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Wednesday, April 4, 2012

CryoBUG at -155°C and dropping

Yesterday afternoon CryoBUG made it down to -155°C in 1 hour and 30 minutes after start-up.


Over the next 1/2 hour it continued to drop, getting down to -159°C before showing signs of something freezing out (not sure if it was the oil or the butane). Yeah you heard that right, I've switched over to hydrocarbons in this charge (butane and ethane). I also went back to a single-stage AutoC, because quite frankly 2-stages without an auxiliary condenser just doesn't work all that well.

The reason for all these changes, is that I just wasn't having much success at getting the "BUG" to work with HFCs only. HFCs in the area of interest, all have relatively warm freezing points. With the BUG's simple design and very few separation points, it just kept freezing up around -95 to -105°C, or showing general signs of instability evidenced by erratic pressure changes.

So in this latest iteration, the refrigerant charge consists of:

R-600 (N-Butane) Boiling Point  = -1°C, Freezing Point = -138°C
R-170 (Ethane) Boiling Point  = -88°C, Freezing Point = -183°C
R-14 (Tetrafluoromethane)  Boiling Point  = -129°C, Freezing Point = -184°C
Argon Boiling Point  = -186°C, Freezing Point = -189°C

Total amount = 90 grams (3.17 ounces), static balance pressure = 235 psig

As can be seen, the butane can still pose a freezing problem if it gets near the evaporator when going below -138°C. So this might be the cause of the freeze-out problem I saw occurring towards the end of my test yesterday. Or it could be the oil. Or it could be overcharged, with the warmer boiling refrigerants flooding out the Phase Separator and spilling over into the next stage. Not really sure at this point.

The latest design has gone back to Andrija Fuderer's single-stage AutoC (patent #3203194), which has shown excellent promise, assuming that the freeze-out problems can be solved. In this current revision (V5), I have reduced the tubing size of the heat exchangers, and gone with a very simple phase separator design. I also decided to keep all the cap tubes external to the HX, unlike what I did on previous versions where the #1 cap tube was slid inside the suction side of the Cascade Condenser. Keeping the cap tubes on the outside allowed me to down size the HX, while still having minimal pressure drop on the suction circuit.



Yesterday's test was without any kind of oil management system in place, so in other words, the compressor's discharge line goes directly to the air cooled condenser. The cascade condenser and the subcooler are actually formed as one tube-in-tube heat exchanger, with cap tube #1 entering through a small hole drilled into the outer (suction) tube about mid way.

And now for the moment you have all been patiently waiting for. I give you the test chart from yesterday's test run.

CryoBUG 4/3/2012 Test Chart (Hydrocarbon Charge)

Pull down was very fast.

28 minutes to -100°C
49 minutes to -130°C
57 minutes to -140°C
72 minutes to -150°C
90 minutes to -155°C

Running pressures were 24/176 psig when the evaporator hit -155°C, and compressor current draw was very reasonable at 2.4 amps.

Highest peak discharge pressure was 332 psig, and peak current draw equaled 3.4 amps (this was within 15 minutes of start-up). One thing to keep in mind is that this is all being done with a single 4.8cc/rev compressor rated for 4810 BTU/Hr.

I think for my next test I'm going to substitute R-290 (propane) for the R-600 (butane), since the propane has a colder freezing point of -186°C. This will let me determine if the oil is the cause of the freeze-out problem, by ruling out the refrigerants.

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