Removing Moisture from Sealed Chamber 1

[Zibraz]

I'm in a design concept stage at the moment. I have a Stirling Cooler which will be running at -120c. The head of the cooler will be enclosed with a box. At this temperature carbon dioxide and water will instantly freeze on the surface and form a large spherical ice ball. I'm looking at ways to prevent this ice build up.



At the moment my ideas are as follows.

[ul][li]Seal it from the otherside using o-rings and sealant tape, them pump the inside of the chamber with nitrogen or some other dry gas.[/li]

[li]Seal it from the outside and create a vacuum.[/li]

[li]Fill any cavities with a physical foam (Although at such low temperatures the foam may break down).[/li][/ul]

I did look at some omniphobic coating, but I doubt they would be robust enough to handle repelling moisture at such low levels. Can does anyone have any other ideas?

Thanks

 

[EdStainless]

You might get by with a vacuum that isn't very low (easier to seal) and adding desiccants/absorbents to the chamber.

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[Compositepro]

Many desiccants are available to do this. Some can be regenerated and reused,like calcium chloride or silica gel, but these may not trap the water tightly enough that it still may still freeze on your cooler. Calcium oxide is a very strong desiccant. Changing temperature in the box will cause air to expand and contract, so the box needs to be sealed against pressure leakage or the vent flow must be dried also. A continuous purge of dry air or nitrogen from high pressure cylinders would be a very fast and robust process. Keep in mind that wood and paper are generally about 10% water.

 

[zeusfaber]

Or simply seal it from the outside (include an expansion bladder to stop it breathing fresh air in every time the temperature changes) and then sit back to admire your handiwork. Once the moisture and CO[sub]2[/sub] in the trapped volume has come out as a light frost, the air in your enclosure will be as dry and CO[sub]2[/sub]-free as you could ever wish for.

It will only turn into a block of ice if you get the sealing wrong.

A.

 

[zeusfaber]

I'm not convinced CO[sub]2[/sub] would freeze onto your head at that temperature even if you left it exposed to the atmosphere. Provided your ambient CO[sub]2[/sub] concentration is within acceptable limits from an occupational health perspective (the EH40 8 Hr TWA is 5000 ppmV), then the frost point will be comfortably below your head temperature - roughly in the range -137 C (400 ppm) to -125 C (5000 ppm).

A.

 

[IRstuff]

I'm not convinced CO2 would freeze onto your head at that temperature even if you left it exposed to the atmosphere.

You'd be surprised, I think. We used to have an issue with Stirling cooler heat lift at 65K, because the little bit of N2 in the cold finger well would freeze out between the cooler cold finger and the thermal contact to the inside of the Dewar. We wound up using a copper paste to fill the gap and prevent gases from freezing out.

In any case, the cooling capacity of a Stirling cooler is usually set as low as possible, since it has a pretty large dissipation, both during cooldown and during operation if the heat load is high. Since the cold finger is where the expander is, it's unclear how you would enclose only the tip of the cold finger and not the rest. If you do enclose the rest, then there will be a continual large thermal load from any trapped gases inside the cryospace. We typically evacuate any cold space down to relatively hard vacuum to prevent convection. The interior of the space would typically be gold, or similar, coated to minimize radiated heating. A properly designed Dewar can achieve less than 150 mW of thermal loading from the ambient to the cold finger. The typical Dewar for infrared sensors is a concentric cylinder shaped Dewar with a well for the Stirling cooler cold finger whose tip would sit inside the well, making thermal contact with the bottom of the well, and the infrared sensor would sit on the opposite side of that interface, inside the vacuum space, see image below. You also want to minimize the volume of the vacuum space to minimize pump-down volume and maximize getter life.

FYI, the vacuum port is a tube coming out of the Dewar and crimped off after pump-down. That's an art, by itself, btw.

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[zeusfaber]

IR, at the sort of temperature you're talking about there, I'm not surprised you still had enough nitrogen in the space for some of it to drop out. At -120C, there's a bit more vapour pressure available.

Interestingly, given its typical dewpoint of -70 C, a dry nitrogen purge (one of the options being considered by the OP) would act as a continuous trace source of fresh condensate - potentially enhanced by whatever moisture found its way back in through the vent.

A.