Coolant

A coolant is a substance, typically liquid, that is used to reduce or regulate the temperature of a system. An ideal coolant has high

While the term "coolant" is commonly used in automotive and

Sulfur hexafluoride is used for cooling and insulating of some high-voltage power systems (circuit breakers, switches, some transformers, etc.).

Steam can be used where high specific heat capacity is required in gaseous form and the corrosive properties of hot water are accounted for.

Two-phase

Some coolants are used in both liquid and gas form in the same circuit, taking advantage of the high specific latent heat of boiling/condensing phase change, the enthalpy of vaporization, in addition to the fluid's non-phase-change heat capacity.

Heat pipes are a special application of refrigerants.

Water is sometimes employed this way, e.g. in boiling water reactors. The phase change effect can be intentionally used, or can be detrimental.

Phase-change materials use the other phase transition between solid and liquid.

Liquid gases may fall here, or into refrigerants, as their temperature is often maintained by evaporation. Liquid nitrogen is the best known example encountered in laboratories. The phase change may not occur at the cooled interface, but on the surface of the liquid, to where the heat is transferred by convective or forced flow.

Liquids

Water is the most common coolant. Its high heat capacity and low cost make it a suitable heat-transfer medium. It is usually used with additives, like corrosion inhibitors and antifreeze. Antifreeze, a solution of a suitable organic chemical (most often ethylene glycol, diethylene glycol, or propylene glycol) in water, is used when the water-based coolant has to withstand temperatures below 0 °C, or when its boiling point has to be raised. Betaine is a similar coolant, with the exception that it is made from pure plant juice, and is not toxic or difficult to dispose of ecologically.^[1]

• Very pure
  electrical conductivity, is used to cool some electrical equipment, often high-power transmitters and high-power vacuum tubes
  .
• pressurised water reactors the most common type, use ordinary (light) water. Some designs, e.g. CANDU reactor
  , use both types; heavy water in the nonpressurized calandria tank as the moderator and a supplementary coolant, and light water as the primary heat transfer fluid.

Polyalkylene glycol (PAG) is used as high temperature, thermally stable heat transfer fluids exhibiting strong resistance to oxidation. Modern PAGs can also be non-toxic and non-hazardous.^[2]

Cutting fluid is a coolant that also serves as a lubricant for metal-shaping machine tools.

Oils are often used for applications where water is unsuitable. With higher boiling points than water, oils can be raised to considerably higher temperatures (above 100 degrees Celsius) without introducing high pressures within the container or loop system in question.^[3] Many oils have uses encompassing heat transfer, lubrication, pressure transfer (hydraulic fluids), sometimes even fuel, or several such functions at once.

• Mineral oils serve as both coolants and lubricants in many mechanical gears. Some vegetable oils, e.g. castor oil are also used. Due to their high boiling points, mineral oils are used in portable electric radiator-style space heaters in residential applications, and in closed-loop systems for industrial process heating and cooling. Mineral oil is often used in submerged PC systems as it is non-conductive and therefore won't short circuit or damage any parts.
  • Polyphenyl ether oils are suitable for applications needing high temperature stability, very low volatility, inherent lubricity, and/or radiation resistance. Perfluoropolyether oils are their more chemically inert variant.
  • A eutectic mixture of diphenyl ether (73.5%) and biphenyl (26.5%) is used for its wide temperature range and stability to 400°C.
  • Polychlorinated biphenyls and polychlorinated terphenyls were used in heat transfer applications, favored due to their low flammability, chemical resistance, hydrophobicity, and favorable electrical properties, but are now phased out due to their toxicity and bioaccumulation.
• Silicone oils and fluorocarbon oils (like fluorinert) are favored for their wide range of operating temperatures. However their high cost limits their applications.
• Transformer oil is used for cooling and additional electric insulation of high-power electric transformers. Mineral oils are usually used. Silicone oils are employed for special applications. Polychlorinated biphenyls were commonly used in old equipment, which now can possess risk of contamination.

Fuels are frequently used as coolants for engines. A cold fuel flows over some parts of the engine, absorbing its waste heat and being preheated before combustion. Kerosene and other jet fuels frequently serve in this role in aviation engines. Liquid hydrogen is used to cool nozzles of rocket engines.

Waterless coolant is used as an alternative to conventional water and ethylene glycol coolants. With higher boiling points than water (around 370F), the cooling technology resists boil over. The liquid also prevents corrosion. ^[4]

Freons were frequently used for immersive cooling of e.g. electronics.

Molten metals and salts

Liquid

For certain applications the stems of automotive poppet valves may be hollow and filled with sodium to improve heat transport and transfer.

For very high temperature applications, e.g.

Liquid nitrogen, which boils at about -196 °C (77K), is the most common and least expensive coolant in use. Liquid air is used to a lesser extent, due to its liquid oxygen content which makes it prone to cause fire or explosions when in contact with combustible materials (see oxyliquits).

Lower temperatures can be reached using liquified neon which boils at about -246 °C. The lowest temperatures, used for the most powerful superconducting magnets, are reached using liquid helium.

Some significant improvements are achievable; e.g. silver nanorods of 55±12 nm diameter and 12.8 µm average length at 0.5 vol.% increased the thermal conductivity of water by 68%, and 0.5 vol.% of silver nanorods increased thermal conductivity of ethylene glycol based coolant by 98%.^[9] Alumina nanoparticles at 0.1% can increase the critical heat flux of water by as much as 70%; the particles form rough porous surface on the cooled object, which encourages formation of new bubbles, and their hydrophilic nature then helps pushing them away, hindering the formation of the steam layer.^[10] Nanofluid with the concentration more than 5% acts like non-Newtonian fluids.

Solids

In some applications, solid materials are used as coolants. The materials require high energy to vaporize; this energy is then carried away by the vaporized gases. This approach is common in

Dry ice and water ice can be also used as coolants, when in direct contact with the structure being cooled. Sometimes an additional heat transfer fluid is used; water with ice and dry ice in acetone are two popular pairings.

References

External links

Look up coolant in Wiktionary, the free dictionary.

• CO₂ as a natural coolant — FAQs