oil failure

[hem057]

have a walk in freezer, semi hermetic, oil failure trips only in colllllld weather, oil level is fine in sight glass, 408a refrig., head pressure control, around 150 head, when weather warms up, unit runs fine and never trips, weather gets cooooollllld, and then trips nightly, have good net oil pressure, 40 psi (55 - 15)
any thoughts on what is causing this I should mention is is sentronic

[taz24]

have a walk in freezer, semi hermetic, oil failure trips only in colllllld weather, oil level is fine in sight glass, 408a refrig., head pressure control, around 150 head, when weather warms up, unit runs fine and never trips, weather gets cooooollllld, and then trips nightly, have good net oil pressure, 40 psi (55 - 15)
any thoughts on what is causing this I should mention is is sentronic


Does it have an oil sep? and does it have crank case heaters?
Have you actualy be present when it failed or have you just attended to it in the morning?
One last question. Does it have an oil filter in the crank? If it does have you proved it to be clean?

Cheers taz.

[Gary]

During the off cycle, refrigerant will migrate to the coldest point. That coldest point should never be the compressor. Hence the need for a crankcase heater. Does the unit have a crankcase heater?

Does the unit pump down in the off cycle? This eliminates liquid from the low side of the system and minimizes migration.

Is the evaporator above or below the compressor? Liquid gathering in the coil can be transported by gravity to the suction line and compressor during the off cycle if the evaporator is higher, causing liquid slugging on startup.

[US Iceman]

Certainly sounds like an oil dilution issue. When refrigerant dissolves in the oil, the viscosity of the oil is reduced. That's why you are experiencing oil pressure faults.

As Gary stated, use or check the crankcase heater. If you do have one of these installed, make sure it is tight to the compressor. I have seen some where the spring clips get loose amd allow the heater to separate away from the crankcase, which does no good, even if it is energized.

A pump down cycle is the other protection from this fault also. If the evaporator has a possibility of collecting gas it will condense. When the compressor starts back up, the compressors gets hit with the liquid.

By pumping out the evaporator, the liquid is stored in the receiver.