A discharge oil separator removes oil from the discharge gas coming out of the compressor. It does so immediately after the gas leaves the compressor. Oil is separated from the refrigerant by reductions in velocity, changes in direction of flow, and impingement on baffles, mesh pads, or coarse screens. Their use reduces the amount of oil reaching the low side. They help maintain oil volume in the compressor oil sump. They also muffle the sound of gas flow.
Oil separators can be very valuable as a means of reducing oil fouling in the evaporator. This is especially true with flooded systems, low-temperature systems, and systems with long runs of piping or other factors that tend to cause oil return problems.