The vacuum insulated evaporator (VIE) is a device which hospitals use to store large amounts of oxygen (cf. gas oxygen manifold).

It stores oxygen as a liquid and therefore needs to be insulated, rather like an insulated flask, otherwise the oxygen will rapidly warm and pass from the liquid phase to the gaseous phase and be lost to the atmosphere. It consists of an outer carbon steel shell and an inner stainless steel shell, with vacuum insulation in between. The liquid oxygen is at a temperature of around −150 °C to −180 °C. It is in the liquid phase as it is below its critical temperature. The critical temperature of oxygen is −118 °C. The critical temperature is the temperature above which the gas cannot be liquified irrespective of the amount of pressure applied. If the temperature rises within the VIE more oxygen will boil off and the pressure will rise. The VIE is not designed to withstand these high pressures and will vent off excess oxygen.

Mandatory safety features

To avoid explosions or other dangerous events, the VIE must be kept in an open space with no overhead wires (to prevent sparks from igniting surrounding materials, which will combust easier in the prescense of liquid oxygen), no surrounding drains or trenches (pooled liquid oxygen will increase the combustibility of surrounding materials) and ringed by a fence of non-combustible material. Parking vehicles, open flames, smoking, and other potentially hazardous activities are normally prohibited anywhere near the VIE.