CO₂ as a refrigerant (R-744) has been around since 1850. However, the emergence of lower-pressure, “safer” refrigerants, such as R-502, R-12, and R-22, drastically reduced its demand. Eventually, due to environmental concerns, each of the aforementioned gases has been phased out, and, ironically enough, R-744 — which boasts a global warming potential (GWP) of 1, has emerged as a viable replacement, especially in domestic, case-controlled refrigeration systems.

While CO₂ offers significant environmental benefits, it’s not without its warts, i.e., flammability rating, high-pressure operation, sensitivity to water contamination, and more.

As more and more case-controlled facilities turn to CO₂ refrigeration, HVAC contractors are tasked with adequately educating themselves on these systems so that they can meet the country’s budding installation and service needs.

Liquid, Vapor, or Gas?

Brett Wetzel, manager of technical troubleshooting and training, CoolSys, has 20-plus years of experience in the HVACR industry. For the last eight years, he’s served as a lead trainer for CoolSys, teaching technicians the ins and outs of the industry’s refrigerants.

Due to its emergence, he’s spent a lot of time deciphering R-744. Using that knowledge, Wetzel created “Introduction to CO₂ Refrigeration” and “Advanced TC-CO₂ Refrigeration” courses, which he is currently teaching to students in-person at the company’s training centers in Texas. These courses will be available at the company’s California training center in 2024.

“Over the last year, we’ve trained approximately 270 people on CO₂ refrigeration systems,” said Wetzel, who also serves as the host of the Advanced Refrigeration Podcast. “I try to emphasize the physics and numbers to really demonstrate how and why everything is working the way it is.”

When it comes to CO₂ refrigeration systems, Wetzel said pressure is among technicians’ chief concerns.

“Once CO₂ hits a certain operating point, there’s no correlation relationship in the pressure-temperature [PT] chart,” he said. “It’s important to know whether the refrigerant is in a liquid or gas state, and, with CO₂, once we hit 87°F saturated, we’re unsure whether it’s in a plasma, vapor, or gas state. To figure that out, we either have to open it up or take scientific measurements.”

As a result, techs often struggle to understand the state of the gas as it migrates from the gas cooler through a high pressure valve (HPV) to the flash tank, said Wetzel.

“It’s a difficult concept to theorize because the math doesn’t necessarily add up,” he said. “If you have 75° saturated condensing temperature, that HPV is trying to control 5° subcooling, but then it opens up further and goes all the way down to 38° saturated in the flash tank. Theoretically, if the liquid is warmer than 38°, it should be a vapor, but, in this case, that doesn’t hold fully true.”

During subcritical operation, approximately 70% of the refrigerant is liquid while the remaining 30% goes to waste, he said.

“That HPV, or high pressure valve, is there to reduce the pressure,” said Wetzel. “It’s essentially trying to control subcooling when it’s subcritical.” In transcritical mode, the HPV starts modulating to maintain a setpoint defined by an equation to achieve a virtual subcooling using only the pressure input as a control point.

And in transcritical applications, liquid production reduces down to about 50-50 but can dip as low as 40% liquid production and 60% vapor.

“In this case, that’s why these systems run with more compressors on the medium-temperature side than the low-temperature side,” said Wetzel. “Once it hits transcritical, that extra mass of vapor has to be compressed by something, because we have to pull that gas down to 38° saturated, which is typically set for most flash tanks.”

Training Timeline

Wetzel is confident most experienced refrigeration technicians should be able to grasp the concepts of CO₂ refrigeration rather quickly.

“Most experienced technicians should attend a three- to five-day seminar, at the very least, depending on how knowledgeable they are,” he said. “There are a lot of new concepts involved. Technicians familiar with the intricacies of refrigeration system controls and sequences of operation will pick it up quicker than those who are not.”

Peter Savage, project manager, controls division, AAA Refrigeration Service Inc. in Bronx, New York, agrees, acknowledging the training timeline is highly dependent on a technician’s experience and curiosity.

“The electrical part of CO₂ refrigeration systems, as well as the controls, often trip up our newer guys,” he said. “Getting guys to understand what pressures and states should be running at what point in the cycle can be challenging.

“Guys need to be able to understand the sequence of operations from day one,” he continued. “These are complex systems. If a technician adjusts just one set point, it could have a major impact on many other aspects of the system’s operation down the line.”

While R-744 systems are complex, when it comes to the refrigeration cycle itself, they’re very similar to most other refrigeration equipment, said Savage.

“Whether it’s a CO₂ rack or an air conditioner on your roof, the refrigeration cycle is pretty much the same,” he said. “CO₂ units utilize flash gas bypass valves and HPVs, but those are really the only two components that differentiate them from ‘normal’ systems.”

While challenges do exist, Savage said techs should not be fearful of CO₂ systems.

“If you’re familiar with variable frequency drives, and those types of technologies, you should be fine,” he said. “And now’s the time to get acclimated with CO₂ because customers will be bidding on these systems very soon, if they’re not already. Acquiring the necessary education early will solve a lot of your problems.”