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Lowering of the Oil Levels in Refrigeration Compressors

Lowering the oil level in the cup of a compressor causes a lack of lubrication and of cooling, which results in an abnormal increase in operating temperature, in the seizure of bearings and in the connecting rod block.

In these conditions it will be possible to notice the surfaces of the bearings, which will show small scratches or grooves instead of the "incrustations" of aluminum characteristics of the washing caused by the coolant.

These small grooves can also be caused by the presence of foreign elements in the cooling circuit, but the difference in color between the two motor shafts is certainly the element that allows to diagnose a lubrication defect due to lack of oil and, over a longer period of time, of mechanical breakage.

Causes of Lowering of the Oil Level

The main causes are: short cycle operation, excessive tendency to foam oil, an oil leak in the exhaust mechanism and prolonged operation at minimum load; all associated to an imperfectly designed cooling circuit.

In the event of prolonged operation with a short cycle, the oil can be sucked in by the compressor and sent in the circuit at a higher rate than the return one; this causes a lowering of the oil level in the cup.

Short-cycle operation can result from a lack of refrigerant charge, from a too small differential on the regulation thermostat, from insufficient thermal loads, etc. These conditions are accompanied by a low refrigerant flow rate and consequently by reduced circulation speeds of gases in the pipes.

If the system is subjected to rapid load variations, which cause frequent shutdowns and starts of the compressor, short-cycle operation can be eliminated by installing a warm gas bypass system.

The excessive tendency to foam the oil in the cup also causes a lowering in the cup; in fact the foam is quickly dragged with the sucked gases and is thus pumped into the circuit fridge. If foam build-up persists, the oil level in the cup may drop significantly. At the start of the compressor, a certain presence of foam is normal, however, after the excess of refrigerant contained in the oil has evaporated, this foam should disappear. Of course if the flow of refrigerant is checked and the appropriate oil is used.

When the foam in the oil persists, this may be due both to the use of an oil quality other than the recommended one, and to an excessive dilution of the oil itself caused by the refrigerant.

The hydraulic cylinder unloading mechanism, of the type used on certain large capacity compressors, can represent a cause of the lowering of the oil in the cup. In fact, if the O-ring seal is defective, the oil under pressure that reaches the exhaust mechanism could flow into the intake chamber and be dragged into the cooling circuit. If the amount of this loss is greater than the return flow rate of the oil in the circuit towards the compressor, the level in the cup will drop to such a point that it will no longer be able to maintain oil pressure.

Lowering of the Oil Levels in Refrigeration Compressors

Author, Riccardo Tigani  Linkedin Page
General Manager at Linea3C Srl

1st August 2019

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