Temperature, humidity and room furnishings factor into the risk that flammable refrigerants could pose to buildings and their occupants. The velocity of a refrigerant leak and the degree to which it mixes or settles in the surrounding air volume also affect the probability of ignition and consequent results.
Researchers with the U.S. Air-conditioning, Heating and Refrigeration Technology Institute (AHRTI) have found that, in some scenarios, flames can spread more rapidly than the “low flammability” definition of no more than 10 centimetres or 3.9 inches per second, and that leaks might be particularly problematic in reach-in or walk-in coolers since flammable concentrations could build in these confined spaces and disperse outward when the door is opened. They call for more modelling after discovering that a single system could release refrigerant as either vapour or a vapour/liquid mixture, which alters the assumed risk profile of some of the products under scrutiny.
Last year, AHRTI researchers conducted extensive tests of the ignition risks of flammable refrigerants poised as potential replacements for products with high global warming potential (GWP). The work is part of the low-GWP alternative refrigerants evaluation program and the associated report, released in June 2017, outlines lessons learned thus far along with recommendations for more research and/or further assessment of safety standards and building codes.
The project included calibration tests of refrigerant leaks to measure and compare the effects of different rates of diffusion, locations of release and sizes of leak openings; and parametric tests to assess how ambient temperature and humidity, physical surroundings, leak magnitude and intermixing with lubricating oil contribute to the likelihood and severity of ignition. Researchers also devised four commercial and three residential whole-room scenarios — for example, a packaged terminal air conditioning (PTAC) unit in a motel room — in which they gauged the likely release from a theoretical source then used a candle flame and electrical arcs in an effort to trigger ignition.
“The residential scenarios included refrigerant leakage in residential utility closet spaces; whereas the commercial applications included leakage in a kitchen of a small restaurant, walk-in cooler and convenience store. These applications represent residential split, roof-mounted commercial kitchen air conditioning units and walk-in and reach-in type refrigeration units,” the report explains.
Tests were based on the premise that there is a plausible likelihood of leaks from both idle and operating equipment, including due to accidental ruptures to systems during home renovations and refrigerant release during recharging. Leaked refrigerant could accumulate in a space where the system is located and/or be drawn into ductwork and disseminated throughout a building. However, researchers explored an extreme and largely unanticipated worst case scenario.
“Because of the way we approached these experiments, some relatively low probability events were forced to occur,” they observe. “Several of the tests would actually be outside of proposed standards — i.e. more refrigerant was leaked into the space without the mitigation proposed for by the standard.”
Those standards are likewise tagged for reassessment and revision as research continues.
“Cursory evaluation of hazard mitigation systems suggests refrigerant detection systems will need to have a faster response time than the 30 second response time that had been originally envisioned. More investigation is needed,” they add.