CO 2 CO2 cooling experiences in the LHCb Velo Thermal Control System Engineering Forum: experiences from cooling systems for LHC detectors CO2 cooling experiences in the LHCb Velo Thermal Control System Bart Verlaat National Institute for Subatomic Physics (NIKHEF) Amsterdam, The Netherlands CO 2 CERN, 30 October 2008

Table of Contents CO2 cooling and the 2PACL method LHCb-VELO Thermal Control System (VTCS). Commisioning results of the VTCS. Conclusions.

Why Evaporative CO2 Cooling? The lightest way of cooling is: Evaporate at high pressure! Why? Vapor expansion is limited under high pressure Volume stays low Low mass flow Pipe diameter stays low High latent heat Mass stays low Carbon Dioxide

Saturation curves in the PT Diagram for CO2, C2F6 & C3F8 (3 used or considered refrigerants at CERN) 75 CO2 50 C2F6 Pressure (Bar) 25 C3F8 -60 -40 -20 20 40 Temperature (°C)

Property comparison Refrigerant “R” numbers: R744=CO2 R218=C3F8 Latent Heat of Evaporation Saturation Temperature (ºC ) Latent heat of evaporation (kJ/kg) Better Refrigerant “R” numbers: R744=CO2 R218=C3F8 R116=C2F6 Density Ratio (ρvapour/ ρliquid) Saturation Temperature (ºC ) ρvapour/ ρliquid Better Liquid Viscosity Saturation Temperature (ºC ) Liquid Viscosity (Pa*s) Better Surface Tension Saturation Temperature (ºC ) Surface Tension (N/m) Better dT/dP Saturation Temperature (ºC ) dT/dP (ºC/bar) Better

2 evaporative cooling principles used in LHC detectors Atlas Inner Detector: Vapor compression system Fluid: C3F8 Liquid Vapor Compressor Heater Pressure BP. Regulator Warm transfer 2-phase Enthalpy Cooling plant Detector LHCb-VELO: 2PACL pumped liquid system Fluid: CO2 Liquid Vapor Compressor Pump Pressure Chiller Liquid circulation 2-phase Cold transfer Enthalpy Cooling plant Detector

The 2-Phase Accumulator Controlled Loop (2PACL) Long distance P7 P4-5 5 Heat out Heat out Condenser 6 Heat in 4 Heat in 2 3 evaporator 2-Phase Accumulator 1 Heat exchanger Restrictor Pump 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 (P4-5 = P7) Vapor Liquid Pressure 2 3 2-phase 4 P7 5 1 6 Enthalpy

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

The LHCb-VELO Detector (VErtex Locator) Detectors and electronics Temperature detectors: -7ºC Heat generation: max 1600 W 23 parallel evaporator stations capillaries and return hose VELO Thermal Control System CO2 evaporator section

The Velo Detector Heat producing electronics Detection Silicon Particle tracks in the VELO from an LHC injection test (22 august ’08) CO2 evaporator (Stainless steel tube casted in aluminum)

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 (Nominal irradiated -25°C) Maintenance free in inaccessable detector area

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

LHCb-VTCS Overview (VELO Thermal Control System) Accessible and a friendly environment Inaccessible and a hostile environment 2.6 m PLC 4m thick concrete shielding wall 3.6 m 2 Evaporators 800 Watt max per detector half 2 R507A Chillers: 1 water cooled 1 air cooled 2 CO2 2PACL’s: 1 for each detector half 2 Concentric transfer lines 55 m VELO

Concentric transfer line 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 Protective cover 2-phase return line 25mm Armaflex NH Isolation Ø14mm Ø6mm Ø16mm Ø4mm Ø66mm VTCS total upward column: 4m+2.6m = 6.6m ΔT ≈ 0.7°C → ≈ 0.1°C/m (C3F8 ≈ 2.3°C/m) Sub-cooled liquid feed line 4m 2.6m Terminals as built

VTCS Schematics 2x CO2 2PACL’s connected to 2 R507A chillers (Redundancy) Lots of sensors and valves

VTCS construction CO2 2PACL’s Stainless steel piping with: (Orbital) Welding Vacuum Brazing Swagelok Cajon VCR fittings and line components Lewa liquid CO2 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 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)

LHCb-VTCS Cooling Components Accumulators VTCS Evaporator Valves Pumps Condensers CO 2

VTCS Units Installed @ CERN Freon Unit CO2 Unit July- August 2007 CO 2

VTCS 2PACL Operation From start-up to cold operation (1) + 2 Pump head pressure (Bar) 2 4-Accumulator liquid level (vol %) 7 4 - Accumulator pressure (Bar) 7 5 – Evaporator temperature (°C) 4 1 Pumped liquid temperature (°C) 1 4- Accumulator Control: + = Heating - = Cooling 7 _ 7 -7 4 CO 2 7 +7 2 1 time A B C D 19 Start-up in ~2 hours

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

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

Temperature (°C), Power (Watt), Level (vol %) 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! (3 sept 08) 80 Accu Heating/Cooling 60 Accu level 40 Detector half heat load (x10) 20 Module Heat load Temperature (°C), Power (Watt), Level (vol %) Silicon temperature -7°C SP=-5°C -20 Evaporator temperature SP=-25°C -40 0:30 1:00 1:30 2:00 Time (Hour)

Fluctuations from the untuned chiller VTCS performance overview for a setpoint of -5°C (Detector switched on, fully powered) CO2 heat transfer dT=1.4°C Evaporator Pressure 31.15 bar = -4.18°C Cooling block dT=0.04°C Cooling block temperature = -2.8°C 1 hour dP=0.6 bar = 6.2 m static heigth CO2 liquid temp= -42°C Fluctuations from the untuned chiller Detector offset from accu control: 0.7°C CO2 liquid dT=4.5°C Evaporator liquid inlet temp = -4.40°C Evaporator vapor outlet temp = -4.44°C Accumulator Pressure 30.54 bar = -4.90°C

Summary The VTCS has successfully passed the 1st 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 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)

What did we learn: 2PACL dynamics work better with the accumulator connection at inlet of condenser (instead of outlet as it is now) No satarated 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-cooling1” 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.

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 CO2 circulator for general purpose laboratory use. Participation in future CO2 cooling systems Atlas IBL, Goat (GOssip in ATlas), Next-64, RELAXed 2PACL upgrade: Replace HFC chiller by CO2 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 CO2 compressor needs oil as lubricant CO2 chiller has extended lower temperature range (CO2 ~-50°C , HFC~-40°C )

Back-up Slides

Property Comparison (1) R744 (CO2) R218 (C3F8) R116 (C2F6) Source Refprop NIST R744 (CO2) R218 (C3F8) R116 (C2F6) Critical Point 31ºC @ 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

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

Example of and Atlas upgrade stave (2) D2.7mm x L25mm = 80167 W/m2 25mm 75mm Heat exchange length CO2 D4.3mm x L25mm = 50337 W/m2 D2.7mm x L75mm = 26722 W/m2 C2F6 D4.3mm x L75mm = 16779 W/m2 D7.7mm x L25mm = 28110 W/m2 C3F8 D7.7mm x L75mm = 9370 W/m2 25mm HX length C3F8 7.7 75mm HX length Mass flux @ -35ºC Φ’ CO2= 506 kg/m2s Φ’ C3F8= 661 kg/m2s Φ’ C2F6= 186 kg/m2s CO2 C2F6 Critical Heat Flux (Bowring/Ahmad): CHF(CO2) = 313 kW/m2, x=1.1

VTCS Accumulator Control Set point Temperature Cooling spiral for pressure decrease (Condensation) Tset 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 Pressure Temperature Pset Evaporator Pressure PID + _ Heating Cooling ΔPfault + _ + + Thermo siphon heater for pressure increase (Evaporation) Paccumulator Pressure drop