CO 2 CO2 Cooling for HEP experiments Bart Verlaat TWEPP-2008 Topical Workshop on Electronics for Particle Physics CO2 Cooling for HEP experiments Bart Verlaat National Institute for Subatomic Physics (Nikhef) Amsterdam, The Netherlands CO 2 Naxos, 18 September 2008

Table of Contents Introduction to evaporative CO2-Cooling Introduction to the LHCb Velo Thermal Control System (VTCS) Commissioning results of the VTCS. Conclusions and outlook.

(Silicon) Particle Detectors and Cooling (Silicon) Particle detectors have specific needs for thermal control: Many distributed heat sources over large volumes. Low temperature gradients between these sources. Permanent cooling To avoid thermal runaway of the irradiated silicon Cooling pipes should have low mass inside detectors Cooling pipes should have low structural impact Radiation resistant cooling fluid Multiple electronic stations (All need cooling) Atlas SCT Alpha Magnetic Spectrometer Silicon Tracker

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 Later on in this presentation this will be illustrated by calculations

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)

What happens inside a cooling tube? Heating a flow from liquid to gas Isotherm Pressure (Bar) 2-phase Enthalpy (J/kg) 90 60 Tube temperature Temperature (°C) Low ΔT 30 Increasing ΔT Fluid temperature -30 Target flow condition Dry-out zone Sub cooled liquid 2-phase liquid / vapor Super heated vapor

How to design a cooling tube? P1 P2 Pressure (Bar) P3 ΔP=P2-P3 Enthalpy (J/kg) h2 h3 For evaporative cooling 5 items are important: Controlling inlet enthalpy (h2) by expanding from liquid (P1,T1) Setting mass flow (Øm) as function of maximum heat load (Qmax) Controlling outlet pressure (P3) Minimize pressure drop (ΔP) Optimize heat transfer (α) with contact area.

2-Phase flow prediction 2-phase flow theory = Well understood single phase flow theory x Magic empirical correlations in a black box Pressure drop: For example: Friedel Correlation Heat transfer: For example: Kandlikar Correlation 2-Phase coëfficiënt order 1~10

How to get the ideal 2-phase flow in the detector? Traditional method: (Atlas) Vapor compression system Liquid Vapor Compressor Heater Pressure BP. Regulator Warm transfer 2-phase Enthalpy Cooling plant Detector 2PACL method: (LHCb) Pumped liquid system Liquid Vapor Compressor Pump Pressure Chiller Liquid circulation 2-phase Cold transfer Enthalpy Cooling plant Detector

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 22 August 2008 Heat producing electronics Detection Silicon 22 August 2008 The VELO has seen particle tracks from an LHC test! CO2 evaporator (Stainless steel tube casted in aluminum)

VELO Cooling Challenges VELO electronics must be cooled in vacuum. Good conductive connection Absolute leak free Maximum power of the electronics: 1.6 kW Silicon sensors must stay below -7°C at all times (on or off). To avoid thermal runaway of the irradiated silicon Adjustable temperature for commissioning. Maintenance free in inaccessible detector area

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

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

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

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

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 20 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 21 Start-up in ~2 hours

March ’08: Commissioning of the VTCS Detector under vacuum and unpowered 20 10 Temperature (°C) -10 -20 -30 -40

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 set point 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 height 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

VTCS Summary The installation at CERN started in July 2007. The VTCS has successfully passed the 1st commissioning phase between march and June ’08 and is ready to be used in the experiment Operational temperature range is between 0°C and -30°C set point (+10°C with the back-up chiller) It has run for 2½months continuously without any problem (only 3 interruption due to power or cooling water failures) It behaves very stable (<0.1°C fluctuation), with the chiller still to be tuned. The silicon temperature is below the required -7°C @ -25°C set point temperature. (This is consistent with the prediction)

Some Lessons Learned The accumulator sometimes gives the pump a 2-phase mixture => cavitation. Problem is solved by connecting the accumulator to the inlet of the condenser instead of the outlet where it is now. Operational temperature range of the evaporator is larger than expected. This is due to the “Duck Foot Cooling1” principle of the transfer line. The main pressure drop of 2-phase flow in the return line was not caused by friction but by the heights of the upward columns combined. Minimization of the upward columns is more important in the design. Were CO2 gives only about 1°C/bar (≈0.1°C/m), the use of other fluids will make upward 2-phase flow impossible without a big temperature penalty. 1 The way a duck can have cold feet without loosing body heat, by exchanging heat between the in- and outlet bloodstream.

Outlook The VTCS is not yet finished, some things have to be done: Implementing automatic back-up procedure. Changing the accumulator connection. Tuning the chiller. Analyze data for publication. Construction of a mini desktop 2PACL CO2 circulator for general purpose laboratory use. Other CERN detectors (Atlas/CMS) have shown interest in the VTCS for their inner tracker upgrades. Challenge: Scaling of the 1.6kW VTCS to a 100kW system. An example of fluid trade-off is given in an Atlas upgrade stave calculation example.

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

Thank you for your attention: Questions?