How does an air conditioner work?

What is an air conditioner?
Conceptually, many people think of air conditioners as devices that cool rooms or buildings. However, in order to understand the principals behind air conditioning, it is helpful to view them as "heat movers". That is, air conditioners move heat from one area to another. For example, if you have a window air conditioner installed in a room, it moves heat from inside the room to the outside. If you are in the room, the effect you feel is that of cooling. Outside, the heat gathered from the room is dissipated.
The process of air conditioning also has dehumidifying effects.

How does an air conditioner work?
Short answer:
An air conditioner blows air through its evaporator coil, which absorbs heat from the air. It then dissipates that heat outside by blowing air over its condenser coil.
Long Answer:

A very simple diagram of a cooling system

At its most basic level, an air conditioner is simply a compressor-driven cooling system.
This system has a few basic components that are linked together

  • Refrigerant runs through the system and provides the cooling
  • The compressor is the "engine" that pushes and pulls the refrigerant through the system
  • The compressor is linked directly to the condenser (often called the condenser coil), which condenses the gaseous refrigerant into a liquid at high pressure
  • The evaporator (often called the evaporator coil) is a large diameter tube that allows the liquid, highly compressed refrigerant to rapidly expand to a gas

A better diagram of a window air conditioner

Most air conditioners also have fans to blow over the evaporator coil to blow the cooled air into the room and to blow over the condenser coil to help dissipate the heat outside. Additionally, there are very small diameter tubes called metering tubes or capillary tubes immediately before the evaporator. These tubes ensure that the refrigerant is under great pressure before it hits the evaporator (which has a much lower pressure) and they help control the flow of refrigerant through the system. There is also usually another large diameter chamber called the accumulator immediately before the compressor. The accumulator ensures that the refrigerant is in its gaseous state before it hits the compressor.

For the purpose of explaining how this system works, we'll follow a linear path, starting with the gaseous refrigerant at the compressor, but keep in mind that system is sealed and continuous. Unless there is a leak, none of the refrigerant escapes- it just circulates through the system over and over, as long as the compressor is running.

The refrigerant is in a gaseous state when it is pulled into the compressor. The compressor pressurizes the gas, raising its temperature, and the condenser coils dissipate most of the excess heat and condense the gas to a liquid. Usually a fan blows over the condenser coil to help get rid of the heat. Most air conditioners have a fan that blows over this assembly to help dissipate the heat. Continuing through the tubing of the system, this liquid is still relatively hot, but it is pressurized, and pressurized liquids have a higher boiling point than non-pressurized (or less-pressurized) liquids. The liquid then travels to the capillary tubes, which are very narrow, to regulate the flow of refrigerant through the system and to ensure a large pressure differential between the capillary tubes and the evaporator. When the liquid refrigerant passes into the large diameter tubing of the evaporator coil, it evaporates immediately, because the pressure dropped, dropping the boiling point of the refrigerant, and causing the refrigerant to boil (causing a state change from liquid to gas). Evaporation is an endothermic reaction (a reaction that absorbs heat) so the air conditioner's fan blows air over the outside of the coil and heat is absorbed from the air. This colder air is then blown into the room that is being cooled. The now-gaseous refrigerant continues through the system to the accumulator, which ensures that it is entirely gaseous, because otherwise the compressor would seize up (a gas can be compressed, but liquid cannot).

You can see the actual cooling that is associated with air conditioners occurs as air is blown by the fan over the evaporator. This is also where the dehumidification takes place. Most air has some degree of water vapor in it, and when the warm moist air is blown over the evaporator, which is cold, the water vapor condenses into liquid water. This water is usually run off to the outside, although some air conditioners use it to help cool the compressor before disposing of it.
Something else to note is that the air conditioner fan can certainly run without the compressor. This promotes air circulation, but does not really cool.

Another important part of an air conditioner is the thermostat. The thermostat is the device that tells the air conditioner when to turn the compressor on and off. The thermostat senses the ambient temperature in the room and shuts the compressor off if it is cold enough and turns the compressor back on when it is too hot.