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Flammability

Where non-flammability is required, some HFCs, HFC/HFO-blends, HFOs and HCFOs are available and have proven technical performance. For some applications, mildly flammable A2L fluorocarbons (lower-GWP HFCs, HFC/HFO-blends and HFOs) can be used and offer excellent technical performance, with lower or ultra-low GWPs compared to non-flammable HFCs but have constraints due to their mild flammability. Highly flammable (hydrocarbons) and toxic (ammonia) refrigerants, due to their safety profiles, have more constraints on charge size and applications. This is because the consequences of a loss of containment due to a leak or during servicing and maintenance can be severe, particularly for larger refrigerant quantities.

Flammability Properties

International and European safety legislation and standards such as ISO 5149, EN 378 and IEC 60335-2-40:2024 define requirements for RACHP equipment, in case of accidental leakage. For IEC 60335-2-40 Safety: Particular requirements for electrical heat pumps, air-conditioners and dehumidifiers, from the 2022 version onwards, it allows an increase in the maximum charge size for A2L and A3 refrigerants (ISO 817), but, given their flammability characteristics, mitigation measures have been put in place to allow for the increases in charge limits without creating new risks.

Flammability properties determine the ease of ignition (minimum ignition energy) and the pressure rise for ignition in confined spaces. If a refrigerant is flammable, the classification properties used are:

  • Lower flammability limit (LFL)
  • Heat of combustion (HoC)
  • Burning velocity (BV)

These are a subset of flammability properties which can also include:

  • Upper flammability limit (UFL)
  • Minimum ignition energy (MIE)
  • Peak constant volume pressure rise for an enclosed combustion
  • Deflagration index, measures the relative explosion severity, calculated from the maximum rate of pressure rise and the test vessel volume

Chart: Flammability data for some refrigerants

Some of these properties are linked. The charts show flammability data for refrigerants R-290 (A3), R-152a (A2), R-717 (B2L), R-32 (A2L), and R1234yf (A2L). This data supports the proposed links between flammability properties and helps explain why only three properties are used for refrigerant classification:

  • Lower flammability limit (LFL)
  • Heat of combustion (HoC), peak constant volume pressure and deflagration index all relate to how energetic combustion is.
  • Burning velocity
    • inversely correlated with the minimum ignition energy (higher burning velocities were more easily ignited by lower ignition energies)
    • appears to be correlated to LFL and deflagration index
      little to no correlation was found between BV and the auto-ignition temperature, with the reason likely being the nature of the standard test for measuring AIT, which is conducted by heating the mixture on all sides.

Auto-ignition temperature (AIT) and Hot surface ignition temperature (HSIT): AIT was originally used as one of the tests to classify refrigerants as non-flammable by UL- Underwriters Laboratories. When non-flammable HFCs were introduced, their AIT was lower than CFCs and UL classified them ‘practically non-flammable’. This had no relevance to their refrigerant classification by ASHRAE 34. More recently, for RAC equipment safety standards, it has been recognised that a test appropriate to a dynamic leak event has much greater relevance. AIT is a static contained event; in contrast hot surface ignition is a dynamic event where liquid refrigerant is sprayed onto a hot surface. HSIT is closer to real leakage events for RAC applications. HSIT shows that for a range of A2L refrigerants, HSITs are > 800°C. In addition, there is an ASTM standard for HSIT (ASTM Standard D8211-18). Independent flammability testing is also carried out, for example by SAE CRPs (Co-operative Research Programs) and the Japan Society of Refrigerating and Air Conditioning Engineers as part of risk assessments for the use of flammable refrigerants.

Flammability References

Journal of Loss Prevention in the Process Industries 49 (2017) 662 – 674, Flammability and explosion characteristics of mildly flammable Refrigerants, S.G. Davis, J.L. Pagliaro T.F. Debold, M. van Wingerden, K. van Wingerden. Note, some of the flammability data in this reference is different to that usually quoted for refrigerants, but different test methods can result in different results although with the same trends

Risk Assessment of Mildly Flammable Refrigerants Final Report 2016, March 2017, The Japan Society of Refrigerating and Air Conditioning Engineers
Summary of SAE CRP1234 Refrigerant Evaluation and Risk Assessment, July 15, 2010