As shown in Figure 1, at atmospheric pressure, CO₂’s solid temperature can reach minus 109.3°F. Technicians should wear proper personal protective equipment (PPE) and follow the manufacturer’s procedures to prevent skin burns and other hazards.

CO₂ refrigeration phase diagram. (Courtesy Effecterra)

Finally, CO₂ is an asphyxiant and, in confined spaces, can lead to oxygen deficiency. Ensure the room and surrounding area housing the system is properly ventilated.

Validate, Inspect, and Vacuum

Before initiating the charging process, it’s imperative that everyone involved fully understands the equipment and process. A visual inspection of the equipment and a thorough review of the manufacturer’s specifications will help techs familiarize themselves with the equipment.

“It’s critically important that techs get familiar with CO₂ racks,” said Brett Wetzel, troubleshooting and technical training manager, CoolSys. “They must know where all the lines go and how the system is controlled before getting started.”

Evan Aschow, lead engineer, Effecterra, said techs should have a general idea of a system’s starting temperatures and pressures before proceeding.

“For most commercial refrigeration systems, the medium-temperature discharge side, constituting the high side, typically benefits from protection at a minimum of 1,740 psi,” he said. “The Intermediate side, representing the liquid side, is safeguarded by pressure reliefs in the flank tank at 650 psi. The low-temperature suction side is commonly secured at 435 psi. Conversely, the medium-temperature suction side is shielded at 650 psi.”

Once an inspection has been completed, the refrigerant’s quality must be validated.

Travis Kisner, regional service sales director at JAX Refrigeration Inc., said his team always employs a third-party certifier to complete any necessary material safety data sheets and test the refrigerant’s quality and compliance.

After the refrigerant has been validated, it’s time to begin the vacuum process. Attach a vacuum pump and evacuate any air or noncondensable gases from the system. The vacuum process ensures the refrigerant is introduced into a system with the proper conditions.

“Our techs run a vacuum to 300 microns and also run a standing pressure test,” said Kisner. “Once we reach 300 microns, we let the system stand for 48 hours to ensure we’re moisture-dry.”

To ensure complete systematic evacuation, Aschow recommends utilizing dry nitrogen with multiple evacuations.

“Employing a holding charge and subsequently checking for sustained pressure the following day helps identify leaks before starting up the equipment,” he said. “If there are any doubts about moisture content, always use a filter dryer.”

Before embarking on the vacuum process, make sure the liquid line is fully closed, said Wetzel.

“Once you’ve shut the entire liquid side down, run your vacuum and ensure you have no more liquid in the system,” he said. “To check this, shut off the suction line. If the pressure increases, that is a sign you have more liquid in that system. When done properly, the expansion valve should let all the refrigerant through, and it should be all suction gas. At this point, per the EPA, you can legally blow the charge via the evaporator, because we can’t put a recovery machine on CO₂ systems.”

Fill the System with Vapor

Once the system is dry, techs should begin to fill the system. Start by connecting the charging equipment to the service ports on the high and low sides of the system. Then, slowly open the refrigerant supply valve and introduce the refrigerant — in the form of vapor — into the system.

“We use a spider tool and slowly fill the low and high sides evenly to 140 psi,” said Kisner. “Then, again, we let the system sit, allowing it to settle.”

Introducing a vapor charge first helps reduce any chance the gas will solidify within the system. This phenomenon occurs because of R-744’s triple point, the unique set of conditions (specific temperature and pressure) at which the refrigerant coexists in equilibrium as a solid, liquid, and gas. For CO₂, the triple point occurs at a temperature of approximately minus 56.6°C (minus 69.9°F) and a corresponding pressure of about 60.4 psig. The triple point is a fundamental reference point for defining temperature scales, as it provides a stable and reproducible set of conditions for calibration purposes.

R-744 is at danger of turning to dry ice if the system pressure dips too low (i.e., below 60 psig), which is why techs should continue to introduce vapor until all sections of the system have equalized to at least 140 psig.

“Prior to opening any component that may have had liquid in it to the atmosphere, it’s crucial to ensure the removal of any liquid and allow the section to warm up, facilitating the boil-off of any remaining liquid,” said Aschow. “This precautionary step helps avoid complications associated with the formation of dry ice during system maintenance. If you are not planning on opening the system to the atmosphere, simply adding pressure from elsewhere in the system [be it from the liquid or vapor line] will eliminate any dry ice. Such circumstances may occur after the system has been running and a section isolated and evacuated on an evaporator with a check valve in series with the ball valve. After turning the system back on, slowly opening the expansion valve is a sufficient way to increase the pressure in the evaporator.”

Kisner said techs must pay close attention to every step because, in mere minutes, one minor mistake can lead to a major dry ice obstacle.

“If a line wasn't purged well enough, or if you have just a little bit of condensate in that liquid line, you may be in trouble,” he said. “The pipes in a CO₂ system are smaller than ammonia, and it doesn’t take much to develop a plug.”

When transitioning from a vapor to liquid CO₂, many techs may consider introducing a pressure reducer. Aschow advises against this practice.

“When employing liquid bottles, refrain from using a pressure reducer and exercise caution to prevent liquid entrapment during the removal of piping or filling hoses,” he said. “After the receiver has reached approximately 140 psi, charging the rest of the system with liquid is appropriate.”

Once the system is up around 140 psi, Kisner said he monitors the system for an hour before proceeding.

“Once we’ve conducted the standing hour test, we push liquid in on the high side,” he said. “We like to rely on the 40% line on the receiver, as a lot of these systems have sub-receivers on the evaporators. At the end, make sure everything is equalized and full of liquid.”

Ensuring the system has the proper type, amount, and quality of oil is another important step in the charging process.

“Techs need to ensure the proper amount of oil present in the compressors and reservoir,” said Aschow. “A tech can confirm operation of the compressor crankcase heaters by verifying oil temperature is at least 36°F above ambient temperature.”

When the oil heaters are activated, begin addressing the oil, said Kisner.

“CO₂ systems are finicky and have a lot of unique characteristics,” he said. “To be sure the system is operating as it should, I recommend using the oil that’s prescribed by the manufacturer.”

Monitor the System & Check the Pressures

Throughout every step of the charging process, techs should constantly monitor pressures.

On the second day, Kisner aims to run the system at 55°F. If everything is working as it’s supposed to, he begins to tinker with the operating conditions.

“We let it run for about a day and a half at 55° and make modifications as we go,” he said. “Every 36 hours, we drop the temperature by about 10°, experimenting with different pressure-temperature correlations, making sure there aren’t any leaks or disturbances in the system.”

If everything checks out, the system has been charged correctly. If it does not, a tech should troubleshoot the system, following the proper sequence of operations.

Despite the fear that many technicians harbor when first encountering a CO₂ system, Kisner said techs should trust their instincts and continue to improve upon their knowledge base.

“Know your sequence of operations for each different system and carefully follow each step,” he said. “Understand the volume before starting, so that you don’t have to come back and add more. CO₂ can be intimidating at first, but if you simply step back and think about each aspect, it gets easier as you go.”