Causes for oil loss

There are several possible causes for oil loss of a compressor. Some common causes which should be avoided:

Inadequate refrigerant pipes

The system should have sufficiently dimensioned discharge gas, suction gas and liquid lines to prevent excessive pressure drops, yet the diameters should be small enough to maintain adequate velocity of refrigerant flow to return the oil to the compressor. The pressure drop should be smaller than corresponding 1 K (2° F) in saturated temperature.

A split rising line has the effect of a downsized rising line at minimum load, while providing about the same pressure drop as a full sized line at full load. The smaller rising line is sized to ensure oil return at minimum capacity; the larger rising line is sized so that the combined "flow area" of both of these lines is approximately equal to that of the main suction gas line.

Suction gas lines are more critical than liquid and discharge gas lines from a design standpoint. Refrigerant lines should be sized to provide a minimum pressure drop at full load and to return oil from the evaporator to the compressor under minimum load conditions.

When operating with external frequency inverter, qualification tests should be done to ensure sufficient gas velocity at minimum speeds and mass flows that might occur during lightly loaded applications, which may further reduce gas velocities below previously expected levels.

Short operating cycle

During long periods of short cycling, the compressors can pump oil into the system at a faster rate than it is being returned, resulting in reduced oil levels in the compressors. To ensure optimum lubrication of the compressor inner parts as well as good oil return, Orbit compressors should be started max. 8 times per hour and remain switched on for a minimum operating time, see Operation Instructions ESB-130. Running the compressor for a shorter time (e.g. to maintain very tight temperature control) can result in progressive loss of oil and damage to the compressor.

Each time a compressor starts, suction pressure and therefore oil sump pressure are quickly reduced. The pressure drop reduces the saturation temperature, resulting in the oil-refrigerant mixture flashing into foam and vapor. With frequent starts, a percentage of oil may be pumped out of the compressor. If the compressor operates for a sufficient time to stabilize the system, oil will return to the compressor – but if the operating time is very short, the oil may still be trapped in the system. If this cycle is repeated, the compressor will progressively pump out oil, resulting in insufficient oil in the sump to ensure reliable operation.

Short cycling can have many causes. e.g. low refrigerant charge, low pressure limiter by setting a narrow differential thermostat control, the minimum load conditions etc. All of these are accompanied by low refrigerant mass flows that, in turn, result in lower gas velocity and poor oil return from the system. In case of rapid load fluctuations, which cause frequent starts and stops, cycling can be eliminated by a capacity control (e.g. an external frequency inverter).

Liquid refrigerant entering the compressor

One of the major causes of compressor failure is damage caused by to much liquid refrigerant entering the compressor. This reduces oil level and lubricating quality of the compressor bearings. The amount of liquid which a compressor can tolerate depends on its design, internal volume, and oil charge. Liquid migration, flooding, and slugging have to be avoided by means that depend e.g. on system design and the specific problem. An oil heater is recommended when the compressor is switched off to prevent reverse refrigerant migration back into the compressors and mixing with the oil.

A refrigeration system should be fine-tuned after installation and commissioning. Checking and setting controls at commissioning is important to ensure the efficient running and long lifetime of the system.