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.
We investigate and address the performance limitations of the ATLAS silicon tracker fluorocarbon evaporative cooling system operation in the cooling circuits of the barrel silicon microstrip (SCT) sub-detector. In these circuits the minimum achievable evaporation temperatures with C3F8 were higher than the original specification, and were thought to allow an insufficient safety margin against thermal runaway in detector modules subject to a radiation dose initially foreseen for 10 years operation at LHC.We have investigated the cooling capabilities of blends of C3F8 with molar admixtures of up to 25% C2F6, since the addition of the more volatile C2F6 component was expected to allow a lower evaporation temperature for the same evaporation pressure.A custom built recirculator allowed the in-situ preparation of C2F6/C3F8 blends. These were circulated through a representative mechanical and thermal setup reproducing an as-installed ATLAS SCT barrel tracker cooling circuit. Blend molar compositions were verified to a precision of 3.10−3 in a custom ultrasonic instrument.Thermal measurements in a range of C2F6/C3F8 blends were compared with measurements in pure C3F8. These indicated that a blend with 25% C2F6 would allow a reduction in evaporation temperature of around 9̂C to below -15̂C, even at the highest module power dissipations envisioned after 10 years operation at LHC. Such a reduction would allow more than a factor two in safety margin against temperature dependant leakage power induced thermal runaway.Furthermore, a blend containing up to 25% C2F6 could be circulated without changes to the on-detector elements of the existing ATLAS inner detector evaporative cooling system.
The silicon tracker of the ATLAS experiment at CERN Large Hadron Collider will operate around -15°C to minimize the effects of radiation damage. The present cooling system is based on a conventional evaporative circuit, removing around 60 kW of heat dissipated by the silicon sensors and their local electronics. The compressors in the present circuit have proved less reliable than originally hoped, and will be replaced with a thermosiphon. The working principle of the thermosiphon uses gravity to circulate the coolant without any mechanical components (compressors or pumps) in the primary coolant circuit. The fluorocarbon coolant will be condensed at a temperature and pressure lower than those in the on-detector evaporators, but at a higher altitude, taking advantage of the 92 m height difference between the underground experiment and the services located on the surface. An extensive campaign of tests, detailed in this paper, was performed using two smallscale thermosiphon systems. These tests confirmed the design specifications of the full-scale plant and demonstrated operation over the temperature range required for ATLAS. During the testing phase the system has demonstrated unattended long-term stable running over a period of several weeks. The commissioning of the full scale thermosiphon is ongoing, with full operation planned for late 2015.
We describe an ultrasonic instrument for continuous real-time analysis of the fractional mixture of a binary gas system. The instrument is particularly well suited to measurement of leaks of a high molecular weight gas into a system that is nominally composed of a single gas. Sensitivity < 5 × 10−5 is demonstrated to leaks of octaflouropropane (C3F8) coolant into nitrogen during a long duration (18 month) continuous study. The sensitivity of the described measurement system is shown to depend on the difference in molecular masses of the two gases in the mixture. The impact of temperature and pressure variances on the accuracy of the measurement is analysed. Practical considerations for the implementation and deployment of long term, in situ ultrasonic leak detection systems are also described. Although development of the described systems was motivated by the requirements of an evaporative fluorocarbon cooling system, the instrument is applicable to the detection of leaks of many other gases and to processes requiring continuous knowledge of particular binary gas mixture fractions.
Precision sound velocity measurements can simultaneously determine binary gas composition and flow. We have developed an analyzer with custom electronics, currently in use in the ATLAS inner detector, with numerous potential applications. The instrument has demonstrated ~0.3% mixture precision for C 3 F 8 /C 2 F 6 mixtures and < 10 -4 resolution for N 2 /C 3 F 8 mixtures. Moderate and high flow versions of the instrument have demonstrated flow resolutions of ± 2% F.S. for flows up to 250 l.min -1 , and ± 1.9% F.S. for linear flow velocities up to 15 ms -1 ; the latter flow approaching that expected in the vapour return of the thermosiphon fluorocarbon coolant recirculator being built for the ATLAS silicon tracker.
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