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Author: Key Account Manager Manuel Eckstein, Inside Sales Administrator Katherine Lee
To reduce environmental impact, the use of many common refrigerants has been restricted in recent years. The Heating, Ventilation, Air Conditioning, and Refrigeration (HVACR) industry are under increasing pressure to carefully select refrigerants, moving away from widely-used hydrofluorocarbons (HFCs) in favor of refrigerants with lower Global Warming Potential (GWP). Safe use of these relatively new refrigerants requires revised leakage detection technologies. These safety requirements are codified by new standards (such as UL 60335-2-40 3rd edition and IEC-60079-29-1). Following the latest decision of the California Air Resources Board (CARB), these new requirements will be incorporated into building standards as early as 2025 for residential and commercial air-conditioning equipment.
In this article, we’ll take a look at the different classes of refrigerants used in the HVACR industry, current shifts in their usage, and some of the leak detection technologies which may be essential in enabling the industry to comply with new environmental and safety regulations.
When ozone-depleting chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) were phased out in response to the Montreal Protocol, hydrofluorocarbons (HFCs) were introduced to replace them. However, HFCs are potent greenhouse gases, and worldwide efforts are currently underway to reduce their usage.
Although the present contribution of HFCs to climate change is small, this is expected to rapidly increase if left unchecked. As a result, their use must be restricted to mitigate global warming. The Kigali amendment (October 2016) to the Montreal Protocol mandates a strict global phasing-down of HFCs.
As a result, the HVACR industry is turning towards alternative systems that make use of refrigerants with lower Global Warming Potential (GWP). But the adoption of low-GWP refrigerants presents its challenges.
ASHRAE standard 34 broadly categorizes refrigerants according to their flammability and toxicity. Each refrigerant is assigned an identifying reference letter and number: the letter designates a toxicity class, and the number denotes flammability.
This safety classification system was recently updated to include the “2L” subclass of flammability, denoting class 2 refrigerants that are flammable but burn very slowly.
Traditional refrigerants – that is, HCFCs, CFCs, and the HFCs which initially replaced them – were chosen for safety and durability. As a result, they generally have low flammability and low toxicity. For example, the most commonly used CFC refrigerant, dichlorodifluoromethane, is categorized in safety group A1.
There are a few viable candidates for refrigerant fluids that meet the dual requirements of low ozone-depletion potential and low global warming potential; including fluorinated alkenes (HFOs), hydrocarbons (e.g. propane), and low-GWP HFOs. The industry has been actively working to develop a new class of low-flammability refrigerants, known as A2L, to meet the lower GWP challenge – but the most practical options have one thing in common: they are more flammable than traditional refrigerants.
Safe use of environmentally responsible refrigerants
Moving away from environmentally harmful HFCs will see many industries opting for flammable A2L-class refrigerants. The safety requirements of HVACR equipment using these flammable low-GWP refrigerants are codified by several new standards. In the USA, these include UL 60335-2-40 (3rd edition) and IEC-60079-29-1.
One of the primary risks associated with the use of these refrigerants is combustion or explosion following leakage. The standards mentioned above prescribe the use of leak detection equipment in HVACR applications in order to prevent this.
Though the standards themselves don’t yet force anyone to use leak detection systems. However, with regards to residential and commercial air-conditioning equipment for example, state building codes will incorporate these standards as soon as 2025. The California Air Resources Board (CARB), for example, is pursuing an aggressive timeline to phase out HFCs in favor of A2L refrigerants. The proposed changes put OEMs under pressure to quickly adapt to the use of flammable refrigerants.
Refrigerant leak detection technology
The two main candidate technologies for low-GWP refrigerant gas detection are Metal Oxide Semiconductor (MOS) and Nondispersive Infrared (NDIR).
Metal Oxide Semiconductor sensors operate on the principle that the electrical resistivity of certain semiconductor materials changes in response to surface interactions between the semiconductor and certain gases. Measuring the electrical resistance of a metal oxide semiconductor can therefore indicate the concentrations of gases and vapors in the air.
MOS sensors are generally cheap to produce, and the technology is adaptable to a wide range of refrigerants. New MOS sensors have been developed to detect A2L refrigerants including HFC-32 and HFC-1234yf.
However, MOS sensors are not without their drawbacks. First among these is susceptibility to “drift”, which refers to natural de-calibration over time. All (gas) sensors must be calibrated so that they give an accurate reading in response to certain concentrations of gas. However, MOS sensors are particularly prone to loss of accuracy as the properties of the sensor gradually change in response to ambient conditions; which requires them to be regularly re-calibrated.
The second issue with MOS sensors is low selectivity. They are often ineffective at distinguishing between target gases and other Volatile Organic Compounds (VOCs) such as ethanol, which can result in false positives.
In addition to these issues, MOS sensors degrade when exposed to refrigerants and other gases. This can result in permanent failure of an MOS-based gas detector after a single exposure to high refrigerant concentrations. This effectively means that MOS sensors are ‘single-use’: a single refrigerant leakage event can render MOS sensors unusable and in need of immediate replacement.
Nondispersive Infrared sensors are spectroscopic. Different gas molecules have characteristic infrared absorption characteristics: NDIR sensors measure the decrease in IR transmission over a short path and use this information to determine the concentrations of target gases in the air.
NDIR sensors have much higher selectivity than MOS sensors and are already widely used for this reason. Importantly, they don’t experience degradation in the same way as MOS sensors and don’t suffer from the same drift issues. However, this comes at a price: NDIR sensors are much more expensive than MOS sensors.
The near future of refrigerant gas detection
We can confidently expect an increase in the use of flammable low-GWP refrigerants in HVACR applications in the not-too-distant future. However, it is not yet clear whether NDIR- or MOS-type devices will be the dominant technology for the detection of these gases. Several other sensor technologies are currently under development for refrigerant detection applications, including those that measure local atmospheric Thermal Conductivity (TC) to determine the concentration of target gases.
We can anticipate that the response time given by a sensing solution may be a critical factor as OEMs need sufficient time to trigger the required mitigation actions when an alarming level is reached. Thermal conductivity-based sensors have been shown to outperform other sensor types in this regard, which may give them the edge over MOS or NDIR sensors for these applications.
Whichever technology emerges as the most common, this will be a developing market in the coming years due to the incorporation of new safety standards into legislation. It will be interesting to see how gas detection technology will adapt to new challenges in the HVACR industry.