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
Katja Klein
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)
Katja Klein
CO2 Studies with 4 Pipes

4. 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

5. The CO2 Set-up at Aachen
Katja Klein
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
Katja Klein
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 & 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

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

12. 4-Pipe Measurement with x = 0.26
Choose start flow so that x =  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

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
Katja Klein
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
Katja Klein
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
Katja Klein
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
Katja Klein
CO2 Studies with 4 Pipes

17. Back-up slides

18. CO2 Phase Diagram
Katja Klein
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
Katja Klein
CO2 Studies with 4 Pipes

20. Dependencies in the System
Katja Klein
CO2 Studies with 4 Pipes