CO₂ Refrigeration: A Proven Sustainable Alternative​

CO₂ (R-744) — non-toxic, non-flammable and with a GWP of 1, CO₂ has demonstrated its effectiveness in both low-temperature (LT) and medium-temperature (MT) applications. CO₂ as a natural refrigerant poses very little threat to the environment.

The term “natural refrigerant” refers to substances that naturally occur in the environment. Unlike the synthetic refrigerants that have commonly been used in refrigeration applications — including hydrofluorocarbons (HFCs) and chlorofluorocarbons (CFCs) — ammonia (NH₃ or refrigerant name R-717), propane (refrigerant name R-290) and carbon dioxide (CO₂ or refrigerant name R-744) are three naturally occurring refrigerants that pose very little threat to the environment.

CO₂ -based refrigeration systems have been successfully deployed in commercial and industrial applications in Europe for nearly two decades. Because of its low critical point and high operating pressure, CO₂ refrigeration strategies must be designed to account for its unique characteristics. In light of current environmental regulations, the popularity of these systems has increased significantly in North America in recent years.

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.

CO₂ has unique performance and operating characteristics that differentiate it from HFCs and dictate system design. It has higher density than a typical HFC refrigerant, which translates into the use of smaller compressors. However, the motor is similar in size since the work being done is approximately the same. CO₂’s higher density means that smaller pipe diameters can be used, especially on the suction side of the system. Due to its high pressures, system components need to be rated to tolerate a higher maximum pressure rating.

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.