All reciprocating compressors will allow some oil to pass into the discharge line along with the discharge gas. Mechanical oil separators are used extensively. However, they are never completely effective. The untrapped oil passes through the condenser, liquid line, expansion device, and into the evaporator.
In a properly designed direct-expansion system, the refrigerant velocity in the evaporator tubes and the suction line is sufficiently high to ensure a continuous return of oil to the compressor crankcase. However, this is not characteristic of flooded systems. Here, the surge drum is designed for a relatively low vapor velocity. This prevents entrainment of liquid refrigerant droplets and consequent `carryover into the suction line. This design also prevents the return of any oil from the low side in the normal manner.
If oil is allowed to concentrate at the insert-bulb location of the level master control, overfeeding with possible flood-back can occur. The tendency to overfeed is due to the fact that the oil does not convey the heat from the low-wattage heater element away from the bulb as rapidly as does pure liquid refrigerant. The bulb pressure is higher than normal and the valve remains in the open or partially open position.