2. Advantages of Small Diameter Tubes in
Transcritical Refrigeration Cycles
Heat Transfer Technologies

3. 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

4. 1. Transcritical R744 Refrigeration
Two Stage Booster
Transcritical System
Source: NREL
https://www.nrel.gov/docs/fy15osti/63821.pdfJune 17-18, 2019 Atlanta, GA

5. 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

6. 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.

7. • 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

8. 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

9. 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

10. 3. Copper Tubes in Gas Coolers
Source: LU-VEJune 17-18, 2019 Atlanta, GA

11. 3. Copper Tubes in Gas Coolers
June 17-18, 2019 Atlanta, GA Source: LU-VE

12. 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

13. 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

14. 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

15. 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

16. ↓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

17. • 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

18. 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

19. 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

20. 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

21. 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

22. • 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

23. 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

24. 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

25. 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

27. Supporting material R744
Source: Annex of IPCC (2005)(7) (Intergovernmental Panel on Climate Change)