It is a dynamic machine: it accelerates the air and then converts speed into pressure, rather than trapping a volume and reducing it.

Screw and piston compressors are positive displacement. They capture a definite volume of air and physically reduce it, so the flow they deliver is set by their geometry and speed, and the pressure is whatever the system demands.

A centrifugal machine works the other way round. An impeller spinning at high speed throws air outward, giving it kinetic energy; a diffuser then slows that air, converting velocity into pressure. Flow and pressure are linked by the machine's characteristic curve rather than being independent.

The consequences are that centrifugal machines suit very large flows, have no contacting parts in the gas path and are therefore oil-free, run for very long periods between overhauls - and cannot simply throttle back the way a positive-displacement machine can.

A flow instability at low throughput where the air momentarily reverses through the machine - violent, damaging, and to be prevented rather than survived.

A centrifugal compressor develops pressure as a function of flow. As flow is reduced, the machine reaches a point where it can no longer sustain the discharge pressure against the system, and the flow briefly reverses. Pressure then collapses, forward flow re-establishes, and the cycle repeats - rapidly and with considerable force.

The result is heavy axial loading on the rotor, severe vibration, rising temperatures and rapid damage to bearings and impellers.

So every centrifugal installation has surge protection: a control system that monitors the operating point against a surge line and opens a blow-off valve to maintain flow through the machine when demand falls too low.

That blow-off vents compressed air to atmosphere, so operating well below full load wastes energy - which is why these machines are matched to steady base loads.

The large end of industrial compressed air - well above where screw compressors are economical, and rising to the very largest process installations.

Below a certain size a centrifugal machine is not competitive: the precision, the high rotational speeds and the control system cost too much relative to a screw compressor. Above it, the position reverses, because centrifugal efficiency holds up as size increases while the mechanical difficulties of a very large screw or piston machine multiply.

So they are found where compressed air is consumed in bulk: steel and metals, chemicals and petrochemicals, glass, cement, mining and minerals processing, large air separation plants, and utility air systems for whole works.

They are also common in dedicated process duties - supplying a single large continuous consumer - because that is precisely the steady base load the machine suits.

Many large sites run centrifugal machines for base load and screw compressors for trim, which gives efficiency at scale plus the ability to follow demand.

Yes in the gas path - there is no contact between moving parts and the air, so no oil is introduced by the compression process.

The impellers do not touch anything. Compression happens through aerodynamic action alone, so unlike a screw or piston machine there is no rubbing surface in contact with the air stream and nothing to lubricate there.

Oil is present in the machine, lubricating the bearings and, on geared units, the gearbox - but it is contained in its own system, separated from the gas path by seals.

That makes centrifugal machines inherently oil-free in the same structural sense as an oil-free screw compressor: the guarantee is the absence of oil in the air path rather than the presence of filtration.

As always, the class achieved should be confirmed against the compressed air purity standard and the seal arrangement understood, particularly on older machines where seal condition matters.

Condition monitoring rather than frequent intervention - these are large rotating machines and are managed as such.

With no contact in the gas path, there is little to wear in the compression elements themselves, and overhaul intervals are long. The routine work is in the supporting systems: lubrication oil condition and filtration, the cooling water circuits for the intercoolers, instrument air and the control system, and the inlet filtration that protects the impellers.

Monitoring matters more than servicing. Vibration monitoring on the bearings, temperature monitoring through the stages, and continuous surveillance of the operating point against the surge line are standard, because these machines give warning through trends long before they fail.

Inlet air filtration deserves particular attention: a high-speed impeller is damaged by particulate, and erosion changes the machine's characteristic gradually.

Overhaul is a specialist operation planned well in advance rather than a maintenance task.