Nitrogen Dewars

Liquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density at its boiling point of 0.807 g/mL and a dielectric constant of 1.4. Liquid nitrogen is often referred to by the abbreviation, LN ₂ and has the UN number 1977.

At atmospheric pressure, liquid nitrogen boils at 77 K (−196 °C; −321 °F) and is a cryogenic fluid which can cause rapid freezing on contact with living tissue, which may lead to frostbite. When appropriately insulated from ambient heat, liquid nitrogen can be stored and transported, for example in vacuum flasks. Here, the very low temperature is held constant at 77 K by slow boiling of the liquid, resulting in the evolution of nitrogen gas. Depending on the size and design, the holding time of vacuum flasks ranges from a few hours to a few weeks.

Liquid nitrogen can easily be converted to the solid by placing it in a vacuum chamber pumped by a rotary vacuum pump. Liquid nitrogen freezes at 63 K (−210 °C; −346 °F). Despite its reputation, liquid nitrogen's efficiency as a coolant is limited by the fact that it boils immediately on contact with a warmer object, enveloping the object in insulating nitrogen gas. This effect, known as the Leidenfrost effect, applies to any liquid in contact with an object significantly hotter than its boiling point. More rapid cooling may be obtained by plunging an object into a slush of liquid and solid nitrogen than into liquid nitrogen alone.

Nitrogen was first liquefied at the Jagiellonian University on 15 April 1883 by Polish physicists, Zygmunt Wróblewski and Karol Olszewski.

Uses

Liquid nitrogen is a compact and readily transported source of nitrogen gas without pressurization. Further, its ability to maintain temperatures far below the freezing point of water makes it extremely useful in a wide range of applications, primarily as an open-cycle refrigerant, including:

• used to store cells at low temperature for laboratory work
• in cryogenics
• as a source of very dry nitrogen gas
• the immersion freezing and transportation of food products
• the cryopreservation of blood, reproductive cells (sperm and egg), and other biological samples and materials
• as a coolant for overclocking a central processing unit, a graphics processing unit, or another type of computer hardware
• as a method of freezing water pipes in order to work on them in situations where a valve is not available to block water flow to the work area.
• in cryotherapy for removing unsightly or potentially malignant skin lesions such as warts and actinic keratosis.
• in the process of promession, a way to dispose of the dead.
• cooling a high-temperature superconductor to a temperature sufficient to achieve superconductivity.
• the cryonic preservation of humans and pets in the hope of future reanimation.
• as a coolant for vacuum pump traps and in controlled-evaporation processes in chemistry.
• as a coolant to increase the sensitivity of infrared homing seeker heads of missiles such as the Strela 3.
• as a coolant to temporarily "shrink" mechanical components during machine assembly and allow improved interference fits.
• as a coolant for super computers

Safety

Since the liquid to gas expansion ratio of this substance is 1:694, a tremendous amount of force can be generated if liquid nitrogen is rapidly vaporized. In an incident in 2006 at Texas A&M University, the pressure-relief devices of a tank of liquid nitrogen were sealed with brass plugs. As a result, the tank failed catastrophically, and exploded. The force of the explosion was sufficient to propel the tank through the ceiling immediately above it.

Due to its extremely low temperature, careless handling of liquid nitrogen may result in cold burns. As liquid nitrogen evaporates it will reduce the oxygen concentration in the air and might act as an asphyxiant, especially in confined spaces. Nitrogen is odorless, colourless and tasteless, and may produce asphyxia without any sensation or prior warning.

Vessels containing liquid nitrogen can condense oxygen from air. The liquid in such a vessel becomes increasingly enriched in oxygen (boiling point = 90 K) as the nitrogen evaporates, and can cause violent oxidation of organic material.

See also

• Computer cooling
• Liquid nitrogen economy

References

1. ^ "Dielectric Constants" . http://www.apgsensors.com/ltr2/access.php?file=pdf/dielectric-constants.pdf .  
2. ^ Umrath, W. (1974) Cooling bath for rapid freezing in electron microscopy. Journal of Microscopy 101, 103–105.
3. ^ Wainner, Scott; Robert Richmond (2003). The Book of Overclocking: Tweak Your PC to Unleash Its Power . No Starch Press. pp. 44. ISBN 188641176X.  
4. ^ "Information Specific to Liquid Nitrogen". Harvard University. 30 July 2003 . http://www-safety.deas.harvard.edu/services/nitrogen.html .  
5. ^ Brent S. Mattox. "Investigative Report on Chemistry 301A Cylinder Explosion" (reprint). Texas A&M University . http://ucih.ucdavis.edu/docs/chemistry_301a.pdf .  
6. ^ British Compressed Gases Association (2000) BCGA Code of Practice CP30. The Safe Use of Liquid nitrogen dewars up to 50 litres. ISSN 0260-4809.

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