A.P.Colijn 1

CO2 Cooling for an ATLAS upgrade

• Plant requirements

• CO2: Properties

• CO2: Consequences of the properties

• CO2: From LHCb to ATLAS

(or the CO2 conspiracy)

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Requirements for ATLAS

1. Cool many distributed heat sources spread over large volumes

2. Low material in the detector3. Small temperature gradients over long distances4. Radiation hard5. Reliability

Will try to convince you that CO2 has it all!

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ΔH(-25C) = 280 kJ/kg

Enthalpy [kJ/kg]

Pre

ssur

e [b

ar]

P = 17 bar

liquid

2-phase gas

CO2 properties: p-H diagram

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C3F8 properties: p-H diagram

ΔH(-25C)=100 kJ/kg

P = 1.7 bar

liquid

2-phase

gas

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Run conditions @ -25CC3F8 CO2

Pevaporation 1.7 bar 17 bar

ΔT for ΔP=+-0.1bar +1.4 C / -1.5C +0.2 C / -0.2 C

ΔT for ΔP=+-1.0bar +12 C / ~-20 C +1.8 C / -1.9 C

ΔH for evaporation 100 J/g 280 J/g

Flow for 100 W 1.0 g/sec 0.4 g/sec

Volume flow 0.6 cm3/sec 0.4 cm3/sec

Major difference for CO2 with respect to C3F8 cooling is the increase by a factor of 10 of the evaporation pressure for T=-25C.

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High Pressure

CO2 plant must be able to withstand about 100bar (C3F8 in ATLAS 20bar)

+ low temperature gradients (dP/P small because P is high)+ low tube diameter (because we can allow large dP)+ low tube thickness (because of low tube diameter)+ high tube flexibility (reduce mechanical stress)

- need more pipe at your heat sinks - higher pressure = higher tube thickness

Some b.o.e. calculations to follow to get some feeling for the numbers….

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Back-of-the-envelope-calculation

• What design consequences would it have to replace C3F8 with CO2?

• Calculations assume CuNi pipes, just as for SCT

• Cool approximately 100W / cooling loop

• Disclaimer: – calculations need more refinement– calculations must be supported by measurements

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Pipe diameter: pressure drop

4

8

R

LFP m

ΔP = pressure drop

Fm = mass flow

η = viscosity

R = pipe radius

L = pipe lenght

Pressure drop for a 1g/sec CO2 flow is of the order of 0.08 bar. Even with a factor 10 more flow temperature differences of a couple of degrees can be expected!

Assume a pipe diameter of 0.9mm, which is 1/4th of the C3F8 cooling pipes we have now on the SCT endcap

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Pipe diameter: pipe thickness

max

2

P

STd

d = pipe diameter

T = pipe wall thickness

S = tensile strength

Pmax = max pressure

With the same pressure as in the C3F8 system we could reduce the pipe thickness by a factor four

A CO2 system has to withstand much higher pressure, so we have to multiply the pipe thickness again by a factor of six

T = 0.12 mm

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Material: CuNi pipes for CO2 and C3F8

)2(4

22/ 283

TdTdL

m CuNiCOFC

d=3.6mm

T = 0.07mmI: CuNi for CO2

at 100 bar

d=0.9mm

T = 0.12mm

II: CuNi for C3F8

at 20 bar

g/cm18.0

g/cm015.0

II

I

m

m

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SCT EndcapAdisk1

Low diameter piping is much more flexible: goes like R5. So no “funny” bends needed in your structure to absorb mechanical stress.

Pipe flexibility: stress

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Heat absorption

P = required cooling powerH = heat transfer coefficient in Wm-2K-1

(calculate/measure it… O(5000Wm-2K-1)d = pipe diameterΔT=change in temperature

To absorb 10W you would need 4 cm of large diameter CuNi pipe, versus 14 cm small diameter pipe if CO2 is used…….

Solutions R&D:- “snaky” cooling pipes at cooling contact- large contact area with module- or….

TdH

PL

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CO2: From LHCb to ATLAS

1. Carbon copy the LHCb plant– (by then) tested technology– “Cold” input lines– Relatively small return lines

2. Design single-stage freezer– “Warm” input lines

– Need oil-less CO2 compressor (!)

– Need relatively big return lines

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Enthalpy [kJ/kg]

Pre

ssur

e [b

ar]

liquid

2-phase gas

CO2 plant: compressor vs pump based

A

BC

DA

B C

D E

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NIKHEF CO2 effort

• Interest from NIKHEF ATLAS group– physicist (Fred Hartjes, Auke-Pieter Colijn, Els Koffeman)

– physicist/engineer (Bart Verlaat expertise…)

• LHCb cooling work finishes in near future

• We plan to setup CO2 test-bed at NIKHEF:– for NIKHEF detector R&D (Gossip)

– for ATLAS upgrade development: support back-of-envelope calculations of this talk with measurements

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