In the United States, to support the Kigali Amendment to the Montreal Protocol and the Paris Climate Agreement, states have formed alliances to address the issue of global warming. Its most important goal is to control greenhouse gas emissions, such as the commonly used hydrofluorocarbon (HFC) refrigerants . According to current policies and measures (as shown in Figure 1), European Union and North America will stop using refrigerants with a Global Warming Potential (GWP) greater than 750 from 2024 to 2025. In addition, according to the "National Plan for China to Implement the Montreal Protocol", China's control plan of HFCs is also on the agenda.
Figure 1. Regulatory Timeline
Therefore, in response to these government regulations, manufacturers are developing and manufacturing systems with low GWP refrigerant. The challenge faced by the products that use alternative HVAC/R refrigerants is that those low GWP refrigerants typically exhibit more flammable characteristics than refrigerants designated as ASHRAE A1 safety group. For many existing air conditioning products that use R-410A, the most promising low-GWP alternatives include R-32 and R-454B, classified as ASHRAE A2L. Although A2L is slightly more flammable than traditional A1 refrigerants, it is less flammable than A3 hydrocarbon refrigerants such as R290 (propane) (refer to Figure 2). The European Union establishes IEC 60335-2-40 standard to specify the design, evaluation, testing, and certification of HVAC equipment using low-GWP refrigerants. The corresponding standard in the United States is UL 60335-2-40; in China, it is GB 4706.32. One of the risks associated with these standards is refrigerant leakage. Therefore, the standards address the use of refrigerant leakage detection systems and specifies their qualification specifications. Especially in UL 60335-2-89, it is clearly stipulated that when the maximum releasable charge is greater than a certain value, the refrigerant leakage detection system must be installed indoors or in cabin.
Figure 2. Refrigerant Safety Group
The critical component of the refrigerant leakage detection system is the refrigerant leakage detection sensor. Currently, sensing technologies for A2L refrigerant leakage monitoring mainly include non-dispersive infrared (NDIR), metal oxide semiconductor (MOS, which can be classified as Chemiresistive principle), and thermal conductivity (TC) technology:
1) The production cost of MOS sensors is very low (due to their simple principle), but its disadvantages are also prominent. First, the long-term stability of MOS sensors is poor and prone to lose accuracy over time. Secondly, MOS sensors have poor selectivity and are easily affected by other volatile organic compounds (VOC, such as alcohol). The biggest problem is that when exposed to high concentrations of refrigerants and other gases, MOS sensor performance will degrade or even permanently fail. It means that MOS sensors are "disposable" and may require human and material resources to replace sensors after a refrigerant leak event.
2) Gas sensors using thermal conductivity technology also have the advantage of low cost. The principle of heat exchange was initially used for flow detection and is a mature detection technology. However, this technology is not mature for gas detection yet, and there are no cases of high-volume applications on the market. Therefore, "to be the first one who eats crabs" is not necessarily the best choice.
3) NDIR technology is a common way to measure gas concentration and can be used to detect gases of various compound types. Because it is based on infrared absorption measurement principles, NDIR gas sensors have high accuracy, fast response time, and good selectivity. Its disadvantage lies in its high cost. Regarding the detailed comparison of above detection technologies, and their pros and cons, please refer to published report from Air-conditioning, Heating and Refrigeration Technology Institute - "leak detection of A2L refrigerants in HVACR equipment ".
Unitense Innovation's NDIR gas sensor U9601 series and U9603 series can be used for A2L refrigerant leakage monitoring, such as R32, R454A, R454B, R454C, R1234yf, etc. The sensor adopts patented optical design, circuit design, digital signal processing algorithm, and compensation algorithm, has passed multiple harsh environmental tests and life tests, conforms to IEC60079-29-1:2016 and IEC 60335-2-40:2022, and has superior stability and reliability. The specific technical information of the sensor can be found in the following product specification sheet and technical papers. We can provide low-cost customized solutions at competitive prices according to customer requirements.
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 Underwriters' Laboratories, "Understanding UL 60335-2-40 Refrigerant Detector Requirements", https://www.ul.com/news/understanding-ul-60335-2-40-refrigerant-detector-requirements, 2019.
 Trane Technologies, "HVAC Industry Update", https://www.trane.com/content/dam/Trane/Commercial/global/newsroom/blogs/REFR-PRB001F-EN_05052020.pdf, 2020.
 Mark W and Rebecca F, "leak detection of A2L refrigerants in HVACR equipment", AHRTI: https://www.ahrinet.org/sites/default/files/2022-07/AHRTI_9009_Final_Report_0.pdf, 2017