the thermosiphon cooling system of the atlas...
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THE THERMOSIPHON COOLING SYSTEM OF THE ATLAS EXPERIMENT AT THE CERN LARGE HADRON COLLIDER
13th International Conference multiphase flow in industrial plants
Sestri Levante (Genova) 17-19 September 2014
Cecilia Rossi
Academy of Sciences of the Czech Republic, 110 00 Prague, Czech Republic
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
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
Overview
The Thermosiphon cooling system of the ATLAS experiment at the CERN Large Hadron Collider
C.Rossi - MFIP 2014 – 19 September 2014
4 main experiments: - ATLAS - CMS - ALICE - LHCb
CERN Large Hadron Collider (LHC)
4 main experiments: - ATLAS - CMS - ALICE - LHCb
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
The ATLAS Experiment
ATLAS Inner Detector (ID)
Inner Detector (ID): - Pixel - SCT (SemiConductor Tracker) - TRT (Transition Radiation Tracker)
C.Rossi - MFIP 2014 – 19 September 2014
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)
C.Rossi - MFIP 2014 – 19 September 2014
The Pixel and SCT cooling system
The cooling fluid must be radiation resistant, have good dielectric properties. Experimental tests identified octafluoropropane (C3F8, R218) as the most suitable fluid for current operations: - very good chemical stability under ionizing radiation, - non-flammable, - non-toxic, - non-corrosive
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.
The cooling system should guarantee long term continuous operation with minimal maintenance periods.
C.Rossi - MFIP 2014 – 19 September 2014
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
Target evaporation temperature @ on-detector cooling pipes -25°C (saturation pressure of 1.67 barabs with C3F8 coolant)
Flow can be slightly modified, setting pressure of inlet liquid with a Pressure Regulator (PR)
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
B-C Condensation Condenser tank E-E’ Evaporation On-detector cooling channels
C-D1 Subcooling Condenser tank E’-F Heating Heat exchanger
D1-D2 Expansion Pressure Regulator F-F’ Heating Electric heater
D2-D3 Subcooling Heat exchanger F’-A Expansion Backpressure Regulator
D3-E Expansion Capillaries A-B Compression Compressor
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INTERNAL PART All the components directly connected to each of the 204 individual detector cooling circuits: • 1 recuperative heat exchanger, • 1,2 or 3 capillaries (depending
on the cooling loop), • on-detector cooling channels, • 1 electric heater
EXTERNAL PART 7 compressors working in parallel and a condenser
The ATLAS ID evaporative cooling system is divided into two main parts separated by the distribution /collection racks equipped with PRs and BPRs
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 barabs ; min aspiration pressure 1 barabs)
Haug model QTOGX-160/80 (Pmax=17 barabs ; Pmin=0.8 barabs)
The compressors have been improved and are presently working satisfactorily but still represent the weakest component of the system.
Thermosiphon
C.Rossi - MFIP 2014 – 19 September 2014
Compressors kept as backup solution
EXTERNAL PART INTERNAL PART
Actual compressor-driven evaporative cooling system
Gravity-driven thermosiphon evaporative cooling system
C.Rossi - MFIP 2014 – 19 September 2014
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
C.Rossi - MFIP 2014 – 19 September 2014
The full scale thermosiphon
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 (C6F14) “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: C6F14 closed loop used to condense the C3F8 through heat exchange across the tubes in the
condenser. C6F14 is used as a transfer fluid mainly for its chemical similarity to C3F8;;
4. Thermosiphon primary circuit: condensing C3F8 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 on-detector 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 thermosiphon is composed of 4 separated circuits:
C.Rossi - MFIP 2014 – 19 September 2014
The full scale thermosiphon
Operating point Pressure [barabs]
Temperature [°C]
Density [kg/m3]
Enthalpy [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
Thermodynamic cycle of the thermosiphon circuit and corresponding schematic. Thermosiphon circuit (A-I). Beyond these points C3F8 enters the internal cooling circuits. Fluid exits the detectors at point M point (E’ in the previous compressor evaporative cycle)
pressure drop along vapour return line (vapour column weight and frictional pressure drop)
counter flow heat exchanger
condenser and subcooling. storage when system is stopped.
Increase of hydrostatic pressure
I-A: by-pass to rapidly ramp down at start-up. Stable performance even when SCT and Pixel trackers are off (minimum thermal load).
Compressors reliability → 1st reason for thermosiphon → also higher margin on required pressure/temp. @ detector The target pressure (1.67 barabs) is specified at the end of on-detector cooling channels (point M).
→ Pressure drop in return line increases the operating temperature of the silicon detectors.
Compressor-driven cooling system → baseline pressure = min operable compressor pressure (1 barabs); Thermosiphon cooling system → baseline pressure = 500 mbarabs (point A)
→ Required evaporation pressure easier to achieve
In order to verify the feasibility of the full scale thermosiphon, two prototypes were built:
Main difference:
• Total available height • Cooling power
Mini-thermosiphon
2 kW thermosiphon
C.Rossi - MFIP 2014 – 19 September 2014
Mini-thermosiphon
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.
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.
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 barabs 0.309 barabs
Nominal liquid pressure at the supply manifold 13.5 barabs 16 barabs
Nominal vapour pressure at the return manifold 0.8 barabs 0.5 barabs
Liquid temperature at the supply manifold +20°C +20°C
Vapour temperature at the return manifold +20°C +20°C
The full scale thermosiphon
Brine and Chiller circuit: Ground level TS Condenser: 12m above 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.
C.Rossi - MFIP 2014 – 19 September 2014
C.Rossi - MFIP 2014 – 19 September 2014
Back up slides
Compressor driven evaporative system
(A-C) Internal circuit (D1 – F’)
Thermosiphon circuit (A-I)
Internal circuit (J – O)
Evap TS
D1 I
D2 J
D3 K
E L
E’ M
F N
F’ O C.Rossi - MFIP 2014 – 19 September 2014
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.
Power consumption of the thermosiphon plant
C.Rossi - MFIP 2014 – 19 September 2014
The full scale thermosiphon
Operating point Pressure [barabs]
Temperature [°C]
Density [kg/m3]
Enthalpy [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
Thermodynamic cycle of the thermosiphon circuit and corresponding schematic. Thermosiphon circuit (A-I). Beyond these points C3F8 enters the internal cooling circuits. Fluid exits the detectors at point M point (E’ in the previous compressor evaporative cycle)
Increase of hydrostatic pressure
ΔT in AB and GH is different (AB = 40 °C; GH = 31 °C): C3F8 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 C3F8 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