1. CO 2 Controlling a 2-phase CO2 loop using a 2-phase accumulator
Refrigeration Creates the Future
Controlling a 2-phase CO2 loop using a 2-phase accumulator
Bart Verlaat
National Institute for Particle Physics (NIKHEF)
Amsterdam, The Netherlands
IIR Conference
23/08/2007, Beijing, China CO 2

2. The Large Hadron Collider related experiments at CERN in Geneva
Introduction to Nikhef National Institute for Particle Physics Amsterdam, the Netherlands
The Large Hadron Collider related experiments at CERN in Geneva
27 km
Ca. 100m
Nikhef participates in particle physics experiments world-wide:
Particle accelerator experiments
Astro-particle physics experiments
Goals of the particle physics research:
What is the matter made of?
How does matter work?
Where did matter come from?
Alpha Magnetic Spectrometer on the International Space Station

3. Particle Detectors and Refrigeration
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
Permanent cooling (0ºC, With or without heat load)
Irradiated detectors will get damaged when becoming warm
Low mass inside detectors
Light weight evaporators, mini-channels
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 carbon dioxide loops at Nikhef, because CO2 is:
Radiation hard
Has excellent thermodynamic properties for micro-channels.
low dP
low dT
low mass
low liquid/vapor density ratio
Multiple electronic stations
(All need cooling)

4. 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.

5. 2-Phase Accumulator history
Vapor
A 2-phase accumulator was developed for controlling in capillary pumped loops (CPL) for satellite thermal control. (1980)
The 2-phase accumulator was adopted as loop pressure control device in the CO2 cooling loop for the AMS-Satellite Tracker detector cooling (AMS-TTCS).
As far as we know, the AMS-TTCS was the first application of a 2-phase accumulator in a mechanically pumped loop.
Lab-tests for the AMS-TTCS showed an excellent performance of the 2PACL technology for earth use.
The LHCb-Velo detector cooling was designed using the 2PACL technology, and is installed at CERN in July First results have shown good performance.
Capillary material
Condenser
Accumulator
Evaporator
Liquid
Capillary Pumped Loop
AMS-TTCS 2PACL

6. The AMS-Tracker Thermal Control System (AMS-TTCS) A 2-Phase Accumulator Controlled Loop in Space
CO2 Evaporator

7. The AMS-Tracker Thermal Control System (AMS-TTCS) A 2-Phase Accumulator Controlled Loop in Space
Evaporator tube
Component box on satellites exterior
Evaporator section
Pump
Accumulator
Heat exchanger
Space radiator
AMS-Detector mechanical Structure

8. 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

9. 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

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

11. LHCb-VTCS Cooling Components2
LHCb-VTCS Cooling Components
Accumulators
VTCS Evaporator
Valves
Pumps
Condensers

12. VTCS Units Installed @ CERN
Freon Unit
CO2 Unit
July- August 2007

13. VTCS 2PACL Operation A B C D Pump head pressure (Bar)
System pressure (Bar)
Accumulator Level (%)
Accu liquid temp. (ºC)
Pump inlet temp. (ºC) A B C D

14. 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

15. 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
Thermo siphon heater for pressure increase
(Evaporation)

16. 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)

17. Accumulator in space No gravity, No liquid level!
Wetting of heater and un-flooding of cooler by capillary channels. (Like in heat pipes)
Capillary structure at accumulator outlet for guaranteed liquid filling in loop
Inner wick
Tube wick
Vessel prior to welding
TTCS Accumulator
Evaporation with heatpipe heater
Condensation with Peltier elements
Liquid path
Liquid connection

18. Conclusions
The 2PACL method turned out to be a good method of controlling a 2-phase mechanically pumped loop.
2PACL is easy to set-up and use. This is ideal for lab-experiments.
Not proven, but it must work for other fluids too.
Thank you!