Safety aspects of CO₂ cooling

Humen in the lab safety

What are the safe CO₂ levels for humen ?

Suppose TRACI (C0₂ cooling machine) breaks/explodes and about 300g of CO₂ gets into the working room ? Take a small room: 3x2x4m = 24 m³. Air in such room weights about 1.3 kg/m³ x 24 m³ = 31kg. 0.3g of C0₂ makes 0.3kg/31kg = 9700 ppm which is:

• 2 times above maximum allowed concentration within a 8 hour working period: 5000 ppm
• 3 times below slightly intoxicating, breathing and pulse rate increase, nausea: 30,000 ppm
• 10 times below unconscious, further exposure death: 100.000 ppm

RF foil and primary vacuum safety

If a microchannel cooled module develops a leak: how long does it take for the RF foil to break ?

RF foil is rated at 5 mBar and has a volume of about 1.0x0.25x0.25 m = 0.125 m³

Gaseous CO₂ weights about 1.8kg/m³ at room temperature thus to make 5 mBar in 0.125 m³ you need about 1.1 g of CO₂.

Liquid CO₂ properties

Pressure drop calculator

Assumptions:

• flow is at 30 bar limited by a 1 meter pipe with 1.4 mm inner diameter
• CO₂ is liquid at -20 degrees with density of 1032 kg/m³ and kinematic viscosity of 0.115 mm²/s

Results:

• flow is 88.1 kg/hour thus 24.5 g/sec - note, that flow is turbulent, thus this result is just an estimate

Now the RF box begins to fill up at this rate and CO₂ becomes gas at room or higher temperature.

It appears that the above leak will make 25 mBar in the first second thus breaking the RF box.

Safety system challenges

• detect CO₂ leaks within 10-20 ms
• shut the broken cooling pipe/module or the whole distribution
• if whole distribution shut: quickly (within 10-20 ms) evacuate CO₂ from all pipes behind the shutdown valve

-- PawelJalocha - 10 Jan 2014