Oil foaming in Compressor sight glass
I just replaced a Copeland short head compressor with a new tall head compressor. Compressor had a failed valved which destroyed the top of the piston. During diagnostic, oil pressure was good at start up then dropped after compressor settled in tripping low oil pressure safety. I assume this indicated refrigerant in oil washing out oil causing oil pressure drop.
New compressor is installed and I am getting foaming in the oil sight glass. This is a 2 compressor Mammoth, water cooled condenser box car unit. This is the No. 2 compressor which is a 3 head with 2 electric unloaders. I have run all my test with compressor fully loaded (although light load on system) and No. 1 compressor off. DX coil for 2 circuits is mixed (not stacked).
Refrigerant pressures are approx 60/225 (depending on ambient). SH at DX outlet is about 20-21 degrees and 23-25 at compressor inlet. SC was 8-15 degrees (I have gauge on discharge and at liquid line filter drier and D/C was about 25 psig higher than LL so not sure what SC is for sure. Still need to verify gauge accuracy (was at the end of the day yesterday & had no time to verify that yet)).
I took apart the TXV and cleaned as best as possible since I had such dirty oil and looked like it may have been sticking. Still oil foam.
Crankcase heater is active when compressor is off.
Oil looks good for approx 30-45 seconds after start up then begins foaming.
Valved off suction service valve to 20 psig suction and still foams. If I open service valve approx. 1 turn open, foam clears for 3-5 seconds, then foams again. It will do this until service valve is open approx 25%, then no change with oil foam after that when valved opened in 1 turn incriments.
In comparison to other indentical unit (although slightly higher air volume and a couple of years older), same size compressors, same size TXV, and no foaming. Bottom of crankcase feels warmer on other unit compressor even when 2 heads unloaded, and No. 1 compressor for this unit. The new compressor crankcase temperature is considerably cooler, which makes me believe that I could still be getting refrigerant in the oil (just a thought). New compressor does have POE oil from factory and did not change. I heard that this oil is prone to foaming.
I have adjusted TXV and can get adjustment, with stem front seated, max SH was about 35 degrees, minimum was about 12 with stem open all the way. I have not yet removed bulb from suction line to see TXV response.
I was told by RSD Copeland guy briefly (he was on bereavment but answered phone anyway) and he was saying he thinks TXV is too big, although this is a factory installed item and neither the TXV or compressor has ever been replaced. He also said I should have 8-10 degrees at DX coil outlet (unless I misunderstood him) and 20+ SH at compressor. I am not sure how I am going to pick up 10+ degrees of SH on an insulated refrigerant line, inside a cabinet and not influenced by ambient temps or any type of hot gas bypass (the unit has one but only on No. 1 compressor). I still need to contact him again since that was a couple of weeks ago (I was on vacation last week) and hopefully he is thinking a little clearer now, so I am not relying on that information for now.
might want to take superheat suction temp from the 6 o'clock position of the suction about 6" from the compressor....might have liquid droplets mixing as it flashes off in the hot oil.don't know if your loading /unloading on that style compressor but sometimes that changes the clear oil also....but as you said its clear on start and then foams up sounds like one cycle of the refrig coming around again..
2 reasons for oil foaming.
1. liquid refrigerant (or at the very least, a very dense refrigerant coming back)
2. too much oil and you are getting a 'splashing effect' from the running train.
my money is on #1. you had broken parts before. you are about to have broken parts again. i would question your method/accuracy of your instruments.
as for gaining superheat. with unloaders and stuff, you may not be able to get more superheat from the suction line when unloaded. when the compressor fully loads up, you will have a greater pressure drop in the suction line and then you will get 'more superheat'. with larger suction lines, the copper wall gets thicker and more difficult to get accurate readings. start taking discharge superheat readings...they will tell you more and verify your suction line readings.
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I don't know much about this kind of unit... but that won't stop me from asking questions lol.
What is the oil sump temp when it's off vs when it's running?
Is there an oil cooler?
How high is the oil level? (getting beat by the crank?)
I'm not sure what kind of crank case heater it has, but if the oil sump temp doesn't get too high, can you run the CCH while the compressor is running to help drive the freezone out of the oil?
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So the old comp had mineral oil, but new comp has POE?
There is no oil cooler, have to check sump temp, oil level is at most 1/3 when not foaming, crancase heater is insertion type at bottom of sump, the standard semi hermetic type CCH.
I was told that the oil was compatable with existing oil and there was no need to change it.
All readings were taken with compressor fully loaded and compressor 1 was off line so that I could get as much load on the new compressor as possible.
I agree with the liquid refrigerant theory, but I can't figure out where to go from here because everything else looks good.
I have tried attaching thermocouple at various positions on the suction line with no real change in line temp. I have tried 2 differant thermocouples with no change in readings. I have the thermocouple attached to the line with a rubber strap to get good contact with refrigerant line and to insulate the thermocouple tip for the most accurate reading possible. I checked SH at DX coil discharge with a 4 degree lower SH than at compressor.
I have the required SH at compressor, and even had foaming when TXV was set at 30-35 degrees of SH. I get foaming when the suction valve is closed approx 80-85% which I would think would minimize any liquid back to the compressor as the pressure drop should flash off some of it with the pressure drop, not to mention the restriction caused by the suction valve being closed almost all the way.
I have removed some of the refrigerant until I had constant flashing in the refrigerant sight glass to eliminate the possibility of any over charge of refrigerant.
Again, I was told that the POE oil does foam more than mineral, so maybe I am just chasing ghosts.
It is very possible that there is oil saturation in the refrigerant line. Since the valves on one head were non existant, I am sure I was pushing some oil out of the compressor since part of the valve was lodged in between the piston and the cylinder bore, although oil level was good in the compressor before it was changed out.
Discharge SH? Not sure what that is, do you mean sub cooling?
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you said that you are manually loading this compressor even tho there is only a light load on it, have you tried testing with a full load and let the unit run normally let both compressors run and check your SA and RA, also wouldn't hurt if you put some heat conductive compound on those remote bulbs for the TXV....might be causing a problem by loading that #2 without much of a load and locking out #1, shut her down for a while get a good load then let her rip and see if you get different results...sometimes its the simple things, just a thought good luck
Was the old system Mineral oil?
Mineral oil and POE do not mix. If you did not flush the system when you changed the compressor you could have problems.
You did not mention if proper refrigeration practices were followed when changing out the compressor. Like evacuation of the system, changing out driers ect. We can not assume that you have done these things.
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Oil pressure during the oil failure events?
Oil pressure fluctuate during the foaming?
What type of oil failure safety?
I am limited on what I can do on this system as it is for our clean room and it is temperature/humidity critical. In other words, if I throw the temps off in the clean room, we would have to stop production and possibly have to throw away any product on that run, so, I can not technically load up the room, or shut the cooling down to load it up. What I have done is lock out the #1 compressor so that I can load up the #2 compressor. We are trying to maintain 55 degree supply air temp and load the rooms with reheat coils for temp control and keep a load on the A/C for humidity control.
I was told by RSD, that the oil in the compressor was compatible with the existing oil, so I did not change out the oil.
All proper refrigeration practices were followed. Compressor was pumped down for replacement with out removing refrigerant. Filter drier cores were replaced and put into vacuum to less than 500 microns at the compressor and filter drier.
The oil failure issues were on the original compressor, no oil failures on new compressor, but I have not been monitoring oil pressures to see if it is dropping after start up and oil foaming...good point, I will check that...
Copeland Discus M#6DP3R-35ME-TSK-800 I was told that the ME pertained to the oil.
This is a Mammoth Box Car unit, water cooled condenser, and again, we are maintaining supply air temperature, this unit is not "comfort cooling". There is no accumulator, receiver, or suction drier on this refrigerant circuit.
The TXV is a Sporlan OVE-40-CP100. The bulb is secure at approx the 3 o clock position. I re-insulated the bulb and the SH stays pretty steady at about 23 degrees with no hunting. If I open the blower door on the negative side of the blower, SH changes as it should and then takes less than a minute to settle back in after the door closes. Everything I have tried on the TXV indicates that it functioning properly, and I am running at the specified SH from Copeland.
Again, I am thinking that it may just be a characteristic of this new oil which I have never dealt with before.
so there is no oil return issue?
i see foaming in alot of low temp compressors, like a layer atop the meniscus, running POE.
Oil pressure never fluctuates, on these systems and have been running for many years without issues......unless they run low on refrigerant and RGT starts to climb, ie little suction gas cooling.
I would monitor the oil pressure, but wouldn't be too concerned otherwise.
BTW i didn't see what refrigerant you are dealing with?