1. 2016/12/6 Yasuhiro Sugimoto @LCWS2016
R&D status of a gas-compressor based 2-phase CO2 cooling system for FPCCD vertex detector
2016/12/6
Yasuhiro Sugimoto
@LCWS2016

2. Introduction

3. Why we need CO2 cooling? Operation temperature
CTI of FPCCD due to radiation damage is a function of temperature
A simple simulation of CTI based on Shockley-Read-Hall theory shows around ℃ is optimal
2-phase CO2 cooling gives constant temperature cooling
~1x1011 e/cm2

4. Why we need CO2 cooling? Material budget / dead region
Heat sources of FPCCD VTX locate mainly at both ends of ladders (on-chip amps and front-end ASICs)
Cooling by 2-phase CO2 at both ends of ladders seems an attractive solution
2-phase CO2 can go through very thin cooling tube (OD 2mm or less)  Only 0.3%X0 increase of material budget of the end-plate
Gas cooling requires much thicker tube  Dead space between FTD and beam pipe

5. Advantages of CO2 cooling
Large latent heat ~300 J/g (x3 of PFC)
High pressure ~1 −40℃ (~5 ℃)
Less evaporated gas volume
Less temperature drop due to pressure drop
 We can use thin cooling tube
Much less Global Warming Potential
CO2
C2F6
C3F8
Latent
321 J/g
~100 J/g
~110 J/g
Triple point
-56.4℃
-97.2℃
-160℃
Critical point
31.1℃
19.7℃
71.9℃
1 MPa
~0.5 MPa
~0.1 MPa
GWP 1
12200
8830

6. Schematic diagram Liquid pump based system (NIKHEF) Heat load
Detector side
Liquid pump based system
(NIKHEF)
Heat load
Expensive
Heat load
Heat load
Detector side
Gas compressor based system
(KEK)
Inexpensive
(Cooling water line)
Close to room temp.

7. Merit of gas-compressor system
We don’t need expensive low temperature chiller
Cooling water near room temperature, which must be available in the detector hall of ILC, is enough to liquefy CO2
We don’t need strict thermal insulation for transfer tubes between liquefier plant and detector
Flexible transfer tube off the shelf can be used
Transfer tubes can be placed along the cable chain  Big advantage in push-pull operation
Transfer tube for Belle-II
Cable chain

8. R&D at KEK Collaboration R&D goal Progress
Members from LC-TPC, FPCCD VTX, Belle-II VTX, and KEK cryogenic group
Started in 2010
R&D goal
Development of 2-phase CO2 cooling system using a gas compressor for CO2 circulation for practical applications
Progress
Development of a “blow system”(2011~)
1st prototype of circulating system (2013~)
2nd prototype of circulating system (2015~)

9. 2nd Prototype

10. 2nd Prototype
Consists of 3 units: liquefier unit, flow meter unit, cooling unit
3 units are connected with metal core flexible tubes (1/4” for liquid, 3/8” for gas)
20m 5m
15m

11. 2nd Prototype
Flowmeter Unit
Liquefier Unit
Cooling Unit

12. 2nd Prototype Gas compressor Haskel gas booster AGD-7
Driven by compressed air
Exhausted air is used for cooling of gas cylinder
Pdischarge=7Pair+Psuction

13. Phase diagram 200 J/g Condenser HEX Transfer tube + flow meter
Needle valve
Gas compressor
Detector
HEX
Heater
200 J/g

14. Cooling power Cooling power measurement using dummy load
Cooling temperature: ~−40℃
Flow rate: 1.4 g/s
Dry out power ~170 W  Less than expectation of 280 (=200x1.4) W
>1kW for cooling temperature of +15℃
Possible reasons of deficit of the cooling power at −40℃
Heat load of transfer tube (Liquid CO2:15℃, ambient temperature:27℃) and other low temperature part T4 T5 T2
Dummy
load T3
T (℃)
P (W)

15. Cooling power
Reality（？）
120 J/g

16. Transfer tube
Dp (MPa)
Pressure drop in metal core flexible tube is one of concerns
However, measured pressure drop is reasonably low
Smaller than resolution of flow meters at low flow rate
At high flow rate, pressure drop is dominated by flow meter for liquid CO2, and still smaller than the resolution for CO2 gas
5.4 MPa liquid
1.0 MPa gas
Flow rate (g/s)
Dp (MPa)
5.9 MPa liquid
5.0 MPa gas
Flow rate (g/s)
(Resolution: 0.15 MPa for liquid, MPa for gas)

17. R&D for improvement

18. Pressure control
Pressure control of 2-phase CO2 by manual back-pressure valve is somewhat unstable
Automatic pressure controller was studied
Back pressure is controlled by external voltage setting
It works very well
Setting
Pressure
Temperature
Flow rate

19. Degradation of O-rings
O-rings made of elastomer are used for gas compressor and safety valves in our system
Degradation of O-rings, called as explosive decompression (ED), causes gas leak
Explosive decompression
At high pressure environment, CO2 gas immigrates into elastomer
When pressure is reduced suddenly, CO2 dissolved inside the elastomer comes out as micro bubbles, expands, and damage the elastomer from inside

20. Degradation of O-rings
O-ring used for a safety valve

21. Degradation of O-rings
How to mitigate the risk?
Safety valve
Replace with metal seal safety valves
Gas compressor
Find out better material for O-rings
Higher Shore durometer hardness
Different materials: Kalrez, Chemraz, etc.
Frequent overhaul
Safety valve with elastomer seal (left)
and metal seal (right)

22. Other subjects to be done
Development of low mass heat exchanger
Very massive stainless plate HEX is used now
HEX made of double-layer tube will be studied
Small and silent liquefier
Remote control of the system
Thermal contact between cooling tube and endplate, endplate and ladders

23. summary

24. Summary
We have successfully developed a prototype of 2-phase CO2 cooling system using a gas compressor for CO2 circulation
Cooling power at −40℃ has been measured, which is satisfactory for FPCCD VTX cooling
Degradation problem of O-rings has to be solved
R&D for many issues are still to be done