1. Aachen Status Report: CO2 Cooling for the CMS Tracker at SLHC
Lutz Feld, Waclaw Karpinski, Jennifer Merz and Michael Wlochal
RWTH Aachen University, 1. Physikalisches Institut B
13 October 2010 MEC Upgrade Meeting

2. Outline Test System at RWTH Aachen University Results
Goals and specifications
Schematic design
Set-up
Results
Temperature distribution over detector pipe
Pressure drop
Dryout, heat from environment
Parallel cooling branches
Summary and Outlook
Jennifer Merz

3. R&D at RWTH Aachen University
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:
Operation of parallel cooling branches
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 CO2 System
Chiller 1:
Chiller temp.  vapour pressure  system temp.
Expansion Vessel:
Saturated mixture of CO2 liquid and vapour
pressure, bar
Enthalpy, kJ/kg
Heat Exchanger:
Subcooling of incoming CO2 (only liquid in pump)
Dissipation of detector heat load
Up to 500 W
heat load
Heat Exchanger:
Warm incoming CO2 to nominal temperature ( given by chiller 1)
Partial condensation of returning CO2
Jennifer Merz 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. Improved CO2 Test System
Aluminum vacuum box currently under construction
CO2-Bottle
CO2-Flasche
Connection to detector pipes
Mount for
detector pipes
Flanges for
electrical feed through
New detector pipes for parallel piping
Jennifer Merz

9. Dryout Measurement Dryout: pipe walls not in touch with liquid anymore
 No heat dissipation by evaporating CO2
Rise in detector temperature
x: vapour quality
x=0
x=1
liquid
gas
Temperature distribution over detector 14 12 10 8 6 4 2
14 thermistors along pipe
CO2 temperature: +20°C
Heat load: 100W
Detector temperature, °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

10. Dryout Measurement - Results
We observed high heat input from environment
Amount can be estimated from dryout measurements
Corrections are rather big (60, 80, 100 W applied with power supply)
Needs crosscheck in vacuum box
-20°C
+20°C
Heat Load, W
Flow, g/min
Jennifer Merz

11. Temperature Distribution
+20°C
-20°C
-40°C
100W
80W
60W
Detector Temperature, °C
Pipelength, m
-20°C
100W
80W
60W
Detector temperature almost constant with applied heat load
Effect bigger at lower temperatures
Comparison with theory still needs to be done
Detector Temperature, °C
Pipelength, m
Jennifer Merz

12. Pressure Drop along Detector Pipe
2-phase flow: pressure drop = temperature drop
Measure pressure gradient  precise control of detector temperature
Determine Δp between inlet and outlet of detector pipe
L=5.8m
di = 1.7mm
-20°C, 100W
-20°C, 80W
-20°C, 60W
+20°C, 100W
+20°C, 80W
+20°C, 60W
Pressure Drop, bar
Pressure sensor
from “Aplisens”
Flow, g/min
Pressure drop measurement with dedicated pressure sensors
Results comparable with old results (Δp from ΔT)
Jennifer Merz

13. Parallel Cooling Branches
Keep pressure drop constant
Apply heat load and determine flow
High heat load  low mass flow
Influence on parallel piping
 Insert restrictions in each branch
 We plan to operate parallel cooling branches with our test system
L=5.8m
di = 1.7mm
Flow, g/min
-20°C, Δ=1.0bar
+20°C, Δp=0.3bar
Heat Load, W
Jennifer Merz

14. Summary CO2 test system fully commissioned and operational
Measurements down to low temperatures show: reasonable cooling power at -40°C
Pressure drop measurements: comparable with “old” results, need comparison with theory
Dryout Measurements: estimate for heat input from environment (needs crosscheck in vacuum box)
Jennifer Merz

15. 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
Perform more measurements on pressure and temperature drop along different pipes: - Vary inner diameter and form/bending - Operation of parallel cooling branch
Comparisons with theory
Repeat measurements with improved system
Jennifer Merz