LU-VE has experience producing heat exchangers for refrigeration and HVAC, especially with natural refrigerants like CO2. They presented on CO2 heat exchangers and their work on the NXTHPG project. Their CO2 test facility in Europe allows them to test heat exchangers under stationary conditions. Software predictions of capacity and pressure drop generally match test results well within 10%. Testing of CO2 evaporators showed capacity increases with Reynolds number as expected. Their 5mm tube CO2 coil for the NXTHPG project was smaller and lighter than the standard design while maintaining performance. CFD analysis helped optimize the fan configuration to increase COP by 2%.

1. High efficiency heat
exchangers for CO2
Mostra Convegno Comfort (MCE), Milano 18/03/2016
Stefano Filippini

2. LU-VE has a great experience in the production of heat
exchanger for refrigeration, air-conditioning and heat
pump application, especially with environmentally
friendly fluids.
This presentation focus on the following themes:
• CO2 heat exchangers
• Application of R&D to NXTHPG - Project

3. LU-VE CO2 test ring
•The test equipment is made up of a climatic room
operating at a constant temperature. It is the only plant
with such characteristics available in Europe dedicated
to heat exchangers.
•The exchanger to be tested is set inside the room.
•An auxiliary exchanger (counter effect) is also present in
the room to ensure perfectly stationary conditions.
Balance between capacity measured on test unit and
auxiliary exchanger.

4. Software prediction
Deviation between measured and predicted value is limited.
Capacity Pressure drop
0
10
20
30
40
50
60
0 10 20 30 40 50 60
EXPERIMENTALCAPACITY
THEORETICAL CAPACITY
3 rows 3
4 rows 2
4 rows 1
+10%
-10%
with software
calibration
0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
-60% -40% -20% 0% 20% 40% 60%
EXPERIMENTALPRESSUREDROP
ERROR PRESSURE DROP
3 rows 3
4 rows 2
4 rows 1
with software
calibration
EXPERIMENTALCAPACITY[kW]
EXPERIMENTALPRESSUREDROP[Bar]
ERROR PRESSURE DROPTHEORETICAL CAPACITY [kW]
Gas coolers:
test results in transcritical operation

5. Result of test campaign dedicated to CO2 evaporators
Evaporator: test results
0.500
0.700
0.900
1.100
1.300
1.500
9000 14000 19000 24000
RATIOCAPACITYCALC/
TEST
REYNOLDS NUMBER
0.500
0.600
0.700
0.800
0.900
1.000
1.100
1.200
1.300
1.400
1.500
0.00 0.20 0.40 0.60 0.80DTSUR/DT1

6. LU-VE is partecipating as industrial partner in the NXTHPG project.
Three finned heat exchangers have been provided:
• CO2 coil for Case 4
• CO2 coil for Case 5
• Propane coil for Case 1
NXTHPG project

7. NXTHPG project: Case 5
COIL TYPE:
• Refrigerant: CO2
• Fin pack dimensions:
2000x1200x69
• Tube diameter: 5,0 mm
• Numbers of tubes: 60
• Numbers of rows: 4
• Numbers of circuits: 40
• Fin spacing: 3,0 mm

8. Geometry
25x21,65
tube Ø9,52
Geometry 20x17,32
tube Ø5
Capacity [kW] 33,40 33,17
Tube diameter [mm] 10,12 5,00
Header volume [dm3] 0,74 0,74
Header diameter [mm] 28 28
Tubes volume [dm3] 25,08 7,13
Total coil internal volume [dm3] 25,82 7,87
Internal volume difference -69,5%
Coil weight [kg] 78,06 45,71
Coil weight difference -41,4%
Air pressure loss [Pa] 66,53 62,70
Air pressure loss difference -5,8%
Advantage of 5mm geometry - Case 5

9. Case 5: Comparison with expected results
Performance of the heat exchanger is generally in line with the power calculated with our in-house software.
It seems that the unit performs as we expect.
Data are greatly dispersed, this is due to experimental uncertainties (outlet quality for instance).

10. These pictures give an evaluation of air distribution on the
whole surface. There is an acceleration near the edges due
to the curvature of the fluid coming from the ambient. The
average velocities are higher for the modified solution,
moreover the zones where the highest velocities are
achieved for the two solutions (at the discharge of the heat
exchanger) are quite different: (i) for the improved solution
this zone is located at the top, (ii) for the standard solution is
located just above the lower half of the surface.
Velocity field at the inlet and at the outlet of
the heat exchanger, Case 4 and 5 NXTHPG
Modified solution: inlet (sx) – outlet (dx)Standard solution: inlet (sx) – outlet (dx)

11. Fan conveyor modifications, NXTHPG Case 5
 the evaporation pressure increases
New conveyor configuration
gives 2% COP increase

12. 1. After intensive R&D activity new coil geometry
Minichannel® with 5mm tube Ø has been developed
2. Measured data of coil performances confirm the
predicted data
3. CFD approach allows to improve in general product
efficiency, specially concerning air distribution
Conclusion