A fan is a machine used to create flow within a fluid, typically a gas such as air. The fan consists of a rotating arrangement of vanes or blades which act on the fluid. The rotating assembly of blades and hub is known as an impeller, a rotor, or a runner. Usually, it is contained within some form of housing or case. This may direct the airflow or increase safety by preventing objects from contacting the fan blades. Most fans are powered by electric motors, but other sources of power may be used, including hydraulic motors and internal combustion engines. Fans produce flows with high volume and low pressure (although higher than ambient pressure), as opposed to compressors which produce high pressures at a comparatively low volume. A fan blade will often rotate when exposed to a fluid stream, and devices that take advantage of this, such as anemometers and wind turbines, often have designs similar to that of a fan.
Typical applications include climate control and personal thermal comfort (e.g., an electric table or floor fan), vehicle and machinery cooling systems, ventilation, fume extraction, winnowing (e.g., separating chaff of cereal grains), removing dust (e.g. in a [vacuum cleaner]), drying (usually in combination with heat) and to provide draft for a fire. While fans are often used to cool people, they do not actually cool air (if anything, electric fans warm it slightly due to the warming of their motors), but work by evaporative cooling of sweat and increased heat convection into the surrounding air due to the airflow from the fans. Thus, fans may become ineffective at cooling the body if the surrounding air is near body temperature and contains high humidity.