Aachen Status Report: CO2 Cooling for the CMS Tracker Lutz Feld, Waclaw Karpinski, Jennifer Merz, Michael Wlochal RWTH Aachen University, 1. Physikalisches Institut B 21 July 2010 MEC Upgrade Meeting

Outline Test System in Aachen Results Summary and Outlook Goals and specifications Schematic design Set-up Performance Results Dryout Pressure and temperature drop Summary and Outlook Jennifer Merz

R & D in Aachen Ongoing: Gain experience with a closed recirculating CO2 system Determine lowest operating temperature Find out ideal operating conditions ( stable system), depending on heat load and CO2 temperature Midterm plans: Measurements on pipe routing inside the tracker (number of bendings, bending radius, inner diameter, ...) Determine optimal cooling contact between cooling system and heat dissipating devices (different materials, different types of thermal connections, ...) Contribute to final module design for tracker at SLHC Jennifer Merz

System Specifications Maximum cooling power: 500W CO2 temperature in detector: -45°C to +20°C Precise flow and temperature control Continuous operation Safe operation (maximum pressure:100bar) Jennifer Merz

Schematic View of the CO2 System Chiller 1: Chiller temperature vapour pressure  system temperature Expansion Vessel: Saturated mixture of CO2 liquid and vapour ΔQ 2 3 pressure, bar ΔQ 1 4 6 5 Enthalpy, kJ/kg Heat Exchanger : Subcooling of incoming CO2 (only liquid in pump) Dissipation of detector heat load Heat Exchanger : Warm incoming CO2 to nominal temperature ( given by chiller 1) Partial condensation of returning CO2 Up to 500 W heat load 5

CO2 Test System (I) CO2-Bottle CO2-Flasche Expansion Vessel 16cm CO2 Detector 42cm 7.6cm Heat Exchanger 19cm Jennifer Merz

Electrical connections CO2 Test System (II) Thermistors CO2-Bottle CO2-Flasche Users panel Electrical connections 6m stainless steel pipe, 1.7mm inner diameter 14 Thermistors along the pipe: Measurement of temperature distribution Simulation of uniform heat load, by current through pipe ( ohmic losses) Box for insulation Jennifer Merz

Influence of Chiller 2 Chiller 2 should only subcool CO2 to ensure liquid in the pump Keep chiller 1 temperature (= detector temperature) constant Vary chiller 2 temperature and observe detector temperature For chiller 2 temperatures from +15 to -5°C: detector temperature at +20°C For lower chiller 2 temperatures: the detector temperature drops drastically Chiller 1 @ +20°C Average detector temp., °C In the following: ΔT = 10K between chillers  detector temperature determined by chiller 1 More investigation at different temperatures needed Temp. of chiller 2, °C Jennifer Merz

Heat Load at -45°C Low temperatures can be reached 14 12 10 8 6 4 2 14 thermistors along pipe -45°C CO2 temperature 13 11 9 7 5 3 1 Detector temperature, °C Additional heat input from environment influences measurements, especially at low temperatures 6m long pipe 1.7mm inner diameter ~ 50 g/min flow Increase heat load by 140W Heat Load Low temperatures can be reached No significant change in detector temperature with applied heat load Jennifer Merz

Dryout Measurement Dryout: pipe walls not in touch with liquid anymore gas x=0 x=1 x: vapour quality Dryout: pipe walls not in touch with liquid anymore  No heat dissipation by evaporating CO2 Rise in detector temperature Temperature distribution over detector 14 12 10 8 6 4 2 14 thermistors along pipe Detector temperature, °C CO2 temperature: +20°C 13 11 9 7 5 3 1 Keep heat load constant Decrease flow step by step Determine where detector temperature rises over nominal value Decrease flow Time, s Jennifer Merz

Measurement of Dryout @ +20°C 30 W 40 W 50 W 60 W 70 W 80 W 90 W 100 W CO2-Temperature: +20°C The higher the heat load, the larger the flow at which dryout is observed Jennifer Merz

Measurement of Dryout @ 0°C 60 W 80 W 100 W CO2-Temperature: 0°C At a lower operating temperature: smaller slope, dryout is observed at smaller flows Jennifer Merz

Comparison Flow vs. Heat Load Determine flow for which just no dryout is observed (from plots on previous slides) For future detector layout safety factor will be applied to avoid dryout by all means inside detector volume Flow, g/min CO2 @ +20°C CO2 @ 0°C Heat Load, W The higher the heat load, the more flow is needed to dissipate the power At a lower operating temperature less flow is needed to dissipate certain heat load Jennifer Merz

Pressure Drop along Detector Pipe Temperature distribution over detector pipe 14 12 10 8 6 4 2 14 thermistors along pipe 13 11 9 7 5 3 1 Detector temperature, °C Determine pressure drop from temperature distribution over detector pipe No heat load -20°C CO2 temperature No heat load -20°C CO2 temperature Decrease flow Time, s Δp, bar In a 2-phase system: pressure drop = temperature drop Measurement of pressure gradient important to precisely control detector temperature Determine Δp between in- and outlet Jennifer Merz Flow, g/min

Pressure Drop with Heat Load -20°C CO2 temperature 100 W 50 W 20 W 0 W Heat input from environment visible for small flows Heat load affects pressure drop The higher the heat load, the higher the pressure drop Jennifer Merz

Pressure Drop: Comparison with Theory Theory curves: Thome model -30°C -20°C -10°C 0°C x=0.15 x=0.10 x=0.09 x=0.05 Measurement agrees with theory for high flows Measured Δp higher for small flows Discrepancy can be explained by creation of vapour due to heat input from environment  higher flow resistance Jennifer Merz

Summary of Results Results for: L=5.5m, di = 1.7mm, Φ = 50g/min Temperature [°C] Pcool* [W] Δp [bar] ΔT [K] +20 50 0.2 0.02 0.4 -20 1.0 0.9 -45 70 1.3 6 *With a safety factor of 2, corresponding to a maximum vapour quality of 0.5 inside detector volume Jennifer Merz

Summary CO2 test system fully commissioned and operational First measurements to low temperatures show: reasonable cooling power at -45°C Pressure drop measurements: at higher temperatures (0°C, -10°C): good agreement, small heat input from environment at lower temperatures (-20°C, -30°C): worse agreement, significant heat input Dryout Measurements: important to determine point of dryout for a given pipe layout, more measurements will be done and compared with theory For the given layout (L=5.5m, di = 1.7mm, Φ = 50g/min) at least 70W (incl. safety factor) can be dissipated at -45°C with a pressure drop of 1.3bar Jennifer Merz

Outlook Improvements of test system ongoing: - Vacuum box for detector pipe: minimize heat input from environment - New heat exchanger: less massive, should allow faster measurements - Install dedicated pressure drop sensor: improve accuracy of measurement Perform more measurements on pressure and temperature drop along different pipes: - Vary inner diameter and form/bending - Investigate influence of parallel piping on performance Determine optimal cooling contact between heat dissipating devices and cooling system Jennifer Merz

Back-Up... Jennifer Merz

Temperature-Pressure-Diagram Jennifer Merz

Dryout – Comparison with Theory Want to compare results to so-called Flow Pattern Maps Different flow regimes can be identified 50 g/min 60 g/min 70 g/min 80 g/min 90 g/min 100 g/min CO2-Temperatur: +20°C Mass Velocity kg/(m2s) Vapour Quality Jennifer Merz

Dryout – Comparison with Theory 60 g/min 80 g/min 100 g/min CO2-Temperatur: 0°C Mass Velocity kg/(m2s) Vapour Quality Jennifer Merz

Heat Load at -45°C Low temperatures can be reached 14 12 10 8 6 4 2 14 thermistors along pipe -45°C CO2 temperature 13 11 9 7 5 3 1 Detector temperature, °C Zusätzlicher wärmeeintrag, insbes. Bei tiefen temperaturen 6m long pipe 1.7mm inner diameter ~ 50 g/min flow Increase heat load by 140W Remark: Enormous temperature differences between pipe and room temperature (here 60K!!) lead to heat input from environment despite insulation Detector pipe will be placed into vacuum box soon Further: heat load means power that was applied from power supply (heat input from environment not taken into account so far) Heat Load Low temperatures can be reached Constant (?) detector temperature with applied heat load Jennifer Merz