aonomus
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Vacuum trap refrigeration system
Not that I've actually built one, but in my scouring of the internet to find out information about refrigeration systems for building a chiller I have
found a nice document describing an engineers conversion of a window AC unit to a working cold trap for a vacuum pump.
http://www.mytekcontrols.com/resources/documents/misc/Alcoho...
Hope everyone enjoys the read, even if replicating it is out of reach for most.
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watson.fawkes
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Quote: Originally posted by aonomus  | | Not that I've actually built one, but in my scouring of the internet to find out information about refrigeration systems for building a chiller I have
found a nice document describing an engineers conversion of a window AC unit to a working cold trap for a vacuum pump. [...] Hope everyone enjoys the
read, even if replicating it is out of reach for most. |
It wasn't built by engineers, from what I can tell,
but by skilled technicians. That's not an insult; it's a perfectly good project. The actual modifications are hardly out of reach. The bulk of the
project is plumbing, albeit more than house plumbing, as refrigeration systems make extensive use of brazing. Brazing is not difficult, but it does
take practice to make it work. The materials of the project are all off-the-shelf refrigeration components, and not fancy ones either.
The only thing that most readers might be unfamiliar with is the autocascade. A cascade chiller is just a sequence of refrigerator loops, each cooling
the next. The refrigerant in each loop gets colder with each step; the refrigerant is chosen appropriately. An autocascade is the same design, but
with a single compressor. There are still multiple expansion orifices (for Joule-Thomson expansion cooling) and so multiple sources of cooling for
each stage. There's a fair bit of information out there on the internet, largely because the overclocking crowd has been homebrewing autocascades for
a while.
The only real issue I see with the system in operation is that the vacuum conductance of all those skinny pipes is going to be horribly low. In
practice, I'd not rely on vacuum conductance, therefore, and use a low-volume N2 purge so that you're not in the ballistic flow regime and yet don't
overtax the vacuum pump. I'm sure that they can get most of the alcohol out by itself, but they'd be able to get all of it out with a purge.
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smuv
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This is a project I have wanted to do for a while; nice to read a detailed report.
I agree with Watson about it being well within reach of anyone mechanically inclined. One thing though, you can soft solder refrigeration joints,
brazing is not a requirement. The key is using a non-lead containing alloy, and making sure you don't contaminate the refrigeration system with flux.
Silver tin alloys are fine for refrigeration work. Brazing will definitely give a stronger joint, but we are talking ~200psi, which is fine for soft
soldering.
Edit: I realize my post is biased towards the U.S. In countries where r410A is more common, things change; these systems run at a lot higher
pressures. Brazing might be more of a necessity here.
[Edited on 2-18-2011 by smuv]
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aonomus
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Well, one of my backburner projects is to convert a 'broken' (seized compressor, replacement already in hand, just awaiting time to braze and replace)
aquarium chiller into a functional lab recirculating chiller with sufficient heat capacity to handle a fast distillation, or a cold bath (with an
autocascade or 2).
Right now I'm at an impass, trying to decide whether it is more practical to just use a large reservoir of coolant as a thermal ballast to avoid rapid
compressor cycling, or, run the compressor continuous duty and use a hot gas bypass valve to provide fine temperature regulation.
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