Monday, January 16, 2012

CryoBUG charge tuning (Day 2)

Based on yesterday's test results, I made some more adjustments to the refrigerant charge as follows...

New Refrigerant Charge:
R-134a  135 Grams
R-23        40 Grams
R-14        40 Grams
Argon       8 Grams (+/- 1 Gram due to inaccuracy of the scale at these low readings)

B.P. = 205 psig

Initial start-up head pressure was much lower at 302 psig, and peaked about 15 minutes into the test run at 321 psig (suction = 12 psig). Also temperature pull-down was improved, hitting an evaporator temperature of -100°C within 24 minutes, and -130°C at just under an hour. If you look at the chart below, you'll also see a much smoother ramp-down of all the interstage temperatures (for TC locations, reference: V1 Diagram).

Chart of CryoBUG 1/16/2012 Test Results

Today's test run was for 3 hours, and resulted in a very stable evaporator temperature of -143°C, with pressures of 9/185 psig, and compressor current draw of 2.49 amps. Still seeing a very low gradient across the evaporator of about -2°C, which suggests a fairly low load for the system mass flow, or perhaps still some flooding caused by an excess of R-14. I'm tending to lean more towards a low load as the ultimate cause.

What I'm really liking, is the much more stable pressures and temperatures, even when leveling out at the end of the test run. On my previous -150°C success, I was seeing some pretty erratic pressure swings, with certain interstage temperatures following suit. This didn't bode well for the long term stability of the unit.

Tomorrow I think I'll try adding a bit more Argon to this charge to see what will happen. Hoping to get a bit closer to -150°C, but without the instability issues.


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