It sounds like you have a serious issue with the design and layout of the refrigerant piping. More than likely if the piping has been improperly sized it has resulted in the lack of oil return to the compressors. Usually this problem will be the result of a low load condition and will dramatically increase when compressor‘s are staged off. This will result in low velocity of refrigerant through the system as previously stated by others. It is the velocity of refrigerant that brings back oil at an equal rate as it leaves the compressor. There are many things to consider when designing the piping arrangement, such as capacity of all components and correct size of pipe so as to maintain a certain pressure loss throughout the system and also maintain velocity. Velocity usually should not be less than 1000 FPM for suction lines. As for the liquid line it is important to maintain pressure loss at no more than 45 PSI so as to maintain sub cooling and if exceeding this pressure it is certain that flash gas will be present before the TXV.
Do you have a two circuit system; each with tandem compressors…..but it’s not clear how many TXV’s you have per circuit. It is important to properly adjust superheat settings and if not set correctly as you know will result in flood back of refrigerant which will wash oil from bearings. Verify the safety margin of the TXV’s and that it is feeding correctly. If using the same TXV verify that superheat has been increased to compensate for the higher temperature Glide.
The superheat will be lower with 407C compared to R-22 if using the same TXV.
Remember that 407C has a high temperature glide of 10 °. The setting of the TXV using R-22 and the initial super heat setting above the saturated temperature was say 10 °. Now with the blend of 407C the same evaporator temperature is achieved - but now this only provides 2 ° F of superheat above the blends vapor temperature.
Refrigerant blends need two columns pressure/temperature charts:
Zeotropic Blends have different temperatures for saturated liquid and saturated vapors at constant pressure.
•Bubble Point (or Liquid) gives pressure for saturated liquid; used as the reference point for sub-cooling calculations.
• Dew point (or Vapor) gives pressure for saturated vapor; used as the reference point for superheat calculations.
407C has a slightly lower discharge temperature, so you will need to ensure that the TXV has sufficient capacity in low ambient condensing temperature which is expected in the winter months. If the system utilizes fan cycling or head pressure control valves and fixes the minimum condensing temperature at say 70 ° (140 psi), the TXV capacity will need to be considered at this condition.
Sometimes the addition of a hot gas by-pass will improve oil return when under a part load. You may want to consider adding a suction accumulator. At start up and after a long period of down time it is likely for refrigerant to condense in the evaporator and suction line which will result in slugging at start up. But now we are getting technical and that’s a long story for another day. Be sure to install a suction filter drier.
Is the condenser located above the compressor and have you considered the net lift or drop. Also what about the evaporator? Are there traps in place for oil return where needed? What is the developed length from compressor to condenser and from condenser to the evaporator?
Troubleshooting is not part of the repair…understand the symptoms and you will find a solution.
please be careful quoting numbers like those above...there are so many variables. i know of instances where < 200 fpm in the suction line was the most perfect choice...and places where 1000 fpm was not enough to bring the oil back in all cases.
Originally Posted by alcomech
traps in the suction or discharge line can be very dangerous if not sized or installed correctly. even if they were, many compressor these days (including scrolls) hold so little oil that you could fill the traps and not have enough oil for the compressors. then when all of the compressors come on, you slug the compressors with oil since there isn't enough room for the extra oil. i prefer oil separators in difficult situations.
Originally Posted by alcomech
i agree on the suction filter drier. i think an oversized (and properly installed) suction line filter drier is very important at this point as well.
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I understand, just throwing out some ideas.
Been there, done that......
If and when you change the compressor don't be surprised if they recommend and send you a pipe changing kit. The two compressors are in parallel correct? If so, they may possibly have you change the oil pipe connection from where it may be, high on the compressors and they may have you put caps on those ports, take out the sight glass on the compressor and put a new line lower, screwed in at the old sight glass threads.
I may be completely off the mark here.
But if they do indeed have you do that, in my little mind it tells me that they know they had a design problem and the oil wasn't getting to each compressor evenly and the one suffered to the point that it messed up the bearings to the point that you are at now.
Just an observation from what I've gone through on a McQuay unit I've worked on.
And yes! I too think the compressor is toast. You may keep it running for a short while, but it's on it's way out.
Hate to say it but it sounds like a rushed through installation. One where all of the parameters were not looked at. And I can't blame the contractor 100%. Few want to pay for a job done right.
Large split systems like this require much thought and consideration on the application and installation if they are to succeed and live long. Especially with a refrigerant retrofit.
Mrak: We got the oil analysis back indicating minimal moisture, high acid, high aluminum high iron and silicon. Mcquay just sent us 2 more compressors. Had a quality assurance guy come out from Mcquay to look over the job for a second time. Took refrigerant line sizes and lengths, pics etc. They now want a sequence of operation report. Unsure what the outcome is going to be on this. We arent installing the new compressors until we get feedback from Mcquay. Also fyi, the second circuit of tandem compressors is piped exactly the same as the first circuit, same refrigerant charge amount, same everything! No problems on that circuit. When we do replace these compressors we intend on doing multiple oil changes, installing removable core filter driers and installing some other type of controls possibly. We did look at the oil line and were thinking the samething about that. Well have to wait and see.
Originally Posted by Mrak
Mrak, tell me some more about your particular experience with the Mcquay unit you referred to?
Originally Posted by Mrak
That McQuay unit is a model: #
At the moment I'm a bit rushed for time, wrapping up the Friday. I did take a picture that may or may not help. When I get to my computer I try getting them to you. Let me know.......
If I knew what I was doing I could get those photo's on this page to show you. Probably just as well that I can't figure out how since the photos aren't all that revealing. I didn't take the cage cover off so it doesn't show up all that well. In any case you've got the mod.# in the above thread. It's a package unit chiller with 4 copelamatic compressors. 2 per evaporator. I still think they must have realized they had a design problem as I mentioned earlier
Originally Posted by acguy951
. If someone else has had similar experience be sure to say something.
Hey Mark, it’s not uncommon for a semi hermetic Coplematic compressor when installed in a parallel tandem application to have an oil equalization tube installed as you described, and in place of the sight glass which interconnect the oil sump of the two compressors. This is usually required when two compressors are connected to the same circuit and so that oil is returned to the compressors at an equal rate. This may appear to be simple however there are conditions which will present problems depending on the load and will vary under a part loaded condition. This is why it is important to consider the strategy of compressor staging. Unless you have an oil separator, you will want to consider equally loading the compressors so that oil will be distributed equally.
What is the Model No of the compressors which you have in place?
Also you realize that the OP has Scroll compressors and not semi hermetic’s. It is very likely that there is an OEM oil equalizer line installed between the two compressors. This brings up a question as to the Model No. of the compressors which he has installed as replacement to the O6E.
Troubleshooting is not part of the repair……understand the symptoms and you will find a solution.