Reciprocating Compressors for Oil-Free Applications
For oil-free applications, special precautions have to be taken. Figure 7 shows how particular designs produce a high degree of cleanliness in the air. No lubrication is necessary in the upper cylinder because the pistons are fitted with PTFE rings; the combination of these rings, austenitic liners and a crosshead guide to minimise piston side loads extends operating life without complicating maintenance. To prevent oil entering the cylinder, a distance-piece incorporating rubber seals is fitted between each cylinder and crankcase. Pure air is produced straight from the compressor without requiring further separation equipment.
An alternative to PTFE piston rings is the use of a labyrinth seal on the piston. Fine grooves are cut in the cylinder wall and the piston skirt, and usually on the piston rod as well.