Coolant is a substance that comes in liquid or gas form and used in regulating or reducing temperature of the system. The ideal coolant is something that has low viscosity, low-cost, chemically inert, non toxic, has high thermal capacity and neither promotes nor cause corrosion of 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. As for the coolants, it can actually stay on its phase, gaseous or liquid and at times, it undergoes a transition with latent heat which then adds the cooling efficiency. Now say for example that the latter is put into use to achieve below ambient temperature, then it is known as refrigerant.
One typical type of coolant that is used today is air. Air cooling is making use of either forced circulation by using fans or convective airflow or otherwise known as passive cooling. Hydrogen is being used as well for high performance gaseous coolant. Thermal conductivity is higher in comparison to other gases because it has low density and high specific heat density and for that, it’s low viscosity that is a big benefit for systems using rotary machines that are susceptible to windage losses. The hydrogen cooled turbo generators are at the moment most widely used electrical generator in big power plants.
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. Carbon dioxide is actually used to be mixed with AGR reactors and Magnox. In regards to sulfur hexafluoride however, this is utilized to insulate and even cool high voltage power systems; to give you examples, it can be switches, transformers, circuit breakers and so on. Whenever there is a need for high specific heat capacity in gaseous form, then expect that steam is what used.
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. Antifreeze solution is made out of suitable organic chemicals oftentimes diethylene glycol, propylene glycol or ethylene glycol in water. A similar coolant is betain but with an exception that it’s manufactured using pure plant juice. The pure de-ionized water is utilized to cool electrical equipment because of its low electrical conductivity.