CO2 Series: Properties of CO2 as a Refrigerant

Refrigeration Guide

Sep 4, 2024

The use of CO2 as a refrigerant (R-744) in commercial refrigeration is expected to rise significantly in North America over the next several years. With a global warming potential (GWP) of 1 and zero ozone depletion potential (ODP), CO2 is one of several emerging refrigerants capable of enabling food retailers to meet their sustainability goals and comply with environmental regulations.

Although CO2 refrigeration has been a known commodity on the world stage for many decades, many supermarket owners, operators and service technicians in North America have had very little (if any) exposure to it. The characteristics of CO2 refrigeration are quite different from traditional hydrofluorocarbon (HFC)-based systems, which has created reservations about applying it within North American food retail outlets.

Today, as North American food retail stakeholders transition away from high-GWP HFC systems, they have many questions about emerging CO2 refrigeration technologies. This article is designed to demystify CO2 refrigeration and provide the basic information needed to help establish a foundational understanding.

Compared to legacy HFC refrigerants, R-744 has a multitude of distinct thermo-physical properties and performance characteristics.

Among its many distinguishing characteristics, R-744 has a low critical point of 87.8°F — which from a refrigeration perspective, is a relatively low temperature. When the ambient (outside) temperature rises above approximately 75°F, system conditions cause the refrigerant to enter the gas cooler as a supercritical fluid (at or above 87.8°F), where its pressure and temperature relationships can rise and fall independently of each other. R-744 is at saturation when it is below the critical point, which is referred to as subcritical mode; above 87.8°F, the refrigerant transitions to transcritical mode.  

CO2 system operating pressures are significantly higher than traditional HFC systems (i.e., those using R-404A or R-410A). Although this often gives service technicians pause when they first encounter CO2 transcritical booster systems, understanding where those pressures occur can give them a greater comfort level. As medium-temperature (MT) compressors discharge into a gas cooler on the roof, pressures could reach 1,400 psi 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 refrigerant pressure exiting the gas cooler to around 550 psi and transfers it to a receiver tank, commonly referred to as a flash tank. Liquid refrigerant exits at the bottom of the flash tank via a main liquid line at 40°F (550 psi) and feeds all the MT evaporators, which operate at about 22°F (420 psi). It also feeds all the low-temperature (LT) evaporators, which operate near -20°F, at an even lower pressure of 200 psi. Although system pressures are higher than what most technicians are accustomed to, proper training and tools should mitigate the reservations of CO2’s high pressures.

R-744 has a high triple point at which the refrigerant's gas, liquid and solid states coexist — occurring at -69.8°F and 60.4 psi. Although -69.8°F is well below normal operating ranges, its corresponding saturation pressure is not 60.4 psi. The hyper-reactivity of a CO2 system can cause the pressure to reach 60.4 psi 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.

R-744 has other unique performance and operating characteristics that dictate system design strategies. Because it has a higher density than typical HFC refrigerants, CO2 requires 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.

If you have any further questions about properties of CO2 as a refrigerant, or if you have any other questions regarding refrigeration, you can contact the Refrigeration Support Team for further advice. Alternatively, you can contact your local branch for support.