coolant

A coolant, or heat transfer fluid, is a fluid which flows through a device in order to prevent its overheating, transferring the heat produced by the device to other devices that utilize or dissipate it. An ideal coolant has high thermal capacity, low viscosity, is low-cost, and is chemically inert, neither causing nor promoting corrosion of the cooling system. Some applications also require the coolant to be an electrical insulator.

The coolant can either keep its phase and stay liquid or gaseous, or can undergo a phase change, with the latent heat adding to the cooling efficiency. The latter, when used to achieve low temperatures, is more commonly known as refrigerant.

Gases

Air is a common form of a coolant. Air cooling uses either convective airflow (passive cooling), or a forced circulation using fans.

Inert gases are frequently used as coolants in gas-cooled nuclear reactors. Helium is the most favored coolant due to its low tendency to absorb neutrons and become radioactive. Nitrogen and carbon dioxide are frequently used as well.

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.

Liquids

The most common coolant is water. Its high heat capacity and low cost makes it a suitable heat-transfer medium. It is usually used with additives, like corrosion inhibitors and antifreezes. 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.

Very pure deionized water, due to its relatively low electrical conductivity, is used to cool some electrical equipment, often high-power transmitters.

Heavy water is used in some nuclear reactors; it also serves as a neutron moderator.

Oils are used for applications where water is unsuitable.

• Mineral oils serve as both coolants and lubricants in many mechanical gears. Castor oil is also used.
• Silicone oils are favored for their wide range of operating temperatures. However their high cost limits their applications.
• Fluorocarbon oils are used for the same reasons.
• High-power electric transformers use transformer oil for cooling and additional electric insulation.

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

Liquid fusible alloys can be used as coolants in applications where high temperature stability is required, eg. some fast breeder nuclear reactors. Sodium or sodium-potassium alloy NaK are frequently used; in special cases lithium can be employed. Another liquid metal used as a coolant is lead, in eg. lead cooled fast reactors, or a lead-bismuth alloy. Some early fast neutron reactors used mercury.

For very high temperature applications, eg. molten salt reactors or very high temperature reactors, molten salts can be used as coolants. One of the possible combinations is the mix of sodium fluoride and sodium tetrafluoroborate (NaF-NaBF₄).

Freons were frequently used for immersive cooling of eg. electronics.

Refrigerants are coolants used for reaching low temperatures by undergoing phase change between liquid and gas. Halomethanes were frequently used, most often R-12 and R-22, but due to environmental concerns are being phased out, often with liquified propane or other haloalkanes like R-134a. Anhydrous ammonia is frequently used in large commercial systems, and sulfur dioxide was used in early mechanical refrigerators. Carbon dioxide (R-744) is used as a working fluid in climate control systems for cars, residential air conditioning, commercial refrigeration, and vending machines.

Heat pipes are a special application of refrigerants.

Liquid gases are used as coolants for cryogenic applications, namely applications using superconductors, or extremely sensitive sensors and very low-noise amplifiers. The most common and least expensive coolant in use is liquid nitrogen. Liquid air is used to lower degree, due to its oxygen content which makes it prone to exploding in contact with combustible materials. Lower temperatures can be reached using liquefied neon. The lowest temperatures, used for the most powerful superconducting magnets, are reached using liquid helium.

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 and chambers of rocket engines.

See also

• Loss of coolant — a type of a nuclear reactor failure

External links

• CO₂ as a natural coolant — FAQs

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