Liquid nitrogen is the same nitrogen that makes up almost 80% of our atmosphere, just much colder. Nitrogen boils at minus 196 degrees Celsius. So, to make it into liquid it has to be cooled down to at least this temperature. At normal temperature, nitrogen is a gas. Like any gas, its molecules are spaced relatively far apart, and move very rapidly, because of this they do not interact with or attract each other. To turn nitrogen gas into a liquid, the molecules have to slow down and this is done by cooling them.

The process commences with compressing air, this pushes the air molecules close together, the air then flows down a tube into a chamber. For the gas to expand in the chamber, the molecules require energy which they get from absorbing the heat from the chamber, cooling it. The process is repeated until the nitrogen gas begins to condense into liquid. One volume of liquid nitrogen gives approximately 700 volumes of gas (700kw hrs to produce 1 metric tonne of gas).

General
Liquid nitrogen is inert, except when heated to very high temperatures where it combines with some of the more active metals, such a lithium and magnesium, to form nitrides; it is also colourless; odourless; non-corrosive; non-flammable and extremely cold.   Nitrogen makes up the major portion of the atmosphere (78.03% by volume, 75.5% by weight).

Although nitrogen is non-toxic and inert, it can act as a simple asphyxiant by displacing the oxygen in air to levels below that required to support life. Inhalation of nitrogen in excessive amounts can cause dizziness, nausea, vomiting, loss of consciousness and death. At low oxygen concentrations unconsciousness and death may occur in seconds and without warning. There are no known toxicological effects from liquid nitrogen.

Containers
Cryogenic liquid cylinders are insulated, vacuum jacketed pressure vessels. They come equipped with safety relief valves and rupture discs to protect the cylinders from pressure build up. These containers operate at pressures up to 350 psig and have capacities between 80 and 450 litres of liquid.