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.


Sunday, February 5, 2012

CryoBUG 2-stage results mixed

Click on Image to Enlarge
Well I tried converting the unit to a 2-stage AutoC design as shown in my recent V2 piping diagram, but I can't say I was really very impressed by the test results. First off, not having an auxiliary condenser preceding the 1st phase separator, makes Cascade Condenser #1's output tend to follow the evaporation temperature of the first refrigerant only (R-134a). So in other words, there doesn't seem to be much if any of the other refrigerants mixing with the R-134a, and this yields a rather warm 2nd phase separator as a result. And without any R-134a in this 2nd separation point, plus the fact that the temperature is too warm to condense the R-23, the system kinda stalls out.

So I changed out the R-134a for R-507 in the hopes that with a lower boiling point, the 2nd phase sep would come down into a more favorable range for condensing the R-23. End result was some improvement, but not nearly enough to really get the system running optimally.

Now the logical conclusion that I came to, would be to add another heat exchanger (HX) preceding the 1st phase separator, thus giving the unit an auxiliary condenser. This approach would most likely have a very high probability of success. However I'm trying to keep this unit as simple, and as small as possible. Adding yet another HX just doesn't fit into this plan.

So I think I'll be going back to the Version 1 layout, but based on some things I saw in my V2 tests, I think I'll be upping the flow for both the phase separator and evaporator cap tubes.

New cap tubing is on order, and hopefully we'll be seeing a retest in about week. I will also be doing a complete oil change to low temperature PVE, and might play around a little bit more with using R-507 as my first refrigerant instead of the R-134a (or perhaps in combination with it).