The term transitional substances was coined by the London amendment (1990) to the Montreal Protocol for some refrigerants and initially referred to hydrochlorofluorocarbons (HCFC) and their blends. It was believed that the temporary transition to HCFCs, substances with lower than CFCs ozone-depleting potential (ODP), would allow better preparation for the full phase-out of ozone-depleting substances. But already two years later when the Copenhagen amendment was adopted, the term transitional substances disappeared from Article 1 of the Montreal Protocol, and HCFCs were included in the list of substances consumption and production of which would be phased out according to the schedule.
By today the term has lost its rigorous definition, and if take into account requirements of the Kigali amendment and other regulations, and development trends in the refrigeration industry, it may be applied to synthetic refrigerants with low global warming potential (GWP), i.e. some HFCs, hydrofluoroolefins (HFOs), and HFC- and HFO-containing blends.
Physical and chemical properties
|R32 (difluoromethane)||R1234yf (2,3,3,3-tetrafluoropropene)||R1234ze (1,3,3,3-tetrafluoropropene)||R1233zd (trans-1-chloro-3,3,3-trifluoropropene)|
|Relative molecular mass||52,02||114,04||114||130,5|
|Boiling point at 101.3 kPa||-52°C||-30°C||-19°C||-18°C|
|Critical pressure (abs.)||57,82 bar||33,82 bar||36,36 bar||35,7 bar|
|Composition||31,1% R1234yf / 69,9% R-32||56% R1234yf / 44% R134a||75,5% R1234yf / 21,5% R-32 / 3% R-744||26% R32 / 26% R125 / 21% R134a / 7% R1234ze / 20% R1234yf||67% R32 / 7% R125 / 26% R1234yf||42% R134a / 58% R1234ze|
|Average molecular mass of blend||62,6||108,4||87,5||189,9||N/A||239,1|
|Boiling point at 101.3 kPa||-50,9°C||-29,2°C||N/A||N/A||N/A||N/A|
|Temperature glide, K||1,5||0||N/A||N/A||N/A||N/A|
|Critical temperature||77 °C||96,5 °C||85,6 °C||83,7 °C||77,1 °C||104,4 °C|
|Critical pressure (abs.)||42,5 bar||37,66 bar||46,6 bar||46,6 bar||52,2 bar||38,2 bar|
Impact on human beings and environment
Hydrofluorocarbon R-32 and most pure hydrofluoroolefins are A2L safety group: low-toxic (non-toxic), mildly-flammable. 2L refrigerants have a lower flammability limit of 0.10 kg/m3, specific heat below 19 MJ/kg, and burning velocity below 10 cm/s.
HFO blends were developed to fill the need in a non-flammable refrigerant that, where practicable, retains such HFO properties as zero ozone-depleting potential and low global warming potential.
Transitional substances are not only flammable but are hazardous to inhale. They are usually heavier than air, colorless, and odorless, so proper ventilation must be ensured at the workplace especially below the ground level.
As the evaporation temperature of most single-component transitional substances and blends is below 0°C at normal pressure, contact with a liquid refrigerant can cause cold burn, and eye and airways damage.
HFCs and HFOs are safe to the ozone layer. According to the TEAP Working Group 1 contribution to the IPCC’s Sixth Assessment Report, Climate change 2021: the Physical Science Basis, 100-year time-horizon GWP of R-32 is taken as equal to 771, although the Kigali amendment uses another value, 675.
100-year time-horizon GWP of pure HFOs is next to none (usually equal to 1) because these substances don’t persist in the atmosphere. GWP of HFO-containing blends that may contain HFCs and other components can be measured in hundreds.
Use of transitional substances
Many manufacturers of air-conditioners consider hydrofluorocarbon R-32 (difluoromethane) as a refrigerant for domestic and small commercial air conditioners because safety requirements prohibit the use of this flammable refrigerant in systems with a larger charge (e.g. VRF-systems).
Since January 1, 2025, the Regulation (EU) No517/2014 on fluorinated greenhouse gases will not allow using HFCs with GWP over 750 in split systems containing up to 3 kg of refrigerant. With a 100-year time-horizon GWP of 675, R-32 will still be used in domestic air conditioners after 2025. But its future cannot be called a bright one. According to IPCC’s Sixth Assessment Report, its 100-year time-horizon GWP is 771, which is higher than the current limit specified in the Regulation, 750, and the Regulation itself is under review and is expected to become stricter.
Hydrofluoroolefin R-1234yf became one of the first HFO-refrigerants in the market. It was developed to satisfy requirements of the European Directive 2006/40/EC on mobile air conditioners that prohibits selling new automobiles with 150+ GWP refrigerant at the European market. According to the latest estimates, GWP of R-1234yf is below 1.
The substance is also used as a refrigerant in stationary refrigeration and air-conditioning equipment and as a component of refrigerant blends.
Hydrofluoroolefin R-1234ze was developed to replace HFC-refrigerant R-134a and can be used as a blowing agent in polymer foams, a propellant in sprays, a component in tyfons (pneumatic warning alarm systems), and dry powder extinguishers.
As a refrigerant, R-1234ze is used in chillers, heat pumps, refrigeration systems in supermarkets.
With the significant difference in thermal characteristics, R-1234ze is not a drop-in replacement for R-134a in operating equipment.
Hydrochlorofluoroolefin R-1233zd is a non-flammable ozone-safe refrigerant with very low GWP designed to use in chillers with centrifugal compressors that are usually installed in refrigeration systems of large buildings.
R-454B is an ozone-safe zeotropic blend with low GWP, 466, developed to replace R-410A in chillers and heat pumps. With lower discharge temperature, higher critical temperature, and efficiency at low evaporation temperature, R-454B is more preferable for heating than R-32.
R-513A is an ozone-safe zeotropic blend with low GWP, 630, designed to replace R-134a in domestic, commercial, and industrial refrigeration systems, central air conditioners, and chillers. The refrigerant can be used in direct expansion systems and systems with flooded evaporators.
R-455A is a mildly flammable (2L) zeotropic blend for new high-, medium- and low-temperature refrigeration equipment. With GPW of 146 (below 150), it satisfies the strictest requirements of the current F-gas regulation. The refrigerant can be used in single-packaged refrigerated cabinets, compressor-condenser units, small chillers, and heat pumps.
R-448A is a non-flammable zeotropic blend with low GWP, 1,386, designed to replace HFC R-404A (GWP 3,922) in new and operating refrigeration equipment.
R-452B is an ozone-safe mildly flammable (2L) zeotropic blend with low GWP developed to replace HFC R-410A in air-conditioning and heating systems with positive-displacement compressors. With the same performance features as R-410A, this refrigerant can be used after affordable minimum upgrade and improvement of equipment.
R-450A is a non-flammable zeotropic blend of R-134a and R-1234ze designed to replace R-134a and with 60% lower GWP, 601, and similar performance features. The refrigerant can be used in air- or water-cooled heat pumps, central refrigeration and heating systems, vending machines and coolers, CO2 high-temperature cascades, medium-temperature refrigeration equipment.
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