During the compression stroke of a reciprocating machine, the gas becomes hotter and some of the oil on the cylinder wall will pass out with the discharge gas. Some oil carry-over will occur with all lubricated compressor types, and in small self-contained systems it quickly finds its way back to the compressor. Start up after a long idle period can result in a large amount of oil carryover for a short period due to foaming. With larger more complex systems with remote evaporators oil it is desirable to fit an oil separator in the discharge line to reduce carry-over to the system (see Figure 5.5).
Figure 5.5 Oil separator (Henry)
The hot entering gas is made to impinge on a spiral to lose much of the oil on the surface by centrifugal force. Some 95-98% of the entrained oil may be separated from the hot gas and fall to the bottom and can be returned to the crankcase. The oil return line is controlled by the float valve, or it may have a bleed orifice. In either case, this metering device must be backed up by a solenoid valve to give tight shut-off when the compressor stops, since the separator is at discharge pressure and the compressor oil sump at suction pressure.
On shutdown, high-pressure gas in the separator will cool and some will condense into liquid, to dilute the oil left in the bottom. When the compressor restarts, this diluted oil will pass to the sump. In order to limit this dilution, a heater is commonly fitted into the base of the separator on large installations.
Oil-injected screw compressors invariably have oil separators and these handle continuous oil carry-over from the injection process. They are frequently built-in to the compressor assembly, particularly with semi-hermetic air-conditioning types. Recirculation back to the injection ports and bearings is continuous. For low-temperature screw compressors the oil is normally cooled during the recirculation process. For installations, which might be very sensitive to accumulations of oil, a two-stage oil separator can be fitted and up to 99.7% of the entrained oil can be removed. However efficient the separation process, a small quantity of oil will be carried over, and the system design must accommodate oil circulation.
Posted in Refrigeration and Air Conditioning