How to incorporate flammable refrigerants
Incorporating flammable refrigerants can help improve energy efficiency and costs, but there are many aspects that need to be considered.
As many industries look to become more energy efficient and reduce their carbon footprint, alternative materials often must be found to meet new standards and demands. Industries such as the HVAC and commercial refrigeration sectors, for example, are shifting to refrigerants with lower global warming potential (GWP), which is a measure of infrared energy the emissions of a gas will absorb relative to carbon dioxide (CO2). The changes to refrigerants, and subsequent considerations that arise with the use of these alternatives, are being addressed through new industry standards and regulations.
New regulations for the industry have been introduced with the objective of finding and using refrigerants with a GWP of less than 750. Currently, the refrigerant most used by these industries, R-410A, has a GWP of 2,088, making it necessary to find alternatives to meet new GWP requirements. The most available alternative refrigerants with GWP ratings less than 750 have properties that present challenges to the HVAC and commercial refrigeration industry. Refrigerants are classified according to specific properties such as toxicity and flammability.
These classifications have been defined under ASHRAE 34 and ISO 817 standards for the designation and safety classification of refrigerants. R-410A is classified as an A1 refrigerant which means it has no flame propagation. Meanwhile, refrigerants with lower GWPs are ofren classified as A2L, A2, or A3, which means they have properties which make them flammable (A2L – mildly flammable, A2 – flammable, A3 – highly flammable). Given these properties, A2L refrigerants have become the most sought-after alternative because they are only considered as mildly flammable having a maximum burning velocity of less than 10 centimeters per second. As such, even though they are designated to be only mildly flammable, popular A2L refrigerants with lower GWPs require additional precautions for operating and storing equipment to allay hazards associated with refrigerant leaks.
Traditional vs. alternative refrigerants
There are multiple differing characteristics between traditional refrigerants and alternatives, such as density, ignition, and physical risk. Each characteristic comes with increased risks that must be considered. For example, the most sought-after alternative refrigerants are heavier than air, which increases risk for hazards should a leak occur close to the floor or ground, where air dilution is limited. These alternative refrigerants each have unique lower flammable limits such that if enough refrigerant is leaked into a space, and the ratio of the amount of refrigerant to air increases above the lower flammability limit, it doesn’t take much to ignite.
Potential sources of ignition must be considered during installation, operation, service, and tear-down. An additional hazard to consider is if a leak and ignition does occur, refrigerants produce hydrogen fluoride (HF) as a biproduct, which can cause severe skin burns, as well as lung damage that may result in respiratory distress or cardiac arrest.
Safety concerns like those described above have not been overlooked by the industry. They are addressed and assessed under the Safety Standard for Refrigeration Systems, ASHRAE 15, and the specific appliance/type standards UL 60335-2-40 and UL 60335-2-89.
These will be coming into effect over the next few years. UL 60335-2-40 becomes mandatory in January of 2024 and applies to: air conditioners, heat pumps, liquid chillers, hydronic fan coil units, hot water pumps, dehumidifiers, supplemental heaters, and equipment with flammable refrigerants. UL 60335-2-89 will come into effect in September 2024, and applies to electrically operated commercial refrigeration appliances that have an incorporated compressor, refrigerating appliances that are supplied in two or more units for assembly as a single appliance in accordance with the manufacturer’s instructions (split system), and partial units intended for installation in a field-erected system.
Both standards include guidance around flammable refrigerants and allowed charge limitations, depending on product type and refrigerant classification. There are significant requirements for installation and operation manuals (IOMs), temperature requirements of components and surfaces, safety control sensor locations, flame arrest enclosure verifications, and toxicity charge limits. They include safety requirements, testing guidance and acceptable limits.
Safety requirements for flammable refrigerants
Under both standards, safety requirements for systems using flammable refrigerants are determined based on the total system refrigerant charge. As such, there are three charge-level classifications, based on the lower flammability limit properties of the refrigerant: m1, m2, and m3. An m1 designation has the lowest charge, m2 the middle, and m3 is the maximum possible charge allowed. The refrigerant’s charge designation and the installation parameters, such as room area, will define safety mitigations measures for the equipment. These methods must be provided with the appliance from the factory or during installation.
Under this system, a unitary air conditioning system with a charge less than m1 would not require additional safety mitigation controls. However, if the charge designation is m2 or m3, the equipment must have a mitigation control in place for a potential refrigerant leak. This might include a detection system that could identify the presence of a leaked refrigerant, shut down the system and activate a circulation blower to dilute concentrations below flammability limits. Another mitigation safety control option would be to design a system to run continuous airflow circulation through the system. The required mitigation is dependent on the total system charge once installed, room area available for dilution should a leak occur, and incorporated system safety designs such as continuous air circulation or safety shutoff valves in the refrigerant system.
Other safety requirements relate to general construction and installation and include guidance around items such as:
- Tubing, which must be enclosed or protected from mechanical damage
- Low temperature solder alloys with a melting point of less than 427 °C, which may not be used for pipe connections
- Fusible plugs added during installation or provided with the equipment from the manufacturer, which must be vented to the outdoors.
Specific safety requirements will depend on the product in question and the refrigerant used in its manufacture and operation. To ensure products adhere to these safety requirements, the standards include testing and certification guidance.
Flammable refrigerant equipment testing
Equipment using flammable refrigerants must be evaluated to ensure compliance to the safety standards. They must be assessed to determine any potential leak sources in the refrigerant system, which then need to be evaluated with potential ignition sources also within the equipment. To achieve this, the testing lab must simulate all potential combinations of leak sources and ignition points to verify components are isolated from the leak source, placed in approved flame arrest enclosures, or the components themselves are designed and listed suitable for use in areas of known flammable vapors, aka hazardous locations.
Critical points to evaluate for a leak include: a joint in the refrigerant system tubing; a bend more than 90 degrees; or a point judged to be a weak point in the refrigerant containing system due to thickness of the metal, exposure to damage, sharpness of a bend, or the manufacturing process. Ignition sources can be electrical components that produce sparks or arcs such as brush-type motors, switches, and relays. Ignition sources may also be surfaces of components or cabinets that exceed the auto ignition temperature (AIT), such as electric heaters or UV lights.
Equipment using flammable refrigerants must also be assessed to ensure it will withstand vibration during both transport and use. To simulate transport, the product must be tested in its final packaging for 180 minutes to ASTM D4728-06. To assess vibration during use, vibrations are measured at all critical points while the appliance is in operation to ensure they do not cause excessive resonance in the piping connected to the compressor.
The standards also include guidance for the labeling of systems with flammable refrigerants. Markings must be present to alert professionals and users to the presence of flammable refrigerants before routine service or repairs. Equipment using A2L refrigerants, need to have the word “WARNING” near all service ports, pipes, hoses, and other devices through which the refrigerant is serviced. Products with A2 and A3 refrigerants must have the word “DANGER” in these same locations. Symbols indicating the use of flammable refrigerants in the product must be used and service ports for A2 and A3 refrigerant systems must be painted or colored a visible red when they are capable of being serviced.
It is important to keep in mind that changes and amendments to the standards in the coming years, until they come into effect, is likely. As such, it is important to stay up to date on developments within the standard to ensure continued success in assessing products. Working with a trusted partner to stay on top of changes, and complete evaluations can help ensure the process goes smoothly, getting products to market efficiently.
Reducing a carbon footprint with alternative refrigerants comes with several challenges. While more environmentally friendly, these substances introduce mild risk that must be addressed. Evaluating systems under UL 60335-2-40 and/or UL 60335-2-89 requires a full assessment of equipment, including its construction, markings, instructions, and manual(s). Proper understanding of the use and safety considerations around these alternate refrigerants will also be key.
Craig Grider is assistant chief engineer for Intertek, a CFE Media content partner.