Refrigerants: Classification, Properties, Substitution, and Environmental Trends
Introduction
Refrigerants are working substances that provide heat transfer in a closed thermodynamic cycle through evaporation and condensation. For the engineer, their physical and chemical properties are key: saturation pressure, boiling point, critical temperature, heat of vapourisation, density, compatibility with materials and oils, and stability during operation. These parameters directly affect compressor performance, heat exchanger geometry, the type of oil used and the overall energy efficiency of the system.
Refrigeration agents evolved from natural substances – ammonia (NH₃), carbon dioxide (CO₂) – to synthetic compounds that provided ease of use and safety, but created environmental problems: ozone depletion and high greenhouse gas emissions. Today, the industry is moving towards substances with minimal global warming potential (GWP) while maintaining energy efficiency and an acceptable level of safety according to ISO 817 classification (toxicity and flammability).
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Basic concepts
Refrigerant – circulates in the refrigeration cycle and transfers heat from the evaporator to the condenser. Its main function is phase transition at moderate pressure, providing a high thermal effect in a compact equipment size. The main characteristics of the refrigerant are saturation pressure at boiling point, specific heat of vapour formation, critical temperature, vapour density, viscosity and heat capacity. It is the ratio of these properties that determines the specific refrigerating capacity and the energy efficiency ratio (EER, COP).
By origin refrigerants are divided into synthetic and natural. Synthetic ones include hydrochlorofluorocarbons (HCFC), hydrofluorocarbons (HFC) and hydrofluoroolefins (HFO). They are artificially produced, have stable characteristics, low toxicity and good oil compatibility. Natural agents – ammonia (R717), carbon dioxide (R744), propane (R290) – exist in nature and do not deplete the ozone layer, but have higher safety requirements due to toxicity or flammability. The physical structure distinguishes between single-component agents and mixtures – zeotropic (temperature slip at phase transition) and azeotropic (behave as one substance). The setting of the thermostatic control valve and the refuelling methods depend on this.
GWP (Global Warming Potential) – the indicator reflects the contribution of the refrigerant to the greenhouse effect relative to CO₂, whose GWP is taken as 1. For example, the GWP of R134a is 1430 and R32 is 675, i.e. the emission of 1 kg of R134a is equivalent to 1430 kg of CO₂ in terms of climate impact. The higher the GWP, the stricter the restrictions on handling, leakage and replacement. In a technical sense, the GWP does not affect system performance, but determines whether the substance can be used in new installations after the specified withdrawal dates.
ODP (Ozone Depletion Potential) is an indicator that quantifies the ability of a chemical to deplete the ozone layer in the Earth’s stratosphere by comparing it to the effect of a reference substance, Freon-11 (CFC-11), which is assigned an ODP of 1.0. A higher ODP value indicates a greater potential for harm to the ozone layer.
Fig. 1 – GWP and ODP of the most popular refrigerants
F-gas (fluorinated gases) – the term encompasses allfluorinated agents regulated by European Union regulations – primarily Regulation (EU) No 517/2014 and its updates. These regulations restrict the production and import of fluorocarbons with high GWP and establish a phase-down schedule. Manufacturers are obliged to switch to alternative substances or mixed agents with GWP below 750 for domestic and commercial equipment. Violation of quotas directly affects the availability of CFCs and the cost of servicing.
Montreal Protocol (1987) – an international protocol to the 1985 Vienna Convention for the Protection of the Ozone Layer, designed to protect the ozone layer by phasing out certain chemicals that deplete the ozone layer (ODP > 0).
Kigali Amendment (2016)– its addition to the Montreal Protocol, which for the first time introduced a global restriction on the use of HFCs under the GWP criterion.
Paris Agreement (2015) – enshrined the obligation of countries to reduce greenhouse gas emissions, including refrigerant leaks. Thus, modern refrigeration equipment should be designed taking into account not only thermodynamic but also environmental criteria: zero ODP, low GWP, compliance with F-gas and ISO 817 safety standards.
Fig. 2 – Prioritisation of regulatory documents aimed at F-gas reduction
Evolution of refrigerant generations
The history of development of refrigeration agents is closely related to the technological capabilities and environmental constraints of their time. The first generation, until the middle of the XX century, was based exclusively on natural substances – ammonia (R717), carbon dioxide (R744), sulphur dioxide (SO₂) and hydrocarbons (propane – R290 and isobutane-R600a). These agents provided high energy efficiency and low operating costs, but were extremely inconvenient in domestic and commercial applications due to toxicity (NH₃, SO₂) and fire hazard (propane, isobutane). The high operating pressure of CO₂ further limited its use due to the complexity of compressor equipment and the low critical temperature level (31 °C).
With the development of organic chemistry in the 1930s, synthetic freons (CFCs and HCFCs) – fully or partially chlorinated hydrocarbons – appeared. They became the second generation of refrigerants, providing safety, stability and compatibility with materials. The most common representative was R22 (CHClF₂), which combined moderate operating pressures, good thermal balance and the possibility of use in a wide range of temperatures. These substances practically replaced ammonia from household and light commercial equipment. However, by the late 1980s, CFC and HCFC molecules had been shown to deplete the stratospheric ozone layer by releasing chlorine through photolysis. As a result of the Montreal Protocol (1987), their production and use began to be phased out.
HCFCs were replaced by the third generation, HFCs (hydrofluorocarbons). These agents do not contain chlorine and have zero ozone depletion potential (ODP = 0). The classic representatives were R134a, R404A, R407C, R410A. They completely displaced R22 and CFC-agents from new systems. From the engineering point of view HFCs turned out to be convenient: low toxicity, good thermodynamic characteristics, stability and availability of equipment. However, further research showed that these substances have an extremely high global warming potential (GWP of 1300 to 4000), which made them subject to the next wave of restrictions – this time climate rather than ozone. The European F-gas regulation introduced in 2014 and the Kigali Amendment to the Montreal Protocol (2016) set a timetable for reducing the production of HFCs with high GWPs by 80-85% by the middle of the 21st century.
In response to the new requirements, a fourth generation, HFOs (hydrofluoroolefins) and their blends, has emerged. The key difference of HFOs is the presence of double carbon bonds in the molecule, which makes them chemically less stable in the atmosphere and sharply reduces the lifetime of the molecule, and thus the GWP (less than 10). The best known representatives are R1234yf, R1234ze(E), R454B, R513A. These substances are designed to replace traditional HFCs while maintaining close performance characteristics. In parallel, there is a return to natural agents – ammonia, carbon dioxide and propane, which due to the development of safety systems and microchannel heat exchangers are again becoming economically and environmentally justified.
Technical analysis of synthetic refrigerants
R22 (CHClF₂)
R22 is a one-component hydrochlorofluorocarbon, one of the most successful and long-lived refrigerants of its generation. The operating temperature range is wide: from -40 °C in the evaporator to 60 °C in the condenser, which made it universal for domestic, commercial and industrial systems. The thermodynamic characteristics of R22 ensured high COP values and stable operation of both reciprocating and scroll type compressors. However, the presence of chlorine gives it a non-zero ozone depleting potential (ODP ≈ 0.05). GWP is about 1810, which is extremely high by modern standards. Production and circulation of R22 has been banned in the EU since 2015, in most countries – completely since 2020. Substitutes: R407C, R422D, R438A, but they require oil change and recalculation of cooling capacity.
R407C (R32/R125/R134a – 23/25/52%)
R407C is a zeotropic blend designed as a replacement for R22. The temperature slip at phase transition is about 7 °C, which requires careful adjustment of the TRV and application of corrected pressure/temperature tables. Pressure and performance are close to R22, but the thermophysical properties are slightly inferior: COP is 2-3 % lower and heat transfer coefficients are 10 % lower due to higher viscosity. GWP – 1774. Energetically R407C is effective in medium temperature systems, but in recent years it has lost its relevance due to high GWP and sensitivity to leaks (change of mixture composition). Substitutes – R32, R452B and HFO mixtures of A2L class.
R404A (R125/R143a/R134a – 44/52/4%)
R404A is a zeotropic blend that became the standard for commercial refrigeration in the 1990s-2010s. It is characterised by high cooling capacity, stable operation in two-stage and cascade schemes, compatibility with polyester oils. The main disadvantage is extremely high GWP ≈ 3920, which makes it one of the most “dirty” HFCs. Pressure and temperature characteristics allow its use in the vaporisation range from -40 °C to -5 °C, but its energy efficiency is lower than that of R22. From 2020, R404A has been virtually phased out in new systems, being replaced by R448A (GWP ≈ 1387) and R449A (GWP ≈ 1397), which provide COP increases of 5-10 %.
R410A (R32/R125 – 50/50%)
R410A is an azeotropic blend, completely replacing R22 in domestic air conditioning. It is characterised by high pressure (50-60% higher than R22). More energy efficient (COP 5-10 % relative to R22), but requires equipment designed for higher pressures and polyester oils. GWP ≈ 2088. Despite its good performance, its use in new installations in the EU is restricted from 2025 onwards due to exceeding the threshold GWP = 750. It is being replaced by R32 and R454B.
R134a (CH₂FCF₃)
R134a is a single component chlorine-free HFC with zero ODP, and GWP ≈ 1430. Has long been the universal choice for medium temperature systems, cold rooms, chillers and car air conditioners. Provides good stability and high quality heat transfer at moderate pressures. The main disadvantage is high GWP. In Europe, the use of R134a in new automotive systems has been banned since 2017. Substitutes: R513A (GWP ≈ 630), R450A (GWP ≈ 605), R1234yf and R1234ze(E).
R513A (R134a/R1234yf – 56/44%)
A pseudoazeotropic HFO mixture designed for direct replacement of R134a without equipment modification. GWP ≈ 630, ODP = 0. Thermodynamic parameters are close to the original substance, the difference in COP does not exceed 2 %. Safe, non-toxic and non-flammable (ASHRAE A1 safety classification). Operating pressures are identical, allowing replacement in existing chillers and cooling units.
R32 (CH₂F₂)
R32 is one of the base components of many blends, but is increasingly being used on its own. It is a single component HFC with GWP ≈ 675, high specific heat and excellent energy efficiency. Pressure is 10-15 % higher than R410A, but COP is also higher. Safety class A2L – low flammable, which requires leakage control and charging restrictions. Used in split systems and new generation heat pumps.
R454B (R32/R1234yf – 68/32%)
Azeotrope positioned as a direct alternative to R410A. GWP ≈ 466, ODP = 0, operating pressures are similar and energy efficiency is 2-4% higher. Safety class A2L – flammable. The main advantage is compatibility with existing compressor platforms with significantly reduced climatic impact. The disadvantage is charge volume limitations and stricter fire safety requirements for the premises.
R1234ze(E) and R1234yf (HFO)
These are representatives of the fourth generation – hydrofluoroolefins. Their GWP is in the range of 1-7, ODP = 0. Due to their low density and high volatility they provide high COP. R1234yf is used in automotive systems, R1234ze(E) in chillers and heat pumps. Both belong to class A2L, requiring fire safety measures. Difficulty in handling is due to solubility in oils and the need for strict humidity control. The prospects of these substances are extremely high – they are considered as a long-term replacement for R134a and R410A in charge-limited installations.
Natural refrigerants
In recent years, there has been a return of interest in the use of natural (or “natural”) refrigerants, which were used even before the era of freons – in the late XIX and early XX century. Their revival is primarily driven by environmental and regulatory factors: the desire to reduce the greenhouse gas potential (GWP) of systems and to move away from the regulation of F-Gas. The main natural refrigerants include ammonia (NH₃, R717), hydrocarbons (primarily propane R290 and isobutane R600a) and carbon dioxide (R744).
Ammonia (NH₃, R717)
Ammonia is one of the oldest and most efficient refrigerants with outstanding thermodynamic properties. Its specific heat of vapourisation and adiabatic efficiency make it possible to achieve high energy efficiency of systems, especially in industrial and high capacity refrigeration systems. At low boiling points (down to -40 °C), ammonia remains stable and provides good condensation behaviour even at temperatures above 40 °C.
The main limitations of ammonia are related to its toxicity and corrosiveness to non-ferrous metals. Its use is prohibited in the domestic and commercial sectors due to the risks to personnel and the need for strict safety measures. However, NH₃ is still the standard in industrial systems, especially in food processing and logistics plants: modern cascade and dual-circuit schemes allow ammonia to be localised in the engine room and a secondary coolant to be used in the consumer circuit.
Ammonia is chlorine and fluorine free, has a GWP = 0 and is not subject to F-Gas regulation. According to ISO 817 classification it belongs to class B2L – moderately toxic and slightly flammable. Current trends aim to increase the tightness and automation of NH₃ systems, making them safer and more economically attractive in the long term.
Propane (R290)
Propane is a representative of hydrocarbon refrigerants with a low GWP ≈ 3 and excellent thermodynamic characteristics close to R22 and R134a. It is compatible with most standard oils and materials, has high energy efficiency and good flowability, which makes it attractive for small and medium capacity refrigeration machines, heat pumps and commercial cabinets.
The main limitation remains its explosion hazard: R290 is class A3 (highly flammable). This requires strictly limited filling volumes and special design measures, from ventilation to intrinsically safe automation. Nevertheless, the development of micro-circuits and hermetically sealed systems has made it possible in recent years to actively introduce propane even in domestic air conditioners and heat pumps on the European market.
Unlike ammonia, propane can be easily integrated into existing Freon systems without requiring complex equipment. At the same time, it is not subject to F-Gas regulation and is seen as one of the key areas for decarbonisation of the refrigeration industry.
Carbon dioxide (CO₂, R744)
CO₂ represents a special class of natural refrigerants. Its GWP is taken as 1 (reference level), it is chemically neutral and non-toxic. The main feature of R744 is its supercritical operation, as its critical temperature is only 31 °C. This requires a different cycle design (transcritical), where condensation is replaced by a cooling process in a gas cooler.
The main advantages of CO₂ are environmental friendliness, high heat flux density and the possibility to use it in systems with small pipe cross-sections. The disadvantages are extremely high operating pressures (up to 100 bar in the hot line) and reduced energy efficiency at high ambient temperatures. Therefore, CO₂ systems are optimal in cold and temperate climates or in cascade solutions where R744 is used as a low-temperature circuit together with NH₃ or R290.
Modern developments aim to optimise cycles with heat recovery and two-stage expansion, allowing CO₂ to gradually gain ground in supermarkets, transport and even heat pumps.
Comparative analyses and trends in refrigerant substitution
The development of refrigeration technology in the last three decades can be characterised as a consistent transition from highly efficient but environmentally hazardous CFCs to more “cleaner” formulations – first on the basis of hydrofluorocarbons (HFCs), and then to hydrofluoroolefins (HFOs) and natural substances. Three criteria have come to the fore: global warming potential (GWP), operational safety and availability of service infrastructure.
Fig. 3 – Preferred application areas of refrigerants by region
Energy efficiency and performance characteristics
From a thermodynamic point of view, ammonia (R717) and propane (R290) have the best energy efficiency (COP, EER). They provide higher specific cooling capacity at lower mass flow rates. Older generation freons – R22 and R404A – have good performance at moderate boiling temperatures, but their energy efficiency decreases in partial modes and at higher condensing temperatures. Blends of R407C and R410A have shown a good compromise between safety and performance, but due to high GWP (1774 and 2088 respectively) are gradually being displaced.
R134a has long remained the benchmark in chillers and cars, but its GWP = 1430 makes it unprofitable under the F-gas Regulation. New HFO substitutes such as R513A (GWP ≈ 630) and R1234yf/ze (GWP < 10) show similar characteristics with noticeably lower climatic impact. At the same time, R32, while remaining a monofreon (as opposed to blends), has a higher heat transfer coefficient and allows for a 20-30 % reduction in system charge, but has a safety class A2L (moderately flammable).
Environmental sustainability and F-gas Regulation
The main regulatory driver is the European Regulation (EU) No 517/2014 (F-gas) and its 2024 update, which aims to reduce HFC emissions by 95% by 2050. Restrictions are introduced in the form of phased reduction of quotas for the production and import of CFCs with high GWP. Already from 2027 the use of refrigerants with GWP > 750 in domestic air conditioners and heat pumps is prohibited, and from the 2030s – in most commercial refrigeration systems. This automatically excludes R410A, R404A and partly R407C.
Fig. 4 – Positioning of the main refrigerants by GWP and density with flammability classes (A1-A3, B1-B2L).
Substitution programmes are based on the transition to HFO components (R1234yf, R1234ze, R454B) and natural agents. At the same time, R32 and R454B are considered as an intermediate solution – they provide GWP reduction by about 3-4 times compared to R410A, but will also be displaced in the perspective of 10-15 years. R513A and R1234ze occupy the niche of chillers and medium capacity refrigeration systems, where non-flammability and stability are critical.
Safety
The requirements of ISO 817 and EN 378 standards dictate the necessity of refrigerant selection taking into account its toxicity class (A/B) and flammability (1/2L/2/3). The safest are R134a, R513A and R1234ze (A1, A2L), moderately flammable – R32, R454B (A2L), highly flammable – hydrocarbons (A3). Ammonia is distinguished as B2L: toxic but with a low propensity to combustion.
In practice, the choice of refrigerant today is determined by a combination of three factors: the allowable GWP, the charge volume and the location of the equipment. For systems in rooms with people (retail halls, offices), A1/A2L with limited charge is preferred; for industrial systems, NH₃ and CO₂; for small hermetically sealed systems, propane.
Figure 5 – ASHRAE 34 classification of refrigerants
Outlook and technology trends
By 2030, the global refrigerant market is expected to undergo a profound restructuring. Most manufacturers (Daikin, Carrier, Bitzer, Danfoss) are already preparing equipment optimised for R32, R454B, R513A and R290. For industrial applications, ammonia and CO₂ remain the main focus, while in the commercial sector the trend towards hydrocarbon micro circuits is increasing. In large capacity chillers, R1234ze and blends with HFO components are gradually being introduced to provide the required energy efficiency with minimal GWP.
In the long term, the industry is moving towards three sustainable directions:
- elimination of fluorinated substances with GWP > 150;
- standardisation of safety systems for moderately flammable agents (A2L, A3);
- increasing the use of secondary circuits and microcascade circuits where the main refrigerant is isolated.
Table 1 – Quota reductions for fluorinated gases (HFC)according to Regulation (EU) 517/2014 and its updates.
| Refrigerant | Type | GWP | Status | Stage / date of withdrawal from circulation (EU) | Comment |
| R22 | HCFC | 1810 | Total ban | From 2015 (service banned from 2020) | Residues are only allowed in existing systems without refuelling. |
| R404A | HFC | 3920 | Prohibited in new installations | From 2020 | Banned for commercial systems with GWP > 2500; replacements – R448A, R449A. |
| R407C | HFC | 1774 | Gradual reduction | Restricted by 2030 | Allowed in repairs and chillers until HFC quotas end. |
| R410A | HFC | 2088 | Ban in new installations from 2027 | 2027 – ban in domestic air conditioners and heat pumps (GWP > 750). | Substitutions: R32, R454B. |
| R134a | HFC | 1430 | Gradual phase-down | 2030 – actual cessation of mass use | Substitutes: R513A, R1234yf, R1234ze(E). |
| R513A | HFO/
HFC |
630 | Authorised | No restrictions (GWP < 750) | Used as a replacement for R134a. |
| R32 | HFC | 675 | Authorised until 2040s | Subject to reduction after 2035 | Temporary agent for systems to replace R410A. |
| R454B | HFO/
HFC |
466 | Authorised | No restrictions | Transitional low-GWP agent, alternative to R410A. |
| R1234yf R1234ze(E) | HFO | <10 | No restrictions | Complies with regulations up to 2050 | Long-term refrigerants with minimal climatic impact. |
| NH₃ (R717) | Natural | 0 | Unrestricted | Not subject to F-gas | Requires compliance with safety regulations. |
| R290 (propane) | Natural | 3 | No restrictions | Not subject to F-gas | Charge volume restrictions (A3). |
| R744 (CO₂) | Natural | 1 | No restrictions | Not subject to F-gas | Used in cascades and heat pumps. |
Thus, the technical evolution of refrigeration systems is no longer driven so much by COP as by the regulatory context and safety requirements. Refrigerant is no longer seen as a fixed component – it is a process variable that adjusts to the balance of efficiency, ecology and regulation.
Conclusion
Older generation refrigerants such as R22, R404A and R410A provided good energy efficiency and ease of use, but their high greenhouse gas potential (GWP > 2000) makes their continued use economically and regulatoryly impractical. They are being phased out not because of technological backwardness, but because they upset the balance between performance and climate resilience.
The intermediate generation – R407C, R134a, R32, R513A, R454B – reflects the industry’s attempt to find a compromise between safety, efficiency and ecology. These agents are still in place, especially in service and modernisation of existing systems, but their lifecycle is limited to the current decade. By 2030-2035, the majority of the market will switch to refrigerants with GWP < 150, which is predetermined by both European and global initiatives to fulfil the Montreal Protocol and the Kigali Amendment.
The present and future of the industry lies with HFOs and natural refrigerants. Ammonia (R717) and CO₂ (R744) are gaining ground in an industry where energy efficiency and scale are important. Propane (R290) is becoming the standard for hermetically sealed systems and small capacity heat pumps. In the segment of chillers and VRF systems, R32 and R454B dominate as temporary solutions, but will gradually be displaced by HFO compositions based on R1234yf and R1234ze.
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Author of the article:
Dmytro Lychak, CEO of EVROPROM
10.11.2025