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At the recent Purdue University Compressor and Refrigeration Conference, some 500 attendees from 26 countries heard presentations of more than 200 papers on a wide range of HVACR topics. Dozens of those dealt with ways to make HFC refrigerants work better. But an even greater number of papers looked at alternatives to HFCs, with CO2 being the most talked about.
In effect, the four days of sessions was a cautionary tale. Many wanted HFCs to remain viable refrigerants long after HCFCs are phased out. But many sensed a long life for HFCs is not a sure thing.
In remarks at a joint session for attendees of the 16th International Compressor Engineering Conference and the 9th International Refrigeration and Air Conditioning Conference, Francois Billiard, director of the International Institute of Refrigeration, pointed to mixed signals in Europe as one cause for concern.
“Many countries are not in favor” of a global position on HFCs, he said. He noted that governments in Denmark, Austria, and Switzerland have already set in motion efforts “that could phase out HFCs in their countries within five years.”
Furthermore, efforts to get an overall European consensus are being hampered by some who label HFCs “a threat,” primarily because of the refrigerants’ global warming potential. So the search is on, he said, for non-HFC technology.
The future of HFCs also came into blunt assessment in some of the papers presented. In a paper on “eco-friendly” refrigerants, engineers from India said the choice of an HFC as “a non-ODP (ozone depletion potential) refrigerant for replacing CFCs in the long term is a misplaced hope.”
Researchers at Purdue, in a paper on piston cylinder expansion devices, noted, “Although HFCs have zero ozone depletion potential, they have significant global warming potential and their future use has been questioned as well.”
THE CO2 SOLUTIONAt the last Purdue compressor and refrigeration conferences, held two years ago, talk surfaced on CO2 as a post-HFC refrigerant. A tour of the school’s Herrick Lab showed a number of studies underway working with CO2. Since then, several projects have gone on-line in Europe using CO2 in conjunction with ammonia (NH3).
This year, the paper from two educators from universities in India noted, “Natural refrigerants like NH3, CO2, hydrocarbons, and water — which have no ozone depletion potential and negligible global warming potential — are the best long-term alternatives which can replace CFCs.”
They added that the importance of CO2 as an attractive alternative has increased “because of its environmental and economic properties. Particularly its use in supercritical ranges is a consideration in mobile air conditioners and other applications.”
While noting that HFCs “have considerable global warming potential (GWP),” they voiced support for HFC-152a. “It has a very small GWP and could be considered as a medium-term substitute if its flammability can be controlled. Research in China has shown that binary and ternary mixtures of R-152a with any nonflammable refrigerant performs better with increased COP and reduced power consumption if used in place of CFC-12.”
Researchers from the Department of Technical Thermodynamics in Germany said, “CO2 as a natural refrigerant was rediscovered and has achieved a very high potential to substitute currently used refrigerants in the area of mobile/automotive air conditioning and refrigeration.
“This development is caused by the excellent thermodynamic, transport, and environmental properties of CO2.” Their research involved two types of heat exchangers used with the refrigerant in an effort to develop better equipment.
Engineers at CPI Engineering Services, Midland, MI, explored the issue of lubricants for CO2 refrigeration. They said, “The selection of a lubricant for the CO2 compressor could have a major impact on the reliability and performance of the system. The importance of this issue is further highlighted by the exceptional solvency of CO2, its high operating pressure, and the acidic nature of CO2 in the presence of moisture.”
To investigate these issues, the researchers performed lab bench tests, compressor tests, and field trials. Their findings were that “compressor/system manufacturers and lubricant suppliers are making progress with CO2 systems that give dependable and efficient performance. Results show that various lubricant chemistries and/or formulations can successfully lubricate CO2 compressors.”
A team of engineers from Purdue acknowledged that “the system performance of CO2 systems is typically poor compared to the current conventional air conditioning systems” that use HCFCs or CFCs.
“One of the most effective ways to achieve parity with CFC and HCFC systems is to replace the expansion valve with an expansion device that minimizes entropy creation and allows for energy recovery during the expansion process.”
The researchers designed and built what they described as “a piston-cylinder-type work-output expansion device. The device is based on a highly modified small, four-cycle, two-piston engine with a displacement of 2 x 13.26 cm3 that is commercially available. The work-output expansion device replaced the expansion valve in an experimental transcritical CO2 cycle and increased the system performance by up to 10%.”
Sidebar: Fine-Tuning HFC ResearchWEST LAFAYETTE, IN — While considerable research focused on CO2, other papers presented at the Purdue conference looked at ways to make HFCs work better. Researchers from the University of Strathclyde in Glasgow, Scotland, took issue with those predicting a near-term demise of HFCs. “They will remain in use for a significant period of time and retain a significant share of the refrigeration market.”
One focus of the project was to find HFC blends with a lower total equivalent warming index (TEWI). The researchers said this would present such a refrigerant in a more favorable light among opponents who are concerned about the global warming impact of currently available HFCs. The blend being looked at was R-134a/R-227ea. The organization promised more detailed findings at a later date.
Another project, from Calor Gas and University College London, looked at the temperature glide issue of refrigerants. They claim to have come up with “a new balancing method that enables components using a refrigerant with temperature glide to be matched correctly. In addition, the influence of superheat, desuperheat, and pressure drop is incorporated into the new method.”
The question of oil return in one of the newer HFCs was raised in a paper presented by representatives of Honeywell in Buffalo, NY. They cited R-417A, a blend of -125, -134a and -600. After an initial wave of research, it was reported that “a better understanding of the impact of the slower oil return on compressor reliability is required before any conclusions can be drawn about the use of these refrigerants. It was shown more clearly that in systems with accumulators and receivers, oil return could be problematic. Using anything other than miscible lubricants in these systems would be a major concern.”
Sidebar: Addressing The CO2-Ammonia IssueWEST LAFAYETTE, IN — One of the first efforts at using CO2 in commercial applications has been in conjunction with ammonia systems. Much attention was paid to that matter at the International Institute of Ammonia Refrigeration (IIAR) Conference this past spring, at which time some plants using the technology had come on-line.
At the Purdue conference, a paper on the ammonia-carbon dioxide approach was presented by a student and two professors from the Environment & Chemical Engineering School in China. Among their findings:
“An NH3/CO2 two-stage, low-temperature refrigeration system takes advantage of the environmental qualities of ammonia and its safety issues. This refrigeration system minimizes the charge quantity for NH3 by filling only in the high-temperature stage, using CO2 as a boiling secondary refrigerant/coolant in the low-temperature stage, and possibly to keep NH3 away from safety-requiring areas.
“With the thermodynamic simulation, the performance of the NH3/CO2 two-stage refrigeration system is calculated and analyzed by changing the mean temperature of evaporator-condenser between the two-stage system and the temperature of the condenser, which provides a theoretic basis for the optimal designation and practical operation of a cascading refrigeration system.”
— Peter Powell
Publication date: 09/02/2002