Coolant is a substance that comes in liquid or gas form and used in regulating or reducing temperature of the system. The best coolants you can get is something that have low viscosity, low-cost, chemically inert, non toxic, has high thermal capacity and not promoting or causing corrosion of the cooling system. Some applications need the coolant to be electrical insulator too.
When the term coolant is used, we often associate it to automotive as well as HVAC applications but industrial coolant is a lot different since it’s more of a technical aspect and used generally in low and high temperature manufacturing applications. The term is covering cutting fluids at the same time. Coolants might be able to stay on its phase, gaseous or liquid or it can even undergo a transition with latent heat that adds to the efficiency of cooling. When the latter is utilized for achieving below ambient temperature, it’s automatically called as refrigerant.
A common type of coolant in the market is air. Air cooling is making use of either forced circulation by using fans or convective airflow or otherwise known as passive cooling. For the high performance gaseous coolant, hydrogen is what being used most of the time. Because thermal conductivity is higher in comparison to other gases, it is expected to have low density and high specific heat density allowing it to have low viscosity which many rotary machines are taking advantage of due to its vulnerability to windage loss. Hydrogen cooled turbo generators are widely used for electrical generators in big power plants at the moment.
As for nuclear reactors that are gas cooled, what is used in this purpose is inert gases. The helium has low tendency of absorbing neutrons and becoming radioactive. Actually, carbon dioxide is also used for mixing it with AGR reactors and Magnox. Sulfur hexafluoride on the other hand is being used for cooling and insulating high voltage power systems similar to switches, transformers, circuit breakers and so on. Steam can also be used where there is high specific heat capacity required in gaseous form.
A popular type of coolant used is water. The low cost and high heat capacity it has makes it ideal for heat transfer medium. Usually, it’s used with additives similar to antifreeze as well as corrosion inhibitors. In most cases, diethylene glycol, propylene glycol or ethylene glycol mixed in water is the components of antifreeze. A similar coolant is betain but with an exception that it’s manufactured using pure plant juice. Pure de-ionized water is used for cooling electrical equipment due to its low electrical conductivity.