1. 1 Engineering Forum: experiences from cooling systems for LHC detectors Bart Verlaat National Institute for Subatomic Physics (NIKHEF) Amsterdam, The Netherlands CO 2 cooling experiences in the LHCb Velo Thermal Control System CERN, 30 October 2008

2. 2 Table of Contents CO 2 cooling and the 2PACL method LHCb-VELO Thermal Control System (VTCS). Commisioning results of the VTCS. Conclusions.

3. 3 Why Evaporative CO 2 Cooling? The lightest way of cooling is: Evaporate at high pressure! Why? Vapor expansion is limited under high pressure Volume stays low Pipe diameter stays low Carbon Dioxide High latent heat Low mass flow Mass stays low dP/P is small

4. 4 75 50 25 0 0 20 40 -20 -40 -60 Temperature (°C) Pressure (Bar) Saturation curves in the PT Diagram for CO 2, C 2 F 6 & C 3 F 8 (3 used or considered refrigerants at CERN) CO 2 C2F6C2F6 C3F8C3F8

5. 5 Property comparison Refrigerant “R” numbers: R744=CO 2 R218=C 3 F 8 R116=C 2 F 6 Density Ratio (ρ vapour/ ρ liquid ) Saturation Temperature (ºC ) ρ vapour / ρ liquid Better Surface Tension Saturation Temperature (ºC ) Surface Tension (N/m) Better Liquid Viscosity Saturation Temperature (ºC ) Liquid Viscosity (Pa*s) Better Latent Heat of Evaporation Saturation Temperature (ºC ) Latent heat of evaporation (kJ/kg) Better dT/dP Saturation Temperature (ºC ) dT/dP (ºC/bar) Better

6. 6 2 evaporative cooling principles used in LHC detectors DetectorCooling plant Warm transfer ChillerLiquid circulation Cold transfer LHCb-VELO: Atlas Inner Detector: Vapor compression system Fluid: C 3 F 8 2PACL pumped liquid system Fluid: CO 2 Heater Compressor Pump Compressor BP. Regulator Liquid Vapor 2-phase Pressure Enthalpy Liquid Vapor 2-phase Pressure Enthalpy DetectorCooling plant

7. 7 The 2-Phase Accumulator Controlled Loop (2PACL) 2PACL principle ideal for detector cooling: -Liquid overflow => no mass flow control -Low vapor quality => good heat transfer -No local evaporator control, evaporator is passive in detector -Very stable evaporator temperature control at a distance (P 4-5 = P 7 ) Condenser Pump Heat exchanger evaporator Restrictor 2-Phase Accumulator Heat in Heat out 1 23 4 5 6 Liquid Vapor 2-phase Enthalpy Pressure 1 23 4 5 6 P7P7 P7P7 P 4-5 Long distance

8. 8 Electron Hadron Proton beam LHCb Detector Overview Muon LHCb Cross section Goals of LHCb: Studying the decay of B-mesons to find evidence of CP-violation (Why is there more matter around than antimatter?) 20 meter Vertex Locator

9. 9 VELO Thermal Control System CO 2 evaporator section Detectors and electronics 23 parallel evaporator stations capillaries and return hose Temperature detectors: -7ºC Heat generation: max 1600 W The LHCb-VELO Detector (VErtex Locator)

10. 10 Particle tracks in the VELO from an LHC injection test (22 august ’08) The Velo Detector Heat producing electronics CO 2 evaporator (Stainless steel tube casted in aluminum) Detection Silicon

11. 11 VELO Cooling Challenges VELO electronics must be cooled in vacuum. –Good conductive connection –Absolute leakfree Maximum power of the electronics: 1.6 kW Silicon sensors must stay below -7°C at all times (on or off). Adjustable temperature for commisioning. –-5°C to -30°C in vacuum (Nominal -25°C) –+10°C to-10°C under Neon vented condition Maintenance free in (inaccessable) detector area

12. 12 VTCS Evaporator 23 parallel evaporator stations  inner =1mm (L~1.5m) Inlet capillaries  inner =0.5mm (L~2m) Aluminium casted evaporator block details vacuum feed through capillaries and return hose liquid inlet (¼”x0.035”) 2-phase outlet (⅜”x0.035”) PT100 cables

13. 13 4 m 2.6 m 2 R507A Chillers: 1 water cooled 1 air cooled 2 CO 2 2PACL’s: 1 for each detector half 55 m Accessible and a friendly environment Inaccessible and a hostile environment 2 Evaporators 800 Watt max per detector half VELO LHCb-VTCS Overview (VELO Thermal Control System) PLC 2 Concentric transfer lines 4m thick concrete shielding wall

14. 14 Concentric transfer line Ø4mm Ø6mm Ø14mm Ø16mm Ø66mm Sub-cooled liquid feed line 2-phase return line 25mm Armaflex NH Isolation Protective cover Transfer line is a 55m long concentric 2-fase liquid-vapor line Functions: Transferring liquid to evaporators Regulate liquid temperature Pick up environmental heat in return line for unloaded evaporator cooling Provide low-pressure drop return flow for distant evaporator pressure control Terminals as built VTCS total upward column: 4m+2.6m = 6.6m ΔT ≈ 0.7°C → ≈ 0.1°C/m (C 3 F 8 ≈ 2.3°C/m) 2.6m 4m

15. 15 VTCS Accumulator Control Thermo siphon heater for pressure increase (Evaporation) Cooling spiral for pressure decrease (Condensation) Accumulator properties: Volume: 14.2 liter (Loop 9 Liter) Heater capacity: 1 kW Cooler capacity: 1 kW System charge: 12 kg (@23.2 liter) System design pressure: 135 bar + + + _ + _ Heating Cooling Set point Temperature Temperature Pressure Evaporator Pressure Pressure drop PID P set T set P accumulator ΔP fault

16. 16 VTCS Schematics 2x CO 2 2PACL’s connected to 2 R507A chillers (Redundancy) Lots of sensors and valves

17. 17 VTCS construction CO 2 2PACL’s –Stainless steel piping with: (Orbital) Welding Vacuum Brazing Swagelok Cajon VCR fittings and line components –Lewa liquid CO 2 pump (100 bar) –In house designed accumulator (130 bar) –Reinforced SWEP condenser (130 bar) Now commercial available at SWEP. –55 meter concentric transfer line –Aluminium casted cooling blocks –Test pressure 170 bar Chillers designed in house with standard commercial chiller components. –Copper piping with hard solder joints –Danfoss line components –Bitzer compressors –SWEP heat exchangers Siemens S7-400 series Programmable Logic Controller (PLC)

18. 18 LHCb-VTCS Cooling Plant Pumps Accumulators Valves Condensers CO 2 2PACL systems Freon chiller systems

19. 19 VTCS Units Installed @ CERN July- August 2007 CO 2 Unit Freon Unit

20. 20 VTCS 2PACL Operation From start-up to cold operation (1) Start-up in ~2 hours A B C D time 20 2 Pump head pressure (Bar) 4 - Accumulator pressure (Bar) 1 Pumped liquid temperature (°C) 5 – Evaporator temperature (°C) 4-Accumulator liquid level (vol %) 4- Accumulator Control: + = Heating - = Cooling _ + 4 1 2 7 7 7 7 1 2 7 4 +7 -7

21. 21 Accumulator Cooling = Pressure decrease VTCS 2PACL Operation From start-up to cold operation (2) A B C D Start-up in ~2 hours A B C D time 21 2 - Pump head pressure (Bar) 4 - Accumulator pressure (Bar) 1 – Pumped liquid temperature (°C) 5 – Evaporator temperature (°C) 20 °C 0 °C -20 °C -40 °C 1 2 4 A B C D Path of 5 4 4 1 2 5 2 5 4 1 5 5 Enthalpy Pressure Set-point range

22. 22 March ’08: Commisioning of the VTCS Detector under vacuum and unpowered

23. 23 SP=-25°C SP=-5°C (3 sept 08) Accu level Module Heat load Silicon temperature Evaporator temperature Accu Heating/Cooling 24 June ’08: After a succesful commisioning of the detector at -25°C, the setpoint is increased to -5°C. And has been running since then smoothly! 80 60 40 20 0 -20 -40 0 0:30 1:00 1:30 2:00 Time (Hour) Temperature (°C), Power (Watt), Level (vol %) Detector half heat load (x10) -7°C

24. 24 CO 2 liquid dT=4.5°C Cooling block dT=0.04°C 1 hour Evaporator Pressure 31.15 bar = -4.18°C Cooling block temperature = -2.8°C CO 2 liquid temp= -42°C Evaporator liquid inlet temp = -4.40°C Evaporator vapor outlet temp = -4.44°C Accumulator Pressure 30.54 bar = -4.90°C Detector offset from accu control: 0.7°C CO 2 heat transfer dT=1.4°C VTCS performance overview for a setpoint of -5°C (Detector switched on, fully powered) dP=0.6 bar = 6.2 m static heigth Fluctuations from the untuned chiller

25. 25 Summary The VTCS has successfully passed the 1 st commissioning phase and was ready to be used in the experiment in July 2008 Operational temperature range is between -5°C and -30°C set point for the water cooled chiller It has run for 3½months continuously with only minor problems It behaves very stable with the chiller still to be tuned (evaporator stability less than 0.05°C) The silicon temperature is below the required -7°C @ -25°C set point temperature. (This is consistent with the prediction)

26. 26 What did we learn: 2PACL dynamics work better with the accumulator connection at inlet of condenser (instead of outlet as it is now) –No saturated liquid feed from accu to pump. –Free pre-cooling at cold start-up due to thermal capacity of condensers. Concentric transfer tube heat exchanger works beyond expectations as the so-called “Duck-foot- cooling 1 ” pinciple is boosting the operational temperature range. The current system is not always initiating boiling at higher operational temperatures (>-10°C), resulting in temporary reduced heat-exchange. 1 The way a duck can have cold feet without loosing body heat, by exchanging heat between the in- and outlet bloodstream.

27. 27 The “Cool” future The VTCS is not yet finnished, some things have to be done: –Implementing automatic back-up procedure. –Changing the accumulator connection. –Tunning the chiller. –Analyse data for publication. Construction of a mini desktop 2PACL CO 2 circulator for general purpose laboratory use. Participation in future CO 2 cooling systems –Atlas IBL, Goat, Next-64, RELAXed 2PACL upgrade: –Replace HFC chiller by CO 2 chiller No integration of chiller and 2PACL! –2PACL is much more stable (<0.05’C) –2PACL has liquid overfeed and needs no boil-off heaters –CO 2 compressor needs oil as lubricant CO 2 chiller has extended lower temperature range (CO 2 ~-50°C, HFC~- 40°C )

28. 28 Back-up Slides

29. 29 Property Comparison (1) R744 (CO 2) R116 (C 2 F 6 ) R218 (C 3 F 8 ) Source Refprop NIST R744 (CO2) R218 (C3F8) R116 (C2F6) Critical Point31ºC @ 73.8 bar 71.9ºC @ 26.4 bar 19.9ºC @ 30.5 bar Triple point-56.6ºC-147.7ºC-100ºC Boiling temperature @ 1 bar -78.4ºC Subli- mation ! -36.8ºC-78.1ºC

30. 30 CO 2 =2.7mm C 2 F 6 =4.3mm C 3 F 8 =7.7mm ΔT=-2°C 2x 20 wafers à 17 Watt 1 Atlas stave : 2 meter length Cooling Q = 680 Watt Tube = 4 meter Example of and Atlas upgrade stave (1) CO 2 C3F8C3F8 C2F6C2F6 C3F8C3F8 C2F6C2F6 Calculations based on -35°C and 75% vapor quality at exit Mass flow @ -35ºC Φ CO2 =2.9 g / s Φ C3F8 =8.7 g / s Φ C2F6 =9.6 g / s Pressure Drop Temperature Drop Generates

31. 31 7.7 CO 2 C3F8C3F8 C2F6C2F6 D2.7mm x L25mm = 80167 W/m 2 D2.7mm x L75mm = 26722 W/m 2 D4.3mm x L25mm = 50337 W/m 2 D4.3mm x L75mm = 16779 W/m 2 D7.7mm x L25mm = 28110 W/m 2 D7.7mm x L75mm = 9370 W/m 2 CO 2 C3F8C3F8 C2F6C2F6 25mm HX length 75mm HX length Example of and Atlas upgrade stave (2) Mass flux @ -35ºC Φ’ CO2 =506 kg / m 2 s Φ’ C3F8 =661 kg / m 2 s Φ’ C2F6 =186 kg / m 2 s Critical Heat Flux (Bowring/Ahmad): CHF(CO 2 ) = 313 kW/m 2, x=1.1 25mm 75mm Heat exchange length