Because without oil in the chamber the rotors cannot be allowed to touch, so something else has to hold them in the correct relationship.

In an oil-injected compressor the male rotor drives the female rotor directly through their meshing profiles, with the oil film preventing metal-to-metal wear. That is simple and it works because the oil is always there.

Remove the oil and direct contact would destroy the rotors quickly. So an oil-free machine drives the two rotors through a set of precision timing gears outside the compression chamber, holding them in exact phase with a small clearance between them at all times. The rotors never touch each other or the housing.

The cost is precision. The gears, the bearings and the rotor machining must all be tight enough to maintain that clearance under load and thermal growth, which is a large part of why the machine is more expensive.

The timing gears themselves are lubricated, but that lubrication is sealed away from the air path.

Because without injected oil there is nothing carrying the heat of compression away, so the temperature rise across a single stage would be unacceptable.

Compressing air heats it. In an oil-injected machine the oil absorbs that heat as it is generated, keeping discharge temperature moderate even at a high pressure ratio. Take the oil away and the air alone carries the heat, so the temperature rise is much greater for the same compression.

A single dry stage taken to normal working pressure would reach temperatures that damage the rotor coatings, threaten the bearings and reduce efficiency sharply, since hot air is harder to compress.

Splitting the work into two stages with an intercooler between them solves it: each stage does less compression and therefore generates less heat, and the air is cooled back down before the second stage. The result is acceptable discharge temperatures and better efficiency than one hot stage would achieve.

No - and the difference is about failure modes rather than about the oil content achieved on a good day.

A well-maintained oil-injected compressor with good downstream filtration can deliver air with very low oil content, and for many purposes that is genuinely sufficient.

The distinction is what happens when something goes wrong. A coalescing element can rupture, a carbon element becomes saturated and stops adsorbing with no visible change, a drain can fail. In each case oil passes downstream and nothing announces it. Where the air contacts food, pharmaceuticals or a semiconductor wafer, the contamination is discovered in the product.

An oil-free machine has no oil in the air path to pass. The guarantee is structural rather than dependent on maintenance being correct.

That is the argument, and it is why regulated industries specify oil-free rather than filtered - it is a risk decision, not a performance comparison.

The most demanding oil content classification in the compressed air purity standard - effectively a manufacturer's declaration of oil-free, verified by testing.

Compressed air purity is classified for particulate, water and oil, each on its own numbered scale, with lower numbers meaning cleaner air. For oil, the classes cover decreasing concentrations, and the most stringent designation requires oil content below the lowest specified level - in practice, below what can be reliably detected.

A machine claiming it should be able to show independent test evidence covering total oil content including vapour, not just aerosol, and measured at the machine outlet under realistic conditions.

When specifying, state the required class for all three contaminants rather than asking for 'oil-free' in general terms, because the phrase is used loosely. And confirm the class applies to the whole installation, since the compressor's output can be recontaminated by an unsuitable distribution system downstream.

Generally yes at the same duty, and it is worth knowing that before comparing quotations on capital cost alone.

The oil in an injected machine seals the rotor clearances very effectively. Without it, the same clearances leak, so an oil-free machine has to work harder for the same delivered air - the internal leakage is real and it costs energy.

The two-stage arrangement recovers some of that, because compressing in two steps with intercooling is thermodynamically more efficient than one large step, but the net position usually still favours the oil-injected machine.

Against that, an oil-free installation needs less downstream filtration, and filtration itself costs pressure drop and therefore energy - so the whole-system gap is narrower than the machine-to-machine gap.

The decision should not be made on efficiency in any case. If the process requires oil-free air, efficiency is a cost of meeting the requirement, not a reason to reconsider it.