Three things simultaneously, which is why it is described as a working fluid rather than just a lubricant.

It seals. The two rotors turn with a small clearance between them and the housing, and without something filling that gap, compressed air would leak back past the rotors and the machine would be inefficient. Oil fills the clearances and forms a seal, allowing a useful pressure ratio in a single stage.

It cools. Compressing air generates a great deal of heat, and in a dry machine that heat would raise discharge temperatures beyond what the materials and the air can tolerate. Oil injected into the chamber absorbs the heat as compression proceeds and carries it out to the cooler.

It lubricates the bearings and the rotor contact.

Because it does all three, the oil circuit is central to the machine rather than ancillary, and its condition determines efficiency, discharge temperature and life.

In two stages - bulk separation by gravity and direction change, then a coalescing separator element for the aerosol.

Air and oil leave the airend as a mixture and enter the separator vessel tangentially. The sudden expansion and change of direction throws most of the oil out of the stream, and it collects in the bottom of the vessel - that is the bulk of the oil recovered.

What remains is a fine aerosol that will not drop out by gravity. It passes through a separator element, a coalescing filter that merges the droplets until they are heavy enough to fall, and the collected oil is returned to the airend through a scavenge line.

That scavenge line is a common fault point: if it blocks, oil accumulates in the element, is carried over into the air, and the machine appears to be consuming oil.

The element is a scheduled replacement item, and a rising pressure differential across it is the indicator that it is due.

A small carryover is inherent, typically a few parts per million by weight, and it is managed downstream rather than eliminated at the compressor.

No separation is perfect. A well-maintained oil-injected machine with a sound separator element delivers air containing a low but non-zero oil content, both as remaining aerosol and as oil vapour, which a coalescing element cannot remove at all.

For most industrial uses - tools, cylinders, general plant air - that is entirely acceptable. Where lower oil content is needed, downstream treatment handles it: coalescing filters for aerosol and activated carbon for vapour, selected to the required air quality class.

What downstream filtration cannot do is make the air genuinely oil-free in the sense a critical process requires, because a filter failure or a saturated carbon element passes oil with no warning. Where the consequence of oil contamination is severe, the argument is for an oil-free machine rather than for more filtration.

Yes - continuous full-load duty is exactly what it is built for, and it is one of the main differences from a reciprocating machine.

The injected oil removes the heat of compression as it is generated, so discharge temperature stabilises at a moderate level and stays there. There are no reciprocating parts, no valves opening and closing, and no rubbing contact in the compression chamber - the rotors do not touch each other or the housing.

That means a hundred per cent duty cycle is normal, and these machines are routinely run twenty-four hours a day for years.

The practical consequence for selection is that a screw compressor should be sized close to the actual demand and run loaded, not oversized. A screw machine running lightly loaded or spending much of its time unloaded is inefficient - it consumes significant power while producing nothing - which is precisely the problem variable speed drive was introduced to solve.

Because compressed air demand varies and a fixed-speed screw compressor wastes a great deal of energy whenever it is not fully loaded.

A fixed-speed machine runs at one speed. When demand falls below its output it unloads - continuing to turn, consuming a substantial fraction of full-load power, while delivering no air. On a site whose demand swings through the day, a fixed-speed compressor can spend much of its life doing exactly that.

A variable speed machine matches its output to demand by changing rotor speed, so it delivers what is needed and consumes roughly in proportion. It also holds system pressure within a narrow band rather than cycling between limits, which allows the whole system to run at a lower average pressure - and lower pressure is itself a significant energy saving.

The payback is usually short where demand genuinely varies. Where demand is constant and the machine runs fully loaded continuously, a fixed-speed unit is simpler and cheaper and the drive adds little.