in some cases, more than 30 times per second — to maintain a more precise temperature and a higher level of controllability over the process. This type of controller also uses different percentages of power from the heater to gain a higher amount of precision.

In addition, switching the power quickly will extend the life of the electric heater by minimizing thermal stress on the internal resistance wire. Due to the rapidly changing current being sent to the heater, the coil used to heat the material is not subjected to wide temperature swings as it is simply being turned on and off. These lower temperature swings result in far more precise control over the material involved in the process and less wear and tear on the heating elements used.

Another feature that an advanced PLC controller can bring to the table is cascade control, which allows the controller to intelligently regulate the temperature of both a given material and the heating element in the process. This is particularly useful when dealing with aggressive chemicals that will react negatively if the heating element exceeds a certain temperature. In cases when the material needs to be heated quickly, the PLC will monitor and control based on the material temperature only until the heating element surface reaches an unsafe level, at which point the system will automatically base the process control on the heating element temperature to avoid any process malfunctions. Cascade control is used successfully in many applications offering a high level of process control while simultaneously maximizing the runtime of the equipment.

With modern advancements in process heating technology, often it is difficult to overlook a piece of equipment that could make a vital difference in the outcome of an operation. If a process heating application is carefully designed with each of the above considerations in mind, the process will consistently produce higher quality results.

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