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8th IIR Gustav Lorentzen Conference on Natural Working Fluids, Copenhagen, 2008

GL2008

8th

IIF/IIR Gustav Lorentzen Conference on Natural Working Fluids

7-10 September 2008

Copenhagen, Denmark

Paper number: xxx

CO2 COOLING FOR THE LHCB-VELO EXPERIMENT AT CERN.

B. Verlaat*, A. Van Lysebetten, and M. Van Beuzekom.

*Author for correspondence

National institute for subatomic physics (Nikhef),

Kruislaan 409

1098 SJ Amsterdam,

The Netherlands,

E-mail: [email protected]

ABSTRACT CO2 as evaporative coolant has gained interest for the use in high energy particle physics

experiments. It is applied as coolant in the Vertex Locator (Velo) of the LHCb detector, which is

an experiment at the Large Hadron Collider at CERN in Geneva.

Silicon particle detectors like the Velo have special requirements on cooling. The sensors which

are spread over a large volume must be kept at a stable cold temperature (-7 ºC) at all times while

the attached electronics generate a substantial amount of waste heat which has to be taken by the

cooling system. The cooling infrastructure in the detector needs to be of low mass and the

construction materials including the coolant need to be radiation resistant. CO2 as coolant is a

good option for this application as it can withstand a large amount of radiation and has excellent

thermal behavior in small diameter tubes.

The CO2 cooling system for the Velo uses the 2-Phase Accumulator Controlled Loop (2PACL)

method. This method supplies low quality CO2 into the evaporator at a constant pressure and

requires no active components inside the detector.

This paper describes the design of the Velo detector and the Velo Thermal Control System

VTCS). It explains the 2PACL method and test results of the VTCS are presented and experience

gained during VTCS commissioning are described.

1. LHCB AND THE VERTEX LOCATOR.

A novel CO2 cooling system is developed for cooling the Vertex Locator (VELO) of the LHCb

experiment. LHCb is a particle detector studying Cp violation which must give the answer to the

question why the universe exist only out of matter, and why anti matter has disappeared. The

LHCb detector (LHCb Coll., 2003) is one of the 4 new build particle detectors constructed

around the collision points of the new Large Hadron Collider (LHC) at CERN in Geneva.


1.1 The Vertex Locator

The VELO (LHCb Coll., 2001) is the sub detector closest to the collision point in LHCb. It

exists of 21 double and 2 single silicon wafer layers which are situated approximately 1 cm away

from the LHC proton beam. The silicon wafers are mounted on a module containing the read out

electronics and mechanical support. The silicon modules are situated in a vacuum which is

separated only by a 0.3mm foil from the LHC beam vacuum. The maximum pressure difference

between the 2 vacuums is 5 mbar, which results in a very complicated system for pumping down

and inflating the vacuum systems simultaneously. The reason for a secondary vacuum for the

silicon modules is the out gassing of the silicon modules, and the ability of installing detector

hardware without exposing the LHC beam vacuum volume to the outside air. The 0.3 mm

aluminum foil around the silicon stations acts also as a faraday cage protecting the silicon and

electronics from the electromagnetic interference of the proton beam.

The silicon stations suffer from a high dose of ionizing radiation induced by the LHC proton

beam. The radiation causes damage to the silicon crystal structure resulting in an increase of

leakage current (The ROSE coll. 1999). Permanent cooling of the sensors is needed to avoid the

outcome of this damage. A silicon temperature less than -7ºC is sufficient to minimize the effects

of radiation damage.

1.2 Velo module design. The silicon modules consist of a carbon fiber TPG

1 laminate at which on both sides an electronics

hybrid with a silicon sensor are glued. At the bottom of this laminate the CO2 cooling evaporator

is mounted on one side and the carbon fiber support paddle on the other side. The paddles are

mounted on a stiff aluminum base frame. Figure 3 show a picture of a constructed Velo half with

the discussed items clearly visible. The beetle read-out chips are located on the hybrid at the edge

of the silicon and generate a substantial amount of waste heat which needs to be taken away by

the CO2 cooling system. The aluminum base is the positional reference of the modules and must

therefore be maintained on room temperature. This is achieved by heaters since the thermal

connection via the paddles will otherwise cause the base to cool down by the CO2 evaporators.

Table 1 and 2 show the temperature requirements and the heat dissipation of the Velo

components.

1. Thermal Pyrolytic Graphite, High conductive material λx,y=1800

W/mK , λz=10

W/mK

Fig 2: Schematic side cut of the VELO

experiment (beam vacuum white

/ detector vacuum yellow)

Figure 1: Artists impression of the

VELO experiment the

Secondary

Vacuum

LHC Beam

Vacuum

Silicon


Table 1: Operational and survival temperature limits of VELO key components

Survival temperatures: Operational

temperatures: Before irradiation: After irradiation:

Silicon Wafers -12°C / -5°C

(Nominal tip = -7°C )

-30°C / 100° Long-term: -30°C / 0°C

Short term: 0°C / 24°C

(100 hours)

Hybrid -20°C / 80°C -40°C / 50°C

RF-Foil <20°C NA

Module base frame 18°C -22°C (Stable) NA

Table 2: Power dissipation of VELO hardware inside the detector vacuum

Per VELO

module

Per PU

module

RF-Foil Module

Base

Total for

VELO

Nominal power 21.8 W 13.3 W 8 W 70W 1048 W

Maximum power 27.5 W 16.2 W 8 W 139W 1394 W

1.2 CO2 evaporator design. Each silicon module is connected to a

dedicated parallel evaporator branch.

One branch consists of a 1 meter long

stainless steel capillary of 1.5x0.25mm,

which is embedded in an aluminum

plate connected to the carbon fiber/ TPG

laminate. The aluminum plate is casted

around the tube in a special developed

procedure by melting the aluminum

around the tube in a vacuum oven.

Melting the aluminum under vacuum

will cause the aluminum to join to the

stainless steel forming inter metallic

phases. The melting procedure will give

a lot of freedom in pipe geometry inside

the aluminum and a perfect thermal

contact between the pipe and the aluminum plate. The cooling block shape is achieved by the

shape of the casting mould. The way the casting is done looks similar to baking of cookies, this is

the reason why the aluminum blocks are called cooling cookies. Each evaporator branch has a 1.3

meter restriction capillary of 1x0.2 mm at the inlet for a good flow distribution over all the

evaporator branches. The presence of the evaporator in a vacuum system gives high constrains to

the leak tightness therefore the complete evaporator assembly (figure 4) is made of stainless steel

tubes all joined together with vacuum brazing or orbital welding. No connectors are present

inside the vacuum system. The inlet manifold connected to the inlet capillaries is outside the

vacuum vessel and is accessible. This way it is possible to connect cooling to individual channels,

a feature which is used by module commissioning. During laboratory commissioning the cooling

was achieved by a CO2 bottle blow system.

Figure 3: Assembled Velo half with silicon

modules, cooling, and module base


2. VELO THERMAL CONTROL SYSTEM

The Velo Thermal Control System

(VTCS) (Van Beuzekom, 2007) is

a cascade system of three

hydraulic systems. A chiller

condenses the evaporated CO2

generated in the Velo module

evaporators back to liquid and

reject the waste heat to the cold

water system of CERN. This

chiller is a commercial type

chiller with a gas compressor,

water condenser, evaporators and

expansion valves. The water

system is called the primary

cooling system, the chiller the

secondary and the CO2 loop the

tertiary cooling system. Figure 6

shows a schematic block scheme

representation of the cascade systems, with the main heat flows and the system temperature

distribution.

The tertiary cooling system is a hybrid 2-phase / single phase mechanically pumped loop using

CO2 as working fluid. It can operate in a single phase or in a two-phase cooling mode. During

start-up it operates in single phase mode, once cooled down it can be set to a 2-phase loop for a

more efficient heat transfer and accurate temperature control.

Figure 4: VTCS CO2 evaporator assembly Fig 5: Velo Module with alumi-

num cooling cookies.

-40°C -20°C 0°C 20°C

Tertiary

system

(CO2)

Secondary

system

(R404a)

Primary

system

(Water)

Work

(Pump)

Detector

waste heat

Environmental

heat leak

Work &

heat

(Compressor)

Environmental

heat leak

Qt1

Qt2

Qt3

Qs2

Qs3

Qs1=Qt1+Qt2+Qt3+Qt4

Control

heat

Qt4

Qp1=Qs1+Qs2+Qs3+Qs4

Control

heat

Qs4

-40°C -20°C 0°C 20°C

Tertiary

system

(CO2)

Secondary

system

(R404a)

Primary

system

(Water)

Work

(Pump)

Detector

waste heat

Environmental

heat leak

Work &

heat

(Compressor)

Environmental

heat leak

Qt1

Qt2

Qt3

Qs2

Qs3

Qs1=Qt1+Qt2+Qt3+Qt4

Control

heat

Qt4

Qp1=Qs1+Qs2+Qs3+Qs4

Control

heat

Qs4

Figure 6: Block scheme and heat balance

representation of the VTCS


The main unit of the VTCS is

placed 50m away from the Velo

detector behind a thick concrete

shielding wall. This wall is shielding

the system from the radiation of the

LHC. Behind this shielding wall it is

also possible for people to work.

The VTCS is designed such that all

active hardware is located in this

safe zone. The cooling hardware in

the experimental area consists only

of tubes and passive devices such as

restrictors and check valves. Local

sensors (pressure and temperature)

are only for monitoring and are not

important for the cooling system

operation. Malfunctioning of

inaccessible vital hardware is so

reduced to an absolute minimum.

The CO2 evaporators in the detector are connected to the VTCS via 50 meter long concentric

transfer lines. The liquid feed transfer tube (1/4”x0.035”) is situated inside the vapor return tube

(16mx1mm). The concentric construction acts as a long counter flow heat exchanger needed for

conditioning the evaporator inlet flow to a low vapor quality.

The flow in the evaporator must be of low vapor quality to achieve a stable liquid expansion in

the flow distribution capillaries. The cooling system must maintain the silicon wafers cold,

powered or unpowered. The evaporator outlet vapor quality is therefore variable. The evaporator

temperature to achieve the silicon temperature requirement is between -25ºC and 30°C (Verlaat,

2005). This temperature can be set in the system by the accumulators and is called the VTCS’ set-

point temperature.

2.1 The 2PACL principle. To achieve liquid expansion over the capillaries and a low vapor quality together with a remote

controlled pressure in the evaporators the 2-Phase Accumulator Controlled Loop (2PACL)

principle is developed at NIKHEF (Verlaat, 2007). This principle is controlling the system

pressure and hence the

evaporator pressure using

a 2-phase accumulator.

This vessel is parallel

mounted to the system and

contains per design always

a content of liquid and

vapor. The guaranteed

presence of a saturated

mixture in the accumulator

makes the system pressure

to be a function of the

accumulator temperature. In this way the system pressure can be tuned independent from the

freon chiller temperature cooling the CO2 condenser.

Co

nd

en

se

r

PumpHeat exchanger

Flooded evaporator

Restrictor

2-Phase Accumulator

He

at

in

He

at

in

He

at

ou

t

He

at

ou

t

1

3 59

1013

Co

nd

en

se

r

PumpHeat exchanger

Flooded evaporator

Restrictor

2-Phase Accumulator

He

at

in

He

at

in

He

at

ou

t

He

at

ou

t

1

3 59

1013

Figure 8: Simplified representation of the 2PACL method

2-phase

gas

R404a

chiller22

33

6677

11

88

44

2-phase2-phase

liquid liquidliquid

2-phaseCondenser Evaporators

Concentric tubePump

Restriction

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

R507a

Chiller

2-phase

gas

R404a

chiller22

33

6677

11

88

44

2-phase2-phase

liquid liquidliquid

2-phaseCondenser Evaporators

Concentric tubePump

Restriction

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

R507a

Chiller

Figure 7: Block scheme and heat balance

representation of the VTCS


The 2PACL system works as long as the freon chiller is colder than the accumulator temperature.

The freon chiller condenses and sub-cools the CO2 so the pumps can operate free from cavitation.

The remaining sub cooling after the pump is heated up by the concentric transfer line such that

the evaporator inlet remains in saturation. The operating condition of the evaporator is

independent of the amount of pump sub cooling. This sub cooling may vary in a wide range so a

controlled condenser temperature is not needed. The accumulator temperature is the only accurate

controlled item in the CO2 loop.

2.2 VTCS construction The VTCS chiller and CO2 loop are installed in racks on one of the LHCb service platforms. The

CO2 loop is constructed from orbital weld components from Swagelok and Cajon VCR

connectors. The CO2 pump used is a Lewa CO2 membrane pump. The accumulators are self

engineered stainless steel vessels

containing cooling spirals and a

thermo siphon liquid heater. There are

2 identical CO2 loops, each cooling 1

Velo detector half. Both CO2 loops

are cooled by 1 water cooled chiller

for normal operation and an air-

cooled back-up chiller for

redundancy. The chillers are self

engineered R507a chillers using

standard refrigeration hardware such

as Bitzer compressors, Danfoss line

components and SWEP plate heat

exchangers. The CO2 condensers are

also SWEP plate heat exchangers

reinforced for the high CO2 system

pressure. The design pressure for the

CO2 loop is 135 bar, the test pressure

170bar.

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

103

104

2x104

h [kJ/kg]

P [kP

a]

-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

Internal heat exchanger brings evaporator pre-expansion per definition right above saturation

(3-5)=-(10-13)Satu

ration lin

e

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 [kP

a]

-40°C

-30°C

-20°C

-10°C

0°C

10°C

0.2 0.4 0.6


1

23

4

5

67

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

103

104

2x104

h [kJ/kg]

P [kP

a]

-40°C

-30°C

-20°C

-10°C

0°C

10°C

0.2 0.4 0.6


1

23

4

5

67

Accumulator pressure = detector temperature

Internal heat exchanger brings evaporator pre-expansion per definition right above saturation

(3-5)=-(10-13)Satu

ration lin

e

Capillary expansion brings evaporator in saturation

Detector load (9-10)

1013

9

8

53

1

Pump is sub cooled

Figure 9: VTCS operation in the Pressure-Enthalpy diagram for CO2

Tertiary CO2 unit

Secondary freon unit

Controls PLC

Accumulators

CO2 pumps

Primary water

supply

CO2 transfer line

Tertiary CO2 unit

Secondary freon unit

Controls PLC

Accumulators

CO2 pumps

Primary water

supply

CO2 transfer line

Fig. 10: The VTCS cooling plant installed at LHCb


2.2 VTCS installation commissioning and testing The VTCS installation started at the LHCb

experiment in July 2007. Commissioning

work is ongoing but the system was

successfully operated for the detector

commissioning in March and May 2008.

The VTCS is operated during the vacuum

period at the end of March 2008 at several

set point temperatures varying from 0ºC to

-30ºC. In this period it was not yet possible

to switch on the detector to induce load on

the system. Figure 11 and 12 show the

temperature results of this vacuum

commissioning period. The first half of the

detector was commissioned in May 2008

with a cooling temperature of +10ºC.

During this commissioning the vacuum

system was under atmospheric pressure. To

avoid too much pressure difference

between the 2 vacuum systems the cooling

temperature was limited to +10ºC. Detector

commissioning with cooling under vacuum

is foreseen in July 2008.

Due to the absence of the detector the VTCS is tested using dummy by pass heaters. These

heaters are mounted on top of the Velo parallel to the evaporator and are a good representation of

the detector heat load. Figure 13 shows the response of the VTCS to a nominal heat load change

from 0 to 600 Watt. The system pressure is raised and compensated by cooling the accumulator.

10 15 20 25 30 35 40 45 50 55 60 65 70-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

15

20

25

30

Time from 29 March clock time (Hours)

Mea

su

red

Tem

pe

ratu

re (

ºC)

Pump Outlet Temperature (TRTT120pvss)

C-Side RF-Foil temperature (TRTT043pvss)

C-Side Module Base Temperature (TRTT038pvss)

Sp=-10ºC

Sp=-20ºC

Sp=-30ºC

Sp=0ºC

Sp=-25ºC

Sp=-30ºC

Sp=0ºC

2 Days + 6 hours cooling under vacuum at several set point temperatures

Start-up

10 15 20 25 30 35 40 45 50 55 60 65 70-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

15

20

25

30

Time from 29 March clock time (Hours)

Mea

su

red

Tem

pe

ratu

re (

ºC)



C-Side Module Base Temperature (TRTT038pvss)

Sp=-10ºC

Sp=-20ºC

Sp=-30ºC

Sp=0ºC

Sp=-25ºC

Sp=-30ºC

Sp=0ºC


Start-up

Sp=-10ºC

Sp=-20ºC

Sp=-30ºC

Sp=0ºC

Sp=-25ºC

Sp=-30ºC

Sp=0ºC


Start-up Condenser Out- / Pump Inlet Temperature (TRTT112pvss)



Condenser Out- / Pump Inlet Temperature (TRTT112pvss)



Condenser outlet / Pump sub cooling

Pump outlet

RF-foil

Accumultor saturation temperature (TRPT102tsat) = Setpoint

TR Evaporator Temperature (TRTT048pvss)

Condenser Inlet (TRTT101pvss)

Accumultor saturation temperature (TRPT102tsat) = Setpoint

TR Evaporator Temperature (TRTT048pvss)

Condenser Inlet (TRTT101pvss)

Accumulator saturation (Set-point)

Evaporator temperature

Condenser inlet

Figure 12: Transient temperature results of the commissioning under vacuum.

-30 -25 -20 -15 -10 -5 0-50

-40

-30

-20

-10

0

10

Setpoint Temperature (ºC)

Measure

d T

em

pera

ture

(ºC

)

TR Condenser Inlet (TRTT101pvss)

TR Pump subcooling (VRTT112pvss)

TR Pump Outlet (TRTT115pvss)

TR Evaporator Inlet (TRTT047pvss)

TR Evaporator outlet (TRTT048pvss)

TR Accu Saturation (TRPT102tsat) = Setpoint control

Condenser inlet Pump sub cooling Pump outlet Evaporator inlet Evaporator outlet Accumulator saturation

Figure 11: Steady state temperature results of

the commissioning under vacuum.


-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)

Te

mp

era

ture

('C

)

-1000.00

-500.00

0.00

500.00

1000.00

1500.00

Po

we

r (W

att

), L

ev

el

(‰)

Evaporator Temp (ºC)

Accu Temp ≈ Set-point (ºC)

Detector Power (Watt)

600 Watt

Accu level (‰)

Accu Cooling Power (Watt)

Pumped Liquid Temp (ºC)

-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)

Te

mp

era

ture

('C

)

-1000.00

-500.00

0.00

500.00

1000.00

1500.00

Po

we

r (W

att

), L

ev

el

(‰)

Evaporator Temp (ºC)

Accu Temp ≈ Set-point (ºC)

Detector Power (Watt)

600 Watt

Accu level (‰)

Accu Cooling Power (Watt)

Pumped Liquid Temp (ºC)

Figure 13: VTCS response to a load step

The accumulator level has

increased and the

evaporator pressure is back

to normal. The pump sub

cooling is increased due to

increase of the freon

chillers’ compressor

suction pressure.

The VTCS was tested

under several heat loads

and set point temperatures.

At the nominal set point of

-25ºC the system was able

to remove 1600W. The

lowest achieved set point

was -40ºC. These numbers

are far beyond the systems

requirement, so dummy

load tests have shown that

the VTCS is able to meet

the requirements of table 1

and 2.

3. CONCLUSION AND DISCUSSION

The VTCS has demonstrated that the 2PACL method with CO2 is a good principle of controlling

the temperature of the Velo silicon modules. The benefits from this system such as small cooling

channels and no active components inside the detector has been notified by other particle

detectors as a possible future cooling system. Both the 2 large CERN experiments Atlas and CMS

are considering a similar system for their upgrade inner detectors. The challenge will be to

upgrade the discussed principle from a 2.5 kW system towards a 100 kW system.

4. REFERENCES

[1] LHCb Coll. 2003, LHCb Reoptimized Detector design and performance, CERN/LHCC

2003-30

[2] LHCb Coll. 2001, The LHCb VELO technical design report, CERN/LHCC 2001-0011

[3] The ROSE Coll 1999, R&D On Silicon for future Experiments, CERN/LHCC note, 2000-

009

[4] B. Verlaat 2005, Thermal performance testing of the VTCS evaporator and VELO

module, NIKHEF EN05-01.

[5] B. Verlaat 2007, Controlling a 2-phase CO2 loop using a 2-phase accumulator,

International Conference of Refrigeration 2007, Beijing, ICR07-B2-1565

[6] M. Van Beuzekom, A. Van Lysebetten, B. Verlaat 2007, CO2 cooling experience

(LHCb), The 16th International Workshop on Vertex detectors, Lake Placid, NY, USA,

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