Forum on Tracking Detector Mechanics 2013 University of Oxford June 19 th -21 th C 2 F 6 /C 3 F 8 saturated fluorocarbon cooling blends for the ATLAS SCT Steve McMahon RAL/STFC 1

I am not Alex Bitadze ! This talk was scheduled to be given by Alexander Bitadze Alex works for SUPA School of Physics & Astronomy, University of Glasgow Alex made all of the measurements show here as part of his PhD and prepared most of the material in this talk. Any mistakes in the presentation today are purely mine European Organization for Nuclear Research The real Alex Bitadze 2

3 In an evaporative cooling system the ON detector temperature is controlled by the ON detector pressure (lower pressure→ lower temperature). This pressure is controlled by a BPR Back Pressure Regulator which is typically some distance from the detector. In our case the coolant is C 3 F 8. During the commissioning of the SCT it became clear that the pressure drops between the end of the ON detector cooling circuits and The distribution racks were larger than expected. This is only true for the barrel circuits where the mass flow is higher (44 of 116 SCT circuits). The situation became even slightly worse when we were forced to move the heaters. Too large a pressure drop means that at the high-end of power dissipation we cannot realize the lowest required temperatures of -25 o C on the detector. We then became concerned that at the end of life of the detector (TDR : 10yrs → 700fb -1 ) there would be very little head room against thermal runaway. Note this would only be a problem at the end of the life of the detector.

4 It was/is not possible to change any of the mechanical parts of the cooling system within the ATLAS cavern. The only way to recover the situation was to change the coolant itself. Use a mixture of C 2 F 6 and C 3 F 8 In a fluorocarbon blend one gets a lower evaporation temperature for the same evaporation pressure.

5 This setup has been used to perform the tests for the SCT Barrel cooling system. We have duplicated the in-pit installation as far as possible above the ground. Before making any changes build a full scale replica of the cooling system in SR1 at Point 1 at CERN

6 Schematic of cooling system assembly in SR1 laboratory

7 Detector Circuit = 500W

8 Schematic of cooling system assembly in SR1 laboratory Counter Flow HEX

9 Schematic of cooling system assembly in SR1 laboratory Inlet

10 Schematic of cooling system assembly in SR1 laboratory Exhaust

11 Schematic of cooling system assembly in SR1 laboratory PA2

Fluorocarbon Blending Machine 12

The P&I for the Fluorocarbon Blending Machine C3F8C3F8 C2F6C2F6 13

14 Compressor with single stage (9 bar output): liquid booster pump (3m below condenser): liquid delivered through 50m un-precooled tubes to thermal test stand at other end of SR1

15 Comparison of Pressure-enthalpy diagrams in pure C3F8 and 25%C2F6/75%C3F8 for circulation in the SR1 blend machine There is a single stage of compression followed by liquid boost from pump located 3m below condenser. The un-cooled liquid delivery lines before simulated ID volume with local heat exchanger Pressure Enthalpy Diagrams for pure C 3 F 8 & Fluorocarbon mixtures.

16 C2F6/C3F8 blends made by liquid phase mass mixing of the two components into the condenser. Saturation pressure of the molar blend verified 2 nd verification of molar blend ratio in vapour phase before circulation through thermal test stand (using local dummy load on blend machine) Continuous verification of molar blend ratio in (exhaust) vapour phase during circulation through thermal test stand located 60m from blend machine No evidence for preferential loss of either component during long term circulation Making and monitoring C 2 F 6 /C 3 F 8 blends

We developed a combined on-line acoustic flow-meter and fluorocarbon mixture analyser For precise measurements of proportion in C 2 F 6 /C 3 F 8 mixtures and for measurements of flow values, new device “Sonar Analyser” was assembled, tested and installed in system. If a blend is used, real time concentration information would be transferred to the evaporative cooling system PLC to control the circulating C 2 F 6 /C 3 F 8 mixture proportion. 17

A combined on-line acoustic flow-meter and fluorocarbon mixture analyser Ultrasonic flow meter linearity comparison with a Schlumberger ∆G16 gas meter and Bronkhorst F-113AI- AAD-99-V flowmeter: C3F8 vapour at 20°C, 1.1barabs Maximum flow 230 lmin -1 (30 gs -1 ): rms precision: ≤ 1% of full scale in both comparison instruments. Comparison between measured sound velocity data and theoretical predictions in C3F8 / C2F6 mixtures. Sound velocity measurement uncertainty of ± 0.05ms -1 gives mixture uncertainty ± 0.3% at 20% C2F6 conc. 18

19 A Standard set of measurements would be taken with With different input liquid pressure : 10/11/12/13 bara With different power load on modules : 0W, 3W, 6W, 9W, 10.5W and different back pressure (dome pressure): “Open Bypass” (1.3bara)/1.6 / 2 / 2.5 / 3 / 4 / 5 / 6 bara

Data for SCT Barrel Structure (0% - 25% of C 2 F 6 ) Figure1. Maximum temperature over the SCT Barrel stave, in case of 13bar abs inlet pressure and different dome pressure applied to Back Pressure Regulator. Figure2. Maximum temperature over the SCT Barrel stave, in case of 13bar abs inlet pressure and different dome pressure applied to Back Pressure Regulator. 20

Figure3. Maximum temperature over the SCT Barrel stave, in case of 13bar abs inlet pressure and different back pressure before the Back Pressure Regulator. Figure4. Maximum temperature over the SCT Barrel stave, in case of 13bar abs inlet pressure and different back pressure before the Back Pressure Regulator. Data for SCT Barrel Structure (0% - 25% of C 2 F 6 ) 21

Figure5. Maximum temperature over the SCT Barrel stave, in case of 13bar abs inlet pressure and different pressure over the Stave*. Figure6. Maximum temperature over the SCT Barrel stave, in case of 13bar abs inlet pressure and different pressure over the Stave*. * P_A2 pressure sensor (see appendix) Data for SCT Barrel Structure (0% - 25% of C 2 F 6 ) 22

Figure7. Temperature over the SCT Barrel stave, in case of 13bar abs inlet pressure and 1.2bar abs (Open by-pass) back pressure in system. Figure8. Temperature over the SCT Barrel stave, in case of 13bar abs inlet pressure and 2bar abs back pressure in system. Data for SCT Barrel Structure (0% - 25% of C 2 F 6 ) 23

Figure9. Difference in Maximum Temperature over the SCT Barrel stave, in case of 13bar abs inlet pressure and different back pressure (Dome) in system. Data for SCT Barrel Structure (0% - 25% of C 2 F 6 ) 24

Figure10. Maximum Temperature over the SCT Barrel stave changing according to C 2 F 6 concentration. In case of 13bar abs inlet pressure and 1.2bar abs (Open by-pass) back pressure in system. Data for SCT Barrel Structure (0% - 25% of C 2 F 6 ) 25

Figure11. Temperature over the SCT Barrel stave changing according to C 2 F 6 concentration. In case of 13bar abs inlet pressure and 1.2bar abs (Open by-pass) back pressure in system. With Theoretical prediction band. Data for SCT Barrel Structure (0% - 25% of C 2 F 6 ) 26

Figure12. Temperature over the SCT Barrel stave changing according to C 2 F 6 concentration. In case of 13bar abs inlet pressure and 1.2bar abs (Open by-pass) back pressure in system. With Theoretical prediction band. Data for SCT Barrel Structure (0% - 25% of C 2 F 6 ) 27

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Gas Mixture Analyzer and Flow-meter 29

Conclusion Excessive pressure drops in the cooling circuits of the Barrel SCT have driven us to consider fluorocarbon blends as a solution that would allow us to operate the detector at the coldest required temperatures as set out in the TDR. We have shown that if we were to need to go to these temperatures blends of C 3 F 8 and C 2 F 6. in the ratio ~ 3::1 would be able to achieve this without changes to the ON detector structures. We have also shown a similar performance for the Pixel structures. We have also demonstrated control systems based on measuring the speed of sound in a gas that would allow us to control the fractions of the different constituent fluorocarbon gases. New information that has come to light since this work was started (a reduction in the expected luminosity before the tracker will be replaced, a better understanding of the on detector power load and the effects of radiation damage on silicon) make us believe that this option will not be required. The construction of a Thermo-Siphon cooling system as a replacement for the current compressor based system is well advanced and on target for commissioning later this year. Operating blends with this system was foreseen in the design. 30

Back Up Material 31

R. Bates 1, M. Battistin 2, S. Berry 2, J. Berthoud 2, A. Bitadze 1, P. Bonneau 2, J. Botelho-Direito 2, N. Bousson 3, G. Boyd 4, G. Bozza 2, E. Da Riva 2, O. Crespo-Lopez 1, C. Degeorge 7, C. Deterre 5, B. DiGirolamo 1, M. Doubek 6, G. Favre 2, J. Godlewski 2, G. Hallewell 3, S. Katunin 8, N. Langevin 9, D. Lombard 2, M. Mathieu 3, S. McMahon 10, K. Nagai 11, D. Robinson 12, C. Rossi 13, A. Rozanov 3, V. Vacek 6, M. Vitek 6 and L. Zwalinski 2 1 School of Physics and Astronomy, University of Glasgow, UK Marseille, June 25th CERN, Switzerland 3 CPPM, France 4 Department of Physics and Astronomy, University of Oklahoma, USA 5DESY, Germany 6 Czech Technical University, Czech Republic 7 Physics Department, Indiana University, USA 8 PNPI, Russia 9 IUT, University of Aix-Marseille, France 10 STFC Rutherford Appelton Laboratory, UK 11 University of Innsbruck, Austria 12 Department of Physics and Astronomy, University of Cambridge, UK 13 Department of Mechanical Engineering, Università degli Studi di Genova, Italy Other People Involved