Air Conditioners

Have you ever been outside on a hot day and stood near a room air conditioner? The air conditioner is actually heating the outdoor air. Near the air conditioner is the last place you'd want to be on a hot day!

Where does the hot air come from? It must come from inside the house. But as you know from the second law of thermodynamics, heat only flows from a lower temperature (indoors) to a higher temperature (outdoors) if work is done on the system.

Figure 16 shows how a room air conditioner operates. The compressor raises the temperature and pressure of the refrigerant, turning it into a hot, high-pressure gas. The temperature of the condenser coil is higher than the outside air temperature, so heat flows spontaneously from the coil to the outside air. A fan increases the rate at which heat flows. As thermal energy is removed from the coil, the refrigerant cools and condenses into a liquid.

Figure 16 In a window air conditioner, outside air is heated as a fan blows it through the condenser coil. Inside the room, a fan draws in warm air through the evaporator coil. The fan blows cooled air out into the room.
Interpreting Diagrams What work is done by the compressor?

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The liquid refrigerant then flows through the expansion valve and decreases in temperature. As the cold refrigerant flows through the evaporator coil, it absorbs thermal energy from the warm room air. The fan sends cold air back into the room. The refrigerant becomes a vapor, and the process starts all over again.

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