CO2 Refrigeration Fundamentals: Servicing Tips

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In the first blog in this series, we discussed the many distinguishing properties of CO₂ (or refrigerant R-744) — including its high system pressures, low critical point and triple point. These characteristics introduce a multitude of unique servicing considerations that differ significantly from traditional hydrofluorocarbon (HFC)-based systems. In this installment, we’ll review some key tips that technicians need to be aware of when servicing CO₂ transcritical booster systems. You can also learn more about a variety of related CO₂ topics in our new CO₂ Chats video series.

From a refrigerant storage best practices perspective, R-744 tank storage is similar to standard HFC storage, including stacking procedures, safety precautions and keeping them chained off in a designated storage area. But that’s where the similarities end. Because CO₂ tanks are designed to handle its high pressures, they weigh significantly more than standard HFC bottles. Empty CO₂ tanks can weigh close to 150 lbs.; when loaded with 50 lbs. of refrigerant, each cylinder can potentially weigh nearly 200 lbs.

Many supermarkets prefer to have an entire system charge on hand, which could potentially be up to 2,000 lbs. Storing that would require 40 cylinders totaling a weight of 8,000 lbs. — or 4 tons. It’s important for contractors to understand where to store the reserve refrigerant and if it will affect building codes by having that much CO₂ in one space. And if stored on a mezzanine, it must be capable of handling the total storage weight.

How do you charge a CO₂ refrigeration system?

When charging a CO₂ refrigeration system, the most important consideration a technician should keep in mind is the triple point pressure of CO₂. 60.4 psi is the pressure at which CO₂ will turn to dry ice. As a result, contractors must be careful not to charge with liquid CO₂ when the system is below this pressure, and instead charge with vapor until the system reaches triple point. Failure to do so will result in the formation of dry ice. There are various anecdotes about technicians — who are more familiar with charging HFC systems — charging a CO₂ system with liquid and causing the formation of dry ice.

Begin charging by introducing CO₂ vapor into the system, and then build system pressure to 60.4 psi and beyond based on equipment manufacturer recommendations — up to 145 lbs. Then, it will be safe to switch to liquid CO₂ to finish charging the system quickly and effectively without the risk of dry ice formation.

What is trapped liquid in a CO₂ refrigeration system?

CO₂’s coefficient of expansion (COE) is higher than a typical HFC refrigerant. One potential scenario that can occur in a CO₂ system is when liquid refrigerant gets trapped in between two valves. In this instance, the pressure can increase 145 psi for every 1.8 °F increase in temperature. As a result, some systems may need to be fitted with appropriate pressure relief valves at the location of the trapped liquid to assist with system operation and service.

How do you detect leaks in CO₂ systems?

Since there is an abundance of CO₂ already present in the atmosphere, R-744 refrigerant can be difficult to detect and requires the use of a capable leak detection system. CO₂ is colorless, odorless and heavier than air, requiring leak detectors to be mounted 18 inches off the ground and below the breathing level. Like HFC systems, it’s important to immediately detect and mitigate CO₂ leaks as they occur.

Manufacturers such as Emerson have designed CO₂-specific leak detection technology that quickly can sense the presence of higher levels of carbon dioxide in a machine room or a walk-in box. Emerson offers both a stand-alone CO₂ leak detection solution as well as devices that can be seamlessly integrated into a building management system (BMS), such as the Lumity™ supervisory control platform.

Are there safety issues to be aware of when handling CO₂ refrigerant?

Because CO₂ refrigeration systems operate at extremely high pressures, technicians should take precautions when handling CO₂. Even when the system is shut off, standstill pressures are extremely high and need to be handled carefully. In addition, CO₂ can displace oxygen and release it in excessive amounts because it’s heavier than air. As a result, technicians should avoid handling it in confined spaces. But with proper training and equipment design, CO₂ can be used safely.

For more information on CO₂ servicing tips and best practices, please view the companion topic in our CO₂ Chats video series. The next installment of the CO₂ Refrigeration Fundamentals blog series will focus on system operation. To learn more about Emerson’s comprehensive CO₂ products and capabilities, please visit Climate.Emerson.com/CO2Solutions.