The most obvious cause is that the system is leaking oil. Telltale signs would be oil on the outside of the system, either in a puddle or possibly in areas where a pipe breach is possible (at joints for example). Examine the system closely for external signs of oil.
Another possible cause of oil loss is improper piping and design. In this case, oil may not be able to continue back through the system because of the nature of gravity.
To understand the concept more readily, think of a garden hose. In order for the water to come out of the hose while it's pointed upward, the water needs to be flowing at a certain pressure. The hose's small diameter makes it possible for water coming out of a household spigot to move upward, especially if the diameter is decreased still more at the end through the nozzle (or by your thumb). If you replaced the garden hose with a fire hose, the water would not have enough pressure to move through the fire hose; the water would fall back and pool at the lowest part of the hose. Likewise, if a refrigeration system has oversized suction line diameters, the refrigerant will not be able to carry the oil back to the compressor. The oil will return to the evaporator.
Asking the oil to move further than it can under a given pressure can also lead to compressor oil loss. To assist nature, some systems use oil traps in the suction line in combination with suction risers (a length of vertical pipe). As refrigerant gas returns from the evaporator, the drops of oil it contains will collect and get mixed in the trap. This breaks up the oil into smaller droplets, which can be carried up the riser pipe more easily by the refrigerant gas.
Traps should be placed every 20 feet in air conditioning system suction risers, or every 10 feet in refrigeration system risers. If the oil comes back, it could be due to poor design or, possibly, poor system redesign.
If you made a conversion to a new refrigerant, you also need to use the appropriate oil per the manufacturer's instructions, and make sure there is enough of it. A small amount of oil will be distributed throughout a normal working system.
If you are losing flow, it could be due to a broken rod. The pounds of refrigerant moved are lowered, and the system is not returning oil from the evaporator; there typically is a lower temperature where the oil lays because it coats the pipe. The delta T goes up across the evaporator. The system is not transferring heat as it should. You may measure low superheat coming out of the evaporator.
If you are called after the compressor has already stopped working, as always, we recommend opening up the compressor to gather more system information. If the problem is due to an oil loss, you may see the following telltale signs inside the compressor:
Without enough oil in the crankcase to properly lubricate the load-bearing surfaces, wearing and scoring happen. There just isn't enough refrigerant mass flow in the system to return oil to the compressor as fast as it is pumped out. Without enough oil, those surfaces become visibly scored or worn.
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