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Lubrication Problems in Refrigeration Compressors

The most common problems regarding lubrication are: oil dilution, lowering of oil level and loss of viscosity due to excessive heating of the compressor.

Oil dilution is certainly the most common problem in refrigeration systems.
Since the oil is very similar to the coolant, it is normal that, after a prolonged shutdown, the oil becomes excessively diluted with the coolant and loses many of its lubricating properties. Also, by certain ranges of normal temperatures, depending on the type of oil used, it is possible that an oil mixture refrigerant saturates, causing the two fluids to separate. The richest mixture of refrigerant (which is more dense) is then found at the bottom of the cup, while the richest mixture of oil (which is less dense) remains in surface. This dilution can be even more severe when the compressor is started after suction of liquid refrigerant accumulated by migration in the evaporator.

When a compressor is started with the oil in these diluted conditions, the richest mixture of refrigerant is sucked by the oil pump: being an excellent solvent, the refrigerant causes a washing of bushings.
Moreover, the very diluted oil tends to foam excessively when the pressure in the cup decreases and it can make the pump no longer work for a certain time.
If under these already rather critical conditions is added the fact of the excess accumulated refrigerant due to the migration into the evaporator, a mechanical failure due to lack of lubrication would not be out of the question.

In case the crankshaft has undergone a coolant wash, the important thing to point out is the way in which the aluminum of the connecting rod is literally “encrusted” around the surface of the crank pin. The pin has no particular trace of discoloration caused by heating, because the failure occurred almost instantaneously and the evaporation of the refrigerant has absorbed most of the heat produced by friction.
The presence of scratches or grooves in the plunger of the piston and in the area of ??the segments is rather frequent when excessive dilution of the oil and coolant liquid shots occur. These inconveniences are caused by washing the upper parts of the cylinder and the segments by the liquid refrigerant, for which there is an almost total lack of lubrication.
The segments then scrape the walls of the cylinder and cause small scraps, which in turn have an abrasive effect on the bell mouth of the piston. This can lead to total seizing of the piston and rupture of the shell.

Lubrication Problems in Refrigeration Compressors

Author, Riccardo Tigani  Linkedin Page
General Manager at Linea3C Srl

15th May 2019

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