Ongoing refrigerant changes are impacting the heating, ventilation, air conditioning and refrigeration (HVACR) industry and raising questions about the future.
Increasing societal pressure to control climate change is driving new regulatory policies to restrict and lower the global warming potential (GWP) impact of fluorocarbon refrigerants that are used in a wide variety of equipment and industries, including the HVACR industry.
In response, the industry is developing and examining a new class of lower GWP refrigerants. As this transition to next-generation, low GWP refrigerants moves forward, there are many questions about changing refrigerants options and requirements to use them safely.
Flammability is one area that requires additional explanation. While it is still early in the refrigerant transition process, this article highlights some important considerations, particularly flammability.
Not all next-generation refrigerants are flammable. There are numerous ultra-low GWP refrigerants (defined in this article as having a GWP of less than 10) that are nonflammable.
Some flammable next-generation refrigerants are blended with nonflammable refrigerants, much like many of the refrigerant blends we use today. For example, the blended R-410A mixes a flammable refrigerant (R-32, ASHRAE Class 2L) with a nonflammable refrigerant (R-125, ASHRAE Class 1).
ASHRAE Standard 34 defines flammability in three separate classes:
- Class 1 (No Flame Propagation)
- Class 2 (Lower Flammability)
- Class 3 (Higher Flammability)
ASHRAE has established a new 2L sub-classification for refrigerant flammability to address new next-generation refrigerants that have lower flammability characteristics.
Therefore, throughout this discussion it’s important to keep in mind that flammability is a continuum and not a set of absolutes as determined by Standard 34.
What is driving the refrigerant transition?
With growing concerns about the impact on the environment and climate change, pressure has been mounting for years to reduce the use of high-GWP refrigerants across many applications and industries. In response, all 197 member countries, including the United States and Canada, agreed to amend the Montreal Protocol (an international treaty designed to reduce the production and consumption of ozone-depleting substances) to phase down hydrofluorocarbons (HFCs).
The U.S. Environmental Protection Agency (EPA) made several changes to management requirements for refrigerants in Section 608 of the Clean Air Act, effective January 1, 2019, to include the following:
- Extending the requirements previously in place for chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) to include all replacement substances, including HFCs and the new hydrofluoroolefin (HFO) options. Hydrocarbons in small, self-contained systems are given an exception for venting.
- Reduced leak trigger rates, which in turn requires enhanced leak tightness requirements. This may push or incentivize the industry to move to technologies that are more hermetic with fewer joints and seals, for better long-term refrigerant containment.
- New requirements for mandatory leak inspections on equipment and increased record keeping requirements.
Refrigerant selection: a balancing act
While the HVACR industry evaluates next-generation refrigerant alternatives, the challenge is to balance environmental benefits with safety, sustainability and design requirements. It’s likely that tradeoffs between GWP, flammability and efficiency will be needed to be made in selecting refrigerants.
When considering refrigerant alternatives for the future, policy makers, the public and manufacturers must select refrigerants with the best balance of the following:
- Environmental performance (direct environmental impact such as inconsequential ozone depleting and reduced GWP)
- Safety for consumers (flammability and toxicity)
- Energy efficiency (indirect environmental impacts such as reduced CO2 emissions and high ambient operations)
- Intellectual property considerations
- Transition costs (impact on industry and consumers)
- Product sustainability (long operational life, reliability, maximizing recyclable content and repurposing components)
What should you know about flammability?
Safety — including the issues of flammability and toxicity — is a key consideration when evaluating next-generation refrigerants.
The HVACR industry has been asked to consider lower GWP refrigerants with both class 2 (lower flammability) and class 3 (higher flammability) flammability.
At a high level, the industry will likely select refrigerants that both meet regulations and are nonflammable or that have the lowest level of flammability possible. The lower the flammability, the lower the risk.
Refrigerant flammability is classified by ASHRAE Standard 34 (ASHRAE-34, 2016) or the newly published ISO 817 (2014). Both standards use similar methodologies, but there are some small differences between them.
ASHRAE 34 divides flammability into three defined classes with an additional subclassification:
- Class 3: higher flammability
- Class 2: lower flammability
- Class 2L: lower burning velocity (BV) class 2s with burning velocities less than or equal to 10 centimeters per second
- Class 1: no flame propagation
The 2L subclassification may become a separate class within ASHRAE Standard 34 to match a similar recent change to ISO 817, which made 2L a separate class.
Historically, most refrigerants used in HVACR products were Class 1, or nonflammable.
Ammonia, which is a class 2L material, has been used in large industrial refrigeration systems for over 100 years. The safety of these systems is heavily controlled as the result of ammonia’s toxicity rather than its flammability. These systems often are a low exposure risk since they are used in applications with low occupancies.
There is limited industry experience with ammonia’s flammability risks because these controls on toxicity limit the ability for ammonia to form flammable mixtures. In the end, flammable events have occurred with ammonia installations, which give some insight into system risks with very large class 2L refrigerant charge.
Hydrocarbons, which are Class 3 materials, have recently been used more in systems like small domestic refrigerators or freezers with very small refrigerant charge sizes.
Flammability safety in these systems is controlled by restricting the charge size to a low enough level to dramatically reduce the risk of propagating a flammable mixture beyond the equipment and limiting a potential flammable event’s severity. The use of Class 3 refrigerants has not expanded much beyond these applications because of the severe safety implications of using large refrigerant charge sizes with these materials.
There are some manufacturers that offer larger charge hydrocarbon systems, but these products have not become mainstream and likely will not because of the need to provide extensive safety procedures compared to an alternate system that might be using an equivalent GWP refrigerant with reduced safety restrictions.
While some nonflammable, ultra-low GWP refrigerants exist, these are lower-pressure refrigerants, and they are typically used only in centrifugal chiller applications. These refrigerants cannot cover the whole range of HVACR product needs.
The HVACR industry is actively investigating the safety of flammable refrigerants. The industry is determining the risks of flammable refrigerants by understanding the probability of potential occurrences and severity of events in various application situations including servicing and handling.
Challenges with using flammable refrigerants
Because of the challenges associated with flammable refrigerants, the HVACR industry launched several risk assessment projects. Flammability risk can be defined by considering and controlling three factors:
- Likelihood of a flammable event from a refrigerant leak that reaches the LFL.
This can be controlled numerous ways: through the use leak sensing in combination of either air circulation or air ventilation to mix or dilute air/refrigerant mixture as it forms, restricting the refrigerant charge, controlling the room area and volume, placing the unit outside or in a controlled equipment room, reducing leaks and joints, and eliminating opportunities to service the unit.
- Presence of an ignition source that is greater than the MIE needed to start combustion of the refrigerant. This can be managed by restricting or enclosing product ignition sources and removing sources or control in the room.
- Impact of the severity of a potential event, which includes the probability of complete combustion, the pressure rise, and the potential to create a secondary fire in the presence of combustible materials, which is a function of time and temperature. This can be controlled by designing the application to handle pressure rise (through venting, for example), and by placing the unit outside.
Several groups — AHRTI, ASHRAE and the U.S. Department of Energy (DOE) — are working together to research the various impacts of refrigerant flammability.
A changing landscape
New refrigerant technology is developing rapidly. Some refrigerants are starting to emerge as potential next-generation solutions. Many of these choices are lower-pressure, nonflammable solutions with vapor pressures similar to R-123 and ultra-low GWP that are ideal for chiller applications with larger refrigerant charge sizes, or they are nonflammable refrigerant blends, with vapor pressures similar to R-134a and moderate GWP of less than 750.
The industry continues to study the use of flammable refrigerant options. It’s important to keep in mind that flammability is a continuum with no specific natural flammable limits, and that not every refrigerant has the same flammability risks.
As standards and codes continue to change, there are many factors to consider as the industry works to find the best balance between minimizing environmental impacts, maintaining safety, and managing product costs.
The HVACR industry will likely have to adjust product refrigerant charge sizes in most direct refrigerant expansion applications to meet the standards. Some direct refrigerant expansion applications where refrigerant charge sizes are quite large, such as large splits, VRF systems, and large distributed commercial refrigeration systems, may not be available in their current form in the future because of flammability requirements.
Optimization of GWP, flammability and performance is possible for next-generation refrigerant options, with the proper research to develop flammability design tools. This work is being conducted on a path forward to enable transition to low-GWP refrigerants for the industry. In the end, using nonflammable low-GWP refrigerants that meet the regulatory requirements in high efficiency products is the easiest way to quickly meet environmental goals.
The preceding article is provided by Trane, a leading global provider of indoor comfort solutions and services.