1. CO 2 CO2 cooling experiences in the LHCb Velo Thermal Control System
Engineering Forum: experiences from cooling systems for LHC detectors
CO2 cooling experiences in the LHCb Velo Thermal Control System
Bart Verlaat
National Institute for Subatomic Physics (NIKHEF)
Amsterdam, The Netherlands CO 2
CERN, 30 October 2008

2. Table of Contents CO2 cooling and the 2PACL method
LHCb-VELO Thermal Control System (VTCS).
Commisioning results of the VTCS.
Conclusions.

3. 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

4. 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)

5. Property comparison Refrigerant “R” numbers: R744=CO2 R218=C3F8
Latent Heat of Evaporation
Saturation Temperature (ºC )
Latent heat of evaporation (kJ/kg)
Better
Refrigerant “R” numbers:
R744=CO2
R218=C3F8
R116=C2F6
Density Ratio (ρvapour/ ρliquid)
Saturation Temperature (ºC )
ρvapour/ ρliquid
Better
Liquid Viscosity
Saturation Temperature (ºC )
Liquid Viscosity (Pa*s)
Better
Surface Tension
Saturation Temperature (ºC )
Surface Tension (N/m)
Better
dT/dP
Saturation Temperature (ºC )
dT/dP (ºC/bar)
Better

6. 2 evaporative cooling principles used in LHC detectors
Atlas Inner Detector:
Vapor compression system
Fluid: C3F8
Liquid
Vapor
Compressor
Heater
Pressure
BP. Regulator
Warm transfer
2-phase
Enthalpy
Cooling plant
Detector
LHCb-VELO:
2PACL pumped liquid system
Fluid: CO2
Liquid
Vapor
Compressor
Pump
Pressure
Chiller
Liquid circulation
2-phase
Cold transfer
Enthalpy
Cooling plant
Detector

7. 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

8. 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

9. 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

10. The Velo Detector Heat producing electronics Detection Silicon
Particle tracks in the VELO from an LHC injection test (22 august ’08)
CO2 evaporator
(Stainless steel tube casted in aluminum)

11. VELO Cooling Challenges
VELO electronics must be cooled in vacuum.
Good conductive connection
Absolute leakfree
Maximum power of the electronics: 1.6 kW
Silicon sensors must stay below -7°C at all times (on or off).
Adjustable temperature for commisioning.
-5°C to -30°C (Nominal irradiated -25°C)
Maintenance free in inaccessable detector area

12. VTCS Evaporator vacuum feed through capillaries and return hose
liquid inlet (¼”x0.035”)
2-phase outlet (⅜”x0.035”)
Inlet capillaries
Finner=0.5mm (L~2m)
Aluminium casted evaporator block details
PT100 cables
23 parallel
evaporator stations
Finner=1mm (L~1.5m)

13. 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

14. Concentric transfer line
Transfer line is a 55m long concentric 2-fase liquid-vapor line
Functions:
Transferring liquid to evaporators
Regulate liquid temperature
Pick up environmental heat in return line for unloaded evaporator cooling
Provide low-pressure drop return flow for distant evaporator pressure control
Protective cover
2-phase return line
25mm Armaflex NH Isolation
Ø14mm
Ø6mm
Ø16mm
Ø4mm
Ø66mm
VTCS total upward column: 4m+2.6m = 6.6m
ΔT ≈ 0.7°C → ≈ 0.1°C/m (C3F8 ≈ 2.3°C/m)
Sub-cooled liquid feed line 4m
2.6m
Terminals as built

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

16. VTCS construction CO2 2PACL’s
Stainless steel piping with:
(Orbital) Welding
Vacuum Brazing
Swagelok Cajon VCR fittings and line components
Lewa liquid CO2 pump (100 bar)
In house designed accumulator (130 bar)
Reinforced SWEP condenser (130 bar)
Now commercial available at SWEP.
55 meter concentric transfer line
Aluminium casted cooling blocks
Chillers designed in house with standard commercial chiller components.
Copper piping with hard solder joints
Danfoss line components
Bitzer compressors
SWEP heat exchangers
Siemens S7-400 series Programmable Logic Controller (PLC)

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

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

19. 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 19
Start-up in ~2 hours

20. 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 20
Start-up in ~2 hours

21. March ’08: Commisioning of the VTCS Detector under vacuum and unpowered

22. 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)

23. Fluctuations from the untuned chiller
VTCS performance overview for a setpoint 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 heigth
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

24. Summary
The VTCS has successfully passed the 1st commissioning phase and was ready to be used in the experiment in July 2008
Operational temperature range is between -5°C and -30°C set point
It has run for 3½months continuously with only minor problems
It behaves very stable with the chiller still to be tuned (evaporator stability less than 0.05°C)
The silicon temperature is below the required -25°C set point temperature. (This is consistent with the prediction)

25. What did we learn:
2PACL dynamics work better with the accumulator connection at inlet of condenser (instead of outlet as it is now)
No satarated liquid feed from accu to pump.
Free pre-cooling at cold start-up due to thermal capacity of condensers.
Concentric transfer tube heat exchanger works beyond expectations as the so-called “Duck-foot-cooling1” pinciple is boosting the operational temperature range.
The current system is not always initiating boiling at higher operational temperatures (>-10°C), resulting in temporary reduced heat-exchange.
1 The way a duck can have cold feet without loosing body heat,
by exchanging heat between the in- and outlet bloodstream.

26. The “Cool” future
The VTCS is not yet finnished, some things have to be done:
Implementing automatic back-up procedure.
Changing the accumulator connection.
Tunning the chiller.
Analyse data for publication.
Construction of a mini desktop 2PACL CO2 circulator for general purpose laboratory use.
Participation in future CO2 cooling systems
Atlas IBL, Goat (GOssip in ATlas), Next-64, RELAXed
2PACL upgrade:
Replace HFC chiller by CO2 chiller
No integration of chiller and 2PACL!
2PACL is much more stable (<0.05’C)
2PACL has liquid overfeed and needs no boil-off heaters
CO2 compressor needs oil as lubricant
CO2 chiller has extended lower temperature range (CO2 ~-50°C , HFC~-40°C )

27. Back-up Slides

28. Property Comparison (1)
R744 (CO2)
R218 (C3F8)
R116 (C2F6)
Source Refprop NIST
R744 (CO2)
R218 (C3F8)
R116 (C2F6)
Critical Point
73.8 bar
26.4 bar
30.5 bar
Triple point
-56.6ºC
-147.7ºC
-100ºC
Boiling 1 bar
-78.4ºC
Subli-mation !
-36.8ºC
-78.1ºC

29. 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

30. 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

31. 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