All residential window air conditioners have a cooling system made up of four primary components, a compressor, an evaporator, a metering device, and a condenser. Air conditioner cooling systems are better understood if you think of them as devices that remove warmth from the air rather than cooling the air.
The compressor is the motor (or engine) of the cooling system. It is normally located in the middle of the air conditioner. It is almost always black in color and about the size of a football. The compressor is running whenever the air conditioner selector switch is set to "cool" and the thermostat is calling for a colder room temperature. It is normally very quiet. When running, it is compressing a refrigerant from a low pressure gas to a high pressure gas. The high pressure gas is then cooled to a liquid inside the condenser. The liquid then travels through the condenser.
The condenser is a series of tubes with fins attached to them, similar to a radiator. It is always located on the back of the air conditioner. The condenser appears as a set of silver fins mounted to the back of the air conditioner. In its liquid state the refrigerant gives off heat. This heat is conducted away from the refrigerant tubes by the silver fins. Once the liquid refrigerant has traveled the length of the tubes it is forced through a tiny copper tube called a capillary tube, which is the metering device. The capillary tube is attached from the end of the condenser to the beginning of the evaporator.
The evaporator is always located on the front of the air conditioner. It also has silver fins. When the liquid refrigerant comes out of the small capillary tube it is injected into the larger tubes of the evaporator. When the liquid enters the larger tubes it is allowed to expand back into a gaseous state. In its gaseous state the refrigerant absorbs heat. The gaseous refrigerant travels through the evaporator tubes back to the compressor. Because the evaporator is absorbing heat, it is very cold to the touch. The temperature drop causes any humidity in the air to collect on the evaporator - sometimes called sweat. There is a fan inside the air conditioner that circulates the air for the evaporator and condenser coils.
The metering device is usually a small tube between the condenser outlet and evaporator inlet. The length and diameter determine how much refrigerant will be allowed into the evaporator or cooling coil. On larger systems this can be a valve that opens and closes according to the temperature of the outlet of the cooling coil, which ensures the proper flow of refrigerant to the coil.
When the unit is running, the circulating fan and compressor are running simultaneously. The fan motor has two fan blades attached to it on either end. The fan blade on the inside part of the unit continually draws room air over the evaporator coils, which are cold. The fan blade on the outside part of the unit continually draws fresh outside air over the condenser coils, which are warm. Because the evaporator coils are cold, they cause moisture in the room to collect on them, much like a cup of ice water on a warm, humid day. When the amount of moisture increases, it begins to drip down off of the coils into the bottom pan of the air conditioner.
The thermostat on a window air conditioner works by sensing the air temperature entering the air conditioner. As the air entering the unit reaches the set temperature it will cause the compressor to turn off. The blower may continue to run depending on the selection chosen on the control panel. Digital thermostats work on a similar principle but display a more precise temperature.
The air conditioner selector switches allow the user to choose the fan speed. The compressor always runs at the same speed regardless of the settings. If low cool is chosen, for example, the fan runs at a slower speed but the compressor still offers the same cooling capacity. There are other switches to control louver operation and other features on some units.
