How Does a Window Air Conditioner Work?
This may come as a surprise, but the main purpose of your air conditioner, as envisioned by its inventor, Willis B. Carrier, is to regulate humidity. To satisfy Carrier's definition of an air conditioner, the appliance should also ventilate and clean the air as well as cool it. Your window air conditioner accomplishes all of these functions by employing a refrigeration system similar to the one in your refrigerator, but the systems are different.
The Refrigeration Cycle
When you turn on your window air conditioner, most of the electricity goes to the compressor, which condenses an inert fluid refrigerant and pumps it through a system of coils. After passing through the condensing coil, the fluid passes through a pinhole aperture into the evaporator coil. The reduced vapor pressure in this coil causes the liquid to vaporize, an endothermic process that draws heat energy from the surrounding air. The rapid cooling condenses moisture out of the air, which collects on the coil and drains away to the outside. Meanwhile, the vaporized coolant continues its journey through the evaporator coil back to the compressor, which condenses it back into a fluid again to begin the cycle anew.
Fans and a Filter
For the air conditioner to efficiently exchange heat between the inside of the building and the outside, it must have a fan to actively dissipate heat from the condenser coils. You can easily feel this heat, generated by the condensation of the coolant, emerging through the outside grill when the machine is running. Another fan draws air from inside the building and directs it across the evaporator coil. Before it reaches the coil, the air passes through a filter that removes particulate matter. The filter serves two purposes: It protects the machinery inside the air conditioner and it provides cleaner air to breathe.
The Dry Setting
Despite the importance that Carrier placed on humidity regulation, most modern air conditioners primarily regulate temperature, and humidity regulation is incidental. Because air conditioners can't actually regulate both temperature and humidity efficiently at the same time, most units have an energy-saving dry mode that enhances dehumidification. When you select this mode, the fan circulating air around the evaporator coils slow down, allowing more moisture to drip off the coils and drain away. When you select the cool mode, on the other hand, the fan operates at full speed and blows some of the moisture dripping from the coils back into the room along with the cool air.
Getting the Most from an Air Conditioner
The filter through which the air passes on its way to the evaporator coil needs to be cleaned or replaced regularly, or the airflow will decrease and the air conditioner won't cool efficiently. Most machines also have an internal vent that you can open or close. When this vent is closed, the machine recirculates air from inside the room -- this is the best setting to use when starting the unit, because it will cool the air faster. After the machine has been running for a while, open the vent to allow fresh air into the room along with the recirculated, cooled air coming from the evaporator coil.