Cooling Without the Combustion: How EARTH Researchers are Facilitating the Development of Safer Green Refrigerants

Researchers at the U.S. National Science Foundation (NSF) Environmentally Applied Refrigerant Technology Hub (EARTH) have developed an effective testing apparatus for assessing the flammability levels of refrigerants, as published in a recent study in the journal of Industrial & Engineering Chemistry Research. This new equipment accurately determines the flammability limits of refrigerants, which is critical data for ensuring the safe use of environmentally-friendly refrigerants in next-generation cooling technologies.

In the 1990s, hydrofluorocarbon (HFC) refrigerants were developed to replace ozone depleting substances such as chlorofluorocarbons (CFCs). Today, the high global warming potential (GWP) of certain HFC refrigerants has led to the phase down of HFC refrigerants over the next ten years. 

A challenge with alternatives such as hydrofluoroolefin (HFO) refrigerants is that some are mildly flammable (ASHRAE classification A2L) compared with HFC refrigerants which were nonflammable (ASHRAE classification A1).

As research is underway to find safe refrigerant alternatives, industry members are interested in accurately measuring flammability limits as established by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE). 

“It's important to know the flammability of a refrigerant and which ASHRAE category it fits in, because that category determines where and how that refrigerant can be used and how much of it can be used,” said Mark B. Shiflett, Foundation Distinguished Professor of Chemical Engineering at the University of Kansas and Director of the EARTH Engineering Research Center. 

According to Shiflett, the initiative for NSF EARTH to develop a precise flammability measurement apparatus was drawn from three needs. First, it is important to explore the measurements carefully and look at the impact of temperature, humidity, spark source, and other variables that can have an important impact on the flammability limits.

“We're going to be developing new refrigerants,” explained Shiflett. “So we're going to want to test to see if they are flammable or not, as well as determine their flammability category as defined by ASHRAE.”

“And finally,” he continued, “relative to developing new refrigerants, is there the possibility of making a flammable component less flammable by mixing it with something else? This gives us the opportunity to study the flammability of mixtures, where mixtures like a flammable refrigerant are blended with a non-flammable refrigerant: How much of the non-flammable refrigerant is necessary in order to make a flammable refrigerant non-flammable?” 

With funding assistance from the NSF, the Wonderful Institute for Sustainable Engineering at KU, and the EARTH member company, Hudson Technologies, who reclaims refrigerants and is headquartered in Champaign, IL, the research team at EARTH, led by Kevin Turner, a Ph.D. student in chemical engineering at the University of Kansas, designed, constructed, and extensively tested a new refrigerant flammability measurement apparatus.

“I worked with one of our staff scientists, who assisted me with the design of the parts for the apparatus” explains Turner. “ASHRAE standards dictate specific parts and what materials we should be using, though some of it is subject to interpretation. Additionally, technicians at KU helped me with machining certain parts when I needed something specific.”

“Once we started testing the measuring process, we had to be mindful that measuring flammability is a harsh environment,” he continued. “There were a lot of tests to determine what is safe, and how to effectively capture photos on camera to review different flame attributes, as well as accurately analyze the images. After it was built, we spent two months testing, refining, and improving the apparatus to attain repeatable and precise flammability measurements.”

The research team identified several improvements that could be made to flammability measurement apparatuses currently in use by others, which led to modifications in the test vessel's stopper assembly, testing chamber, ignition system, and humidification system.

“It’s important to design an apparatus that provides reproducible results. Our biggest improvement was to the actual stopper assembly,” explained Turner. “And in the testing chamber, we have to make sure we are controlling the temperature. Other apparatus support the flask on the bottom, but ours supports it on the top, so it hangs to ensure that the air circulates all around the sphere.”

Testing the apparatus design and continual refinements were critical. In doing so, the team focused on measuring the flammability of the refrigerant HFC-32, because the flammability limits are well known. Alignment with published data indicates success in the new measurement process.

“That's the advantage of being at a university and part of a Ph.D. thesis: We have the time to do an extensive amount of testing,” said Shiflett. “Often, companies aren’t able to spend two months just focused on trying to measure a known system to qualify equipment.”

He continued, “NSF EARTH is trying to develop the next generation of refrigerants that are environmentally safe, as well as safe from a human aspect. People have to handle refrigerants. They have to fix and repair equipment. And so knowing the flammability characteristics of the refrigerant is key to knowing how to build the refrigerant system, how to service the system, and how to safely and effectively recover the refrigerant. The test equipment we’ve built is producing high quality results and available to our industry partners for measuring refrigerant flammability.”

Please direct queries regarding this study to Prof. Mark Shiflett, mark.b.shiflett@ku.edu.

EARTH Engineering Research Center

The Environmentally Applied Refrigerant Technology Hub (EARTH) is an NSF- and corporate-funded Engineering Research Center (ERC). EARTH is dedicated to revolutionizing how refrigerants are formulated, manufactured, applied, monitored, and recycled to dramatically reduce the environmental footprint of the global cooling sector. Led by the University of Kansas, this consortium of partner research universities includes the University of Notre Dame, University of Maryland, Lehigh University, University of South Dakota, and the University of Hawai′i.