40 TH ANNIVERSARY 1979 - 2019

Transcritical and Subcritical CO2 Systems Explained

Today, more and more businesses and industries are turning to CO2 Refrigeration Systems to meet their needs. In concrete, Transcritical and Subcritical CO2 Systems. Unlike traditional refrigerants, CO2 is becoming a more strategic alternative. Systems based on this refrigerant are versatile and adaptable. Therefore, it’s no surprise that various markets, from commercial to industrial applications, currently use them.

These systems use CO2 to absorb and remove heat from a space, transferring it elsewhere through a cycle of compression, condensation, expansion, and evaporation. Two types – transcritical and subcritical – operate effectively under different temperature and pressure conditions. We invite you to discover more in-depth information on CO2 as a natural refrigerant.

In this blog post, we explore the key nuances and how these two types of systems are game-changers for efficiency and sustainability. In the end, our invitation is clear. Share experiences, and we can effectively impact the refrigeration sector together.

Two crucial notes to start off. One, pressure is the key element in reading the differences between Transcritical and Subcritical CO2 Systems. Two, the critical temperature and pressure of CO2 are approximately 31°C (87°F) and approximately 73 bars, respectively.


Understanding Transcritical CO2 Systems

Remember the pressure factor? That is one of the main reasons why these systems are unique. They operate above the critical point of CO2 because the pressure discharge of the compressor is very high. To prevent overheating, a gas cooler must constantly cool the fluid in this type of cycle.

Today, Transcritical CO2 cycles are used in small to large-capacity applications, with recent growth in Industrial Refrigeration. For example, last year, SCM Frigo added high-capacity industrial transcritical CO2 racks to the portfolio. The iBooster can provide up to 1.5MW (427TR) for medium temperature and up to 600kW (171TR) for low temperature.

What are the key components of a Transcritical CO2 System? The compressor, Gas Cooler, Expansion Valves (high pressure, flashgas, and electronic expansion valves), and the Evaporator, with advanced controls. All work together to create a cycle that ensures efficient cooling. With the right design, these systems can adjust to varied climates, from cooler to warmer. This means greater efficiency. And, also, the capacity of this type of system can be adjusted to suit commercial and industrial refrigeration needs. In this way, Transcritical CO2 systems are in itself an efficient, sustainable, and reliable solution.


Looking into Subcritical CO2 Refrigeration

These systems operate below the critical point, maintaining CO2 in a liquid state throughout the refrigeration cycle. They have a two-stage process: first, it involves a gas compression phase, cooling and condensing back into a liquid; second, expansion and evaporation to absorb heat from the refrigerated space.

What are the key components?

  • Condenser: responsible to send CO2 gas back into a liquid form, releasing heat;
  • Compressor: designed to handle lower pressures;
  • Expansion valve: regulate the flow of the high-pressure liquid CO2;
  • Evaporator: absorbs the heat from the refrigeration space and cools it down.

Subcritical CO2 systems are effective in environments where temperatures are lower, with design conditions below the critical point of CO2. That’s how they maximize and guarantee efficiency. In practice, as it takes lower operating pressures and temperatures, it consumes less energy consumption and reduces the costs of maintenance. These systems are a sustainable solution, an advantage they share with the Transcritical systems.

These systems are typically used in cascade, where there are two independent refrigeration circuits. So that condensation is ensured by another fluid. Which can be water, ammonia, or another fluid. The simplest way to condense CO2 in this type of system is to integrate it into a booster-type CO2 system.


There is no other reality: the future of refrigeration depends on sustainability, efficiency, and innovation. Both Transcritical and Subcritical CO2 Systems offer specific advantages, are focused on a greener future, and are suitable for different climates and operational requirements.

Some questions and doubts on your mind? Contact us on LinkedIn or find out more about our Training Sessions in the Beijer Ref Academy. Together, we can make a difference in the future of all, especially, in the refrigeration industry.

Building Connections
On LinkedIn

Become a CO² Specialist

Beijer Ref Academy will offer technicians and installers the opportunity to learn how to operate CO2 refrigeration systems in different configurations.