Temperature Control Gets Smart

Engineers responsible for chemical processes are learning that one easy way to improve product quality and increase yields is by tightening heating process control. In applications in which electromechanical thermostats control heating with simple on-off control, digital temperature controllers can provide fine temperature regulation, adjustable ramps for heat application and gradual cool-downs.

Digital temperature controllers have fallen in price, so an engineer can upgrade a control system with only a modest investment. What's more, sophisticated proportional-integral-deriviative (PID) controllers are available with push-button displays and Windows-compatible software, eliminating the need to write system control code.

Among the processes that can benefit from smart temperature controls are fermentation, petroleum refining, rubber production, polymerization and synthetic fiber production. Different types and levels of control intelligence are useful for different applications. Therefore, an engineer needs to know which smart control features will be most advantageous to his or her process, and which types of programming features best fit the plant's operating environment.

Clearly, process plants must consider many features when selecting a digital temperature controller. But attention to process economics and application details help the specifier avoid overspending on intelligence features of marginal value.

Broadly speaking, "smart control" refers to digital controllers with features that go beyond basic open/closed systems. Smart temperature controllers can be programmed to provide a variety of outputs in response to changing conditions. Often this includes "predictive" logic that compensates for errors, load changes or thermal shifts. Auto-tuning of control loop PID coefficients and diagnostics for internal or external faults also might be possible.

Digital controllers might allow real-time sensing in the process loop, and provide electrical excitation for transducers. Of particular importance in process applications are controllers with alarm functions, as well as those with the ability to record or communicate event data such as temperature, date, time, channel number and alarms to other devices or to supervisory control software.

Consider a reactor application in a pilot plant, where various time and temperature recipes are being tested to arrive at optimal results. Some reactors require heat to start the reaction, so electric cartridge or insertion heaters often are used for this purpose. Once the reaction starts, the process could be exothermic.

Temperature control for such pro-cesses must be multifaceted. See Fig. 1.

First, specific heating loop PID parameters are needed to bring the temperature up to the precise reaction point, without overheating and in the shortest practicable time. In this case, the engineer should select a controller that provides auto-tuning of PID coefficients to optimize convergence to the set-point temperature. See Fig. 2.

Once the reaction is going, it must be throttled back by a cooling loop, usually connected to the same controller as the heating loop. Typically, the engineer will want to program alarm conditions into the controller. He or she will need a control relay or digital output to open a valve that neutralizes the reaction if it gets too hot (Fig. 2).

The resolution, accuracy and repeatability of the controller are important considerations, and they, too, are driven by process requirements. A plastic injection machine, for example, does not require temperature control as precise as that required by a chemical storage tank. Typically, a digital controller's temperature accuracy is specified in terms such as (0.2 percent of span +1 least significant digit [LSD]). More precise controllers with a 0.1-degree precision are available for applications requiring tight temperature regulation.

Repeatability is key in all these applications. In other words, each time the controller is reset to a given temperature, an actual measurement will display the same temperature as the last time, within a narrow tolerance band. See Fig. 3.

Other temperature controller selection issues include physical size, the types of sensor inputs, ease of installation and power controller interface issues.

Older temperature control systems commonly have a bulb and capillary sensing element that provides on-off control for a single loop. For process materials that tolerate wide temperature swings, this level of control might be adequate. However, these systems can be upgraded easily with new DIN temperature controllers, which provide much better resolution.

Many microprocessor-based DIN systems currently are available, most of which are easy to install and allow universal sensor inputs, including thermocouples, resistance temperature detectors (RTDs), voltage and current.