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Tuesday, February 28, 2012

CryoBUG moving back to 2-Stage design

Well after giving it quite a bit of thought, I've decided to pursue the 2-Stage design for a while more and see where it takes me. I have a couple of reasons for this decision, with the major one being consideration for either the oil or the warmer boiling refrigerant eventually freezing and causing plugging in the colder stages.

By having an additional phase separator, I feel very certain that none of the warmer freezing components will ever make it into the colder regions. This also allows me to go with a 5th refrigerant in my mixture which I'll call HFC-X for now. It has characteristics very similar to either HCFC-123 or HFE-7000. However being an HFC, the new alternative refrigerant is ozone friendly, unlike HCFC-123. It's also lower then most other HFCs when it comes to Global Warming Potential, having a GWP of 650.
 
Being an azeotropic ternary blend, the HFC-X has a component as part of the blend that gives it excellent solubility with hydrocarbon based lubricants. And last but not least, it is also considered to be a nonflammable substance.

High Boiler Comparison Chart

In the chart above, I have listed the various refrigerants that fall into what I call the "High Boilers", basically refrigerants having a positive boiling point temperature. The first three CFC-11, CFC-113, and HCFC-123 have either been phased out or are soon to be. The last two are both suitable for use in a modern day autocascade refrigeration system as the first component of the mixture. HFE-7000 however has very poor solubility with any of the lubricants normally used for refrigeration compressors (Mineral Oil, AB, POE, or PVE). It is also very expensive, especially when purchased in smaller quantities such as 10lbs at $250 on up. But it does have a relatively low freezing point, which is important, especially when the first phase separator is proceeded by an auxiliary condenser (not present in CryoBUG's 2-Stage design).

The HFC-X azeotrope is relatively cheap, readily available, and is very miscible with all the commonly used oils. It's only draw back is the warmer freezing point, which makes it imperative that it be separated from the other refrigerants well before getting down to this temperature. Also the point of evaporation should be warmer than the freezing point of this refrigerant. Thus having an auxiliary condenser prior to the first phase separator might cause problems where the phase separator's condensate is evaporated downstream (typically around -60°C).

So why use this additional refrigerant?
By using a warmer boiling component such as HCFC-123, which was a common refrigerant in Polycold charges prior to 2010, we can achieve a lower vapor pressure then if something like HCFC-22 or HFC-125 were used alone. In a 60/40 ratio, with the warmer component being the larger proportion, the boiling point will still be in the -40's, but the vapor pressure will be a mere fraction of what it would have been if an appropriate amount of HCFC-22 were used without the High Boiler being present. A secondary effect is seen as better heat transfer in the compressor and air cooled condenser where the temperatures are elevated.

So I am hoping that I'll have some preliminary test data using this new refrigerant in the next couple of weeks. Wish me luck.

 
 
 
 

10 comments :

Adam said...

HFC32 again eh? ;)

Michael St. Pierre said...

Hi Adam,
No not HFC32. It's a blend of 3 components which form a near azeotrope. It took me quite a while to stumble upon it, since it was not listed as a refrigerant.

NoL said...

My bad, thought I had it matching GWP but that was a silly thought. Hope it works, I'm looking forward to a week in my workshop in early april and planning on making a bunch of video guides to revive the vapor phase guys on XS.
Looking forward to more updates on your cryobug.

Michael St. Pierre said...

Yeah as you well know, to really match it up you'd have to factor in the boiling point and the freezing point (or what is sometimes referred to as melting point). I would help you out, but I kinda want to keep this one secret for now, because it'll be the key to creating an HFC only charge that gets outside of some existing patents relating to AutoC's.

Currently I am doing a small tear down on the "BUG", but it should be back up and running in another week.

Good luck on getting the XS Phase Change forum hopping again. Other than you, Gray, and a small handful of others, it sure has been quite.

NoL said...

No worries, I'm just the type to poke sticks at secrets. Won't try and reveal it if I do come up with it after now.

I've been trying to find more of the "new rules" for hydrocarbons now that R22 and such is gone and the new rules are in effect. I find very little against the use of them. Hopefully I'll make some headway in that direction.

Cryobug looks awesome as ever, although I was sad to see you rip apart a -150C unit such as you had.

One of these days you'll have to let me know if your up for a "new project" and I'll drop a flex line, evaporator with mount, and "CPU load block" in the mail.

All the best.

Michael St. Pierre said...

Even though I was successful at reaching -150 C with a single stage AutoC, I just didn't trust that it would have continued to do so over the long term. Freeze out issues with some of the refrigerants and certainly the oil were problematic, and imminent. Adding at least one more stage of phase separation will be good insurance against these kind of problems.

Although this may well predicate the need for an extra HX prior to the additional phase sep, as well as a better spread of refrigerant boiling points to really tie it all together.

NoL said...

I look forward to it.
Have you ever worked with R236?

Michael St. Pierre said...

Yes R236 is one of the prime ingredients in the Polycold "Green Charge" and is also covered in their patent, along with HFE-7000 as one of the possible alternates. It works very well in combination with R125 as the warmer boiling components within an AutoC's mixed refrigerant charge.

alternative refrigerants said...

why wont every one pay attention to rf142?

Michael St. Pierre said...

Hello "alternative refrigerants". I think you meant to say R-142b unless I'm mistaken. Since this is an HCFC it can no longer be used in Europe, and will soon be the same in the US. Whereas R-125 being an HFC is still legal as part of a mixed refrigerant charge, or even by itself.

And R-142b has a freezing point of -130C, which makes it non-suitable for my single phase separated autocascade design. This is also the reason why I'll be going from using R-600 to R-600a (Iso-Butane) which has a freezing point of -160C.

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