As food retailers embark on low-GWP refrigerant transitions, CO2 refrigeration is advancing as a sustainable alternative to HFC systems. For many store owners and their service networks, CO2 (or refrigerant R-744) is a relatively new technology that presents unique operational challenges and considerations.
In recent years, system and component manufacturers have made great strides to shorten the CO2 learning curve, such as addressing regional climate challenges, simplifying system installations, and developing an ever-expanding list of application best practices. Whether you’re new to CO2 refrigeration, an early adopter, or a skilled practitioner, understanding the differences between CO2 and HFC systems can help facilitate a smooth transition.
CO2 Properties
Compared to HFC systems, CO2 booster systems have distinct characteristics, high-pressure management strategies, and design and charging considerations, all of which stem from the properties of the refrigerant R-744. (See Table 1.)
TABLE 1: Key differences in refrigerant properties between R-744 and R-404A, a legacy HFC. (Courtesy of Copeland)
CO2 booster systems run at high operating pressures exceeding 1,400 psi in supercritical (i.e., transcritical) mode on a hot summer day. R-744 has a low critical point of 87.8°F and 1,056 psig, compared to R-404, which has a much higher critical point of 161.6 °F and 503 psig. When R-744 is below the critical point of 87.8 °F, a system runs in subcritical mode. When the ambient temperature rises above ~75°F (as detected at the gas cooler), R-744 changes to a supercritical fluid, and the system enters supercritical mode.
R-744 has a high triple point of 60.4 psig (-69.8 °F), which is essential to be aware of during system charging. A system cannot be charged with liquid while its pressures are below 60.4 psig. If that happens, R-744 will turn into dry ice in the charging hose, which will stop the refrigerant flow and cause potential system problems. Technicians should charge with vapor until the system reaches at least 100 psig.
Lessons Learned
Copeland’s system integration experts have assisted with more than 500 global CO2 field installations, collaborating with leading manufacturers, contracting companies, and service technicians. Every installation is a learning opportunity, and we’re eager to share some key lessons in order to help simplify CO2 adoption.
One of the most critical takeaways is that sensors are not optional. Proper installation and placement of pressure transducers or temperature sensors are essential for ensuring stable CO2 booster system operation. The temperature sensor at the gas cooler outlet is critical, along with several other gas cooler and flash tank sensors. Unlike an HFC system, if these are not installed or operating correctly, CO2 systems can quickly become unstable and potentially cause a system shutdown. Accurate sensor data is essential to CO2 system management and high-pressure control.
It is also important to keep a close eye on oil management. Proper oil management is vital in CO2 booster systems, especially since it’s subject to seasonal variances. Compressors have higher oil carry-over rates when they run in supercritical mode and must be managed closely during summer or high-ambient months.
When the oil reservoir is equalized to the flash tank, the flash tank pressure also impacts a booster system’s oil pressure flow. If the flash tank pressure is too low, the system won’t be able to generate adequate oil flow. CO2 system components and sections are generally more interdependent than those typically encountered in an HFC system.
Finally, if you’re ready to start up a system, make sure you dot your i’s and cross your t’s. CO2 booster system installers and commissioners have discovered that they must be much more thorough than when powering up an HFC system. Before flipping the “on” switch, every sensor, connection, cable, and gauge must be checked. Following a CO2 start-up procedure checklist, such as the one developed by Copeland, can help guide and ensure successful system startups.
Best Practices
Copeland’s extensive CO2 field experience, product expertise, and advanced test labs have helped us build a knowledge base of CO2-specific system design and application best practices. These best practices include optimizing systems to adapt to climate conditions.
Because of R-744’s refrigerant properties, the ambient temperature can significantly impact CO2 booster system performance — particularly in warm climates. Recently, original equipment manufacturers and system designers have employed design strategies that optimize system efficiencies and performance based on the local environment, the most common of which include:
- Adiabatic (wetted) cooling on the gas cooler;
- Parallel compression on the medium-temperature (MT) suction; and
- A combination of parallel compression and adiabatic.
Extensive climate research by manufacturers has identified best-fit strategies per location and validates their potential benefits in various regions.
Another best practice is to embrace electronic control systems. Addressing R-744’s reactivity to various ambient and operational conditions requires greater automation than traditional HFC systems. Integrated electronic controls are required to manage system pressures; control variable fan speeds; effectively stage compressors on/off; modulate high-pressure valves (HPVs), bypass gas valves (BGVs), and electronic expansion valves (EEVs); maintain consistent flash tank and oil reservoir pressure; and ensure smooth compressor modulation.
A fully integrated control ecosystem simplifies the application of CO2 refrigeration and automates system operation. Copeland, for example, leverages system data in our CO2 test labs to refine machine-learning algorithms, support smoother system control, and apply lessons learned to update supervisory software.
Finally, it is important to mitigate noise and vibration. CO2’s high operating pressures can cause compressors to vibrate excessively and produce sound levels that can disrupt a retail environment. Built-in compressor design strategies and seamless VFD integration, such as those offered by Copeland, can support best-in-class low sound and vibration.
A fully integrated approach to CO2 refrigeration that encompasses compression, drives, valves, controls, monitoring, and field service plays a key role in simplifying system applications. By gathering insights from field installations and research initiatives, controls software and machine learning algorithms can be updated in order to fine-tune system startup, commissioning, and management. These advancements simplify system installation and help flatten the learning curve for CO2 refrigeration systems.