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. Liquid nitrogen is the best known example encountered in laboratories. Liquid gases may fall here, or into refrigerants, as their temperature is often maintained by evaporation. Phase-change materials use the other phase transition between solid and liquid. The phase change effect can be intentionally used, or can be detrimental. Water is sometimes employed this way, e.g. Heat pipes are a special application of refrigerants. Many otherwise excellent refrigerants are phased out for environmental reasons (the CFCs due to ozone layer effects, now many of their successors face restrictions due to global warming, e.g. Carbon dioxide (R-744) is used as a working fluid in climate control systems for cars, residential air conditioning, commercial refrigeration, and vending machines. Anhydrous ammonia is frequently used in large commercial systems, and sulfur dioxide was used in early mechanical refrigerators. Halomethanes were frequently used, most often R-12 and R-22, often with liquified propane or other haloalkanes like R-134a. Refrigerants are coolants used for reaching low temperatures by undergoing phase change between liquid and gas. 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. Steam can be used where high specific heat capacity is required in gaseous form and the corrosive properties of hot water are accounted for. Sulfur hexafluoride is used for cooling and insulating of some high-voltage power systems ( circuit breakers, switches, some transformers, etc.). Carbon dioxide is used in Magnox and AGR reactors. Helium has a low tendency to absorb neutrons and become radioactive. Inert gases are used as coolants in gas-cooled nuclear reactors. Hydrogen-cooled turbogenerators are currently the most common electrical generators in large power plants. Its thermal conductivity is higher than all other gases, it has high specific heat capacity, low density and therefore low viscosity, which is an advantage for rotary machines susceptible to windage losses. Hydrogen is used as a high-performance gaseous coolant. Air cooling uses either convective airflow (passive cooling), or a forced circulation using fans. The latter, when used to achieve below- ambient temperature, is more commonly known as refrigerant.Īir is a common form of a coolant. This coolant can either keep its phase and stay liquid or gaseous, or can undergo a phase transition, with the latent heat adding to the cooling efficiency. It has varying oil content from nil oil (synthetic coolant). Water-soluble coolant is oil in water emulsion. Industrial cutting fluid has broadly been classified as water-soluble coolant and neat cutting fluid. While the term "coolant" is commonly used in automotive and HVAC applications, in industrial processing heat-transfer fluid is one technical term more often used in high temperature as well as low-temperature manufacturing applications. Some applications also require the coolant to be an electrical insulator. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, chemically inert and neither causes nor promotes corrosion of the cooling system. Substance used to reduce or regulate the temperature of a systemĪ coolant is a substance, typically liquid, that is used to reduce or regulate the temperature of a system.
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