4 points in the CO2 transcritical system

4 points in the CO2 transcritical system


The design of a CO2 transcritical system is a challenge even for a company like ours, an expert in industrial and commercial refrigeration.

The correct implementation and quality of the equipment depends not only on efficiency, but also on the safety and profitability of the industry where it is implemented. When evaluating the feasibility of this system in a refrigeration project, it is advisable to consider these 4 key points.

Why CO2?

It is not only the concern for natural gases and their role in environmental care. The physicochemical properties of carbon dioxide make it an excellent compound for transporting heat, so its use becomes beneficial for everyone. You gain efficiency in commercial or industrial refrigeration, and the planet ? gains in its conservation.

Temperature and critical point of CO2

Before working with CO2, it is important to know its characteristics:

The critical temperature of CO2 is approximately 31 ° C.

The critical pressure, also approximate, is 73 bar.

Cooling systems that base their operation on this compound operate in different ways as they do above or below the critical point, which is nothing more than the point where the liquid-vapor phase change is maintained in equilibrium.

2. CO2 system: transcritical or subcritical?

In the use of CO2 as a refrigerant gas, we can locate two applications: subcritical and transcritical:

In a subcritical system, the temperature of CO2 in the isothermal stage after fluid compression is below the critical temperature.

In transcritical systems, the CO2 cools but does not condense at the gas cooler outlet, remaining above the critical temperature.

3. Pressure in CO2 transcritical systems.

Transcritical systems, like subcritical systems, evaporate CO2 below its critical point. However, in these systems, the compressor pressure discharge is very high (above 1069 psia / 73.7 Bar) and above the critical point, where CO2 exists without a clear distinction between the liquid and gaseous state. This results in the “transformation” of the CO2 into a kind of hazy vapor, denser than the gaseous state.

Consequently, the superheated fluid must be constantly cooled in a gas cooler, instead of being cooled and condensed into liquid, as in a subcritical cycle. The compressed and cooled fluid receives a pressure reduction below the critical point where a part of the fluid condenses into liquid to be fed to the evaporator.

4. Cycle: from transcritical to subcritical

If the gas heat sink is sufficiently cold, the cycle could be transcritical to subcritical and then, condensation of some part of the fluid into liquid could occur in the gas cooler. This does not cause any operational problems if the pressure regulating valve at the cooler outlet is properly controlled.

5. Transcritical cycle and heat exchange with the outside.

You can also make a CO2 cycle that exchanges heat with the outside. In this case it is a transcritical cycle, as in certain periods of the year where the outside temperature is near or around 31.1 ° C.

The following figure shows a simplified one-stage for a transcritical cooling circuit superimposed on a pressure-enthalpy diagram. Heat from the gas cooler is rejected to a heat sink at a temperature higher than the critical temperature.

Interested in more information? You can contact our engineering and development department for commercial and industrial refrigeration installations.