Answer
Oil-Free Labyrinth-Piston Compressors (e.g., Sulzer / Burckhardt design): These are preferred for LPG/Ammonia operations. The piston features a series of labyrinth grooves that create non-contact sealing against the cylinder wall. This design ensures that no lubricating oil comes into contact with the cargo vapor, preventing downstream cargo contamination.
Oil-Injected Screw Compressors: Used on some modern vessels, but they require complex, high-efficiency multi-stage oil separators to ensure zero oil carry-over into the cargo tanks.