The refrigeration oil sight glass indicated plenty of oil in the compressor crankcase.
But when the compressor started running the oil disappeared!
It began to flash and boil, then leave.
Given a chance, refrigerant always migrates to the coldest place in a system.
That's why pump down systems isolate the refrigerant in the receiver during off cycles.
Otherwise the refrigerant would slowly fill the evaporator. When the compressor started, liquid would flood back in the suction line and possibly damage the compressor.
Now think about a low temp freezer. The refrigerant gas returning to the compressor is so cold, the suction line forms ice.
This cold refrigerant cools the compressor and the refrigeration oil.
During off cycles the refrigerant migrates into the oil.
This mixture of oil and refrigerant reaches a state of equilibrium. The pressure at the surface of the oil equals the pressure witin the oil.
When the compressor starts, everything changes.
The refrigerant pressure above the oil decreases causing the refrigerant in the oil to leave.
This departure is not slow or measured. It is fast and violent. The oil begins to boil and foam. A large amount simply travels with the refrigerant as it leaves the compressor.
You can watch it disappear!
Keep the oil temperature warmer than the surrounding refrigerant.
Warm oil will not attract refrigerant.
That's why crankcase heaters are so important! They keep the oil where it's suppose to be, in the compressor.
If the oil doesn't return quickly the unit can trip the oil safety control.
It can also give the false impression that more oil is needed. When additional oil is added, it only magnifies the problem.
This excess oil can damage the compressor and flood the system.
A flooded system causes a reduction in capacity as it coats the system.
On more than one occasion I have removed a large excess volume of oil after installing a crankcase heater.
In my opinion the crankcase heater is the most overlooked and under appreciated component in refrigeration equipment!
Refrigeration oil gone in a flash? Better warm up to that crankcase heater!
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