In physics, chemistry, engineering, and thermodynamics, heat is energy produced or transferred from one body, region, set of components, or thermodynamic system to another in any way other than as thermodynamic work.
In ordinary language, as distinct from technical language, heat has a broader meaning. This can lead to confusion if the diversity of usage of words is forgotten.
Thermodynamically, energy can be produced or transferred as heat by thermal conduction, by thermal radiation, by friction and viscosity, and by chemical dissipation.
The engineering discipline of heat transfer recognizes heat transfer by thermal conduction, by convection, through mass transfer of fluid, and by thermal radiation.
Heat transfer by conduction and by radiation from a hotter to a colder body is spontaneous. The second law of thermodynamics requires that the transfer of energy from one body to another with an equal or higher temperature can only occur with the aid of a heat pump by mechanical work, or by some other similar process in which entropy is increased in the universe in a manner that compensates for the decrease of entropy in the cooled body, due to the removal of the heat from it. For example, energy may be removed against a temperature gradient by spontaneous evaporation of a liquid.
A mechanical fan is a machine used to create flow within a fluid, typically a gas such as air.
A fan consists of a rotating arrangement of vanes or blades which act on the air. 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 and solar power.
Fans produce air flows with high volume and low pressure, as opposed to compressors which produce high pressures at a comparatively low volume. A fan blade will often rotate when exposed to an air 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 conduction 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.