Air cooling is a method of dissipating heat. It works by making the object to be cooled have a larger surface area or have an increased flow of air over its surface, or both. An example of the former is to add fins to the surface of the object, either by making them integral or by attaching them tightly to the object's surface (to ensure efficient heat transfer). In the case of the latter it is done by using a fan blowing air into or onto the object one wants to cool. In many cases the addition of fins adds to the total surface area making a heatsink that makes for greater efficiency in cooling.
In all cases, the air has to be cooler than the object or surface from which it is expected to remove heat. This is due to the second law of thermodynamics, which states that heat will only move spontaneously from a hot reservoir (the heat sink) to a cold reservoir (the air).
Air is mainly used for air-cooled internal combustion engines (ICE), particularly those powering aircraft, because it is a readily available fluid and is often at a suitable temperature to be used efficiently. While many such ICE are called "liquid cooled" the cooling liquid is usually cooled by air passing through a radiator or heat exchanger. Examples of direct air cooling in modern automobiles are rare. The most common example is the Flat engine or Boxer engine, once used extensively by Porsche and still in use on BMW motorcycles. Notable past models include the Volkswagen Beetle and related models, the Citroën 2CV, the Chevrolet Corvair and the Porsche 911 until 1998.
Gas turbine engines (e.g. turbojets, turbofans, etc.) incorporate turbines, which are exposed to the hot gases exiting the combustion chamber. Where necessary, relatively cold air is bled from the compression system and used to cool the turbine blades and vanes, to prevent them from melting.
Derating at high altitude
When operating in an environment with lower air pressure like high altitude or airplane cabins. The cooling capacity has to be derated compared to that of sea level.
A rule-of-thumb formula 1 - (h/17500) = derating factor. Where h is the height over sea level in meters. And the result is the factor that should be multiplied with the cooling capacity in [W] to get the cooling capacity at the specified height over sea level.