oil separator troubleshooting

[coldkeeper1]

Hello to everybody.

I´ve read that in general refrigeration systems work better if the refrigeration oil is kept where it´s really helpful: inside the compressor. As in most cases truck-engine driven transport refrigeration compressors don´t have sightglasses to help in the observation of the oil level, what should be the symptoms of a system where the oil separator fails to return the oil from the discharge line to the compressor? In such a case, my guess is that the oil will get accumulated in both the evaporator and condenser, reducing heat transfer and therefore cooling performance. But as a reduction in cooling performance could also be related to other causes, is there any way to pinpoint the origin of this poorly performance to oil separator troubles? Thank you.

[Tesla]

Hi Coldkeeper1
Not to sure about trucks but on a chiller when run at lo loads the oil tends to log in the evaperator, to return the oil we need to run at above 80% load for about 4 hours. This returns the oil then we can check the compressor. Thank goodness oil will be a thing of the past in 20 years - check turbocor.

[GXMPLX]

No, the oil separator is not the REAL cause of poor heat transfer. The oil separator cannot be 100% efficient for there are some parts of the system you need oil. The oil you have in the condenser is hot, low viscosity and should´nt be a problem (usually). Oil in the evaporator it is a big problem in low temperature applications. The real cause of this problem is low refrigerant velocity and unless you have something inside the oil separator that blocking refrigerant's normal flow, the cause of this is: abnormal low suction pressure, anything that can affect compressor flow like faulty capacity control, anything that can affect expansion device's refrigerant inyection like not being well fed, or even not having enough thermal load like having ice on the coil, or anything that can disturb the cool, high viscocity's oil flow like poorly designed pipe. The probability this is caused by the oil separator is slim.

[coldkeeper1]

Thank you very much fellows for your helpful comments. In the past few days I´ve been trying to find out the reason for a cooling capacity reduction in a refrigerated truck unit: with the box empty, it takes almost 2 hours to lower the temperature from 26 to 16 deg. C. I know for sure that this unit was recently overhauled with new compressor, TXV valve, CPR valve, oil and refrigerant charge, that´s why I´m running out of options while trying to troubleshoot this problem. After reading some manuals, I began to ask myself if there was an oil logging problem in the heat exchangers. Since the system has an oil separator installed, I figured out that for such a logging problem to occur, this separator must be damaged, but didn´t know how to verify its condition without actual removal of it.

[GXMPLX]

If oil logging is the problem, it is easy to verify if you can open the system. Just blow with nitrogen and MEASURE all the oil that comes out the evap. If not enough comes out then blow the suction line and so on. The compressor manufacturer usually publishes how much oil is in the compressor when new and the oil separator has an extra charge (depends on size but is enough to begin to open the float). Not more than this should be in your system because it is compact.

Unfortunately oil logging is not the ONLY cause for poor heat transfer.

Sometimes there are external clues that can help you pinpoint oil logging, observing that the lower parts of the pipes don't cool or frost like the upper side, or lower circuits, etc.