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.

Contents

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

Related Research Articles

<span class="mw-page-title-main">Heating, ventilation, and air conditioning</span> Technology of indoor and vehicular environmental comfort

Heating, ventilation, and air conditioning (HVAC) is the use of various technologies to control the temperature, humidity, and purity of the air in an enclosed space. Its goal is to provide thermal comfort and acceptable indoor air quality. HVAC system design is a subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer. "Refrigeration" is sometimes added to the field's abbreviation as HVAC&R or HVACR, or "ventilation" is dropped, as in HACR.

<span class="mw-page-title-main">Heat exchanger</span> Equipment used to transfer heat between fluids

A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contact. They are widely used in space heating, refrigeration, air conditioning, power stations, chemical plants, petrochemical plants, petroleum refineries, natural-gas processing, and sewage treatment. The classic example of a heat exchanger is found in an internal combustion engine in which a circulating fluid known as engine coolant flows through radiator coils and air flows past the coils, which cools the coolant and heats the incoming air. Another example is the heat sink, which is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant.

<span class="mw-page-title-main">Dehumidifier</span> Device which reduces humidity

A dehumidifier is an air conditioning device which reduces and maintains the level of humidity in the air. This is done usually for health or thermal comfort reasons, or to eliminate musty odor and to prevent the growth of mildew by extracting water from the air. It can be used for household, commercial, or industrial applications. Large dehumidifiers are used in commercial buildings such as indoor ice rinks and swimming pools, as well as manufacturing plants or storage warehouses. Typical air conditioning systems combine dehumidification with cooling, by operating cooling coils below the dewpoint and draining away the water that condenses.

<span class="mw-page-title-main">Chiller</span> Machine that removes heat from a liquid coolant via vapor compression

A chiller is a machine that removes heat from a liquid coolant via a vapor-compression, adsorption refrigeration, or absorption refrigeration cycles. This liquid can then be circulated through a heat exchanger to cool equipment, or another process stream. As a necessary by-product, refrigeration creates waste heat that must be exhausted to ambience, or for greater efficiency, recovered for heating purposes. Vapor compression chillers may use any of a number of different types of compressors. Most common today are the hermetic scroll, semi-hermetic screw, or centrifugal compressors. The condensing side of the chiller can be either air or water cooled. Even when liquid cooled, the chiller is often cooled by an induced or forced draft cooling tower. Absorption and adsorption chillers require a heat source to function.

<span class="mw-page-title-main">Refrigerator</span> Appliance for cold food storage

A refrigerator, colloquially fridge, is a commercial and home appliance consisting of a thermally insulated compartment and a heat pump that transfers heat from its inside to its external environment so that its inside is cooled to a temperature below the room temperature. Refrigeration is an essential food storage technique around the world. The lower temperature lowers the reproduction rate of bacteria, so the refrigerator reduces the rate of spoilage. A refrigerator maintains a temperature a few degrees above the freezing point of water. The optimal temperature range for perishable food storage is 3 to 5 °C. A similar device that maintains a temperature below the freezing point of water is called a freezer. The refrigerator replaced the icebox, which had been a common household appliance for almost a century and a half. The United States Food and Drug Administration recommends that the refrigerator be kept at or below 4 °C (40 °F) and that the freezer be regulated at −18 °C (0 °F).

Difluoromethane, also called difluoromethylene, HFC-32Methylene Fluoride or R-32, is an organic compound of the dihalogenoalkane variety. It has the formula of CH2F2. It is a colorless gas in the ambient atmosphere and is slightly soluble in the water, with a high thermal stability. Due to the low melting and boiling point, (-136.0 °C and -51.6 °C respectively) contact with this compound may result in frostbite. In the United States, the Clean Air Act Section 111 on Volatile Organic Compounds (VOC) has listed difluoromethane as an exception (since 1997) from the definition of VOC due to its low production of tropospheric ozone. Difluoromethane is commonly used in endothermic processes such as refrigeration or air conditioning.

<span class="mw-page-title-main">Air handler</span> Device used to regulate and circulate air as part of an HVAC system

An air handler, or air handling unit, is a device used to regulate and circulate air as part of a heating, ventilating, and air-conditioning (HVAC) system. An air handler is usually a large metal box containing a blower, furnace or A/C elements, filter racks or chambers, sound attenuators, and dampers. Air handlers usually connect to a ductwork ventilation system that distributes the conditioned air through the building and returns it to the AHU, sometimes exhausting air to the atmosphere and bringing in fresh air. Sometimes AHUs discharge (supply) and admit (return) air directly to and from the space served without ductwork

<span class="mw-page-title-main">Absorption refrigerator</span> Refrigerator that uses a heat source

An absorption refrigerator is a refrigerator that uses a heat source to provide the energy needed to drive the cooling process. Solar energy, burning a fossil fuel, waste heat from factories, and district heating systems are examples of convenient heat sources that can be used. An absorption refrigerator uses two coolants: the first coolant performs evaporative cooling and then is absorbed into the second coolant; heat is needed to reset the two coolants to their initial states. Absorption refrigerators are commonly used in recreational vehicles (RVs), campers, and caravans because the heat required to power them can be provided by a propane fuel burner, by a low-voltage DC electric heater or by a mains-powered electric heater. Absorption refrigerators can also be used to air-condition buildings using the waste heat from a gas turbine or water heater in the building. Using waste heat from a gas turbine makes the turbine very efficient because it first produces electricity, then hot water, and finally, air-conditioning—trigeneration.

<span class="mw-page-title-main">Forced-air</span> HVAC system

A forced-air central heating system is one which uses air as its heat transfer medium. These systems rely on ductwork, vents, and plenums as means of air distribution, separate from the actual heating and air conditioning systems. The return plenum carries the air from several large return grills (vents) to a central air handler for re-heating. The supply plenum directs air from the central unit to the rooms which the system is designed to heat. Regardless of type, all air handlers consist of an air filter, blower, heat exchanger/element/coil, and various controls. Like any other kind of central heating system, thermostats are used to control forced air heating systems.

Economizers, or economisers (UK), are mechanical devices intended to reduce energy consumption, or to perform useful function such as preheating a fluid. The term economizer is used for other purposes as well. Boiler, power plant, heating, refrigeration, ventilating, and air conditioning (HVAC) uses are discussed in this article. In simple terms, an economizer is a heat exchanger.

<span class="mw-page-title-main">Vapor-compression refrigeration</span> Refrigeration process

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<span class="mw-page-title-main">Air source heat pump</span> Most common type of heat pump

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<span class="mw-page-title-main">Thermal expansion valve</span> Component of air conditioning and refrigeration systems

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<span class="mw-page-title-main">Slurry ice</span> Refrigerant

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<span class="mw-page-title-main">Direct exchange geothermal heat pump</span>

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<span class="mw-page-title-main">Freeze stat</span> Temperature sensing device

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<span class="mw-page-title-main">International Institute of Refrigeration</span> Company

The International Institute of Refrigeration (IIR), is an independent intergovernmental science and technology-based organization which promotes knowledge of refrigeration and associated technologies and applications on a global scale that improve quality of life in a cost-effective and environmentally sustainable manner, including:

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References

  1. ↑ "StackPath". machinedesign.com. Retrieved 2023-01-04.
  2. ↑ "Benefits of reduced diameter copper tubes in evaporators and condensers. - Free Online Library". www.thefreelibrary.com. Retrieved 2023-01-04.
  3. 1 2 3 "Assembly Magazine | Manufacturing automation and design | ASSEMBLY". www.assemblymag.com. Retrieved 2023-01-04.
  4. 1 2 3 Hipchen, John (2010). “Small Tubes of Copper in ACR Applications," Webinar presented by ACHR News and Appliance Design.
  5. 1 2 3 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. 1 2 3 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.; et al. (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. PMC   3378845 . 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. Bibcode:2017JPhCS.923a2016L. doi: 10.1088/1742-6596/923/1/012016 .
  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" (PDF). Science and Technology for the Built Environment. 23 (6): 933–945. doi:10.1080/23744731.2017.1300012. hdl:11577/3232958. S2CID   136381840.
  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. 1 2 3 4 5 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. 1 2 Microgroove™ Update Newsletter: Volume 2, Issue 1, January 2012: http://www.microgroove.net/sites/default/files/4600_microgroove_nl_jan2012_2.pdf
  25. 1 2 3 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. 1 2 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 Archived 2012-04-17 at the Wayback Machine
  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. 1 2 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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