In recent years, the utilization of a number of common refrigerants has been restricted in order to reduce environmental impact. The Heating, Ventilation, Air Conditioning and Refrigeration (HVACR) industry are subjected to growing pressure to choose refrigerants carefully.
They are leaving behind widely-used hydrofluorocarbons (HFCs) in favor of refrigerants with lower Global Warming Potential (GWP).
Revised leakage detection technologies are needed for the safe employment of these relatively new refrigerants. These safety requirements are codified by new standards (such as UL 60335-2-40 3rd edition and IEC-60079-29-1).
These new requirements will be added to building standards for residential and commercial air-conditioning equipment as early as 2025, following the latest decision from the California Air Resources Board (CARB).
The different classes of refrigerants employed in the HVACR industry, current shifts in their usage and some of the leak detection technologies which may be vital to enable the industry to comply with new safety and environmental regulations will be outlined in this article.
Hydrofluorocarbons (HFCs) were introduced to replace ozone-depleting hydrochlorofluorocarbons (HCFCs) and chlorofluorocarbons (CFCs) when they were phased out in response to the Montreal Protocol.1
Yet, HFCs are potent greenhouse gases and global efforts are currently happening to decrease their usage.2
Even though the current contribution of HFCs to climate change is small, if left unchecked, this is expected to increase rapidly. Their utilization must be restricted as a result in order to mitigate global warming.
A strict global phasing-down of HFCs is mandated by the Kigali amendment (October 2016) to the Montreal Protocol.3
The HVACR industry is looking for alternative systems that utilize refrigerants with lower Global Warming Potential (GWP) because of this amendment.4,5 The adoption of low-GWP refrigerants brings about some challenges.
Refrigerants are categorized broadly by ASHRAE standard 34, according to their flammability and toxicity.6 Each refrigerant is assigned an identifying reference letter and number: the number denotes flammability and the letter designates a toxicity class.
Image Credit: ASHRAE
Recently, this safety classification system was updated to include the ‘2L’ subclass of flammability to represent class 2 refrigerants, which are flammable but burn very slowly.6
Traditional refrigerants were chosen for safety and durability, such as HCFCs, CFCs and the HFCs, which initially replaced them. They typically have low toxicity and low flammability because of this. For instance, dichlorodifluoromethane, the most commonly utilized CFC refrigerant, is categorized in safety group A1.
There are not many viable candidates for refrigerant fluids that meet the dual requirements of low global warming potential and low ozone-depletion potential; including hydrocarbons (e.g., propane), fluorinated alkenes (HFOs) and low-GWP HFOs.7
In order to meet the lower GWP challenge, the industry has been working hard to develop a new class of low-flammability refrigerants, known as A2L. The most practical choices all have one thing in common as they are more flammable than traditional refrigerants.4,7,8
Safe Use of Environmentally Responsible Refrigerants
Many industries will adopt flammable A2L-class refrigerants as they move away from environmentally harmful HFCs.
A number of new standards are employed to codify the safety requirements of HVACR equipment using these flammable low-GWP refrigerants. In the USA, these include UL 60335-2-40 (3rd edition) and IEC-60079-29-1.4,9,10
Explosion or combustion following leakage is one of the key risks associated with the use of these refrigerants. In order to prevent this, the standards mentioned previously advise the utilization of leak detection equipment in HVACR applications, though the standards themselves don’t yet force anyone to employ leak detection systems.
Yet, with regards to commercial and residential air-conditioning equipment, for instance, state building codes will use these standards as soon as 2025.
For example, to phase out HFCs in favor of A2L refrigerants, The California Air Resources Board (CARB) is pursuing an aggressive timeline.11 The changes proposed put OEMs under pressure to adapt to the use of flammable refrigerants quickly.
Refrigerant Leak Detection Technology
Nondispersive Infrared (NDIR) and Metal Oxide Semiconductor (MOS) are the two main candidate technologies for low-GWP refrigerant gas detection.12,13
Metal Oxide Semiconductor sensors work on the principle that the electrical resistivity of certain semiconductor materials alters in response to surface interactions between certain gases and the semiconductor.
Therefore, measuring the electrical resistance of a metal oxide semiconductor can establish the concentrations of vapors and gases in the air.
Producing MOS sensors is usually low cost and the technology is adaptable to a large variety of refrigerants. In order to detect A2L refrigerants, including HFC-32 and HFC-1234yf, new MOS sensors have been developed.13
Image Credit: Sensirion Inc
MOS sensors are not without their drawbacks. The first of these is their susceptibility to ‘drift,’ which refers to natural de-calibration over time. All (gas) sensors must be calibrated so that they produce an accurate reading in response to certain concentrations of gas.
As the properties of the sensor gradually change in response to ambient conditions, MOS sensors are especially prone to loss of accuracy, which means they must be re-calibrated regularly.14,15
Low selectivity is the second issue with MOS sensors. They are sometimes ineffective at distinguishing between target gases and other Volatile Organic Compounds (VOCs) like ethanol, which can lead to false positives.13 Further to these issues, when exposed to refrigerants and other gases, MOS sensors degrade.
This can lead to permanent failure of an MOS-based gas detector after a single exposure to high refrigerant concentrations, which means that MOS sensors are ‘single-use.’ A single refrigerant leakage event can render MOS sensors unusable.
As nondispersive Infrared sensors are spectroscopic, different gas molecules have characteristic infrared absorption characteristics. NDIR sensors quantify the reduction in IR transmission over a short path and utilize this information to establish the concentrations of target gases in the air.
Image Credit: MDPI
NDIR sensors possess a much higher selectivity than MOS sensors and for this reason, they are already widely employed. Crucially, they do not experience degradation in the same way as MOS sensors and so they do not suffer from the same drift issues.16 This comes at a price; NDIR sensors are a lot more costly than MOS sensors.
The Near Future of Refrigerant Gas Detection
An increase in the use of flammable low-GWP refrigerants in HVACR applications is expected in the near future, but it is not yet clear whether NDIR- or MOS-type devices will be the dominant technology for the detection of these gases.
At present, numerous other sensor technologies are under development for refrigerant detection applications, including those that measure local atmospheric Thermal Conductivity (TC) to establish the concentration of target gases.17
It is anticipated that the response time supplied by a sensing solution may be vital, as OEMs require enough time to trigger the mitigation actions needed when an alarming level is reached.
In this regard, thermal conductivity-based sensors have demonstrated a better performance than other sensor types, which may give them the edge over MOS or NDIR sensors for these applications.18
Whichever technology emerges as the most common, due to the incorporation of new safety standards into legislation, this will be a developing market in the coming years. It will be interesting to see how gas detection technology will adapt to new challenges in the HVACR industry.
- Calm, J. M. & Hourahan, G. C. Physical, Safety, and Environmental Data for Current and Alternative Refrigerants. 22 (2011).
- Hydrofluorocarbons. Climate & Clean Air Coalition https://www.ccacoalition.org/en/slcps/hydrofluorocarbons-hfc.
- The Kigali Amendment (2016): The amendment to the Montreal Protocol agreed by the Twenty-Eighth Meeting of the Parties (Kigali, 10-15 October 2016) | Ozone Secretariat. https://ozone.unep.org/treaties/montreal-protocol/amendments/kigali-amendment-2016-amendment-montreal-protocol-agreed.
- Understanding UL 60335-2-40 Refrigerant Detector Requirements. UL https://www.ul.com/news/understanding-ul-60335-2-40-refrigerant-detector-requirements.
- Update on the Air-Conditioning Safety Standards for HVAC Equipment. UL https://www.ul.com/news/update-air-conditioning-safety-standards-hvac-equipment.
- ASHRAE New Refrigerant Designations and Safety Classifications. https://www.ashrae.org/file%20library/technical%20resources/refrigeration/factsheet_ashrae_english_20200424.pdf.
- Table 1 COP and volumetric capacity of selected low-GWP fluids and current HFC and HCFC fluids in the basic, liquid-line/suction-line heat exchanger (LL/SL) and economizer (Econ.) cycles.
- McLinden, M. O., Brown, J. S., Brignoli, R., Kazakov, A. F. & Domanski, P. A. Limited options for low-global-warming-potential refrigerants. Nature Communications 8, 14476 (2017).
- UL Standard | UL 60335-2-40. https://standardscatalog.ul.com/ProductDetail.aspx?productId=UL60335-2-40.
- IEC 60079-29-1:2016+AMD1:2020 CSV | IEC Webstore. https://webstore.iec.ch/publication/66754.
- Manufacturers Ask CARB to Extend Air Conditioning Deadline. https://www.achrnews.com/articles/143552-manufacturers-ask-carb-to-extend-air-conditioning-deadline?v=preview.
- Technology, I. E. Refrigerant leak detection: IR vs SEMICONDUCTOR SENSORS. Envirotech Online https://www.envirotech-online.com/news/gas-detection/8/net/refrigerant-leak-detection-ir-vs-semiconductor-sensors/46545.
- Izawa, K. SnO2-Based Gas Sensor for Detection of Refrigerant Gases. Proceedings 14, 32 (2019).
- Liu, H., Chu, R. & Tang, Z. Metal Oxide Gas Sensor Drift Compensation Using a Two-Dimensional Classifier Ensemble. Sensors 15, 10180–10193 (2015).
- Peterson, P. J. D. et al. Practical Use of Metal Oxide Semiconductor Gas Sensors for Measuring Nitrogen Dioxide and Ozone in Urban Environments. Sensors (Basel) 17, (2017).
- Dinh, T.-V., Choi, I.-Y., Son, Y.-S. & Kim, J.-C. A review on nondispersive infrared gas sensors: Improvement of sensor detection limit and interference correction. Sensors and Actuators B: Chemical 231, 529–538 (2016).
- The selection and use of flammable gas detectors. https://www.hse.gov.uk/pubns/gasdetector.pdf.
- Hrnjak, P. Creative Thermal Solutions. Air-Conditioning, Heating and Refrigeration Technology Institute 46.
This information has been sourced, reviewed and adapted from materials provided by Sensirion Inc.
For more information on this source, please visit Sensirion Inc.