Classification of flammable refrigerants has taken a new momentum due to the use of flammable refrigerants in some refrigerating applications such as domestic refrigeration. ASHRAE Standard 34 has been continuously updated for ten years to integrate both improved test methods and the use of new refrigerants and blends. ASHRAE flammability class 2 includes a wide flammability range of moderately flammable substances, and additional criteria are required to create an orderly ranking of flammability within this class. The burning velocity (BV) is a fundamental property that can be used for this purpose. This paper presents the main results of tests that have permitted the measurement of the BV of six pure substances and three blends. Three classes of flammability appear when using the maximum BV for ranking. BV is not a substitute for the lower flammability limit (LFL) and the heat of combustion (HOC) but is a complementary and essential criterion, permitting a continuous scaling of flammability from moderate to highly flammable substances.

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