Grades 6–8 · Machines
How does an air conditioner move heat outside?
An air conditioner can move heat from a cool room into air that is already warmer outside. It does this by changing the pressure and temperature of a circulating refrigerant. The indoor and outdoor air streams stay separate; refrigerant in closed pipes carries energy between them.
In the diagramAn air conditioner carries room heat outdoorsClosed refrigerant loop · invented energy for one cycleRoom sideOutdoor sideEvaporatorCondenserCompressorPressure dropWorkAir recirculatesAir takes heat awayRoom 6 kJ + work 2 kJ = outdoors 8 kJ
Indoors, a fan moves room air over the evaporator coil. The refrigerant is at low pressure, which lets it boil at a low temperature. Boiling requires energy, so it absorbs heat from the warmer air passing over the coil. The air returns to the room cooler while refrigerant vapor leaves the evaporator.
An electrically powered compressor squeezes this low-pressure vapor, raising its pressure and temperature. Now the refrigerant can be hotter than the outdoor air. At the outdoor condenser coil, heat flows from refrigerant into that air, assisted by another fan. As energy leaves, the refrigerant condenses to liquid.
The liquid then passes through a pressure-reducing device. Its pressure falls, some of the liquid flashes into vapor, and a cold low-pressure mixture reaches the indoor coil again. The two pressure levels allow the refrigerant to absorb heat indoors and release it outdoors at different temperatures. The compressor supplies the work needed to keep that cycle going.
Over a complete cycle, the refrigerant returns to its starting condition. The outdoor heat release includes both the heat removed from the room and the supplied work. In the illustrated account, six kilojoules removed indoors plus two kilojoules of electrical work becomes eight kilojoules released outside. The outdoor unit's warmth is therefore part of the same process that cools the room.
About this explanation
Starting knowledge: Understand heat transfer, liquid and gas changes, and simple energy sums.
Sources
- US Department of Energy — Home Cooling 101: How an Air Conditioner Works
- US Department of Energy · Lawrence Berkeley National Laboratory — Technical Support Document for Room Air Conditioners, Volume 2
- Trane — AC Expansion Valve