Sciencemadness Discussion Board

Building a cold trap

TheIdeanator - 19-10-2018 at 20:12

Recently I acquired a working 10,000btu/hr (2.9kW) window ac unit to make a cold trap out of. The only instance I've seen of this is here. Has anyone else done this sort of project?

Ubya - 19-10-2018 at 23:59

https://youtu.be/mzeRvdnIP-I

if you were able to put two smaller units in series (and change the refrigerant accordingly) you could get much lower temperatures, equivalent of dry ice and ethanol. to get -70 or -80 celsius with just one stage you would need a really big pressure difference in the system, and so a big and watts sucking compressor

XeonTheMGPony - 20-10-2018 at 05:33

Here ya go, cold traps are harder then ya think to get a good design going but worth it.

Attachment: Alcohol_Cold_Trap.pdf (684kB)
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XeonTheMGPony - 20-10-2018 at 05:43

Quote: Originally posted by Ubya  
https://youtu.be/mzeRvdnIP-I

if you were able to put two smaller units in series (and change the refrigerant accordingly) you could get much lower temperatures, equivalent of dry ice and ethanol. to get -70 or -80 celsius with just one stage you would need a really big pressure difference in the system, and so a big and watts sucking compressor


and a short lived inefficient unit while at it! A simple 3 stage auto cascade will do this much better, or even a 2 stage cascade system with much improved energy utilization.

The gasses are the most expensive part, first stage propane and N-Butane can be used (The N-Butane @ 0.5c boiling point, is used for compressor cooling at the small cost of a tiny bit of capacity)

Second stage could be R-507 or R-410a for a happy -50c range for the easier to source gasses.

Ubya - 20-10-2018 at 06:14

Quote: Originally posted by XeonTheMGPony  
Here ya go, cold traps are harder then ya think to get a good design going but worth it.


mhh well this is interesting, i did not think of this design, much appreciated thank you

RogueRose - 20-10-2018 at 09:12

If you want a really good lab grade cold trap (like the one in the first video link) check out surplus auctions for Universities and colleges. I see these quite often and they usually don't sell for a whole lot, maybe $50-100 at times (and obviously more for some newer larger models). They often sell as "untested" but they are usually pulled from working labs as I've never heard anyone buying one that hasn't functioned - unless it was sold as such.

Maybe even look for a broken one and repair it?

TheIdeanator - 20-10-2018 at 15:22

Quote: Originally posted by XeonTheMGPony  
Here ya go, cold traps are harder then ya think to get a good design going but worth it.


I wish there were more pictures there, I can't tell what's really going on. It seems considerably more complicated than I'm willing to troubleshoot with the cascading heat exchangers and mixed refrigerant. I was just going to re-route the plumbing and hope for the best.

Ubya - 20-10-2018 at 15:55

Quote: Originally posted by TheIdeanator  
Quote: Originally posted by XeonTheMGPony  
Here ya go, cold traps are harder then ya think to get a good design going but worth it.


I wish there were more pictures there, I can't tell what's really going on. It seems considerably more complicated than I'm willing to troubleshoot with the cascading heat exchangers and mixed refrigerant. I was just going to re-route the plumbing and hope for the best.


an air conditioner does not produce sub zero temperatures on the evaporator, otherwise it would frost during normal operation. re-routing the plumbing would be ok if you wanted to make a water cooler for distillations,
but to freeze water or organic solvents in a vacuum you need lower temperatures, that's why dry ice and acetone or liquid nitrogen are used for this porpuse

TheIdeanator - 20-10-2018 at 21:26

Quote: Originally posted by Ubya  
Quote: Originally posted by TheIdeanator  
Quote: Originally posted by XeonTheMGPony  
Here ya go, cold traps are harder then ya think to get a good design going but worth it.


I wish there were more pictures there, I can't tell what's really going on. It seems considerably more complicated than I'm willing to troubleshoot with the cascading heat exchangers and mixed refrigerant. I was just going to re-route the plumbing and hope for the best.


an air conditioner does not produce sub zero temperatures on the evaporator, otherwise it would frost during normal operation. re-routing the plumbing would be ok if you wanted to make a water cooler for distillations,
but to freeze water or organic solvents in a vacuum you need lower temperatures, that's why dry ice and acetone or liquid nitrogen are used for this porpuse


They can, actually,
https://advanced-air.com/uploads/Coil_Frost_656_DJFss.jpg
but when sized and charged appropriately they shouldn't because it hinders their effectiveness as an AC.

The system in the video I linked was icing over and he shows the thermocouple on the cooler itself running at -38 °C which is ~4 °C above the bp of propane (the refrigerant he's using).

[Edited on 21-10-2018 by TheIdeanator]