Cold Operation of a Recirculating CO 2 System at Aachen

1. Cold Operation of a Recirculating CO2 System at Aachen Lutz Feld, Waclaw Karpinski, Jennifer Merz, Michael Wlochal RWTH Aachen University, 1. Physikalisches Institut B 10.02.2010CMS Upgrade Cooling and Mechanics Meeting

2. Jennifer Merz Outline • Description of CO2 test system in Aachen • Specifications of new chiller • Dryout measurements at room temperature • First measurement at low temperature • Conclusion and outlook

3. Jennifer Merz CO2 Test System Aachen - Specs • Goals: • Gain experience with a recirculating (closed) CO2 cooling system • Find the lowest possible operating temperature • Determine ideal operating conditions ( stable operation) depending on heat load and coolant temperature • Specifications: • Maximum heat load: 500W • Coolant temperature at detector: -45°C - +20°C • Precise temperature control and flow measurement • Continous operation • Safe operation (maximum pressure 100bar)

4. Schematic of Test System Chiller 1:Chiller temperature  vapour pressure  system temperature Expansion Vessel: Filled with saturated mixture of CO2-liquid and -vapour -45°C ΔQ ΔQ Burst Disk CO2 Bottle • Heat Exchanger 1: • Heats liquid CO2 to appropriate temperature • Partial condensation of returning CO2 Vacuum Pump Chiller 2:Heat removal Burst Disk Heat Exchanger 2 Heat Exchanger 1 -50°C • Heat Exchanger 2: • Removes heat from the system • Cools incoming CO2 down to about -50°C Detector: 500 W heat load Flow Meter CO2 Pump Burst Disk 4

5. Jennifer Merz CO2 Cooling Test System CO2-Bottle CO2-Flasche CO2 Bottle Expansion Vessel Detector Heat Exchanger 1 Vacuum-Pump

6. Jennifer Merz Cooling of Expansion Vessel CO2-Flasche • Copper piping replaced by copper shell • Improvement of thermal contact • Reduction of flow resistance Vacuum-Pump

7. Jennifer Merz CO2 Cooling Test System CO2-Bottle CO2-Flasche • 6m long stainless steel pipe with 1.5mm outer diameter, in insulated box • 14 thermistors to measure temperature distribution over pipe • Electrical connections to simulate heat load Vacuum-Pump

8. Jennifer Merz Chillers: unistat 815 • Originally ordered chiller had to be replaced; cooling power was not sufficient • New chiller was damaged during transport • “Old” chiller had to be repaired by company • Chillers now achieve required low temperatures CO2-Bottle CO2-Flasche • Delay in commissioning, system is fully operational since end of January • Chiller specifications: (both chillers identical) unistat 815 • Company: Peter Huber Kältemaschinenbau, Offenburg, Germany • Temperature range: -85°C to 250°C • Cooling power: 1.5kW @ -20°C • 1.4kW @ -40°C • 1.2kW @ -60°C • Pump: max. 40 l/min, max. 0.9bar

9. Dryout Measurement at Room Temp. • Dryout: Pipe walls not in touch with liquid anymore •  no power dissipation by evaporating CO2 • temperature rises Heat load: 70W Liquid Gas 14 12 10 8 6 4 2 13 11 9 7 5 3 1 • Vary revolutions per minute of CO2 pump •  variation of CO2 flow • Keep heat load constant • Determine when temperature rises over certain level Heat load: 70W

10. Jennifer Merz Dryout Measurement at Room Temp. 30W 40W 50W 60W 70W •  The lower the flow, the earlier a rise in temperature is observed • The higher the heat load, the more flow is needed Vacuum-Pump

11. Jennifer Merz First Measurement at Low Temperature • Expansion vessel at -27°C • Chiller 2 at -44°C • No stable operation • Fast variations in CO2 flow • Beginning of dryout visible • System behaviour not yet understood • Operation at low temperatures possible (down to -45°C) if bypass is open; nearly no flow through detector pipe • Chillers work fine (see also next slide) • Flow resistance too high? • More investigation and measurements needed Vacuum-Pump

12. Chiller Performance Chiller 1 • Both chillers show excellent performance • Low temperatures • Stable operation Nominal Temp. Internal Temp. Chiller 2 Nominal Temp. Internal Temp. Temp. at Exp. Vessel Vacuum-Pump

13. Jennifer Merz Conclusions and Outlook • CO2 test system in Aachen is finally fully operational • First measurements show that system works in principal • New chillers manage to reach low temperatures • No stable operation at low temperatures possible yet • Investigate how to bring low temperature to detector • Install filter to avoid water and other disturbing particles in system • Determine pressure drop and temperature distribution for different pipe routings and diameter

14. Jennifer Merz Back Up....

15. CO2 Enthalpy-Pressure Diagram • Design of cooling plant in p-H-digram • Enthalpy: • H = U + pV “internal energy + expansion work” • ΔH is exchanged heat at constant pressure liquid Pressure, bar liquid + gas gas Enthalpy, kJ/kg

16. 4 • 2 • 3 • 5 • 6 • 1 CO2 Enthalpy-Pressure Diagram Re-Circulating System (closed system) • Design of cooling plant in p-H-digram • Enthalpy: • H = U + pV “internal energy + expansion work” • ΔH is exchanged heat at constant pressure liquid ΔQ Pressure, bar liquid + gas ΔQ gas Detector load (4-5) Enthalpy, kJ/kg

17. Jennifer Merz CO2 Cooling System Advantages: • CO2 : low density  small contribution to material budget • Operation at high pressures  small pipe diameter • Low temperatures (-45°C)  good for sensor performance Current tracker (C6F14) vs. tracker with DC-DC converters cooled with CO2 • Power consumption with DC-DC converters: 84W per petal • Including 18.8W power loss of converters, due to converter efficiency of 80% • One aluminium cooling block per converter implemented to dissipate converter power loss TEC Cooling TEC Total Material Budget -38.5% -14.3% Current tracker with C6F14 Layout with DC-DC converters with CO2 A total reduction of 14.3% seems possible with CO2cooling and DC-DC conversion.