Wednesday, February 26, 2014

CryoBUG Life Testing Stopped

Well as predicted, on the second round of life testing the gradual accumulation of oil getting up to the final cap tube, caused enough blockage to reduce the flow of refrigerant feeding the Cold Head. The end result: gradual reduction in temperature under moderate load conditions over the first 2-3 weeks of 24/7 operation, followed by a rapid warm-up of at least 20°C when flow could no longer keep up with load conditions.

Vapor-Liquid Phase Separator
The oil blockage occurs when it gets into an area having cold enough temperatures to freeze it. The usual method of trying to avoid this is to use a coalescing oil separator, something that would be difficult to do in CryoBUG, and even more so in the upcoming CryoSPRITE due to size constraints. So what I'll be doing instead, is to try to improve my refrigerant phase separation design, of which I only have one in the system.

The image to the right shows a fairly sophisticated vapor-liquid phase separator (courtesy of Wikipedia). I will be trying to implement some of the techniques shown by this example into a small enough package to work in the CryoSPRITE, and eventually into CryoBUG. Since I have very little space available, I will need to be creative with my approach, and will likely try to combine some of what is independently shown in the diagram into a commonly shared aspect.

It is a pity that I have to do this even with an all hydrocarbon charge, something that I thought would insure good oil return back to the compressor before it had a chance to freeze. But this is the way things often are when doing product development. It is also most likely a result of my one phase separator autocascade design, which is really pushing the envelope at -150°C.

Most autocascades designed for ultra-low temperature operation use up to 3 refrigerant phase separators, as well as 3 cascade HX's. Having the extra phase separation normally gives you ample opportunity to separate out the oil along with the condensed refrigerants, well before it gets into a cold enough stage to freeze.

But I have no intention of giving up so easily on my simple one phase separator design. So we'll see how things go when I get into testing CryoSPRITE, which should be fairly soon, since I now have all the parts required for assembly.

Sunday, February 23, 2014

CryoBUG requires regeneration after 21 days

Well it appears that a very small amount of oil gets past CryoBUG's phase separator, and over time accumulates in the final cap tube feeding the Cold Head, ultimately freezing and blocking passage of refrigerant. The effect is gradual, taking nearly 3 weeks to cause a cooling problem.

Presently I have been running a 2nd life test going onto 18 days now, with an adjusted refrigerant charge. Thus far things are okay, but signs have already begun to appear that are similar to what was seen in the first test run. These signs signal a possible problem to come. I'll know for sure in about another week of 24/7 operation.

The problem isn't catastrophic, and the unit will recover by allowing it to warm up overnight and then restarting. However it would be better if a solution can be developed. And just such a solution is being implemented in the new CryoSPRITE prototype, which if successful can be incorporated into CryoBUG.

So life testing will continue.

Stay tuned for more news to come.