Monday, January 23, 2012

CryoBUG completed tuning for V1

Version 1 of CryoBUG has now officially completed it's tuning phase experiments.

In this go around, I accomplished what I consider to be a very stable "sub -140°C" unit charge, which is one where the pressures and temperatures are stable, without making wild excursions as seen in some of the previous tests. Although -150°C is the goal for this unit, the reality of achieving this with my very simple hardware is probably not realistic. At least not in what I consider to be a stable and reliable production ready version. So with that in mind, here is the charge that seems to work well for the -140°C regime.

110 Grams R-134a
60 Grams R-23
40 Grams R-14
8 Grams Argon

BP = 235 psig

Here is the 2nd day's confirmation test run with this charge (for TC locations, reference: V1 Diagram)...

Chart of CryoBUG 1/23/2012 Test Results

Within 1 hour the evaporator was down to -131°C. And when it finally stabilized after running four and half hours,  it achieved an ultimate temperature of -143.5°C, and running pressures of 5/205 psig. Temperatures only varied by +/- 0.5° worst case, and the suction pressure variance was only +/- 0.5 psi.

The unusually low suction pressure is the result of only running with the original #1 cap tube coming off of the phase separator (I have the extra parallel cap tube that was added later capped off for this test). My gut feeling tells me the system would benefit from more flow, but not so much if the added flow is to cap tube #1 like it used to be. I think it's time to add some active feed going directly to the top of what is now the Auxiliary Condenser, thus making it into a Cascade Condenser (see proposed Version 2 system diagram below). My thoughts are that this will allow for better separation and utilization of the R-23.

So in essence what I'll be doing, is to add another Phase Separator before the original Auxiliary Condenser, and another cap tube feeding from it to the end of this HX (this HX becomes Cascade Condenser #1). I believe this will be a far more effective way to add additional cap tube flow to the system, while enhancing separation of the R-23 from the R-14, and insuring good oil return back to the compressor. This should also yield a system that is better suited for -150°C temperatures, although still retaining a simple design.

Hopefully I'll get a chance to test out this theory over the next couple of days or weeks.

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.

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.

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.

Sunday, January 15, 2012

CryoBUG charge tuning

For the time being I'm pretty happy with CryoBUG's hardware configuration, but I know there is more tweaking left to do with the refrigerant charge.

So with that in mind, today I ran a test with reduced R-23 and increased R-14 to get a better feel for what is really needed.

New Refrigerant Charge:
R-134a  125 Grams
R-23        40 Grams
R-14        55 Grams
Argon       8 Grams (+/- 1 Gram due to inaccuracy of the scale at these low readings)

B.P. = 250 psig

Here's a chart showing all the recorded temperature points during this test run.

Chart of CryoBUG 1/15/2012 Test Results

Notice the warm-up about 10 minutes into the run of what I call CC SUCT (Cascade Condenser Suction), and then it's turn-around about 20 minutes later. It has also been rather surprising how warm my Phase Separator operates with this new design, something that was quite unexpected, but seems to be a characteristic of this system. And although the BUG was still ramping down and getting colder, I decided to shut it down at 2 hours into the run. BTW for a better explanation of the plotted temperature points please refer to the following diagram.

CryoBUG Thermocouple Location & Identification Diagram

At the 2 hour mark I was seeing compressor pressures of 14/207 psig, and a compressor current draw of 2.58 amps. Evap-In was at -140°C and the Evap-Out was just a bit warmer at -138°C (apparently a flooded evaporator). Most likely these temps would have come down another 4-5 degrees in the next hour, but I'd seen all I needed to at this point and made the decision to terminate the test run.

So what are my feelings about the results of this enriched R-14 charge? Well as I kinda suspected, but wanted to confirm, additional R-14 is not beneficial to my goal of creating a stable -150°C unit. In fact it was most likely hampering me from seeing a lower evaporator temperature then what I did today. However the lesser amount of R-23 in my opinion did help out, and this I believe was reflected in much more stable pressures and temperatures this go around.

I think my next test will be to reduce the R-14, while leaving all the other refrigerants in the same proportions.