Exploring Cascade Cryogenics: From Methylene Chloride to Liquefied Air at 70K

[SupaVillain]

http://www.pbs.org/wgbh/nova/physics/reaching-ultra-low-temperatures.html

is this thing accurate? It uses methylene chloride as first refrigerant, then ethene, and then oxygen, and gets to liquefy air...down to 70kelvins

after researching the very small info on the net about cascade refrigeration systems, this is the only source that says one can get down to liquefying air. In the dewars process i believe the lowest temperature needed is 77kelvins or 192 celsius. I'd love to know if this is possible since cascade cooling seems the easiest and cheapest process for cryogenics.

 

[streety]

There is a lot of practical advice at http://www.xtremesystems.org/forums...ge-Cooling&s=8f2366fc1e1b11bf60ad81cff4364507

One thing to consider is that each stage not only cools the gas of the next stage, but also the heat produced by the compressor of that stage, and all subsequent stages. The consequence of this is that the thermal energy the final stage is able to extract is only a fraction that of the first stage.

You mention elsewhere you are interested in purifying Xenon. As Xenon is a very rare element you will need to cool down a very large volume of air to collect workable volumes of Xenon. To maintain efficiency significant effort will be needed in designing a heat exchanger such that your outflowing xenon-depleted air effectively pre-cools the inflowing air. There may be benefits to cooling the inflowing air at each stage of the system, although a single heat exchanger for the air path may be both efficient and easier to produce.

 

[SupaVillain]

Yes but is methylene chloride, ethene, and oxygen the makeup needed for reaching 70 kelvin? That is my question. I know of these other sites

 

[streety]

I'm no expert, but by my rough calculation I would say yes, this should work. You would need a compressor on the oxygen stage capable of drawing a vacuum of 0.05 atm/5 kPa/50 mbar. That should give you a lower limit of 68.8 K.

However, if building such a system myself this gas mix would not be my first choice. I don't think methylene chloride is widely used so I would go with a more common option for the first stage. The second stage seems reasonable, but the proximity of a flammable gas with oxygen in the third stage makes me somewhat uncomfortable. I would switch it out with a non-flammable option. Nitrogen should undergo a phase change at 70 K and 0.38 atm/38.5 kPa/380 mbar, so may actually perform better while also being safer.

 

[SupaVillain]

I've looked at a lot of stuff one those sites and they do use very different refrigerants but they also don't go as low temp as i want to go, i was assuming it was this certain gas selection that would make temperatures like 70 kelvin possible

 

[streety]

I don't think there is anything unique about this combination of gases. While not all combinations are going to work, there is more than just one.

What you're aiming for at each stage is a phase change between the high pressure warm side and the low pressure cold side. As the pressure drops, so to will the temperature at which the phase change occurs. In practice, you don't want the pressure to be too low or the system will be horrendously inefficient. For the third stage the list of gases that undergo a phase change at reasonable pressure and the temperature you want will be relatively short. For the first stage, almost anything will work.