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Friday, January 20, 2012

CryoBUG charge tuning & other things

Well I wont go into all the details, because a lot has transpired since my last post. But to summarize the events, here is a list of the tests conducted over the last several days...

January 17th -- Same charge as used on previous day, but boosted the Argon amount.

Chart of CryoBUG 1/17/2012 Test Results

This charge got us colder as expected, but also aggravated the pressure and temperature fluctuations as seen previously. If you look at the Evap-In ("Blue" Line) you will see that it is very jagged. This indicates that rapid temperature fluctuations are occurring ( for TC locations, reference: V1 Diagram).

January 18th -- Reduced the R-23 and R-14 down to 30 Grams each.
Resulted in slower cool down and generally poor results. Evaporator temperatures were barely able to make it to -120°C.

January 19th -- Just for grins back to a no R-23 charge: 130 Grams R-134a, 80 Grams R-14, 8 Grams Argon.
Stalled out around -117°C with pressure instabilities early on. Tried adding 30 more Grams of R-134a, which then caused wild fluctuations in the suction pressure of 10 to 30 psig. Began wondering if there was liquid hang-up in the Temprite 340 being used as the phase separator.

January 20th -- Tilted liquid outlet side of Temprite 340 downward, and also pinched off one of the parallel #1 cap tubes (1/2 the flow). Same charge as previous day, but with 30 Grams more R-134a.


Suction pressure was very low at 3-5 psig, and within 30 minutes, pressure fluctuations began. After running for an hour, the evaporator leveled out at -117°C. I then added 40 Grams more R-134a which almost immediately resulted in a mid stage freeze-up of what I strongly suspect was R-134a that had overflowed the Temprite, and then gotten into an area colder than -92.5°C (freezing point of R-134a). The freeze-up was quite evident, since the suction pressure dropped into a fairly deep vacuum (15" hg), and the discharge pressure also dropped substantially (lost about 70 psi). This effect didn't last long (maybe 20-30 seconds) before a noise was heard emanating from within the stack like something suddenly flushing through, followed by the suction pressure drastically rising well into the positive pressure region.

Summary
These series of tests were conducted to get a better understanding of what is actually needed in this new AutoC design, and what are the main problems associated with it. The first test was just a slight tweak in Argon level to the charge as used in the 1/16/2012 test run. This was an attempt to push the evaporator a bit closer to -150°C, without the instability as seen before. On the second test, I was trying to determine how little of the R-23 and R-14 would be required to keep the system working. And then later I tried going back to my earlier R-134a/R-14 charge, but this time with Argon added (trying to eliminate refrigerants). I tried increasing the amount of R-134a to see if it would fill the gap of no longer having any R-23 in the charge. I also tried reorientating the Temprite 340 which is being used as my phase separator, in order to better allow liquid to flow out of it. And along with the tilting of the Temprite 340, I simultaneously reduced the cap tube flow for CT #1 thinking that the reduction in liquid hold-up for the phase separator would require less flow in the cap tube because of the better liquid seal.

So what I found out: R-23 is essential for proper operation. Trying to bridge the gap between R-134a and R-14 without R-23 creates inefficiency in the over all operation of the unit. And adding extra R-134a to try to make up for this, tends to exceed the flow rate of the two paralleled cap tubes that feed out of the phase separator, thus causing overflow and poor separation (this can be seen as wild pressure fluctuations). Theoretically it would seem reasonable to increase the flow of the first cap tube even more, but I think this would merely result in a higher then needed suction pressure, with poorer low temperature performance. This would be especially true if the compressor mass flow was not to be increased as well.

In several of my earlier tests, I wouldn't see the pressure instabilities occur until I was down in the -140°C and colder territory. This leads me to believe that what was actually happening, is that more R-23 was dissolving into the R-134a and causing the total liquid amount in the separator to increase. This increase was just enough to start slopping some liquid over the top, and then went on to create a 100% liquid seal on the final cap tube, as seen by the sudden increase in suction pressure. However this increased suction pressure also caused temperatures to warm up, which then resulted in less R-23 dissolving into the R-134a, and less liquid to cause an overflow. So the pressures and temperatures would begin to swing back to normal once again, only to repeat the process a little bit later.

Based on what I have seen, I think my next test will be to reduce the R-134a and use a larger proportion of R-23. Perhaps in this way I can better control the liquid level in the phase separator, while still obtaining good temperatures in the cascade condenser. Since I am back to a single cap tube for CT #1, I think I'll leave it that way and see how it works out with this new charge strategy.

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