THE THERMOSIPHON COOLING SYSTEM OF THE ATLAS EXPERIMENT AT THE CERN LARGE HADRON COLLIDER M. Battistin, S. Berry, A. Bitadze, P. Bonneau, J. Botelho-Direito, G. Boyd, F.Corbaz, O. Crespo-Lopez, E.Da Riva, C. Degeorge, C. Deterre, B. DiGirolamo, M. Doubek, G. Favre, J. Godlewski, G. Hallewell, S. Katunin, D.Lefils, D. Lombard, S. McMahon, K. Nagai, D. Robinson, C.Rossi, A. Rozanov, V. Vacek, L. Zwalinski Cecilia Rossi Academy of Sciences of the Czech Republic, 110 00 Prague, Czech Republic 13 th International Conference multiphase flow in industrial plants Sestri Levante (Genova) 17-19 September 2014
The Thermosiphon cooling system of the ATLAS experiment at the CERN Large Hadron Collider Overview 1. Introduction 2. The Pixel and SCT cooling system 3. The full scale Thermosiphon - Working principle of the thermosiphon - 2 kw and mini thermosiphon 4. Conclusion
CERN Large Hadron Collider (LHC) 4 main experiments: - ATLAS - CMS - ALICE - LHCb
The ATLAS Experiment General purpose experiment built to investigate a wide range of physics reactions produced by high-energy proton-proton collisions. Main elements of ATLAS: - Detectors - Inner Detector (Tracking), - Calorimeter, - Muon spectometer - Magnet System ATLAS Inner Detector (ID) Inner Detector (ID): - Pixel - SCT (SemiConductor Tracker) - TRT (Transition Radiation Tracker)
The Pixel and SCT cooling system Thermal requirements: - Pixel & SCT must be cooled to -15 C to cope with radiation exposure of silicon over 10 years operation at LHC (warmer temperature operation is possible at present) - Temperature uniformity must be no worse than 2 C along the evaporators - The cooling system must remove a total dissipation of 62.4 kw (204 parallel cooling loops) The system must introduce minimum extra material into silicon trackers to minimize production of background particles that would deteriorate ID performance and surrounding calorimeters. Evaporative cooling system Advantages: - Wide range of operating temperature - Good thermal uniformity - Mass flow 10-20 times less than in mono-phase liquid cooling system (smaller pipes)
The Pixel and SCT cooling system The cooling fluid must be radiation resistant, have good dielectric properties. Experimental tests identified octafluoropropane (C 3 F 8, R218) as the most suitable fluid for current operations: - very good chemical stability under ionizing radiation, - non-flammable, - non-toxic, - non-corrosive The cooling system should guarantee long term continuous operation with minimal maintenance periods. To achieve the presently-required silicon substrate operating temperature (-7 C or lower) an evaporation temperature of -25 C is required in the on-detector cooling channels.
Evaporative fluorocarbon cooling system presently cools the Pixel and SCT detectors. System based on compression-condensation cycle: similar to standard industrial direct expansion cooling plant Temperature set for each circuit with a Back- Pressure Regulator (BPR) in the exhaust vapour return tube Flow can be slightly modified, setting pressure of inlet liquid with a Pressure Regulator (PR) B-C Condensation Condenser tank E-E Evaporation On-detector cooling channels C-D Target 1 evaporation Subcooling temperature Condenser tank @ on-detector E -F Heating cooling pipes Heat exchanger D-25 C (saturation pressure of 1.67 bar abs with C 3 F 8 coolant) 1 -D 2 Expansion Pressure Regulator F-F Heating Electric heater D 2 -D 3 Subcooling Heat exchanger F -A Expansion Backpressure Regulator D 3 -E Expansion Capillaries A-B Compression Compressor Recuperative heat exchanger to increase overall efficiency and maximize phase-change enthalpy Electrical heater: evaporates and raises above the cavern dew point residual liquid - avoid condensation on ext. surface of return pipes. Controlled by PLC system - feedback from temperature sensors placed on external surface of heaters and on downstream tubes
The ATLAS ID evaporative cooling system is divided into two main parts separated by the distribution /collection racks equipped with PRs and BPRs EXTERNAL PART 7 compressors working in parallel and a condenser High radiation environment Oil free compressor to prevent accidental mixing of coolant and lubricating oil The compressors were especially modified to satisfy the very demanding compression ratio (max output pressure 15 bar INTERNAL PART abs ; min aspiration pressure 1 bar abs ) Haug model QTOGX-160/80 (P max =17 bar abs ; P min =0.8 bar abs All ) the components directly connected to each of the 204 The compressors have been improved and are presently working satisfactorily individual but still represent detector the cooling weakest circuits: component of the system. 1 recuperative heat exchanger, 1,2 or 3 capillaries (depending on the cooling loop), on-detector cooling channels, Thermosiphon 1 electric heater
Gravity-driven thermosiphon evaporative cooling system Actual compressor-driven evaporative cooling system Compressors kept as backup solution INTERNAL PART EXTERNAL PART
The full scale thermosiphon The ATLAS thermosiphon evaporative cooling circulator takes advantage of the peculiarities of LHC experiments. The great height difference (92 m) between the underground cavern housing the experiment and the surface allows natural circulation of the coolant with no active components (pumps or compressors) in the primary loop. Compressors
The full scale thermosiphon The thermosiphon is composed of 4 separated circuits: 1. Water circuit: cooling the first stage of the chiller circuit: water from cooling towers at ~25 C: 2. Chiller circuit: two stage compression cycle to cool down perfluorohexane (C 6 F 14 ) brine heat transfer liquid to -70 C. The chiller operates in cascade: the fist stage using R404a and the second stage R23; 3. Brine circuit: C 6 F 14 closed loop used to condense the C 3 F 8 through heat exchange across the tubes in the condenser. C 6 F 14 is used as a transfer fluid mainly for its chemical similarity to C 3 F 8; ; 4. Thermosiphon primary circuit: condensing C 3 F 8 at surface to produce a liquid column from surface to cavern (exit pressure hydrostatic column of 92 m of fluid). Liquid evaporates in the unchanged ondetector cooling channels and returns to surface as vapour by differential pressure. System must supply high pressure liquid to on-detector components, while guaranteeing the required evaporation pressure.
The full scale thermosiphon Increase of hydrostatic pressure condenser and subcooling. storage when system is stopped. pressure drop along vapour return line (vapour column weight and frictional pressure drop) counter flow heat exchanger Thermodynamic cycle of the thermosiphon circuit and corresponding schematic. Thermosiphon circuit (A-I). Beyond these points C 3 F 8 enters the internal cooling circuits. Fluid exits the detectors at point M point (E in the previous compressor evaporative cycle) I-A: by-pass to rapidly ramp down at startup. Stable performance even when SCT and Pixel trackers are off (minimum thermal load). Pressure Temperature Density Enthalpy Compressors Operating reliability point 1 st [bar reason abs ] for thermosiphon [ C] also [kg/m higher 3 Physical State ] margin [kj/kg] on required pressure/temp. @ detector The A target pressure (1.67 0.5 bar abs ) is specified 20 at the end 3.90 of on-detector 310.8 cooling channels Superheated (point vapour M). B 0.495-20 4.65 280.6 Superheated vapour Pressure drop in return line increases the operating temperature of the silicon detectors. C 0.309-25 2.85 277.4 Superheated vapour D 0.309-60 1699 140.3 Saturated liquid E 0.4-65 1717 135.7 Sub-cooled liquid F 16.1-62 1712 139.0 Sub-cooled liquid G 16.1-51 1672 149.2 Sub-cooled liquid H 16 Required evaporation -20 pressure 1552 easier to 179.4 achieve Sub-cooled liquid I 16 20 1365 222.0 Sub-cooled liquid I 0.5-51 7.89 222.0 Two-phase x=0.6 Compressor-driven cooling system baseline pressure = min operable compressor pressure (1 bar abs ); Thermosiphon cooling system baseline pressure = 500 mbar abs (point A)
In order to verify the feasibility of the full scale thermosiphon, two prototypes were built: Mini-thermosiphon 2 kw thermosiphon Main difference: Total available height Cooling power
Mini-thermosiphon Small scale system both in terms of height and cooling power. Tests done over an evaporation temperature range from 15 C to -30 C, focussed on the start-up and shut down phases of the plant. 2 kw thermosiphon Built to explore the behaviour of the plant in a prototype with a similar height of the final full scale system (70 m). Main differences with the final plant: - cooling capacity, - absence of the heat exchanger between the supply and return rack. Two different modes: 1. by-pass cycle : to facilitate system start-up 2. test section cycle : trackers simulated with a dummy load compose of two parallel heaters loops each dissipating 1kW. The test allowed prediction and optimization of the final cooling plant and confirmed the running constrains seen in the mini-thermosiphon. Main design parameters 2kW thermosiphon full scale thermosiphon Total cooling capacity of the system 2kW 62.4kW Nominal condensation temperature -48.8 C -60 C Pressure at the condenser/tank 0.57 bar abs 0.309 bar abs Nominal liquid pressure at the supply manifold 13.5 bar abs 16 bar abs Nominal vapour pressure at the return manifold 0.8 bar abs 0.5 bar abs Liquid temperature at the supply manifold +20 C +20 C Vapour temperature at the return manifold +20 C +20 C
The full scale thermosiphon TS Condenser: 12m above ground level Brine and Chiller circuit: Ground level Connection to existing system: 80m underground Water circuit: Ground level
Conclusion Compressors of existing evaporative cooling system will be replaced by a gravity-driven thermosiphon recirculator. System takes advantages of special features of the LHC experiments (around 100m underground). Advantages: Expected long term reliability absence of active components in the main loop; Lower cooling temperature lower baseline pressure at the outlet of the on-detector cooling system; Coolant loss reduction reduced number of connection and reduced maintenance; Improved cleanliness no pollution caused by wear to reciprocating components. Experimental tests: Two small scale thermosiphon plants built to verify the feasibility of the system. Tests demonstrated operation over the required detector operating temperature range and provided valuable experience on the thermosiphon plant. Unattended stable operation over a period of weeks was demonstrated. Future plans: Water, brine and chiller circuit commissioning is going on. Welding problems for the thermosiphon condenser delayed thermosiphon circuit commissioning. Full plant operation planned for beginning of 2015.
Thank you! Cecilia Rossi cecilia.rossi@cern.ch
Back up slides
Compressor driven evaporative system (A-C) Internal circuit (D1 F ) Thermosiphon circuit (A-I) Internal circuit (J O) Evap D1 D2 D3 E TS I J K L E M F N F O
Power consumption of the thermosiphon plant The main purpose of the thermosiphon plant is not to have a system that is efficient from the energetic point of view, but to cope with the very special request of the ATLAS experiment.
The full scale thermosiphon Increase of hydrostatic pressure ΔT in AB and GH is different (AB = 40 C; GH = 31 C): C 3 F 8 Vapour : B(-20 C, 0.5bar) Cp = 0.723 kj/kg K, A(20 C, 0.5bar) Cp = 0.784 kj/kg K Ave Cp = 0.750 kj/kg K C 3 F 8 Liquid : G(-51 C, 16bar) Cp = 0.941 kj/kg K, H(-20 C, 16bar) Cp = 1.009 kj/kg K Ave Cp = 0.975 kj/kg K Cp in liquid phase (GH) is ~ 25% higher than in vapour phase (AB), temperature change will be ~75% lower Thermodynamic cycle of the thermosiphon circuit and corresponding schematic. Thermosiphon circuit (A-I). Beyond these points C 3 F 8 enters the internal cooling circuits. Fluid exits the detectors at point M point (E in the previous compressor evaporative cycle) Operating point Pressure Temperature Density Enthalpy [bar abs ] [ C] [kg/m 3 ] [kj/kg] Physical State A 0.5 20 3.90 310.8 Superheated vapour B 0.495-20 4.65 280.6 Superheated vapour C 0.309-25 2.85 277.4 Superheated vapour D 0.309-60 1699 140.3 Saturated liquid E 0.4-65 1717 135.7 Sub-cooled liquid F 16.1-62 1712 139.0 Sub-cooled liquid G 16.1-51 1672 149.2 Sub-cooled liquid H 16-20 1552 179.4 Sub-cooled liquid I 16 20 1365 222.0 Sub-cooled liquid I 0.5-51 7.89 222.0 Two-phase x=0.6