Ignition hazard due to air infiltration in refrigeration systems working with flammable refrigerants.

With the implementation of the revised EU F-gas Regulation and the proposed PFAS restrictions currently under consideration, the wider use of flammable refrigerants is expected in the near future. During refrigeration system operation, pressures below atmospheric may occur in the evaporator, particularly in low-temperature applications and during transient operating modes such as pump-down. This operating condition is commonly regarded as undesirable due to the potential ignition hazard caused by air infiltration. Previous studies have shown that, under specific conditions, autoignition of oil–refrigerant–air mixtures may occur, leading to so-called diesel-type combustion. This article presents a case study of a eutectic refrigeration system used in transport refrigeration to assess whether ignition conditions can arise during operation and to identify effective risk mitigation measures. The formation of flammable mixtures on both the low- and high-pressure sides of the system is examined. Theoretical analysis indicates that conventional low- and high-pressure safety switches prevent the formation of a combustible mixture during compression provided that the air infiltration hole diameter is below 1.4 mm. However, preventing the formation of flammable mixtures on the high-pressure side requires an additional safety system capable of detecting non-condensable gas infiltration. A method for selecting the parameters of such a system is proposed. Experimental results demonstrate that liquid subcooling in the condenser provides a rapid and reliable indication of non-condensable gas infiltration and can be used for this purpose. This work aims to support the safe application of flammable refrigerants in low-temperature refrigeration systems.