1. CO 2 Development of the Velo Thermal Control System (VTCS)
CO2 cooling activities at NIKHEF
Development of the Velo Thermal Control System (VTCS)
Bart Verlaat
National Institute for Particle Physics (NIKHEF)
Amsterdam, The Netherlands CO 2
CERN, 13 March 2008

2. (Silicon) Particle Detectors and Cooling
(Silicon) Particle detectors have specific needs for thermal control:
Many distributed heat sources over large volumes.
Serial evaporators
Low temperature gradients between these sources.
Low pressure drop, constant heat transfer coefficients
Permanent cooling (<0ºC, With or without heat load)
Irradiated detectors will get damaged when becoming warm
Low mass inside detectors
Light weight evaporators, low volume, => mini-channels
Low structural impact
Small diameter tubing, wiggly structure
Radiation resistant cooling fluid
Alpha Magnetic Spectrometer
Silicon Tracker
Particle detection surface
(Low material, homogeneous and stabile temperature)
The above mentioned properties have led to the development of CO2 loops at Nikhef, because CO2 is:
Radiation hard
Has excellent thermodynamic properties for micro-channels.
Low dT/dP
Low mass
Low liquid/vapor density ratio
Low viscosity
High latent heat
High heat transfer coefficient
Multiple electronic stations
(All need cooling)

3. Typical temperature distribution of a heated tube
Saturated vapor
Saturated liquid
Partial dry-out
Tube temperature
Temperature (°C )
Fluid temperature
Tube length (m)
Target flow condition
Dry-out zone
Sub cooled liquid
2-phase liquid / vapor
Super heated vapor

4. 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

5. Property comparison (2)
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. Example of and Atlas upgrade stave
2x 20 wafers à 17 Watt
Cooling
Q = 680 Watt
Tube = 4 meter
Refrigerant “R” numbers:
R744=CO2
R218=C3F8
R116=C2F6
1 Atlas stave : 2 meter length
4mm ID Tube
2mm ID Tube
Calculations based on 75% Vapor quality at exit
Mass -35ºC
Φ R744= 2.9 g/s
Φ R218= 8.7 g/s
Φ R116= 9.6 g/s
dP calculation according to Friedel/Blasius

7. Heat transfer and dry-out of CO2 in the VTCS evaporator (1mm ID tube)

8. Cooling Fluid Choice: Facts, advantages and disadvantages
When looking to the presented data CO2 seems the most promising candidate for detector cooling.
Small diameter tubing
Isothermal behavior (Low dT)
Although CO2 has relative high heat transfer coefficients, the possible small diameters (=small heat exchange surface), need special attention.
CO2 is easy to use especially for testing, it is cheap and allowed to vent into the atmosphere.
High system pressure not a problem in small tubes.
CO2 can not be liquid in atmospheric conditions, a leak is in general not problematic. It produces snow as in a fire extinguisher

9. How to get the ideal 2-phase flow in the detector?
Vapor compression system
Always vapor needed
Dummy heat load when switched off
Oil free compressor, hard to find
Direct expansion into detector with C3F8 compressor
Warm transfer lines
Boil-off heater and in detector
Temperature control by back-pressure regulator
Atlas method:
Liquid
Vapor
2-phase
Pressure
Enthalpy
Compressor
Heater
BP. Regulator
Detector
Warm transfer over distance
Cooling plant
Pumped liquid system
Liquid overflow, no vapor needed
No actuators in detector
Oil free pump, easy to find
Standard commercial chiller
CO2 liquid pumping
Cold concentric transfer line
No components in detector
Temperature control by 2-phase accumulator
LHCb method:
Liquid
Vapor
2-phase
Pressure
Enthalpy
Compressor
Pump
Detector
Chiller
Liquid circulation
Cold transfer over distance
Cooling plant

10. The 2-Phase Accumulator Controlled Loop (2PACL)13 10
Flooded evaporator
Heat out
Condenser
Heat out 9
Heat in
Heat in 2 5 1
Heat exchanger
Restrictor
Pump
2PACL principle ideal for detector cooling:
Low vapor quality for serial evaporators.
No local evaporator control, evaporator is passive in detector.
No maintenance in hostile area
No actuators in radiation zone.

11. LHCb Detector Overview
Electron
Hadron
Proton beam
Goals of LHCb:
Studying the decay of B-mesons to find evidence of CP-violation
LHCb Cross section
Vertex Locator
Muon
20 meter

12. The LHCb-VELO Thermal Control System (LHCb-VTCS) A 2-Phase Accumulator Controlled Loop
Detectors and
electronics
Temperature detectors: -7ºC
Heat generation: 1600 W
23 parallel evaporator stations
capillaries
and return hose
VELO Thermal Control System
CO2 Evaporator section

13. LHCb-VTCS Overview A 2-Phase Accumulator Controlled Loop
Accessible and a friendly environment
Inaccessible and a hostile environment
R507a
Chiller
Cooling plant:
Sub cooled liquid CO2 pumping
CO2 condensing to a R507a chiller
CO2 loop pressure control using a 2-phase accumulator
Evaporator :
VTCS temperature ≈ -25ºC
Evaporator load ≈ Watt
Complete passive CO 2

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

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

16. VTCS 2PACL Operation CO 2 Start-up in ~2 hours A B C D A B C D time 16
Pump head pressure (Bar)
System pressure (Bar)
Accumulator Level (%)
Accu liquid temp. (ºC)
Pump inlet temp. (ºC) A B C D CO 2
time A B C D 16
Start-up in ~2 hours

17. VTCS Evaporator performance (Stability and response to heat-load changes)
Evaporator Temp (ºC)
Accu Temp ≈ Set-point (ºC)
Detector Power (Watt)
600 Watt
Accu level (‰)
Accu Cooling Power (Watt)
Pumped Liquid Temp (ºC)
(A/Left side)
@Setpoint =-25ºC:
Accumulator temperature: ºC
Evaporator temperature (No Load): ºC
Evaporator temperature (600 W Load): -23.0ºC
Stabilization time from 0 to 600 Watt: ca. 7min
Temperature stability : <0.25ºC CO 2

18. VTCS Transfer line Operation (Internal heat exchanger)B C
Accumulator set-point B
[5] Evaporator liquid in (ºC)
[10] Evaporator pressure (Bar) C
[14] Accumulator pressure (Bar)
Cooling plant side
Evaporator side
[10] Evaporative temp. (ºC)
[13] Condenser Inlet (ºC)
Transfer line temperature profile
[1] Pump inlet (ºC)
A: Condenser and evaporator single phase
B: Evaporator 2-phase, condenser single phase
C: Both evaporator and Condenser 2- phase CO 2

19. VTCS Accumulator Control
2PACL Start-up
Cooling spiral for pressure decrease
(Condensation)
Pump head (Bar)
Accumulator Pressure (Bar)
Heater temp. (ºC)
Accu Level (%)
Decrease heater power near critical point to prevent dry-out
Liquid temp. (ºC)
Heater power (%)
Pump inlet (ºC)
Accumulator Properties:
Volume 14.2 liter (Loop 9 Liter)
Heater capacity 1kW
Cooling capacity 1 kW CO 2
Thermo siphon heater for pressure increase
(Evaporation)

20. VTCS filling and sizing
Single-phase cold operation is worst-case for minimum level
(Heater need to be submerged all the time)
Two-phase cold operation is worst-case for maximum level
(Significant part of the cooling coil need to be in vapor phase)
VTCS Design
Loop fill ratio: 500 gram/liter
Loop fill ratio: 650 gram/liter
Loop fill ratio: 725 gram/liter
Loop fill ratio: 575 gram/liter
Over critical filling
Under critical filling
Accumulator Liquid level
Under critical fillings (<468 g/L) cause dry-out of accumulator near critical point.
Fillings just above critical density show best performance (500 – 600 g/L)
Ratio accumulator volume / loop volume: >1.5 (AMS-TTCS & LHCb-VTCS) CO 2

21. Conclusions CO2 is a very good cooling fluid for detector cooling
Low thermal gradients
Small tube sizes
High heat transfer
The 2PACL method turned out to be a good method for circulating the cooling fluid.
Easy to operate
Standard industrial components
Stable operation
Large operational temperature range
Passive in detector
Heat load independent
Easy start-up and cool-down procedure
2PACL is easy to set-up and use. This is ideal for lab-experiments.
Not proven, but the 2PACL method it must work for other fluids too.

22. What brings the future (1) ?
Future projects at NIKHEF:
Development of a desktop CO2 cooler for laboratory and prototype use.
Upgrade Altas SCT cooling (CO2?)
Small cooling projects: Medipix,....
Studies on small cooling pipes. Understand the pro‘s & con’s. Verify lacking theory.
Heat exchange
Flow pattern Cooperation with CMS?
Pressure drop

23. What brings the future (2) ?
How to communicate in the future and benefit from each other in the development phase.
Organize a general detector cooling workshop and present eachothers experiences (Atlas, CMS, LHCb, Allice, etc……)
Follow closely the refrigeration technologies at IIR- conferences (International Institute of Refrigeration)
GL-2008 Copenhagen, Natural refrigerants mainly CO2
Heat transfer conferences, Conference on Heat Transfer and Fluid Flow in Microscale, Whistler, Canada,
Hefat South Afrika………etc, etc….
Adopt new technologies developed for commercial CO2 cooling.
Aluminum micro channel heat exchangers
Primary CO2 chillers for better operation around -40’C (tricky area for Freon)