The document describes an improved oil level detection and equalization system for CO2 high pressure shell hermetic rotary compressors. It presents the working principle of using temperature measurements during fluid expansion to detect oil presence or absence. Test results on a CO2 booster system show the system can accurately detect oil levels in under 60 seconds for most tests. The system also modulates a valve to equalize oil levels between compressors, with testing showing this approach is effective at low compressor rotation speeds. In conclusion, the simple temperature-based approach provides reliable oil detection and equalization to help ensure safe compressor operation.

1. Carel Industries S.p.A.
Improved oil level detection and equalization system for
CO2 high pressure shell hermetic rotary compressor
Giovanni ROSSANESE, Luca MATTIELLO, Willy MUVEGI, Luigi NALINI
8th IIR Conference: Ammonia and CO2 Refrigeration Technologies, 2019, Ohrid, R. Macedonia

2. 8th IIR Conference: Ammonia and CO2 Refrigeration Technologies, 2019, Ohrid, R. Macedonia
Outline
• Introduction
• Working principle
• Test Rig Description
• Oil Presence Detection– Logic & Test Results
• Oil Equalization Procedure – Logic & Test Results
• Conclusions

3. 8th IIR Conference: Ammonia and CO2 Refrigeration Technologies, 2019, Ohrid, R. Macedonia
Introduction
• BLDC rotary compressors
→ efficiency and cost effectiveness
→ small/medium cooling capacities → compressor racks

4. 8th IIR Conference: Ammonia and CO2 Refrigeration Technologies, 2019, Ohrid, R. Macedonia
Introduction
• BLDC rotary compressors
→ efficiency and cost effectiveness
→ small/medium cooling capacities → compressor racks
• Low pressure and low temperature
• Oil is injected into the shell directly

5. 8th IIR Conference: Ammonia and CO2 Refrigeration Technologies, 2019, Ohrid, R. Macedonia
Introduction
• BLDC rotary compressors
→ efficiency and cost effectiveness
→ small/medium cooling capacities → compressor racks
• Low pressure and low temperature
• Oil is injected into the shell directly
• High pressure and high temperature
• Oil is withdrawn from the shell

6. 8th IIR Conference: Ammonia and CO2 Refrigeration Technologies, 2019, Ohrid, R. Macedonia
Working Principle
• Expansion of oil/CO2 mixture
2
4
6
8
10
12
200 300 400 500 600
Pressure[MPa]
Enthalpy [kJ/kg]
< 𝑇𝑖𝑛 𝐶𝑂2
𝑇𝑜𝑢𝑡

7. 8th IIR Conference: Ammonia and CO2 Refrigeration Technologies, 2019, Ohrid, R. Macedonia
Working Principle
• Expansion of oil/CO2 mixture
2
4
6
8
10
12
200 300 400 500 600
Pressure[MPa]
Enthalpy [kJ/kg]
< 𝑇𝑖𝑛 𝐶𝑂2
≈ 𝑇𝑖𝑛 𝑂𝑖𝑙
𝑇𝑜𝑢𝑡

8. 8th IIR Conference: Ammonia and CO2 Refrigeration Technologies, 2019, Ohrid, R. Macedonia
Working Principle
• Expansion of oil/CO2 mixture
• It requires flow control devices
• Modulating valve is installed at the
compressor oil balance pipe
𝑙𝑜𝑤 𝑓𝑙𝑜𝑤 → 𝑑𝑒𝑡𝑒𝑐𝑡𝑖𝑜𝑛
ℎ𝑖𝑔ℎ 𝑓𝑙𝑜𝑤 → 𝑒𝑞𝑢𝑎𝑙𝑖𝑧𝑎𝑡𝑖𝑜𝑛
2
4
6
8
10
12
200 300 400 500 600
Pressure[MPa]
Enthalpy [kJ/kg]
< 𝑇𝑖𝑛 𝐶𝑂2
≈ 𝑇𝑖𝑛 𝑂𝑖𝑙
𝑇𝑜𝑢𝑡

9. 8th IIR Conference: Ammonia and CO2 Refrigeration Technologies, 2019, Ohrid, R. Macedonia
Test Rig Description
• CO2 booster system with air-cooled gas
cooler
• Standard industrial control boards,
equipment and instrumentations
• MT/LT – 30 kW/6kW
• PAG oil ISO VG100
• Sight glasses for visual inspection

10. 8th IIR Conference: Ammonia and CO2 Refrigeration Technologies, 2019, Ohrid, R. Macedonia
Test Rig Description

11. 0
10
20
30
40
50
60
70
80
90
100
0:00 0:30 1:00 1:30 2:00 2:30 3:00 3:30 4:00 4:30 5:00 5:30 6:00 6:30 7:00
Temperature[°C]
DeltaTemperature[K]
Time [mm:ss]
Oil Valve Inlet Temp
Oil Valve ΔT calc
8th IIR Conference: Ammonia and CO2 Refrigeration Technologies, 2019, Ohrid, R. Macedonia
Oil Presence Detection
• Control predicts the pure CO2 temperature drop at given pressures
by modelling an isenthalpic expansion
(Inputs) 𝑇𝑖𝑛, 𝑃𝑑𝑖s, 𝑃𝑠𝑢𝑐𝑡 → (Output) Δ𝑇𝑐𝑎𝑙𝑐
Rot speed [Hz] = 43 – 48
𝑃𝑑𝑖𝑠𝑐ℎ [MPa] = 7.8
𝑃𝑠𝑢𝑐𝑡 [MPa] = 2.8

12. 0
10
20
30
40
50
60
70
80
90
100
0:00 0:30 1:00 1:30 2:00 2:30 3:00 3:30 4:00 4:30 5:00 5:30 6:00 6:30 7:00
Temperature[°C]
DeltaTemperature[K]
Time [mm:ss]
Oil Valve Inlet Temp
Oil Valve ΔT exp
Oil Valve ΔT calc
Threshold ΔTcalc/n
8th IIR Conference: Ammonia and CO2 Refrigeration Technologies, 2019, Ohrid, R. Macedonia
Oil Presence Detection
• Control predicts the pure CO2 temperature drop at given pressures
by modelling an isenthalpic expansion
(Inputs) 𝑇𝑖𝑛, 𝑃𝑑𝑖s, 𝑃𝑠𝑢𝑐𝑡 → (Output) Δ𝑇𝑐𝑎𝑙𝑐
• Gradually closing of the valve OD until Δ𝑇𝑒𝑥𝑝 is below the threshold
• Valve OD range 85.4 – 31.3%
Δ𝑇𝑒𝑥𝑝 <
Δ𝑇𝑐𝑎𝑙𝑐
𝑛
Rot speed [Hz] = 43 – 48
𝑃𝑑𝑖𝑠𝑐ℎ [MPa] = 7.8
𝑃𝑠𝑢𝑐𝑡 [MPa] = 2.8

13. 8th IIR Conference: Ammonia and CO2 Refrigeration Technologies, 2019, Ohrid, R. Macedonia
Test – Working Conditions
MT1 MT2 MT3 MT4 MT5 LT1 LT2
𝑇𝑒𝑣 [°C] -5.0 -5.0 -8.0 -11.0 -15.0 -25.0 -35.0
𝑃𝑑𝑖𝑠 [MPa] 8.0 6.5 9.8 8.0 6.5 2.7 2.7
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
0.6 1.0 1.4 1.8 2.2 2.6 3.0 3.4 3.8 4.2
DischargePressure[MPa]
Suction Pressure [MPa]
Compressor Envelope
MT
LT
Rotation speed 30/60/90 Hz
MT1
MT2MT5
MT4
MT3
LT1LT2

14. 8th IIR Conference: Ammonia and CO2 Refrigeration Technologies, 2019, Ohrid, R. Macedonia
Oil Missing Detection – Test & Results
Δ𝑇𝑒𝑥𝑝 ≈ Δ𝑇𝑐𝑎𝑙𝑐

15. 8th IIR Conference: Ammonia and CO2 Refrigeration Technologies, 2019, Ohrid, R. Macedonia
Oil Missing Detection – Test & Results
Δ𝑇𝑒𝑥𝑝 ≈ Δ𝑇𝑐𝑎𝑙𝑐
MT1 MT2 MT3 MT4 MT5 LT1 LT2
𝑇𝑖𝑛 [°C] 88.8 72.4 128.5 108.4 100.4 59.3 82.6
Δ𝑇𝑒𝑥𝑝 [K] 35.1 28.7 35.5 33.8 27.5 9.4 12.5
dev [K] 2.0 0.6 4.7 1.6 0.7 0.4 0.9

16. 8th IIR Conference: Ammonia and CO2 Refrigeration Technologies, 2019, Ohrid, R. Macedonia
Oil Presence Detection – Test & Results
↑ 𝐶𝑂2
-------
↓ 𝑂𝑖𝑙
• 201 tests collected (165 for MT, 36 for LT)

17. 8th IIR Conference: Ammonia and CO2 Refrigeration Technologies, 2019, Ohrid, R. Macedonia
Oil Presence Detection – Test & Results
τ < 30 s < 40 s < 60 s
MT 84.2% 93.3% 98.2%
LT 27.8% 58.3% 88.9%
↑ 𝐶𝑂2
-------
↓ 𝑂𝑖𝑙
• 201 tests collected (165 for MT, 36 for LT)
• Short duration reduces efficiency loss (lower gas by–pass)

18. 8th IIR Conference: Ammonia and CO2 Refrigeration Technologies, 2019, Ohrid, R. Macedonia
Oil Equalization Procedure

19. 8th IIR Conference: Ammonia and CO2 Refrigeration Technologies, 2019, Ohrid, R. Macedonia
Oil Equalization Procedure

20. 8th IIR Conference: Ammonia and CO2 Refrigeration Technologies, 2019, Ohrid, R. Macedonia
Oil Equalization Procedure
• Performed only if more than one compressor active and oil detection ends
with not aligned outcomes
• Oil equalization is directly verified
0%
20%
40%
60%
80%
100%
0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00
OpeningDegree[%]
Time [mm:ss]

21. 8th IIR Conference: Ammonia and CO2 Refrigeration Technologies, 2019, Ohrid, R. Macedonia
Oil Equalization Procedure – Test & Results
• Test done by manually
forcing the imbalance
• Low rotation speed is
critical
MT LT
𝒓𝒑𝒔 [𝐇𝐳] ✓ Total ✓ Total
30 2 9 11 4 1 5
60 15 0 15 6 0 6
90 14 1 15 6 0 6
• Detection procedure can act as equalization as well
• If the equalization is not effective ( ) → safe operation by oil
refill from the oil separator

22. 8th IIR Conference: Ammonia and CO2 Refrigeration Technologies, 2019, Ohrid, R. Macedonia
Conclusions
• Detection and equalization of oil levels for each compressor
• Simple and reliable method based on temperature
measurement during fluid expansion
• Equalization effectiveness is verified for each compressor
(and not presumed)
• Modulation gives other advantages such as
→ short response time
→ limited gas bypass
→ adaptability to current working conditions
• Life test has to be performed

23. THANK YOU
Giovanni ROSSANESE, Luca MATTIELLO, Willy MUVEGI, Luigi NALINI
Carel Industries S.p.A.
8th IIR Conference: Ammonia and CO2 Refrigeration Technologies, 2019, Ohrid, R. Macedonia
giovanni.rossanese@carel.com, luca.mattiello@carel.com,
willy.muvegi@carel.com, luigi.nalini@carel.com
International Institute
of Refrigeration
www.iifiir.org
Institute International du
Froid
iif-iir@iifiir.org