Troubleshooting; Lubrication Problems; Oil Dilution; Refrigerant Migration - Emerson Copeland Discus Series Application Manuallines

7

Troubleshooting

The prevention of failures is one of the primary responsibilities of the installer. Otherwise the
user will not get the benefit of factory guaranteed quality.
7.1

Lubrication problems

Compressors are delivered with an initial oil charge. The correct oil level is shown in section
2.6.11.
Some, but not all lubrication problems are listed below:

Oil pump-out due to high on/off cycling rate. The number of cycles should be limited to
10 - 12 per hour. A high cycling rate will pump oil into the system and lead to lubrication
failure. Oil leaves the compressor at start-up and the short running time is insufficient to
return the oil to the compressor via the suction side, the result being lubrication damage.

Incorrect calculation of pipe sizes. It should be remembered that the entire system will be
coated in oil to some extent. Oil viscosity changes with temperature. More oil stays in the
system than was originally expected.

Low gas velocity. System gas velocity changes depending on temperature and load
(capacity control). In low load conditions the gas velocity may not be high enough to return
oil to the compressor.

Faulty or badly designed oil return system.

Incorrect pipe work.

Leaks.
In time, lubrication problems lead to failure of the main moving parts. A standard oil pressure
switch protects the compressor against low oil pressure if the problem lasts for some
considerable time.
The typical breakdown symptom of a compressor with inadequate lubrication is failure of the
bearing furthest away from the oil supply, the nearest having just enough oil to be properly
lubricated.
7.2

Oil dilution

During the off-cycle a certain refrigerant concentration is always present in the compressor oil.
This depends on the compressor temperature and crankcase pressure. The rapid reduction of
pressure on start-up causes the refrigerant to evaporate from the oil. This causes oil foaming
which can be seen in the compressor oil sight glass. The oil pump draws in much diluted oil and
foam and cannot build up oil pressure. If this cycle is repeated often enough bearing failure will
eventually occur. To prevent this type of failure a crankcase heater and/or a pump down system
should be fitted.
7.3

Refrigerant migration

When the compressor is switched off for a long period refrigerant can condense in the
crankcase. Example: R22 with a crankcase pressure of 8.03 bar and a temperature of 22°C, the
crankcase would contain a mixture of 35% R22 and 65% oil by weight. If the compressor body is
colder than the evaporator, refrigerant will move from the evaporator to the compressor
crankcase. Refrigerant migration normally occurs when the compressor is installed in a cold
area. A crankcase heater and/or a pump down cycle provide good protection against refrigerant
migration.
7.4

Inadequate suction superheat

The suction superheat should not fall below 10K. Low superheat will cause valve plate, piston,
cylinder wall and connecting rod damage. Low superheat can be caused by a defective or badly
adjusted expansion valve, incorrect sensor bulb mounting or by very short refrigeration lines. If
refrigeration lines are very short the installation of a heat exchanger or an accumulator would be
recommended.
D6.3.4/0715-0716/E 27