By letting some gases through the fibre wall faster than others - a difference in permeation rate rather than a physical filter.

Each fibre is a tube with a wall made from a polymer chosen for its selectivity. Compressed air is fed into the bore. Gas molecules dissolve into the polymer at the inner surface, diffuse through the wall, and emerge on the outside.

Different gases do that at very different rates. Oxygen, water vapour and carbon dioxide permeate comparatively quickly, so they cross the wall and are collected outside the fibres and vented. Nitrogen permeates far more slowly, so most of it stays in the bore and travels down the fibre to the outlet as product.

It is not a sieve and nothing is trapped - the process is continuous, and the separation happens along the length of the fibre. That is why there is no cycle, no regeneration and no switching, and why a membrane is on specification almost immediately.

For simplicity, instant availability and tolerance of intermittent duty - not for purity.

A membrane has no valves cycling, no control sequence, no vessels to pressurise and vent, and no moving parts to wear. That makes it quiet, compact and largely maintenance-free apart from the filtration protecting it.

It comes on specification within moments of starting, where a PSA machine needs a stabilisation period. So for duty that starts and stops - a machine used a few hours a day, a mobile installation, a standby system - a membrane wastes nothing while a PSA machine vents product at every start.

It also suits remote and unattended sites, offshore and mobile applications, and anywhere servicing is expensive.

What it does not do is reach the purities PSA achieves. Once the requirement moves into low residual oxygen territory, the membrane's flow collapses and PSA becomes the only sensible option.

Useful purities for a wide range of industrial duties, but well short of PSA's high end - and the flow penalty for pushing it is severe.

Membrane purity is adjusted by changing how much gas is allowed to pass through the module: slowing the flow gives the oxygen more time to permeate out, so the product is purer. Speeding it up gives more product at lower purity.

That relationship is steep. Moving a membrane system towards its upper purity limit reduces the delivered flow dramatically, and the compressed air consumed per unit of nitrogen rises correspondingly. There comes a point where adding more membrane modules to reach a purity is more expensive than buying a PSA machine.

Typical membrane applications - tank and pipeline blanketing, tyre inflation, fire prevention systems, marine inerting, general packaging - sit comfortably in the range where membranes are efficient. Laser cutting and electronics do not.

Oil aerosol above everything, then liquid water and certain vapours - and the damage is permanent.

Hydrocarbon aerosol reaching the fibres coats the polymer surface and interferes with the dissolution and diffusion the separation depends on. Selectivity falls, and it does not recover. Unlike a fouled adsorbent bed, which can sometimes be regenerated, a contaminated membrane module is replaced.

Liquid water in the fibre bores blocks flow and can damage the fibres physically. Some solvent vapours attack the polymer directly.

So the feed air specification is a hard requirement rather than a recommendation: coalescing filtration to remove aerosol, activated carbon to remove oil vapour, and adequate drying - all to the manufacturer's stated standard, with elements changed on schedule rather than when they look dirty.

Because the failure is gradual and irreversible, monitor purity as the early indicator. A membrane that is slowly losing purity at constant flow is reporting contamination.

Where simplicity, size or remoteness matters more than purity.

Marine and offshore applications are a classic case - tank inerting on vessels, blanketing on platforms - where the equipment must run unattended, survive motion and vibration, and be serviced rarely. Mobile and skid-mounted nitrogen units use them for the same reasons.

Fire prevention systems in archives, server rooms and cold stores use membrane nitrogen to hold a reduced-oxygen atmosphere continuously. Tyre inflation, both commercial and aviation, is another established use.

Process applications include tank and pipeline blanketing, chemical and pharmaceutical inerting where moderate purity suffices, and food packaging at the less demanding end.

They are also frequently chosen simply for space: a membrane module bank is compact and can be mounted in places a twin-vessel PSA machine cannot go. In every case the decision starts by confirming the required purity sits inside the membrane's efficient range.

By controlling the flow through the module - slower flow gives higher purity.

Separation happens progressively along the length of the fibre: the longer the gas spends travelling down the bore, the more oxygen has permeated out through the wall. So restricting the outlet flow raises purity, and opening it up lowers purity while delivering more gas.

That makes adjustment simple - a flow control valve and a purity indicator are effectively the whole control system - and it is why a membrane responds almost immediately to a change.

The relationship is steep at the high-purity end, so a small reduction in flow buys a modest purity gain while a large one may be needed for a further step. Feed pressure and temperature also affect it: higher pressure and warmer air both increase permeation.

Because temperature matters, a system set up in winter may behave differently in summer, which is worth checking rather than discovering.

No - they are replaced on performance rather than on time, and with clean air they last a long while.

There is nothing in a membrane module that wears mechanically. The fibres do not cycle, flex or move, so in a properly protected installation the module simply continues to work.

What ends its life is contamination. Oil aerosol and vapour degrade selectivity irreversibly, and the effect accumulates. The symptom is a gradual loss of purity at a given flow, or a gradual loss of flow at a given purity.

So the maintenance regime is: protect the module with the specified filtration, change those filter elements on schedule, and monitor purity as the condition indicator. Replace the module when performance no longer meets the requirement.

A module that has degraded quickly is reporting a filtration problem, and replacing it without fixing that simply consumes another module.