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

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 Katja Klein CO2 Studies with 4 Pipes

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) Katja Klein CO2 Studies with 4 Pipes

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

The CO2 Set-up at Aachen Katja Klein CO2 Studies with 4 Pipes

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

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 Katja Klein CO2 Studies with 4 Pipes

The CO2 Set-up at Aachen Katja Klein CO2 Studies with 4 Pipes

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 Katja Klein CO2 Studies with 4 Pipes

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

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 Katja Klein CO2 Studies with 4 Pipes

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 Katja Klein CO2 Studies with 4 Pipes

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 Katja Klein CO2 Studies with 4 Pipes

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 Katja Klein CO2 Studies with 4 Pipes

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 Katja Klein CO2 Studies with 4 Pipes

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 Katja Klein CO2 Studies with 4 Pipes

Back-up slides

CO2 Phase Diagram Katja Klein CO2 Studies with 4 Pipes

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 Katja Klein CO2 Studies with 4 Pipes

Dependencies in the System Katja Klein CO2 Studies with 4 Pipes