Every compressor throws out some oil with the gas it discharges; this amount is roughly 1–3% of the circulating refrigerant. The oil passes easily through the condenser and the liquid line, mixed with the liquid refrigerant. The difficult part is the evaporator and the suction line: the refrigerant has evaporated, the oil has thickened in the cold, and the only thing that will carry it to the compressor is the velocity of the gas. If the oil returning is less than the oil leaving, the crankcase level falls a little every day and in the end the bearings are left without oil.
This is why the design rules are built on velocity. A gas velocity of at least 4 m/s is required in horizontal suction lines and at least 8 m/s in vertical lines rising upwards. Horizontal lines are given a slope of about 0.5–1 cm per metre in the direction of flow, towards the compressor. An oil trap (U-bend) is formed at the foot of vertical risers, and on tall risers the trap is repeated every 3–4 metres. Choosing the pipe larger than necessary reduces the pressure loss but lowers the velocity and leaves the oil behind on the way.
Part load makes the job harder. When capacity control or an inverter brings the compressor down to half capacity, the gas velocity halves too; oil that returns at full load can no longer climb the riser. The solution is a double suction riser, a minimum frequency limit for the inverter and an oil recovery run at full speed at set intervals. At evaporating temperatures of −25 °C and below, on long lines and in central systems an oil separator is fitted in the discharge line; the separator sends most of the oil back to the crankcase before it goes into the system.
On site the symptom is the oil level in the sight glass falling over a matter of days and the oil pressure safety tripping. Adding oil as a first reaction is mostly wrong: the oil has not been lost, it has collected in the evaporator and the lines; after a defrost or a long full-load run it comes back all at once, and this time the excess oil harms the compressor. First the pipe diameter, slope, traps and superheat are checked; the level is corrected after the cause has been found.