From 1974 onward, due to the research of Mario Molina and Sherwood Rowland, the world of industrial refrigeration was significantly impacted. These researchers from the University of California (Irvine, USA) confirmed the harmful effects of chlorofluorocarbons (CFCs) on the ozone layer. Finally, in 1987, practically the entire world formalized the Montreal Protocol to gradually replace fluorinated refrigerants (CFCs) with less harmful substances. Consequently, carbon dioxide, classified in ASHRAE regulations as refrigerant R-744, became a natural alternative to CFC gases.
After the Montreal Protocol, increasingly strict restrictions have been imposed, leading to a renewed interest in natural refrigerants. These natural refrigerants had been sidelined for many decades because fluorinated refrigerants offered better technical advantages.
What is the R-744 refrigerant or carbon dioxide?
In 1754, Joseph Black experimentally discovered carbon dioxide, which he called “fixed air.” Additionally, this Scottish scientist made a significant contribution to thermodynamics, developing the concepts of latent heat and specific heat. However, it was Antoine Lavoisier, the father of modern chemistry, who first classified substances into pure chemical elements and compounds. Lavoisier, building on the experiments of Joseph Priestley and Henry Cavendish, correctly explained the combustion process, discovering chemical elements like oxygen, carbon, and hydrogen, as well as compounds like carbon dioxide.
At ambient temperature and pressure, the R-744 refrigerant is a colorless and odorless gas, heavier than air. The chemical formula of this refrigerant is CO2, a non-reactive and stable substance. In other words, the molecule of this substance is composed of two oxygen atoms bonded to one carbon atom. CO2 does not react with metals or plastics, and it does not decompose when subjected to high pressure and temperature.
Evolution of the R-744 refrigerant in industrial refrigeration
In 1835, the French scientist Adrien Thilorier first produced “dry ice” accidentally by introducing liquid carbon dioxide into an empty container. Liquid carbon dioxide had been obtained previously through the chemical reaction between sodium bicarbonate, water, and concentrated sulfuric acid. From that moment on, the powerful refrigerating properties of carbon dioxide began to be observed. “Dry ice” is nothing but solidified carbon dioxide at ambient pressure, reaching a temperature of -78.5 °C.
Although there is a British patent registered in 1850, the first CO2 refrigeration system was built by Carl Von Linde in 1881. This was a compression and expansion system using CO2. Subsequently, this technology gradually expanded year after year until the mid-1920s. Then, the use of CO2 or ammonia systems declined due to the emergence of synthetic CFC refrigerants.
By 1960, CO2 refrigeration systems had almost disappeared. However, from 1993, there was a renewed interest in CO2 refrigeration systems worldwide. This resurgence was supported by the research of Norwegian scientist Gustav Lorentzen. Lorentzen perfected a simpler and more efficient method for working with CO2 within a transcritical thermodynamic cycle.
Ecological benefits of CO2 refrigeration systems
The R-744 refrigerant has an Ozone Depletion Potential equal to zero (ODP=0), meaning it does not affect the ozone layer. Furthermore, the Global Warming Potential is equal to one (GWP=1), which serves as the reference value for other refrigerants.
To grasp the significance of this, consider the indicators of other refrigerants. For example, currently used fluorinated refrigerants have a GWP that can range between 400 and 1500. However, a natural refrigerant like propane has a GWP equal to 3, while ammonia has a GWP equal to 0.
Thermodynamic properties of CO2
The R-744 refrigerant is a gas that operates at much higher pressures than any fluorinated refrigerants, ammonia, propane, etc. Additionally, carbon dioxide has a high density, nearly double that of air at atmospheric pressure. Consequently, the enthalpy of evaporation is approximately 5 times higher than that of any other refrigerant. Thus, a given mass of CO2 circulating through a refrigeration system has a greater refrigerating effect than other refrigerants.
In the Mollier Diagram of CO2, it is observed that this refrigerant has a critical point at a pressure of 73.77 bar and a temperature of 30.98 °C. Beyond this critical point, the R-744 refrigerant can no longer condense. Consequently, this creates two well-defined operating zones: subcritical operation (below this point) and transcritical operation (above this point). This characteristic causes industrial refrigeration, in regions with temperatures equal to or lower than 25 °C, to operate in subcritical mode. However, in hotter regions, CO2 refrigeration systems operate in transcritical mode.
It is also important to note that CO2 has a triple point at a pressure of 5.18 bar and a temperature of -58.58 °C. This point indicates that if we expand liquid CO2 to a pressure below 5.18 bar, CO2 will turn into a solid state and obstruct the system.
Intersam, a leading company in CO2 gas coolers and evaporators
In systems with R-744 refrigerant, as well as with any other refrigerant, it is essential to have high-efficiency heat exchangers. In this regard, at Intersam, a leading company in CO2 gas coolers and evaporators, we offer equipment with high heat transfer coefficients manufactured under the strictest safety and quality standards to work with CO2, ammonia, HFC, propane, etc. If you have any doubts or concerns, you can contact us, and we will be happy to assist you.