A method of treating air on board on a vehicle, before the air is used in compressed form in the vehicle, wherein the air in a first step is compressed by means of a compressor, which for the purpose of its operation is supplied with lubricating oil, thereafter is conveyed through a particle separator, and finally relieved from vaporised water by means of a filter,
- wherein, the compressed air in said particle separator is relieved from lubricating oil accompanying the air from the compressor by being rotated in a separation chamber by means of a rotating member, said rotating member comprising separation discs that form a plurality of interspaces through which said compressed air relieved from said particles is conveyed.