Advantages of using copper tubing for R744 refrigerant

By R744.com team, Jan 12, 2012, 12:33 3 minute reading

According to a press release from the International Copper Association (ICA) the benefits of using copper tubes, and in particular small copper tubes, for natural refrigerant applications include high conductivity, high burst strength and low tube volume. Hoop strain hardening is another critical advantage, highlighted in an achrnews.com webinar.

Thanks to technological improvements with respect to high-pressure compressors and high-pressure expansion valves (HPEVs) the use of R744 as a climate-friendly refrigerant in a variety of applications has become feasible. Within R744 systems copper has proven to be durable and reliable. According the achrnews.com webinar, the unique benefits of using copper include:
  • Proven durability & reliability
  • Workability in annealed state
  • Corrosion resistance
  • Anti-microbial properties
  • Superior heat transfer coefficient: Stainless Steel (SS): 25 W/mK; Aluminum (Al): 237 W/mK; Copper (Cu): 398 W/mK
  • Strong brazed joints
  • Ability to make field repairs
MicroGroove and ICA highlight suitability of small diameter copper tubes for high pressures in R744 transcritical refrigeration cycle
 
In the transcritical refrigeration cycle pioneered by Gustav Lorentzen the compressor does not condense the gas into a liquid but instead cools the gas. The pressure in the heat exchangers for this step, called “gas coolers” is very high, in the range of 120 bar. The burst pressure ratings may be several times higher, whilst pressures for the evaporation step is typically in the 45 to 60 bar range.
 
Heat exchanger coil technology with copper tubes is well suited for both R744 gas cooler and evaporator applications. Since burst pressure increases as tube diameter decreases smaller tubes are particularly appropriate for R744 applications. In addition, because the heat transfer coefficient is high for R744, typically the tubes for R744 do not require inner grooving.
 
Copper alloy hardened by iron
 
Gas cooler tubes can be made of a special copper alloy hardened by small percentages of iron, as demonstrated in a paper presented at the 2011 IIR International Congress of Refrigeration (ICR) in Prague by LU-VE S.p.A. in Uboldo, Italy, a major manufacturer of air conditioning and refrigeration products. The ICR paper discusses the design characteristics, including material selection required for the 120 bar working pressure of a CO2 gas cooler. In this study the evaporator coil was an off-the-shelf design also made with copper tubes.
 
Hoop strain hardening allows copper piping systems support high-pressure refrigerants 
 
Copper’s ability to strain harden and strengthen to fit the environment is another key benefit of the material, highlighted in an achrnews.com webinar entitled “How copper piping systems support high pressure refrigerants”, presented by Chris Mueller, Head of Product Development Mueller Industries.
 
When an annealed copper system is subjected to high pressure the copper expands in diameter. This radial expansion creates strain hardening as the grains are realigned. The strain hardened copper is stronger as a result of the grain realignment. Further strain hardening will continue until the stronger system is stabilised at the pressure used. With fully annealed copper this phenomenon can increase strength by up 7 times.
 
Importance of grain size 
 
In copper piping the average grain size is critical to performance. Comparing two samples of copper with grain sizes of 0.070mm and 0.055mm, the 0.055mm grain size improves performance by 20%. 
 
About ICA
 
The International Copper Association, Ltd. (ICA) is the leading organization for promoting the use of copper worldwide. ICA’s mission is to promote the use of copper by communicating the unique attributes that make this sustainable element an essential contributor to the formation of life, to advances in science and technology, and to a higher standard of living worldwide. Visit www.copperinfo.com for more information about ICA.

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By R744.com team (@r744)

Jan 12, 2012, 12:33




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