Oil return
Also known as: oil carry-over, oil trap
Short definitionThe oil that the compressor throws out with the gas travelling round the pipework and coming back to the crankcase. If the suction line is not designed for it, oil stays in the evaporator and pipes and the compressor runs short of oil.
When a pipe diameter is chosen, oil matters as much as pressure drop. The compressor sends oil into the system with the gas, at a rate of 1–3% of the refrigerant it circulates. On the liquid side the oil mixes with the refrigerant and moves along easily. After the evaporator it thickens in the cold and only the velocity of the gas carries it. That is why 4 m/s is taken as the minimum in a horizontal suction pipe and 8 m/s in a riser.
A horizontal line is sloped towards the compressor by 0.5–1 cm per metre; a U-shaped trap is made at the foot of the riser and repeated every 3–4 metres on a tall one. With an inverter or stepped compressor the velocity is checked at the lowest step, and a double riser is run if needed. In freezer rooms and on long lines an oil separator is fitted on the discharge side. If the crankcase level is falling, oil is not added straight away, because oil lying in the lines comes back all at once later.