Chiller oil logging

[WCEbloke]

G'day all,
I've got a curly one here, I've spoken to a few Fridgys now and have had a range of suggestions, come conflicting with others, I thought I might get this communities advice before taking the next step, I've seen there are some very experienced and clever people on this site, There's a bit to try and explain, but lets see how we go;
THE HISTORY:
I'm working on a Trane RTUB-216 chiller using a screw compressor. Last summer we lost a fair bit of oil from the oil gasket. On inspection, I found there was next to no oil in the oil separator or compressor sump (~300ml). There is no sight glass and the oil safety had not tripped. It is very difficult to determine how much oil is in this system once it has run. There is a procedure in the manual on how to check the initial oil charge involving connecting flexible lines and a sight glass to the oil separator. This is meant to only be done when the unit is in vacuum (so no refrigerant is dissolved in the oil), and all the oil is in the oil lines, filter, and oil sump. The name plate states it takes 8 ltrs of oil. I used the initial oil charging procedure in the manual to charge the unit and ended up putting nearly 8 Ltrs in to the system to get the oil separator up to the desired level. The system ran great for a few months until winter and we started getting 'low evap refrig temp' faults. This temp is measure by a probe located just downstream from the EXV and is used to determine the SST for the Trane controller.

This fault has been intermittently occurring (usually in colder morning ambient temps) right through to this summer and I'm now concerned oil has logged somewhere in the system causing the low suction temp. We've still had no oil safety trips and the unit seems to run ok when it does. I've been recording the sub cooling and Superheat and both are fine. Recently I removed the refrigerant again and confirmed that it was not short of refrigerant. I found very little oil in the compressor or oil separator again.

THE QUESTIONS:
Where is the most likely place the oil is sitting right now?
Why is there no oil in the oil system?
Why did it not trip on oil safety when it was running.
Is it possible the oil is being pushed up to the condenser (air cooled, approx 3.5 mtrs above compressor) during pumpdown and then returning on startup through the evaporator, leaving some logged in the evaporator causing the fault?
What methods can I use to remove the oil from the evaporator if it is logged there.
How should I go about adding or checking the oil level on a system like this to ensure not too much or too little oil is added?

THE SUGGESTIONS SO FAR:
-Add 0.95Ltrs of oil to the sump of the compressor (this is the 'initial factory/field oil charging procedure'), recharge it with Refrigerant and test run it. It may have something to do with the head pressure control. (the condenser coil is in poor condition)
-Do not add any more oil, enough has been added last time, the compressor will not be damaged, the oil safety will protect the compressor.
-Change the oil filter.
-The oil is sitting in the condenser.
-Try moving the oil out of the evaporator by turning the water pumps off and flooding the evaporator with liquid refrigerant
-Try moving the oil out of the evaporator by strapping an electric heater to the water inlet pipe
-Try moving the oil out of the evaporator by using the Trane controller to open the EXV 100% and flooding the evaporator.


If anyone has read to this point, good effort!
I will keep yas posted on the progress.
Cheers.

[RANGER1]

WCEbloke,
Manual from web.
Check oil level procedure.
If chiller off & chiller water temperatures increased, when turn on, oil should come back, then check oil level as per manual.
Manual suggests 16L total oil charge.
I would be concerned oil level safety never trips, is there a way to physically check operation (not assume).
Not a good idea to flood machine with refrigerant trying to get oil back.

https://manualmachine.com/trane/rtub...0-user-manual/

[Grizzly]

Hi WCEbloke.
I am not well versed on Trane Chillers. But I have in my time worked upon many others and there has been lots of posts historically regarding the Trane Screw and its issues regarding checking their oil Levels.
As you yourself mention.
May I suggest that you ignore your Oil Levels temporarily and Run the Screw with a large load.
The higher swept volume of the refrigerant gases should then start to return any Oil that is logged in the evap.
Which is a common problem when chillers are required to run in lower than normal Ambient operating Temps!
Also check when looking at the conditions you describe. The difference between the saturated Suction Temperature and that of the chilled Water off temp.
If you start seeing 3,4 of 5c diff ( when the norm is 1 to 2c) then you have a good indication that the evap is Oil logged or The water flow rate is low.
Which in itself is a quite common cause of your Suction Temp/ pressure failures.
So don't assume its Oil logged check the flow as well.
Just some points to check and i hope you get somewhere.
Grizzly

[Tycho]

Like Grizzley, I am not well versed on Trane chillers, and Frankly, I know next to nothing about them.

I have to say that all I know about Trane chillers are that they are DX, judging by this picture trane.jpg
from the first page of the manual that Ranger posted.

really, a system like this shouldn't have any oil logging.

What refrigerant is it running on?

Everything that leaves the compressor and bypasses the oil separator (if there is one) should return to the compressor through the suction.

There is one caveat though.. if the system is constantly running on low conditions, oil can collect in the evaporator/chiller. but this should easily be fixed by forcing the chiller to run on high loads.

[RANGER1]

In the manual I posted it has a few troubleshooting idea's as well.
It appears there is a oil flow valve internally after oil filter?
If it were a Bitzer then I would test oil flow control annually by closing oil to compressor(if there is a valve in your case I assume there is) & compressor should stop within a few seconds (if Bitzer).

At least then you know equipment is safe & secure.

It may also have a high discharge temperature if low on oil flow.

With the current hot conditions in Australia, would have thought plenty of heat load at the moment.

[WCEbloke]

Hi WCEbloke.
I am not well versed on Trane Chillers. But I have in my time worked upon many others and there has been lots of posts historically regarding the Trane Screw and its issues regarding checking their oil Levels.
As you yourself mention.
May I suggest that you ignore your Oil Levels temporarily and Run the Screw with a large load.
The higher swept volume of the refrigerant gases should then start to return any Oil that is logged in the evap.
Which is a common problem when chillers are required to run in lower than normal Ambient operating Temps!
Also check when looking at the conditions you describe. The difference between the saturated Suction Temperature and that of the chilled Water off temp.
If you start seeing 3,4 of 5c diff ( when the norm is 1 to 2c) then you have a good indication that the evap is Oil logged or The water flow rate is low.
Which in itself is a quite common cause of your Suction Temp/ pressure failures.
So don't assume its Oil logged check the flow as well.
Just some points to check and i hope you get somewhere.
Grizzly

Hey Grizzly, I'm trying to understand why the TD between SST and water off should be only 1-2c. We run our chilled water at 7-10c which would require a 4-9c SST, which is a little high. the SST in the past has been arount the 0c mark I believe. with the Low refrig temp recent faults, it has dropped in to the minuses but I haven't actually witnessed it falling to the required -5c to trip out, only found the faults at a later date.