1

CO2 cooling activities at NIKHEF

Bart VerlaatNational Institute for Particle Physics

(NIKHEF)Amsterdam, The Netherlands

Development of the Velo Thermal Control

System (VTCS)

CERN, 13 March 2008

(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• 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

Alpha Magnetic Spectrometer

Silicon Tracker

Particle detection surface(Low material, homogeneous

and stabile temperature)

Multiple electronic stations(All need cooling)

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

Tube

tem

pera

ture

Flui

d te

mpe

ratu

reSat

ura

ted

liq

uid

Par

tial d

ry-o

ut

Sat

ura

ted

vap

or

Sub cooled liquid 2-phase liquid / vapor Super heated vapor

Dry-out zoneTarget flow condition

Tem

per

atur

e (°

C )

Tube length (m)

Typical temperature distribution of a heated tube

Property Comparison (1)R744 (CO2)

R116 (C2F6)

R218 (C3F8)

Source Refprop NIST

R744 (CO2)

R218 (C3F8)

R116 (C2F6)

Critical Point

31ºC @ 73.8 bar

71.9ºC @26.4 bar

19.9ºC @30.5 bar

Triple point -56.6ºC -147.7ºC -100ºC

Boiling temperature @ 1 bar

-78.4ºC Subli-mation !

-36.8ºC -78.1ºC

Property comparison (2)

Refrigerant “R” numbers:R744=CO2

R218=C3F8

R116=C2F6

-40 -30 -20 -10 0 10 200

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

DratioR744

DratioR218

DratioR116

Density Ratio (ρvapour/ ρliquid)

Saturation Temperature (ºC )

ρva

po

ur/

ρliq

uid

Be

tte

r

-40 -30 -20 -10 0 10 200

0.002

0.004

0.006

0.008

0.01

0.012

0.014

SurfTR744

SurfTR218

SurfTR116

Surface Tension

Saturation Temperature (ºC )

Sur

face

Ten

sion

(N

/m)

Be

tte

r

-40 -30 -20 -10 0 10 200.5

1

1.5

2

2.5

3

3.5

4

4.5

5x 10

-4

DVliqR744

DVliqR218

DVliqR116

Liquid Viscosity

Saturation Temperature (ºC )

Liqu

id V

isco

sity

(P

a*s)

Be

tte

r

-40 -30 -20 -10 0 10 200

50

100

150

200

250

300

350

dHR744

dHR218

dHR116

Latent Heat of Evaporation

Saturation Temperature (ºC )

Late

nt h

eat

of e

vapo

ratio

n (k

J/kg

)

Be

tte

r

-40 -30 -20 -10 0 10 200

5

10

15

20

25

dT/dP

Saturation Temperature (ºC )

dT/d

P (

ºC/b

ar) Be

tte

r

Refrigerant “R” numbers:R744=CO2

R218=C3F8

R116=C2F6

-40 -35 -30 -25 -20 -15 -10 -5 0-2

-1.5

-1

-0.5

0

Saturation Temperature (̀ C)

Pres

ure

drop

(bar

)

dPresR744

dPresR218dPresR116

-40 -35 -30 -25 -20 -15 -10 -5 0-30

-25

-20

-15

-10

-5

0

Saturation Temperature (̀ C)

Tem

pera

tre d

rop

(̀C)

dTempR744

dTempR218

dTempR116

4mm ID Tube

-40 -35 -30 -25 -20 -15 -10 -5 0-20

-15

-10

-5

0

Saturation Temperature (̀ C)

Pre

sure

dro

p (b

ar)

dPresR744

dPresR218

dPresR116

-40 -35 -30 -25 -20 -15 -10 -5 0-80

-70

-60

-50

-40

-30

-20

-10

0

Saturation Temperature (̀ C)

Tem

pera

tre

drop

(̀C

)

dTempR744

dTempR218dTempR116

2mm ID Tube

2x 20 wafers à 17 Watt

1 Atlas stave : 2 meter length

Cooling

Q = 680 WattTube = 4 meter

bverlaat@nikhef.nl04-03-2008

Calculations based on 75% Vapor quality at exit

Example of and Atlas upgrade

stave

Mass flow @ -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

Measured and Calculated Heat Transfer Coeficients for the VTCS Evaporator as a function of heatflux and massflux (Tevap =-25°C)

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0 100 200 300 400 500 600 700 800

Mass flux (kg/m2s)

HTC

(W/m

2K)

HTC Measured (17.3 kW/m2)

HTC Kandlikar (17.3 kW/m2)

HTC Measured (21.9 kW/m2)

HTC Kandlikar (21.9 kW/m2)

VTCS Evaporator Performance (Heatload 24 Watt)

-25

-20

-15

-10

-5

0

5

10

15

1 201 401 601 801 1001 1201 1401 1601 1801 2001 2201 2401 2601 2801 3001

Time (seconds)

Te

mp

era

ture

(ºC

)

.

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Mas

sflo

w (

g/s

)

.

Block 1 Temperature

Block 3 Temperature

Block 5 Temperature

CO2 Temperature

Hybrid Temperature

Silicon Temperature

Massflow (g/s)

Nominal flow condition Reduced flow condition

Critical Flow condition

(~0.22 g/s)

Serious dry-out

Flow increase restoresmodule and silicon temperature.

Module and silicon temperature is seriously affected by dry-out.

Last cooling block shows serious signs of dry-out, module is not yet affected

Last cooling block shows the first signs of a near dry-out

Block 1Block 3Block 5

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

tube)

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

How to get the ideal 2-phase flow in the

detector?

Detector

Cooling plant

Warm transfer over distance

Detector

Cooling plant

Chiller Liquid circulationCold transfer over distance

Direct expansion into detector with C3F8 compressorWarm transfer linesBoil-off heater and in detectorTemperature control by back-pressure regulator

CO2 liquid pumpingCold concentric transfer lineNo components in detectorTemperature control by 2-phase accumulator

LHCb method:

Atlas method:

Liquid Vapor

2-phase

Pre

ssu

re

Enthalpy

Liquid Vapor

2-phase

Pre

ssu

re

Enthalpy

Vapor compression system•Always vapor needed•Dummy heat load when switched off•Oil free compressor, hard to find

Pumped liquid system•Liquid overflow, no vapor needed•No actuators in detector•Oil free pump, easy to find•Standard commercial chiller

Hea

terCompressor

Pump

Compressor

BP. Regulator

The 2-Phase Accumulator Controlled Loop(2PACL)

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.

Con

dens

er

PumpHeat exchanger

Flooded evaporator

Restrictor

2-Phase Accumulator

He

at

in

He

at

in

He

at

ou

t

He

at

ou

t

1

2 59

1013

-450 -400 -350 -300 -250 -200 -1505x102

103

104

2x104

h [kJ/kg]

P [k

Pa]

-40°C

-30°C

-20°C

-10°C

0°C

10°C

0.2 0.4 0.6

Tertiary VTCS in P-H diagram

1

23

4

5

67

Accumulator pressure = detector temperature

I nternal heat exchanger brings evaporator pre-expansion per definition right above saturation(3-5)=-(10-13)

Capillary expansion brings evaporator in saturation

Detector load (9-10)

1013

9

8

53

1

Pump is sub cooled

-450 -400 -350 -300 -250 -200 -1505x102

103

104

2x104

h [kJ/kg]

P [k

Pa]

-40°C

-30°C

-20°C

-10°C

0°C

10°C

0.2 0.4 0.6

Tertiary VTCS in P-H diagram

1

23

4

5

67

-450 -400 -350 -300 -250 -200 -1505x102

103

104

2x104

h [kJ/kg]

P [k

Pa]

-40°C

-30°C

-20°C

-10°C

0°C

10°C

0.2 0.4 0.6

Tertiary VTCS in P-H diagram

1

23

4

5

67

Accumulator pressure = detector temperature

I nternal heat exchanger brings evaporator pre-expansion per definition right above saturation(3-5)=-(10-13)

Capillary expansion brings evaporator in saturation

Detector load (9-10)

1013

9

8

53

1

Pump is sub cooled

Electron

HadronProton beam

Proton beam

LHCb Detector Overview

Muon

LHCb Cross sectionGoals of LHCb:Studying the decay of B-mesons to find evidence of CP-violation

20 meter

Vertex Locator

VELO Thermal Control System CO2 Evaporator section

Detectors and electronics

23 parallel evaporator stations

capi

llarie

s

and

retu

rn h

ose

•Temperature detectors: -7ºC •Heat generation: 1600 W

The LHCb-VELO Thermal Control System (LHCb-VTCS)

A 2-Phase Accumulator Controlled Loop

LHCb-VTCS Overview A 2-Phase Accumulator Controlled

Loop

2-phase

gas

R404a chiller

22

33

6677

11

88

44

2-phase2-phase

liquid liquid liquid

2-phase

Con

den

ser Evaporators

Concentric tubePump

Rest

rict

ion

AccumulatorCooling plant area

Transfer lines(~50m) VELO area

55

liquid

Evaporator :• VTCS temperature ≈ -25ºC• Evaporator load ≈ 0-1600 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

R507aChiller

LHCb-VTCS Cooling Components

VTCS Evaporator

Pumps

Accumulators

Condensers

Valves

VTCS Units Installed @ CERN

July- August 2007

CO2 Unit

Freon Unit

VTCS 2PACL Operation

50 100 150 200 250 300 350 4005x100

101

102

2x102

h [kJ/kg]

P [bar

]

-40°C

-20°C

0°C

20°C

40°C

0.2 0.4 0.6 0.8

VTCS start-up and operating cycles

1

3,5

810,13

1

3 5

8

9,10,13

1

35

8

9,10131

3 5

8

9 10 13

CB

D E

A

50 100 150 200 250 300 350 4005x100

101

102

2x102

h [kJ/kg]

P [bar

]

-40°C

-20°C

0°C

20°C

40°C

0.2 0.4 0.6 0.8

VTCS start-up and operating cycles

1

3,5

810,13

1

3 5

8

9,10,13

1

35

8

9,10131

3 5

8

9 10 13

CB

D E

A

-50

-25

0

25

50

75

100

0:00 0:15 0:30 0:45 1:00 1:15 1:30 1:45 2:00 2:15 2:30 2:45 3:00

Pump head pressure (Bar)

System pressure (Bar)

Accumulator Level (%)

Accu liquid temp. (ºC)

Pump inlet temp. (ºC)

A B C D

2-phase

2-phase

Sub-cooledliquid

Condense

r

Evapora

tor

Pump

Accumulator

liquid

Heater

Heat exchanger

R50

7a

chill

er

1 3

10

9

85

13

2-phase

2-phase

Sub-cooledliquid

Condense

r

Evapora

tor

Pump

Accumulator

liquid

Heater

Heat exchanger

R50

7a

chill

er

11 33

1010

99

8855

1313

VTCS Evaporator performance

(Stability and response to heat-load changes)

-40.00

-38.00

-36.00

-34.00

-32.00

-30.00

-28.00

-26.00

-24.00

-22.00

-20.00

1:40:48 PM 1:48:00 PM 1:55:12 PM 2:02:24 PM 2:09:36 PM 2:16:48 PM 2:24:00 PM

Time (hh:mm:ss)

Tem

per

atu

re (

'C)

-1000.00

-500.00

0.00

500.00

1000.00

1500.00

Po

wer

(W

att)

, Lev

el (

‰)

VTCS_TL_PT102.P TL_PT102 Accu Pressure

VTCS_TL_PT102.P Tsat(TL_PT102)

VTCS_TL_PT104.P TL_PT104 Pump HeadPressure

VTCS_TL_TT045.T TL_TT045 Evaporator

VTCS_TL_TT112.T TL_TT112 CO2 Pump InletTemperature

VTCS_TL_HT105.Power TL_HT105 AccuHeater

VTCS_TL_LT101.Level TL_LT101 Level

VTCS_TL_TT125.T TL_Detector heater

Evaporator Temp (ºC)

Accu Temp ≈ Set-point (ºC)

Detector Power (Watt)

600 Watt

Accu level (‰)

Accu Cooling Power (Watt)

Pumped Liquid Temp (ºC)

@Setpoint =-25ºC:Accumulator temperature: -24.8ºC Evaporator temperature (No Load): -23.4ºCEvaporator temperature (600 W Load): -23.0ºCStabilization time from 0 to 600 Watt: ca. 7minTemperature stability : <0.25ºC

(A/Left side)

VTCS Transfer line Operation(Internal heat exchanger)

-50

-25

0

25

50

75

0:00 0:15 0:30 0:45 1:00 1:15 1:30 1:45 2:00 2:15 2:30 2:45 3:00

[10] Evaporator pressure (Bar)

[13] Condenser Inlet (ºC)

[1] Pump inlet (ºC)

2-phase

gas

R507a chiller

1

2-phase

2-phase

liquid

2-phase

Condense

r

Evapora

tor

Concentric tube

Pump

Rest

rict

ion

Accumulator

liquid

2

3 4

10

9

8765

11

1214

13

2-phase

gas

Transfer lines(Ca. 50m)Cooling plant area VELO area Inside VELO

Heater

By-p

ass

2-phase

gas

R507a chiller

11

2-phase

2-phase

liquid

2-phase

Condense

r

Evapora

tor

Concentric tube

Pump

Rest

rict

ion

Accumulator

liquid

22

33 44

1010

99

88776655

1111

12121414

1313

2-phase

gas

Transfer lines(Ca. 50m)Cooling plant area VELO area Inside VELO

Heater

By-p

ass

Acc

umul

ator

se

t-po

int

[5] Evaporator liquid in (ºC)

A B C

B

C

Transfer line temperature profile

A: Condenser and evaporator single phase

B: Evaporator 2-phase, condenser single phase

C: Both evaporator and Condenser 2- phase

[14] Accumulator pressure (Bar)

[10] Evaporative temp. (ºC)

A

Eva

pora

tor

side

Coo

ling

pla

nt s

ide

VTCS Accumulator Control

Accumulator Heater Performance

0.0

50.0

100.0

150.0

200.0

250.0

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00

Accumulator Pressure (Bar)

Ther

mal

Res

ista

nce

(mK

/W)

-100.00

-50.00

0.00

50.00

100.00

150.00

Hea

ter

Tem

pera

ture

Gra

dien

t ('C

)

Resistance (mK/W)

dT heater (ºC)

1000 W750 W

500 W

400 W

250 W

100 W

Resistance @ 1000, 750, 500, 400 & 250 W

Resistance @ 100 W

-40

-20

0

20

40

60

80

100

0:00 0:05 0:10 0:15 0:20 0:25 0:30 0:35 0:40 0:45

2PACL Start-up

Heater power (%)

Accu Level (%)

Heater temp. (ºC)

Liquid temp. (ºC)

Pump inlet (ºC)

Accumulator Pressure (Bar)

Pump head (Bar)

Decrease heater power near critical point to prevent dry-out

Thermo siphon heater for pressure increase(Evaporation)

Cooling spiral for pressure decrease(Condensation)

Accumulator Properties:

• Volume 14.2 liter (Loop 9 Liter)• Heater capacity 1kW• Cooling capacity 1 kW

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)

Over critical filling

Under critical filling

Accumulator Liquid level

(Fill rate 500 gram/liter)

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0

Accumulator Volume (L)

Liq

uid

leve

l (%

)

.

(Fill rate 575 gram/liter)

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0

Accumulator Volume (L)

Liq

uid

leve

l (%

)

.

(Fill rate 725 gram/liter)

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0

Accumulator Volume (L)

Liq

uid

leve

l (%

)

.

(Fill rate 650 gram/liter)

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0

Accumulator Volume (L)

Liq

uid

leve

l (%

)

.

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

• 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)

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.

What brings the future (1) ?

• Future projects at NIKHEF:– Development of a desktop CO2 cooler for

laboratory and prototype use. (<1kW@-35’C)

– 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

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)