1. 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 MEC Upgrade Meeting

2. 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

3. 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

4. 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

5. 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

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

7. 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

8. 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 +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

9. Heat Load at -45°C Low temperatures can be reached14 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

10. 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

11. 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

12. 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

13. 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
+20°C
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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. Back-Up...
Jennifer Merz

21. Temperature-Pressure-Diagram
Jennifer Merz

22. 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

23. 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

24. Heat Load at -45°C Low temperatures can be reached14 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