Refrigerant: function and operation
Refrigerants play a crucial role in A/C systems by transporting heat from the evaporator to the condenser. In doing so, 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 into 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 economic feasibility.
- Environmental impact: Impact on the environment, such as ozone depletion and greenhouse effect.
- Regulations: Laws and regulations that restrict or prohibit the use of certain refrigerants.
GWP (Global Warming Potential):
The GWP (Global Warming Potential) is determined by scientific organizations such as the IPCC, which use advanced climate models to calculate the impact of gases on global warming. GWP measures the warming capacity of different gases in comparison to carbon dioxide (CO₂), for which the GWP is set at 1. This helps in making environmentally friendly choices and limiting the emissions of gases with a high GWP.
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, which means that one kilogram of R12 has the same warming effect as 10,900 kilograms of CO₂. This corresponds to the emissions of a petrol car driving 68,000 km with a fuel consumption of 1 in 15. The GWP of R744 is 1, because this refrigerant is identical to CO₂ and is therefore compared with itself.
The following section describes the most commonly used refrigerants in vehicles, where GWP has played an important role in the move 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 encountered at most in classic vehicles.
- R12 used to be widely used in vehicle air conditioning systems. It is a refrigerant consisting of carbon atoms (C), chlorine atoms (Cl) and fluorine atoms (F), with the chemical formula CCl₂F₂. R12 belongs to the group of CFCs (chlorofluorocarbons), also referred to as CFC12. Due to its harmful effect on the ozone layer (GWP of 10,900), R12 has been banned since 1 January 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, which makes it a more environmentally friendly alternative. The formula of R134a is C₂H₂F₄, and it is also referred to 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 affect the ozone layer, it has a higher GWP value (Global Warming Potential) of 1430. This means it has a significant greenhouse effect, although it is less harmful than R12.
- R1234yf is a hydrofluoro-olefin (HFO) and has replaced R134a since 1 January 2011. It has a much lower GWP value of 4, which means 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 CO₂ (R744). R1234yf is slightly flammable and its chemical formula is C₃H₂F₄. It complies with the F-gas regulations, but the fire risk is slightly higher compared to other refrigerants.
- R744 (CO₂), better known as carbon dioxide (CO₂), is an environmentally friendly refrigerant with a GWP value of only 1, which makes 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, which requires special system designs, because 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 refrigeration systems, as well as in vehicle air conditioning, including in electric vehicles with a heat pump.
Molecular composition:
Internationally, refrigerants are indicated with an “R”, which stands for “Refrigerant”. The numbers after the “R” indicate the molecular composition of the refrigerant. The meaning of the numbers is as follows:
- The first digit after the “R” indicates the number of carbon atoms minus one. If there is no first digit, the number of carbon atoms is 1.
- The second digit indicates the number of hydrogen atoms plus one.
- The third digit indicates the number of fluorine atoms.
Chlorine atoms are not listed in the R code. Below are the molecular compositions of the four refrigerants we encounter in automotive technology.
R12:
The chemical formula for R12 is CCl₂F₂, which means that the molecule consists of one carbon atom, no hydrogen atoms, two fluorine atoms and two chlorine atoms. The chlorine atoms are not indicated in the R code, but they are part of the molecular structure.
- First digit (1): Indicates the number of carbon atoms minus one (C₁ – 1 = 0). If there is no first digit, this means that the number of carbon atoms is 1.
- Second digit (1): Number of hydrogen atoms minus one (H₀ – 1 = -1, so 0 hydrogen atoms)
- Third digit (2): Number of fluorine atoms (F₂)
R134a:
Chemical formula: C₂H₂F₄, which means that 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 (C₂ – 1 = 1)
- Second digit (3): Number of hydrogen atoms plus one (H₂ + 1 = 3)
- Third digit (4): Number of fluorine atoms (F₄)
- a: Indicates the specific isomer of tetrafluoroethane, namely 1,1,1,2-tetrafluoroethane
R1234yf:
The chemical formula for R1234yf is C₃H₂F₄, which means that 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 (C₃ – 1 = 2)
- Third digit (3): Number of hydrogen atoms plus one (H₂ + 1 = 3)
- Fourth digit (4): Number of fluorine atoms (F₄)
- 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 (CO₂):
R744 is a special designation for carbon dioxide (CO₂). The chemical formula for R744 is CO₂, which means that the molecule consists of one carbon atom and two oxygen atoms. This refrigerant contains no hydrogen or fluorine atoms.
- First digit (7): The “7” indicates that this is an inorganic refrigerant. For inorganic compounds, the digits are not interpreted in the same way as for organic refrigerants.
- Second digit (4): In this specific case, “744” is simply a standardized code and does not have the same meaning as for organic refrigerants. It simply indicates that it is CO₂.
- Third digit (4): See the explanation of the second digit
Maintenance with an A/C charging station:
An A/C charging station is required to evacuate and refill the A/C system. 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 past months or years. A properly functioning system may leak a maximum of 10% per year. In the case of leaks, however, this percentage can increase much faster. If the A/C system has never been maintained (by which we mean evacuating and refilling the system), it can become so empty after a few years (for example up to 8 years) that it no longer functions properly. This can lead to damage to the A/C compressor. With some luck, the system can function again by refilling it. It is therefore recommended to have the system serviced every 2 to 4 years.
An A/C check is not the same as maintenance. Sometimes, during a check only the air temperature is measured and a conclusion is drawn on whether the system is working properly. When having maintenance carried out, therefore clearly ask whether the system will be evacuated and refilled.
For more explanation about the air conditioning system, see the chapter air conditioning.
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