Why feed air decides nitrogen performance
On-site nitrogen generators do not make nitrogen; they separate it from compressed air. A pressure swing adsorption (PSA) unit pushes air through beds of carbon molecular sieve that hold back oxygen while nitrogen passes, then switches beds and vents the captured oxygen. A membrane unit passes air through bundles of hollow fibres in which oxygen, water vapour and carbon dioxide permeate faster than nitrogen. In both cases the generator consumes several volumes of compressed air for every volume of nitrogen it delivers, and that ratio rises sharply as purity rises.
This means the air system is usually the largest energy user in a nitrogen installation and the most common cause of complaints. When a plant reports “the generator cannot hold purity”, the cause is very often low or fluctuating inlet pressure, too little air at peak demand, wet air, oil aerosol or high inlet temperature rather than a fault in the separation package itself.
- Lower inlet pressure reduces the separation capacity of both PSA beds and membranes, so purity falls or output must be cut.
- Water and oil aerosol poison carbon molecular sieve and foul membrane fibres, causing slow, often permanent loss of capacity.
- High inlet temperature reduces PSA performance and can push a membrane outside its permitted operating window.
- Undersized air supply shows up first at bed changeover, start-up and peak product demand, not at the average.
What the generator needs from the air system
Every generator maker publishes an inlet specification. Before choosing or checking a compressor, collect these values for the exact model, at the purity and nitrogen flow you actually need.
| Feed-air parameter | Why it matters | What to confirm |
|---|---|---|
| Inlet pressure range | Separation capacity depends strongly on pressure; many industrial PSA units are rated around 7–10 bar (101.5–145 psig) | Minimum pressure at the generator inlet during bed switching and peak demand, not at the compressor |
| Feed-air flow | Air consumption rises steeply with purity | Air flow at your purity and output, with its reference conditions (FAD or normal m³/h) |
| Pressure dew point | Moisture loads the adsorbent or fibres | Typical requirement is refrigerated-dryer quality (PDP about +3 °C) or better; some packages specify desiccant drying |
| Oil content | Oil aerosol and vapour degrade adsorbent and membranes | Coalescing filtration and, where specified, an activated-carbon stage; ISO 8573-1 class as stated by the maker |
| Particles | Dust and pipe scale block filters and beds | Particulate filtration class and clean distribution pipework |
| Inlet temperature | Hot air reduces separation efficiency and may exceed membrane limits | Maximum and minimum inlet temperature in summer and winter operation |
State flow on a clear basis. A generator brochure may quote air in normal cubic metres per hour while a compressor quotes free air delivery in cfm or m³/min at its own inlet conditions. Convert both to the same reference before comparing; mixing them can hide a shortfall of a meaningful size.
How to size the compressed-air supply
Average air-to-nitrogen ratios are a starting point, not a design. Use the generator’s actual demand pattern.
- Define the nitrogen duty. Required purity (for example 95, 99, 99.9 or 99.99 percent), product flow, delivery pressure, daily pattern and what happens if supply is interrupted.
- Get the feed-air figure at that duty. Ask the generator supplier for air flow and inlet pressure at your purity, flow and site ambient temperature.
- Add the treatment losses. Filters and dryers drop pressure. A heatless desiccant dryer also uses a significant share of the dried air as purge, which must come from the compressor on top of the generator demand.
- Check the dynamic demand. PSA bed switching and pressurisation draw air in pulses. A correctly sized air receiver between the dryer and generator smooths this and keeps the compressor from cycling hard.
- Set the compressor pressure. Work back from the minimum generator inlet pressure, adding filter, dryer and pipe losses. Remember that every extra bar costs about 7 percent more compressor power, so do not over-pressure “for safety”.
- Decide redundancy. If nitrogen is critical (inerting, blanketing, food packaging), plan a standby compressor, a bypass arrangement or a cylinder or liquid nitrogen back-up.
Where the plant already has a compressed-air network, decide whether the generator draws from the main header or has a dedicated compressor and treatment train. A dedicated supply protects purity from other users’ demand swings; a shared header saves capital but needs pressure control and priority valving so that a production surge elsewhere does not starve the generator.
Typical feed-air treatment train
A robust layout for most PSA and membrane generators runs in this order, with each item sized for full flow and the worst summer conditions:
- Wet air receiver to drop out bulk water and buffer demand.
- General-purpose filter, then a high-efficiency coalescing filter to remove oil aerosol.
- Refrigerated or desiccant dryer to the dew point the generator specifies.
- Activated-carbon filter where the maker requires oil-vapour removal, then a particulate after-filter.
- Dry air receiver close to the generator inlet, with a pressure gauge or transmitter at the generator connection.
Membrane generators often include a heater to hold a constant fibre temperature; check that the air arriving is within its range. PSA units are sensitive to oil slip, so filter-element changes must not be stretched. An oil-free compressor removes the oil-aerosol risk at source but still needs drying and particulate filtration, because water and dust are not removed by oil-free compression.
Indian site conditions that change the design
Summer ambient temperatures across Telangana and Andhra Pradesh commonly reach 40–45 °C, and above 45 °C on open sites, and compressor rooms run hotter. Hot intake air raises compressor discharge temperature, increases the water load on the dryer and raises the air temperature reaching the generator. Plan for:
- Good compressor-room ventilation and ducted hot-air exhaust so that the compressor and dryer do not recirculate their own heat.
- Refrigerated dryers rated for high ambient and inlet temperature, or correction factors applied to the selection.
- Intake filters and pre-filters that suit dusty sites, cleaned or replaced on a schedule.
- Monsoon humidity, which increases condensate volume; drains and separators must handle it.
- Voltage dips and short power cuts; confirm how the generator and compressor restart and how long the nitrogen receiver can carry the process.
Symptoms, likely causes and what to check
| Symptom | Likely feed-air cause | What we check |
|---|---|---|
| Purity drops at peak demand | Inlet pressure falls during bed switching or high product draw | Pressure trend at generator inlet, receiver size, compressor capacity and control band |
| Gradual loss of output over months | Oil or water reaching the adsorbent or membrane | Filter differential pressure, element age, dryer dew point, drain function, oil carry-over |
| Purity worse in summer afternoons | High inlet temperature or dryer overloaded | Air temperature at inlet, dryer approach temperature, room ventilation |
| Compressor short-cycling | Pulsed PSA demand with little storage | Receiver volume, pressure band, control settings |
| Water at generator drain or filters | Failed drain, dryer bypassed or overloaded | Drain valves, dryer alarms, bypass valve positions |
Always measure at the generator inlet, not only at the compressor. A compressor gauge showing 8 bar can sit upstream of a blocked coalescing filter delivering well under that to the generator.
Commissioning and acceptance checks
Agree acceptance on both sides of the boundary. On the air side, record inlet pressure (minimum during cycling), flow, dew point, temperature and filter differential pressures. On the nitrogen side, the generator supplier verifies purity with a calibrated analyser, product flow and delivery pressure. Run the test through start-up, steady operation and a demand step, and in hot conditions if possible.
- Pressure logged at the generator inlet over several complete PSA cycles.
- Dew point measured at the dryer outlet with a suitable instrument.
- Oil and particle quality sampled to ISO 8573 methods where the duty is sensitive.
- Product purity and flow recorded at the same time as the inlet values.
Product-gas suitability for food, pharmaceutical or medical use is a separate approval governed by the relevant standards and the user’s quality system; correct feed air is necessary but not sufficient.
How Reddy Compressors helps
We work on the air side of nitrogen installations as an independent multi-brand specialist. From our office at Tarnaka, Secunderabad, our team of more than 20 service engineers and technicians covers the whole of Telangana and Andhra Pradesh, and we supply equipment and spares across India.
- Selection and supply of new and used/refurbished oil-injected and oil-free compressors of ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other makes, sized from the generator maker’s feed-air data.
- Dryers, filters, receivers and drains matched to the generator’s inlet specification.
- Air audits that log pressure, flow and dew point at the generator inlet to find the real cause of purity loss.
- Service and AMC for the compressor and treatment train, with filter and drain maintenance on schedule.
- Rental compressors from our own fleet for commissioning, shutdowns or while a failed unit is repaired.
- Nitrogen booster compression where a higher product pressure is required.
We do not design or warrant the separation package itself; we coordinate with the generator supplier so that the air arriving at their inlet matches what they specify.
What to send us
- Generator make, model and type (PSA or membrane) and its inlet specification sheet.
- Required nitrogen purity, flow, delivery pressure and operating hours.
- Existing compressor, dryer and filter nameplate photos, if any.
- A sketch of the air and nitrogen pipework with pipe sizes and receiver volumes.
- Any purity or pressure readings, and when problems occur.
- Site location and room ambient conditions.
Frequently asked questions
How much compressed air does a nitrogen generator need?
It depends on the technology, the purity and the inlet pressure. Air consumption per unit of nitrogen rises steeply as purity moves from 95 towards 99.99 percent, and membranes generally need more air than PSA at high purity. Ask the generator maker for feed-air flow at your exact purity, output and ambient, then add dryer purge and losses before sizing the compressor.
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Why is my nitrogen generator not reaching purity?
The most common causes are low or fluctuating inlet pressure, too little air during bed switching, wet air from a failed drain or overloaded dryer, oil aerosol reaching the adsorbent, and high inlet temperature. Log pressure and dew point at the generator inlet over several cycles before blaming the generator. Gradual loss over months often points to contamination of the sieve or membrane.
Do I need an oil-free compressor for a nitrogen generator?
Not always. Many generators run well on oil-injected screw compressors with good coalescing and activated-carbon filtration, kept on schedule. An oil-free compressor removes the oil risk at source, which suits food, pharmaceutical and electronics duties, but it still needs a dryer and particulate filters. Follow the generator maker’s stated inlet quality and the product-gas requirements of your process.
What dew point does a PSA nitrogen generator need?
Many PSA generators specify air dried to refrigerated-dryer quality, typically a pressure dew point around +3 °C, combined with oil and particle filtration. Some packages, or plants with very cold or critical duties, call for desiccant drying to −20 °C or −40 °C. The generator’s own inlet specification is the reference; do not assume one value suits every model.
Can the nitrogen generator share the main plant compressor?
Yes, if the header can hold the generator’s minimum inlet pressure at all times, including when other users surge. Use a dedicated filter and dryer train or confirm the existing one is suitable, add a receiver near the generator and consider a priority or pressure-sustaining valve. For critical nitrogen, a dedicated compressor with standby is often simpler to manage.
Why does my nitrogen output fall in summer?
Hot intake air raises compressor discharge and dryer inlet temperatures, so the dryer may struggle and warmer, wetter air reaches the generator. PSA separation efficiency also falls with temperature, and membranes have upper limits. Improve compressor-room ventilation, check the dryer rating at actual summer conditions and measure air temperature at the generator inlet during the hottest hours.
Related pages
Field guide
On-site nitrogen: Choosing the technology and feeding it properly
Nitrogen is about 78% of the air, and a nitrogen generator separates it from the oxygen, moisture and carbon dioxide. Cylinders and liquid deliveries work, but they bring transport, storage and supply risk. On-site generation makes nitrogen where it is used, from compressed air. The generator is only as good as the air fed to it, which is why this page sits with compressed-air engineering.
Three ways to make nitrogen
| Method | How it works | Typical fit |
|---|---|---|
| PSA (pressure swing adsorption) | Compressed air passes through twin vessels of carbon molecular sieve, which holds back oxygen, water vapour and CO2 while nitrogen goes through; the vessels swap between producing and regenerating | High purity: electronics, laser cutting, laboratories, packaging, chemical and pharmaceutical use |
| Membrane | Hollow polymer fibres let oxygen, water vapour and CO2 permeate faster than nitrogen, leaving nitrogen-rich gas at the outlet | Moderate purity: simple, compact, few moving parts, quick start |
| Cryogenic air separation | Liquefied air is distilled | Very large volumes and very high purity; large industrial sites |
Purity ranges quoted by makers vary by model and flow. As a rule, membranes serve lower purity and PSA serves higher purity, and getting higher purity from the same generator means less nitrogen for the same feed air. Always size from the maker’s curve for your purity and flow, not from a brochure range.
The air path
What the feed air must be
- Oil-free to the maker’s limit. Oil vapour fouls carbon sieve and membranes; use the filtration the maker specifies and keep it fresh
- Dry. Water also harms the sieve, so the dryer pressure dew point matters
- Stable pressure and enough flow. A lean or unstable supply drops purity or output
- Clean. Dust and condensate shorten life; check drains and differential pressure across filters
For compressor choice and treatment see what oil-free means and aftercooling and condensate removal. For higher delivery pressure see nitrogen booster compression.
Keeping a nitrogen plant reliable
- Watch purity continuously and calibrate the analyser on schedule
- Trend dew point and filter differential pressure
- Track sieve or membrane health and replace on evidence
- Find and fix leaks on the feed side; leakage wastes compressor energy
- Review alarms and keep the maintenance plan current
Typical uses include modified-atmosphere packaging, laser-cutting assist, pipeline purging, chemical blanketing, electronics, battery manufacturing, heat treatment and tank inerting. Reddy Compressors can size the feed-air system for a nitrogen generator; send the required flow, purity, pressure and duty.
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.
Nitrogen generator feed-air compressor sizing
Work out the compressor FAD and discharge pressure a nitrogen generator needs, from the nitrogen flow and air factor or from the maker’s feed-air figure, adding dryer purge and losses the air factor does not cover.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Feed-air basis: Nitrogen flow × air factor
- Nitrogen flow per generator (in the flow unit below): 100
- Air factor: feed air ÷ nitrogen, same basis: 3
- Feed-air flow per generator from the data sheet (in the flow unit below): 300
- Flow unit: scfm (standard reference)
- Reference temperature of the scfm figure (°C): 20
- Number of generators running together: 2
- Losses outside the air factor, e.g. dryer purge (% of compressor delivery): 0
- Design margin on flow (%): 0
- Site inlet temperature (°C): 20
- Site inlet pressure (bar a): 1.013
- Generator minimum inlet pressure (bar g): 8
- Pressure drop through filters, dryer and pipe (bar): 0.5
Result
- Feed air per generator: 300 scfm = 100 × 3
- Total nitrogen flow: 200 scfm from 2 generators
- Total feed air before other losses: 600 scfm
- Compressor FAD to enquire at site inlet: 16.99 m³/min = 600.1 cfm at 20 °C, 1.013 bar a
- Minimum compressor discharge pressure: 8.5 bar g = 123 psig (8 + 0.5 bar losses)
Method. Feed air per generator = nitrogen flow × air factor (same reference basis), or the feed-air flow from the data sheet. Total = per generator × number running together. Losses outside the air factor (for example heatless desiccant dryer purge, as a share of compressor delivery): total ÷ (1 − loss). Margin: × (1 + margin). Conversion to FAD at site inlet: Q_FAD = Q_ref × (p_ref ÷ p_inlet) × (T_inlet ÷ T_ref), absolute values; Nm³/h is 0 °C and 1.01325 bar a; for scfm enter the reference temperature its quoter uses (at 1.01325 bar a). Minimum compressor discharge pressure = generator minimum inlet pressure + filter, dryer and pipe losses. The air factor must come from the selected generator’s data at your purity, pressure and temperature; it rises steeply with purity.
What to take from it. Size on the worst summer inlet temperature and check the pressure at the generator inlet during bed switching, not at the compressor. Do not add pressure “for safety”: about 7% more power per bar.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 7 November 2026. How our engineering content is reviewed.
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.
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.