Full Scale Thermosyphon Cooling Plant Description

Contents 1. Introduction................................................................................................................................1

2. Full Scale Thermosyphon Detailed Description ..............................................................................2

2.1. Thermosyphon Circuit ..........................................................................................................3

2.1.1. Condenser....................................................................................................................6

2.1.2. Pipes............................................................................................................................6

2.2. Brine Circuit.........................................................................................................................7

2.3. Chiller Circuit .......................................................................................................................7

3. Process and Instrumentation Diagram and its Components ...........................................................8

4. Electrical Power ........................................................................................................................ 10

5. Integration ................................................................................................................................ 11

6. References ................................................................................................................................ 11

1. Introduction

The Thermosyphon Project1 concerns the cooling of the Inner Detector of ATLAS experiment, in LHC

Point 1 (Switzerland).

The silicon part of the Inner Detector (ID) of the ATLAS experiment is presently cooled by a fluorocarbon

evaporative system, keeping -25 °C in the in the detector and the inlet and outlet tubes at 20 °C. The

heat load is 62.4 kW while the evaporation temperature in the boiling channels is -25 °C.

A 95m high two-phase thermosyphon shall replace the present system.

The Thermosyphon Project has three separated circuits: the main Thermosyphon circuit, the Brine

circuit and the Chiller one:

DRAFT

• The Thermosiphon circuit uses C3F8. Its basic working principle consists on condensing the C3F8

at the surface. This produces a liquid column from the surface to the cavern, increasing the

pressure due to the height difference. Then the C3F8 evaporates in the detector while cooli ng it

and goes back to the surface as gas by differential pressure.

The Brine circuit uses liquid Perfluorohexane C6F14. This circuit removes the heat from the

Thermosyphon circuit via a heat exchanger that will be then transferred to the chiller circuit.

• The Chiller circuit uses a cascade refrigeration circuit to cool down the Perfluorohexane circuit.

A general overview of the Thermosyphon cooling plant is presented in Figure 1.

Figure 1. Full Scale Thermosyphon scheme in LHC Point 1.

The main components of the Thermosyphon cooling plant: the Condenser, the Brine circuit and the

Chiller circuit are located at the surface level where all the heat is removed from the system.

The existing evaporative cooling system can be switched to the Thermosyphon, once it is fully

commissioned, by operating two valves on the supply and return distribution lines.

2. Full Scale Thermosyphon Detailed Description

In order to reach and to decrease even more the detector’s working temperature with this new cooling

system, the return pressure (that will set the evaporation pressure at the detector) at the return line

Vapour Return

Liquid SupplyPin Tin

Pout

x204 lines

x204 lines

Present Plant: - High circuit pollution risk- Risk of oil free compressor failure- Frequent compressor maintenance

2-Stage Redundant Chiller(-70°C)

ΔH

P1

P2Dummy Load

P3

P2 > P3 > P1

Chiller Circuit Brine Circuit Thermosyphon Circuit

To detector From detector

12/10/2010 60kW Thermosyphon 6

AB

C

DE

F G H I

JK

LM N O

PI‘

New Plant: Thermosyphon- Natural circulation of the fluid - No working components - Low maintenance (filters)

ATLAS Surface

USA15

PX15

UX15

ATLAS I.D.

needs to be decreased. In order to do so, the condenser of the Thermosyphon will need to condense the

return vapour at -60 °C and to sub-cool it to -65 °C (to allow a stable performance of the Thermosyphon

cooling plant). At this saturation the correspondent saturation pressure of the C3F8 at the condenser is

0.31 bar(a).

In order to keep the condenser at -65°C, the chiller will have to cool the Brine circuit to -70 °C. The heat

load on the detector is 62.4 kW which corresponds to a mass flow rate equal to 1.2 kg/s of the C3F8. The

required chiller power to condense and sub-cool the C3F8 from 20 °C to -65 °C on the Thermosyphon

Condenser is therefore 170.5 kW.

The two stage chiller will be responsible for the heat removal at the desired temperature. In between

the Chiller circuit and the Thermosyphon circuit is the Brine circuit. The Brine circuit allows the precise

regulation of the temperature on the Thermosyphon Condenser.

In order to keep the detector cold even when its electronics are switched off, the Thermosyphon cooling

plant has a “warm operation” mode that provides 50 kW of cooling power at -20 °C. This mode can be

independent from the supply of the cooling water from the cooling towers, by using an air cooled

condenser on the first stage of the chiller.

The control software for the Thermosyphon, Brine, Chiller, and Water circuits is conforming to the

CERN’s UNICOS framework (UNified Industrial COntrol System). The human-machine interface will be

based on a SCADA built on PVSS.

2.1. Thermosyphon Circuit

The liquid C3F8 at the outlet of the condenser is at -65 °C and will fall to the bottom of the circuit, at the

USA15 service cavern, increasing its pressure until the required 15 bar(a). The liquid supply line going to

the detector requires a fluid temperature of 20 °C (the supply lines are not insulated). So the liquid C3F8

needs to be heated to this temperature. Two electrical heaters and one recuperation heat exchanger

where then added to the main liquid line at the bottom of the thermosyphon circuit. The recuperation

heat exchanger heats the liquid C3F8 by the use of the return vapour. Cooling the return vapour will also

reduce the required power for the condensation.

In order to allow a stable performance of the thermosyphon a by-pass that keeps a minimum load on

the system is used. The by-pass contains a Dummy Load that consists on heat exchanger and electrical

heaters submerged in a water-glycol bath. To keep an easy access to the system’s components the by-

pass and all the electrical heaters and heat exchangers on the liquid and vapour line are located at the

same place: in the USA15 service cavern.

A scheme of the cooling system and its thermodynamic diagram is shown in Figure 2. Table 1 shows the

thermodynamic conditions on all the points of the system and Table 2 shows the power evaluation on all

the system’s components.

Figure 2. Scheme of the Thermosypnon cooling system (left) and thermosynamic diagram (right).

Table 1. Thermodynamic conditions on all the points of the Thermosyphon circuit.

Table 2. Power evaluation on all the Thermosyphon circuit’ components.

A

B

C

D

E

F

G

H

I

12/10/2010 60kW Thermosyphon 6

AB

C

DE

F G H I

JK

LM N O

PI‘

Operating point Pressure

(bar) Temperature

(°C) Density (kg/m3)

Thermodynamic State

A 0.5 20

3.90 Superheated Vapour

B 0.51 -20

4.65 Superheated vapour

C 0.309 -25

2.85 Superheated vapour

D 0.309 -60

1699 Saturated liquid

E 0.4 -65

1717 Sub-cooled liquid

F 16.1 -62

1712 Sub-cooled liquid

G 16.1 -51

1672 Sub-cooled liquid

H 16 -20

1552 Sub-cooled liquid

I 16 20

1365 Sub-cooled liquid

I’ 0.5 -51

7.89 Two-phase 66% vapour

An important parameter for the FTS is the design pressure. This pressure depends on the maximum

condensation temperature/pressure combined with the hydrostatic pressure difference due to the

height of the condenser. Figure 3 shows the variation of the pressure (condensation pressure plus the

pressure correspondent to the hydrostatic pressure difference) for different condensation temperatures

for pure C3F8.

Figure 3. Maximum pressure as a function of the condensation temperature for pure C3F8.

For the maximum condensation temperature of 35°C, the maximum pressure at the bottom of the plant

is less than 25 bars. So, for pure C3F8 the plant components would be PN25.

However, and despite the fact that the actual plant uses pure C3F8 as a cooling medium, blends with

other fluorocarbons may be required in the future to improve the cooling performance on the detector.

It is therefore possible that a mixture of C3F8/C2F6 is used to increase the evaporation pressure at the

same evaporation temperature. For a given temperature, a higher the concentration of C2F6 implies a

higher saturation pressure of the blend. So, it would be possible to decrease the evaporation

temperature without decreasing the evaporation pressure.

Taking the blends into account, the Full Scale Thermosiphon is PN40. This means that it would be

capable of using a high concentration (up to 40%) of C2F8 in C3F8, corresponding to a higher

condensation and hydrostatic pressures.

Operation

line

Description Power

[kW]

Component

Point A to B Cooling of return vapour 36.2 Vapour Cooling HX

Point B to C Expansion on the return line 4.5 Return Pipe

Point C to D Condensation 165 Condensing Coil

Point D to E Sub-Cooling 5.5 Sub-Cooling Coil

Point C to E Condensation & Sub-cooling 170 Condenser

Point E to F Heating along the liquid line 4 Supply Pipe

Point F to G Heating from -62 to -51°C 12.2 Electrical Heater

Point G to H Heating the liquid with the return vapours 36.2 Sub-Cooling HX

Point H to I Heating from -20 to 20°C 51.1 Electrical Heater

Point I’ to A Evaporation and super heating to 20°C 107 Dummy Load

0

5

10

15

20

25

-80 -60 -40 -20 0 20 40 60

Max

imu

m P

ress

ure

[bar

]

Condensation Temperature [°C]

2.1.1. Condenser

The condenser of the Thermosyphon is the most important component of the Thermosyphon loop,

being responsible for maintaining the detector’s evaporation pressure. It will also be capable of storing

all the fluid in the system. The total mass of C3F8 in the Thermosyphon is 2750 kg: 1565 for the liquid

line, 645 kg for the vapour line, and 540 kg for the leak compensation. The construction drawing of the

Condenser is shown in Figure 4.

Figure 4. Construction drawing of the FSTS Condenser.

The Condenser has an approximate volume of three cubic meters, a length of seven meters, and a

weight of 3500 kg. It is made in stainless steel 316L and 316Ti and it is insulated with a layer of

FOAMGLASS. In order to minimize its weight and cost, because of its large size, the design pressure is 25

bar.

A triangular steel structure placed on the top of the building 3184 is used to place the condenser.

2.1.2. Pipes

The thermosyphon supply and return pipes are seamless stainless steel 304L with the following

dimensions:

- Supply pipe DN50: ID = 56.3mm; OD = 60.3mm (2mm thickness).

- Return pipe DN200: ID = 211.56mm; OD = 219.08mm (3.76mm thickness).

The design pressure for both supply and return pipes is PN40. The design temperature range for the

supply and return pipes are -70 °C to 35 °C and -35 °C to 35 °C, respectively.

Both the supply and return pipes are covered with an insulation layer of FOAMGLAS with a thickness of

80 mm and 75 mm, respectively.

The pipe route has the 3DModel Number: ST0322617_01 and can be found on the EDMS document

number 1142340.

A piping and piping supports stress analysis including the calculations for the earthquake resistance

following the Eurocode 8 has been done3, 4.

The pipe weldings are 100% X-ray inspected.

2.2. Brine Circuit

The Brine circuit is placed between the Thermosyphon circuit and the Chiller circuit. It is responsible for

removing the heat removal from the Thermosyphon Condenser to the Chiller. The cooling fluid of the

Brine circuit if the Perfluorohexane C6F14.

The circuit consists on the use of a liquid pump that sends the C6F14 at -70 °C to the Thermosyphon

Condenser on the top of the building 3184 that then goes back to the Chiller, located at the ground

floor. After the pump there is an electrical heater of 40 kW of power that is responsible for the fine

tuning of the supply temperature, allowing also a fast increase of the C6F14 temperature when needed.

The expansion vessel of the Brine circuit is placed on the roof of building 3184, in same structure of the

Thermosyphon Condenser.

The brine circuit pipes are DN125 (ID = 134.5 mm; OD = 139.7 mm; 2.6 mm thickness). They are

designed for a temperature range of -75 °C to 35 °C. The design pressure for both the return and supply

pipes is PN16. They are covered with an insulation layer of 70 mm of FOAMGLAS. Like the

Thermosyphon pipes, the Brine circuit pipes and their supports, a stress analysis including the

calculations for the earthquake resistance following the Eurocode 8 has been done 5.

2.3. Chiller Circuit

The Chiller circuit is responsible for the heat removal of the Thermosyphon. It has a cooling power of

170 kW at a temperature of -70 °C. It is later cooled by the cooling water from the cooling towers.

The chiller consists on a two stage vapour compression cycle using R404a on the first stage (high

temperature) and R23 on the second stage (low temperature). It can run from 20 to 100 % of its cooling

capacity at all temperature set points: from 20 to -70 °C. The first stage is equipped with an air cooled

condenser that is capable of providing 50 kW of cooling power at -20°C. The first stage can work

independently of the cooling water. For the operation of the control valves it uses a 6 bar pneumatic dry

air supply.

Figure 5 shows the chiller configuration scheme.

Figure 5. Chiller configuration scheme.

The chiller unit has 12 meters of length, 2.5 meters width, and three meters height. The total weight is

15 tons. It has two connections for the inlet and outlet of the brine circuit and two connections for the

inlet and outlet of the cooling water. During normal operation the Chiller consumes 101.4 m3/h of

cooling water.

3. Process and Instrumentation Diagram and its Components

The Process and Instrumentation Diagram (P&ID) contains all the Thermosyphon cooling plant circuits

designed at CERN: the Thermosyphon, the Brine, and the Water circuits. The P&ID diagram is shown in

Figure 6.

Figure 6. Full Scale Thermosyphon Process and instrumentation Diagram2.

In general, on all the circuits of the thermosyphon cooling plant pressure and temperature transmitters

where placed each time the thermodynamic change on the specific location. When the temperature of

the circuit can reach low values, the pressure transmitters are connected to line throughout a capillary

tube. The temperature transmitters are installed inside a “Doit de Gant” allowing its removal from the

line without the need of stopping the plant or draining the circuit line.

Shutoff valves have also been installed in strategic parts of the circuit in order to allow the separation of

the different parts of the circuit in case any intervention is needed to be done. All the valves are

pneumatically actuated and its closing time can be controlled.

On the liquid lines, in between two shutoff valves a double safety valve has been installed to release the

pressure in case of cold liquid trap. The safety valves are connected after a three -way valve to allow the

switch between them.

On the thermosyphon and brine circuits the perfluorocarbons are filtered by particle filters and

dehydrators, installed in double parallel circuits. Differential pressure transmitters are placed between

the filters to monitor their performance. On the water circuit, differential pressure transmitters hav e

also been placed between the inlet and outlet of all the heat exchangers due to the risk of fouling on

these components.

The plant is equipped with flow meters on the thermosyphon and water circuits.

The electrical heaters on the Thermosyphon and Brine circuits are protected with thermal switches.

In general, for the Brine and Thermosyphon circuits, where perfluorocarbons are used, the leak

tightness requirement of all the mechanical components was lower than 10-7 mbar.lt/s.

4. Electrical Power

For the two operation modes of the Thermosyphon cooling plant, the normal and the warm operations,

there are two different power supplies: the standard and he emergency one. The required emergency

and the UPS power supplies are also available as standard power during the normal operation.

The FSTS requires electrical power on the surface and underground areas. The electrical cupboards for

the power supply at the surface area are located in building 3185 and for underground the electrical

cupboard is located in USA15 service cavern. All control cupboards are secured by an UPS power supply.

The Table 3 two shows the required electrical power for the different components of the FSTS cooling

plant6.

Table 3. Full Scale Thermosyphon Electrical Power Consumption.

Component Connection Location

Standard Installed Power [kW]

Emergency Power [kW]

UPS Power [kW]

Chiller I Blg. 3185 231 237 -

Chiller II Blg. 3185 231* 237* -

Brine pumps Blg. 3185 - 2x30 -

Brine Heater Blg. 3185 40 - -

Water Pumps Blg. 3185 2x10 - -

Thermosyphon Heater I USA 15 15.5 - -

Thermosyphon Heater II USA 15 - 56 -

Thermosyphon Dummy Load USA 15 100* 25 -

Thermosyphon and Chiller I Control Cupboard (surface)

Blg. 3185 - - 5

Brine and Water Control Cupboard Blg. 3185 - - 5

Chiller II Control Cupboard Blg. 3185 - - 5*

Thermosyphon Control Cupboard(underground)

USA 15 - - 5

TOTAL Blg. 3185 291 297 10+5*

TOTAL USA 15 115.5 81 5

5. Integration

6. References

Bld 3184

SH1

SDX1

Condenser Location

Location of the FSTS Electrical and Control Cupboards

ChillerBrine StationUnderground Gallery

Thermosyphon Condenser

Brine pipe routing

Blg. 3184 (SH1)

USA 15 Level 3

Thermosyphon by-pass components

Primary water HX(inside the underground gallery)

Underground gallery(dashed line)

Service Hole

Water pumps location(inside SH1)

Chiller 1

Brine Station

[1] General Description of the Full Scale Thermosiphon Cooling System for Atlas SCT and Pixel.

Thermosyphon Project Technical Note. EDMS 1083852.

[2] FSTS P&ID. EDMS 1101188.

[3] FSTS Piping System Report. EDMS 1148755.

[4] FSTS C3F8 Pipe Stress Analysis. EDMS 1163494.

[5] FSTS Brine pipe calculation specification. EDMS 1164132.

[6] FSTS Electrical Power Requirement. EDMS 1159012.

[7] FSTS Integration folder: https://edms.cern.ch/nav/P:CERN-0000076703:V0/P:CERN-

0000090874:V0/TAB3.