1. CO 2 CO2 Cooling for HEP experiments Bart Verlaat TWEPP-2008
Topical Workshop on Electronics for Particle Physics
CO2 Cooling for HEP experiments
Bart Verlaat
National Institute for Subatomic Physics (Nikhef)
Amsterdam, The Netherlands CO 2
Naxos, 18 September 2008

2. Table of Contents Introduction to evaporative CO2-Cooling
Introduction to the LHCb Velo Thermal Control System (VTCS)
Commissioning results of the VTCS.
Conclusions and outlook.

3. (Silicon) Particle Detectors and Cooling
(Silicon) Particle detectors have specific needs for thermal control:
Many distributed heat sources over large volumes.
Low temperature gradients between these sources.
Permanent cooling
To avoid thermal runaway of the irradiated silicon
Cooling pipes should have low mass inside detectors
Cooling pipes should have low structural impact
Radiation resistant cooling fluid
Multiple electronic stations
(All need cooling)
Atlas
SCT
Alpha Magnetic Spectrometer
Silicon Tracker

4. Why Evaporative CO2 Cooling?
The lightest way of cooling is:
Evaporate at high pressure!
Why?
Vapor expansion is limited under high pressure
Volume stays low
Low mass flow
Pipe diameter stays low
High latent heat
Mass stays low
Carbon Dioxide
Later on in this presentation this will be illustrated by calculations

5. Saturation curves in the PT Diagram for CO2, C2F6 & C3F8
(3 used or considered refrigerants at CERN) 75
CO2 50
C2F6
Pressure (Bar) 25
C3F8
-60
-40
-20 20 40
Temperature (°C)

6. What happens inside a cooling tube? Heating a flow from liquid to gas
Isotherm
Pressure (Bar)
2-phase
Enthalpy (J/kg) 90 60
Tube temperature
Temperature (°C)
Low ΔT 30
Increasing ΔT
Fluid temperature
-30
Target flow condition
Dry-out zone
Sub cooled liquid
2-phase liquid / vapor
Super heated vapor

7. How to design a cooling tube?P1 P2
Pressure (Bar) P3
ΔP=P2-P3
Enthalpy (J/kg) h2 h3
For evaporative cooling 5 items are important:
Controlling inlet enthalpy (h2) by expanding from liquid (P1,T1)
Setting mass flow (Øm) as function of maximum heat load (Qmax)
Controlling outlet pressure (P3)
Minimize pressure drop (ΔP)
Optimize heat transfer (α) with contact area.

8. 2-Phase flow prediction
2-phase flow theory =
Well understood single phase flow theory x
Magic empirical correlations in a black box
Pressure drop:
For example: Friedel Correlation
Heat transfer:
For example: Kandlikar Correlation
2-Phase coëfficiënt order 1~10

9. How to get the ideal 2-phase flow in the detector?
Traditional method:
(Atlas)
Vapor compression system
Liquid
Vapor
Compressor
Heater
Pressure
BP. Regulator
Warm transfer
2-phase
Enthalpy
Cooling plant
Detector
2PACL method:
(LHCb)
Pumped liquid system
Liquid
Vapor
Compressor
Pump
Pressure
Chiller
Liquid circulation
2-phase
Cold transfer
Enthalpy
Cooling plant
Detector

10. LHCb Detector Overview
Electron
Hadron
Proton beam
Goals of LHCb:
Studying the decay of B-mesons to find evidence of CP-violation
(Why is there more matter around than antimatter?)
LHCb Cross section
Vertex Locator
Muon
20 meter

11. The LHCb-VELO Detector (VErtex Locator)
Detectors and
electronics
Temperature detectors: -7ºC
Heat generation: max 1600 W
23 parallel evaporator stations
capillaries
and return hose
VELO Thermal Control System
CO2 evaporator section

12. The Velo Detector 22 August 2008 Heat producing electronics
Detection Silicon
22 August 2008
The VELO has seen particle tracks from an LHC test!
CO2 evaporator
(Stainless steel tube casted in aluminum)

13. VELO Cooling Challenges
VELO electronics must be cooled in vacuum.
Good conductive connection
Absolute leak free
Maximum power of the electronics: 1.6 kW
Silicon sensors must stay below -7°C at all times (on or off).
To avoid thermal runaway of the irradiated silicon
Adjustable temperature for commissioning.
Maintenance free in inaccessible detector area

14. The 2-Phase Accumulator Controlled Loop (2PACL)
Long distance P7
P4-5 5
Heat out
Heat out
Condenser 6
Heat in 4
Heat in 2 3
evaporator
2-Phase Accumulator 1
Heat exchanger
Restrictor
Pump
2PACL principle ideal for detector cooling:
Liquid overflow => no mass flow control
Low vapor quality => good heat transfer
No local evaporator control, evaporator is passive in detector.
Very stable evaporator temperature control at a distance (P4-5 = P7)
Vapor
Liquid
Pressure 2 3
2-phase 4 P7 5 1 6
Enthalpy

15. VTCS Accumulator Control
Set point Temperature
Cooling spiral for pressure decrease
(Condensation)
Tset
Accumulator properties:
Volume: 14.2 liter (Loop 9 Liter)
Heater capacity: 1 kW
Cooler capacity: 1 kW
System charge: 12 kg liter)
System design pressure: 135 bar
Pressure
Temperature
Pset
Evaporator Pressure
PID + _
Heating
Cooling
ΔPfault + _ + +
Thermo siphon heater for pressure increase
(Evaporation)
Paccumulator
Pressure drop

16. LHCb-VTCS Overview (VELO Thermal Control System)
Accessible and a friendly environment
Inaccessible and a hostile environment
2.6 m
PLC
4m thick concrete shielding wall
3.6 m
2 Evaporators
800 Watt max per detector half
2 R507A Chillers:
1 water cooled
1 air cooled
2 CO2 2PACL’s:
1 for each detector half
2 Concentric transfer lines
55 m
VELO

17. VTCS Schematics 2x CO2 2PACL’s connected to 2 R507A chillers (Redundancy) Lots of sensors and valves

18. LHCb-VTCS Cooling Components
Accumulators
VTCS Evaporator
Valves
Pumps
Condensers CO 2

19. VTCS Units Installed @ CERN
Freon Unit
CO2 Unit
July- August 2007 CO 2

20. VTCS 2PACL Operation From start-up to cold operation (1)+
2 Pump head pressure (Bar) 2
4-Accumulator liquid
level (vol %) 7
4 - Accumulator pressure (Bar) 7
5 – Evaporator
temperature (°C) 4
1 Pumped liquid temperature (°C) 1
4- Accumulator Control:
+ = Heating
- = Cooling 7 _ 7 -7 4 CO 2 7 +7 2 1
time A B C D 20
Start-up in ~2 hours

21. VTCS 2PACL Operation From start-up to cold operation (2)
Pressure B C
Accumulator
Cooling =
Pressure decrease 5
20 °C A
0 °C 2
Path of 5 4
Set-point range D
-20 °C 5 1 4
-40 °C
Enthalpy D
2 - Pump head pressure (Bar) B 2 A C 2
4 - Accumulator pressure (Bar) 5 5
5 – Evaporator
temperature (°C) 4 CO 2 4 1
1 – Pumped liquid temperature (°C) 1
time A B C D 21
Start-up in ~2 hours

22. March ’08: Commissioning of the VTCS Detector under vacuum and unpowered20 10
Temperature (°C)
-10
-20
-30
-40

23. Temperature (°C), Power (Watt), Level (vol %)
24 June ’08: After a succesful commisioning of the detector at -25°C, the setpoint is increased to -5°C. And has been running since then smoothly!
(3 sept 08) 80
Accu Heating/Cooling 60
Accu level 40
Detector half
heat load (x10) 20
Module Heat load
Temperature (°C), Power (Watt), Level (vol %)
Silicon temperature
-7°C
SP=-5°C
-20
Evaporator
temperature
SP=-25°C
-40
0:30
1:00
1:30
2:00
Time (Hour)

24. Fluctuations from the untuned chiller
VTCS performance overview for a set point of -5°C (Detector switched on, fully powered)
CO2 heat transfer
dT=1.4°C
Evaporator Pressure
31.15 bar = -4.18°C
Cooling block
dT=0.04°C
Cooling block temperature = -2.8°C
1 hour
dP=0.6 bar = 6.2 m static height
CO2 liquid temp= -42°C
Fluctuations from the untuned chiller
Detector offset from accu control: 0.7°C
CO2 liquid
dT=4.5°C
Evaporator liquid
inlet temp = -4.40°C
Evaporator vapor
outlet temp = -4.44°C
Accumulator Pressure
30.54 bar = -4.90°C

25. VTCS Summary The installation at CERN started in July 2007.
The VTCS has successfully passed the 1st commissioning phase between march and June ’08 and is ready to be used in the experiment
Operational temperature range is between 0°C and -30°C set point (+10°C with the back-up chiller)
It has run for 2½months continuously without any problem (only 3 interruption due to power or cooling water failures)
It behaves very stable (<0.1°C fluctuation), with the chiller still to be tuned.
The silicon temperature is below the required -25°C set point temperature. (This is consistent with the prediction)

26. Some Lessons Learned
The accumulator sometimes gives the pump a 2-phase mixture => cavitation. Problem is solved by connecting the accumulator to the inlet of the condenser instead of the outlet where it is now.
Operational temperature range of the evaporator is larger than expected. This is due to the “Duck Foot Cooling1” principle of the transfer line.
The main pressure drop of 2-phase flow in the return line was not caused by friction but by the heights of the upward columns combined. Minimization of the upward columns is more important in the design. Were CO2 gives only about 1°C/bar (≈0.1°C/m), the use of other fluids will make upward 2-phase flow impossible without a big temperature penalty.
1 The way a duck can have cold feet without loosing body heat,
by exchanging heat between the in- and outlet bloodstream.

27. Outlook The VTCS is not yet finished, some things have to be done:
Implementing automatic back-up procedure.
Changing the accumulator connection.
Tuning the chiller.
Analyze data for publication.
Construction of a mini desktop 2PACL CO2 circulator for general purpose laboratory use.
Other CERN detectors (Atlas/CMS) have shown interest in the VTCS for their inner tracker upgrades.
Challenge: Scaling of the 1.6kW VTCS to a 100kW system.
An example of fluid trade-off is given in an Atlas upgrade stave calculation example.

28. Example of and Atlas upgrade stave (1)
Q = 680 Watt
Tube = 4 meter
2x 20 wafers à 17 Watt
Cooling
Calculations based on -35°C and 75% vapor quality at exit
1 Atlas stave : 2 meter length
CO2
CO2
ΔT=-2°C
C2F6
C3F8
C2F6
C3F8
Pressure Drop
Generates
Temperature Drop
Mass -35ºC
Φ CO2= 2.9 g/s
Φ C3F8= 8.7 g/s
Φ C2F6= 9.6 g/s
CO2=2.7mm
C2F6=4.3mm
C3F8=7.7mm

29. Example of and Atlas upgrade stave (2)
D2.7mm x L25mm = W/m2
25mm
75mm
Heat exchange length
CO2
D4.3mm x L25mm = W/m2
D2.7mm x L75mm = W/m2
C2F6
D4.3mm x L75mm = W/m2
D7.7mm x L25mm = W/m2
C3F8
D7.7mm x L75mm = 9370 W/m2
25mm HX length
C3F8
7.7
75mm HX length
Mass -35ºC
Φ’ CO2= 506 kg/m2s
Φ’ C3F8= 661 kg/m2s
Φ’ C2F6= 186 kg/m2s
CO2
C2F6
Critical Heat Flux (Bowring/Ahmad): CHF(CO2) = 313 kW/m2, x=1.1

30. Thank you for your attention:
Questions?