Risk Assessment of Refrigeration Systems Using A2L Flammable Refrigerants

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Risk Assessment of Refrigeration Systems Using A2L Flammable Refrigerants


In accordance with the Montreal Protocol, which addresses threats of ozone depletion, governments
world-wide instituted a phase-out of the use of chlorodifluoromethane (R-22) – including in commercial
reach-in and walk-in cooler applications – beginning in 1996. As a result of this action, most newly
manufactured reach-in and walk-in coolers in the United States use R-134a or R-404A as their
refrigerants (US EPA, 2010a). R-134a (1,1,1,2-tetrofluoroethane) has an ozone depletion potential (ODP)
of 0 but has a global warming potential (GWP) of 1,430 (IPCC, 2007).1 R-404A is a blend of 44% R-125
(pentafluoroethane), 52% R-143a (1,1,1-trifluoroethane), and R-134a. It has an ODP of 0 but a GWP of
3,922 (US EPA, 2010a). There is, therefore, world-wide interest in developing new low-GWP
refrigerants to address global climate change concerns. One class of potential replacement refrigerants
exhibit relatively low GWP but mild flammability (i.e., American Society of Heating, Refrigerating and
Air-Conditioning Engineers [ASHRAE]-34/ISO-817 Class 2L). These refrigerants would provide a
significant environmental benefit if they could be successfully adopted for use in stationary refrigeration
applications (Powell, 2011). One low-GWP 2L refrigerant, R-1234yf (2,3,3,3-tetrafluoropropene), has
been identified as suitable for use in automotive air conditioning (Gradient, 2009; US EPA, 2011), but
significant differences between automotive and commercial reach-in and walk-in cooler systems preclude
direct extrapolation between these uses. An earlier evaluation of R-152a (a Class 2 refrigerant) for use in
home refrigerators (ADL, 1991) reported a low risk of fire or explosion – less than one fire per million
refrigerators per year from leaks during operation and system service. While informative, the ADL study
was conducted more than two decades ago and may not reflect current technologies or procedures,
particularly for non-residential refrigeration systems using ASHRAE 2L refrigerants. More recently,
Colbourne and Suen (2004) described a risk assessment of R-290, R-600a, and R-1270 in small indoor
refrigeration systems, determining that a fire could occur up to 82 times per million refrigeration units per
year. However, this risk assessment only considered refrigerants of class 3 rather than the 2L refrigerants
of interest here which may pose lower flammability risks. The current risk assessment, carried out as a
cooperative industry effort coordinated by the Air-Conditioning, Heating and Refrigeration Institute
(AHRI), explores more broadly whether 2L refrigerants may be used safely in commercial cooler
applications, given current technologies.
As used in the context of this evaluation, “risk” is the likelihood or probability that leaked refrigerant
from a commercial reach-in or walk-in cooler system is ignited. Risks are evaluated and quantified
through the process of risk assessment. Like all risk assessments, the risk assessment of a potential
alternative refrigerant is a multi-step process. An early step in the process is to consider the possible
scenarios under which the refrigerant could leak and be ignited. It is then necessary to gather data to
support a quantitative estimation of the risk associated with that particular event. Once all of the potential
scenarios are identified and the necessary data are collected, the data are brought together to develop a
mathematical estimate of potential risk.