Examining NH3/CO2 Cascade Systems
With those words, Per Nielsen of York Refrigeration-Marine & Controls notched up the role of carbon dioxide (CO2) as a refrigerant at the International Institute of Ammonia Refrigeration’s (IIAR’s) 25th annual conference. Nielsen was one of two presenters offering papers on the combination of ammonia and CO2 in cascade systems.
The CO2 factor gained attention because of two considerations. The first one put ammonia-based technology on a more level footing with other refrigerants. “With the ammonia/CO2 cascade now being a viable option,” said Nielsen, “there is no longer a reason for large halocarbon systems, and there is no longer a reason to have ammonia in the production area. This cascade can do the job more efficiently, quickly, and safely.”
The second factor is the possibility of the demise of HFC refrigerants in Europe. Paul Homsy of Nestle (who also presented a paper on the topic) said, “HFCs will be banned in 2006 in Denmark and possibly in all of Europe by 2010. Like pure ammonia, the ammonia/CO2 combination is a proven solution for food retailers and industrial refrigeration applications.”
On LandHomsy described a plant that had been operating on R-22, but was targeted for a complete overhaul. “We required that the new system be more reliable and use less energy than the R-22 system. We also wanted to increase freeze-drying production capacity.
“Finally, we would only consider a system that uses environmentally friendly refrigerants. An ammonia/CO2 cascade design met all of our criteria.”
Homsy added, “It was also the first of its kind because ammonia/CO2 designs had not yet been used for freeze-drying applications.”
The configuration included a CO2-based low stage consisting of four oil-free reciprocating compressors, including one standby unit that drew suction vapor from three surge drums. Each surge drum had recirculating pumps to distribute liquid CO2 to more than 20 evaporators. The compressors discharge to two shell-and-tube cascade heat exchangers, with CO2 liquefied by evaporating ammonia.
The ammonia-based high stage had four screw compressors, including one as a standby that drew evaporated ammonia from the heat exchangers and discharged to three evaporative condensers.
The process was a challenging one. Fortunately, the end user was willing to give higher priority to the result than to the costs along the way. At one point, the contractor hired a “trained person dedicated to ensuring that the delicate task of installing valves goes well.
“That technician dismantles the valve, giving only the valve body to the installation fitters,” he continued. “The more delicate valve head parts are stored safely. After the welding is complete, he inspects both the pipe, using an endoscope, and the valve body, particularly the seat, for signs of debris and/or damage. If the pipe is clean and free of any foreign bodies, and the valve body is not damaged, he carefully installs the valve head.”
The necessary adjustments were made to get the new technology up and running. The system has been operating successfully for more than two years, Homsy said.
“We believe that this project demonstrates the technical viability of low-temperature industrial ammonia/CO2 refrigeration systems.”
At SeaThe same technology was taken to sea aboard a Norwegian fishing vessel, Neilsen said. This application included two ammonia screw compressors with computer control, one of which was equipped with a variable-speed drive; six reciprocating CO2 compressors, three of which were modified for dual use to provide cooling, or for dedicated use in a hot-gas defrost system; three pumps connected at the suction end of a low-pressure receiver that can hold 5 tons of CO2; and a shell-and-tube cascade heat exchanger with vented double-wall tubing.
“A fisherman will not tell you how much cooling capacity he needs,” Neilsen said. “He will tell you, ‘I have so many square meters of production area; how many tons of frozen fish per 24 hours can your system provide?’”
Like its land-based counterpart, Nielsen and his colleagues experienced a variety of challenges in making the ammonia/CO2 technology work. “Once we realized that the problems with CO2 refrigeration could be easily solved, we were able to enjoy its substantial benefits.”
He listed these benefits:
Publication date: 06/23/2003