Pressure Swing Adsorption units separate air using a special sieve that adsorbs oxygen preferably to nitrogen. When high-pressure air flows through the sieve, oxygen molecules are caught while nitrogen molecules pass on. The sieve continues to adsorb oxygen until a saturation point is reached. After that, the entering air stream is cut off and the oxygen is able to leave the tank at low pressure. In a PSA unit, two connected tanks, containing sieves, work together to produce a near-continuous stream of nitrogen. When one tank has become saturated and is releasing adsorbed oxygen, the entering air stream is switched to the other tank for oxygen adsorption.
PSA units are best used to produce nitrogen in low-volume situations. They have a very long lifetime, even in cold temperatures; the sieve is simply replaced a number of times a year. The units are also easy to install on-site for operations that need nitrogen on-demand. Furthermore, they are readily available and have a relatively short purchase and delivery time. PSA's main disadvantage occurs when large flow rates of nitrogen are needed and nitrogen becomes significantly cheaper to buy from a cryogenic source.