Most generators are of the Pressure Swing Adsorption (PSA) type, oxygen is produced from the sieve bed under pressures as high as 8 barg (116 psig), and the zeolite sieve is regenerated against ambient pressure. Such high pressures require an air compressor and air dryer to be used; major running costs that are eliminated with VSA technology.

The VSA process produces oxygen at lower pressure (1.5 barg) and totally regenerates the sieve in a mild vacuum (0.5 barg). In a VSA generator the compressor can be replaced by a blower, meaning that capital and operating costs are reduced. The overall costs of buying and operating a VSA generator are lower than PSA systems.

Air enters the generator via an inlet vent, passes through the four way valve V-1, through a single stage, dry rotary lobe blower, an air cooler and then into the adsorber bed, again via valve V-1. On entering the adsorber vessel water vapour, carbon dioxide and hydrocarbons are removed in a desiccant layer. Thus only oxygen and nitrogen molecules flow over the sieve bed, where the nitrogen is adsorbed and the oxygen passes out of the vessel and through the open valve CV-2 to fill the O₂ buffer tank.

The bed quickly becomes saturated with nitrogen molecules. These now have to be removed in a vacuum step to regenerate the bed. Valves CV-1 and CV-2 are closed, and the four way valve V-1 rotates such that the blower now acts in reverse and "sucks" nitrogen molecules off the sieve bed, in addition to removing water vapour, carbon dioxide and hydrocarbons off the desiccant layer. All these are exhaust gases that are vented out of the system through the same blower (now acting as a vacuum blower) that the air originally entered.

In the purge step valve CV-2 opens to allow oxygen molecules to flow back into the adsorber vessel to help remove any last traces of nitrogen in the system.

Finally, in the re-pressurisation step, the whole system is brought back up to atmospheric pressure by valve V-1 rotating into a position such that air can rush back into the inlet. At the same time oxygen is allowed into the top of the bed, thus ensuring that there is no movement of the sieve bed (sometimes a problem in PSA systems). The product valve PIC-1 remains open throughout the whole of the above cycle so that a continuous flow of oxygen product is maintained.