1. Recirculating CO2 System
Cold Operation of a
Recirculating CO2 System
at Aachen
Lutz Feld, Waclaw Karpinski,
Jennifer Merz, Michael Wlochal
RWTH Aachen University, 1. Physikalisches Institut B
CMS Upgrade Cooling and Mechanics Meeting

2. Outline Description of CO2 test system in Aachen
Specifications of new chiller
Dryout measurements at room temperature
First measurement at low temperature
Conclusion and outlook
Jennifer Merz

3. CO2 Test System Aachen - Specs
Goals:
Gain experience with a recirculating (closed) CO2 cooling system
Find the lowest possible operating temperature
Determine ideal operating conditions ( stable operation) depending on heat load and coolant temperature
Specifications:
Maximum heat load: 500W
Coolant temperature at detector: -45°C - +20°C
Precise temperature control and flow measurement
Continous operation
Safe operation (maximum pressure 100bar)
Jennifer Merz

4. Schematic of Test System
Chiller 1: Chiller temperature  vapour pressure  system temperature
Expansion Vessel:
Filled with saturated mixture of CO2-liquid and -vapour
-45°C ΔQ 2 3 ΔQ 1 4 6 5
Burst Disk
CO2
Bottle
Heat Exchanger 1:
Heats liquid CO2 to appropriate temperature
Partial condensation of returning CO2
Vacuum
Pump
Chiller 2: Heat removal
Burst Disk 5
Heat
Exchanger 2 6
Heat
Exchanger 1
-50°C
Heat Exchanger 2:
Removes heat from the system
Cools incoming CO2 down to about -50°C 3 4
Detector:
500 W
heat load 1
Flow Meter
CO2
Pump
Burst Disk 2 4

5. CO2 Cooling Test System CO2-Bottle CO2-Flasche Vacuum-Pump CO2 Bottle
Expansion
Vessel
Detector
Heat
Exchanger 1
Vacuum-Pump
Jennifer Merz

6. Cooling of Expansion Vessel
CO2-Flasche
Copper piping replaced by copper shell
Improvement of thermal contact
Reduction of flow resistance
Vacuum-Pump
Jennifer Merz

7. CO2 Cooling Test System CO2-Bottle CO2-Flasche
6m long stainless steel pipe with 1.5mm outer diameter, in insulated box
14 thermistors to measure temperature distribution over pipe
Electrical connections to simulate heat load
Vacuum-Pump
Jennifer Merz

8. Chillers: unistat 815
Originally ordered chiller had to be replaced; cooling power was not sufficient
New chiller was damaged during transport
“Old” chiller had to be repaired by company
Chillers now achieve required low temperatures
CO2-Bottle
CO2-Flasche
Delay in commissioning, system is fully operational since end of January
Chiller specifications: (both chillers identical) unistat 815
Company: Peter Huber Kältemaschinenbau, Offenburg, Germany
Temperature range: -85°C to 250°C
Cooling power: -20°C
-40°C
-60°C
Pump: max. 40 l/min, max. 0.9bar
Jennifer Merz

9. Dryout Measurement at Room Temp.
Dryout: Pipe walls not in touch with liquid anymore
 no power dissipation by evaporating CO2
temperature rises
Heat load: 70W
Liquid
Gas 14 12 10 8 6 4 2 13 11 9 7 5 3 1
Vary revolutions per minute of CO2 pump
 variation of CO2 flow
Keep heat load constant
Determine when temperature rises over certain level
Heat load: 70W

10. Dryout Measurement at Room Temp.
30W
40W
50W
60W
70W
 The lower the flow, the earlier a rise in temperature is observed
The higher the heat load, the more flow is needed
Vacuum-Pump
Jennifer Merz

11. First Measurement at Low Temperature
Expansion vessel at -27°C
Chiller 2 at -44°C
No stable operation
Fast variations in CO2 flow
Beginning of dryout visible
System behaviour not yet understood
Operation at low temperatures possible (down to -45°C) if bypass is open; nearly no flow through detector pipe
Chillers work fine (see also next slide)
Flow resistance too high?
More investigation and measurements needed
Vacuum-Pump
Jennifer Merz

12. Chiller Performance Chiller 1 Both chillers show excellent performance
Low temperatures
Stable operation
Nominal Temp.
Internal Temp.
Chiller 2
Nominal Temp.
Internal Temp.
Temp. at Exp. Vessel
Vacuum-Pump

13. Conclusions and Outlook
CO2 test system in Aachen is finally fully operational
First measurements show that system works in principal
New chillers manage to reach low temperatures
No stable operation at low temperatures possible yet
Investigate how to bring low temperature to detector
Install filter to avoid water and other disturbing particles in system
Determine pressure drop and temperature distribution for different pipe routings and diameter
Jennifer Merz

14. Back Up....
Jennifer Merz

15. CO2 Enthalpy-Pressure Diagram
Design of cooling plant in p-H-digram
Enthalpy:
H = U + pV “internal energy + expansion work”
ΔH is exchanged heat at constant pressure
liquid
Pressure, bar
liquid + gas
gas
Enthalpy, kJ/kg

16. CO2 Enthalpy-Pressure Diagram
Re-Circulating System
(closed system)
Design of cooling plant in p-H-digram
Enthalpy:
H = U + pV “internal energy + expansion work”
ΔH is exchanged heat at constant pressure
liquid 4 1 6 5 3 2 ΔQ
Detector load (4-5)
Pressure, bar
liquid + gas
gas
Enthalpy, kJ/kg

17. TEC Total Material Budget
CO2 Cooling System
Advantages:
CO2 : low density  small contribution to material budget
Operation at high pressures  small pipe diameter
Low temperatures (-45°C)  good for sensor performance
Current tracker (C6F14) vs. tracker with DC-DC converters cooled with CO2
Power consumption with DC-DC converters: 84W per petal
Including 18.8W power loss of converters, due to converter efficiency of 80%
One aluminium cooling block per converter implemented to dissipate converter power loss
TEC Cooling
TEC Total Material Budget
-38.5%
-14.3%
Current tracker with C6F14
Layout with DC-DC converters with CO2
A total reduction of 14.3% seems possible with CO2 cooling and DC-DC conversion.
Jennifer Merz