The working fluid enters the evaporator inside the room as a low-pressure liquid at approximately outside air temperature. (1) The evaporator is typically a snake-like pipe. The fluid immediately begins to evaporate and expands into a gas. In doing so, it uses its thermal energy to separate its molecules from one another and it becomes very cold. Heat flows from the room to this cold gas. The working fluid leaves the evaporator as a low-pressure gas a little below room temperature and heads off toward the compressor. (2) It enters the compressor as a low-pressure gas roughly at room temperature. The compressor squeezes the molecules of that gas closer together, increasing the gas's density and pressure. Since squeezing a gas involves physical work, the compressor transfers energy to the working fluid and that fluid becomes hotter. The working fluid leaves the compressor as a high-pressure gas well above outside air temperature. (3) The working fluid then enters the condenser on the outside, which is typically a snake-like pipe. Since the fluid is hotter than the surrounding air, heat flows out of the fluid and into the air. The fluid then begins to condense into a liquid and it gives up additional thermal energy as it condenses. This additional thermal energy also flows as heat into the outside air. The working fluid leaves the condenser as a high-pressure liquid at roughly outside air temperature. (4) It then flows through a narrowing in the pipe into the evaporator. When the fluid goes through the narrowing in the pipe, it's pressure drops and it enters the evaporator as a low-pressure liquid. The cycle repeats. Overall, heat is been extracted from the room and delivered to the outside air. The compressor consumes electric energy during this process and that energy also becomes thermal energy in the outside air. The maximum coefficient of such an air conditioner is E_max = T_room / (T_outside – T_room).
Refrigerators and heat pumps work in the same way.

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