Temperature Control for Temperature Control Systems
Figure 1. The basic components of a chiller consist of a temperature control module, a pump to circulate the fluid, a refrigeration system to cool the fluid and a reservoir to hold a volume of cold fluid in the chiller.
Industrial chillers are used to circulate a constant-temperature fluid in a closed loop with liquid-cooled instruments and tools in order to increase process repeatability and reproducibility. The basic components of a chiller (figure 1) consist of:
A pump to circulate the fluid.
A reservoir to hold a volume of cold fluid in the chiller.
A refrigeration system to cool the fluid.
- A temperature control module.
A standard, off-the-shelf temperature controller can be used as a temperature control module. Controllers with a variety of options are offered by any number of manufacturers. Nearly if not all temperature controller include a temperature display, are panel mounted and accept temperature inputs from resistance temperature detectors (RTDs) or thermocouples. Various communications options also are available.
A helpful feature of many controllers is a proportional-integral-derivative (PID) autotune function. With this feature, the user can allow the controller to calculate the optimum response to system disturbances such as varying process loads, setpoint changes and noise.
After autotuning is completed, the user can make additional adjustments to the tuning parameters to ensure that the process fluid temperature is within the process control limits.
Once set, however, the user’s work is not done. The autotune feature will continue to attempt to tune the temperature controller at the operating point being evaluated. Yet process dynamics often are affected by conditions that only exist at certain times or under specific circumstances.
If special conditions exist in a process and can be sensed or predicted in some way, a method of adaptive (feed-forward) control can be used to supplement the reactive (feedback) control used in a typical control scheme. Alternately, chillers serving applications with advanced needs such as these may be better suited for programmable logic controllers (PLCs) or operator interface terminals (OITs).
The use of a PLC provides flexibility to satisfy the needs of a more demanding control environment (figure 2). It allows for the seamless integration of non-process, temperature-related process measurements such as pressures, flow rates and ambient temperature, as well as critical events such as pump overload or a safety shutdown, in an adaptive control scheme.
PLCs also provide a means of data collection and communication. They are easily connected to host systems via myriad communications options. Fieldbus connections such as Profibus, DeviceNet and Lonworks are available as well as non-proprietary serial and Ethernet links. Using a PLC may allow the user to better integrate the chiller with the tool.
Figure 2. PLCs or OITs often are used on chillers serving applications with advanced needs.
Temperature StabilityWhile temperature control is a critical function for many industrial processes, temperature stability is as important for some processes. Increased stability generally correlates to increased cost.
For instance, many applications require a temperature that is stable to ±0.5°C of a given setpoint temperature at a specified heat load. This level of stability can be accomplished by measuring the temperature in the reservoir and cycling an on-off valve in the refrigeration system (figure 3). A basic PID algorithm in the temperature control module cycles the on-off valve as needed. The reservoir provides a volume of constant temperature fluid to help reduce the impact of any small temperature changes due to heat load changes from the tool.
While the reservoir helps maintain a constant temperature for the fluid being supplied to the tool, it also masks large temperature spikes in the fluid returning from the tool. These changes result from varying heat loads.
For example, a laser in operation may add a constant 300 W of heat load to the fluid for five minutes. The laser power may then run at 500 W for five minutes, and then be brought back down to 300 W for another five minutes. This cycle may repeat over and over.
Increasing the heat load by 67 percent will suddenly change the fluid temperature returning to the chiller. Because the fluid temperature in the reservoir changes more slowly, the temperature control module is slower to respond to these changes.
Figure 3. Temperature stability can be accomplished by measuring the temperature in the reservoir and cycling an on-off valve in the refrigeration system.
Temperature stability can be increased for dynamic processes by measuring the temperature of the fluid at the exit of the refrigeration system and prior to the reservoir (figure 4). The temperature control module sees the spikes in fluid temperature and can respond accordingly. Therefore, the temperature stability of the fluid being supplied to the tool can be doubled by simply changing the measurement point (figure 5).
It is important to note that if a chiller is designed for ±0.5°C stability using an on-off valve such as a refrigeration solenoid valve, increasing the stability further will cause more frequent cycling and reduce the life of the valve. This issue can occur when using the autotune feature on an off-the-shelf temperature controller. Therefore, the chiller should only provide the stability needed to keep the process in control.
Figure 4. Temperature stability can be increased for dynamic processes by measuring the temperature of the fluid at the exit of the refrigeration system and prior to the reservoir.
Ambient Temperature Losses
Another common issue related to chiller temperature control is environmental or ambient temperature loss between the chiller and the tool. This typically occurs when the process fluid is much hotter or colder than room temperature and there is a significant amount of distance and piping between the chiller and the tool. Sometimes the chiller is even on a different floor than the tool itself.
For example, a fluid exiting the chiller at 59°F (15°C) may warm up to 68°F (20°C) by the time it reaches the tool due to room air raising the fluid temperature. Yet the process requires fluid to be delivered to the tool at a constant temperature of 59°F (15°C), not 68°F (20°C).
For this issue, an offset can be entered into the temperature control module. A setpoint temperature of 59°F (15°C) is entered for the chiller, but the chiller will actually control to 50°F (10°C). The process will get the needed 59°F (15°C) fluid, and the chiller will communicate a value of 59°F (15°C) on the display or through communications.
A number of processes such as those used in medical devices, certain lasers and semiconductor equipment are more sensitive to temperature. For recirculating chillers, stability to ±0.1°C of a given setpoint temperature at a specified heat load is not uncommon. However, as previously mentioned, it is recommended that you ensure that this level of stability is really needed for a given process because there can be an increased cost for either chiller components or development.
A PID algorithm in the temperature control module will control either an on-off valve or a modulating valve in the refrigeration system. A modulating valve typically is a stepper valve, providing finer control at the operating point. Also, it is not limited to a specific number of cycles over the life of the valve like an on-off valve. The modulating valve does take longer to fully open and close as compared to an on-off valve. Therefore, it will take longer for the chiller to ramp to a new temperature given a step change in the setpoint.
Figure 5. The temperature control module sees the spikes in fluid temperature and can respond accordingly. The temperature stability of the fluid being supplied to the tool can be doubled by simply changing the measurement point.
Temperature control can be complicated in chillers when controlling multiple fluid loops or operating over a large temperature range with highly variable heat loads. In this case, PLCs and OITs are used because they can be programmed to control multiple control devices and have multiple PID loops. These devices provide ultimate flexibility as they are only limited by the program created for them.
It is important to determine system operating points and required stability carefully. Incorrectly specifying these items may result in the tool temperature being out of control or unnecessary costs being added on. An experienced chiller manufacturer can provide the correct custom or standard chiller selection based on the right inputs. PH