As noted in Part 1, carbon dioxide (CO₂) has served as a dependable and safe refrigerant since the Industrial Revolution. Even though modern food retailers are increasingly adopting CO₂ (R-744) as their preferred refrigerant, skepticism remains as to how well this refrigerant works. This second installment of a two-part series will delve into additional misconceptions surrounding the use of CO₂.

Myth: Pressure in a CO₂ System Is Dangerously High

Pressure is required to operate a CO₂ system, prompting concerns about the potentially high pressures — up to 1,600 psi — particularly on hot summer days. There is a fear that such pressures could pose a danger to anyone who comes into close contact with the coolers or freezers. The truth is that all sections of the CO₂ system that require high pressure are located either in an equipment room or on the roof, posing no risk to shoppers or employees.

Another way to consider this fear is by looking at paintball and scuba diving. A paintball gun powered by a CO₂ cylinder (positioned under the shooter’s arm) experiences 894 psi at an ambient temperature of 75ºF. No one is worried the paintball gun’s CO₂ cylinder is too dangerous; in fact, some newer model paintball guns are powered by high-pressure air or nitrogen – between 3,000 and 4,500 psi.

For scuba divers, a compressed air tank resting behind their heads can have pressure ranging anywhere from 2,400 to 3,500 psi. Again, no one says these pressure levels are dangerously high.

Reality: The pressure in a CO₂ system located inside a grocery store is actually lower than the pressure used to power paintball guns and scuba tanks, and we don’t think twice about playing paintball or scuba diving.

Myth: Leak Rates in CO₂ Systems Are Greater Than 100%

This myth needs some clarification of terms. There is a difference between leak rates and emission rates. Leaks can occur in any system and can go unnoticed for days, months, or years, which may result in a large rate of fugitive gases being released into the atmosphere. Emission rates, on the other hand, concern three areas:

1. Actual leaks in piping or fittings.
2. Service and repair activities in which gases may need to be vented to the atmosphere.
3. Power failures that force the relief valve to open, emitting gases into the atmosphere.

As a real-world example, a system for a food retailer holds approximately 400 pounds of CO₂ total. When surveyed, a contractor said he adds approximately one 40-pound cylinder per year, which is a leak rate of 10% — nowhere near 100%.

To demonstrate emissions due to a power failure, Hillphoenix tested a CO₂ system with a 650-psi (45 bar) relief valve on the system’s suction line. When the system was shut down, the 650-psi relief valve activated after 7.5 hours.

An added benefit of CO₂ is that it’s odorless, non-corrosive, non-toxic, and nonflammable, which means no reclamation is needed.

Reality: It is necessary to differentiate between leak rate and emission rate. When you do, it becomes apparent that a typical leak rate for a CO₂ system is closer to 10%, and if emissions occur during service or repair, there is no need for reclaim.

Myth: Refrigerant-Grade CO₂ Isn’t Widely Available in North America

In 2020 and 2021, outside factors contributed to this myth, including contaminated wells and a shortage of CO₂ cylinders. There were also notorious supply chain issues. But CO₂ supply worries are essentially a thing of the past.

For many years, refrigerant systems required 99.99% pure Grade 4 Coleman/instrument-grade CO₂. This level of purity was needed to ensure that the CO₂ did not contain excessive impurities or moisture. However, there has been a change in the grades of CO₂ approved for use in refrigerant systems. A regulation change now allows 99.9% pure refrigerant-grade CO₂ to be used, as it has comparable purity and moisture levels.

Also, in cases of emergency or a shortage of higher-grade CO₂, lesser grades may be used as long as a filter drier is used during charging activities, and the user verifies that no air or non-condensable gases are present in the CO₂ after charging.

Reality: Refrigerant-grade CO₂ is widely available from many suppliers in North America, and both refrigerant- and Coleman-grade CO₂ are now acceptable for refrigeration systems.

Myth: Adiabatic Gas Coolers Use Too Much Water and Are Required in Warm Climates

Operators have used adiabatic gas coolers for years to keep system pressures lower and save on energy costs. Adiabatic gas coolers must be used in regions where the ambient design temperature can reach or exceed 101ºF, according to ASHRAE. Of the 2,943 climate stations that ASHRAE operates in the U.S. and Canada, only 14% (roughly 410 sites) — most of which are in the western U.S. and Canada — experience 101ºF ambient temperatures.

There are also cherry-picked examples, like a store in a hot southwest climate that uses a lot of water. What may go unsaid, however, is that the facility is massive — three times the size of a typical store. And if an adiabatic gas cooler is operated in a normal-sized store and in a higher water-saving mode, significantly less water will be used.

Other methods to keep operating pressures low during hot summer days are utilized worldwide and are currently being field trialed in North America.

Reality: Those who argue adiabatic gas coolers must be used in warmer climates — and that they are water hogs — are being selective with the data and not telling the entire story.

Myth: CO₂ Transcritical Systems with Dry-Gas Coolers Require Water Spray During High Ambient-Temperature Conditions

Some people are convinced that a sprinkler is needed on a system with a dry-gas cooler to keep the system pressure low when the ambient temperature gets close to 100ºF.

To refute this myth, data was gathered from many refrigeration installations with dry-gas coolers in the Upper Midwest. Even as the ambient temperature approached 101ºF, the dry-gas cooler continued to operate as expected, and there was no need for a sprinkler. Even at that elevated temperature, the high-pressure valve never came close to fully opening.

Reality: A properly sized and maintained dry-gas cooler does not require water spray as ambient temperatures rise toward 101ºF.

The bottom line is that CO₂ is a safe and reliable refrigerant that many modern food retailers are finding to be an ideal choice for their refrigeration systems.