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.