Atmospheric Air mainly consists of Oxygen and Nitrogen gases along with small quantities of water vapour, Carbon Dioxide, Argon, Helium, etc. Oxygen and Nitrogen from the air separated due to difference in boiling points by distillation through a fractional column.

Atmospheric air is sucked in by multi stage Compressor through a filter and is compressed to the design pressure. The Compressed Air then passed through after cooler, Moisture Separator, Cascade Cooler, Freon Chilling unit, Moisture Separators, and them to the Molecular Sieve Battery for removal of Carbon Dioxide, Hydrocarbons and Moisture from the process Air. This pure Air then passes through the 1^st Heat Exchanger, where the out going Nitrogen cools it and Oxygen. Part of this cooled Air is passed through Expansion Engine and the other part through the 2^nd Heat Exchanger. Both the Expansion Engine and 2^nd Heat Exchanger help in further cooling down the Air, which is finally released to the bottom of the column through an expansion valve. The Air becomes liquid at this stage. The column consists of two parts, the lower column and upper column. In between the lower and upper columns there is a condenser, which acts as a reflux for the lower column and as a re-boiler for the upper column. The liquid air at the bottom of lower column separates through the trays to give crude Oxygen at the bottom and approximately 90% pure Nitrogen at the top. Crude Oxygen termed as rich liquid is then expanded through an expansion valve from the lower column to the middle of the upper column to the top of the upper column. Due to difference in the boiling points, the pure Nitrogen boils over and accumulates at the top of the upper column and Oxygen, which accumulates at the bottom of the upper column.

Both Nitrogen and Oxygen are removed through separate paths in Heat Exchangers, for cooling the incoming air. Oxygen is compressed to a prescribed settled pressure by a liquid pump and is directly filled into cylinders. Nitrogen is however available at a pressure of approximately 0.5kg/cm2 and the same can be compressed into cylinders with help of an independent high-pressure compressor.