The International Institute of All-Natural Refrigeration (IIAR) is updating its safety standard for CO2 (R744) to reflect new approaches to high pressures in transcritical CO2 systems.
The updates were explained by Eric Smith, IIAR’s Vice President and Technical Director, at the ATMOsphere (ATMO) America 2026 conference in Tarrytown, New York on June 2.
IIAR, which for many years developed ANSI-approved safety standards for ammonia (R717) refrigeration used in industrial applications, has expanded its scope to create standards for CO2 and hydrocarbons like propane (R290) that can be applied to industrial and commercial refrigeration.
The CO2 standard (Safety Standard for Closed-Circuit Carbon Dioxide Refrigeration Systems) was published in 2021 and adopted by model codes (such as the International Mechanical Code and the Uniform Mechanical Code) in 2024. IIAR is close to publishing its hydrocarbon standard, Smith said.
The CO2 safety standard was developed for “unlisted systems,” which encompasses custom-designed and field-erected equipment. It covers design, inspection, testing, maintenance and installation.
“If you want to affect the industry, this is how you do it.”
Eric Smith, IIAR’s Vice President and Technical Director
In April IIAR approved an Addendum A for the CO2 standard, with plans to publish it soon, Smith said. Addendum A updates the calculation of relief capacity, the minimum required discharge capacity of a relief valve in pounds-of-air-per-minute for transcritical CO2 systems. “It is a calculation that must be done to figure out what size relief valves you need to protect a vessel or other pieces of equipment from rupturing, should they over-pressure,” he explained.
To calculate relief capacity, you multiply the diameter and length of a vessel by a standard capacity factor (f). When IIAR’s CO2 standard was published in 2021, “we were relying on an f-factor of one, which was incorrect,” said Smith. “It’s applicable for certain pressure and temperature, but it was based on older ASHRAE standards.”
In 2022, ASHRAE developed capacity factors for a range of temperatures and pressures for CO2 but only went up to 935psi (64.5bar), which falls short of the critical point of CO2 (1,072psi/74bar) when the system enters transcritical operation.
“What the capacity factors were for transcritical remained elusive for a number of years” until IIAR research committee determined appropriate values above the critical point, said Smith. The IIAR CO2 standard’s Addendum A includes a chart that includes the specific capacity factors needed at transcritical operation.
Closing CO2 gaps
IIAR’s research committee is working on closing “a couple of gaps” in CO2 knowledge as it relates to high pressures, Smith said. One of those involves attaching relief piping to the header on relief valves when venting to the atmosphere. This is not considered appropriate “because there’s a possibility that dry ice forms when [CO2] releases from high pressure to a very low pressure,” he explained, adding that the dry ice could clog the valve.
The IIAR is working with the University of California at Berkeley to study this phenomenon and “see when and where the formation of dry ice might occur and under what conditions” so that new guidelines can be developed, Smith said. Resolving this issue would help the refrigeration industry “because it doesn’t make a lot of sense to not be able to header your relief valves” with discharge piping.
The IIAR research committee also plans to develop a tool for designing low-side equipment to better manage pressure spikes in the event of power outages. “If you lose power on the low side, then when that refrigerant comes to equalization, pressure gets very high,” Smith explained. The committee is looking at how to relieve the pressure without “blowing the CO2” into the atmosphere. “We’re hoping to get that done within the next year or so.”
Smith invited attendees at ATMO America 2026 to join one of the IIAR committees. “If you want to affect the industry, this is how you do it,” he said. “Get involved with the committees that do this work [like] the research committee [and] the standards-writing committee. It’s rewarding.”
