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ATLAS MeetingATLAS MeetingAugust 12, 2008

University of Santa Cruz

Cooling System WorkCooling System WorkMarco Oriunno, SLACMarco Oriunno, SLAC

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Carbon Dioxide is under consideration to replace fluorocarbon as a refrigerant fluid for the upgrade of the ATLAS inner detector (Pixel/Strips)

Direct advantages inside the tracker volume compared to the present fluorocarbon based system:

Lower temperatures easily achievable O(-50oC) -> high protection against the risk of thermal runway

Higher refrigeration capability -> smaller pipes (integration/material budget)

Reduced mass flow per refrigerated power -> pressure drops & pipe size

Compact cooling system plant

Distinctive features:

High standstill pressure (10÷60 bars)

Natural gas, not flammable, dielectric and not toxic

Negligible Global Warming Potential and Ozone Depletion Impact

Fluorocarbons will be soon banned, the refrigeration industry has started the CO2 Rush as the next refrigeration standard.

Considerations I

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Although the CO2 advantages are acknowledged, there is not yet any official endorsement of CO2 cooling system for ATLAS

Several groups are already working on cooling calculations, but only few experimental installations are available only in Europe: NIKHEF, Liverpool both with basic blown systems

CMS also has expressed interest in carbon dioxide cooling for the Tracker upgrade: CERN plans to organize few one-day technical forums starting next October.

On the path of growing the ATLAS upgrade effort at SLAC, the involvement of SLAC in the CO2 cooling project has a two-fold advantage : it is a complement to the US ATLAS activities and encourages the growth of user presence on the site.To develop the possibility of a SLAC involvement took an explorative trip on week June 23-27 :

•One day meeting at NIKHEF to discuss ( with Nigel and Georg) the ATLAS cooling needs, technical visit of the CO2 Blown system.

•Meeting with the CMS engineers involved in the CO2 upgrade to establishing communication, plan to exchange information to solve common problems

•Technical Visit at CERN of the CO2 plant of LHCb VELO detector, running stable at -30oC

•Independent private discussions with the ATLAS colleagues based at CERN (Neal, Vic, Andrea, Christophe) on the features of the present cooling system and various integration aspects

Attempt to take the most updated and unbiased picture of the ATLAS cooling upgrade project

Considerations II

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STEP 1, Development of a blown CO2 plant with small refrigeration capacity,

Unavoidable move to coalesce people and resources around the project

Characterization of the boiling parameters and the heat transfer

Pipes characterization under high pressure: materials, sizesThermal test of small detector prototypes from other US

ATLAS group

STEP 2, development of a real vapor-compression plant with larger refrigeration capacity, few kW : a non trivial extrapolation of the previous exercise.

Test of full scale stave/disk prototypesCharacterization of components : capillaries, heat exchangers,

evaporators

Parallel activity, Participation to the definition of specs for the upgraded cooling plant:

Thermal issues detector related : mat. budget, heat transfer, pressure drops

Plant design: choice of compressors, heaters, heat exchangers, piping, integration.

SLAC PLAN

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SLAC PLAN, details(*)

(*) To be consolidated after real knowledge of the %effort of people involved. I counted myself at 100% which is not the case at the present

Today 08.12

Start experimental

Data

Procurement Launch $ $ $

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Step 1, The Blown System

The maximum refrigeration power is limited and defined by a reasonable time to swap the CO2 cylinder: for 10 watts are needed 0.085 g/s, i.e. 80 hours continuously

Start with a dummy heater on a ¼” pipe to commission the system at few mg/s

Boiling heat transfer studies :

Several mathematical models available for different boiling regimes

Very scarce data in the open literature

Effect of evaporator shape and surface material finishing

Pressure drop studies with different evaporator shapes and sizes

Thermal test of stave module detector prototypes: thermal resistance fluid-sensor

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Step 2, A medium size vapor-compression cycle

As soon as large size stave prototypes become available in the US ATLAS community, the blown CO2 plant must be upgraded to a medium size vapor-compression plant of ~10kW

• The design will require some time because of the several details to be proper addresses

• The safety and installation issues will require also more attention and time

• The financial investment will be more important

Such architecture may well be adopted for the ATLASup ID, therefore it is worth to start to look at general technical solutions which can be scaled up or down for both systems.

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We have started think about the global architecture of the final cooling system

Many options and variants are possible using CO2 :

1. Adopt the same design as for the fluorocarbon, but at least try to improve the known weak points

2. Adopt a modified vapor-compression cycle (trans critical cycle)

3. Adopt a completely different solution, similar to the LHCb VELO, with an Accumulator Pressure Loop

Performances and costs for all the options should be fairly compared before to make the final choice but it could be a long process.

We need soon a document with the minimal functional requirements of the cooling system upgrade : heat loads, temperature range, radiation environment, tracker opening scenario, B-layer insertion, beam pipe bake out, thermal barriers and controls.

Such document should be brief and leave out the technical solutions as much as possible.

ATLAS ID cooling plant, first thoughts

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At the present we consider the option 1, the less steep to climb because :

1. It is a well proven scheme in refrigeration industry for large installation

2. There are not practical limitations to the refrigeration power ( worth to recall that ATLAS ID-up is close to 0.25 MW)

3. Allow naturally the reliable location of the plant in a radiation protected environment

4. Minimize the integration space through warm transfer line

5. Long time experience and trained people in the ATLAS community

6. The factorial risk increase innovating on fluid and cooling plant architecture at the same time, i.e. know how, people and costs.Since we believe that a vapor-compression remains after all a

good solution for the ATLAS ID upgrade, exploring the conceptual design for such a system, not only will provide some preliminary answers but also the guidelines for the Step 2, a medium size vapor-compression cycle.

ATLAS ID cooling plant, first thoughts

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Oil free compressor

condenser

stave -25°C

capillary

gas heater

PBPR setpoint

Pressureregulator

PPR setpoint

Distribution rack

INNER TRACKER

Subcooling

Distribution rack

Shielded and accesible area

Experimental cavern

Heat Exchanger

Oil free compressor

condenser

stave -40°C

capillary

PBPR setpoint

Pressureregulator

PPR setpoint

Distribution rack

INNER TRACKER

Distribution rack

Shielded and accesible area

Experimental cavern

Heat Exchanger

Item Power (kW)

Pixel 10

Barrel Strips 70

Disks 50

Heat losses 20

TOTAL 150

Total with safety factor

250

CO2 plant with unchanged C3F8

architecture

CO2 plant with upgraded

architecture

Heaters and Heat Exchangers outside tracker volume

Heat loads expected

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LCO2receiver

Condenser

ReciprocatingCompressor 152 kWA

B

C

D

Main line liquid

USA15 - UX15

4 x Ø30/34

P = 150 mbar

Main line gas

USA15 - UX15

P = 120mbar

250 Nm3/h

capillary

E

F

inlet distribution inside detector

P = 1bar

vapor quality 0.9

PR

P= 10 barT = 20oC

P=58 barT=192°C

P=58 bar

T=18°C

T=20°C

P=58 barT=20°C

6.44 m3/h

Water HEX 20oC

Pressure Regulator

BPR

Back Pressure Regulator

vapor quality 0.38

Stave -40 oC

Refrigeration Load = 250 kWC02 mass flow = 1.3 kg/s

T

CO2

Water

L

83 kW

7

2

5 4

6

8 1

320.0 [°C]

-40.0 [°C]

192.0 [°C]

21.0 [°C]

18.0 [°C]

CYCLE ANALYSIS : ONE-STAGE CYCLE

T5 :

T1 :

T2 :

TE :

TC :

0.42 [kg/kg]X6 :

QE :

QC : 384 [kW]

240 [kW]

W : 158.5 [kW]

QSGHX : 0 [kW]

1.521COP* :COP : 1.514

0.9817 [kg/s]m :

T4 : 18.0 [°C]

Department ofMechanical Engineering

192.0 [°C]T3 :

REFRIGERANT : R744

20.0 [°C]T7 :

21.0 [°C]T8 :

> DX EVAPORATOR

Technical Univ ersity

SUB-DIAGRAM

WINDOWS

CARNOT : 0.391TOOL C.1

CoolPack

© 1999 - 2001

of Denmark

Version 1.46

LOG(p),h-DIAGRAM

P=10 barT= -40°C

P=10 bar

T= -40°C

P=10 barT= 20°C

H2O flow = 35 m3/h

Pressure ratio (P2/P1) = 5.8

Water HEX 16oC

H2O flow = ?? m3/h

384 kW

M.Oriunno, SLACJuly 15 2008

P=58 barT=20°C

P=10 barT= 20°CP=10 bar

T= 20°C

Total thermal load 240 kW

Ditribution rackX 4

Main line liquid

UX15 - Detector

60 x Ø4/6

P = 384 mbar

60 x Ø4/6

Ditribution rackX 4

Global architecture

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Parameter Symbol Min Max C3F8 Refrigeration load Qe 150 kW 250 kW 60 kW Evaporation temperature Te -40oC -40oC -25oC CO2 mass flow mf 0.789 kg/s 1.315 kg/s 1.056 kg/s Compressor Power Qc 91 kW 152 kW 60 kW Compressor Volumetric flow Vf 150 Nm3/h 250 Nm3/h 530 Nm3/h Compression ratio R 5.8 5.8 6 Compressor Max Temperature Tm 185oC 185oC 90oC Condensation temperature Tc 20oC 20oC 52oC Gas Cooling Heat rejected Qg 154 kW 257 kW 41 kW Condensation Heath rejection Tl 123 kW 205 kW 90 kW Sub-cooling Qu 16 kW 27 kW 35 kW Super-heating Qb 52 kW 87 kW NA

Estimated PerformancesDistinctive features:

Low mass flow

Low volumetric flow

None or light sub-cooling but in the experimental cavern

Heater and/or heat exchanger outside the cold volume

Significant increase of the condensers capacity

Higher gas rejection temperature at the compressors

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Parameters of adiabatic capillary flow:Inlet pressure of sub cooled liquid … pin = 58

barInlet temperature of sub cooled liquid … Tin =

18°CCapillary tube inner diameter … ID = 0.60 mm

and 0.80 mmRelative inner wall roughness: ε ID = 0.003

ID=0.6 mm

ID=0.8 mm

CapillariesCapillaries are fully passive devices and therefore are the most reliable devices to drop pressure inside an inaccessible and harsh environment.

Length very dependent on several parameters like inlet conditions and mass flow

Preliminary simulations done by Vic Vacek, CTU Prague (private communication) confirm that the final capillary length for CO2 is not so different form C3F8 ~2-3 meters

V.Vacek

V.Vacek

V.Vacek

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The high standstill pressure of the CO2 (~60 bar) is generally raising questions and concerns about the size and the reliability of pipes and connectors

One should note that high pressure piping with ~ 150 bars and beyond, are routinely handled in large chemical process plants with a satisfying level of reliability and safety

Of course this should not relax the effort of to design the pipe work in very detail, especially of the part inside the cold volume, which will not be anymore accessible after the sealing and the irradiation

Pressurized piping must be compliant with the code ASME B31.3 where the material grades, the minimum wall thickness and the allowable stresses are given as function of pipe diameter and the design gage pressure. For straight pipe under internal pressure the minimum wall thickness is given by:

)1((2 YPSEW

Pdt

Where d is the inner diameter, P the design pressure, S the allowable stress, Y, E and W are coefficients depending on the quality of the material. The following figures show the size of the pipes for the two cases of high pressure (60 bar) and low pressure (10 bar) :

High Pressure and piping

3 4 5 60

0.2

0.4

0.6

0

t di Sss( )

t di Scu( )

t di Sal( )

63 di 12 14 16 18 20

0

0.02

0.04

0.05

0

t di Sss( )

t di Scu( )

t di Sal( )

2012 di High Pressure 60 bar for Stainless steel, Copper and Aluminium Low Pressure 10 bar for Stainless steel, Copper and Aluminium

Copper

Steel

Al

Copper

Steel

AlMaterial Xo

(cm) Min wall thickness mm

(ASME B31) Radiation length/Xo

Stainless Steel 1.76 0.10 1.13 % Copper 1.35 0.19 2.81 % Aluminum Alloy 8.9 0.30 0.67 %

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We can adopt for CO2 the same distribution used for the C3F8.

The Inner detector is subdivide in four quadrant fed independently by a 30/34 mm liquid line and a 72/76 mm vapor line.

Distribution racks fan in to ~ 50 liquid lines 4/6 mm and fan out 18/20 mm vapor lines.

The racks contain also the pressure and the back pressure regulators valve which set, independently for each channel, the mass flow and the evaporation temperature

Pressure drops calculations

Phase # channels Mass flow per channel (g/s) Max Length (m) Pipe ID (mm) Pressure drop (mbar)

Compressor to Racks liquid 4 250 177 30 220Racks to Inner Detector liquid 200 5 25 4 534Internal piping Inner detetctor liquid/vapor 200 5 ~ ~Inner Detector to racks vapor 200 5 25 18 8Racks to Compressor vapor 4 250 177 72 91

pressure drops

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Conclusions

Plan to develop at SLAC an US ATLAS CO2 cooling facility

Blown CO2 system in construction -> expected to run end of September

Larger plant in design phase, main components have been sized

Preliminary calculations shows that for the final ATALSup ID, adopting CO2 in a vapor-compression cycle similar to the present plant running with C3F8, is feasible and offers many advantages due to the better physical properties of CO2.

It provide also enough margin to eliminate the weak points shown so far by the present C3F8 system

It fit well in super high irradiated environment like SLHC

It minimize the unavoidable take of risk stemming from the adoption of too many technological unknowns.