Physics:Internally grooved copper tube

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Internally grooved copper tubes, also known as "microfin tubes", are a small diameter coil technology for modern air conditioning and refrigeration systems. Grooved coils facilitate more efficient heat transfer than smooth coils.[1][2] Small diameter coils have better rates of heat transfer than conventionally-sized condenser and evaporator coils with round copper tubes and aluminum or copper fin that have been the standard in the HVAC industry for many years. Small diameter coils can withstand the higher pressures required by the new generation of environmentally friendlier refrigerants. They have lower material costs because they require less refrigerant, fin, and coil materials. They enable the design of smaller and lighter high-efficiency air conditioners and refrigerators because the evaporator and condenser coils are smaller and lighter.

With MicroGroove technology, heat transfer is enhanced by grooving the inside surface of the tube. This increases the surface to volume ratio, mixes the refrigerant, and homogenizes refrigerant temperatures across the tube.[3][4][5][6]

Tubes with MicroGroove technology can be made with copper or aluminium. Copper fins are an attractive alternative to aluminium due to the better corrosion resistance of copper and its antimicrobial benefits.[7][8][9][10][11]

Design

To use smaller tubes instead of conventional-sized tubes in air conditioners, heat exchangers must be redesigned including the fin and tube circuits.[12] Design optimization requires the use of computational fluid dynamics to analyze airflow around the tubes and fins, as well as computer simulations of refrigerant flow and temperatures inside the tubes. This is important because the overall heat transfer coefficient of a coil is a function of the convection of the refrigerant inside the tube to the tube wall, conduction through the tube wall, and dissipation through the fins.[13][14][15]

Engineering considerations for using Microgroove include:

  1. Determining the best ratio of transverse tube pitch to longitudinal tube pitch by fin efficiency analysis.[16]
  2. Optimizing transverse and longitudinal tube pitch by performance analysis and material cost.[16]
  3. Optimizing fin pattern by comparing performances of fins with different patterns through computational fluid dynamics-based simulations.[16]
  4. Testing the performance of heat exchangers with smaller diameter tubes.[16]
  5. Developing empirical equations for predicting performance of heat exchangers with smaller diameter tubes.[16]

Published experiments on MicroGroove coil performance and energy efficiency take into account the effects of fin spacing and fin design, tube diameter, and tube circuitry.[17] Tube circuitry is substantially different than for conventional coils. Coils should be optimized with respect to the number of paths between the inlet and outlet manifolds. Typically, smaller diameter tubes require more paths of shorter lengths. Published research on tube circuitry[18] and fin design for heat exchangers made with 4 mm tubes[19] are available.

Research on a heat exchanger redesign with 5-mm diameter tubes demonstrated a 5% greater heat exchange capacity than that of the same size heat exchanger with 7 mm diameter tubes. Also, the refrigerant charge of the 5 mm diameter tubes was less than the 7 mm diameter tubes.[20] In China, Chigo, Gree, and Kelon are producing air conditioners with coils that have 5 mm diameter tubes.[21]

A variety of fin designs have been developed for use with small-diameter copper tubes. The performance of slotted and louvered fin designs have been evaluated and compared as a function of various fin dimensions. Simulations have been used to optimize fin design performance.[22]

Refrigerants

The phasing out of CFC and HCFC refrigerants (e.g., HCFC-22, also known as R22) due to global warming concerns has helped to spur innovations in cooling technologies.[23][24] Natural refrigerants such as carbon dioxide (R744) and propane (R290), as well as R-410A, have become attractive replacements for air conditioning and refrigeration applications.

Higher pressures are typically required to condense these new environmentally friendly refrigerants compared to those that are being phased out. Small diameter copper tubes are more desirable in applications with higher pressures. For tubes of the same thickness, smaller diameter tubes can withstand higher pressures than larger diameter tubes.[3][4][5][6][25] Hence, as tube diameters decrease, burst pressures increase. This is because working pressure is directly proportional to wall thickness and inversely proportional to diameter. By designing coils with shorter tube lengths, less work is required to circulate the refrigerant. Therefore, refrigerant pressure drop factors due to small diameter tubes can be offset.

Carbon dioxide (R744) refrigerants are used in modern vending machines, refrigerated supermarket display cases, ice-skating rinks, and other emerging applications.[26][27] Microgroove's smaller diameter copper tubes have the strength to withstand the very high gas cooler and burst pressures of R744 while allowing for lower overall refrigerant volumes.[28]

Propane (R290) is an eco-friendly refrigerant with outstanding thermodynamic properties.[29][26] The pressure requirements for R290 are much less than for carbon dioxide, but R290 is extremely flammable. Research has demonstrated that MicroGroove is suitable for R290-charged room air conditioners because the refrigerant charge requirement is dramatically reduced with smaller diameter copper tubes. The risk of tube explosions is dramatically reduced as well.[30][31] Research conducted with propane in MicroGroove has implications for heat exchanger coils used in refrigerators, heat pumps and commercial air conditioning systems.[32]

Weight savings

In a design study of functionally equivalent 5 kW HVAC heat exchangers, tube materials in the coils weighed 3.09 kg for 9.52 mm diameter tube, 2.12 kg for 7 mm diameter tube, and 1.67 kg for 5 mm diameter tube. Tube weight was reduced by 31% when copper tube diameters were downsized from 3/8 inch to 7 mm. Tube weight was reduced by 46% when copper tube diameters were downsized from 3/8 inch to 5 mm. The weights of the fin materials in the coils was 3.55 kg for the 9.52 mm coils, 2.61 kg for the 7 mm coils, and 1.55 kg for the 5 mm coils.[3][4][5][6][33]

Antimicrobial

Copper is an antimicrobial material. Bio buildup can be reduced with copper coils. This helps to maintain high levels of energy efficiency for longer periods of time and avoids energy efficiency drop off over time.

The use of copper coils to inhibit the growth of fungi and bacteria is a recent development in innovative air conditioning and refrigeration products. OEM companies, such as Chigo in China and Hydronic in France, are now manufacturing all-copper antimicrobial air conditioning systems to improve indoor air quality.[24]

Materials

Smaller diameter refrigerant paths can also be realized with extruded aluminium tubes. These have been designed with several microchannels in one flat, ribbon-like tube. Aluminium microchannel technology offers significant advantages over conventional copper-aluminium round tube plate fin coil, including improved heat transfer performance and reduced refrigerant charge.[34] However, copper MicroGroove offers higher heat transfer efficiencies than aluminium microchannel tubes and it enables smaller refrigerant volumes because the tube ends of MicroGroove are connected by small U-joints rather than large headers.[35]

Manufacturing

Copper tubes are often produced by a cast and roll process. Copper ingots are cast into mother tubes and these tubes are then drawn to a final shape, annealed, and enhanced with an inner surface texture to improve heat transfer performance. The production of small diameter copper tubes requires only the addition of one or two additional drawing passes to achieve 5 mm tube diameters.[36][37]

Existing air conditioner coils made of round copper tubes and aluminium fins (CTAF coils) typically are mechanically assembled using tube expansion.[37][25]

The equipment used in manufacturing Microgroove products expands the tubes circumferentially (i.e., the circumference of the tube is increased without changing the length). This "non-shrinkage" expansion allows for better control of tube lengths in preparation for subsequent assembly operations. Tubes are inserted, or laced, into the holes in a stack of precisely spaced fins. Expanders are inserted into the tubes and the tube diameters are increased slightly until mechanical contact is achieved between the tubes and fins. The high ductility of copper allows for this process to be performed accurately and precisely. Heat exchanger coils made in this manner have excellent durability and heat transfer properties.[38][39]

The small-diameter tube project in China involves manufacturers who together account for more than 80 percent of HVAC production of approximately 75 million units. Several OEMs in North America are marketing residential air-conditioner products with copper tubes.[25] Air-conditioner OEMs, including Guangdong Chigo Air Conditioning,[40] the Refrigeration Research Institute of Guangdong Midea Refrigeration Appliances Group,[41] and Shanghai Golden Dragon Refrigeration Technology Co., Ltd.[42] have described the benefits of small-diameter copper tubes versus the standard for various designs and diameters. ACR coils from original equipment manufacturers (OEMs) Gree, Haier, Midea, Chigo and HiSense Kelon are also available.[43]

See also

Further reading

  • Simulation-Based Comparison of Optimized AC Coils Using Small Diameter Copper and Aluminium Microchannel Tubes, by John Hipchen, Robert Weed, Ming Zhang, Dennis Nasuta (2012). The Fourteenth International Refrigeration and Air Conditioning Conference; July 2012; (Purdue)

References

  1. ↑ "StackPath". http://machinedesign.com/news/cool-technology-small-copper-tubes-make-big-impact-air-conditioner-efficiency/. 
  2. ↑ "Benefits of reduced diameter copper tubes in evaporators and condensers. - Free Online Library". https://www.thefreelibrary.com/Benefits+of+reduced+diameter+copper+tubes+in+evaporators+and...-a0272754902. 
  3. ↑ 3.0 3.1 3.2 "Assembly Magazine | Manufacturing automation and design | ASSEMBLY". https://www.assemblymag.com/. 
  4. ↑ 4.0 4.1 4.2 Hipchen, John (2010). “Small Tubes of Copper in ACR Applications," Webinar presented by ACHR News and Appliance Design.
  5. ↑ 5.0 5.1 5.2 Ding, Guoliang et al. (2010). “Condensation Heat Transfer Characteristic of R410A-oil Mixture Inside Small Diameter Smooth Copper Tubes,” ASME-ATI-UIT Conference on Thermal and Environmental Issues in Energy Systems, Sorrento, Italy, May 2010.
  6. ↑ 6.0 6.1 6.2 Ding, Guoliang et al. (2010). “Two-phase Heat Transfer Characteristics of R410A-oil Mixture Flow Condensation Inside Small Diameter Microfin Copper Tubes,” Conference on Thermal and Environmental Issues in Energy Systems, Sorrento, Italy, May 2010.
  7. ↑ "Exploring the wonderful world of antimicrobial coils". Air Conditioning & Refrigeration News 29 (13): 34. 2013. 
  8. ↑ Weaver, L; Michels, HT; Keevil, CW (January 2010). "Potential for preventing spread of fungi in air-conditioning systems constructed using copper instead of aluminium". Lett. Appl. Microbiol. 50 (1): 18–23. doi:10.1111/j.1472-765X.2009.02753.x. PMID 19943884. 
  9. ↑ Schmidt, Michael G. (2012). "Characterization and Control of the Microbial Community Affiliated with Copper or Aluminum Heat Exchangers of HVAC Systems". Current Microbiology 65 (2): 141–149. doi:10.1007/s00284-012-0137-0. PMID 22569892. 
  10. ↑ Longo, G. A.; Mancin, S.; Righetti, G.; Zilio, C. (2017). "R245fa Flow Boiling inside a 4.2 mm ID Microfin Tube". Journal of Physics: Conference Series 923 (1): 012016. doi:10.1088/1742-6596/923/1/012016. Bibcode: 2017JPhCS.923a2016L. 
  11. ↑ Longo, Giovanni A.; Mancin, Simone; Righetti, Giulia; Zilio, Claudio; Doretti, Luca (2017). "Saturated R134a flow boiling inside a 4.3 mm inner diameter microfin tube". Science and Technology for the Built Environment 23 (6): 933–945. doi:10.1080/23744731.2017.1300012. http://paduaresearch.cab.unipd.it/11186/1/Longo_et_al_STBE_2017.pdf. 
  12. ↑ Principle of Designing Fin-and-Tube Heat Exchanger With Smaller Tubes for Air Conditioner Wei Wu, Guoliang Ding, Yongxin Zheng, Yifeng Gao, Ji Song; International Refrigeration and Air Conditioning Conference at Purdue, July 16–19, 2012 http://www.conftool.com/2012Purdue/index.php?page=browseSessions&abstracts=show&mode=list&search=2223
  13. ↑ Ding, G.L., Ren, T., Zheng, Y.X., and Gao, Y.F. (2011). Simulation-Based Design Method for Room Air Conditioner with Smaller Diameter Copper Tubes, 23rd IIR International Congress of Refrigeration; Prague, Czech Republic; July; Paper 2232.
  14. ↑ Wu, W., Ding, G.L., Zheng, Y.X., Gao, Y.F. and Song, J. (2012). Principle of Designing Fin-And-Tube Heat Exchanger with Smaller Diameter Tubes for Air Conditioner; 14th International Refrigeration and Air Conditioning Conference, Purdue Conferences, West Lafayette, Indiana; July; Paper 1217; https://docs.lib.purdue.edu/iracc/1217/
  15. ↑ Cotton, Nigel and Zheng, Wenson (2012). Designing for Efficient Heat Transfer; Appliance Design, July; http://www.appliancedesign.com/articles/93186-designing-for-efficient-heat-transfer
  16. ↑ 16.0 16.1 16.2 16.3 16.4 New Research Underscores Best Practices in Coil Design; Microgroove™ Update Newsletter: Volume 2, Issue 3, June 2012; http://www.microgroove.net/sites/default/files/microgroove_nl_june2012b.pdf
  17. ↑ G.L. Ding et al. (YEAR), 23rd IIR International Congress of Refrigeration, Prague, August 23–26.
  18. ↑ Wei-kun Ding et al. (Xi’an and ICA) "Development of Small-diameter Tube Heat Exchanger: Circuit Design and Performance Simulation," presented at the Conference on Thermal and Environmental Issues in Energy Systems, Sorrento, Italy, May 2010 (CTEI-ES 2010)
  19. ↑ Ju-fang Fan et al.; "Development of Small-diameter Tube Heat Exchanger: Fin Design and Performance Research," presented at CTEI-ES 2010
  20. ↑ Principle of Designing Fin-and-Tube Heat Exchanger With Smaller Tubes for Air Conditioner; Wei Wu, Guoliang Ding, Yongxin Zheng, Yifeng Gao, Ji Song; International Refrigeration and Air Conditioning Conference at Purdue, July 16–19, 2012; http://www.conftool.com/2012Purdue/index.php?page=browseSessions&abstracts=show&mode=list&search=2223
  21. ↑ Cool Technology: Small copper tubes make a big impact on air conditioner efficiency; MachineDesign.com; August 23, 2012; http://machinedesign.com/article/cool-technology-small-copper-tubes-make-a-big-impact-on-air-conditioner-efficiency-0823?page=0%2C3
  22. ↑ Ju-fang Fan, Wei-kun Ding, Wen-quan Tao, Wenson Zheng, Frank Gao, and Kerry Song; Development of small-diameter tube heat exchanger: Fin design and performance research.
  23. ↑ Shabtay, Yoram, Black, J. and Kraft, Frank (2014). New Copper-based Heat Exchangers for Alternative Refrigerants, Fifteenth International Refrigeration and Air Conditioning Conference; Purdue Conferences in West Lafayette, Indiana; Paper 1532; https://docs.lib.purdue.edu/iracc/1532/
  24. ↑ 24.0 24.1 Microgroove™ Update Newsletter: Volume 2, Issue 1, January 2012: http://www.microgroove.net/sites/default/files/4600_microgroove_nl_jan2012_2.pdf
  25. ↑ 25.0 25.1 25.2 FAQs: Thirty Questions with Answers about Economical, Eco-friendly Copper Tubes for Air Conditioner Applications; http://www.microgroove.net/sites/default/files/overview-ica-questions-and-answers-qa30.pdf
  26. ↑ 26.0 26.1 Microgroove™ Update Newsletter: Volume 1, Issue 3, December 2011: http://www.microgroove.net/sites/default/files/4473_ica_microgroove_nl_final.pdf
  27. ↑ Filippini S., Merlo U. (2011). Air Cooled Heat Exchangers For CO2 Refrigeration Cycles; 23rd IIR International Congress of Refrigeration; Prague, Czech Republic; July; Paper 2232.
  28. ↑ "Natural Refrigerant CO2," a handbook edited by Walter Reulens, ATMOsphere 2009. Compressors are described in Section 8.1 (pp. 348-382) and heat exchangers are described in section 8.2 (pp. 383-410) of the Handbook, available online free of charge as a PDF here: www.atmosphere2009.com/files/NaReCO2-handbook-2009.pdf
  29. ↑ Ding, Guoliang et al., (2012). Developing Low Charge R290 Room Air Conditioner By Using Smaller Diameter Copper Tubes, 10th IIR Gustav Lorentzen Conference on Natural Refrigerants; Delft, The Netherlands; Paper 183
  30. ↑ "Principle of Designing Fin-And-Tube Heat Exchanger with Smaller Diameter Tubes for Air Conditioner" by Wei Wu, Guoliang Ding, Yongxin Zheng, Yifeng Gao and Ji Song, The Fourteenth International Refrigeration and Air Conditioning Conference, Purdue University, July 2012; http://www.conftool.com/2012Purdue/index.php?page=browseSessions&abstracts=show&mode=list&search=2223
  31. ↑ Read also: "Developing Low Charge R290 Room Air Conditioner by Using Smaller Diameter Copper Tubes" by Guoliang Ding, Wei Wu, Tao Ren, Yongxin Zheng, Yifeng Gao, Ji Song, Zhongmin Liu and Shaokai Chen; The Tenth IIR Gustav Lorentzen Conference on Natural Refrigerants, June 2012 (GLC)
  32. ↑ Professor Guoliang Ding of Shanghai Jiao Tong University (SJTU) Presents New Research on Smaller Diameter Copper Tubes at the Tenth IIR Gustav Lorentzen Conference on Natural Refrigerants; June 26, 2012; http://www.microgroove.net/press/professor-guoliang-ding-shanghai-jiao-tong-university-sjtu-presents-new-research-smaller-diame
  33. ↑ FAQs: Thirty Questions with Answers about Economical, Eco-friendly Copper Tubes for Air Conditioner Applications; http://www.microgroove.net/sites/default/files/overview-ica-questions-and-answers-qa30.pdf
  34. ↑ Q&A: Microchannel air-cooled condenser; Heatcraft Worldwide Refrigeration; April 2011; http://www.heatcraftrpd.com/landing/2011/air-cooled-condenser/res/pdfs/H-ACCMCX-QA.pdf
  35. ↑ Filippini, S., (2010). A new heat exchanger geometry for next condenser generation with ultra low refrigerant charge; IIR Second Workshop on Refrigerant Charge Reduction Conference; Stockholm, Sweden; June.
  36. ↑ Cotton, Nigel (2014). Smaller Diameter-Copper Tubes Support Manufacturing and Design: A report from the 2014 Purdue Conference; International Appliance Manufacturing; October.
  37. ↑ 37.0 37.1 Shabtay, Yoram and Cotton, Nigel (2015). Trends in the Design and Manufacture of Round-Tube Plate-Fin Coils from Smaller Diameter Copper Tubes; International Appliance Manufacturing; October.
  38. ↑ Cotton, Nigel (2013). Building Better Appliances with Smaller-Diameter Copper Tubes; International Appliance Manufacturing, October 23;. http://www.appliancedesign.com/articles/93807-building-better-appliances-with-smaller-diameter-copper-tubes
  39. ↑ Shabtay, Yoram and Cotton, Nigel (2015). Trends in the Design and Manufacture of Round-Tube Plate-Fin Coils from Smaller Diameter Copper Tubes; International Appliance Manufacturing, October.
  40. ↑ You Shunyi et al. (Guangdong Chigo Air Conditioning Co.) "Application of Small Diameter Inner-Grooved Copper Tubes in Air-Conditioning Systems," presented at Second IIR Workshop on Refrigerant Charge Reduction, Stockholm Sweden, June 2010 (RCR 2010)
  41. ↑ Jia Qingxian et al. "Experimental Research for Lowering Refrigerant Charge with 4 mm Tube Heat Exchanger," presented at RCR 2010
  42. ↑ Wu Yang et al. "Performance and Cost Analysis and Research of Air-Cooled Heat Exchanger Using Small Diameter Copper Tubes," presented at RCR 2010
  43. ↑ The China Refrigeration Expo (CR-2011) in Shanghai, China; in Microgroove™ Update Newsletter: Volume 1, Issue 1, May 2011: http://www.microgroove.net/sites/default/files/microgroove_nl_issue_1.pdf



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