Outer Tracker Preheater Temperatures Tom French CERN 29092020

Outer Tracker: Preheater Temperatures Tom French (CERN) 29/09/2020

Observed Effects & Rationale for Pre-Heaters • Liquid CO 2 can arrive at the detector sub-cooled through coaxial pipes, to bring it up to the saturation temperature, we need to inject heat. CO 2 Set Point -24. 5 °C Dummy Load Power 324 W Pre-Heater Power 0 W Flow Rate 1. 3 g/s Sub-cooled CO 2 liquid over first 1 -2 modules with temperature rise of 3°C, then boiling starts and temperature drops due to pressure drop in pipe. • When it reaches the saturation point, the liquid CO 2 may not start to boil immediately, it can become superheated if heated gently (e. g. by modules or environmental heat leak). We need to apply a “kick” to trigger boiling. 20/06/2019 +13°C superheat CO 2 Set Point -24. 5 °C Dummy Load Power 324 W Pre-Heater Power 0 W Flow Rate 1. 3 g/s Superheating visible on dummy load start-up at -24°C with no pre-heater. Temperature rises by 15°C before suddenly dropping back to the 2 -phase curve seen above. T. French - G. Baldinelli – F. Bianchi 22

Preheater Operation Preheater Test Results – Temperature Plots Principle of operation: • A small heater creates a high heat flux on the tube wall and generates bubbles. This disturbs any superheating downstream of the heater and provides stable temperatures for the detector electronics. Baseline operation: 1. The CO 2 flow is started. 2. The preheater is turned on to prevent superheating. 3. The modules are turned on. • If needed, the preheater can be turned off, or off then on again after the modules are on. • Benefit: can check preheater temperature to see if cooling is working before switching on modules. Alternative operation: 1. The CO 2 flow is started. 2. The modules are turned on. 3. The preheater is turned on to remove any superheating which is present. 3

Example TB 2 S Pipework Return manifold OUTLET TB 2 S Ladder Evaporator tube 2 or 3 ladders in series TB 2 S Ladder INLET Evaporator tube Inlet capillary Preheater outside ladder Inlet manifold Alternative preheater position inside the ladder (Mikko Barinoff) 4 kamil. cichy@cern. ch, thomas. french@cern. ch

Example TBPS Tilted Pipework Return manifold OUTLET TBPS Layer TBPS Ring Preheater on ring tube Inlet capillary Detector flange INLET Evaporator tube Inlet Manifold 5

Temperatures • A temperature sensor on the preheater would give the following readings: Preheater off, cooling off Shows pipe temperature Preheater off, cooling on Shows CO 2 temperature Preheater on, cooling on Check cooling is on before turning modules on Preheater on, cooling off Preheater overheats rapidly (2°C/s) • Therefore, a temperature reading should be present to prevent overheating of the resistors. If they overheat (to about 180°C) the casing melts and the resistance tends to open circuit. • 1 temperature reading is needed per preheater, with which an interlock would turn off both resistors in the case of overheating. There is one preheater (each with 2 resistors) per evaporator section (e. g. 3 ladders in series). • 2 temperature sensors could be used for redundancy. 6

Flow Stop 7

Monitoring CO 2 • Perhaps a useful place to put a temperature sensor if we wanted to monitor the CO 2 would be as shown in the plot below. • On the pipe, >10 mm away from a heat source, to give CO 2 fluid temperature. • A sensor here, combined with a general return manifold T reading, would show the delta T (therefore pressure drop) in each evaporator. • This sensor would also show if large amounts of superheating were happening. Possible sensor position 9

Test Setup: TB 2 S Ladders • Using existing Tracker Outer Barrel rods with the right size cooling pipes (2. 0 x 2. 2 mm made out of Copper-Nickel), installed in an insulated box in the cold room in CERN building 186. The cold room is set to the CO 2 saturation temperature (down to -25°C) to give an essentially adiabatic test environment (room temperature tuned to give no change in liquid CO 2 temperature through the setup). • Modules represented by 72 x 200Ω dummy load resistors providing 2 W nominal (4 W max) per cooling point, controlled by a power supply. • TRACI CO 2 plant used (can achieve ≈ -25°C with 150 W load). • Pre-heater mounted on pipe before 1 st ladder inlet and controlled with a separate power supply. • Temperature distribution mapped using PT 100 RTDs. CO 2 Out 20/06/2019 T. French - G. Baldinelli – F. Bianchi CO 2 In 8 mm pre-heater concept tested (2 x 10Ω resistors clamped to pipe) 10

Preheater Operation – Summary 9: 08 Preheater on with 10 W. Flow was 2. 62 g/s. I left it with just the preheater on for just under 2 hours to see if there were any strange effects, none seen. 11: 13 Module power on with 234 W for 3 ladders. Due to increased resistance, the flow decreased from 2. 62 g/s to 2. 35 g/s. The temperature increase in the preheater is due to this drop in flow rate. 11: 27 Preheater turned off. Cooling insert temperature drops in the first few cooling inserts by about 2°C. 12: 08 Module power off. • Note that there is no superheating in this plot (due to the preheater being on). Without the preheater (or with too small power in the preheater), there is about 4°C of superheating of the fluid, and poorer HTC in the first few cooling inserts. 11

Preheater on BEFORE modules • Transition observed in insert temperature at 5. 9 W for this particular example. Superheating gone! 12