Low Flammability Limit of Mixtures of Natural Refrigerants: Standards vs Theory

Natural refrigerants are being increasingly adopted by heat pump manufacturers. Heat pumps using flammable refrigerants need to satisfy charge limit requirements from standard IEC/EN 60335-2-40 and EN 378. The allowed refrigerant amount for the specific application is directly related to the Low Flammability Level (LFL). A possibility to reduce refrigerant flammability and allow a higher refrigerant charge is to adopt or design mixtures in which a hydrocarbon is mixed with a less flammable refrigerant. This work investigated the differences between LFL data available in international and European standards, and those calculated using theoretical methods. In the case of natural refrigerant mixtures R-432A, R-433B, and R-433C, approved in ASHRAE 34 and ISO817, it was found that the differences were between 26 % and 52 %. Differences in LFL test results for pure refrigerants, caused by the inter-laboratory spread, may partly explain the inconsistencies in the LFL of mixtures. Moreover, if not printing errors, unit conversion errors were found in ASHRAE 34 for the mixtures R-429A, R-430A, R-431A, R-433A, R-433C, and in EN 378-1 for mixture R-433B.