Evaluating Safety in CO2 Systems

The use of carbon-dioxide refrigeration systems has been gaining in popularity in recent years as an alternative to ammonia systems, as companies learn about their energy efficiency, safety benefits and resiliency. Although CO2 , like ammonia, is classified as a “natural refrigerant,” there are a number of differences between the two refrigerants, related to the significantly higher operating pressure of CO2 systems that are critical to understand from a safety perspective. And engineers must take into account those differences when designing and building a C02 system.

“It starts at the design process and continues through to what needs to be understood from an operational standpoint,” says David Blackhurst, director at Star Technical Solutions. “When working with higher pressures you have to define your pressure envelope and the operating parameters under pressure. A typical ammonia system would be designed with a high pressure side rating of 250 psig, but with CO2 it could be around 750 psig for a cascade or subcritical system operating with hot-gas defrosting and as much as 1,750 psig for a transcritical system. So you need to design for components that can withstand those pressures.”

Among the considerations when designing and operating a CO2 system is to avoid situations when liquid CO2 can be trapped, be prepared for cross-contamination by being able to promptly respond to a leak that causes ammonium carbamate, carefully consider the location of pressure relief valves and CO2 detectors and guard against water contamination.

CO2 has a high coefficient of thermal expansion, so the system can fail in areas where liquid becomes trapped and warms up, creating extremely high pressures. Designers must identify those potential areas and either include some form of pressure control within that section or make sure it is not isolated once the system is operating. A fail-safe measure could include having two valves that can’t be closed simultaneously, with one valve locked in open position.

“If you have to close CO2 valves for whatever reason, it must be done in a considered and deliberate manner,” Blackhurst says. “Trapped liquid is a big issue because if liquid on the low pressure side is at -40 degrees, is constrained between two closed valves, and then rises to say 32 degrees, that part of the system will be under enormous pressure.”

Cross-contamination in a cascade system is another CO2 issue. Cascade refrigeration plants feature an intermediate heat exchanger that has condensing CO2 on one side and ammonia on the other side that is evaporating. If a leak occurs, the CO2 will always flow to the ammonia side due to CO2 ’s higher pressure. Therefore, it is important to select the most appropriate type of heat exchanger with the least opportunity of allowing CO2 to leak into ammonia.

If such a leak does take place, a white, crystalline compound called ammonium carbamate is formed that will clog up the internals of the ammonia system. A good way to limit the damage from cross-contamination is to design the overall system with two separate ammonia circuits. If one circuit is contaminated because its intermediate heat exchanger fails, the second circuit would allow the plant to keep running at half capacity.

Another option is to install fast-acting valves in the ammonia lines around the intermediate heat exchanger so the detector can quickly pick up ammonium carbamate and contain the contamination to a localized area.

“If you lose cooling through cross contamination it could take weeks to deal with it,” Blackhurst said.

The design and location of pressure relief valves in a CO2 system differs from an ammonia system. With an ammonia system, typically the pressure-relief valve is local to the vessel and connected to it by a short stub of piping. Ammonia passing through the pressure relief valve discharges either to a diffusion tank or to the atmosphere, via a length of pipe. But CO2 gas passing becomes a solid as it passes through the pressure relief valve and is dropped to atmospheric pressure. Therefore, the pressure relief valve must be positioned at the point of discharge so that the solid material will be blown into the atmosphere. Otherwise, the pressure relief valve could become blocked preventing adequate pressure relief from the vessel in question.

Another difference between CO2 and ammonia systems is the amount of water contamination that each system can tolerate. The CO2 in a refrigeration system should contain less than 10 parts-per-million of water.

If the system is not dry the water in it will manifest in different parts of it as water, ice and a crystallized structure composed of CO2 and water particles that look like ice. “It’s important to know the quality of the CO2 that you’re buying, and also to make sure all the water is out of the system before you begin to charge with CO2 ,” Blackhurst says.

Finally, the proper placement of CO2 detectors is a critical safety measure. Although CO2 is not toxic or flammable, you are still dealing with a vapor that if undetected can cause death at high enough concentrations, he said.

The threshold limit value for CO2 is 5,000 ppm (0.5 percent). That is the highest concentration that a worker can safely be exposed to for up to eight hours. The National Institute for Occupational Safety & Health (NIOSH) IDLH limit for CO2 is 40,000 ppm (4 percent). The setting of CO2 detectors should therefore be a minimum of 5,000 ppm and an absolute maximum of 40,000 ppm.

It is also important to bear in mind that CO2 is 1.5 times heavier than air, so careful consideration must be given to locating sensors where CO2 might leak and concentrate, such as in stairwells or basement areas. One should also consider the level of ventilation in the space.