Odd view in a sight glass

[dkemper]

All, I've a low temp 8 ton single compressor, dual evaporator setup for a freezer that has been plagued with poor performance issues and excessive head pressures.

Externally, the oddest symptom is the view in the sight glasses. Both sight glasses show the refrigerant (R404a) to be a hard white froth/foam -- almost looks creamy. Not "bubbly" -- under charge the unit enough to drop pressures and bubbles can be seen as well in the sight glasses.

System lost a high pressure line some months ago and vented it's refrigerant. After the repair, new filter drier and good solid vacuum followed with new refrigerant, again, the same view in the sight glasses. Three different (same model) compressors have also been installed that I know of, all have continued to have the same conditions.

Sight glasses will clear, but only when high pressure is nearing the operational maximum of the Copeland compressor, about 400 PSI.

I've a near identical unit, same compressor/condenser unit, same make/model evaporators that works perfectly. The capacities of the condenser, compressor, evaporators and expansion valves all appear to add up to a ballanced system at operational temperatures. In short, I do have pretty good faith in the basic hardware of the system.

The only two differences between the units, the "working" one uses 7/8" copper for it's liquid lines and has 1750 RPM evap fans, the "problem child" uses 5/8" copper and 1075 RPM fans.

Is it possible the much smaller liquid lines are causing both the performance and pressure issues? Seems to me that it might be forcing excessive liquid velocity along with excessive pressure drop from both the restriction and surface area friction.

Anyone with any other thoughts or ideas?

Many thanks!

[Paulajayne]

Hi

Creamy oil tends in indicate water in the system.

Paula

[fixit]

Hi
There is water in there,
was all the selonids open when you vaccum.
you will have to open the expansion valve and check the filter.
Regards

[dkemper]

Paulajayne,

I'm not sure I'm fighting moisture in the system. After the refrigerant leak, I was able to pull and hold a rock solid vacuum on the system (sub 500 microns). Also, at exceptionally high head pressure, the creaminess fades completely to clear.

lastly, the oil in the crankcase of the compressor is clear.

Still, I can get a bit of down time on the unit, probably last weekend of the month and replace the cores in the filter/drier. Can't hurt

Many thanks for the suggestion!

Win

[Andy W]

To eat 3 compresors, you have problems! sounds like oil is floating about leaving the compressors to possibly run short of oil and seize, if 3 compressors have lost their oil, it possibly migrated elsewhere, any oil seperator fitted? With out looking at a pipe sizing chart I can not comment on the pipe size, what length is the pipe run, what size suction pipe, evaps above or below the compressor, oil traps fitted, condensing unit or remote condenser, does it ever trip out on oil pressure switch, if one fitted is it wired correctly and proven to operate.

[monkey spanners]

Mineral oil with R404A or any HFC refrigerant will give a milky sightglass. The only other thing i can think of is if a drier core has broken up and the dust is going round the system. Maybe you could send a sample of oil/refrigerant off to be tested.

Cheers Jon

[dkemper]

Andy,

First failure that I know of was due to worn valves, same as the latest failure.

The second compressor failed due to slugging, shattered suction valves and damaged outlet valves. It's my understanding it was slugged by incautious re-charging.

The 3nd failure simply would not pump the system down any longer -- It'd pull down enough to shut down on low pressure, but could not pull any vacuum in the system. Not sure it'd even pull down to 0 psig. Again worn valves or piston rings.

But you're right, this unit has problems and they date back much further than my knowledge of it's history.

To the best of my knowledge, it's used POE oil from the start, but I do think I'll see about getting an oil sample. Will have to see where to send it off to for analysis.

The compressor's sight glass does not show a lack of oil in the compressor. And yes, the oil failure is working correctly. I can simulate a oil pressure failure and shut the compressor down. Unit does not have a history of oil pressure resets. I did replace the mechanical safety with a Sentronics one when I replaced the compressor earlier this month.

Sadly, there is not an oil separator, the compressor is about 10 feet above the evaporators. Total distance for the copper is about 35 feet with 5/8" liquid and 1 5/8" suction. Both suction lines merge into a 2 5/8" line that feeds the accumulator. Outlet from the condenser is 7/8" splitting after the filter/drier to the two 5/8" lines. Suction lines do have P traps at the bottom of the rise.

Again, many thanks for the ideas!

Win

[Paulajayne]

Has the type of oil used in this problem compressor been changed?

Paula

[dkemper]

Has the type of oil used in this problem compressor been changed?

Paula

Paula, to the best of my knowledge, it's used POE since installed about 6 years ago.

All three major components were used at the time they were united into their current low temperature application. I believe the original application had the evaporators using R-22 medium temp and the condensing unit was an R-502 low temp. Both would have used oils that would not "play nice" with the R-404a/POE currently in use.

I will see about getting an oil sample analyzed to see if there is a mix of oils, or if there are other contaminates in the oil.

Thanks!

Win

[NH3LVR]

In 1982 I saw some Bohn Units in Alaska that had the Sight Glasses obscured with a Cream colored substance. They belonged to a friend of mine, and were in a small Salmon freezing plant he owned. We were both perplexed, and resorted to reading the manual. It said merely to contact the manufacturer if this condition was found.
Since it was not a problem we ignored it, and the matter passed.
Ten years later I ran across these same units in another plant. The Sight Glasses were still obscured in two of the units.
While doing some repiping I removed the color change elements and wiped them out with a swab. The deposits came off easily. I did not find them to be oily at all.
These units utilized R502, and with the age I am almost positive they had Mineral Oil installed.
Since R502 would be a good solvent, I am at a loss to understand what could have been in the system to cause this residue to stick to the glass.

[dkemper]

NH3LVR, Odd you should mention it, but this unit, along with a pair of identical condensing units are BOHN units. (not that the other 2 show the same symptoms)

Perhaps the one symptom I really cannot explain is why the creaminess goes away at excessive head pressures, or when the unit pumps itself down for a defrost. Under both conditions, the sight glasses on the liquid lines are perfectly clear.

The existance of a non-compressible contaminate, or at least a difficult to compress contaminate is probably the Occam's razor diagnosis, yet with having seen sub 500 micros hold on the entire system followed with new, fresh from the cylinder refrigerant, I find it difficult to believe such contaminates would have found a way to remain in system.

Since I can recover the refrigerant, I'll likely try that once cooler weather settles back into the area and I can afford to deliberately loose 1/3 of the cooling in the freezer. With peak season production in full swing, mid-summer temps, and the Florida sun beating on the ceiling of the freezer, now is NOT the time for a schedule outage.

[NH3LVR]

dkemper:

I think what I observed in the past and what you are currently seeing are entirely different.
I would look for something obstructing the liquid path.
Have you checked for temperature drop across the the liquid line components?
Having the other two identical units should help in the troubleshooting.

[dkemper]

NH3LVR,

I think looking for a temp drop in the liquid lines is likely my next step. I should be able to get a lift into the freezer this Sunday. Will be our first day of no shipping in some months so I won't block any fork lift traffic.

One good thing, for what ever reason, the liquid lines are insulated in the freezer, the only evaps I've got so installed. Will keep the freezer itself from making that much of a drop in the liquid temps.

Heck, if I had any faith in vampire taps, I'd pierce at least one of the liquid lines to check for a pressure drop as well.

Again,

Many thanks,

Win

[Andy]

NH3LVR,

I think looking for a temp drop in the liquid lines is likely my next step. I should be able to get a lift into the freezer this Sunday. Will be our first day of no shipping in some months so I won't block any fork lift traffic.

One good thing, for what ever reason, the liquid lines are insulated in the freezer, the only evaps I've got so installed. Will keep the freezer itself from making that much of a drop in the liquid temps.

Heck, if I had any faith in vampire taps, I'd pierce at least one of the liquid lines to check for a pressure drop as well.


Again,

Many thanks,

Win

Hi dkemper:)

what temperature is your liquid line, it wouldn't hapen to be gas not liquid you have in the liquid line:eek:

Measure all this, if this is the problem, the higher RPM fans should help a lot, along with a suction liquid heat exchanger or a small air cooled condenser feeding in the bottom and out the top on the liquid line after the receiver.

Kind Regards Andy :)

[mohamed khamis]

It sounds that u have problem of oil foaming inside the condenser and liquid line. The creamy sight glass indicates to foaming of oil inside the sight glass. I think the oil is not stable inside the crankcase for some reason and it has tendency to boil off and exit with the discharge vapor inside the condenser and this drops the condenser efficiency and so u offset it by more charging. What is the kind of the compressor and what is the discharge temperature?

Cheers

[displaced paddy]

I Think that if you had a good liquid seal in the receiver, you shouldn't be able to see oil at all. It sounds like maybe this unit has the wrong fans or a dirty condenser or a broken dip tube in the receiver. With 10psig suction and 400psig head, a high compression ratio and scorching discharge temps are probably taking the compressors out. Check for suction pressure drops, while you're there. Socks, Felts and occasionally cardboard might give you a thru volume problem so bad that you could never clear the sight glass. Good luck to ya and stay out of the heat as much as you can

[Gibbo]

I am thinking you have mixed oil mineral and poe in system.