CO2 Refrigeration Fundamentals: Properties of R-744

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The use of CO₂ (or R-744) in commercial refrigeration is expected to rise significantly over the next several years. With a global warming potential (GWP) of 1 and zero ozone depletion potential (GWP), CO₂ can help food retailers to meet their sustainability goals and comply with emerging regulations. Because the fundamentals of CO₂ refrigeration are quite different from traditional hydrofluorocarbon (HFC)-based systems, Emerson has compiled a video series that seeks to address common questions and misperceptions about them. As a companion to the CO₂ Chats Series — which features candid interviews with me and Derek Langenkamp, Emerson’s product manager of semi-hermetic and CO₂ — we’re launching a five-part CO₂ Refrigeration Fundamentals blog series. In this first installment, we’ll look at R-744’s unique properties and how it differs from HFC refrigerants.

What is CO₂’s critical point?

Among its many distinguishing characteristics, R-744 has a low critical point of 87.8 °F. Although that may not seem like a low temperature, it is from a refrigeration perspective. What this means is that when the temperature rises above that 87.8 °F, the refrigerant enters a supercritical fluid phase. In this supercritical zone, its pressure and temperature relationships no longer match — where each 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, R-744 is no longer at saturation and operates in transcritical mode.

Does CO₂ have high operating pressures?

CO₂ system operating pressures are significantly higher than those with HFCs such as R-404A or even R-410A. Because of this, service technicians often have reservations when working on CO₂ transcritical booster systems. Understanding where those pressures occur within a system can give contractors a greater comfort level. Medium-temperature (MT) compressors discharge onto the roof at the gas cooler. Thus, on a 95 °F ambient day, that pressure will be around 1,400 psi typically within a stainless-steel discharge line coming back into the building.

Once in the building, this high-pressure refrigerant goes through an expansion valve into a receiver flash tank, reducing the pressure to around 550 lbs. It is then carried through a liquid line to an MT evaporator, which at about 23 °F is 425 psi. The low-temperature (LT) evaporator operates near -20 °F and will be at an even more reduced pressure of 200 psi. Although system pressures are higher than what most technicians are accustomed to, proper training and tools should relieve concerns some have related to CO₂’s high operating pressures.

What is CO₂’s triple point?

CO₂’s triple point is the point at which the refrigerant’s gas, liquid and solid state coexist — which occurs at -69.8 °F and 60.4 psi. Since -69.8 °F is well outside of normal operating ranges, you might wonder why triple point is a consideration. If the proper steps and cautions are not taken, a CO₂ system can reach 60.4 psi or lower. When this occurs, R-744 is likely to turn into dry ice. Once dry ice forms in a refrigeration system, it stops refrigerant flow and may cause a variety of potential problems.

What are the differences between CO₂ and HFCs?

R-744 has other 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.

For more information on any of CO₂’s unique properties, please view the companion topic in our CO₂ Chats video series. The next installment of the CO₂ Refrigeration Fundamentals blog series will focus on servicing tips. To learn more about Emerson’s comprehensive CO₂ products and capabilities, please visit Climate.Emerson.com/CO2Solutions.