Refrigerant: Function and Operation
Refrigerants play a crucial role in air conditioning systems by transferring heat from the evaporator to the condenser. The refrigerant undergoes two phase changes: in the evaporator, it changes from liquid to gas, and in the condenser from gas to liquid. This process ensures that the absorbed heat is effectively released to the outside air.
In the automotive industry, different types of refrigerants are used, each with specific properties that determine the choice. Important factors include:
- Boiling Point: Important for the phase change in the system.
- Cost: Influences the economic feasibility.
- Environmental Effects: Impact on the environment, such as ozone depletion and greenhouse effect.
- Regulation: Laws and regulations that restrict or prohibit the use of certain refrigerants.
GWP (Global Warming Potential):
The GWP (Global Warming Potential) is established by scientific organizations like the IPCC, which use advanced climate models to calculate the impact of gases on global warming. The GWP measures the warming capacity of different gases compared to carbon dioxide (CO2), for which the GWP is set at 1. This helps in making environmentally friendly choices and limiting emissions of gases with high GWP.
The GWP indicates how much more warming a gas causes compared to CO2 over a period of 100 years. For example, the GWP of R12 is 10,900, meaning that one kilogram of R12 has the same warming effects as 10,900 kilograms of CO2. This is equivalent to the emissions of a gasoline car driving 68,000 km with a consumption of 15 kilometers per liter. The GWP of R744 is 1, as this refrigerant is equal to CO2 and therefore is compared to itself.
The following paragraph describes the most commonly used refrigerants in vehicles, where GWP has played an important role in the transition to newer refrigerants.
Commonly Used Refrigerants in Motor Vehicles:
In the automotive sector, we currently find R134a, R1234yf, and the newer R744 refrigerant. R12 is mostly found in classic vehicles.
- R12a0was once widely used in vehicle air conditioning systems. It’s a refrigerant composed of carbon atoms (C), chlorine atoms (Cl), and fluorine atoms (F), with the chemical formula CCl7f2F7f2. R12 belongs to the group of CFCs (chlorofluorocarbons), also known as CFC12. Due to its harmful effects on the ozone layer (GWP of 10,900), R12 has been banned since January 1, 1995.
- R134a was introduced in 1992 as a replacement for R12. This refrigerant contains HFCs (hydrofluorocarbons) and is less harmful to the environment. R134a contains no chlorine, making it a more environmentally friendly alternative. The formula of R134a is C7f2H7f2F7f4, and it is also known as HFC134a or tetrafluoroethane. The small letter ‘a’ in the name indicates variations in chemical structures with the same atomic composition. Although R134a does not deplete the ozone layer, it has a higher GWP (Global Warming Potential) value of 1430, meaning it has a significant greenhouse effect, although less harmful than R12.
- R1234yf is a hydrofluoro-olefin (HFO) and has been replacing R134a since January 1, 2011. It has a much lower GWP value of 4, meaning that the greenhouse effect of R1234yf is about 300 times lower than that of R134a. Despite this lower GWP value, the greenhouse effect of R1234yf is still about four times higher than that of CO7f2 (R744). R1234yf is slightly flammable and its chemical formula is C7f3H7f2F7f4. It complies with the F-gas regulation, but the fire hazard is somewhat higher compared to other refrigerants.
- R744 (CO7f2), better known as carbon dioxide (CO7f2), is an environmentally friendly refrigerant with a GWP value of only 1, making it an attractive alternative to other refrigerants such as R134a and R1234yf. The molecular structure of R744 is much smaller than that of R1234yf. This refrigerant operates at high pressures up to 140 bar, requiring special system designs, as conventional systems are usually not suitable for such pressure levels. R744 has excellent thermodynamic properties, allowing it to efficiently transfer large amounts of heat with a small volume. This results in compact and efficient systems. It is increasingly used in commercial and industrial cooling systems, as well as in vehicle air conditioning, but also in electric vehicles with a heat pump.
Molecular Composition:
Internationally, refrigerants are designated with an “R,” standing for “Refrigerant.” The numbers following the “R” indicate the molecular composition of the refrigerant. The meaning of the numbers is as follows:
- The first number after the “R” indicates the number of carbon atoms minus one. If there is no first number, the number of carbon atoms is 1.
- The second number indicates the number of hydrogen atoms plus one.
- The third number indicates the number of fluorine atoms.
Chlorine atoms are not mentioned in the R-code. Below are the molecular compositions of the four refrigerants encountered in vehicle technology.
R12:
The chemical formula for R12 is CCl7f2F7f2, meaning the molecule consists of one carbon atom, no hydrogen atoms, two fluorine atoms, and two chlorine atoms. The chlorine atoms are not mentioned in the R-code but are part of the molecular structure.
- First Digit (1): Indicates the number of carbon atoms minus one (C7f1 – 1 = 0). If there is no first digit, it means the number of carbon atoms is 1.
- Second Digit (1): Number of hydrogen atoms minus one (H7f0 – 1 = -1, hence 0 hydrogen atoms)
- Third Digit (2): Number of fluorine atoms (F7f2)
R134a:
Chemical formula: C7f2H7f2F7f4, meaning the molecule consists of two carbon atoms, two hydrogen atoms, and four fluorine atoms. This refrigerant contains no chlorine atoms.
- First Digit (1): Number of carbon atoms minus one (C7f2 – 1 = 1)
- Second Digit (3): Number of hydrogen atoms plus one (H7f2 + 1 = 3)
- Third Digit (4): Number of fluorine atoms (F7f4)
- a: Indicates the specific isomer of tetrafluoroethane, namely 1,1,1,2-tetrafluoroethane
R1234yf:
The chemical formula for R1234yf is C7f3H7f2F7f4, meaning the molecule consists of three carbon atoms, two hydrogen atoms, and four fluorine atoms. This refrigerant contains no chlorine atoms.
- First Digit (1): Indicates that the molecule contains a double bond
- Second Digit (2): Number of carbon atoms minus one (C7f3 – 1 = 2)
- Third Digit (3): Number of hydrogen atoms plus one (H7f2 + 1 = 3)
- Fourth Digit (4): Number of fluorine atoms (F7f4)
- y: Indicates the position of the double bond and the specific structure of the molecule
- f: Identifies the specific isomer of tetrafluoropropene, namely 2,3,3,3-tetrafluoropropene
R744 (CO7f2):
R744 is a special designation for carbon dioxide (CO7f2). The chemical formula for R744 is CO7f2, meaning the molecule consists of one carbon atom and two oxygen atoms. This refrigerant contains no hydrogen atoms or fluorine atoms.
- First Digit (7): The “7” indicates that it is an inorganic refrigerant. For inorganic compounds, the digits are not interpreted in the same way as organic refrigerants.
- Second Digit (4): In this specific case, the “744” is simply a standardized code and does not hold the same meaning as with organic refrigerants. It simply indicates that it is CO2.
- Third Digit (4): See the explanation of the second digit.
Maintenance with an Air Conditioning Charging Station:
To evacuate and fill the air conditioning system, an air conditioning charging station is needed. This device also checks whether the system is free of leaks. After evacuation, the refrigerant and compressor oil are weighed. This provides insight into how much the system has leaked over the last months or years. A well-functioning system should leak no more than 10% per year. However, in the case of leaks, this percentage can increase much faster. If the air conditioning system has never been serviced (meaning the system is evacuated and refilled), it can become so empty over a few years (for instance, up to 8 years) that it is no longer functional. This can lead to damage to the air conditioning compressor. With some luck, the system might function again by refilling it. Therefore, it is advisable to have the system maintained every 2 to 4 years.
A check-up of the air conditioning is not the same as maintenance. In some cases, during a check-up, only the air temperature is measured and a conclusion is drawn as to whether the system is working well. Therefore, when doing maintenance, make sure to ask if the system is being evacuated and filled.
For more explanations about the air conditioning, see the chapter air conditioning.
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