Identifying Suitable Joint Types for Flammable Refrigerants

From ASHRAE Journal Newsletter, June 23, 2020

With the need for low-global warming potential (GWP) refrigerants, researchers have studied the properties of many alternative refrigerants. A2L refrigerants can be suitable alternatives, but they are mildly flammable. A solution to alleviate the flammability concern is to prevent leakage of refrigerant from cooling systems.

In a recent Science and Technology for the Built Environment article, researchers investigate the assembly, durability and leak rate of mechanical joints used in refrigerant and air-conditioning systems to identify which joint types are suitable to be used in systems that employ flammable refrigerants. Researchers Neal Lawrence, Ph.D., Member ASHRAE and Stefan Elbel, Ph.D., Member ASHRAE, discuss the research.

1. What is the significance of this research?

The move towards low-GWP alternative refrigerants has led to significant community interest in 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. Explain the steps of this research project. What did the process look like?

The project began by defining the fitting types and test conditions most relevant to the current interest of ASHRAE and then developing testing methods that allowed for meaningful evaluation of the different types of fittings.

Each fitting was then assembled, with over 300 total fittings being assembled and tested. The amount of time it took to assemble each fitting was recorded, and any leaks after assembly were noted. Fitting assembly was performed by both experienced and inexperienced technicians for each fitting type to observe the effect that experience level has on assembly of each fitting.

A series of robustness tests were then performed on each type of fitting, evaluating the effects of cycling of refrigerant pressure and temperature, repeated freezing and thawing of moisture on the surface of the fitting and vibration on the fittings. The observed leaks in different fitting types resulting from these robustness tests were recorded.

Finally, average leak rates were determined for the different fitting types by measuring the change in R-32 concentration of the hermetically sealed environment the fittings were housed in using infrared photoacoustic multi-gas analyzers. The results of the assembly, robustness, and leakage rate tests were then analyzed, and meaningful conclusions for each fitting type were drawn.

3. 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, HFOs and blends as straightforward replacements for 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 future. 

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 but 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 that long-term durability effects were not part of the currents study, but they could be studied 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 CTS team 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 (robustness) 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.

6. 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 important role of this research on helping increase the use of low-GWP A2L refrigerant alternatives.