The U.S.-based International Institute of All-Natural Refrigeration (IIAR) is currently engaged in a three-year project to determine whether CO2 (R744) forms dry ice in a transcritical refrigeration system’s piping when the gas is released through a relief valve.
The project, which is being done in collaboration with the University of California, Berkeley, was described by Eric Smith, the IIAR’s Vice President and Technical Director, at the IIR-Gustav Lorentzen Conference on Natural Refrigerants, held August 12–14 at the University of Maryland.
“There is speculation that if a CO2 system releases refrigerant through a relief valve, there may be the possibility that dry ice forms in the relief piping, thus negating the effect of the relief valve,” Smith said in a plenary speech on August 12. “Our goal is to figure out if it will happen or not.”
The project is studying “whether – and under what conditions – [dry ice] might form in piping and what the best practices are,” Smith said, adding, “maybe we will find that solid CO2 never forms in a discharge and everything is A-OK.”
The researchers will employ “actual equipment” in collecting data, in addition to computational fluid dynamics (CFD), Smith said. The project will account for pressure, temperature, heat transfer and piping geometries to determine “the appropriate sizing of CO2 relief valves and piping.”
This and other projects are developed by the IIAR’s research committee to create or update the IIAR’s guidelines, standards and handbooks. The committee works with the IIAR’s Natural Refrigeration Foundation, an education, research and training association, to “identify, fund and execute research projects,” and with other IIAR committees to identify “questions and needs” in the industry, said Smith. At the Gustav Lorentzen Conference, he invited university researchers to participate in the IIAR’s research committee and projects.
Recent research projects
Smith also discussed the IIAR’s research projects completed in the past few years, including:
- Condensation-induced hydraulic shock. This phenomenon, caused when hot gas is introduced to a pipe containing cold liquid refrigerant, has led to “a few catastrophic releases” of ammonia (R717), Smith said, adding that it could occur with any refrigerant. The goal of the project was to “understand how much hot gas can be introduced for the defrost process.” It looked at how slowly a motor-operated valve should open to prevent condensation-induced hydraulic shock and what size a solenoid valve should be to mitigate the effects of this event. The results of the project will be added to IIAR’s Refrigeration Piping Handbook.
- Optimizing velocity in wet suction risers. This project helped to determine the optimal velocity of saturated ammonia in wet suction risers in a liquid-overfeed system such that “liquid flow reversal and the resulting [energy] penalties are avoided,” said Smith. The IIAR produced a provision in the Refrigeration Piping Handbook and computer programming to aid designers in selecting appropriate piping for wet risers and avoiding oversized piping. This can save 10% in compressor power, he noted.
- Dispersion of ammonia in cold rooms. “We found ammonia disperses very quickly; in a 30,000ft2 [2,787m2] room it might take three seconds for ammonia to reach concentrations that are detectable,” said Smith. “In the end, the research made a lot of end users feel pretty good because we didn’t end up requiring more detectors than were necessary.”
- Dispersion of ammonia in the atmosphere. The study recommends that emergency exhaust fans generate a vapor velocity of 2,500ft/min (762m/min) to propel ammonia into the atmosphere and minimize off-site consequences. It also said that relief headers should point upwards toward the atmosphere.
- Insulation installation guidelines. The study surveyed the best insulation techniques for piping to improve energy performance and protect equipment.
- Estimating the mass of refrigerant releases. The study produced a tool allowing an end user to quickly determine how much ammonia is released in a leak and whether it needs to be reported to a local or federal agency.
Projected IIAR projects include:
- Investigation of the longevity of internal relief valves.
- An analytic method to determine the pressure drop of two-phase flow through control valves.
- Freezer door infiltration rates (in collaboration with ASHRAE).
- Relative risks of ammonia systems compared to everyday risks.
- Failure-data survey for conducting layers of protection analysis (LOPA).
Smith also provided updates on some of the IIAR’s standards and training:
- In 2024, the IIAR published IIAR-9 for ammonia systems, covering minimum system safety requirements for existing systems.
- A standard for large-scale hydrocarbon systems is out for a second public review and is expected to be published in the summer of 2025.
- An online course on CO2 systems is under development.
Based in Alexandria, Virginia, IIAR in 2023 changed its public name to the International Institute of All-Natural Refrigeration from the International Institute of Ammonia Refrigeration, which had been used since its founding in 1971. This change reflects IIAR’s expanded focus on all natural refrigerants, including CO2 and hydrocarbons and ammonia, in both industrial and commercial refrigeration.
“There is speculation that if a CO2 system releases refrigerant through a relief valve, there may be the possibility that dry ice forms in the relief piping, thus negating the effect of the relief valve.”
Eric Smith, VP and Technical Director, IIAR
