A PSA oxygen generator produces 90–96% pure oxygen on site from compressed air. It does this by trapping nitrogen in zeolite molecular sieves and letting oxygen pass through. Most plants deliver oxygen at about 5.5 barg, run unattended under a PLC, and produce oxygen for roughly ₹8.8 per Nm³ at a power tariff of ₹7/kWh. That is well below cylinders (₹30–60) and liquid oxygen (₹15–25).
This guide explains how the process works, what each component does, how purity and capacity are set, and how to compare PSA with cylinders and liquid oxygen.
How a PSA oxygen generator works
Pressure swing adsorption separates air using a simple fact: at high pressure, zeolite holds on to nitrogen far more strongly than oxygen. Air is about 78% nitrogen and 21% oxygen, so removing the nitrogen leaves an oxygen-rich product. Lowering the pressure releases the nitrogen again, so the same sieve can be reused for years.
A plant has at least two adsorber towers that take turns. While one makes oxygen, the other is cleaned. Valves controlled by the PLC switch them every few seconds, so oxygen flows without interruption.
One cycle in each tower runs like this:
- Pressurisation and adsorption. Clean, dry compressed air enters the tower. Zeolite traps nitrogen, and oxygen passes through to the oxygen receiver.
- Pressure equalisation. Before switching, the full tower shares part of its pressure with the empty one. This recovers compressed air that would otherwise be vented and cuts power use.
- Depressurisation. The used tower vents to atmosphere. The drop in pressure releases the trapped nitrogen.
- Purge. A small stream of product oxygen flows back through the vented tower to sweep out the last nitrogen, so the next cycle starts clean.
- Switch. The towers swap roles and the cycle repeats.
Argon behaves like oxygen in zeolite and passes through with it. Air contains about 0.9% argon, which is why PSA oxygen tops out at about 95–96% rather than 99%+.
Components of a PSA oxygen plant
Every component upstream of the towers exists to protect the zeolite. Oil and moisture are the two things that permanently damage it.
| Component | What it does | Why it matters |
|---|---|---|
| Air compressor | Compresses air to 5–7 barg; screw compressors are preferred for efficiency, low noise and compact size | It uses most of the plant’s power, so its efficiency sets the running cost |
| Refrigerated air dryer | Cools the air so moisture condenses and drains out | Moisture reduces the zeolite’s ability to hold nitrogen |
| Air receiver | Stores dry compressed air as a buffer | Lets the compressor run efficiently when demand changes |
| Line filters | Remove oil, dust and leftover moisture | Oil traces poison zeolite permanently |
| PSA towers | Pressure vessels filled with zeolite molecular sieve and activated alumina | Where the separation happens; the activated alumina layer catches remaining moisture |
| PLC and valves | Run the cycle automatically | Correct timing gives stable purity and lower power use |
| Oxygen receiver | Stores product oxygen for the pipeline | Absorbs demand peaks and keeps supply steady |
| Oxygen analyser and alarms | Monitor purity and pressure continuously | Flag any drift before it reaches the point of use |
Purity, pressure and flow
PSA oxygen purity is set by how hard the plant is run against its design capacity. Drawing less oxygen gives higher purity, and drawing more gives lower purity.
- Medical oxygen: 93% ± 3%, the level hospitals specify for patient use. Pharmacopoeia standards vary by country.
- Industrial oxygen: 90–96%, chosen for the process. Cutting, brazing, wastewater aeration and ozone generation all run well in this range.
- Delivery pressure: about 5.5 barg, which is low and safe compared with 150 barg cylinders. A booster compressor raises pressure when cylinders must be filled.
- Capacity: from a few Nm³/hr to several thousand. Absstem’s Idos range covers 1–5,000 Nm³/hr.
For very large flows, VPSA (vacuum pressure swing adsorption) is often used. It adsorbs at near-atmospheric pressure and uses a vacuum pump to release the nitrogen, which can lower power per Nm³ at high capacity.
Where PSA oxygen is used
- Hospitals, nursing homes and medical colleges: piped medical oxygen to every bed. See medical oxygen generators.
- Steel and metal: cutting, welding and brazing. See steel and metal industry.
- Wastewater treatment: higher dissolved oxygen for faster treatment. See wastewater treatment.
- Aquaculture: oxygenation for fish and shrimp farms. See aquaculture.
- Glass, pulp and paper, and ozone generation: see pulp and paper and ozone generation.
PSA vs cylinders vs liquid oxygen
On-site PSA wins on cost per Nm³, safety and independence from suppliers. Cylinders and liquid oxygen still suit very small or occasional use.
| Parameter | Liquid oxygen | High-pressure cylinders | PSA oxygen generator |
|---|---|---|---|
| Cost per Nm³ | ₹15–25 (depends on distance from the LOX plant) | ₹30–60 (a 7 m³ cylinder costs about ₹200–500 to fill) | About ₹8.8 at ₹7/kWh power |
| Safety risk | High: large expansion from liquid to gas | Very high: 150 barg storage | Low: about 5.5 barg |
| Supplier dependence | Regular refilling | Regular refilling | None: made on site |
| Carbon emissions | High (trucking) | High (trucking) | Very low |
For a full breakdown, read Comparison of the Cost of Oxygen Gas from Different Sources.
Running cost and payback
Power is the main running cost, and almost all of it goes to the air compressor. At ₹8.8 per Nm³ and ₹7/kWh, a PSA plant uses roughly 1.25 kWh per Nm³ of oxygen. Most plants pay back their cost in 12–24 months against cylinders or liquid oxygen.
Idle mode cuts power further. When demand drops, the plant pauses the cycle while the receiver holds pressure, instead of venting oxygen it does not need.
To work out the payback for your own site, use the free ROI calculator or read how to calculate the ROI of a PSA oxygen generator.
How to choose the right capacity
Size the plant on real demand, not on the cylinder count. Over-sizing wastes capital, and under-sizing drops purity at peak.
- Measure average and peak flow in Nm³/hr, and the hours per day the peak lasts.
- Set the purity your process actually needs. Running at 90% instead of 95% can allow a smaller plant.
- Size the oxygen receiver to cover short peaks, so the generator can be sized nearer the average.
- Allow for site conditions. High ambient temperature and altitude reduce compressor output.
- Decide on backup: a cylinder manifold or a second generator for critical uses such as ICUs.
Maintenance and service life
A well-kept PSA plant runs for 10 years or more. Absstem gives a 10-year warranty on the molecular sieve. Maintenance is mostly routine:
- Daily checks of purity, pressure and dew point, done automatically where IIoT monitoring is fitted.
- Changing filter elements on schedule, because they are the sieve’s protection.
- Servicing the compressor and dryer as the manufacturer specifies.
- Inspecting valves and silencers during preventive visits.
The sieve itself should not need topping up every year if the air is kept clean and dry. Absstem Shield covers AMC and CMC for PSA plants of any brand.
Talk to a PSA engineer
For a plant sized to your application, see Idos industrial oxygen generators or MedO medical oxygen generators. You can also email info@absstem.com or call 1800 547 0506.