In commercial refrigeration, CO₂ (refrigerant name R-744) is emerging globally as a sustainable, viable alternative to legacy hydrofluorocarbon (HFC) refrigerants. But for U.S. food retailers and their service networks, CO₂ is a relatively unknown commodity. Most technicians have little to no experience working with CO₂ and have many questions about refrigerant characteristics, system architectures and management. Alleviating these concerns starts with a basic understanding of the fundamentals of CO₂ refrigeration.

Compared to legacy HFCs, R-744 has many distinct thermo-physical properties and performance characteristics.

High operating pressures — CO₂ system operating pressures are significantly higher than traditional HFC systems (i.e., those using R-404A or R-410A). As medium-temperature (MT) compressors discharge into a gas cooler on the roof, pressures could reach 1,400 psig on a 95 °F summer day. MT discharge lines are constructed with stainless steel or special ferrous alloy copper to handle these pressures. Within a facility and/or machine room, a high-pressure expansion valve reduces the pressure to around 550 psig.

Low critical point of 87.8 °F — R-744 is at saturation when it is below the critical point, which is referred to as subcritical mode; above 87.8 °F, it transitions to transcritical mode. When the ambient temperature rises above approximately 75 °F, refrigerant enters the gas cooler as a supercritical fluid (at or above 87.8 °F), where the relationship between pressure and temperature is uncoupled.

High density — R-744 has a higher density than typical HFC refrigerants, requiring the use of smaller compressor displacements — although the motor is sized similarly to carry the workload. 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 must be rated for a higher maximum working pressure.

High triple point of -69.8 °F (60.4 psig) — R-744 has a high triple point, at which the refrigerant’s gas, liquid and solid states coexist. Although -69.8 °F is well below normal operating ranges, its corresponding saturation pressure is not 60.4 psig. The hyper-reactivity of a CO₂ system can cause the pressure to reach 60.4 psig or lower. When this occurs due to improper maintenance, R-744 turns into dry ice, stops the refrigerant flow, and causes a variety of potential problems.

When charging a CO₂ refrigeration system, technicians should keep its triple point pressure in mind and guard against turning the charge to dry ice. When the system is below 60.4 psig, it should be charged with vapor until the system reaches triple point. In fact, most equipment manufacturers recommend charging the system with vapor until it reaches at least 100 psig. Then, technicians can switch charging with liquid to speed up the process.

When handling CO₂ refrigeration systems, technicians should take proper precautions. Even when a system is shut off, standstill pressures are still likely to remain high and should be handled accordingly. R-744’s high density can quickly displace O₂ when released in excessive amounts. Thus, technicians should avoid handling it in confined spaces and ensure they use proper leak detection measures.