An air conditioner is basically a refrigerator without the insulated box. It uses the evaporation of a refrigerant, R407C, to provide cooling.

This is how the evaporation cycle in an air conditioner works:

1. The compressor compresses the refrigerant, causing it to become hot, high-pressure gas (red in the diagram above).

2. This hot gas runs through a set of coils so it can dissipate its heat, and it condenses into a liquid.

3. The refrigerant liquid runs through an expansion valve, and in the process it evaporates to become cold, low-pressure refrigerant gas (light blue in the diagram above).

4. This cold gas runs through a set of coils that allow the gas to absorb heat and cool down the air inside the building.

Mixed in with the refrigerant is a small amount of a lightweight oil. This oil lubricates the compressor.

A window air conditioner unit implements a complete air conditioner in a small space. The units are made small enough to fit into a standard sash window frame. You close the window down on the unit, plug the unit in and turn it on to get cool air. Alternatively they can be placed through a hole in a wall. If you take the cover off of an unplugged window unit, you will find that it contains:

A compressor
An expansion valve
A hot coil (on the outside)
A chilled coil (on the inside)
Two fans
A control unit