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Coolants are gaseous, liquid or solid substances or mixtures that are used to remove heat . The difference from the refrigerant is that a refrigerant in a refrigeration cycle can do so against a temperature gradient, so that the ambient temperature even may be higher than the than the temperature to the object of cooling, while a coolant is only capable, in a refrigeration cycle, the enthalpy along of the temperature gradient to a place of lower temperature.

In addition to coolants, the generic term heat transfer medium also includes heating medium (heating medium) or cooling medium; the assignment is not always clear and is made according to the application or temperature range. In technological processes, mainly water , air or thermal oil is used as a coolant or heat transfer medium. The economically sensible bridging temperature differences between the environment and the coolant are approx. 40 Kelvin with air cooling and approx. 10 Kelvin with water cooling.

Coolant temperatures below the ambient temperature can only be achieved by means of cooling processes operated with suitable refrigerants, also known as link processes. Coolant temperatures of down to below −50 ° C, independent of the environment, can be achieved with cold mixtures . With dry ice even down to −100 ° C (together with ether ).

Depending on the application, other substances such as liquid metals (sodium), salt solutions or liquefied gases (hydrogen, helium) are also used.

In metal cutting, cooling lubricants are used that prevent the tool or the material to be processed from being excessively heated by friction.

Coolants are disposed of in separate containers. The color of the collection container is beige.


Cooling liquids can cool the goods directly or via a heat exchanger .

  • direct contact, e.g. B .:
    • Quenching of hot metal in a water or oil bath (e.g. hardening of suitable steel ).
    • Cooling lubricants and cutting oil are used to cool the workpiece and cutting tool during machining (drilling, sawing, milling, turning).
    • Shower in the lido, the body can reach 37 ° C on the surface, with water from the shower at a temperature of 20 ° C, the skin is noticeably cooled, where the water is fresh soon to only a little above the original water temperature. As the water runs along the body, it warms up, heat extracted from the body. If wind now also brushes the wetted skin, evaporation of water can generate additional heat dissipation and thus further temperature reduction, provided that the incoming air is at most as warm as the skin and is not saturated with water vapor.
  • With heat exchanger: Separation of the material cycles through solid walls between the cooling liquid and the fluid to be cooled ( gas , liquid ), e.g. B .:
    • Internal combustion engine coolant circulation. Air flows through the metallic cooler, which is enlarged with corrugated fins in the surface . Here the ambient air is the cooling medium. The circulating liquid (water- glycol mixture) cooled in this way cools down the hot parts of the engine (especially the cylinder head ) in a circuit driven by a water pump .
    • Nuclear reactors : in most reactors, the heat generated is dissipated by water. In the pressurized water reactor , this (radioactively contaminated) primary circuit is cooled by a non-radioactive secondary circuit via heat exchangers.

cooling water

Water is the most common coolant used. With once-through cooling, the heated water is drained away after a single use. With circulation cooling, the cooling water flows in a cooling circuit and is used to transport thermal energy through convection . In both cases one speaks of water cooling .

A well-known application is the cooling of thermodynamic cycle processes for the production of mechanical energy in power plants or internal combustion engines .

Further examples are assemblies of power electronics , high-power lasers , transmitter tubes and magnetrons .

Cooling water is available in many different compositions, depending on the location and area of ​​use. The use of deionized water or additives prevents system contamination from components of the cooling water . The growth of microorganisms is called fouling and it can be prevented by additives and / or opaque pipes and tanks.

The maximum permissible value for cooling water returned to water bodies in France has been 28 degrees since 2006. At water temperatures of over 28 degrees, the oxygen content of the water is so low that the fish are threatened with suffocation.

Antifreeze is often added to the cooling water in closed cooling circuits to prevent it from freezing in winter, as is the case with cooling fluids in cars or solar fluids .

With open cooling water circuits, lower temperatures can also be achieved due to the evaporation cooling and more intensive mixing of the water to be cooled and the cooling air. Examples are the cooling towers of power plants, cooling machines for ice rinks.

Stationary internal combustion engines sometimes have a pressureless water jacket around the cylinder with an opening from which water evaporates and thus cools the engine ("evaporative cooling "). The evaporated water must be replaced regularly.

Liquid metals

In nuclear reactors, sodium or NaK-78 ( eutectic mixture of 22% sodium and 78% potassium ) is sometimes used in the coolant circuit. The coolant has good heat transfer properties and a wide usable temperature range. Sodium already melts at 98 ° C, but only boils at 883 ° C. NaK-78 melts at −12.6 ° C and boils at 785 ° C.


Oil cooling is used where particularly good electrical insulation properties are required. This is e.g. This is the case, for example, with X-ray tubes , large transformers , but also large capacitors and circuit breakers . Oil does not have the same high heat capacity as water and is also more viscous, but it has a higher boiling point.

In the past, polychlorinated biphenyls (PCB) (e.g. dichlorophenyl) were also used as cooling oils in transformers and power capacitors. They have excellent insulating properties and are also non-flammable. Today their use is banned due to their toxicity.

Synthetic organic esters have been used in transformers for over 25 years as a PCB replacement, but also as a replacement for classic mineral oil . These are saturated pentaerythritol tetra-fatty acid esters, which are characterized by a high fire point (> 300 ° C), flash point (> 250 ° C) and a high ignition temperature (> 375 ° C) and are therefore non-flammable like PCBs. In addition, the ester liquid is non-toxic, easily biodegradable and classified as not hazardous to water. The insulating properties are comparable with those of mineral oil, which is traditionally used as insulating oil in transformers, and with those of PCB .


In aircraft, due to its frost resistance, low viscosity and no fouling, pure ethanol is sometimes used as a coolant.

See also

Web links

Individual evidence

  1. La centrale nucléaire de Golfech stoppée par la caniculeentrale nucléaire de Golfech stoppée par la canicule