Finding the Proper Fit: ASHRAE Research Identifies Fittings for Mildly Flammable Refrigerants

From eSociety, September 2019

As part of the HVAC&R industry’s move to using lower GWP refrigerants, an ASHRAE research project evaluated joint types considered for field installation of systems that use flammable refrigerants.

1808-RP, Servicing and Installing Equipment using Flammable Refrigerants: Assessment of Field-made Mechanical Joints, characterizes and quantifies leak tightness of various types of field-made joints⁠—press, compression and flare fittings⁠—used in HVAC&R systems. Fittings were evaluated based on assembly time, durability and leak rate with R-32. 

Press fittings allowed for the quickest assembly and lowest number of failures during durability testing. But the press fittings had the highest average leak rate compared to other fitting types, according to 1808-RP.

Principal investigator Stefan Elbel, Ph.D., Member ASHRAE, and Neal Lawrence, Ph.D., Associate Member ASHRAE, discussed the research, which was sponsored by MTG.LowGWP, Lower Global Warming Potential Alternative Refrigerants. The 1808-RP team has submitted a technical paper to be published in ASHRAE’s Science and Technology for the Built Environment.

1. What is the significance of this research?

The move toward low-global warming potential (GWP) alternative refrigerants has led to significant interest in ASHRAE A2L refrigerants, the mild flammability of which creates a concern over possible leakage. The purpose of this study was to investigate which types of field-made, mechanical joints are suitable for use in systems where leakage of an A2L refrigerant is a concern. 

The results of the study have provided detailed measurements of refrigerant leak rates for press or crimp fittings, compression fittings and flare fittings. Brazed joints were also investigated to create a baseline.

2. Why is it important to explore this topic now?

The HVAC&R industry is currently in the process of switching to low-GWP alternative refrigerants, and many manufacturers are strongly considering A2L refrigerants such as R-32, hydrofluoroolefins (HFOs) and blends as straightforward replacements to current refrigerants. 

Due to the flammability concerns of these alternate refrigerants, leakage and reliability data on refrigerant system joints, among other parts of the system, is needed in order to best design systems with A2L refrigerants in the near future.  

3. Were there any surprises or unforeseen challenges for you when preparing this research?

It was surprising to see the potential for substantial refrigerant leakage as a result of improperly assembled mechanical refrigerant joints, as well as the large impact the technician training level has on the integrity of some of the investigated fitting types. These results clearly demonstrate the importance of ASHRAE funded research on low-GWP A2L refrigerant alternatives

4. What lessons, facts and/or guidance can an engineer working in the field take away from this research?

Engineers can use the results of this study to select the best mechanical joint type for their system. It was found that press or crimp fittings were fairly resistant to developing leaks due to thermal or mechanical fatigue, while yielding a consistent though tolerable leak rate. Flare and compression fittings could yield almost no detectable leak when properly installed, though these joint types probably should be checked periodically during use to make sure they do not loosen slightly due to thermal or mechanical effects experienced during prolonged use in the system. 

It should be noted, however, that long-term durability effects were not part of the current study but could be looked at in the future.

5. How can this research further the industry’s knowledge on this topic?

The results of this study are intended to guide system manufacturers when selecting joint types for their systems that rely on technicians to complete installation in the field. The study provides a detailed comparison of the leakage and robustness of different options for field-installed joints. 

Furthermore, the Creative Thermal Solutions, Inc. team, the contracting company, has developed a comprehensive test matrix investigating different fitting types/sizes, material combinations, and technician training level. Specially developed test methods were devised to provide harsh yet realistic durability test conditions to expose the test items to accelerated mechanical and thermal fatigue. Resulting leak rates were quantified in a highly accurate refrigerant leak test facility following the harshness tests.