Why is it so difficult, even impossible, to carry out oil-free compression element rebearing?

It all starts when an oil-free compression element (otherwise known as an oil-free air-end) is designed. Atlas Copco's goal is high air supply with the highest efficiency level, resulting in a specific shape of the male and female rotors towards the housing. This design can be further validated in 3D simulations, resulting in extensive prototype testing, followed finally by serial production.

The successful Atlas Copco Z compressor air-end design has minimum distance between the rotating and non-rotating parts as it is important to avoid any air leakage during the compression cycles. Here, for an oil-free screw element, the coating plays an important role: A double Teflon coating will tighten off the rotors towards the housing, and it also contains graphite to reduce friction, but whilst giving the element the chance to “run in to shape”.

To further reduce the chance of air outflows, we have the highest tolerances in place during machining. We can deviate by only a fraction of the diameter of a human hair, which helps to reduce the head clearance, i.e. the gap in the front and back of the rotors towards the element housing.

All these design attributes on our oil-free Z compressor elements are nice to have, but an element needs to perform thousands of hours in challenging conditions, including high temperatures,  too much condensate, or dirt particles from the inlet air. These conditions demand Atlas Copco designed parts, for example bearings, oil seals and gaskets. They close off the moving from the non-moving parts, but also ensure that axial loads are well distributed over the housing design.

The refurbishment of an oil-free screw element means to dis-assemble it, exchange the worn parts and finally assemble the element again. And here the first problems can arise. Whilst the dis-assembling of an element is not difficult, the re-assembly requires the highest tolerances, which can’t be reached without special tools and knowledge. The consequences of bad element assembly are extensive air leaks and a reduced element performance.

The exchanging of worn parts is often one of the key arguments of companies who recondition oil-free compression elements. But from Atlas Copco's own analysis we can see that sometimes cheaper, non-OEM parts are used or, even worse, the old parts are reused! This means that one of the key arguments for refurbishing the element will result in early failure of the element, and the compressor as a result. Unfortunately such practices are invisible to the customer.

Finally, the re-coating of the male and female rotors is often part of the  Z compressor element refurbishment process. Also here our analysis shows that specifications are often not met, a standard single layer Teflon coating is used so no “run into shape” is possible and - due to missing graphite - the friction increases.

The initial savings made by purchasing a cheaper refurbished oil-free screw element will be exceeded after about 13 months by a much higher energy bill, without even taking into account the user's cost incured by repairs or production stoppage.

Says Paul Clark, Business Line Manager Compressor Technique Service Sales & Marketing: "Getting an oil-free screw compressor element refurbished has many pitfalls, does not save on servicing costs, and we do not recommend it. The difference in energy consumption between a reconditioned and a new compressor element can be as much as 20% and there is a high risk of element failure. It is always better to replace a worn oil-free screw compressor element with a new one from the original equipment manufacturer."