Cryogenic liquid cylinders, also referred to as liquid containers or dewars, are double-walled, vacuum vessels with multilayer insulation in the vacuum space. They are designed for the transportation and storage of liquefied gases at cryogenic temperatures. Most liquid cylinders allow for withdrawal of product in either the liquid or gas phase, although some cylinders permit liquid withdrawal only.

There are two primary advantages of liquid cylinders: (1) they hold a large volume of gas at relatively low pressure compared to compressed gas cylinders; (2) they are a source of cryogenic liquid that can be easily handled.

Cryogenic products normally shipped in liquid cylinders include argon, nitrogen, oxygen, helium, and hydrogen in their liquid states. Carbon dioxide and nitrous oxide are also available as refrigerated liquids, but at higher temperature and pressure. Other gases that may use cryogenic liquid cylinders are krypton, methane, neon, and xenon.

Although the liquid cylinders are well insulated, the extremely low temperatures of the cryogenic liquids result in constant heat leak and vaporization. Vaporization rates will vary and may be as low as 0.4% or as high as 3% of the container's volume per day. The product, if not used, will build pressure in the container and periodically vent via the container's pressure relief device. This is a normal and safe function of the container.

Refer to Air Products' Safetygram #27—Cryogenic Liquid Containers (PDF) for further details on the safe handling of liquid cylinders.