A combined ultrasonic flow meter and binary vapour mixture analyzer for the ATLAS silicon tracker

Abstract

An upgrade to the ATLAS silicon tracker cooling control system may require a change from C 3 F 8 (octafluoro-propane) evaporative coolant to a blend containing 10-25 % of C 2 F 6 (hexafluoro-ethane). Such a change will reduce the evaporation temperature to assure thermal stability following radiation damage accumulated at full LHC luminosity.Central to this upgrade is a new ultrasonic instrument in which sound transit times are continuously measured in opposite directions in flowing gas at known temperature and pressure to deduce the C 3 F 8 /C 2 F 6 flow rate and mixture composition. The instrument and its Supervisory, Control and Data Acquisition (SCADA) software are described in this paper.Several geometries for the instrument are in use or under evaluation. An instrument with a 'pinched axial' geometry intended for analysis and measurement of moderate flow rates has demonstrated a mixture resolution of 3.10 -3 for C 3 F 8 /C 2 F 6 molar mixtures with ~20 %C 2 F 6 , and a flow resolution of 2 % of full scale for mass flows up to 30gs -1 . In mixtures of widelydiffering molecular weight (mw), higher mixture precision is possible: a sensitivity of < 5.10 -5 to leaks of C 3 F 8 into part of the ATLAS tracker nitrogen envelope (mw difference 160) has been seen.An instrument with an angled sound path geometry has been developed for use at high fluorocarbon mass flow rates of around 1.2 kgs -1 -corresponding to full flow in a new 60kW thermosiphon recirculator under construction for the ATLAS silicon. Extensive computational fluid dynamics studies were performed to determine the preferred geometry (the ultrasonic transducer spacing and placement together with the sound crossing angle with respect to the vapour flow direction). A prototype instrument with 45º crossing angle has demonstrated a flow resolution of 1.9 % of full scale for linear flow velocities up to 15 ms -1 .In addition a further variant of the instrument is under development to allow the detection and elimination of incondensable vapour accumulating in the condenser of a fluorocarbon recirculator.The combined flowmeter and binary gas analysis instrument has many potential applications, including the analysis of hydrocarbons, vapour mixtures for semi-conductor manufacture and anaesthetic gas mixtures.

The Thermosiphon Cooling System of the ATLAS Experiment at the CERN Large Hadron Collider

“…A blend containing 20-25% C 2 F 6 in C 3 F 8 would enable a lower evaporation temperature for the same pressure, thus satisfying the system requirements. A series of tests has been performed over a range of C 2 F 6 /C 3 F 8 molar concentrations, using a test station that thermally reproduced an installed SCT cooling circuit (Bates et al 2013;2015;Rossi et al, 2014). Future studies of blend circulation in a thermosiphon are envisioned.…”

A custom on-line ultrasonic gas mixture analyzer with simultaneous flowmetry developed for use in the LHC-ATLAS experiment, with wide application in high and low flow gas delivery systems

“…It is installed in the cir cuit circulating the gas mixture to be analyzed and is designed to allow simultaneous on-line analysis and flowmetry through the transmission of sound pulses in opposite directions. In low flow applications the sound path is usually aligned with the gas flow direction [6], [7], while for high flows it crosses the gas flow at an angle, with transducers positioned so as to minimize turbulence and pressure drop in the flowing gas. Our studies have shown that high precision flow measurement is possible without the requirement of the entire sound path length being contained within the flowing gas volume.…”

“…Our studies have shown that high precision flow measurement is possible without the requirement of the entire sound path length being contained within the flowing gas volume. We have developed several instruments with angled and axial geometries [6].…”

“…One such instrument, using a pinched axial geometry has been used with binary gas flows up to 230 I.min" , and demonstrated a flow resolution better than 1 % and a mixture resolution of 3.10' 3 for C3F8/C2F6 molar mixtures containing 20% C2F6 [6]. Other instruments are used to monitor C3F8 leaks into the N2 environmental volumes of the AT LAS pixel and SCT detectors.…”

“…The use of a blend containing up to 25% of the more volatile C2F6 could solve the problem. However, it will be critical to monitor the C3F8/C2F6 mixture ratio and adj ust the blend in real time to ensure the adequate cooling of the detectors [5], [6], [7]. An instrument developed for this purpose has demonstrated a precision in blends of C3F8/C2F6 of around ± 0.3% while allowing simultaneous flow measurement.…”