GasesAir is a common form of a coolant.
Air cooling uses either
convective airflow (passive cooling), or a forced circulation using
fans.
Hydrogen is used as a high-performance gaseous coolant. Its
thermal conductivity is higher than of all gases, it has high
specific heat capacity, and low
density and therefore low
viscosity, which is an advantage for rotary machines susceptible to
windage losses.
Hydrogen-cooled turbogenerators are currently the most common electrical generators in large power plants.
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.
LiquidsThe 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
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 therefore not toxic or difficult to dispose of ecologically.
[1]Very pure
deionized water, due to its relatively low
electrical conductivity, is used to cool some electrical equipment, often high-power transmitters and high-power
vacuum tubes.
Heavy water is used in some nuclear reactors; it also serves as a
neutron moderator.
Polyalkylene Glycol or PAG's are used as high temperature, thermally stable heat transfer fluids exhibiting strong resistance to oxidation. Modern PAG's 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 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]
Mineral oils serve as both coolants and lubricants in many mechanical gears.
Castor oil is 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.
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.
Freons were frequently used for
immersive cooling of e.g. 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.
Molten metals and saltsLiquid
fusible alloys can be used as coolants in applications where high temperature stability is required, e.g. some
fast breedernuclear reactors.
Sodium (in
sodium cooled fast reactors) 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 e.g.
lead cooled fast reactors, or a lead-
bismuth alloy. Some early
fast neutron reactors used
mercury.
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.
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-NaBF
4). Other choices are
FLiBe and
FLiNaK.
Liquid gasesLiquified gases are used as coolants for
cryogenic applications, including
cryo-electron microscopy,
overclocking of computer processors, applications using
superconductors, or extremely sensitive
sensors and very low-
noiseamplifiers.
Carbon Dioxide (chemical formula is
CO2) - is used as a coolant replacement
[4] for cutting fluids. CO2 can provide controlled cooling at the cutting interface such that the cutting tool and the workpiece are held at ambient temperatures. The use of CO2 greatly extends tool life, and on most materials allows the operation to run faster. This is considered a very environmentally friendly, especially when compared to the use of petroleum oils as a lubricant; parts remain clean and dry which often can eliminate secondary cleaning operations.
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.
Liquid hydrogen at -250 to -265 °C can also be used as a coolant. Liquid hydrogen is also used both as a
fuel and as a coolant to cool
nozzles and
combustion chambers of
rocket engines.
NanofluidsAn emerging and new class of coolants are
nanofluids which consist of a carrier liquid, such as water, dispersed with tiny nano-scale particles known as
nanoparticles. Purpose-designed nanoparticles of e.g.
CuO,
alumina,
titanium dioxide,
carbon nanotubes,
silica, or metals (e.g.
copper, or
silvernanorods) dispersed into the carrier liquid the enhances the heat transfer capabilities of the resulting coolant compared to the carrier liquid alone.
[5] The enhancement can be theoretically as high as 350%. The experiments however did not prove so high thermal conductivity improvements, but found significant increase of the
critical heat flux of the coolants.
[6]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%.
[7] 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.
[8]SolidsIn 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
spaceflight, for
ablative atmospheric reentry shields and for cooling of
rocket engine nozzles. The same approach is also used for fire protection of structures, where ablative coating is applied.
Dry ice and
water ice can be also used as coolants, when in direct contact with the structure being cooled.
Sublimation of water ice was used for cooling the
space suits of astronauts in the
Project Apollo.
See alsoReferences
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