Anatomy / How refrigeration works
Inside an air conditioner
An air conditioner is a heat pump. It moves warmth out of your home with four parts and one loop of refrigerant. Step through the cycle, watch the heat change hands, and see why your hand feels what it does.
The cycle, step by step
The four parts
Each component has one specific job. Here is the breakdown - open one for the full story.
Compressor
Squeezes low-pressure vapor until it is hot and dense enough to reject heat outdoors. The cycle's only real work input.
Think of the compressor as a pump for heat. The vapor it draws in carries whatever heat the evaporator absorbed, and by compressing it the compressor makes that vapor hotter and smaller. Pressure and temperature climb together, which is what lets the downstream coil give heat to warm outside air. Every bit of electrical energy the compressor uses ends up, one way or another, adding to the heat moved outdoors.
Condenser coil
A bank of metal fins and tubes that the hot vapor flows through. A fan pulls outside air past, and the refrigerant condenses as it gives up heat.
The condenser is a coil of refrigerant tubes wrapped in thousands of thin metal fins, which spread the contact surface so a little coil can touch a lot of air. Hot vapor enters at the top, air rushes past the fins, and the vapor condenses into liquid as it travels down. The air leaving the outdoor unit is warmer than the air entering it - that is the room's heat, being handed back to the sky.
Expansion valve (TXV)
A metering orifice that chokes high-pressure liquid and drops its pressure. The sudden expansion chills the refrigerant dramatically.
A thermostatic expansion valve (TXV) is a precisely sized opening with a moving needle that meters how much refrigerant flows. As the high-pressure liquid squeezes through, its pressure collapses - and since pressure drives boiling point, the refrigerant now boils at frigid temperatures. A sensor on the suction line nudges the needle open or shut to keep the evaporator optimally fed.
Evaporator coil
The mirror image of the condenser: cold liquid boils inside the fins while the indoor fan pushes room air across them to blow out cooler air.
Where the condenser hands heat outward, the evaporator collects it. Cold low-pressure liquid enters the coil and boils at a low temperature, and the indoor air blowing across the fins is warm enough to feed that boil. Each gram of refrigerant that turns to vapor drags latent heat out of the air with it. The chilled air is pushed back into the room, and the vapor heads to the compressor for the next round.
The ideas that make it work
A few words you will hear from anyone who repairs or designs these - and what they actually mean.
Latent heat
Changing a kilogram of refrigerant between liquid and vapor absorbs or releases a huge amount of energy while the temperature barely moves. This is the quiet engine of the cycle: the evaporator silently drinks the room's heat as liquid boils, and the condenser quietly dumps it as vapor condenses.
Pressure and temperature
As a gas is compressed its pressure and temperature rise together. Raising the refrigerant's pressure makes its boiling point rise, so it condenses easily outdoors. Dropping the pressure at the valve makes the liquid boil at a low temperature - cold enough to soak up the room's heat.
Superheat
On the suction line, the vapor is warmed slightly beyond its boiling point. This guarantees the compressor only ever sees vapor, never damaging liquid slosh. It is measured as the temperature of the vapor above its saturation temperature.
Subcooling
On the liquid line, the condensed refrigerant is cooled a little below its condensation temperature. That margin keeps it liquid right up to the valve, so the metering device gets a clean, predictable flow rather than a froth.