Why the feed-air boundary causes most problems
On-site gas plants sit between two disciplines. The compressed-air side looks after compressors, dryers, filters, piping and receivers. The gas side looks after nitrogen or oxygen purity, product pressure and flow. Problems tend to appear exactly at the boundary. The compressor can look healthy while the generator receives unstable pressure, hot air, water or oil aerosol. Equally, a generator is often blamed for low purity when the real cause is a choked filter, a small receiver or a feed line too narrow for the flow.
The sound method is to start from the gas generator maker’s inlet requirements and work backwards to the compressor:
- Define product-gas demand: flow, purity and delivery pressure, now and later.
- Obtain the generator’s feed-air conditions: flow, minimum and maximum pressure, temperature, dew point, oil and particle limits.
- Convert the generator’s feed-air flow into compressor free air delivery (FAD) at site conditions, with margin for filter and dryer losses.
- Build a pressure budget: compressor discharge minus dryer, filter and pipe drops must still meet generator inlet pressure when filters are near change-out.
- Select dryer, filters and receivers to suit.
- Plan standby, shutdown behaviour and maintenance access.
- Commission with measured pressure, dew point and flow at the generator inlet.
Generator types and what they need from the air
| Technology | How it works | Feed-air sensitivities | Typical treatment |
|---|---|---|---|
| Nitrogen PSA (carbon molecular sieve) | Two beds adsorb oxygen under pressure and regenerate in turn | Oil and water damage the sieve; purity depends on steady pressure and flow | Refrigerated or desiccant dryer, coalescing filters, carbon filter, buffer receiver |
| Nitrogen membrane | Hollow fibres let oxygen and water pass faster than nitrogen | Oil and liquid water foul fibres; output and purity change strongly with feed temperature | Coalescing and carbon filtration, often heater for stable temperature |
| Oxygen PSA / VPSA (zeolite sieve) | Zeolite adsorbs nitrogen; oxygen-rich product passes | Zeolite is very sensitive to moisture and oil; feed often at lower pressure than nitrogen PSA | Low dew point drying, coalescing and carbon filtration; feed by compressor or blower per design |
| Cryogenic air separation | Air is compressed, purified and liquefied, then distilled | Very large, continuous, specialist duty with its own purification | Designed and supplied by the gas-plant specialist as a complete package |
Typical numbers, always to be checked against the generator datasheet: nitrogen PSA and membrane feeds commonly run at about 7–10 bar (101.5–145 psig), and some membrane systems use higher pressures for better yield. Air-to-nitrogen ratios rise sharply as purity rises, so a small change in specified purity can mean a much larger compressor. That is why the purity actually needed by the process should be confirmed before the compressor is sized.
Typical feed-air system architecture
- Compressor – oil-injected screw with strong filtration, or oil-free screw where the generator maker or product gas requires it. Size on FAD at site conditions, not on nameplate kW.
- Wet receiver – smooths compressor cycling and drops out condensate.
- Dryer – refrigerated (typical +3 °C PDP) for many nitrogen PSA and membrane systems; desiccant (typical −40 °C PDP) where the generator or application specifies it, and for oxygen zeolite systems.
- Filters – general-purpose and high-efficiency coalescing filters, then activated carbon for oil vapour. Each with a differential-pressure gauge and automatic drain.
- Dedicated feed-air receiver – close to the generator, so bed switching does not pull the header down.
- Product-gas receiver – on the gas side, buffering the process.
- Segregation – if the generator shares a plant compressor, a regulator and non-return valve protect each side. Product-gas piping must never be cross-connected to compressed-air lines.
Our role versus the gas-plant specialist
Clear scope avoids finger-pointing when purity falls. A typical split looks like this:
| Reddy Compressors (feed-air side) | Gas generator maker or specialist |
|---|---|
| Compressor selection, supply, rental and service | Generator design, sieve or membrane selection |
| Dryer, filters, drains and receivers on the air side | Purity, product-gas pressure and flow guarantee |
| Feed-air pressure, dew point and flow measurement at the inlet | Oxygen analysers, product-gas controls, valves and beds |
| Feed-air piping and compressor controls | Product-gas storage, distribution and safety systems |
| Spare parts for compressors and air treatment | Sieve replacement, membrane modules and generator spares |
We do not design or warrant nitrogen or oxygen generators. We make sure the air reaching them meets the maker’s inlet conditions and we provide the measurements to prove it.
Common problems and fixes
| Symptom | Feed-air causes to check |
|---|---|
| Nitrogen purity falling or flow short | Low inlet pressure from filter drop or undersized compressor; feed air too hot; leaks; demand above design |
| Sieve or membrane damaged early | Oil carryover from a worn separator or saturated carbon filter; liquid water from failed drains; dryer bypass left open |
| Purity swings with bed switching | No dedicated feed receiver; long, narrow feed line; compressor load/unload band too wide |
| Membrane output drops in winter or rises in summer | Feed temperature changes; no heater or temperature control |
| Compressor trips in summer | Hot plant room, fouled coolers, poor ventilation |
| Higher power for the same gas output | Purity set higher than the process needs; leaks; high compressor set pressure |
Reliability, standby and temporary cover
Where nitrogen blankets reactors or tanks, purges lines or protects packaged food, loss of feed air quickly becomes a safety or quality problem. Typical protections include a standby compressor, a liquid nitrogen or cylinder backup on the gas side, alarms on feed pressure and dew point, and a planned tie-in for a rental compressor with dryer and filters. Temporary air must meet the same oil and dew-point limits as the permanent system; a site compressor with no treatment can ruin a sieve bed in hours.
Energy
The compressor uses most of the energy of an on-site nitrogen or oxygen plant, so feed-air efficiency matters:
- Specify the lowest gas purity the process genuinely needs; higher purity needs much more feed air.
- Keep generator inlet pressure at the maker’s value, not higher; about 7% more power per extra bar applies here too.
- Change filters on differential pressure so pressure drop does not force a higher compressor setting.
- Use a VSD compressor where gas demand varies widely.
- Fix leaks on both air and gas sides.
- Compare the specific power of feed air against the gas output to see the true cost per cubic metre of gas.
Indian context
On-site nitrogen and oxygen generation is common in pharmaceutical, chemical, food packaging, electronics, metal heat-treatment and laser-cutting plants across Telangana and Andhra Pradesh, and in hospitals and industry across India. High summer temperatures raise feed-air temperature and load dryers; monsoon humidity increases condensate; and dust loads intake filters. Power dips and DG changeovers can trip both compressor and generator, so restart sequencing should be planned so the generator does not start on poor-quality air.
Many Indian plants add a nitrogen generator to an existing plant-air system rather than installing dedicated compressors. That can work well, but only if the existing compressor has spare FAD at the generator’s inlet pressure during the busiest shift, the dryer can take the extra flow without losing dew point, and the generator gets its own receiver and filtration. A short logging exercise on pressure and flow before purchase tells you whether the shared system can carry the new load.
How Reddy Compressors helps
Reddy Compressors, Tarnaka, Secunderabad, has served industry since 2002 as an independent multi-brand specialist. We sell, rent and service compressors and air treatment from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and others. For gas-plant feed air we:
- Size and supply new or used compressors, dryers, filters and receivers to the generator maker’s inlet conditions.
- Provide rental compressors with treatment from our own fleet for breakdowns, shutdowns and project start-ups.
- Service compressors, dryers, filters and drains under planned maintenance or AMC, with over 20 service engineers and technicians covering Telangana and Andhra Pradesh.
- Measure feed-air pressure, dew point, flow and power during audits.
- Supply spare parts across India, identified from the nameplate and confirmed before quoting.
Our track record. We have supplied compressors to leading companies in industrial gases across Telangana, Andhra Pradesh and beyond – new and used machines, long-term rental, and breakdown cover when a plant’s own compressor fails – backed by our own service team.
What to send us
- Generator make, model, technology and datasheet feed-air requirements.
- Required gas flow, purity and delivery pressure.
- Existing compressor, dryer and filter nameplate photos.
- Sketch of the feed-air line, receivers and any shared plant air.
- Purity, pressure or alarm records and the problem you are seeing.
- Site location and power arrangement.
Frequently asked questions
What compressor size do I need for a nitrogen generator?
Start from the generator maker’s feed-air flow at your required nitrogen purity and flow, not from the nitrogen flow alone. Higher purity needs much more air. Convert that feed-air figure to compressor FAD at site conditions and add margin for dryer purge and filter losses. Send us the generator datasheet and we will check the compressor size against it.
Watch
A short explainer on this topic
Slide-by-slide engineering explainers from Reddy Compressors. More in the video library.
What air quality does a PSA nitrogen generator need?
Most PSA nitrogen generators need dry air, typically from a refrigerated dryer at about +3 °C PDP or a desiccant dryer where specified, plus coalescing and activated-carbon filters to keep oil off the carbon molecular sieve. The exact ISO 8573-1 class, pressure and temperature limits are in the generator datasheet and should be treated as the governing requirement.
Why is my nitrogen generator purity dropping?
On the air side, the usual causes are low inlet pressure from blocked filters or an undersized compressor, hot feed air, leaks and rising demand. Oil or water reaching the sieve or membrane also reduces performance permanently. Measure pressure and dew point at the generator inlet first; if those are correct, the generator itself needs checking by its maker.
Can an oil-injected compressor feed a nitrogen generator?
Yes, many PSA and membrane systems run on oil-injected screw compressors with coalescing and carbon filtration, provided the generator maker accepts it. Filters and separators must be maintained on time, because oil carryover can damage the sieve or membrane. Some oxygen and food or pharmaceutical applications specify an oil-free compressor instead.
Does an oxygen generator need different feed air from a nitrogen generator?
Often, yes. Oxygen PSA and VPSA systems use zeolite, which is very sensitive to moisture and oil, and some run at lower feed pressure than nitrogen PSA. They may need a lower dew point and their feed may come from a compressor or a blower depending on design. Always follow the oxygen generator maker’s feed-air specification.
Can I rent a compressor to keep my nitrogen plant running during a breakdown?
Yes. We supply rental compressors from our own fleet with dryers and filters arranged to meet your generator’s inlet requirements. Untreated site air can damage a sieve bed, so tell us the generator model and air specification when you call, and plan a tie-in point in advance if possible.
Related pages
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Flow and reference-condition conversion
Convert a flow quoted at one set of conditions to another, and between units. Different suppliers quote at different reference conditions, so always compare like with like.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Flow as quoted: 10
- Unit: m³/min
- Quoted at pressure (bar a): 1.013
- Quoted at temperature (°C): 20
- Convert to pressure (bar a): 1.013
- Convert to temperature (°C): 0
Result
- Converted flow: 9.32 m³/min
- 155.3 L/s
- 559.1 m³/h
- 329.1 cfm
Method. Q₂ = Q₁ × (P₁ / P₂) × (T₂ / T₁), with absolute pressure and absolute temperature. Dry-air basis; humidity is ignored. Unit factors: 1 m³/min = 16.667 L/s = 60 m³/h = 35.31 cfm.
What to take from it. A flow quoted at 20 °C and the same air quoted at 0 °C differ by about 7%. Check the reference conditions before comparing two machines.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.
Pressure budget and the cost of excess pressure
Add up the pressure that is lost between the compressor and the point of use, and see what a lower set-point would be worth.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Pressure needed at the point of use (bar g): 6
- Dryer and filter loss (bar): 0.5
- Piping loss (bar): 0.3
- Hose, fittings and regulator loss (bar): 0.3
- Control band and safety margin (bar): 0.3
- Current compressor set-point (bar g): 8
- Compressor input power (kW): 75
- Saving per bar reduced (%): 7
- Running hours per year: 6000
Result
- Total pressure losses: 1.1 bar
- Control band and safety margin: 0.3 bar
- Compressor pressure needed: 7.4 bar g (point of use + losses + band and margin)
- Possible reduction from current set-point: 0.6 bar
- Indicative energy saving: 18,900 kWh per year
- Indicative cost saving: Enter your electricity tariff (cost per kWh) above
Method. Required compressor pressure = point-of-use pressure + sum of losses. Savings = power × (reduction in bar) × saving per bar. A saving of roughly 6–8% of compressor power per bar of pressure is commonly quoted; it is indicative and varies with machine type and control, so edit it to suit.
What to take from it. The saving is real only if the farthest and most demanding point still works at the lower set-point. Prove it before you change the setting.