In refrigeration systems, maintaining optimal oil temperature is crucial for ensuring the smooth operation of compressors and other critical components. Traditionally, oil temperature control has been manually managed using two air-cooled variable frequency drives (VFDs), but implementing automatic oil temperature control in a Distributed Control System (DCS) has presented several challenges. These challenges include measurement reliability, system synchronization, and the complexity of multi-variable control. This article discusses these issues in detail and proposes a solution using advanced control techniques.
Challenges in Automatic Oil Temperature Control
Several factors make it difficult to implement automatic oil temperature control in a DCS system:
- Measurement Reliability: The oil temperature readings are transmitted via Modbus 485 communication, which is not always stable or reliable enough to meet the real-time control requirements.
- Multiple Measurement Points: The system is designed to operate with four separate oil temperature sensors, which increases the complexity of controlling and monitoring these temperatures.
- Variable Control Requirement: It is necessary to control two out of the four oil temperatures simultaneously, which complicates the control process.
- Low Precision of Communication Data: The communication data is not highly precise, adding further difficulty in making accurate adjustments.
- Simultaneous Control of Multiple VFDs: Two air-cooled VFDs need to be adjusted simultaneously to maintain the required oil temperature.
- Safety Concerns in Online Configuration: The system requires online configuration, which introduces potential safety risks.
While the current system can operate manually, achieving automation is not simply a matter of applying existing control logic. The complexity of the system and the variability of conditions require more sophisticated control strategies.
Advanced Control Approach: Solving the Complexities
To overcome these challenges, an advanced control strategy can be employed. In this approach, the four oil temperatures are treated as controlled variables, while the two air-cooled VFDs are treated as manipulated variables. The goal is to control the two active oil temperatures while disregarding the inactive ones, and to synchronize the two VFDs to regulate the oil temperature.
Process Overview
The control system has two main components:
- Manual Control: Initially, the two VFDs are manually adjusted to control the oil temperature.
- Advanced Control: The advanced control system automatically adjusts the VFDs to maintain the oil temperature, ensuring that the two VFDs operate at the same frequency.
The key advantage of advanced control is that it doesn’t require the creation of new variables to synchronize the VFDs. Instead, it leverages control strategies like proportional-integral-derivative (PID) control, setpoint tracking, and ratio control. These strategies can be applied flexibly to ensure that the oil temperatures are maintained within the required range.
Control Synchronization
The advanced control approach uses a combination of control techniques, such as:
- Setpoint Control: The oil temperature is maintained by adjusting the VFDs according to predefined setpoints.
- Ratio Control: This ensures that both VFDs adjust in tandem, maintaining the desired temperature profile.
By gradually adjusting the frequency of the two VFDs, the system can bring both units to the same operating frequency without any disruptive switching or manual intervention. This synchronization is vital for preventing system instability and ensuring the continuous operation of the refrigeration unit.
Advantages of Advanced Control
Compared to the traditional PID control systems used in DCS, advanced control offers several advantages:
- Flexibility: Advanced control allows for dynamic adjustment without needing to modify the DCS control logic each time the operating conditions change.
- Safety: The system can be configured to operate within safe parameters, reducing the risk of over-temperature situations or system damage.
- Simplification: Complex multi-variable control tasks, such as controlling multiple oil temperatures and VFDs simultaneously, are simplified by the advanced control algorithm.
- Cost-Efficiency: The advanced control system can operate with less manual intervention and fewer system adjustments, reducing operational costs.
Multi-Variable Control
In industrial settings, multi-variable control systems are commonly used to manage multiple variables simultaneously. For example, systems that control cooling water temperatures using multiple VFDs or manage liquid levels in tanks with multiple valves are examples of similar control challenges. In the case of refrigeration units, managing multiple oil temperatures and simultaneously controlling two VFDs is a complex task for traditional PID control systems. Advanced control, however, simplifies this by allowing multiple controlled variables to be adjusted in real time.
By using techniques like multi-input multi-output (MIMO) control, the advanced control system can handle the dynamic interactions between multiple variables, such as oil temperature and VFD speed, ensuring stable operation even under varying conditions.
Conclusion
The implementation of advanced control techniques for oil temperature control in refrigeration units offers several advantages over traditional PID-based DCS systems. The use of advanced control allows for more flexible, safe, and cost-effective management of multiple oil temperatures and VFDs. As industries continue to demand more efficient and reliable control systems, leveraging these advanced control strategies will become increasingly important for optimizing performance and reducing operational risks.