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Evaporative CoolingEvaporative cooling is a system in which latent heat of evaporation is used to carry heat away from an object to cool it. The latent heat contains a considerable amount of energy, and carries away more heat than if the same temperature liquid was simply removed physically. The simplest example would be sweat, which the body secretes in order to cool itself. The amount of heat transfer depends on the evaporation rate, which in turn depends on the humidity of the air and its temperature, which is why you sweat more on hot, humid days. Evaporative cooling was in vogue for aircraft designs for some time in the late 1930's. In this case the system was used in order to reduce, or eliminate completely, the radiator which would otherwise create considerable drag. In these systems the water in the engine was kept under pressure with pumps, allowing it to heat to temperatures above 100 Celsius, as the actual boiling point is based on the pressure. The super-heated water was then sprayed though a nozzle into an open tube, where it rapidly boiled and released its heat. The tubes could be placed under the skin of the aircraft, resulting in a zero-drag cooling system. However these systems also had serious disadvantages. Since the amount of tubing needed to cool the water was large, the cooling system covered a significant portion of the plane even though it was hidden. This led to all sorts of added complexity and the systems were always terribly unreliable. In addition this large size meant it was very easy for it to be hit by enemy fire, and practically impossible to armor. British and US attempts to use the system turned to ethylene glycol (Prestone) instead. The Germans instead used clever streamlining and positioning of traditional radiators. Even its most ardent supporters, Heinkel's Gnter brothers, eventually gave up on it in 1940. Evaporative cooling is commonly used in cryogenic applications. The vapor above a reservoir of cryogenic liquid is pumped away, and the liquid continuously evaporates as long as the liquid's vapor pressure is significant. Evaporative cooling of ordinary helium forms a 1-K pot, which can cool to at least 1.2 K. Evaporative cooling of helium-3 can provide temperatures below 300 mK. Each of these techniques can be used to make cryocoolers, or as components of lower-temperature cryostats such as dilution refrigerators. As the temperature decreases, the vapor pressure of the liquid also falls, and cooling becomes less effective. This sets a lower limit to the temperature attainable with a given liquid. External links
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