Coolant

Coolant are gaseous, liquid or solid substances or mixtures of substances which are used for the removal of 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 may be even higher than the temperature of the cooled object, while a coolant is only capable, in a refrigeration cycle, the enthalpy along transport of the temperature gradient to a location of lower temperature.

The preamble heat carrier comprises besides coolant and heating ( heating medium ) or cold vehicle; the assignment is not always clear and is made on the application and temperature range. In technological processes predominantly water, air, or thermal oil is used as the coolant or heat transfer fluid. The economic sense bridgeable differences in temperature between ambient and coolant amount for air cooling about 40 Kelvin, with water cooling about 10 Kelvin.

Coolant temperatures below the ambient temperature can only be operated by using suitable refrigerants refrigeration processes, also called links processes, to achieve. With freezing mixtures is environment independent coolant temperatures down to below -50 ° C can be achieved. With dry even up to -100 ° C ( with ether).

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

In metal cutting, cooling lubricants are used to prevent excessive heating of the tool or the material to be processed by the friction.

Coolants

Coolants can cool directly or via a heat exchanger, the cooling material.

• Direct contact, such as: Shower in the beach, the body can reach 37 ° C at the surface, with water from the shower of 20 ° C temperature, the skin is cooled noticeably well, where the water appears fresh soon to only slightly above the original water temperature. While the water entlangrinnt the body heats it by the body heat extracted. Now wind blows additionally the wetted skin, evaporation of water can generate additional heat dissipation and thus further lowering the temperature, if the incoming air is at most equal warm as the skin and not saturated with water vapor.
• Quenching of hot metal in a water or oil bath ( for example, the curing for a suitable steel).

• Engine coolant circulation. Air flows through the metal, increased with corrugated fins in the surface of the radiator. Here, the ambient air, the cooling medium. The so down -cooled circulating fluid (water -glycol mixture ) is cooled in a driven with a water pump circuit, the hot parts of the engine down (especially the cylinder head).
• Nuclear reactors: in most reactors, the heat generated is dissipated by water. When pressurized water reactor of this ( radioactively contaminated ) primary circuit is cooled via heat exchanger of a non-radioactive secondary circuit.

Cooling water

Water is the refrigerant most commonly used. In the once-through cooling the heated water is discharged after one use. In the cooling circulation, the cooling water flowing in a cooling circuit and serves for the transport of heat energy by convection. In both cases we speak of water cooling.

A known application is the refrigeration of the thermodynamic cyclic processes for the production of mechanical energy in power plants or internal combustion engines.

Other examples are assemblies of power electronics, high power laser, transmitter tubes and magnetrons.

Cooling water is available in many different configurations, depending on the location and area. Contaminants of components of the plants by the cooling water can be prevented by the use of deionized water or additives. The growth of microorganisms is called fouling, it can be prevented by additives and / or opaque lines and tanks.

The maximum value allowed for back -led to water cooling water in France is 26 degrees. At water temperatures of 28 degrees, the oxygen content decreases so much that the fish threatens suffocation.

Often the cooling water is added in a closed cooling circuits antifreeze to prevent freezing in winter.

In open cooling water circuits can be water to be cooled and the cooling air to reach even lower temperatures by evaporative cooling and intensive mixing of. For example, the cooling towers of power plants, chillers for ice rinks.

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

Molten Metals

In nuclear reactors, partly sodium or NaK -78 ( eutectic mixture of 22 % sodium and 78 % potassium) is used in the coolant circuit. The coolant has good heat transfer properties, and a large temperature range usable. Sodium melts at 98 ° C, but not boil below 883 ° C. NaK -78 melts at -12.6 ° C and boils at 785 ° C.

It is also believed that the Soviet Union Komsomolets in the atom - hunting submarine, the only ship of the class, Mike, had built a reactor was cooled with a lead -bismuth mixture. Since 1989 this ship lies at the bottom of the North Sea, after there had been a fire in the reactor chamber.

Oil cooling is used where particularly good electrical insulating properties are required. This is the case, for example, in X-ray tubes, large transformers, but also large capacitors and power switches. Oil, not having the high heat capacity such as water, and is also rather higher viscosity, but has a higher boiling point.

Formerly also polychlorinated biphenyls (PCBs ) have been used (for example, dichlorophenyl) as cooling oil in transformers and power capacitors. You have excellent insulating properties and are also flammable. Today their use is banned because of their toxicity.

As PCB substitutes, but also as a replacement for the classic mineral oil, for more than 25 years, synthetic organic esters are used in transformers. These are saturated Pentaerythrittetrafettsäureester which are distinguished by a high point (> 300 ° C), flash point (> 250 ° C) and a high ignition temperature ( > 375 ° C) and thus distinguished as PCBs are nonflammable. In addition, the ester fluid is non-toxic, biodegradable and easily classified as not hazardous for water. The insulating properties are comparable to those of mineral oil, which is classically used as insulating transformers, and to those of the PCB.

Alcohol

In aircraft to its low viscosity and entfallendem fouling due to its frost resistance, partially pure ethanol used as a cooling liquid.