VIE (vacuum-insulated evaporator)

HTM 02-01 lists the parts of a VIE system: a vacuum-insulated cryogenic storage vessel for the bulk liquid, two or more ambient-heated vaporisers, control equipment for the pressure and flow of gas to the pipeline, and a test point. The vessel is typically a stainless-steel inner pressure vessel inside a mild steel outer shell, with high-performance insulation kept under vacuum between them to reduce evaporation, and liquid oxygen is stored at down to −183 °C. Vacuum loss increases the boil-off rate, and vacuum failure is typically shown by ice on the outer shell. HTM asks for telemetry on all VIEs, so that the hospital and the gas supplier can monitor vessel levels and pressures continuously, and for vaporiser control valves to fail open in a power failure.

HTM adds that hospitals with larger oxygen demands will generally use a bulk medical oxygen VIE. Open Critical Care lists liquid oxygen via VIEs among high-pressure oxygen sources, with cylinders and oxygen plants. WHO interim guidance (2020) notes that cryogenic liquid oxygen is always produced off site and that a bulk tank feeds the pipeline by self-vaporisation, without a power supply.