Friday, April 27, 2012

CryoBUG Controller

Well I've been busy over this last week or so. Besides trying to track down a source of cheap R-600a (Isobutane), I have also been designing an electronic controller for the more finalized version of the CryoBUG.

Here is a list of some of the things I wanted to incorporate into this controller.
  1. Toggle compressor power ON and OFF by pressing momentary push button switch.
  2. Have a bi-color led back light in the switch for "ON" as well as "FAULT" indication.
  3. When experiencing a fault such as an Over Pressure condition, to shut-down the compressor and require user intervention in order to reactivate it.
  4. Have the fault indication retained even after the condition that caused the fault has cleared (fault memory).
  5. Provide redundancy in case the logic circuits fail, so that a fault can still shut-down the compressor.
  6. Provide a remote means of both monitoring and control of the CryoBUG functions.
  7. Utilize a short circuit proof control transformer and a sealed compressor relay for hydrocarbon refrigerant safety.
  8. Maintain low EMF and RF emissions by the use of low speed CMOS switching circuits and linear voltage regulation.
And here is the circuit design that incorporates all of these requirements...

Click on Image to Enlarge

And here is a small video showing the bread boarded prototype in action...

Just a little more background on the remote aspect. This design incorporates a fully isolated interface for remote monitoring and control of all the CryoBUG functions. And in the anticipated end user application, there will be several of these units providing water vapor cryopumping for small vacuum coating chambers on an automated process cycle. It is desirable to have all machinery, pumps, ect. controlled remotely by the central computer control system of this vacuum coating system.

Here are the functions provided by the CryoBUG remote interface:
  1. Control: Compressor ON/OFF (universal 6v-24v AC/DC optically isolated input)
  2. Status: Compressor Running (contact closure)
  3. Status: At set point temperature "Ready to Coat" (contact closure)
  4. Status: Unit Fault (contact closure)
(Edit 4/29/12 Rev 1.1: the remote control pin-out has changed as well as some functionality)
If it is desirable to disable the manual On/Off switch via the remote, then holding the remote in an active high state will do so. With this inverted logic, it would be necessary to momentarily remove the remote On/Off signal (bring it Low) in order to toggle power remotely, and then immediately restore the signal (bring it High). However If simultaneous manual and remote operation is desired, then use normal logic for remote power control (momentary High to toggle power).

As for temperature monitoring and set point, I decided that it would be best to utilize an independent process controller such as the Solo SL4824-RR 1/32 DIN from Automation Direct.

These PID controllers are getting to be very inexpensive for all that they are capable of doing. It just wouldn't be practical for me to try and duplicate this functionality into my CryoBUG controller. Although there are some cheap temperature displays available from China, but most of them are not able to measure much below -40°C.

So in my design I routed the relay contact of the process controller through my board, and make it available as a status contact on my remote connector. The user simply enters a set point on the process controller where they wish to be alerted when the CryoBUG has achieved that temperature. This "Ready" signal let's the vacuum coating system know that water vapor cryopumping is active, and that the coating process can begin.

At this stage, the controller design looks pretty solid, so the next step will be to lay out a printed circuit board and get a few fabricated. I'm using the ExpressPCB software to design my board, and will compare their cost to Futurlec's to see which fabricator I wish to use. As far as I know, these are the only two choices available when using that software package. On the plus side, it is extremely easy to use, so much so, that it's truly a pleasure to work with. Also the ExpressPCB software works great on my Linux machines under Wine.

As for other parts of my CryoBUG's development. I'm still waiting for some R-600a to arrive from England, so further charge related tests are temporarily on hold.

Wednesday, April 11, 2012

CryoBUG Stable -150°C Operation

Well it took quite a few different tests and refrigerant variations, as well as some minor hardware changes to finally get what I consider to be a stable -150°C unit. To qualify for this prestigious stability award, it must have virtually no fluctuation in evaporator temperature (+/- 0.5°C), and meet the target temperature requirement (-150°C or colder). Today we did just that.

Evaporator Temperature -152 C, Total Unit Current Draw 2.36 Amps

Compressor running pressures were equally amazing at 26/180 psig, which is also the reason why the current draw was so moderate at 283 VA (226 Watts, PF=0.8).

CryoBUG 4-11-2012 --- 4.5 hour Test Run
Other then the strange oscillation in the CC SUCT temperature, the Evap was about as Flat-Lined as one could hope for. Reference V5 Piping Diagram for TC locations.

Cool down was also very fast and steady...
-100 C @33 minutes
-110 C @36 minutes
-120 C @41 minutes
-130 C @64 minutes
-135 C @70 minutes
-140 C @77 minutes
-145 C @85 minutes
-150 C @98 minutes
-152 C @just under 2 hours (flat line)

So what did I do to achieve this? Basically one thing, and that was to dramatically reduce the flow in the final cap tube (CT#2). The charge was the same N-butane, Ethane, R-14, Argon blend as my first V5 hydrocarbon test run, and in the same proportions.

The funny thing was it took me in quite a circle trying to zero in on what exactly caused my initial freeze-out problems. I tried using Propane instead of N-butane, which didn't freeze, but also didn't yield temperatures nearly as cold. This was probably due to the higher suction and discharge pressures. So I tried reducing cap tube flow, but still couldn't get down to where I wanted to be while still using Propane in the charge. Next I went back to N-butane, but used less of it. Still I wasn't able to achieve my target temperature. So I added more R-14, and got a tad bit of improvement. And then added more only to see the improvement go away, as the suction pressure once again started climbing too high.

So finally just for grins, I decided to try the original N-butane charge at the same proportions that I had originally started out with, while hoping that just by some weird stroke of luck the resized final cap tube would do the trick. Well it sure did!

The only other thing worth mentioning, is that the compressor discharge temperature was very comfortable, settling in at 51°C. This combined with the good running pressures, should insure a long and happy life for the compressor.

What's next?

I would like to try Isobutane (R-600a) in place of the N-butane (R-600). My gut feeling tells me that this should yield similar test results. And with the Isobutane's lower freezing point of -160°C, I would feel a lot more comfortable in the long term (N-butane freezes at -138°C). Now I just need to get my hands on some of this.

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.