The Evaporator electric fans cause the cool air contained within the conditioned space to flow over the fins of this heat exchanger. Because the refrigerant is made to be colder in the Evaporator than the surrounding cool conditioned air, heat is transferred from the air to the refrigerant causing it to evaporate to vapor form. This is the process, of course, that actually achieves air conditioning.

During the evaporation process, the lubricating oil is also chilled (it never changes phase; the oil always remains in liquid form). Thus, what emerges from the Evaporator is the refrigerant vapor, slightly superheated, and a cold flow of oil that coats the inside surface of the suction line. This cold oil coating is dragged back to the Compressor through the flow of the refrigerant vapor when it re-enters the Compressor. Conventional compressors require this return oil flow not only for lubrication, but also for cooling. It is apparent, however, that the energy consumed in cooling the oil cannot be used to also cool the air and is therefore a loss to the system.

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