A coolant is any fluid used to regulate heat or lower temperatures of physical bodies or enclosed spaces. It is also known as thermal transfer fluid. The coolant can be a gas or liquid substances. As an aside, there are also solid coolants such as the ones found in the cooling systems of modern forms. These solid thermal transfer substances vaporize when exposed to heat, thereby following a fundamental rule in fluid mechanics, in which a quickly expanding fluid absorbs thermal energy from its immediate surroundings.
Among thermal transfer substances, gas coolants are the most widely used. Most people tend to think of coolants as substances flowing inside closed systems. There are open cooling systems that uses air as coolants. One example is the air cooling system found inside many computer cases. Air cooling can work through convective airflow, which means natural movement of air, or with the help of fans.
A popular gas coolant for high performance applications is hydrogen. Amongst gases, hydrogen has the best thermal conductivity and excellent specific heat capacity. This means it is quick to absorb heat and can absorb greater amount of heat. It also has low density to allow machines to operate with minimal hindrance. Many large power plants use hydrogen-cooled electrical generators.
Inert gases, helium, carbon dioxide, and sulfur hexafluoride are other gas coolants used for industrial applications.