{"id":22891,"date":"2022-08-11T11:00:45","date_gmt":"2022-08-11T09:00:45","guid":{"rendered":"https:\/\/www.intarcon.com\/?p=22891"},"modified":"2025-08-11T14:39:51","modified_gmt":"2025-08-11T12:39:51","slug":"r744-refrigerant","status":"publish","type":"post","link":"https:\/\/www.intarcon.com\/en\/r744-refrigerant\/","title":{"rendered":"R744 refrigerant"},"content":{"rendered":"\t\t
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Due to growing concerns about the harmful impact that fluorinated refrigerants can have on the environment, natural refrigerants have made a comeback, especially NH3 and CO2 used for industrial purposes at low temperatures. In this post we will talk about CO2 refrigeration.
Commonly known as R-744 when used as a refrigerant, carbon dioxide (CO2) has become one of the most popular natural refrigerants in recent times, although it should be noted that this technology is not new, as it was already commonly used over a century ago.
The popularity that CO2 has achieved recently as a refrigerant is due to its very low impact on the environment compared to HFCs (hydrofluorocarbons), which are under threat from current regulations (F-gas). CO2 does not damage the ozone layer (Ozone Depletion Potential ODP = 0) and it has a low impact on global warming (Global Warming Potential GWP = 1), taking this last value as a reference for determining the GWP of other gases. Also, the high efficiency of this gas means it has a lower indirect contribution to the global warming of the planet.<\/p>

CO2<\/sub>, a good refrigeration alternative<\/h2>

CO2<\/sub>\u00a0is an odourless, colourless liquid that is heavier than air. Although it is necessary for life on earth, it is also a Greenhouse Gas (GHG) that can modify the environment if its concentration in the atmosphere increases significantly. It is a \u201chigh-safety<\/strong>\u201d refrigerant, i.e. \u201cnon-flammable<\/strong>\u201d and with a \u201clow toxicity<\/strong>\u201d.<\/p>

CO2<\/sub>\u00a0was used as an early refrigerant before freons, but it quickly fell out of use due to its more complex technology. It has\u00a0excellent thermo-physical properties<\/strong>, although it poses difficulties due to its low critical temperature value (30.978\u00b0C) and its high pressures. It has a volumetric capacity that is much higher than that of conventional refrigerants.<\/p>

Advantages<\/h4>

This gas has a high thermal conductivity and high density in the gaseous phase, which results in good heat transfer in evaporators, condensers and gas coolers; so these characteristics allow smaller units to be selected, compared to those that use CFCs, HCFCs and HFCs. Also, due to the fact that it has a low pressure drop, the diameter of the pipes can be reduced.<\/p>

CO2<\/sub> is a good alternative for both commercial and industrial refrigeration,\u00a0but certain safety measures must be considered. It must be taken into account that CO2<\/sub>\u00a0cannot be detected through smell, therefore, as it is denser than air, it can displace oxygen to limits that are harmful to health. As it gives off no odour, it may be the case that, if there is a leak, the technician is not able to detect it. Such characteristics make it vital to pay special attention to leak detection, having an alarm system that can detect and warn in time of the presence of CO2<\/sub>\u00a0and having an emergency ventilation system.<\/p>

Furthermore, the high pressure of the gas when it leaks will cause an explosion, with splashes of refrigerant with residues in solid form at very low temperature at the speed of sound. Please note that CO2<\/sub> should never be charged in liquid form when the system is at a lower pressure than the triple point (5.2 bar). If you were to do so, the liquid entering the system would suddenly change state, turning into dry ice and remaining in that state inside the system.<\/p>

Limitations<\/h4>

Unlike other natural refrigerants,\u00a0CO<\/strong>2<\/sub><\/strong>\u00a0cannot adapt to any unit, either old or current<\/strong>. Units must be designed for the characteristics of this gas and for the high pressures it must withstand. Lastly, in relation to hydrocarbons, CO2<\/sub>\u00a0has the advantage of being able to be used in installations without any charge limitation.<\/p>

<\/p>

What is the history of CO2<\/sub>\u00a0as a refrigerant?<\/h2>

CO2<\/sub>\u00a0has been known since the beginning of mankind, existing in the atmosphere at a concentration of 0.04% by volume.<\/p>

As a refrigerant, it began to be used in the 19th<\/sup> century with mechanical refrigeration. In 1881, Carl Linde built the first machine using CO2<\/sub>. Later, compressors were developed to work with CO2<\/sub> and double-stage systems. Its use became increasingly widespread.<\/p>

After the First World War, synthetic gases were developed, which made it possible to use less robust elements with greater efficiency. This is where the decline of CO2<\/sub>\u00a0as a refrigerant began, with CFC gases taking over. CFC refrigerants dominated the world market for 50 years, until 1974, when it was discovered that the use of these substances was depleting the ozone layer. That is when HFC refrigerants were developed, which are still used today. However, they contribute to global warming and, therefore, their use is limited.<\/p>

\"History<\/p>

Why are we considering using CO2<\/sub>\u00a0in refrigeration systems?<\/h2>

As a result of the\u00a0Kyoto Protocol<\/strong>, agreed by the United Nations Framework Convention in order to fight climate change, the goal is to\u00a0reduce the emissions<\/strong>\u00a0of six greenhouse gases (CO2<\/sub>\u00a0, CH4<\/sub>, N2<\/sub>O, HFC, PFC and SF6<\/sub>).<\/p>

\"\"\"co2<\/span><\/p>

Among the measures taken, as well as the regulations created to achieve these goals, the\u00a0F-gas\u00a0Regulation was implemented in 2006, with subsequent even stricter updates. Its aim is to reduce HFC gas emissions to 1\/3 by 2030.<\/p>

To achieve these goals, restrictions on use and prohibitions have been set at the European level. At the same time, at state level, taxes have been imposed on the use of refrigerants in order to encourage the use of other refrigerants with a lower environmental impact.<\/p>

\"\"<\/p>

If you look at refrigerants with GWP levels < 150, the majority are flammable (HFO), toxic (NH3<\/sub>) or work at high pressures (CO2<\/sub>).<\/p>

\"\"<\/h3>

\"cta-ce-nevamar--1204-x-347-px\"<\/a><\/h3>

Thermodynamic properties of CO2<\/sub><\/h3>

The\u00a0main difference of CO<\/strong>2<\/sub><\/strong>\u00a0compared to other refrigerants is the operating pressure at which it works. However, this makes it a\u00a0high-density<\/strong> gas, resulting in a\u00a0greater cooling effect<\/strong>\u00a0with a low circulating mass.<\/p>

\"\"<\/span><\/p>

The\u00a0enthalpy of evaporation<\/strong>\u00a0per displaced cubic metre (kJ\/m3<\/sup>) is much higher than in other gases. This results in smaller compressor displacements and smaller pipe diameters.<\/p>

\u00a0<\/span><\/p>

\"evaporation<\/b><\/h4>

Pressure-enthalpy diagram (log P-h) (Mollier diagram)<\/h4>

When working with CO2<\/sub>\u00a0for refrigeration, an expanded P-h diagram must be used, i.e. in addition to the standard vapour, liquid-vapour and liquid regions that can be seen in the Mollier diagram of any conventional refrigerant, the region below the critical point (supercritical phase) and the regions below the triple point must also be represented.<\/p>

The following chart shows the physical state of the CO2<\/sub>,\u00a0according to the pressure (P) and the enthalpy (h). By modifying these two variables, four clearly differentiated phases can be obtained:\u00a0solid, liquid, vapour and supercritical fluid<\/strong>; as well as three other intermediate two-phase mixture regions:\u00a0solid-liquid, solid-vapour and liquid-vapour<\/strong>.<\/p>

\"Mollier
With respect to other refrigerants, it is striking that even with very low temperatures we obtain high saturation pressures <\/span>(e.g. 0\u00b0C\u00a0\u27f9\u00a035 bar). Please note the following points from the phase diagram:<\/p>