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A coolant is a substance, typically liquid, that is used to reduce or regulate the temperature of a system. 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. Some applications also require the coolant to be an electrical insulator.
However, once water boils, it is an insulator, leading to a sudden loss of cooling where steam bubbles form. Steam may return to water as it mixes with other coolant, so an engine temperature gauge can indicate an acceptable temperature even though local temperatures are high enough that damage is being done. An engine needs different temperatures.
Once the coolant reaches the thermostat's activation temperature, it opens, allowing water to flow through the radiator to prevent the temperature from rising higher. Once at optimum temperature, the thermostat controls the flow of engine coolant to the radiator so that the engine continues to operate at optimum temperature.
Proper engine coolant and a pressurized coolant system obviate these shortcomings of water. With proper antifreeze, a wide temperature range can be tolerated by the engine coolant, such as −34 °F (−37 °C) to +265 °F (129 °C) for 50% (by volume) propylene glycol diluted with distilled water and a 15 psi pressurized coolant system.
Once the optimum operating temperature is reached, the thermostat progressively increases or decreases its opening in response to temperature changes, dynamically balancing the coolant recirculation flow and coolant flow to the radiator to maintain the engine temperature in the optimum range as engine heat output, vehicle speed, and outside ...
A hospital in Sweden relies on snow-cooling from melt-water to cool its data centers, medical equipment, and maintain a comfortable ambient temperature. [44] Some nuclear reactors use heavy water as coolant. Heavy water is employed in nuclear reactors because it is a weaker neutron absorber. This allows for the use of less-enriched fuel.
Coolant oil may be limited to cooling objects under approximately 200–300 °C, otherwise the oil may degrade and even leave ashy deposits. Pure water may evaporate or boil, but it cannot degrade, although it may become polluted and acidic. Water is generally available should coolant need to be added to the system, but oil may not be.
Both flasks are submerged in a dry ice/acetone cooling bath (−78 °C) the temperature of which is being monitored by a thermocouple (the wire on the left). A cooling bath or ice bath , in laboratory chemistry practice, is a liquid mixture which is used to maintain low temperatures, typically between 13 °C and −196 °C.