Sunday, December 18, 2011

CryoBUG 2nd Test Run

After thinking about the previous day's test run, I decided to not only try a new ratio for the R-134a and R-14, but while running, I also added in some R-23 and Argon. Since I was doing a little bit of adding gases on the fly (not always weighing it in), I'm guessing that the final charge consisted of:

130 Grams R-134a
50 Grams  R-14
40 Grams R-23
5-10 Grams Argon (the weight on this is probably pretty bogus -- Argon's too light for my scale)

As I tweaked back and forth with the various gases, the temperatures just kept getting better (see pressures and temps below).

Green = Evap-Out, Blue = Evap-In

Compressor Running Pressures (17 psig Suction and 235 psig Discharge)

Initially I had started out with only R-134a and R-14, with the R-14 at 40 Grams instead of the 100 Grams I had utilized in the first test. To make up the balance, and to see what would happen if I filled the nearly 100 degree gap between the two refrigerants. I added R-23. This reduced the gap to approximately 50 degrees between each refrigerant's boiling point. The end result was not quite as cold as with more R-14, but pretty close and with lower running pressures. Next began the Argon additions.

Every time I would add Argon the suction would drop a bit, followed by the discharge rising. And if the suction started to wander back and forth (hunting), adding a bit of R-23 and/or R-14 would usually set it straight. And with these additions the temperature kept creeping downwards.

The only thing that began to bother me was the fact that the phase separator temperature was no where near as cold as what I had anticipated. Only making it down to +2°C, instead of -30 to -35°C that had been expected. And even though I was using an extra 30 Grams of R-134a, nothing had changed in this regard. This could only mean one thing, and that was that CT #1 wasn't flowing enough. So I think to remedy this, I'll need to add another cap tube in parallel running along the outside of the cascade condenser's tubing, and drill a small hole in the side for insertion near the termination point of the original. In order to maintain the sub cooling aspect, I'll need to spot braze this additional cap tube to the outside of the cascade's tubing to maintain heat exchange.

Other then this small setback, the unit is working remarkably well, and doing so without over working the compressor (current draw = 2.6 Amps, Compressor Discharge Temp = +55°C).

Still got a ways to go if I'm to meet my goal of -150°C, but I think the journey to do so will be quite fascinating and fun.


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