Performance of a CO 2 System with Four Parallel Pipes and Variable Heat Loads

1. Performance of a CO2 System with Four Parallel Pipes and Variable Heat Loads L. Feld, K. Klein, F. Scholz, M. Wlochal RWTH Aachen University Phase-1 Cooling Meeting, October 29th, 2012

2. History • We started to install a recirculating CO2 cooling system in 2009 • First measurements shown by Jennifer Merz in February 2010 • Measurement problems triggered continuous improvement of the set up • e.g. installation of evacuated box, more powerful chiller, … • Jennifer left for industry at the end of 2011 • Spring 2012: Bachelor work by Franziska Scholz; behaviour during parallel operation of four pipes CO2 Studies with 4 Pipes

3. Motivation • In the detector, the load on parallel pipes can be different • by construction • due to problems, e.g. when power on one part has to be switched off • This leads to reduction of the flow resistance on pipes with lower load • Flow will prefer to go through pipes with lower flow resistance • Pipes with higher heat load will see lower flow  danger of dryout • Can be fixed by adding large flow resistances (capillaries) to each pipe  differences in flow resistance insignificant • We wanted to observe the effect in the lab (without capillaries) CO2 Studies with 4 Pipes

4. The CO2 Set-up at Aachen Fill level sensor Expansion vessel Drain CO2 bottle Chiller 1 Burst disk Vacuum pump Heat exchanger 1 Heat exchanger 2 Detector pipes Flow meter Chiller 2 Vacuum box pump CO2 Studies with 4 Pipes

5. The CO2 Set-up at Aachen CO2 Studies with 4 Pipes

6. The CO2 Set-up at Aachen • 2 x Unistat 815 from Huber • Gather CO2 pump • Expansion vessel from Swagelok • … Original version, before insulation CO2 Studies with 4 Pipes

7. The CO2 Set-up at Aachen • Four stainless steel pipes of 5.25m length • Inner / outer diameter of 1.7mm / 2.0mm • 14 termistors on pipes 1 & 2 4 termistors on pipes 3 & 4 • Variabel heat load applied by sending currents through the pipes (up to 400W per pipe) Vacuum box with 4 pipes CO2 Studies with 4 Pipes

8. The CO2 Set-up at Aachen CO2 Studies with 4 Pipes

9. Dryout Measurement Method • At what vapour fraction x does dryout occur? • The last thermistor on the pipe will see it first • Apply heat load • Reduce flow  • Measure temp. increase on last thermistor • Extract Dryout from linear fit Temperatur of thermistors vs. time Temp. of last thermistor vs. flow P = 220W Temperature [°C] Temperature [°C] 150W 180W 200W 220W Measurement point Flow [g/min] Reduction of flow CO2 Studies with 4 Pipes

10. Dryout on one Pipe Effective heat load: Dryout [g/min] Effective heat load [W] Extract xDryout (and heat from environment) from linear fit: CO2 Studies with 4 Pipes

11. Flow on the four Pipes • Differences in flow observed between the 4 pipes (due to manifold?) • This has to be kept in mind for the following Flow in single pipes for 2800 rev./min Pipe 1 Pipe 2 Pipe 3 Pipe 4  [g/min] Measurement point CO2 Studies with 4 Pipes

12. 4-Pipe Measurement with x = 0.26 • Choose start flow so that x = 0.26 no dryout expected (Note: this is the mean x!) • Apply 60W on each pipe • Remove consecutively heat load on pipes 4  3  1 • Observations: • Flow increases due to reduction of flow resistance • Pressure drop over pipes decreases since pressure built up in pump decreases • T on pipe decreases when its load is removed, due toT = P/(A) betw. pipe and CO2 • T on other pipes decreases, since p on outlet decreases • No dryout Total flow dp over pipes T on pipe 1 T on pipe 2 T on pipe 3 T on pipe 4 Time CO2 Studies with 4 Pipes

13. 4-Pipe Measurement with x = 0.39 • Choose start flow so that mean x close to dryout (determined experimentally) • Apply 60W on each pipe • Remove consecutively heat load on pipes 4  3  2 • Remember: flow 4 < 3 < 2 < 1 • Observations: • Flow and pressure as before • At the start, signs of dryout on pipe 4 (increased T fluctuations) • Removing heat loads shifts dryout signs to pipe with next lower flow • Pipe 4 stabilizes Total flow dp over pipes T on pipe 1 T on pipe 2 T on pipe 3 T on pipe 4 Time CO2 Studies with 4 Pipes

14. 4-Pipe Measurement with x = 0.39 • Effects depend on the order! • Apply 60W on each pipe • Remove consecutively heat load on pipes 1  2  3 • Remember: flow 4 < 3 < 2 < 1 • Observations: • When heat is removed from pipe 1, all pipes show signs of starting dryout • Clear dryout on pipe 4 after heat is removed from pipe 2 Total flow dp over pipes T on pipe 1 T on pipe 2 T on pipe 3 T on pipe 4 Time CO2 Studies with 4 Pipes

15. 4-Pipe Measurement with x = 0.31 • In the end mean x was determined where there is just no sign of dryout: xfour = 0.31 • Much lower than in single pipe measurement (xsingle = 0.76) • Remove consecutively heat load on pipes 4  3  2 Total flow dp over pipes T on pipe 1 T on pipe 2 T on pipe 3 T on pipe 4 Time CO2 Studies with 4 Pipes

16. Conclusions • Effect is clearly observed: reduction of load on one pipe can bring others, that were before stable, into dryout • Dryout occurs at much lower vapour fraction than with a single pipe • Would have been interesting to repeat measurement with capillaries, but Franziska‘s Bachelor thesis had to be finished before • Now we concentrate on development of cooling blocks for the Phase-2 outer tracker module, and on DC-DC converter cooling bridges for Phase-1 pixels CO2 Studies with 4 Pipes

17. Back-up slides

18. CO2 Phase Diagram CO2 Studies with 4 Pipes

19. Pressure Drops in the Set-up • Pressure is built up by pump, partly dropped by filter • For a fixed flow resistance, increasing the revolution number increases the flow and also the pressure drop in the pump • If flow resistance is reduced at a fixed revolution number, the flow increases, and the pressure drop in the pump decreases • Pressure drop over pump and filter must equal pressure drop over pipes CO2 Studies with 4 Pipes

20. Dependencies in the System CO2 Studies with 4 Pipes