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Sunday, March 31, 2019

Risk Assessment of Refrigeration Systems Using A2L Flammable Refrigerants

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There is currently world-wide interest in developing substitutes for materials whose environmental release may contribute to global climate change. The primary refrigerants used in commercial reach-in and walkin coolers are R-404A and R-134a, greenhouse gases with global warming potentials (GWPs) in excess of 1,400. Possible replacements for these refrigerants in commercial applications include ASHRAE Class 2L refrigerants, which have lower global warming potentials but are mildly flammable. Although normal operation poses negligible risk, accidental releases due to equipment fault or fatigue could potentially result in refrigerant ignition if a sufficient ignition source is also present at the time and location of the release. To better understand these risks, Gradient conducted a risk assessment to evaluate the use of three Class 2L refrigerants – R-32, R-1234yf, and R-1234ze(E) – in commercial cooler systems. Three location scenarios were evaluated: a small restaurant kitchen, a lunch counter, and a convenience store. Two types of units were studied: walk-in and reach-in coolers. The work included Computational Fluid Dynamics (CFD) modeling, experimental measurements, and a fault tree analysis (FTA) to quantify ignition risks. The CFD modeling indicated that for large accidental releases of R-32, R-1234yf, and R- 1234ze(E) (i.e., on the order of 50 g/s for R-32, 25 g/s for R-1234yf and 1234-ze(E)), refrigerant concentrations in a small restaurant kitchen, lunch counter, and convenience store can be expected to be substantially below their respective lower flammable limits (LFLs). Incorporating these findings, the FTA estimated that the risks of refrigerant ignition due to an accidental refrigerant leak across the different scenarios ranged from 10-10 to 10-13 events per unit per year for R-1234ze(E), from 10-9 to 10-12 events per unit per year for R-1234yf, and from 10-9 to 10-11 events per unit per year for R-32. For comparison, the overall risk of a significant commercial structure fire in the US is 2 x 10-2 per structure per year. The FTA-estimated risks were driven by the kitchen walk-in cooler scenario, which involved the smallest air volumes and the greatest likelihood of a flame source being present (i.e., a gas cook-top burner or pilot light). Risks for R-32 and R-1234yf were similar, because both are equally capable of being ignited by a flame source that might be found in a restaurant kitchen. Risks for R-1234ze(E) were
lower because this refrigerant is only flammable at temperatures above normal room temperature. Based on CFD modeling, experimental testing, and FTA, the risk assessment indicates that average risks associated with the use of these ASHRAE 2L refrigerants are significantly lower than the risks of common hazard events associated with other causes and also well below risks commonly accepted by the public in general.

Executive Summary
1. Introduction
2. Properties of Alternative Refrigerants Under Study
3. Data Acquisition
4. Fault Tree Analysis
5. Data Gaps
6. Conclusions
Appendix A. Hughes Associates Inc. Detailed Results
Appendix B. Fault Trees and Input Values Table

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