A liquid nitrogen container is also called Dewar vase or simply “dewar”
In wider terms, a Dewar is a container that keeps its contents warmer or colder than the external environment by placing vacuum areas (which allow high thermal insulation) between the contents and the environment. Vacuum is used only for thermal insulation; the content is not vacuum-sealed.
“Dewar” is named after its inventor, physicist and chemist James Dewar, who created it in 1892. It consists of an external container, usually made of metal, with a cavity inside, commonly made of glass or metal. The section between the outer walls of the container and the inner cavity is vacuum-sealed. The vacuum can only conduct the heat by irradiation to minimize radiation loss. The region vacuum-sealed is filled with reflective and matting material such as silver and fiberglass.
The material contained within the cavity achieves thermal equilibrium with the inner walls, having, thus, a very low evaporation rate.
In order to be used, the container must have an opening, which is the main source of heat loss in the container: to restrict this dispersion, the container must be provided with cap made of insulating material, originally cork, today plastic.
The most common example of Dewar is the classic Thermos, so called after the name of the first company that introduced them in 1904, useful for storing liquids at specific temperatures for several hours (cold for about 8 hours, hot for about 24 hours).
As for the dewars for liquid nitrogen, the cap is only placed on the opening (called mouth) because nitrogen constantly evaporates and must be able to leave without hindrance.
These containers are also equipped with a safety valve that preserves their integrity even in case of thermal shocks that cause pressure fluctuations.
The excellent thermal insulation of these dewar ensures that the nitrogen evaporation is very (at room temperature would be instantaneous) low: thanks to this system, liquid nitrogen can be used as a strong refrigerating.
Another type of liquid nitrogen containers are the pressure ones, hermetically sealed to increase the life of nitrogen inside them. As a precaution, normally, pressure vessels are equipped with at least two special safety valves.
