A cryogenic liquid is defined as a liquid with a boiling point below -130oF (-90oC). Common cryogens and their properties are summarized in Table 1.
Cryogen | Boiling point (1 atm) oC (oF) | Critical pressure psiga | Liquid density, g/L | Gas density (27oC), g/L | Liquid-to-gas expansion ratio | Type of gas |
Ar | -186(-303) | 710 | 1402 | 1.63 | 860 | Inert |
He | -269(-452) | 34 | 125 | 0.16 | 780 | Inert |
H2 | -253(-423) | 188 | 71 | 0.082 | 865 | Flammable |
N2 | -196(-321) | 492 | 808 | 2.25 | 710 | Inert |
O2 | -183(-297) | 736 | 1410 | 1.4 | 875 | Oxidizerb |
CH4 | -161(-256) | 673 | 425 | 0.72 | 650 | Flammable |
apound per square inch gauge; bAlthough oxygen does not burn, it will support combustion. Oxygen-enriched atmospheres may lead to violent reactions, such as rapid combustion or explosions, with incompatible materials.
Hazards may include fire, explosion, embrittlement, pressure buildup, frostbite and asphyxiation.
Many of the safety precautions observed for compressed gases also apply to cryogenic liquids. Two additional hazards are created from the unique properties of cryogenic liquids:
Vaporization of cryogenic liquids (except oxygen) in an enclosed area can cause asphyxiation. Vaporization of liquid oxygen can produce an oxygen-rich atmosphere, which will support and accelerate the combustion of other materials. Vaporization of liquid hydrogen can form an extremely flammable mixture with air.
EHS recommends the the following PPE for handling cryogenic liquids:
Cryogenic liquids can be transported, stored, and handled in a variety of containers depending on quantity and desired use. All three types of containers are present at Princeton University.
Dewars are non-pressurized, double walled containers used to contain cryogenic liquids. The are between the walls is under high vacuum for maximum thermal insulation. They have a loose fitting insulated cap that enable gases to escape while preventing moisture buildup at the neck. In many cases they are used at "satellite" cryogenic liquid containers within a lab.
Dewar flasks are smaller, double-walled cryogenic storage containers that usually only maintain the liquid for a few hours. They usually have a metal outer wall and a glass inner wall, with the void space under high vacuum. Special care should be taken when handling as these flasks can implode.
Cryogenic liquid cylinders, sometimes also referred to as Dewars, are insulated, vacuum jacketed, pressurized vessels. They come equippmed with safety relief valves and rupture disks to alleviate pressure buildups. These cylinders come in a variety of sizes ranging from 80-450 liters. They can be equipped to provide gas through an internal vaporizer or provide liquid under their own internal vapor pressure. These cylinders are usually filled by the users at filling stations throughout the campus. All users must wear proper PPE when filling and discharging. Cryogenic cylinders routinely relieve their internal pressure and you may observe a sudden hissing sound and the appearance of a fog. This is completely normal. When using cryogenic cylinders, users should ensure that all valves and pressure relief devices are in place and in good condition. If pressure relief devices fail, catastrophic failure may result.
A few buildings on campus have cryogic storage tanks. These are filled by vendors on a set schedule and are used to fill the end user crogenic cylinders and dewars.
Most cryogenic liquids are odorless, colorless, and tasteless when vaporized. When cryogenic liquids are exposed to the atmosphere, the cold boil-off gases condense the moisture in the air, creating a highly visible fog.