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Old 02-03-13, 10:55 AM   #21
jeff5may
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Mikesolar and/or AC_Hacker,

Yes, the two main places I look are the hvac talk forum and refrigeration engineers forum. The pros in the know all design the PTAC and RAC setups for 30 degrees superheat at the inlet line to the cxr at design conditions (85 deg F/30 deg C ambient). Asked about SLHX, they said it's just another part that could make these things fail. In walk-in freezer systems, though, they are commonly used since they save money. Their main focus is on reliability, then cost, efficiency comes in a distant third. Unless it's a big system that costs a lot to run.

I read in an article I can't find now that a designer noticed the "foam burp" effect. What the pros know is that it is a well known condition and design for it not to happen. This one briefly discusses it in section 4.2

http://docs.lib.purdue.edu/cgi/viewc...8&context=icec

The main idea is that at "high" evap temps (>-10 deg c) and below about 15 k of superheat, the propane boils so rapidly in the evap that the oil doesn't fully separate. The obvious solution is to keep superheat above 15 k. You can do this by either raising the superheat setting on the TXV or by adding SOMETHING in between the evap and cxr. This something will be the SLHX in my unit. With it, I will be able to set the TXV to a lower superheat setting (than 15K).

For an insight into the theoretical arguments surrounding this subject, just google 'R290 suction superheat'. The pros will argue this to death because it depends so much on specific situations. When narrowed to R22a (refrigeration engineer grade propane), the topic quickly devolves into scare tactics and warnings that propane is explosive. They are all against drop-in replacement of r12/r134/r22.

All my experimental systems have been made from old R22-based units. They are plentiful and dirt cheap. Mineral oil, 10 SEER type units.


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Old 02-03-13, 03:27 PM   #22
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AC,
I just read the paper you presented. Is this awesome or what? Exploiting supersonic evaporator flow for efficiency gain! The author admitted he shoulda used a larger accumulator because he slugged his compressor! Priceless...

I hope the next version will support sustained operation. It would be a tremendous innovation.

I don't know if my unit was producing supersonic flow, I highly doubt it, but it was doing the same thing as the author described in his paper. Right at the point where efficiency was greatest, the evap would burp foam. I wasn't slugging my cxr, I was foaming it. I believe it was at around 10 deg F of superheat at the bulb, 15 deg f at the cxr inlet. from this effect, I learned how hard I could push my evaporator.
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Old 02-03-13, 04:32 PM   #23
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Quote:
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Right at the point where efficiency was greatest, the evap would burp foam. I wasn't slugging my cxr, I was foaming it.
So, you had a sight glass, right?

-AC
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Old 02-03-13, 08:59 PM   #24
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It is interesting that, at the end of the first paragraph of section 2, they want to insult the compressor. Not sure how that will help
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Old 02-03-13, 09:22 PM   #25
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So, you had a sight glass, right?

-AC
Well,err,hmm,uh...no. That would have made my cheap reverse a/c unit not so cheap.

In all honesty, I have no way of knowing whether I was slugging my compressor or not. But I do know I was running my evap too hard.
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Old 02-03-13, 09:29 PM   #26
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I wonder if the viscosity issues of R290 and mineral oil are the same with a scroll as they appear to be with the recip? Are you really getting the foaming or is there some other issue. BTW, a sight glass is not very expensive.
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Old 02-04-13, 10:51 PM   #27
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OK, OK, you win. In my next unit, I'll put a sight glass in the suction line right before the compressor. When and if I test the envelope, I'll know what actually got through the line. If I have more trouble with the existing unit, I'll stick one in it and revisit this issue. For now, the 30 degF superheat at the suction inlet will remain.

Mikesolar,
It seems to be an issue at high flow, high temp (>-10 degC), low superheat conditions. With r290 or r600, POE oil is the worst, then PAG, then mineral oil. The mfrs are recommending Shell clavus g68 or suniso 4gs oil in r290 systems due to their higher viscosity, where in r22 systems they recommend clavus g32 or suniso 3gs. Kind of like putting racing oil in your car if you like to rev the engine. If the oil thins out, it will start from a higher initial viscosity and won't get too thin or foam up.
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Old 02-05-13, 05:29 AM   #28
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I started with an R410 pot, drained the POE and put in MO. The fridgie boys say that 5% POE won't be a problem in running the system so i am not concerned about getting it all out BUT, I still haven't turned it on yet. There may be a host of issues most of which, I am guessing, will revolve around sizing Evaps and condensers properly.

I don't know if the oils noted by the authors are available here or are they European only.
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Old 02-05-13, 05:01 PM   #29
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Quote:
Originally Posted by Mikesolar View Post
I started with an R410 pot, drained the POE and put in MO. The fridgie boys say that 5% POE won't be a problem in running the system so i am not concerned about getting it all out BUT, I still haven't turned it on yet. There may be a host of issues most of which, I am guessing, will revolve around sizing Evaps and condensers properly.

I don't know if the oils noted by the authors are available here or are they European only.
From what I know about compressors (not much), the R410 types are set up for a higher compression ratio and higher ultimate pressures (r22 or 290 high side runs in the 200's psi range, r410a runs in the 400's psi range). I have heard that you can run them at lower pressures, though.

The oils I posted are worldwide standards made by American companies. Suniso is made by Sunoco, same guys that supply gas to NASCAR. 3gs is 30 weight, 4gs is 40 weight. Shell Clavus is made by Shell oil, the number is the weight. Any Hvac supplier will know what they are and have something equivalent.
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Old 02-05-13, 07:58 PM   #30
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Quote:
Originally Posted by jeff5may View Post
From what I know about compressors (not much), the R410 types are set up for a higher compression ratio and higher ultimate pressures (r22 or 290 high side runs in the 200's psi range, r410a runs in the 400's psi range). I have heard that you can run them at lower pressures, though.
Mostly right. Compression ratios remain the same, it's just the low and high side pressures are higher in an absolute sense. The other thing about 410A vs 22 or 290 is it has quite a high latent heat capacity so you need less mass flow for the same refrigerating capacity.
This means 410 compressors have a smaller displacement than their R22 counterparts, so a 18KW compressor for 410A will provide a significantly reduced capacity with a lower pressure refrigerant.

I can dig up the figures and give you a calc if you are really interested (or you could run a quick simulation in coolpack with a fixed mass flow compressor and see the difference in Qc when you switch refrigerants)

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