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Created: 9/6/2005

Oxygen is a non-metallic element existing as a colourless odourless diatomic gas (O2) in the lower atmosphere and as a triatomic oxygen (O3) and monatomic oxygen (O) in the upper atmosphere.

Joseph Priestley is usually credited with the discovery of oxygen because he was the first to describe it in 1775 in his work entitled Experiments and Observations on Different Kinds of Air.  However Scheele, a Swedish apothecary, first discovered oxygen in 1772.

Manufacture of oxygen

Oxygen is manufactured by the fractional distillation of air.  Air is compressed to 5 atmospheres and cooled to -181oC using reverse heat exchangers. A two-stage distillation process yields 99.5% O2 (and 0.4% argon).  It is stored at 137 bar in silver cylinders with green shoulders in the United States.

Storage of Oxygen: The VIE

Hospitals store large quantities of oxygen as a liquid in a vacuum insulated evaporator (VIE).

The inner shell of the VIE is made of stainless steel. It is separated from the outer carbon steel shell by an insulated gap with a vacuum of 0.16-0.3KPa. The contents are at 1000KPa and a temperature of -150oC (critical temp -119oC). A gauge indicates the pressure within and a differential pressure gauge (compares top to bottom) indicates the oxygen contents by weight.

Compensatory adjustments made by the VIE system

If there is a period of low oxygen use or  hot weather this will result in the temperature of the liquid oxygen increasing.. This leads to increased pressure in the VIE and at 1500KPa the blowoff valve opens allowing vaporisation into the environment, (the latent heat of vaporisation cools the liquid oxygen).

If there is a period of high oxygen use by the hospital or very cold weather, there will be reflected by a decreased temperature of liquid oxygen and thus a decreased pressure in the VIE system. At 1000KPa the Pressure Raising Valve (PRV) opens, and allows liquid oxygen to shunt through a Pressure Raising Vapouriser (PRV). This allows environmental heat to enter the system. This mechanism ensures that the pressure and temperature of the VIE rises back to normal. All outgoing gases pass through a heat exchanger to warm them.

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