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Advantages of small diameter tubes in transcritical refrigeration cycles

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Yoram Shabtay, Heat Transfer Technologies

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Advantages of small diameter tubes in transcritical refrigeration cycles

  1. 1. Advantages of Small Diameter Tubes in Transcritical Refrigeration Cycles Heat Transfer Technologies
  2. 2. June 17-18, 2019 Atlanta, GA 1. Transcritical R744 Refrigeration 2. Gas Cooler Improvements 3. Copper Tubes in Gas Coolers 4. Gas Cooler Examples 5. Cu-Fe Alloy for Transmission Lines 6. Conclusions Overview
  3. 3. 1. Transcritical R744 Refrigeration Two Stage Booster Transcritical System Source: NREL https://www.nrel.gov/docs/fy15osti/63821.pdfJune 17-18, 2019 Atlanta, GA
  4. 4. June 17-18, 2019 Atlanta, GA Source: NREL https://www.nrel.gov/docs/fy15osti/63821.pdf Close up of gas cooler, high- pressure discharge gas and high-pressure cooled gas 1. Transcritical R744 Refrigeration
  5. 5. June 17-18, 2019 Atlanta, GA Source: https://www.nrel.gov/docs/fy15osti/63821.pdf 1. Transcritical R744 Refrigeration Critical Pressure 72.9 atm 7.39 Mpa 1,071 psi 73.9 bar Critical Temperature 304.25 K 31.10 °C 87.98 °F Above the critical T and critical P, CO2 is a supercritical fluid (SCF) with liquid-like density but gas-like transport properties.
  6. 6. • Reduce tube diameter • Increase number of branch circuits • Efficient fin design • Use modeling to optimize components and complete system under operating conditions (MOGA) 2. Gas Cooler Improvements June 17-18, 2019 Atlanta, GA
  7. 7. Reducing tube diameters from 9.53 mm (3/8”) or 8mm (5/16”) to 5 mm can bring at least: • 45% tube weight reduction (Wall) • 20% fin weight reduction • 45% reduction in internal volume Due to increase in internal and external Heat Transfer Coefficient (HTC) 2. Gas Cooler Improvements June 17-18, 2019 Atlanta, GA Source: Super Radiator Coil
  8. 8. 2. Gas Coolers Improvements • Compact design • Lower refrigerant charge • Higher pressure capable • High-strength copper alloy (CuFe2P) available for even higher pressure June 17-18, 2019 Atlanta, GA Source: Super Radiator Coil
  9. 9. 3. Copper Tubes in Gas Coolers Source: LU-VEJune 17-18, 2019 Atlanta, GA
  10. 10. 3. Copper Tubes in Gas Coolers June 17-18, 2019 Atlanta, GA Source: LU-VE
  11. 11. Value Units Tube dia. 9.52 mm Tube 5 mm Dimensions (mm) 2000 x 1200 x 69 Capacity (kW) 33.40 33.17 Header Volume (dm3) 0.74 0.74 Tube Volume (dm3) 25.08 7.13 Total Coil Internal Volume (dm3) 25.82 7.87 Internal Volume Difference -70% Coil Weight (kg) 78.06 45.71 Coil Weight Difference -41% Air Pressure Loss (Pa) 66.53 62.70 Air Pressure Loss Difference -5.8% 0 20 40 60 80 100 internal Volume Coil Weight Air Pressure Loss Source: LU-VE 4. R744 Gas Cooler Examples June 17-18, 2019 Atlanta, GA
  12. 12. Comparison of 5/16 inch vs 5 mm Using same Coil Dimensions: 18” x 37” (460 mm x 940 mm) 4. R744 Gas Cooler Examples Source: Super Radiator Coil 0 20 40 60 80 100 Tube Weight Fin Weight Total Internal Volume 5/16 inch 5 mmTube Weight -35% Fin Weight -21% Internal Volume -45% June 17-18, 2019 Atlanta, GA
  13. 13. 4. R744 Gas Cooler Examples CO2 Gas Cooler Unit 5 mm tube 5/16 inch Tube % Reduction Capacity BTU/h (kilowatt) 43,000 (12.6) 43,000 (12.6) Design Pressure PSIA (MPa) 1005 (68.4) 1005 (68.4) Coil Size Inch (mm) 18 x 37 (460 x 940) 18 x 37 (460 x 940) Rows 4 4 Fin Density fins / inch 15 12.5 Tube OD Inch (mm) 0.197 (5.0) 0.3125 (7.94) Tube Wall Inch (mm) 0.040 (1.0) 0.049 (1.25) Tube Weight Pounds (kg) 24.5 (11.1) 37.7 (17.1) 35% Fin Material aluminum aluminum Fin Thickness Inches (mm) 0.0039 (0.10) 0.0045 (0.114) Fin Weight Pounds (kg) 7.5 (3.4) 9.5 (4.3) 21% Total Internal Volume liter 1.2 2.2 45% Source: Super Radiator CoilJune 17-18, 2019 Atlanta, GA
  14. 14. 4. R744 Gas Cooler Examples Model Tube Material Design Pressure Test Pressure AG Copper 478 psi (33 bar) 681 psi (47 bar) AG-H (“H” = High Pressure for R410A refrigerant) Copper 652 psi (45 bar) 943 psi (65 bar) XG (“X” for CO2 refrigerant) Copper-Iron 1740 psi (120 bar) 2494 psi 172 bar VCM Copper 478 psi (33 bar) 681 psi (47 bar) VCX (“X” for CO2 refrigerant) Copper-Iron 1740 psi (120 bar) 2494 psi (172 bar) Source: Alfa LavalJune 17-18, 2019 Atlanta, GA
  15. 15. ↓41% Recommende d design MOGA R600a optimization results example, using 5 mm tubes instead of the baseline 6.35 mm tubes 4. R744 Gas Cooler Examples Baseline June 17-18, 2019 Atlanta, GA Source: Sub-Zero
  16. 16. • UNS Alloy C19400 • Chemical Composition (Next Slide) • Good thermal conductivity due to high copper content • Increased strength and temperature stability • Ultimate Tensile Strength 60 ksi vs. 36 ksi for std. copper • Corrosion resistance slightly improved What is Copper-Iron? 5. Cu-Fe Alloy for Transmission Lines Not 5 mm: Thicker Walls and High Strength Alloy Source: Japan Copper Development Association http://www.jcda.or.jp/english/tabid/117/Default.aspx
  17. 17. Copper-Iron vs. Copper UNS C19400 UNS C12200 Cu 97.0 – 97.8% 99.9% Min Fe 2.1 – 2.6% N/A Zn 0.05 – 0.20% N/A P 0.015 – 0.15% 0.015 – 0.04% UTS min., Hard 60 ksi 36 ksi YS min, Hard 46 ksi 30 ksi Elongation min., Hard 5% 8% UTS min., Soft 45 ksi 30 ksi YS min., Soft 15 ksi 9 ksi Elongation min., Soft 30% 45% 5. Cu-Fe Alloy for Transmission Lines June 17-18, 2019 Atlanta, GA
  18. 18. Piping throughout the store is mostly unchanged • Essentially subcritical CO2 • Protected by pressure regulating valve Piping from rooftop rack to rooftop gas cooler … and back • Often the only “transcritical” piping is beyond the rack • Designed for 1740 or 1860 psi • Heat reclaim is wise and can be mandatory by jurisdiction CO2 Transcritical 5. Cu-Fe Alloy for Transmission Lines Limited options for high pressure piping and protection devices 1. Welded carbon steel 2. Welded stainless steel 3. Brazed copper-iron alloy June 17-18, 2019 Atlanta, GA
  19. 19. Tube Type Wall Thickness Color Code Copper- Iron* Extra-heavy (C194) wall thickness Black ACR Type K Heavier copper (C122) wall thickness Green ACR Type L Standard copper (C122) wall thickness Blue Type M Plumbing tube for non-refrigerant applications Red 5. Cu-Fe Alloy for Transmission Lines June 17-18, 2019 Atlanta, GA
  20. 20. Typically pressure ratings decrease for larger diameters • Standard ACR Copper: 700 psi possible only up to 1-3/8 inches • Type K ACR Copper: 700 psi possible only up to 2-5/8 inches • Copper fittings: 700 psi possible only up to 2-5/8 inches Cu-Fe Tubes maintain 1740 or 1860 psi rating across all sizes. • Wall thickness increases rapidly as outer diameter increases • Hence, the wall thickness is greater for high-strength alloy at 1-5/8 inches than for ACR copper at 4-1/8 inches Pressure Rating Simplicity 5. Cu-Fe Alloy for Transmission Lines June 17-18, 2019 Atlanta, GA
  21. 21. • All installation tools and processes are common to standard copper piping • Brazing procedures are unchanged • Thicker material may require more heat. • Standard BCuP or BAg braze alloys are used, according to personal preference or as specified by OEM. • Many installers using 15 percent silver content 5. Cu-Fe Alloy for Transmission Lines June 17-18, 2019 Atlanta, GA
  22. 22. Braze copper alloys to steel manifolds or fittings • High-temperature flux • Use brazing rod with 40-50 percent silver content • Ideally with a nickel component (about 3 percent Ni) 5. Cu-Fe Alloy for Transmission Lines June 17-18, 2019 Atlanta, GA
  23. 23. Transmission line brazing summary: • Certified welders are not required • Copper-iron alloys allows ACR technicians to install, troubleshoot and fix the refrigeration systems The Right Long-term Solution 5. Cu-Fe Alloy for Transmission Lines June 17-18, 2019 Atlanta, GA
  24. 24. 1. The use of smaller-diameter copper tubes can result in lighter gas coolers and reduced refrigerant charge. 2. Additional branch circuits allow for high mass flow rates even as overall size and weight of the gas cooler is reduced. 3. UNS C19400 is a high-strength Cu-Fe alloy that has been adapted for use in gas coolers as well as high-pressure transmission lines. 4. UNS C19400 empowers commercial refrigeration technicians with the ability to install, service and modify all piping associated with transcritcal R744 systems. 6. Conclusions June 17-18, 2019 Atlanta, GA For more information visit www.microgroove.net
  25. 25. Supporting material R744 Source: Annex of IPCC (2005)(7) (Intergovernmental Panel on Climate Change)

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