Carbon dioxide as a refrigerant was popular from the late 1800s to the 1930s, before giving way to CFCs. CO2 is now the subject of renewed interest.
A number of industry experts explored aspects of a CO2 revival during two panel sessions at the annual conference of the International Institute of Ammonia Refrigeration (IIAR).
According to information from the sessions, the initial interest in CO2 is in conjunction with ammonia in a cascade system. But it will only catch on with just the right components and up-to-date technologies, according to industry experts.
Joe Pillis of York Refrigeration Group, York, PA, acknowledged, “This is an old idea that has come back around. All manufacturers are doing CO2 testing to get their knowledge back up to speed.”
On the plus side, Pillis said use of CO2 allows for reduced-sized low-stage compressors and can operate at -65 degrees F “with no risk of air ingress.” It is also considered an environmentally friendly refrigerant.
On the minus side, use of CO2 with ammonia can “add to the cost and complexity of cascade systems.”
He noted, “The key in using CO2 is to keep condensing temperature down to avoid bearing and deflection limits when using screw compressors.” He said that since 1991 York has brought “seven or eight” CO2 systems on-line in the field and “they work fine.”
He added, “The gas acts like R-22 at high suction. The issue is how dense the refrigerant is on the low side. The key is the pressure drop.”
An added issue concerns oil. Pillis said attention is now focusing on PAO (polyalphaolefin) oils more so than POEs or PAGs (polyalkylene glycols) because of PAOs’ immiscibility. But to use PAOs, special oil separators would be needed “due to the risk of oil globules floating in CO2 in the recirculator.”
Monika Witt, of the German company T.H. Witt, described CO2 as needing “standard parts and standard technology with high pressures.”
CO2 pumps, she said, need “open impellers and side channels to avoid axial forces and enable gas purging.” Also needed, she said, are “standard pumps with higher test pressure and float valves originally designed for heat recovery systems, and shell-and-tube heat exchangers.” She added that “CO2 is safe when used properly and correct precautions are taken.”
Gordon Struder of Evapco, Westminster, MD, said, CO2 “may once again become a common refrigerant.” He said operating pressures must be kept at 75.1 psia. Evaporator temperatures at about 0?F require greater tube wall thickness. Oil circulation needs to be minimized through the evaporator, he said, to contain oil contamination.
Different methods of defrosting also need to be considered, such as hot gas with a booster compressor, water with periodic wash downs, and electrical means with small evaporators and commercial coils.
Pega Hrnjak of the University of Illinois reported that research is being done at the college on CO2 applications. Preliminary findings, he said, include “better performance in the evaporator, but worse in the condenser. Heat transfer coefficients are lower than predicted by conventional correlation, but still high.”
George Briley of Technicold Services, San Antonio, TX, focused on off-cycle considerations such as use of an expansion tank, the need to vent to atmosphere with pressure-regulating valves, or use of a small compressor added on to maintain temperature.
Both Chuck Taylor of the Stellar Group, Jacksonville, FL, and Briley noted that the use of a CO2/NH3 cascade allows for a reduction in the ammonia charge and equipment size. Those could be considerations when trying to reduce the amount of ammonia in a system while reducing equipment size.
Taylor said, “Given the increasing costs in our industry, it is time to address the need for alternatives.” He noted that CO2 compressors are much smaller than comparable ammonia compressors. He also noted defrost concerns and “triple point” — a unique situation in which CO2 can turn solid. He added, “We have to rethink how to do leak detection in a vacuum.”
Through it all, he said, it is possible “to build a CO2 system as reliable as today’s ammonia systems.”
Richard Novak of Praxair, LaGrange, IL, noted that CO2 is a familiar refrigerant in food freezing, where it is used as an expendable refrigerant. He encouraged those in the ammonia industry to look at CO2, both its pluses and minuses. “Look at design codes. Look at what you are using in valves and pumps.”
In fact, he said, CO2 is listed in the EPA’s Significant New Alternatives Policy (SNAP) program as a “viable alternative.”
One of the most attractive aspects of its use with ammonia systems is the possibility of a significant reduction of the amount of ammonia at a site, he said. He noted that various regulations governing more than 10,000 lb of ammonia at a site would be not be in force if the amount of ammonia could be less than that. He said CO2 cascades could mean “five to ten times less ammonia at a site,” but with the same cooling capacity.
Further, he said, a CO2/NH3 system is, in effect, a secondary loop concept. “Ammonia does not have to be present through the facility. It can be limited to the engine room, in a controlled area with limited access.”
In the United Kingdom, Nestle company had a large, two-stage, pumped recirculation R-22 system with increasing operational problems at a coffee-making plant. “Headquarters has concluded that R-22 had no long-term future,” Nestle’s Adrian Page told attendees at the International Institute of Ammonia Refrigeration’s (IIAR’s) annual conference. He said he and his colleagues were instructed to come up with a new solution that would be cost-conscious and avoid a system shutdown.
The team looked at options involving HFCs, two-stage ammonia vapor compression, hydrocarbons, closed-circuit air-cycle refrigeration, and CO2.
“Preliminary investigations identified CO2 in cascade with ammonia as a promising choice because of the small size of the plant and associated piping,” said Page. The investigation involved a pilot test to evaluate the design and cost of the system vs. a conventional two-stage ammonia system. “The study confirmed the significant cost benefits of the cascade system,” he said.
Next came a demo unit containing CO2 as part of the cascade cycle that rejected heat to the existing R-22 plant. “Nestle experienced no unexpected problems with CO2 as a refrigerant, and found that operation close to the triple point (a unique situation in which the CO2 can turn solid) is not a concern.”
So the company decided to create a full-size system in the plant.
Among the criteria:
Page noted that while various European codes and safety regulations governed the project, there was actually less red tape than with many conventional projects. “In practice, no regulation restricted the designs for the plant. In the United Kingdom, ammonia is only classified as a major hazard if the total charge is above 50 metric tons. There are no restrictions on the use of CO2.”
It was decided that the normal design pressure for most parts of the CO2 system should be 435 psig, corresponding to a saturated temperature of about 21 degrees F. Then there was the question of limiting system pressure to the design pressure if the plant was not running.
Said Page, “In bulk CO2 storage systems, the pressure is normally maintained at the desired level by a small refrigeration system that condenses gas in the vapor space of the storage vessel.
“If this small system has sufficient capacity to handle the heat gain to the vessel, the pressure can be maintained. The ammonia circuit acts as the ‘small’ refrigeration system.”
Page added, “It was decided that the best way to limit pressure rise is to control ventilation of CO2 vapor from the system. Each vessel is therefore fitted with a CO2 relief valve that is powered by a battery-operated backup power supply.” Each vessel also has a normal, dual-relief-valve system.
Overall, Page said the ammonia system used was of “conventional design, with no novelties attempted.” He said the project involved “building a new compressor plant room and condenser area for the main plant items, and new low-temperature surge drums on the outside of the building. The evaporators were then installed one at a time in the locations vacated by decommissioned R-22 equipment, ensuring that between the old and new systems, they always had sufficient cooling capacity to meet process needs.”
A key concern for the team involved ventilation. “CO2 is one of the friendliest possible refrigerants,” Page said. “It is, however, heavier than air and can be a suffocation hazard. This risk can be managed by ensuring good local ventilation — either by natural airflow or by permanently running fans. In most areas, the company uses detection systems to warn of high concentrations of CO2.”
At the time of the presentation, the system had been operating about a year. “All indications are favorable for long-term operation,” Page said.
— Peter Powell
Publication date: 06/03/2002